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Patent Abstract
A solenoid valve control is described, having a switching output
stage for controlling a solenoid valve, an arrangement for preselecting
a valve opening time, and an arrangement for preselecting a pressure
over the valve. Also provided is an arrangement for determining
a valve control time from the preselected valve opening time and
the preselected pressure over the valve. A method is also described
for controlling a solenoid valve which can be carried out to advantage
with this solenoid valve control.
Patent Claims
What is claimed is:
1. A solenoid valve control, comprising: a switching output stage
for controlling a solenoid valve; an arrangement for preselecting
a valve opening time; an arrangement for preselecting a pressure
over the solenoid valve; and an arrangement for determining a valve
control time from the preselected valve opening time and the preselected
pressure over the solenoid valve.
2. The solenoid valve control according to claim 1, wherein: the
arrangement for preselecting the valve opening time operates in
accordance with an inverse hydraulic model.
3. The solenoid valve control according to claim 1, wherein: the
means for preselecting the pressure over the solenoid valve operates
in accordance with a pressure estimation model.
4. The solenoid valve control according to claim 1, wherein: the
arrangement for determining the valve control time operates in accordance
with a table that lists pressure values and respective correction
values.
5. The solenoid valve control according claim 1, wherein: the arrangement
for determining the valve control time models a time dependence
of a valve current and a current dependence of a valve opening pressure.
6. The solenoid valve control according to claim 5, wherein: the
valve current has an exponential time dependence.
7. The solenoid valve control according to claim 1, wherein: the
arrangement for determining the valve control time operates in accordance
with an approximation function.
8. The solenoid valve control according to claim 1, further comprising:
a freewheeling diode connected in parallel with the solenoid valve,
so that one of an LMV operation (linearized solenoid valve) and
a CPC operation (continuous pressure control) having pulse width-modulated
voltage in parallel with pulse step control can occur.
9. The solenoid valve control according to claim 8, wherein: the
CPC operation is implemented at an inlet valve of a brake cylinder
for an anti-skid system control, and the pulse step control is implemented
at an inlet valve of a brake cylinder for an anti-spin control system.
10. A method of controlling a solenoid valve, comprising the steps:
controlling the solenoid valve including a switching output stage;
preselecting a valve opening time; preselecting a pressure over
the solenoid valve; and determining a valve control time from the
preselected valve opening time and the preselected pressure over
the solenoid valve.
11. The method according to claim 10, further comprising the step
of: preselecting the valve opening time in accordance with an inverse
hydraulic model.
12. The method according to claim 10, further comprising the step
of: preselecting the pressure over the solenoid valve in accordance
with a pressure estimation model.
13. The method according to claim 10, wherein: the valve control
time is determined in accordance with a table that lists pressure
values and respective correction values.
14. The method according to claim 10, further comprising the step
of: modeling a time dependence of a valve current and a current
dependence of a valve opening pressure when determining the valve
control time.
15. The method according to claim 10, wherein: a valve current
has an exponential time dependence.
16. The method according to claim 10, wherein: the valve control
time is determined in accordance with an approximation function.
17. The method according to claim 10, wherein: a freewheeling diode
is connected in parallel with the solenoid valve, so that one of
an LMV operation (linearized solenoid valve) and a CPC operation
(continuous pressure control) having pulse width-modulated voltage
in parallel with pulse step control can occur.
18. The method according to claim 10, wherein: the CPC operation
is implemented at an inlet valve of a brake cylinder for an anti-skid
system control, and the pulse step control is implemented at an
inlet valve of a brake cylinder for an anti-spin control system.
Patent Description
FIELD OF THE INVENTION
[0001] The present invention relates to a solenoid valve control
having a switching output stage for controlling a solenoid valve,
an arrangement for preselecting a valve opening time, and an arrangement
for preselecting a pressure over the valve. The present invention
also relates to a method of controlling a solenoid valve having
the steps: controlling a solenoid valve having a switching output
stage, preselecting a valve opening time and preselecting a pressure
over the valve.
