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Patent Abstract
The invention relates to a solenoid valve made using certain materials
which is capable of operation at high frequencies and which can
be made as a compact unit. The plunger (1) is made from an electromagnetic
material having a saturation flux density greater than 1.2 Tesla
preferably more than 1.6 Tesla, the bore leading from the valve
head chamber (14) to the nozzle orifice (12) has a length to diameter
ratio of less than 5:1 preferably between 2:1 and 4:1 and the nozzle
orifice (12) has a diameter of 80 micrometer or less. The invention
also relates to method for operating a drop on demand ink jet printer
incorporating such a valve.
Patent Claims
1. A method for applying an image forming composition to a pile
fabric using a drop on demand ink printer, characterized in that
the printer is operated at a drop generation frequency of at least
1 kHz.
2. A method as claimed in claim 2, characterized in that the pile
fabric has a pile length of at least 2 mms and the printer is operated
at a pressure of less than 3 bar, notably at from 1.5 to 2.5 bar.
3. A method for applying an image forming composition to a pile
fabric using a drop on demand ink jet printer, comprising the steps
of: a. providing a drop on demand ink jet printer, comprising: 1.
a reservoir; 2. a nozzle orifice; and 3. a valve for regulating
the flow of ink from the reservoir to the nozzle orifice, the valve
comprising: i. a plunger member journalled for axial reciprocation
between a rest and an operative position within a tubular member
supporting an electric coil under the influence of a magnetic field
generated by that coil when an electric current passes through the
coil; ii. bias means to bias the plunger towards its rest position
when no current is applied to the coil, the distal end of the plunger
extending into a valve head chamber having a outlet bore to a nozzle
orifice, the reciprocation of the plunger being adapted to open
or close a fluid flow path from the valve head chamber through that
bore to the nozzle orifice; and wherein, the plunger is made from
an electromagnetic material having a saturation flux density greater
than 1.2 Tesla, the bore leading from the valve head chamber to
the nozzle orifice has a length to diameter ratio of 5:1 or less,
and the nozzle orifice has a diameter of 80 micrometers or less;
and b. operating the printer at a drop generation frequency of at
least 1 kHz
4. A method for printing an image on a pile fabric, using a drop
on demand printer in which the solenoid valve mechanism for controlling
the flow of fluid to the nozzle orifice comprises a plunger member
journalled for axial reciprocation between a rest and an operative
position within an electric coil under the influence of a magnetic
field generated by that coil when an electric current passes through
the coil, the distal end of the plunger extending into a valve head
chamber having an outlet nozzle bore, the reciprocation of the plunger
being adapted to open or close a fluid flow path from the valve
head chamber through that bore, characterized in that: a. the plunger
is of a unitary construction and is made from an electromagnetically
soft material having a saturation flux density greater than 1.4
Tesla, a coercivity of less than 0.25 ampere per meter, and a relative
magnetic permeability in excess of 10,000; and b. the nozzle bore
leading from the valve head chamber to the nozzle orifice has a
length to diameter ratio of less than 8:1.
5. A method of operating a solenoid valve, the method comprising
the step of energizing an electric coil to generate a magnetic field
in order to reciprocally drive a plunger within a coil, wherein
the magnetic field is controlled such that the speed of the plunger
is decreased as the plunger approaches at least one of its extremes
of movement.
Patent Description
RELATED APPLICATIONS
[0001] The present application is a division of U.S. application
Ser. No. 10/492,258, filed Dec. 28, 2004, which is the United States
national stage application of PCT/CA/01544, filed Oct. 15, 2002,
which claims priority to British application 0216935.7, filed Jul.
22, 2002, all of which are hereby incorporated by reference in its
entirety.
[0002] The present invention relates to a device, notably a high
speed solenoid type valve, an ink jet printer incorporating that
valve and methods of operating an ink jet printer, preferably utilising
that valve.
BACKGROUND OF THE INVENTION
[0003] Ink jet printers are non-contact printers in which droplets
of ink are ejected from one or more nozzle orifices so as progressively
to build up a printed image on a substrate moved relative to the
nozzle. One form of ink jet printer comprises a source of ink, typically
a reservoir or bottle of ink, which is pressurised to from 0.1 to
2 bar, notably about 1 bar. The pressure is created, for example,
by pressurising the air space above the ink in the bottle or reservoir.
The ink is fed to the nozzle orifice(s) in a print head through
which it is ejected as a series of droplets onto the surface of
the substrate. The flow of ink through each nozzle orifice is controlled
by a solenoid valve. Typically, such a valve comprises an electromagnetic
plunger journalled for axial movement within an axially extending
electric coil. The distal end of the plunger is located within a
valve head chamber through which ink flows from the reservoir to
the nozzle orifice. When current is fed through the coil, this generates
a magnetic field which acts on the plunger to move it axially and
thus open, or shut, the inlet of a bore from the valve head chamber
to the nozzle orifice. Typically, the magnetic field acts to retract
the plunger against the bias of a coil spring to create a flow path
between the valve head chamber and the nozzle orifice. When the
electric current no longer flows in the coil, the magnetic field
ceases and the plunger returns under the bias of the spring to close
the flow path to the nozzle orifice. Typically, a plurality of nozzle
orifices are formed as one or more rows in a plate, the nozzle plate,
and each nozzle orifice is served by a separate solenoid valve,
so that droplets of ink can be ejected independently from one or
more of the nozzle orifices. Typically, the valves are fed with
ink from the reservoir via a manifold which serves to split and
even the ink flow between each of the valves. The row of nozzle
orifices is typically aligned transversely to the direction of travel
of the substrate so that simultaneous operation of the valves will
cause a row of ink dots to be printed on the substrate.
[0004] The valves are operated so as to deposit dots upon the substrate
at the desired locations on the substrate to build up the elements
of a five, seven, eight or more dot raster image on the substrate.
By suitable timing of the opening of the various valves in the print
head an alphanumeric or other image can be formed on the substrate
to print a date, product batch code, logo, bar code or other image
on the substrate. If desired, several print heads can be combined
in an array so as to print a wider image on the substrate and the
line of nozzle orifices in the print head can be angled to the direction
of movement of the substrate so as to lay down droplets which are
more closely spaced than where the print head is aligned normal
to the line of travel of the substrate.
[0005] For convenience, the term drop on demand printer will be
used to denote in general such types of ink jet printer.
[0006] The size of the printed dot can readily be altered by varying
the duration for which the valve is held open, and hence the amount
of ink that it allows to flow through the nozzle orifice. The form
of the image which is printed can readily be altered by varying
the sequence of operation of the valves in the print head so that
droplets are ejected from the appropriate nozzles in the appropriate
sequence to form the desired image. Such alterations of the images
and the dot sizes can readily be controlled by a computer or microprocessor
operating under an appropriate program or operating system. Such
drop on demand printers are widely available commercially and find
widespread use in printing a wide range of both visible and non-visible
machine-readable images on a wide range of substrates.
[0007] However, as the speed of travel of the substrate past the
print head increases, a point is reached at which the valve cannot
be operated at sufficient speed to eject droplets at sufficient
frequency to form the desired image without creating some distortion.
Typically, the limit for the speed of operation of solenoid valves
in current use in an ink jet printer head is less that 800 to 1000
Hz. With increasing pressure on manufacturers to increase through
put from a given production or packaging line, there is an increasing
need to be able to print the dots onto the substrate at rates greater
than this.
[0008] In an alternative form of ink jet printer known as an impulse
jet printer, a piezoelectric crystal or other transducer is applied
to or forms part of a wall of an ink jet chamber having an ink inlet
and an ink outlet to a nozzle orifice. When a voltage is applied
to the transducer, the transducer expands or flexes and causes a
change in the volume of the ink jet chamber. This causes a droplet
of ink to be ejected from the chamber and to exit through the nozzle
orifice. The transducer can be caused to flex at very high rates
by electronic control of the frequency of the electrical pulses
applied to the transducer, so that such a print head can apply dots
at frequencies up to 15 kHz or more. However, the volume of ink
ejected through the nozzle orifice is dependent upon the extent
of flexing of the transducer. This can be varied by varying the
amplitude of the electric pulse applied to the transducer. However,
each type of transducer operates consistently only within a narrow
percentage, typically plus or minus 50%, of the optimum operating
pulse amplitude, so that only a limited range of dot sizes can be
achieved with a commercially available impulse jet printer. This
limits the number of applications for which a given impulse jet
head can be used for.
[0009] It has been proposed in International Patent Application
No PCT/SE97/01007 to produce a solenoid type valve for a drop on
demand ink jet printer which is claimed to be capable of operating
at frequencies of up to 3 kHz. Such a valve incorporates light weight
components so as to reduce the inertia of the plunger and thus enable
it to accelerate and decelerate rapidly at each extreme of its travel
within the coil. To achieve this, the plunger is formed from two
components, one made from an electromagnetic material so that it
can be caused to move by the magnetic field generated by the current
passing through the coil, and a second lightweight plastic component
for the distal end of the plunger. Such a construction is complex
and expensive. Furthermore, we have found that a print head incorporating
such a valve design does not print acceptable images. For example,
at high frequencies of operation of the valve, the printed dots
are uneven and there are many small satellite dots around each of
the primary dots printed by the print head.
[0010] We have now devised a form of valve which can be operated
at speeds of up to 8 kHz or more and yet can be used in a drop on
demand printer to print uniformly sized droplets over a surprisingly
wide range of dot sizes and operating frequencies. Furthermore,
the valve of the invention can be more compact and with smaller
components than a conventional design of solenoid valve for use
in a drop on demand ink jet printer. This allows high definition
printing to be achieved at high print rates without excessive heat
being generated during operation of the valve. Such a valve enables
drop on demand technology to be used in high speed applications
for which an impulse jet print head had hitherto been considered
the only technically viable form of print head.
