Method and arrangement for controlling a braking system of a vehicle

The invention is directed to a method and an arrangement for controlling a brake system wherein the electrically actuable actuator for the wheel brakes is actuated in the context of a closed-loop control in normal operation. A transfer from the closed-loop control to an open-loop control of the actuator takes place with entry into at least one pregiven operating state, especially an operating state with low wheel rpm.

FIELD OF THE INVENTION 
The invention relates to a method and an arrangement for controlling a 
brake system of a motor vehicle such as a brake system having an electric 
braking force. 
BACKGROUND OF THE INVENTION 
A braking system with electrical braking force for motor vehicles is 
disclosed, for example, in international patent publication WO-A 94/24453. 
There, a brake system is described wherein the braking force is generated 
by electric motors. A suitable procedure for actuating these wheel brakes, 
especially in the region of standstill of the vehicle, is not suggested 
herein. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide measures for actuating wheel 
brakes of a vehicle especially in the region of the standstill of the 
vehicle. 
The method of the invention is for controlling a brake system of a motor 
vehicle driven by an operator, the brake system including at least one 
wheel brake apparatus including an actuable brake actuator. The method 
includes the steps of: actuating the brake actuator in the context of a 
closed-loop control in dependence upon a desired value pregiven by the 
operator; and, switching from the closed-loop control to an open-loop 
control of the brake actuator in at least one operating region of the 
vehicle in dependence upon the desired value. 
The solution provided by the invention makes a suitable procedure for 
actuating the wheel brakes available and is especially suited for the 
region of standstill of the vehicle. This procedure affords special 
advantages when actuating the wheel brakes in the context of a brake 
torque control loop. 
It is furthermore advantageous to provide a clear strategy for the service 
brake and/or parking brake for operating regions in which no reliable 
statements are present as to the braking torque applied on the wheel. 
It is especially advantageous that a clear standstill criterion can be 
given for the vehicle on the basis of the brake torque signal and this 
signal clearly differentiates from the standstill-like blocking of a 
wheel. 
The actuation of the service brakes (similar to control operation) is 
maintained via the control of the brake actuation in the region of 
standstill of the vehicle in an advantageous manner. 
It is especially advantageous that a so-called "stopping ABS" can be 
realized with the solution provided by the invention whereby the stopping 
operation is considerably improved. 
A further advantage is afforded by the solution of the invention for the 
control of a parking brake which can be controlled adaptively in several 
operating situations and/or be controlled with respect to each wheel 
individually. 
It is also advantageous to apply the solution of the invention for wheel 
brakes having electric braking forces (electric-motor brakes).

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 shows an electronic control unit 10 having an input circuit 12, at 
least one microcomputer 14 and an output circuit 16. These components are 
connected to each other via a bus system 18 for data exchange. Further, a 
control unit 20 is shown and, in a preferred embodiment, this control unit 
actuates the electric actuators which preferably include DC motors. The 
actuators are connected to the wheel brakes of the motor vehicle. The 
electric motor 26 (DC motor) is connected via electric connecting lines 22 
and 24 to the control unit 20, as shown exemplary for one wheel brake of 
the vehicle. 
The electric motor 26 is connected via a mechanical connection 28 to the 
brake clamp 30 of the wheel 32. When the electric motor 26 is actuated, 
the brake clamp 30 of the wheel brake is opened or closed depending upon 
the direction of rotation of the motor. As a rule, the actuating device is 
guided into the proximity of its rest position when no current flows 
through the motor (opened brakes). Furthermore, the actuating device 
includes a controllable clutch which holds the brake shoes to the value 
adjusted by the electric motor in predetermined operating situations (for 
example, parking brake function) even when the clutch is not driven. 
In the region of the actuating device, at least one sensor 34 is provided 
which supplies via a line 36 an actual value as to brake actuation to the 
control unit 10 (there, the input circuit). The sensor 34 can be a 
brake-force sensor, a brake torque sensor, a current sensor, et cetera. In 
the preferred embodiment, the sensor 34 detects the braking torque acting 
on the wheel 32. 
In another embodiment, the braking torque is estimated, for example, from 
the braking force, mean friction radius and friction coefficients. The 
actuating device is actuated by the control apparatus 10 via an output 
line 38 extending from the output circuit 16. 
In addition to the control of the wheel brake shown, the wheel brakes of 
the other vehicle wheels are controlled in a corresponding manner via 
output lines 40, 42 and 44. Additional input lines 46, 48 and 50 are 
connected to the input circuit 12. These input lines 46, 48 and 50 conduct 
the control actual values of the other wheel brakes to the control 
apparatus 10. In addition, at least one input line 52 leads to the control 
apparatus 10 from a measuring device 54 for detecting the brake-pedal 
actuation (for example, the pedal path, pedal force, et cetera). Input 
lines 55 to 57 lead to the control apparatus 10 from measuring devices 58 
to 60, respectively. The measuring devices 58 to 60 detect additional 
operating variables necessary for controlling the brake system. These 
operating variables are, for example, wheel speeds, axle loads, yaw rates, 
et cetera. 