BACKGROUND INFORMATION
[0002] Various systems for automatically influencing the driving
properties of a motor vehicle are known. These include, for example,
the anti-skid system (ABS) and anti-spin control (ASC). In these
systems, the pressure in a wheel brake cylinder is adjusted by solenoid
valves whose switching states are influenced by a solenoid valve
control. For example, in the brake system of a motor vehicle there
are reversing valves (USV valves) on the path from a hydraulic pump
to a master brake cylinder and inlet valves (EV valves) to a wheel
brake cylinder for which solenoid valve controls are used. Both
inlet valves and reversing valves, which can be operated linearly,
are known. This property is utilized to an increasing extent in
anti-skid systems (ABS) and anti-spin control (ASC) systems. In
the case of linearly operated valves, the pressure drop over the
valve is essentially a linear function of the electric valve current.
Basically, the valve current can be adjusted through current-controlled
valve output stages. However, such current-controlled valve output
stages are expensive, so there has been a search for other options.
It is already known that the valve current can be switched with
inexpensive switching output stages. These switching output stages
supply the electric valve voltage with pulse width modulation. A
mean current can be adjusted by way of such pulse width modulation,
using a freewheeling diode connected in parallel to the valve coil,
so the desired pressure drop over the valve can be adjusted in this
way. The freewheeling diode permits a current to flow when the output
stage is turned off.
[0003] However, a disadvantage of the required freewheeling diode
is that brief opening and subsequent closing of the valve are influenced
to a great extent by the freewheeling diode. The reason for this
great effect is that the current drops slowly when the control of
the valve is shut down in comparison with a situation without a
freewheeling diode. Since the switching threshold of a valve depends
greatly on the pressure drop over the valve, the effect on the current
characteristic due to the freewheeling diode described here leads
to different switching times, depending on the pressure drop over
the valve. This has resulted in different valve opening times, depending
on the pressure drop over the diode, with identical valve control
times.
[0004] Thus, on the whole, it is problematical to implement a pulse
width-modulated control such as an LMV control (linearized solenoid
valve) or CPC operation (continuous pressure control) in parallel
with a pulse step control in a single solenoid valve control, because
the freewheeling diode which is required for pulse width-modulated
control prevents accurately implementable valve opening times. However,
under some circumstances it may be desirable to use both pressure
modulation principles in one controller, mainly against the background
of preventing use of expensive current-regulated valve output stages.
For example, it is possible to operate an anti-skid system (ABS)
with a pulse width-modulated CPC on the inlet valve. In the case
of anti-spin control (ASC), however, this is not possible because
requirements regarding the accuracy of the set pressures are higher
in anti-spin control (ASC).
SUMMARY OF THE INVENTION
[0005] The present invention is based on the generic solenoid valve
control such that means are provided for determining a valve control
time from the preselected valve opening time and the preselected
pressure over the valve. In this way, it is possible to influence
the variation in the valve opening time as a function of the pressure
drop over the valve, namely by determining a "corrected"
valve control time. The valve control time thus takes into account
the valve current characteristic over time, for example, and therefore
the effect of the switching time on the basis of the dependence
of the switching thresholds on the pressure drop.
[0006] The means for preselecting a valve opening time preferably
use an inverse hydraulic model. Hydraulic models are generally used
in controllers to calculate a pressure on the basis of input parameters
such as a valve current and a valve opening time, for example. With
an inverse hydraulic model, it is possible to calculate a required
valve opening time from the pressure over the valve.
[0007] The means for preselecting a pressure over the valve preferably
use a pressure estimation model. Since the pressure over the valves
cannot be determined directly, pressure estimation models which
are contained in controllers of the related art are suitable means
for estimating the required pressure for further determination of
the valve control time.
[0008] It may be advantageous if the means for determining a valve
control time use a table which lists pressure values and the respective
correction values. Each pressure value can thus be assigned a certain
correction value, for example, which transforms the preselected
valve opening time into a valve control time. It is also conceivable
for each pressure value to be assigned multiple correction values,
one of the correction values being used to determine the time for
opening the valve, for example, and another correction value being
used for the time for closing the valve.
[0009] However, it may also be advantageous if the means for determining
a valve control time model the time dependence of the valve current
and the current dependence of the valve opening pressure. The required
valve control times can thus be determined by determining functional
relationships between the valve current and time or between valve
opening pressure and current. The functional relationships can be
determined mathematically or empirically in good approximation.
[0010] For example, the fact that the valve current may have an
essentially exponential time dependence may be utilized. Mathematical
models can be constructed easily with such exponential relationships.
[0011] It may also be advantageous if the means for determining
the valve control time use an approximation function. The valve
control time can be determined approximately by using approximation
functions, this being sufficient for numerous applications.