SUMMARY OF THE INVENTION
[0011] Accordingly, the present invention provides a valve mechanism
for controlling the flow of fluid therethrough, which mechanism
comprises a plunger member journalled for axial reciprocation between
a rest and an operative position within a tubular member supporting
an electric coil under the influence of a magnetic field generated
by that coil when an electric current passes through the coil, bias
means to bias the plunger towards its rest position when no current
is applied to the coil, the distal end of the plunger extending
into a valve head chamber having a outlet bore to a nozzle orifice,
the reciprocation of the plunger being adapted to open or close
a fluid flow path from the valve head chamber through that bore
to the nozzle orifice, characterised in that:
[0012] a. the plunger is made from an electromagnetic material
having a saturation flux density greater than 1.2 Tesla; and
[0013] b. the bore leading from the valve head chamber to the nozzle
orifice has a length to diameter ratio of 5:1 or less; and
[0014] c. the nozzle orifice has a diameter of 80 micrometres or
less.
[0015] For convenience, the terms distal and proximal will be used
herein to denote that portion of a component which is located downstream
and upstream respectively with respect to the flow of ink or other
fluid through the valve. The valve of the invention is of especial
application in drop on demand ink jet printers which are to be operated
at drop generation frequencies of 1 kHz and higher. For convenience,
the invention will be described hereinafter in terms of a valve
for such an application.
[0016] In existing designs of solenoid valve, it is not possible
to operate the valve for prolonged periods at speeds in excess of
1 kHz. We believe that this is due to the fact that the hysteresis
of the material from which the plunger is made to changes in the
magnetic field applied to it causes the material to become magnetically
saturated so that it no longer can respond to further changes in
the magnetic field applied to it. The use of a material having a
magnetic saturation flux density of greater than 1.2 Tesla, preferably
from greater than 1.4 Tesla, notably 1.4 to 1.8 Tesla, enables the
plunger to continue to respond to changes in the magnetic field
applied to it even after prolonged periods of operation at frequencies
in excess of 3 kHz and more, for example up to 9 kHz, without becoming
magnetically saturated. Furthermore, where the coil/plunger combination
also has a high magnetic inductance, for example 9 milliHenrys or
more, the coil can exert a high driving force on the plunger further
enhancing the rate of response of the plunger to changes in the
electrical current flowing through the coil. These properties enable
a smaller plunger, that is one of smaller mass, to be used, since
a given drive current can exert a greater force on the plunger of
a given size than where a conventional material of construction
is used. These properties result in a plunger which can be accelerated
rapidly at either extreme of its travel with a comparatively low
driving current, thus reducing the heat generated during operation
of the valve.
[0017] We have also found that it is desirable to reduce the gap
between the coil and the plunger to a minimum so as to reduce the
loss of magnetic coupling between the coil and the plunger to a
minimum. This enhances the response of the plunger to changes in
the current applied to the coil and reduces the amplitude of the
current required to drive the plunger within the coil. In conventional
designs of solenoid valve proposed hitherto, the coil is wound upon
a plastic or other bobbin and the bobbin then mounted upon a tube
forming the bore within which the plunger reciprocates. This results
in the formation of a radial gap of 2 mms or more between the inner
face of the coil and the outer face of the plunger. From another
aspect of the present invention, it is preferred to form the coil
directly upon or within the wall of the tubular member within which
the plunger reciprocates as described below, so as to reduce this
radial gap to less than 1 mm.
[0018] The nozzle orifice is typically provided by the open end
of the outlet bore from the valve head chamber. Such a bore can
be provided by a capillary tube outlet to the valve head chamber,
or by the bore in a jewel nozzle of the type conventionally used
in ink jet printer nozzles. Such a jewel can be set into the distal
end of a bore in a nozzle plate upon which the valve mechanism is
mounted. It will be appreciated that the jewel nozzle can provide
both the outlet bore from the valve head chamber and the nozzle
orifice itself, for example when a jewel nozzle is set into a bore
within a nozzle plate so that its proximal face is substantially
flush with the proximal face of the nozzle plate, for example where
it forms all or part of the distal end wall of the valve head chamber.
[0019] We have found that if the volume of the body of fluid retained
in the bore between the valve head chamber and the nozzle orifice
is excessive, that body of fluid possesses sufficient inertia to
damp out rapid movement of the plunger required when the valve is
operated at high frequencies. This causes the fluid to issue erratically
from the nozzle orifice at frequencies of operation in excess of
about 1 kHz. Surprisingly, if the length of the bore is less than
5 times the diameter d of the bore, for example 0.5 to 5 times the
diameter of the bore, the volume of ink retained in the bore between
actuations of the plunger is reduced sufficiently so that it can
respond rapidly to actuations of the plunger.
[0020] Where the plunger is to be actuated at frequencies in excess
of 1 kHz, we have found that if the length to diameter ratio of
the bore is reduced to below about 1.5:1, spraying of the droplets
at the nozzle orifice may occur, even though the frequency of droplet
formation is below than at which an impulse jet printer having an
even smaller length to diameter ratio for the outlet nozzle operates
without causing spraying of the ejected droplets. We therefore prefer
that the length to diameter ratio of the bore be from 2:1 to 4:1.
It is particularly preferred that the length to diameter ratio be
less than about 3:1, preferably 2:1 to 3:1. To reduce the volume
of ink within the bore, we have found that the diameter of the nozzle
orifice should be less than 80 micrometres, preferably about 40
to 60 micrometres. Surprisingly, we have found that the use of these
combinations of bore length to diameter and nozzle orifice sizes
reduces the formation of satellite droplets from the nozzle as well
as reducing the damping effect on the motion of the plunger and
enables droplets of a wide range of sizes to be produced consistently
at frequencies in excess of 1 kHz, notably at 2 kHz or more.
[0021] With conventional valve designs, one problem has been the
imperfect formation of the desired droplets after the valve has
been idle for any length of time. This has been ascribed to drying
out of the ink at the nozzle orifice and many steps have been taken
to reduce this. We have found that the use of the bore and nozzle
orifice dimensions described above has the surprising effect of
reducing the imperfect initial droplet formation. We believe that
this is due to a reduction in the drying out of ink within the valve
mechanism. Furthermore, we have found that the use of a length to
diameter ratio of from 2:1 to 5:1 enables control of the directionality
of the ejection of the droplet from the nozzle orifice to be achieved.
In a conventional design of drop on demand print head, the length
to diameter ratio of the bore is typically about 10:1. It is surprising
that reducing the length of the bore does not result in loss of
directionality of the ejection of the droplet from the nozzle orifice.
[0022] Surprisingly, we have found that the removal of part of
the core of the plunger to form an internal bore within part of
the length of the plunger and thus reduce the mass of the plunger,
does not affect the magnetic properties of the plunger to a significant
extent and that the plunger behaves magnetically as if it were a
solid member. For example, the axial bore can be formed as a blind
ended drilling in a solid rod of a suitable material. Such a form
of construction of the plunger is much simpler than the complex
two component construction described in PCT Application No SE97/01007.
The amount of metal removed from the plunger depends upon a number
of factors, for example the strength required for the resultant
hollow plunger, the magnetic force required to move the plunger
against the bias of the spring and the acceleration and deceleration
required at each end of the travel of the plunger. However, the
bore should not extend significantly into that portion of the plunger
which lies within the coil when the plunger is in its fully retracted
position. We have found that if the bore extends significantly beyond
this point, the magnetic coupling between the coil and the plunger,
and hence the force driving the plunger at the initiation of its
extension stroke towards its extended position, is reduced.
[0023] Removal of material from the plunger reduces the mass and
hence inertia of the plunger. However, removal of material would
have been expected to reduce the magnetic force which can be applied
to the plunger by the coil and hence to result in a plunger that
could not be accelerated sufficiently by the magnetic field applied
to it. We have found that this is counteracted by the use of materials
of construction for the plunger which have a high saturation flux
density as described above. Preferably, the plunger/coil combination
has a high magnetic inductance, typically 9 milliHenrys or more.
Suitable materials for use as the plunger material include magnetisable
iron or steel alloys, notably those of iron and nickel containing
from 40 to 55% of nickel, especially those containing from 45 to
50% nickel and from 55 to 50% of iron. If desired, other metals
such as chromium or aluminium may also be present in minor amounts.
Preferred materials for present use are those which have a saturation
flux density in excess of 1.6 Tesla, for example 1.8 Tesla or more.
The preferred materials also have a coercivity less than 0.25 amperes
per metre, and a permeability in excess of 50,000, for example 100,000
or more. Suitable materials for present use include those ranges
of alloys sold under the Trade Names Permenorm 5000 and Vacofer
SI. Where high Tesla materials are used, it may be desirable to
provide the plunger with a corrosion resistant external layer or
surface.
[0024] If desired, composite materials, such as polymers or sintered
or fritted ceramics or silicon matrices, which have particles of
a suitable ferromagnetic or other magnetisable material dispersed
therein or which have otherwise been modified to have the desired
high saturation flux density and/or magnetic inductance may be used.
For example, a plunger may be formed from laminates of materials
of different properties to achieve the desired overall magnetic
properties.
[0025] The plunger typically comprises a generally cylindrical
member which is made from a suitable magnetic material and which
is a sliding fit within the coil or the tubular member within or
upon which the coil is supported. The plunger can be of any suitable
length and cross section which is preferably congruent to the internal
cross section of the tubular support for the coil. However, it is
particularly preferred to form the plunger with as a low a mass
as is practicable so as to enhance the speed of response of the
plunger to changes in the electric current applied to the coil.
Thus, the plunger may have an axial bore therein as described above.
Alternatively, the plunger may be formed as a polymer, ceramic or
other matrix containing a suitable ferromagnetic material incorporated
therein. However, it is particularly preferred to form the plunger
as a unitary solid body from a soft ferromagnetic material with
a diameter of less than 2.5 mms, notably about 1 mm, and a length
to diameter ratio of more than 3:1, preferably from about 5:1 to
10:1, so that the plunger slides freely within the tubular support
member of the coil.
[0026] In a preferred embodiment, therefore, the invention provides
a compact, light weight solenoid valve of the invention characterised
in that the plunger has a diameter of less than 2.5 mms, notably
about 1 mm, and has a length to diameter ratio of more than 3:1,
preferably from about 5:1 to 10:1.
[0027] The plunger is conveniently formed by machining, rolling
or extruding the desired alloy to form a length of material having
the desired size and shape. During the machining of the preferred
materials of construction to form the plunger for the valve, the
magnetic properties of the material may be affected. It may therefore
be desired to subject the manufactured plunger to some form of post
forming treatment so as to recover the magnetic values. Such treatments
include heat treatment or mechanical impact treatment which cause
a change in the crystal composition of the material. The optimum
form of post forming treatment can be readily determined using simple
trial and error.