In normal operation of the brake system, a desired value for the individual 
wheel brakes is derived by the control apparatus 10 (the microcomputer 14) 
from the degree of actuation of the brake pedal. This takes place, for 
example, by considering the axle loads in accordance with pregiven 
characteristic lines, characteristic fields, tables or computation steps. 
This desired value, which is derived from the degree of actuation, defines 
in the preferred embodiment a desired value for the brake torque to be 
applied. The desired value is corrected, as needed, in the context of an 
ABS and/or VDC control for each wheel individually in accordance with the 
wheel speeds, rates of yaw, et cetera. The abbreviation "VDC" is the 
abbreviation for "Vehicle Dynamics Control" and is explained, for example, 
in the article by A. T. van Zanten et al" entitled "VDC, the Vehicle 
Dynamics Control System of Bosch", SAE Paper No. 950759 (1995). The 
desired value for each wheel individually corrected in this manner is then 
compared for each wheel brake, to the actual value for the brake torque 
measured at the corresponding wheel and the difference in each case is 
supplied to the controller provided for the corresponding wheel. The 
controller forms, in accordance with the difference and a predetermined 
control strategy (for example, PID), a controller actuating quantity which 
is outputted via the output line to the control unit 20. There, the 
control quantity, which defines a current through the electric motor 26, 
is converted by actuating the actuating motor. 
The described brake-torque control loops (for each wheel individually) are 
implemented in another embodiment in the control unit 20 which likewise 
includes at least one microcomputer for this case. 
In the region of low wheel rpms, that is, in the region of standstill of 
the vehicle, no brake torque can be measured at least on a plane. For this 
reason, and for the control of an electrically controlled brake system on 
the basis of the detected brake torque, it is necessary in this operating 
region to find additional solutions in addition to the brake-torque 
control. 
According to the invention, it is proposed to provide a transition from the 
closed-loop control to the open-loop control of the service brake in the 
region of very low wheel rpm and especially in the region of standstill of 
the vehicle. In this connection and as explained below, the actuating 
quantity for each wheel is adjusted in accordance with the desired value. 
With a change of the position of the brake pedal and for building up the 
braking force or reducing the braking force, the actuating quantity for 
all wheel brakes is synchronously changed and with approximately the same 
time-dependent trace. As a suitable criterion for limiting this control 
region, a trace of the brake torque signal is applied in the region of low 
road speed (when the road speed drops below a minimum value). With the 
transition to standstill, the brake pad moves from sliding friction into 
static friction. A static friction peak occurs thereby in the 
time-dependent trace of the brake torque. 
In a further embodiment, the decay of the brake torque as a function of 
time can be applied as a criterion or a mix of both methods can serve as a 
criterion. The decay of the brake torque as a function of time is caused 
by the spring-damped suspension of the vehicle. Thus, standstill is, for 
example, detected when a static-friction peak and a brake-torque trace 
characterizing the decay thereof are present. When standstill is detected, 
there is a movement out of brake torque closed-loop control and the 
actuating variable for the brake actuators is formed in accordance with an 
open-loop control. A defective detection of standstill is avoided by 
applying the road speed as a further criterion. 
The pulses in the area of the brake-torque signal, which arise when 
stopping the vehicle, can also be applied to improve the stopping 
operation. On a wheel, for which such a standstill criterion is detected, 
a switchover from closed-loop control to the open-loop control takes place 
and the brake is released in the context of the open-loop control. A 
renewed braking is initiated at a time point at which a pregiven number of 
wheels are detected to be at standstill in the context of the criterion. 
Two, three or four different wheels are applied to trigger the repeat 
braking. 
A parking brake function is likewise realized in the context of the 
open-loop control at standstill of the vehicle. At standstill, it is 
possible that the vehicle can be loaded so that there is a change of the 
axle loads associated therewith. For this reason, the control value for 
the actuating variable in the case of the parking brake cannot be derived 
from the service brake desired value last measured effectively, 
especially, if the axle loads are not detected. Instead, in this case, a 
predetermined parking brake value must be controlled to which can be 
possibly less than a service brake value because of the static friction at 
the brake linings. If the vehicle is at an incline, then a parking brake 
torque can be measured. An increase of the value of the actuating variable 
for each wheel individually (adaptive parking brake) can be made in 
dependence upon the value of this parking brake torque in one embodiment. 
The adaptive parking brake affords advantages especially in combination 
with a two-value actuating device via an appropriate braking force at 
limited energy consumption. A use of the adaptive parking brake for the 
case of a metered actuating device would be connected with a detection of 
standstill. 
The solution according to the invention is shown in the preferred 
embodiment of a wheel brake with an electric braking force. The invention 
can be utilized with the above-mentioned advantages also with a braking 
system equipped with hydraulic or pneumatic actuators for the wheels 
brakes. Here, a variable is used as a controller actuating variable or 
control actuating variable with which the valve devices are actuated for 
controlling the pressure in the wheel brake cylinders. In accordance with 
the procedure of the invention, this variable is formed in the context of 
a closed-loop control and in special operating situations, in the context 
of an open-loop control. 