[0012] A freewheeling diode is preferably connected in parallel
with the solenoid valve, thus yielding LMV operation or CPC operation
with pulse width-modulated voltage in parallel with pulse step control.
In CPC operation, pulse width modulation takes place continuously,
whereas in LMV operation, pulse width modulation takes place in
phases, while at other times the valve is closed. Since a freewheeling
diode in parallel with the solenoid valve is used for both modes
of operation, the pressure dependence of the switching times of
the valve is especially great. Therefore, the present invention
manifests its advantages especially in such parallel operation of
pulse step control and pulse width-modulated control.
[0013] The present invention also manifests its particular advantages
due to the fact that there is CPC operation at the inlet valve of
a brake cylinder for an anti-skid system control (ABS control),
and there is pulse step control at the inlet valve of a brake cylinder
for an anti-spin control (ASC) system. It is thus possible to eliminate
current-regulated valve output stages in an inexpensive manner.
For ABS control, pulse width-modulated CPC operation may be used
at the inlet valve of the brake cylinder. This is not possible with
anti-spin control (ASC), if no current-regulated valve output stages
are to be used at the inlet valves.
[0014] The present invention is based on the generic method such
that a valve control time is determined from the preselected valve
opening time and the preselected pressure over the valve. In this
way, it is possible to influence the variation in the valve opening
time as a function of the pressure drop over the valve, namely by
determining a "corrected" valve control time. The valve
control time thus takes into account the valve current characteristic
over time, for example, and therefore the effect of the switching
time on the basis of the dependence of the switching thresholds
on the pressure drop.
[0015] A valve opening time is preferably preselected by using
an inverse hydraulic model. With an inverse hydraulic model, it
is possible to calculate a required valve opening time from the
pressure over the valve.
[0016] A pressure over the valve is preferably preselected by using
a pressure estimation model. Since pressure over the valves cannot
be determined directly, pressure estimation models which are contained
in controllers of the related art are suitable means for estimating
the required pressure for further determination of the valve control
time.
[0017] It may be beneficial if a valve control time is determined
by using a table which lists pressure values and the respective
correction values. Each pressure value can thus be assigned a certain
correction value, for example, which transforms the preselected
valve opening time into a valve control.
[0018] However it may also be beneficial if the time dependence
of the valve current and the current dependence of the valve opening
pressure are modeled when determining a valve control time. The
required valve control times can thus be determined by determining
the functional relationships between the valve current and the time
or between the valve opening pressure and the current. The functional
relationships can be determined mathematically or empirically in
good approximation.
[0019] It is especially beneficial for the modeling if the valve
current has an essentially exponential time dependence. Mathematical
models can be constructed easily with such exponential relationships.
[0020] However, it may also be beneficial if a valve control time
is determined by using an approximation function. The valve control
time can be determined approximately by using approximation functions,
this being sufficient for numerous applications.
[0021] The present invention is especially advantageous due to
the fact that a freewheeling diode is connected in parallel with
the solenoid valve, thus yielding LMV operation or CPC operation
with pulse width-modulated voltage in parallel with pulse step control.
In CPC operation, pulse width modulation takes place continuously,
whereas in LMV operation, pulse width modulation takes place in
phases, while at other times the valve is closed. Since a freewheeling
diode in parallel with the solenoid valve is used for both modes
of operation, the pressure dependence of the switching times of
the valve is especially great. Therefore, the present invention
manifests its advantages especially in such parallel operation of
pulse step control and pulse width-modulated control.
[0022] However, the present invention also manifests particular
advantages due to the fact that there is CPC operation at the inlet
valve of a brake cylinder for an anti-skid system control (ABS control)
and there is pulse step control at the inlet valve of a brake cylinder
for an anti-spin control (ASC) system. It is thus possible to eliminate
current-regulated valve output stages in an inexpensive manner.
For ABS control, pulse width-modulated CPC operation may be used
at the inlet valve of the brake cylinder. This is not possible with
anti-spin control (ASC), if no current-regulated valve output stages
are to be used at the inlet valves.
[0023] The present invention is based on the surprising finding
that the valve control time can be determined in the case of a switching
output stage even with valve opening thresholds that have a great
dependence on pressure. This has advantages with regard to the combination
of pulse width-modulated control with pulse step control, because
in pulse width-modulated control, a freewheeling diode is connected
in parallel with the valve coil, which leads a great influence over
time on the current characteristic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 shows a voltage characteristic and a current characteristic
for pulse width modulation.