[0028] The distal end of the plunger may be provided with a plastic
or rubber end face which forms a fluid tight seal when it bears
against the transverse end wall of the valve head chamber. If desired,
that end wall can be provided with one or more upstanding circumferential
ribs or seal members generally concentric with the inlet to the
outlet bore of the valve head chamber so as to enhance the sealing
engagement of the end face of the plunger with the end wall of the
valve head chamber. However, where the bore is provided by a length
of capillary tube, the end of the tube can protrude into the valve
head chamber to provide an annular sealing surface with which the
distal end face of the plunger engages. The optimum form of the
sealing arrangement can readily be determined by simple trial and
error tests and a combination of two or more forms of sealing arrangement
may be used if desired. The raised ribs or other sealing arrangement
can be formed using any suitable technique, for example during the
moulding of the transverse end wall. Surprisingly, we have found
that the use of annular raised sealing ribs on the distal end of
the plunger and/or on the transverse face of the valve head chamber
with which it engages has the effect of reducing the formation of
satellite drops from the nozzle orifice, especially at operation
of the valve at frequencies in excess of 1 kHz.
[0029] The valve mechanism has a coil through which an electric
current is passed to generate the magnetic field which acts upon
the plunger. Such a coil can be of conventional design and construction
and serves to generate the magnetic field required to move the plunger
when a current flows through the coil. Preferably, the coil is a
single winding of a suitable wire wound on a suitable bobbin which
may be removed and the resultant free coil potted in a suitable
resin to provide a radially compact coil having a central axial
bore which provides the tubular member within which the plunger
reciprocates. If desired, the coil can be wound with two or more
taps so that different electric currents can be applied at different
points axially along the coil. For convenience, the invention will
be described hereinafter in terms of a single coil having a single
pair of connectors to connect to a source of current.
[0030] However, as indicated above, we prefer to minimise the radial
gap between the coil and the plunger so as to optimise the magnetic
coupling between the coil and the plunger. This can be achieved
by winding the coil directly upon the tubular member within which
the plunger reciprocates, with a thin insulating interface between
the wire of the coil and the tubular member where a metal tube is
used. Alternatively, the coil can be formed by winding a bare wire
coil upon an insulating tubular member and then retaining the coil
in position by applying a retaining coat of resin or other binder
upon the wound coil. Alternatively, the coil can be wound upon a
mandrel, removed and then potted in a suitable resin which then
forms the wall of the tubular member within which the plunger reciprocates.
In a particularly preferred embodiment, the tubular member is formed
from a ceramic material, for example as a ceramic frit tube. The
coil can be formed by depositing a conductor track, for example
by vapour phase or electrical deposition of a copper or silver conductor
or track, upon the surface of the fritted tube or into grooves etched,
machined, laser cut or otherwise formed in the external surface
of the ceramic tube. Alternatively, the coil may be formed as a
copper, silver, gold or other conductive track upon a flexible circuit
board which is then rolled upon a mandrel to form a cylindrical
tubular member incorporating the coil. In all such designs the gap
between the coil has been reduced, typically to a radial dimension
of less than 1 mm, typically less than 0.5 mm, for example 100 to
200 micrometres, as compared to the 2 mm or greater gap in a conventional
solenoid coil. Such a reduction in the gap results in greater efficiency
in coupling the plunger magnetically to the coil, resulting in lower
power consumption and greater speed of response of the plunger to
changes in the current flowing in the coil. Such constructions also
result in a unitary construction for the coil and the tubular member
within which the plunger reciprocates, thus simplifying construction
and assembly of the valve and enables a more compact construction
to be achieved. Furthermore, since a smaller driving force is typically
required to move the plunger in a valve of the invention, it is
often possible to form the coil with a single layer of wire or other
conductor. This further assists the formation of a compact construction.
[0031] If desired, the tubular member supporting the coil can be
longitudinally extended to provide the radial walls of the valve
head chamber. In one embodiment of such a construction, the tubular
member is formed as a cylindrical tube having one end closed to
form the transverse terminal wall of the valve head chamber, the
wall being pierced by a bore whose free end provides the nozzle
orifice. Such an assembly can readily be formed by electo or laser
etching of a silicon or ceramic member to high accuracy using automated
techniques.
[0032] The valve mechanism of the invention is preferably used
in co-operation with a plurality of closely adjacent valve mechanisms,
each serving one or more discrete nozzle orifices, to form an array
type print head capable of applying a plurality of dots of fluid
simultaneously to a substrate to create a two dimensional image.
Such an array can be formed by mounting the outlet end of the valves
upon a nozzle plate with a bore through the plate providing the
outlet bore from the valve head chamber of the valve to the nozzle
orifice. Typically, a jewel nozzle is set into the nozzle plate
to provide both the bore from the valve head chamber and the nozzle
orifice. In a particularly preferred embodiment, the nozzle plate
is provided with a series of upstanding tubes, each in register
with one of the bores through the plate. The tubes serve as the
tubular member support for the coil of the valve and the plunger
reciprocates within that tube. The distal end portions of the tubes
adjacent the nozzle plate, or the proximal portion of the bore in
the nozzle plate, serves as the valve head chamber of the valve
mechanism. Such arrays can be formed from ceramic or silicon materials
using automated techniques and the nozzle orifice can be provided
either by a jewel nozzle set into a bore through the nozzle plate
or by forming a suitable nozzle orifice in the end of a blind end
bore in the nozzle plate using a laser. Such assemblies can be formed
on a very small scale enabling miniaturisation of the valve structure
to be achieved which aids the printing of small dots, typically
less than 60 micrometres in diameter, on the substrate to be printed,
enhancing the definition of the printed image.
[0033] It is also preferred to provide each valve mechanism with
a metal housing to the coil thereof. This acts not only as a return
path for the magnetic field generated by the coil within it, but
also acts as a magnetic screen so as to reduce cross talk between
the magnetic fields generated by one coil and the coil of an adjacent
valve mechanism. Typically, such a metal housing is made from .mu.
metal, aluminium or stainless steel and also acts as a rigid housing
for the components of the valve mechanism. Thus, the housing can
be of a generally cylindrical form and can be crimped radially inwardly
at each end thereof to retain end pieces and the coil axially clamped
upon one another, one end piece carrying an axial fluid inlet, the
other defining the valve head chamber and carrying an axial capillary
tube or jewel nozzle which forms the outlet bore between the chamber
and the nozzle orifice. Alternatively, the distal end of the metal
housing can be crimped or otherwise secured to the nozzle plate
where the nozzle plate carries upstanding tubular members as described
above.
[0034] The valve mechanism of the invention preferably also comprises
a means, notably a spring, for biasing the plunger towards its rest
position. Typically, the spring is a compression spring and acts
to bias the plunger against the inlet at the proximal end of the
bore to the nozzle orifice, so that the rest position of the plunger
is in the valve closed position. When a current is applied to the
coil, this opposes the bias of the spring and moves the distal end
of the plunger away from the bore inlet to open a flow path from
the valve head chamber to the nozzle orifice. However, it will be
appreciated that the rest position may be the valve open position
and the operative position is the valve closed position. For convenience,
the invention will be described hereinafter in terms of the rest
position being the valve closed position.
[0035] The spring member is pre-tensioned, for example from 50
to 80% of the travel of the compression of the spring is taken up
by the pre-tensioning, since we have found that such pre-tensioning
enables the spring to apply a consistent bias force against the
movement of the plunger over the remainder of the compression of
the spring during movement of the plunger. We have found that the
use of a conical spring is of especial benefit since such springs
can readily be fitted within the dimensions of the valve head chamber
and will tend to be self centring during the assembly of the valve
mechanism, whereas conventional cylindrical coil springs do not.
Furthermore, the use of a conical spring reduces the mass and hence
inertia of the spring; further aiding rapid response of the spring
to movement of the plunger. It is particularly preferred to use
a conical coil spring which is pre-tensioned to the last two turns
of the spring, since we have found that such a spring responds rapidly
to the movement of the plunger and the pre-tensioning enables the
spring to exert a significant bias force over a small additional
compression of the spring.
[0036] However, it will be appreciated that the bias effect could
be applied alternatively or in addition to that applied by the spring
by applying a current to the coil which opposes the movement of
the plunger. Such opposing current can be applied under the control
of electronic switching using conventional techniques and software,
for example as described below.
[0037] The valve of the invention also comprises a valve head chamber
in which the distal end of the plunger co-operates with the bore
leading from the valve chamber to the nozzle orifice to open and
close a flow path for fluid to the nozzle orifice. This valve chamber
may take the form of a simple cylindrical extension of the metal
housing described above to provide the magnetic screen between adjacent
valves in an array of valves or of the tubular support member for
the coil. A transverse end wall carrying the axially extending bore
to the nozzle orifice is secured to that housing, for example by
a crimping operation, to form a closed chamber at the distal end
of the coil. Alternatively, as described above, the tubular support
member can extend axially beyond the coil to form the side wall
of the valve head chamber. A transverse end wall can be formed integrally
with or secured to the distal end of the tubular support member
and the nozzle orifice formed in that end wall, for example by a
laser.
[0038] The outlet to the valve head chamber can be provided by
an axially extending tube, the proximal end of which passes through
the transverse end wall and the distal end of which forms or is
provided with the nozzle orifice. Where the outlet is provided by
an axially extending tube, for example a stainless steel capillary
tube, this can be used to mount the valve assembly upon a nozzle
plate or other support. Alternatively, the metal housing providing
the magnetic screen around the coil may be provided with lugs or
other means for mounting the valve mechanism. Where the valve mechanism
comprises tubular coil supports extending transversely from a nozzle
plate as described above, these and the nozzle plate provide the
means for mounting and securing the valve mechanism in the desired
location.
[0039] In a preferred embodiment of the invention, the valve mechanism
is mounted on a nozzle plate having a plurality of nozzle bores
which are formed substantially simultaneously in a single operation
so that the nozzle plate has a unitary construction without the
use of jewel nozzles. Such a simple unitary nozzle structure can
readily be made using a wide range of techniques and overcomes the
problems associated with misalignment of jewel nozzles in a multi-nozzle
print head. In such a preferred embodiment, the nozzle orifice and
bore through which the ink is ejected upon operation of the valve
are formed as a unitary structure, for example concurrently as a
bore is cut or otherwise formed in a plate upon which the valve
mechanism is to be mounted. For example, the bore/nozzle orifice
is formed in a nozzle plate by a laser, electro-forming or etching,
needle punching or other techniques. The nozzle plate can be from
50 to 400 micrometres thick so as to achieve the desired length
to the bore. At such thickness, the nozzle plate takes the form
of a metal or other foil which is mounted in a suitable support
member to provide a rigid and mechanically strong nozzle plate assembly.
We have found that by forming the nozzle bores simultaneously in
a multi-nozzle nozzle plate, problems due to misalignment of the
bores with one another are minimised.
[0040] We have also found that by selection of the bore forming
technique, the walls of the bore are sufficiently smooth to reduce
flow separation and the formation of eddies at the interface between
the bore walls and the fluid flowing through the bore. Furthermore,
such techniques may also be used to form other features on the nozzle
plate which enhance the operation of the valve. For example, electro-forming
or etching of a metal foil can be used to form the bores/nozzle
orifices in the plate and also to form a raised lip or ridge around
the inlet to the bore leading to the nozzle orifice. This provides
a localised pressure point between the distal end face of the plunger
and the nozzle plate to assist the formation of a fluid tight seal
when the plunger is in the valve closed position. Alternatively,
where a needle is used to form the bore in a metal foil, this will
cause the foil to deform and form a belled entry to the bore which
will assist smooth flow of fluid into the bore from the valve head
chamber. The penetration of the needle through the foil may also
polish the surface of the foil, and hence the internal wall of the
bore which is formed, as the surface of the needle slides over the
material of the foil. Similarly, the use of a laser to form the
bore in a metal, ceramic or plastic foil may also form a polished
surface to the walls of the bore, notably where the laser beam is
pulsed for very short periods, typically less than 1 nanosecond,
to reduce the formation of deposits around the lip of the bore of
material which has been ablated from the plate in forming the nozzle
bore.
[0041] Such assemblies can be formed on a very small scale enabling
miniaturisation of the valve structure to be achieved. It is preferred
to provide the nozzle plate as a metal, ceramic or other foil having
the bores formed therethrough as described above and to mount that
plate so that the bores therein are in register with the distal
ends of the plungers of the valves. In this case, the valve head
chambers can be individually formed in the surface of the foil or
in an intermediate plate located between the valve coil support
members and the nozzle plate. However, we have found that the flow
of ink or other fluid to the individual bores and nozzle orifices
is enhanced if the intermediate plate is formed with a continuous
chamber which provides a combined valve head chamber for all the
valves in the print head assembly. In such a construction, the seal
between the distal end face of each plunger and the registering
bore in the nozzle plate provides adequate isolation of flow through
each of the nozzle bores and orifices.
[0042] If desired the raised sealing ribs or areas on the nozzle
plate can be formed from a flexible material to cushion the impact
of the end face of the plunger against the nozzle plate. Such deformation
may also assist formation of the fluid tight seal where the end
face of the plunger does not carry a rubber or similar pad. If desired,
the pad carried by the end face of the plunger can be formed from
a material which undergoes cold creep or deformation under the load
of the bias spring urging the plunger into the valve closed position.
Such creep may form a nipple or other projection which extends into
the proximal portion of the nozzle bore in the nozzle plate. Upon
reciprocation of the plunger, this projection repeatedly wipes at
least the initial part of the proximal portion of the nozzle bore
and displaces solid deposits which may have deposited upon the wall
of the bore and this may assist in reducing initial drop deformation
after rest periods of the valve. To assist the operation of this
projection, the mouth to the inlet to the bore through the nozzle
plate may be belled, as may occur when a needle is used to form
the bore in the nozzle plate.
[0043] Fluid can be fed to the valve head chamber by any suitable
means, for example by one or more radial inlet ports in the side
wall of the chamber. Alternatively, fluid can be caused to flow
axially past part or all of the plunger within the coil so that
the fluid lubricates the movement of the plunger within the coil
and can also act to cool the coil at high current loadings and/or
high frequencies of operation of the valve. Thus, the bore in the
tubular support for the coil can have a generally circular cross
section and the plunger may have a squared or hexagonal cross section,
axial flattenings, grooves or flutes which form axial fluid flow
paths along the plunger. Where this is done, the proximal end of
the valve mechanism can be provided with an axial inlet to feed
fluid axially into the space(s) between the plunger and the coil.
[0044] Such a valve is capable of being operated at high frequencies,
typically 2 to 9 kHz and finds especial application as the solenoid
valve in a drop on demand ink jet printer head. In such an application,
the valve is desirably as small and compact as possible so as to
reduce the overall size of the print head and the inertia of the
components of the valve mechanism. The valve is incorporated into
any suitable form of drop on demand printer to control the flow
of ink through the nozzle orifices of that printer. As described
above, the valve mechanism can be incorporated into a compact structure
forming an array print head which is operated under the control
of a computer which determines the open time of each of the valves
and the sequence of opening of the valves to print the desired image.
Such a computer can operate in the conventional manner. However,
as described below, the computer may be used to achieve many other
functions in the control of the operation of the printer.
[0045] Accordingly, from another aspect, the invention provides
a drop on demand printer characterised in that the flow of ink through
the nozzles of the printer is regulated by a valve of the invention.
[0046] For convenience, the invention has been described in terms
of such use of the valve mechanism. However, it will be appreciated
that the valve mechanism of the invention may be used wherever a
small, high speed valve is required, for example in the dosing of
measured amounts of a reagent in a chemical or biological analysis
or other process, notably in the assessment of medicaments or in
diagnostic testing or analysis. The valve of the invention also
finds use in applying a predetermined quantity of a reagent in the
verification of the authenticity of a sample.
[0047] Surprisingly, we have found that the application of droplets
of ink at high frequencies to long pile fabrics, for example carpets
and felted or woven fabrics, enables satisfactory application of
the dye to the fibre without the need to use very high viscosity
inks. Thus, in place of inks having viscosities in excess of 250
Cps hitherto considered necessary to achieve good dying of the fibres,
we have found that good results can be achieved using inks of from
60 to 120 Cps applied at frequencies of about 2 kHz. The ability
to use low viscosity inks enables the printing to be achieved using
smaller nozzle orifices, which increases the definition of the pattern
printed on the fabric. It also enables the operator to select inks
from a wider range than hitherto and to operate the printer at lower
ink pressures, which reduces the need for special modification of
the printer and the risk of failure of components.
[0048] Many fabrics, both woven and non-woven, have a surface which
presents free ends of fibres generally normal to the plane of the
fabric. Such fabrics include felted materials where fibres in a
randomly orientated mass are compressed, optionally in the presence
of a bonding agent such as an adhesive; materials woven from strands
made up from a plurality of individual fibres where the surface
of the fabric has been, brushed, teased, abraded or otherwise treated
to separate some of the fibres from within the strands to form a
fluffy surface to the material, for example a brushed nylon; woven
materials made from materials which are inherently fluffy, such
as knitted or woven angora, merino or cashmere wools or cotton terry
towelling; and carpet type materials such as velvets, velours and
tufted carpets where individual lengths of strands or fibres are
knotted, sewn, glued or otherwise secured to a sheet member, typically
a reticulate backing sheet, whereby the free ends of the strands
or fibres form a pile which extends generally normal to the plane
of the backing or where loops of the strands or fibres are formed
extending generally normal to the plane of the backing and the free
ends of the loops severed to form the pile. For convenience the
term pile fabric will be used herein to denote all such types of
material where individual fibres or strands comprising groups of
fibres extend generally normal to the plane of the material to provide
a pile effect surface to the material.
[0049] It is often desired to form patterns or images upon the
surfaces of pile fabrics, for example a coloured pattern. This can
be achieved by interweaving different coloured, textured or other
material strands of wool or other material into the fabric as it
is being made. However, this is difficult and time consuming, especially
where the pattern is complex and/or a plurality of colours or textures
are desired. Such use of a plurality of different strands is becoming
progressively uneconomic in the large scale manufacture of commodity
materials, such as patterned carpets.
[0050] It has therefore been proposed to manufacture the pile fabric
from neutral or uniformly coloured fibres or strands and to apply
a colour to the pile fibres after the fabric has been woven or otherwise
manufactured. The colour is typically an ink applied by any suitable
printing technique. A printing technique which is used is an ink
jet printing technique using a drop on demand type of printer. The
ink is desirably applied at the rate of about 300 to 400% by weight
of the fibre to be coloured and needs to penetrate substantially
uniformly throughout the strands formed from the individual fibres.
If a very mobile ink having a viscosity of about 10 cPs at 25.degree.
C. (as is commonly used in an ink jet printer) is used, it will
run down the length of the strands and form an intense coloration
at the base of the pile, leaving the top portion of the pile inadequately
dyed, and little penetration of the colour into the strands will
take place. It is therefore necessary to increase the viscosity
of the ink in order to ensure that it runs down the fibre at a sufficiently
slow rate for uniform penetration of the ink into the strands and
coverage of the surface of the individual fibres takes place. The
longer the pile, the greater this problem becomes. With long pile
fabrics, that is those with a pile length of about 2 mms or more,
it is necessary to use inks having a viscosity of from 250 to 500
cPs at 25.degree. C.
[0051] Such viscous inks are difficult to jet through the very
fine orifice nozzles of a conventional ink jet printer and pressures
far in excess of those for which the printer is designed would be
required. Furthermore, if a low viscosity ink were applied at such
high pressures, it would issue from the nozzles as high powered
jets and cause the individual strands to bend over and thus prevent
the ink from contacting other strands in the pile. It is therefore
customary to use nozzles having orifices which are progressively
greater as the length and closeness of the pile increases. Thus,
for a carpet having a pile length of 3 mms or more it is necessary
to use an ink having a viscosity of about 300 cPs, a pressure of
about 2 bar and nozzle diameters of typically 500 micrometres in
diameter so that the viscous ink can be ejected in sufficient amounts
to attain the desired loading of ink on the individual strands.