[0025] FIG. 2 shows a voltage characteristic and a current characteristic
and their effect on valve behavior.
[0026] FIG. 3 shows a voltage characteristic and a current characteristic
according to FIG. 2 for the mathematical illustration of possible
models.
[0027] FIG. 4 shows a schematic diagram of a solenoid valve control
according to the present invention.
DETAILED DESCRIPTION
[0028] FIG. 1 shows voltage U and current I plotted qualitatively
over time t, such as those occurring in pulse width-modulated systems.
A mean current, which is established on the basis of pulsed voltage
U, can be influenced in terms of its size by the width of the voltage
pulses. To permit a current to flow even when the output stage is
turned off, a freewheeling diode is preferably connected in parallel
with the valve coil.
[0029] FIG. 2 shows the consequences of the presence of such a
freewheeling diode, namely the marked time dependence of current
I. The control time of the valve is shown as t.sub.anst. Depending
on the pressure relationships over the valve, this control time
t.sub.anst can lead to different valve opening times t.sub.off.
This is due to the fact that a higher current is used to switch
the valve at high pressures than at low pressures. For example,
the valve switches to an opened state later in the case of a pressure
P.sub.H than a pressure P.sub.L. Conversely, the valve closes sooner
at a high pressure P.sub.H than at a low pressure P.sub.L. Opening
time is thus much shorter at a high pressure t.sub.ooff(P.sub.H)
than at a low pressure t.sub.off(P.sub.L).
[0030] FIG. 3 shows a diagram similar to that in FIG. 2 to illustrate
mathematical models. For calculation of valve control time t.sub.anst,
the dependence of current threshold s.sub.an at which the valve
opens on the pressure over the valve is determined first. Furthermore,
current threshold s.sub.ab at which the valve closes again on the
pressure is determined. In good approximation, these thresholds
are essentially a linear function of pressure. However, these current
thresholds can also be determined empirically in model experiments.
The relationship between current threshold s.sub.an and the time
after which the valve opens is given approximately by an exponential
function:
s.sub.an=1-exp(-c.sub.an.multidot.t.sub.an).
[0031] Parameter C.sub.an is determined largely by the inductance
and resistance of the valve coil. The simplest method of determining
it is empirically by using model experiments. Rearranging the equation
yields
t.sub.an=-(1/c.sub.an).multidot.ln(1-s.sub.an)
[0032] Like s.sub.an, the relationship between s.sub.ab and t.sub.ab
is also given by an exponential function:
s.sub.ab=exp(-c.sub.ab.multidot.t.sub.ab).
[0033] Under ideal conditions, parameter c.sub.ab is equal to c.sub.an.
However, there may be deviations due to different electrical relationships.
Rearranging yields:
t.sub.ab=-(1/c.sub.ab).multidot.ln(s.sub.ab).
[0034] The time during which the valve is open is obtained as follows
t.sub.off=t.sub.anst-t.sub.an+t.sub.aus.
[0035] The valve control time can thus be determined as follows:
t.sub.anst=t.sub.off+t.sub.an-t.sub.aus.
[0036] The equations for s.sub.an and s.sub.ab are interchanged
in the case of valves having "inverse" control, i.e.,
where the pressure pulse is induced by removing the voltage, e.g.,
in the case of inlet valves (EV) and reversing valves (USV).
[0037] Since it is complicated to calculate the natural logarithm
within the control, it is advisable to approximate the relationship
described here in the controller by an approximation function or
to store this relationship in the form of a table or in an engine
characteristics map, where t.sub.an-t.sub.aus is listed as a function
of pressure. This table or this engine characteristics map can also
be directly determined empirically, so it is no longer necessary
to determine in detail individual parameters c.sub.an, c.sub.ab,
s.sub.an as a function of pressure and s.sub.ab as a function of
pressure. FIG. 4 shows a schematic diagram of a solenoid valve control.
A solenoid valve 10 is controlled during time t.sub.anst. To this
end, a valve opening time t.sub.off is preselected by a controller
12. In addition, pressure p over the valve is determined from these
valve opening times by using a pressure estimation model 14. This
valve control time t.sub.anst is calculated in means 16 for determining
the valve control time from valve opening time t.sub.off and pressure
p.
[0038] The previous description of embodiments according to the
present invention is used to illustrate the present invention but
not to restrict it in any way. Various alterations and modifications
are possible within the scope of the present invention without going
beyond the scope of the present invention or its equivalents. |