[0052] Whilst the use of large diameter nozzles for high viscosity
inks enables the ink to be deposited on the strands of the pile
to achieve substantially uniform coloration of the individual strands
and fibres, the size of the droplets issuing from the nozzle are
sufficiently large to cause perceptible loss of definition in the
printed pattern. Furthermore, the size of the droplets also results
in adjacent droplets applied to the pile contacting one another
to cause colour bleeding where the droplets are of different colours.
[0053] Surprisingly we have found that the use of a drop on demand
print head which operates at frequencies greater than 1 kHz, notably
a print head incorporating a valve of the invention, enables the
size of the droplets being printed and hence the pressure required
to eject them through comparatively small nozzle orifices to be
reduced. This reduces the problems of colour bleed and enhances
the definition of the printed image or pattern. Furthermore, we
have found that it becomes possible to omit individual printed droplets
from the printed pattern and thus print a blank area within the
image which is not visually perceptible but which acts to provide
a gap within the printed strands to act as a barrier to colour bleeding.
Such a gap may also be printed as a black line defining the edges
of areas printed with different colours, which enhances the perceived
definition of the printed image or pattern.
[0054] Accordingly, from another aspect, the present invention
provides a method for applying an image forming composition to a
pile fabric using a drop on demand ink printer, characterised in
that the printer is operated at a drop generation frequency of at
least 1 kHz. Preferably, the pile fabric has a pile length of at
least 2 mms and the printer is operated at a pressure of less than
3 bar, notably at from 1.5 to 2.5 bar.
[0055] A particularly preferred drop on demand ink jet printer
is one utilising a valve of the invention to control the flow of
an ink through the individual nozzle orifices and the nozzle orifices
have a diameter of from 250 to 600 micrometres, notably about 500
micrometres; and in which the plunger of the solenoid valve has
a diameter of less than 2.5 mms. We have also found that the use
of such a printer enables individual control of the printing of
the dots of the image so that accurate over-printing of dots can
be achieved. It is thus possible to enhance the colour range and
strength which can be achieved. Such a printer thus enables an infinite
scaling of the colour hues which can be achieved.
[0056] The invention can be applied to the application of any form
of image to any pile fabric. However, the invention is of especial
application in the application of a water and/or solvent based ink
composition to form a patterned image on a long pile fabric having
a pile length of about 2 to 5 mms as measured from the top surface
of the sheet member to which the strands or fibres forming the pile
surface of the fabric are secured. Such pile fabrics can be velvets
or twist pile carpets, but for convenience the this aspect of the
invention will be described in terms of printing a multi-colour
pattern on a tufted pile carpet in which the strands containing
a plurality of individual fibres are secured to a reticulate backing
sheet by adhesive. Such carpets can be made by any suitable technique
and the invention can be applied during the fabrication of the carpet
after the strands have been secured to the backing sheet or can
be applied after the carpet has been manufactured in a separate
colour printing operation. As indicated above, the strands are made
from a neutral tint fibre, for example from a natural washed wool
fibre, optionally in admixture with one or more natural coloured
polymer fibres, for example polyester or polyamide fibres. If desired,
the fibres or the strands formed from the fibres may be given one
or more treatments to render the fibres receptive to the ink composition
to be applied to them. The fibres, their formation into strands,
the treatment of the fibres or strands and the formation of the
carpet can all be those conventionally used in the manufacture of
a tufted carpet.
[0057] For convenience, the this aspect of the invention will be
described hereinafter in terms of the application of ink to a neutral
washed wool fibre tufted carpet shortly after the pile has been
formed on a reticulate woven polypropylene backing sheet.
[0058] In a preferred embodiment, this aspect of the invention,
the printer is one in which the solenoid valve mechanism for controlling
the flow of fluid to the nozzle orifice comprises a plunger member
journalled for axial reciprocation between a rest and an operative
position within an electric coil under the influence of a magnetic
field generated by that coil when an electric current passes through
the coil, the distal end of the plunger extending into a valve head
chamber having an outlet nozzle bore, the reciprocation of the plunger
being adapted to open or close a fluid flow path from the valve
head chamber through that bore, characterised in that:
[0059] a. the plunger is of a unitary construction and is made
from an electromagnetically soft material having a saturation flux
density greater than 1.4 Tesla, preferably about 1.6 to 1.8 Tesla,
a coercivity of less than 0.25 ampere per metre, and a relative
magnetic permeability in excess of 10,000; and
[0060] b. the nozzle bore leading from the valve head chamber to
the nozzle orifice has a length to diameter ratio of less than 8:1,
preferably from 1.5:1 to 5:1, notably from 2:1 to 4:1.
[0061] The term magnetically soft is used herein to denote that
the material loses the magnetic field induced in it by the coil
when the current in the coil ceases, in contrast to a permanent
magnet which retains its magnetism.
[0062] The use of materials having high magnetic flux saturation
densities enables the plunger to respond rapidly to changes in the
magnetic field generated by the coil without the generation of excessive
heat. The low coercivity of the plunger material also aids the rapid
rise and fall of the induced magnetic field within the plunger under
the influence of the field generated as a current is passed through
the coil at low applied coil currents. This, coupled with the high
permeability of the material, enables a high magnetic drive force
to be generated rapidly between the coil and the plunger. As a result,
the plunger can be accelerated rapidly by the coil without the need
to apply high drive currents to the coil, typically in excess of
20 amperes, as hitherto considered necessary. This again reduces
the heat energy which is generated as the plunger is moved by the
coil. The low coercivity also permits a reverse magnetic force to
be generated rapidly by reversing the direction of the current in
the coil. This reversed force can be used to slow down the movement
of the plunger as it reaches either or both extremes of its travel
as described below.
[0063] Furthermore, we have found that the above design of valve
can be held in the open position for prolonged periods to print
continuous lines on the substrate which have a length equivalent
to at least three individual printed dots. With conventional solenoid
valves, it has been considered necessary to pulse the current to
the coil so as to form overlapping dots of ink on the substrate.
In practice this often leads to the valves burning out due to the
high currents applied to the coil to move the plunger from its initial
rest position into the valve fully open position. We have found
that the amplitude of the current flowing through the coil required
to hold the plunger in the valve open position is surprisingly much
less, typically 80 to 50% less, than the current required to move
the plunger initially away from its rest position. By applying a
current pulse which has an initial amplitude sufficient to move
the plunger from its rest position to the valve open position and
then reducing this amplitude to a lower value for the remainder
of the pulse, it is possible to hold the valve open for prolonged
periods so as to print lines of ink on the substrate.
[0064] Furthermore, by reducing the length of the nozzle bore,
the pressure drop across the nozzle is reduced, allowing a faster
exit velocity to be achieved at the nozzle orifice. Surprisingly
this is achieved without causing spraying of the droplets, that
is the break up of the droplet at the nozzle orifice into a plurality
of smaller droplets. This enables a higher frequency of droplet
generation to be achieved at a given ink pressure for a given length
of flight path.
[0065] In a conventional drop on demand printer, the operation
of each solenoid valve is triggered in response to a signal from
a computer or microprocessor, which determines which valve is opened
and when so as to print the desired image. We have found that the
control the operation of the valve using software has a number of
other significant benefits which enable the valve of the invention
to deliver high quality printed images at much higher frequencies
that has hitherto been considered possible for a drop on demand
print head.
[0066] Thus, it is particularly preferred to use software to calibrate
the valve so that under specific conditions it delivers a consistent
droplet of ink through the nozzle orifice. With conventional designs
of solenoid valve, it is necessary to compensate for minor variations
in dimensions and materials of the manufactured valve by physically
adjusting the axial travel of the plunger so as to vary the size
of the flow path created when the plunger is withdrawn from sealing
engagement with the transverse end wall of the valve head chamber
or the tube leading to the nozzle orifice. This will affect the
size of the dot ejected from the nozzle orifice and the objective
of the calibration process is to achieve a uniform droplet size
from all the nozzle orifices in a print head under the same printing
conditions. The conventional design of solenoid valve incorporates
a stop within the bore of the tubular support for the coil, which
stop provides a physical limit to the axial movement during retraction
of the plunger. In such a conventional valve design, the air gap
between the proximal end of the plunger and the stop is adjusted,
for example by making the stop a stiff push fit or a screw fit within
the tubular support, so that it can be moved axially within the
bore of the tubular member to achieve the desired air gap. Such
adjustment of the air gap is tedious and time consuming and is prone
to operator error.
[0067] We have found that software can be used to set a specific
point in the retraction of the plunger at which the plunger movement
halts. This point can readily be adjusted by simple modification
of a parameter of the software, for example by keyboard input of
a new value for that parameter. Such adjustment can be achieved
very accurately and the calibration carried out for a number of
sets of printing conditions so that the current pulse size and duration
required to achieve given droplet sizes can be determined and stored,
for example in as a machine readable code on a magnetic disc, look
up table in a memory chip or other storage medium, for future use
with that valve. The calibration can be achieved simply and at smaller
increments of droplet size than is possible with screw adjustment
of the stop in conventional design of solenoid valve.
[0068] In carrying out the calibration, droplets are printed onto
a substrate whilst operating the valve under standard conditions
and at a given electric current pulse amplitude and duration applied
to the coil. The printed dot is examined by any suitable means and
the amplitude and/or duration of the electric pulse raised or lowered
to achieve the desired dot size. Such a process can be carried out
manually. However, it is preferred to carry out this process automatically
by inspecting the printed dot using a CCD camera or other inspection
means and comparing the form of the printed dot with parameters
for the required dot. Such comparison and subsequent adjustment
of the current pulse can be carried out using a suitably programmed
computer. It is especially preferred to monitor the diameter and
circularity of the printed dot and the presence of satellite small
dots adjacent the desired dot using a CCD array or camera and comparing
the dot characteristics with those held in a look up table which
identifies the correction which needs to be applied to the current
pulse applied to the coil to achieve the desired printed dot characteristics.
The optimum variation in the operation of the valve mechanism, for
example to increase or reduce the open time of the valve, can be
determined by trial and error tests. These optimum values of the
variation then stored in a look up table or other storage medium
to provide one of the parameters against which the printed dot and
the operation of the print head is assessed.
[0069] The use of a CCD camera or array and computer to inspect
the droplet of ink as it is ejected and/or the printed dot and to
modify the current applied to the coil of the valve also has applications
during the operation of the valve on-line during printing of images.
Thus, the computer can be programmed to decelerate the movement
of the plunger at each end of its travel. We have found that this
reduces splatter of the ink from the nozzle orifice due to sharp
impact of the plunger against the seal members at the entry of the
bore between the valve head chamber to the nozzle orifice. The use
of software can also be used to compensate for fluctuations in the
viscosity of the ink due to temperature variations or other reasons;
variations in voltage applied to the different coils in an array
of print heads which are operated simultaneously; and to compensate
for other changes in operating conditions, for example the use of
a different ink, which require changes in the form and size of the
electrical pulse applied to the coil of the valve. The use of software
can also be used to hold a valve in the open position to print a
continuous line of ink in place of the series of overlapping dots
achieved with present print head operating techniques; and to vary
the open time of the valve for the initial droplets ejected from
the nozzle orifice following a rest period of the valve.
[0070] In all cases the operation of the valve is modified by the
computer in response to a signal from the CCD camera or other mechanism
used to inspect and monitor the droplet of ink as it is ejected
and/or the printed dot and to compare the observed droplet or dot
to parameters held in a memory of the computer or another storage
medium so as to determine what modification, if any, is required
to the current applied to the coil so as to achieve the desired
dot.
[0071] The invention thus provides a print head of the invention
operated under the control of a computer in combination with a mechanism
for observing the printed dot of ink or other fluid applied to the
substrate, the computer being programmed to detect differences between
the observed dot and the desired dot and to apply a correction to
the current applied to the coil so as to maintain the desired observed
dot parameters.
[0072] Such a combination enables the printed dot quality to be
monitored and corrected on-line during operation of the printer.
Hitherto, the print quality was observed objectively by the operator
of the printer and a correction to the operation of the printer
applied manually. The ability to use the software on-line to achieve
monitoring and correction of print quality is a major benefit to
the operator and can achieve virtually instantaneous correction
of fluctuations in print quality.
[0073] The monitoring and correction may be achieved using conventional
software and hardware techniques and designs. The dot quality can
be monitored continuously and a correction applied in response to
the average of three or more successive dots. Alternatively, the
printed dot quality can be monitored at intervals, for example every
second or at intervals of every twenty operations of the valve,
and any correction applied once the printed dot deviates by more
than say 5% for any one or more of the parameters used to assess
the quality of the printed dot.
[0074] Typically, the monitoring of the printed dot quality will
be used to apply a signal to vary the open time of the valve.
[0075] It will be appreciated that the signal indicating that some
variation of the operation of the valve is required may be provided
from an external source rather than from the on-line scanning of
the printed dot. Thus, a sensor may monitor the operating temperature
of the printer and/or of the ink fed to the valve, since this will
affect the viscosity and hence the jettability of the ink. Alternatively,
such sensors may monitor: the voltage applied to the valve mechanism,
for example the voltage drop which occurs when a plurality of valves
are operated simultaneously from a single power source; the time
for which a specific valve has rested between printing operations,
the frequency of operation of a valve and so on. These sensors may
then address a series of look up tables which then set the variation
of the open time required to reduce defects in quality of the printed
dot if that parameter being monitored varies from a predetermined
optimum value.
[0076] It is preferred that the quality of the printed dot from
each nozzle be monitored individually. However, if desired the printed
dot quality from groups of nozzles may be monitored collectively.
[0077] In the conventional computed control of the operation of
a valve in a drop on demand printer, simple single bit signals are
used to open and shut the valve since all that has been required
hitherto is that the computer instruct the valve when to open and
shut the valve so as to print a dot of the required size. However,
the ability to vary the operation of each valve individually during
the operation of the printer in response to many inter-related factors
requires the transmission of more complex signals than simple open
and shut instructions. We have found that it is desirable to transmit
signals in byte format so that the amount of information transmitted
can accommodate the permutations of operating parameters desired.
Thus, for example, the use of byte form signal transmission offers
256 possible graduations of open time of the valve. This enables
the amount of ink deposited in each printed dot to be varied over
a finely graduated range by providing a look up table with 256 individual
addresses therein from which the computer controlling the operation
of the printer can instruct the open time of the valve to be selected.
This enables a true grey scale image to be printed using a drop
on demand print head, which has not hitherto been considered practical.
The use of byte signal transmission enables a wide selection of
values for variation of a given operating parameter to be transmitted
and responded to rapidly and accurately, further enhancing the speed
and accuracy of operation of the print head. This, coupled with
the high frequency printing of consistent quality dots over a wide
range of sizes and speeds enables the present invention to extend
the use of drop on demand printers into fields of use for which
they have hitherto not been considered possible whilst retaining
the flexibility of printed dot size which cannot readily be achieved
with other forms of printer.
DESCRIPTION OF THE DRAWINGS
[0078] A preferred embodiment of the invention and its operation
under on-line software control will now be described by way of illustration
only and with respect to the accompanying drawings, in which
[0079] FIG. 1 is a diagrammatic axial cross section through a valve
of the invention;
[0080] FIG. 2 is an axial cross section through a drop on demand
ink jet print head incorporating an array of the valves of FIG.
1;
[0081] FIG. 3 is plan view of the nozzle plate of the print head
of FIG. 2;
[0082] FIG. 4 is a diagrammatic block diagram of an array of FIG.
2 in combination with a CCD camera for monitoring the quality of
the printed dots and a computer for establishing what variation
to the frequency, form, shape and amplitude of the electrical pulse
applied to the coil of the valve of FIG. 1 is required to compensate
for any deviation in the quality of the observed printed dot;
[0083] FIGS. 5 to 7 illustrate variations in the construction of
the valve of FIG. 1;
[0084] FIG. 8 illustrates an alternative form of the print head
of FIG. 2;
[0085] FIG. 9 shows a schematic depiction of a solenoid valve which
is suitable for use with the calibration of the valve using software
according to that aspect of the present invention;
[0086] FIG. 10 shows in diagrammatic form an apparatus for use
in this aspect of the invention;
[0087] FIGS. 11 to 13 illustrate alternative forms of the apparatus
of FIG. 10;
[0088] FIGS. 14 and 15 show diagrammatically a valve and printer
in which the current applied to the coil is modified to decelerate
the plunger at either extreme of its travel; and
[0089] FIG. 16 illustrates the form of current pulse applied.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0090] The valve of FIG. 1 comprises a plunger 1 which is journalled
as a close free sliding fit for axial reciprocation in a stainless
steel tube 2. Tube 2 has a thin insulating coating or sleeve (not
shown) formed upon its outer face and supports a coil 3 wound upon
it. Coil 3 is supplied with an electric current from a source (not
shown) under the control of a computer 20, shown in FIG. 4. A stop
4 is mounted at the proximal end of tube 2 to limit the axial retraction
of plunger 1 within tube 2. The coil 3 is encased in a metal cylindrical
housing 5.
[0091] The above assembly is mounted in a support housing 10 which
extends axially beyond the distal end of the coil and has a transverse
end wall 11 which carries a jewel nozzle 12. In the embodiment shown
in FIG. 1, housing 10 has an axially extending internal annular
wall 13 which forms the radial wall of the valve head chamber 14
into which the distal end of the plunger extends. The distal end
of the plunger 1 carries a terminal rubber or other sealing pad
15 which seats against the proximal end face of jewel 12 in sealing
engagement. A pre-tensioned conical spring 16 biases plunger 1 into
sealing engagement with the face of the jewel as shown in FIG. 1,
the rest or valve closed position.
[0092] Plunger 1 is made from a ferromagnetic alloy having a saturation
flux density of 1.6 Tesla such a Permenorm 5000 or similar magnetically
soft ferromagnetic alloy. In order to reduce the mass of the plunger
1, it may have a blind internal bore extending from the distal end
thereof. However, this bore should not extend beyond line A-A shown
in FIG. 1 when the plunger is in its rest position. It is also desirable
that the plunger have a diameter of less than 3 mms, typically about
1 mm, and a length to diameter ratio of about 5:1. For example,
the bore in the jewel nozzle shown in FIG. 1 has an l:d ratio of
2 to 3:1 and the nozzle orifice has a diameter of 60 micrometres.
[0093] Ink is fed under a pressure of 1 bar to an ink gallery 17
encompassing wall 13 and enters the valve head chamber via radial
ports 18. When the plunger is in its rest position as shown in FIG.
1, the pad 15 is in sealing engagement with the face of the jewel
nozzle 12 and thus prevents flow of ink through the nozzle orifice.
In order to enhance the seal between the pad 15 and the jewel 12,
we prefer to provide the proximal face of the jewel with one or
more raised annular sealing ribs (not shown). This has the surprising
effect of reducing satellite droplet formation when the valve is
operated at high frequencies, typically in excess of 1 to 2 kHz.
[0094] Such a valve can be operated at frequencies of from under
1 kHz to over 8 kHz to produce consistently sized droplets in the
size range 20 to 150 micrometres or more by controlling the length
for which the current flows in the coil 3 and the frequency at which
such current pulses are applied to the coil.
[0095] As indicated above, the valve is preferably used in an array
with other valves to form a print head which extends transversely
to the line of travel of a substrate upon which an image is to be
printed. Such an array is shown in FIGS. 2 and 3. In this case the
terminal portion 11 of the housing 10 is provided by a trough-shaped
nozzle plate 30 carrying the nozzles 12 and serving as a manifold
to form the ink flow gallery 17 feeding ink from ink inlet spigots
31 at each end of the nozzle plate via the inlet ports 18 to the
individual valve head chambers 14 of the valves in the array. The
array is provided with a connector 32 by which individual electric
current supplies can be fed to the coils 3 in each of the valves.
In such an array, the housing 4 serves to reduce electrical and
magnetic cross talk between adjacent valves in the array.
[0096] Such valves and arrays can be made by machining appropriate
metal components. However, one alternative form of construction
is to form the tube 2 as a ceramic or silicon member 40 as shown
in FIG. 5. The coil 41 can be formed in grooves 42 cut in the external
surface of the tube 40 so that the air gap between the coil and
the plunger 43 journalled within the tube is reduced. The coil 41
can be a wire coil wound into the grooves 42; or can be a conductive
track which is deposited by any suitable means in the grooves 42.
If desired, the assembly can then be coated with a polymer to retain
and protect the coil within the grooves. In place of a rigid ceramic
or silicon support tube, the tube 40 can be provided by a sheet
of a flexible support medium, for example a suitable fibre filled
polymer or the like, upon which a copper or other conductive track
has been formed. The support medium is then rolled into a cylinder
to form a cylindrical support carrying the coil upon its inner or
outer face. In such designs, the tube 40 can extend axially to form
the radial walls 44 of the valve head chamber and the distal open
end of the tube closed with a jewel nozzle 45. The whole assembly
can then by encased in a stainless steel or other tube 46 which
acts to support the assembly and provide the magnetic return path
as screening for the coil. The ends of the tube 46 can be inwardly
crimped to secure the tube 40, the coil 42 and the jewel 45 in position.
[0097] In place of the above forms of construction, an assembly
of valves can be formed as shown in FIG. 6 by forming a nozzle plate
50 from a silicon or ceramic frit or other material. This plate
is provided with jewel nozzles 51 at the desired spacings along
the plate 50. Plate 50 is provided with upstanding tubular members
52 which form the tubes 40 of the valve design of FIG. 5. The coils
53 are wound or otherwise formed upon the upstanding tubular members
52 and the array is completed as in FIG. 5. The valve head chamber
54 is formed by the terminal distal portions of the tubular members
and radial ink inlet ports may be provided to enable ink to flow
into the valve head chamber. A plunger 55 is journalled in tubular
members 52 for axial reciprocation under the influence of coil 53.
In place of the jewel nozzle forming the closed distal end to the
valve head chamber, the plate 50 can be provided as a continuously
extending plate so as to form closed ends to the upstanding tubular
members 52. These closed ends can then be pierced, for example by
a laser, to form the bores therethrough and the nozzle orifices.
[0098] In place of the radial ink inlet ports to the valve head
chamber 14 or 54, ink can flow axially past the plunger 1 or 55
from an ink inlet to the axially extending space between the tubular
members 2 or 52 and the plungers 1 or 51. To form the axial passages
past the plunger, the bore in tubular member 2 or 52 can have an
oval or polygonal cross section and plunger 1 or 55 has a circular
cross section. However, it is preferred to form plunger 1 or 55
with axial flats to it which provide axial passages between the
plunger and the circular cross section bore of the tubular member
as shown in FIG. 7.
[0099] As indicated above, the operation of the valve is controlled
by a computer 20 in response to a CCD camera or array 21 or other
sensors 22 detecting the quality of the printed dots and/or other
factors such as temperature, voltage, frequency of operation of
the valve which also affect the printed dot quality. Thus, the computer
20 determines which valve to open in the array of FIG. 2 and for
how long so as to print a drop of the desired size at the desired
position on the substrate 23 passing the print head 24. At slow
frequencies of operation, for example below 1 kHz, this will usually
result in a good quality dot being printed on the substrate. However,
as the frequency increases, say to 2 kHz or more, the quality of
the printed dot may suffer, for example due to the sudden closure
of the valve causing the formation of satellite dots. The computer
can respond to this by detecting from the CCD array that such satellite
dots are being formed and causing the shape of the pulse of electric
current applied to the coil to change so that the movement of the
plunger at each extreme of its travel is reduced so as to reduce
the sudden-ness of the closure of the valve by causing the plunger
to soft land against the face of the jewel nozzle or end wall of
the valve head chamber. Alternatively, the computer can respond
to the instruction to print at high frequencies by reducing the
open time of the valve by reference to a look up table 25 which
carries a list of reductions in open time for a range of operating
frequencies. Similarly, the software controlling the operation of
the print head can detect when a valve has been idle for any length
of time and provide, through another look up table, a signal to
increase the open time of the valve for the initial dots printed
by that valve to compensate for any drying out of the ink within
the valve and/or at the nozzle orifice. In such cases, it is preferred
that the information between the computer and the look up table
be exchanged as bytes sized signals so that up to 256 possible permutations
of open time and operating frequency can be accommodated in a single
signal.
[0100] The printer of FIG. 2 was operated with a nozzle bore having
a length to diameter ratio of 10:1, 8:1, 4:1 and 0.5:1 and at a
drive current frequency of 2 kHz. At the 10:1 ratio, the pressure
required to feed the ink through the bore to achieve a consistent
printed dot size was about 10 Bar. However, such a pressure is too
high for use with conventional drop on demand print heads and would
have resulted in rupture of components. If the pressure was reduced
to a more acceptable level, say about 3 Bar, the rate of flow of
ink through the print head was insufficient to provide ink to form
the droplets consistently so that the printed dots were of uneven
size and there were missing dots where the valve had not been able
to acquire ink from the reservoir.
[0101] Where the ratio was 8:1, the pressure required to feed the
ink to the nozzle bore to achieve uniform printed dot size and quality
was 5 Bar, which is at the upper extreme of operating capability
of the components of a drop on demand printer.
[0102] Where the ratio was 4:1, the printer operated successfully
at an ink pressure of 1.5 to 2.5 Bar and could print consistent
dots at coil drive current frequencies of from less than 1 kHz to
7 kHz.
[0103] Where the ratio was 0.5:1, the printer could not be operated,
even at ink pressures of 0.1 Bar without causing spraying of the
ink and the formation of multiple small dots as well as the desired
main dots.
[0104] The use of a preferred form of drop on demand print head
as shown in FIG. 8 in printing images on a carpet pile fabric having
a pile length of 3 mms under on-line software control will now be
described by way of illustration only.
[0105] The printer shown in FIG. 8 is a modification of the print
head shown in FIG. 2 in which The nozzle plate 100 is formed with
a plurality of bores 101 therethrough having a length of 1000 micrometres
and a diameter of 500 micrometres. The plate is made from stainless
steel and the bores are formed either by needle punches or by laser
drilling each hole. Alternatively, the bores 101 can be formed by
electro-etching, which technique can also be used to form the raised
annular ridge 102 around the inlet to each of the bores 101. This
foil nozzle plate is clamped between two stainless steel support
plates 103 and 104. Plate 104 is formed with a single manifold chamber
105 which extends over all the bores 101 formed in plate 100. Alternatively,
the chamber 105 can be formed in plate 100.
[0106] A valve assembly 110 contained in a support housing 111
is secured to plates 100, 103 and 104 with each of the plungers
in a valve mechanism within the assembly in register with a bore
101. The valve mechanisms comprise a coil wound upon a support tube
112 within which a plunger 113 is a loose sliding fit. Each coil
is surrounded by a stainless steel housing 114 which is crimped
to an apertured support plate 115 clamped between housing 111 and
plate 104 to locate and secure each valve mechanism with the plunger
projecting through the aperture in register with a bore 101 in the
nozzle plate 100. The electrical contacts for the coils are fed
from a multi-contact plug and socket from a computer controlled
power source, not shown. The valve head chamber for each valve mechanism
110 is provided by the single manifold chamber 105 which is fed
with ink from each end of plate 104.
[0107] The plungers 113 are made from a ferromagnetic alloy having
a saturation flux density of 1.6 Tesla, a coercivity of 0.2 a/m
and a relative magnetic permeability of 100,000. The alloy is a
45/55 Ni/Fe alloy sold under the trade mark Permenorm 5000 and each
plunger is 2 mm in diameter and 7.5 mms long. The nozzle bore and
orifice in the jewel nozzle have a diameter of 300 micrometres and
an l:d ratio of from 2:1 to 3:1.
[0108] Ink having a viscosity of 250 cPs is fed under a pressure
of 1.5 bar to the manifold chamber 105 and enters the bore 101 when
the plunger 113 is retracted by applying current to coil 112.
[0109] Such a valve can be operated at frequencies of from under
1 kHz to over 8 kHz to produce consistently sized droplets in the
size range 250 to 500 micrometres by controlling the length for
which the current flows in the coil 112 and the frequency at which
such current pulses are applied to the coil.
[0110] The print head of FIG. 8 was used to apply inks having a
viscosity of 300 cPs through a nozzle orifice of 500 micrometres
to apply different coloured inks to the pile of a neutral wool fibre
coloured tufted carpet. The print head was operated at a frequency
of 2 kHz to achieve substantially uniform coloration of the individual
fibres within the pile. The boundaries between different colours
of the printed image were clearly defined and the definition of
the image was excellent. In an alternative operation, the pint head
was programmed not print an ink dot at the boundary between two
colours so as to minimise the risk of colour bleeding between areas
of different colours.
[0111] The calibration of a solenoid valve using software will
now be described with respect to FIGS. 9 to 13.
[0112] FIG. 9 shows a schematic depiction of a solenoid valve 10
which is suitable for use with the method of the present invention.
The valve 910 comprises plunger 920, tube 930 and coils 940. The
plunger 920 comprises a ferromagnetic material (or any other magnetic
material) and is received within the tube 930 so as to be able to
move freely along the axis of the tube. The plunger can be impelled,
for example towards the open end of the tube, by the application
of a current to the coils 940, the current generating a magnetic
field within the tube, which causes a magneto motive force to act
upon the plunger. The timing and frequency of the current pulses
applied to the coils can be controlled by computer (not shown).
The solenoid valve additionally comprises a return mechanism (not
shown), such as a spring, that acts to return the plunger to its
initial position once the plunger has completed its full range of
travel.
[0113] In practice, a print head will comprise a matrix of such
valves that are arranged in a square or rectangular arrangement.
FIG. 10 shows two exemplary valves 210a, 210b from such a print
head matrix 220. Associated with each valve is valve control means
215a, 215b, each of the valve control means being in communication
with a central computer system 230. The operation of each valve
is controlled by the transmission of control pulses from the central
computer system 230 to each of the valve control means 215a, 215b.
The valve control means are responsive to the central computer system
such that the central computer system is able to vary the time that
the valves are held open for. This controlled variation of the valve
enables ink drops of a desired size to be produced for depositing
upon the substrate 250.
[0114] The print head can be calibrated upon manufacture and then
at periodic intervals during its operation. The central computer
system instructs the print head to generate a predetermined matrix
of drops. This test matrix is deposited on a test substrate and
the printed image can be examined to determine the correlation of
the printed image to the original test matrix. If the ratio of the
size of a printed pixel to the size of the respective pixel of the
original test matrix is outside a threshold value then the respective
valve control means can be instructed to change the time that the
valve is to be opened for. If the printed pixel is too small then
the valve open time will be increased (either by the addition of
more time or by multiplying the valve open time by a suitable constant).
Similarly, if the printed pixel is too large, then the valve open
time will be decreased accordingly. The threshold that is used to
determine whether a printed pixel is too small or too large may
be varied in accordance with the nature of the print substrate and/or
the application that the print head is being used for.
[0115] As variations in printed pixel size will depend upon mechanical
variations within the valve, it is possible that a valve may operate
satisfactorily for one size of pixel or within a given range of
valve operating rates. Therefore, the calibration may need to be
repeated across the range of pixel sizes and valve rates that will
be used by the valve. The range of calibration factors that are
required by each valve may be stored in a look-up table, or it may
be possible to determine one or more equations such that the relevant
calibration factor can be calculated given the desired valve operation
rate and pixel size.
[0116] In an alternative embodiment, imaging means 240 may be additionally
coupled to the computer control system and aligned so as to view
the area of the substrate that the print head matrix prints upon.
When a test matrix is printed upon the substrate, the image means
is able to convert that image to an electrical signal that can be
transmitted to the central computer system. The central computer
system can, after any necessary image processing (digitising, filtering,
etc.), compare the printed image with the original test matrix that
is stored within the central computer system. The ratio of pixel
sizes can be determined for each pixel and calibration factors calculated
for each valve as required. The central computer system can then
communicate the calibration factors to the valve control means associated
with the valves that require calibration.
[0117] The valve control means receives, interprets and executes
signals that are received from the central computer system. It will
be readily understood that the valve control means may be implemented
such that each valve has a dedicated control means or alternatively
that a number of valves may be controlled by a single control means.
[0118] In a preferred embodiment, the valve control means comprise
a field programmable gate array (FPGA). FPGAs comprise memory and
logic elements that can be configured by the user to provide a desired
functionality.
[0119] In the preferred embodiment, the FPGA, and associated devices,
is used to control a linear array of 16 valves. Referring to FIG.
11, the valves 610a, 610b, . . . , 610p are controlled by valve
control means that comprise FPGA 616, electrically erasable programmable
ROM (EEPROM) 617, RAM 618, programmable ROM (PROM) 619 and input/outputs
622, 624, 626. The FPGA 616 is connected to each of the valves 610a,
610b, . . . , 610p, EEPROM 617, RAM 618 & PROM 619. All three
input/outputs 622, 624, 626 interface with the FPGA. When the FPGA
is powered up, it loads its internal configuration data from PROM
619 and then follows the sequences that have been loaded from the
PROM. The EEPROM 617 stores a range of data comprising a look-up
table comprising data associated with each of the valves, data specific
to the valve control means and FPGA, status information, etc. The
FPGA will load this data from the EEPROM and then initialise the
RAM 618, by writing zero values into each memory location in RAM.
The FPGA will then wait to receive print data or other commands
from one of the inputs. Input/output 622 is connected to the computer
control system and input/output 624 can be used to connect to a
further valve control means (see below). Input 626 provides a series
of pulses that are used in co-ordinating the printing process. When
the array of valves is printing onto a substrate, the substrate
is normally moved underneath the valves. The series of pulses supplied
to input 626 may be generated from an encoder applied to a shaft
in the apparatus that is moving the substrate relative to the valves.
[0120] FIG. 12 shows a schematic depiction of a number of registers
that are formed with the FPGA when the FPGA configuration data is
loaded from PROM 619. The first register 631 is used to write to
and read from the EEPROM 617 and is also used when initialisation
data is read from the EEPROM. Second register 632 receives print
data from the computer control system, such as the alphanumeric
characters or bitmaps to be printed, or a signal to initiate a printing
process. Second register 632 also writes print data to the RAM and
is used to initialise the RAM during the start-up phase. The third
register receives configuration data from the computer control system
such as data controlling the slant that may be applied to the print
head. Fourth register 634 receives print data from the RAM and passes
it to the fifth register 635, which uses the print data to operate
the valves 610.
[0121] A desired print image (which may include alphanumerical
characters) is entered into the computer control system and this
image is then converted into raster data that is to be communicated
with the valve control means. The valves 610 may be operated for
different periods of time so as to provide the appearance of 16-level
greyscale images. Thus the print data can be supplied in the form
of a raster comprising a 4 bit word for each valve, with the value
of the 4-bit word determining the greyscale that is to be generated
by the valves. The print data is received by the second register
and written into the RAM 618. The RAM is logically arranged in 16
rows, with each of the valves corresponding to a row. There are
a plurality of columns, each of which corresponds to a time slot.
Each raster scan also corresponds to a time slot and the time slot
is determined by the frequency at which the shaft encoder supplies
pulses to the FPGA.
[0122] When print data is received at the FPGA the second register
interprets the greyscale data for each valve, obtaining the time
that each valve must be opened for in order to generate the desired
greyscale from a look-up table held in the first register. In theory,
each valve should be held open for the same period of time in order
to generate the dame greyscale, but mechanical variations in each
valve will lead to each valve having slightly different characteristics.
Calibration factors that account for these differences are held
in the look-up table. The valve times are then written into the
RAM, using as many columns as are necessary to store all of the
rasters. A write pointer is set to the first column of the data.
Each memory location holds the grey scale value for the associated
valve and time slot.
[0123] When the next shaft encoder pulse is received the RAM column
indicated by the write pointer is read to see which of the 16 valves
need to be operated, i.e. which memory locations have non-zero entries.
Once the memory locations have been read then all the memory locations
in the column are overwritten with zero.
[0124] The identity of these valves, along with the time for which
the valves are to be held open are then transmitted to the fourth
register, which may perform further operations on the valve times
in order to correct for valve operation at high speed or a long
time period between subsequent operations of the valve. The valve
times are then passed to the fifth register which calculates the
number of shaft encoder pulses that are equivalent to the valve
times. The valves are then opened for a period of time equal to
that number of shaft encoder pulses.
[0125] As the valves 610 are electromechanical devices, their size
provides a limitation to the print resolution that can be obtained.
Typically, each valve may be provided at an offset of 4 mm from
the adjacent valve(s). If a greater resolution (i.e. smaller pixel
separation is required) then the matrix may be slanted so that the
valves are closer together in one axis. The disadvantage of this
is that if no correction is made to the print rasters then the desired
image will be printed out slanted.
[0126] Such a correction may advantageously be provided using the
RAM to provide a slant to the print raster data. Once the greyscale
data has been translated into valve open times, rather than writing
the valve data into a vertical column, the write data can be offset
across a number of columns. For example, if the desired slant angle
is 45.degree. then the valve open time for the first valve should
be written into the column indicated by the write pointer, the valve
open time for the second valve should be written into the next column
along from the column indicated by the write pointer, and so on,
such that the valve open time is written into the RAM at the desired
slant angle.
[0127] Typically the 16-level greyscale can be provided using valve
open times between approximately 80 .mu.s and 250 .mu.s. It has
been found advantageous to initially open the valve by providing
a first voltage for a first period of time and to provide a second
voltage, that is lower than the first voltage, for a further period
of time in order to hold the valve open. This reduces the possibility
that the valve remains open for longer than is required to provide
the desired greyscale, leading to decreased printing performance.
It has been found particularly advantageous to apply a 36V pulse
for approximately 80 .mu.s and a second pulse of approximately 5V
for the remainder of the time that the valve remains open.
[0128] In a further preferred embodiment, the valve control means
and valves described above with reference to FIG. 11 will be co-located
upon a single circuit board 650. A number of circuit boards can
then be connected in serial and physically located in a vertical
array so that the valves can deposit a two-dimensional matrix on
a print substrate. In such a case (see FIG. 14), one of the boards
650a will be connected via serial input/output 622 to the computer
control system 230 and to the second board via serial input/output
624. The second board 650b will be connected to the first board
via serial input/output 622 and to the third board 650c via serial
input/output 624, and so on. The last board in the serial chain
can detect its position as its serial input/output 624 will have
no connection. On power up the last board in the serial chain assigns
itself address 0 and transmits this address to the preceding board,
which then assigns itself address 1. This process continues, with
the address value being incremented until each board has an assigned
address. The first board 650a will then report its address to the
computer control system such that the system is aware of the number
of connected boards. The system will prefix any communication with
a board with the board's address. Preferably 16 boards are connected
together to provide a 16.times.16 printing matrix.
[0129] The FPGA used in the preferred embodiment was a Xilinx Spartan
II XC2S100 which was preferred as its configuration was determined
by the data loaded from the PROM in start up. Such an FPGA may be
replaced by a cheaper device in which the FPGA is hardwired, for
example by blowing fuses to form logic elements, rather than configurable
through software.
[0130] It will be understood that the above technique for calibrating
a solenoid valve is suitable for use with any type of solenoid valve
and in any application in which solenoid valves are used. However,
the technique is of especial application to the compact high speed
valves of the invention where the small size of the components makes
manual adjustment of the position of pole pieces and other components
difficult and inaccurate.
[0131] As stated above, the software and computer control can be
used to decelerate the movement of the plunger at either or both
extremes of its travel so as to reduce spattering of the ink from
the nozzle orifice due to excessive slamming of the plunger against
its seat.
[0132] According to another aspect of the present invention there
is provided a method of operating a solenoid valve, the method comprising
the step of energising an electric coil to generate a magnetic field
in order to reciprocally drive a plunger within a coil, wherein
the magnetic field is controlled such that the speed of the plunger
is decreased as the plunger approaches at least one of its extremes
of movement. The control of the magnetic field may be achieved in
a number of ways.
[0133] In a preferred embodiment, the magnetic field may be controlled
such that the speed of the plunger is decreased as the plunger approaches
its closed position, in order to reduce the impact as the valve
closes. The magnetic field may be controlled such that the speed
of the plunger is decreased, the magnetic field resisting a force
exerted on the plunger by a return means. Such a method of operating
the valve is now described with reference to FIGS. 14 to 16.
[0134] FIG. 14 shows a schematic depiction of a solenoid valve
710 which is suitable for use with this method of operating the
valve. The valve 710 comprises plunger 720, tube 730 and coils 740.
The plunger 720 comprises a ferromagnetic material (or any other
magnetic material) and is received within the tube 730 so as to
be able to move freely along the axis of the tube. The plunger can
be impelled, for example to |