A realization of the closed-loop control and of the transition to open-loop 
control is shown in FIG. 2 with respect to a flowchart. The subprogram 
described there is run through for each wheel brake individually at 
pregiven time points with the actuation of the brake pedal. In FIG. 2, a 
selected wheel brake is viewed. Corresponding subprograms are provided for 
the other wheel brakes. In a first step 100, the desired value, which is 
derived from the pedal actuation, is detected for the selected wheel brake 
as is the road speed V. Thereafter, in step 102, the actual value of the 
selected brake is detected. In the next step 104, the controller actuating 
variable Yi.sub.R for the selected wheel brake is formed in accordance 
with a pregiven control strategy on the basis of the desired value and 
actual value (corrected, if required, via additional functions such as 
ABS, VDC, et cetera). The standstill criterion is checked in the next 
inquiry step 106. In one embodiment, this check is performed by evaluating 
corresponding marks. These marks are set when, at a selected wheel brake, 
there is a drop of the speed below a threshold value and when a 
static-friction peak occurs in the brake torque signal and/or a decay in 
the brake torque as a function of time. If the standstill criterion is 
satisfied, then, in step 108, the open-loop control is initiated. If the 
standstill criterion is not satisfied, then in accordance with step 110, 
the controller actuating variable Yi.sub.R, which is determined in step 
104, is outputted. Thereafter, the subprogram is repeated with step 100 at 
a pregiven time. 
A preferred embodiment of the control 108 will now be described. 
The control variable Yi.sub.S for the wheel brake is determined on the 
basis of the detected desired value as well as a pregiven time function T 
which is essentially the same for all wheel brakes. The time function 
leads to the change of the control variable when there is a change of the 
desired value. The time function ensures a brake force reduction (or the 
brake force buildup) which takes place at all braked wheel brakes 
essentially in synchronism and in essentially the same time-dependent 
trace or curve. For the determination of the control variable on the basis 
of the desired value, a pregiven characteristic line, a pregiven 
characteristic field, a pregiven table or suitable computation steps are 
provided. The control variable is stored in the pregiven characteristic 
line in dependence upon the desired value and, if required, the axle 
loads. The allocation is determined experimentally so that, at each wheel 
brake, approximately the same brake force, brake torque or the same 
current results. The control variable is changed from the old value to the 
new value in the context of the pregiven time function if a difference 
between the desired value is determined from one of the pregiven program 
runthroughs and the actual desired value. The computed control variable is 
then outputted to the brake actuator device. 
In FIG. 3, the operation of the procedure shown in FIG. 2 is presented in 
the context of quantities plotted as a function of time. Thus, FIG. 3a 
shows the time-dependent trace of the desired value and FIG. 3b shows the 
brake torque as a function of time at a selected wheel. FIG. 3c shows the 
trace of the controller actuating variable at this wheel and FIG. 3d shows 
the trace of the control actuating variable at this wheel. 
The driver actuates the brake pedal from time point t.sub.1 on in order to 
brake the vehicle. Correspondingly, a brake torque in accordance with FIG. 
3b is built up at the wheels because of a corresponding course of the 
controller actuating variable (FIG. 3c). This brake torque is built up in 
the context of the brake torque control. From time point t.sub.2, the 
driver holds the brake pedal in a constant position so that the controller 
actuating variable is interrupted starting at time point t.sub.2 and the 
built up brake torque is maintained in the actuator by closing the clutch. 
Between time points t.sub.2 and t.sub.3, control interventions can take 
place for changes of the desired value and these interventions build up or 
reduce the brake force. At time point t.sub.3, the wheel speed drops 
toward 0 so that a peak in the trace of the torque is detected. This trace 
of the brake torque is brought about because, at this time point, the 
friction between the brake disc and the brake lining goes from a sliding 
friction to a static friction. In this way, the brake torque at this wheel 
is measurably increased. Because of the increased actual value, the 
controller is compelled to reduce braking force. A negative controller 
actuating variable between time points t.sub.3 and t.sub.4 is the result. 
The brake torque can no longer be measured after standstill of the wheel 
so that, pursuant to FIG. 3b, no signal is any longer present which can be 
evaluated. 
The drop below the rpm threshold and the friction peak of the brake torque 
signal are evaluated as standstill criteria so that, after the time point 
t.sub.4, the standstill of the vehicle is detected and a switchover to 
open-loop control takes place. In the region of time point t.sub.4, a 
control actuating variable is therefore pregiven which is maintained in 
correspondence to the desired value up to the time point t.sub.5. In this 
way, the brake force built up at the wheel is maintained. Between the time 
points t.sub.5 and t.sub.6, the driver releases the pedal so that the 
control actuating variable is correspondingly decontrolled. This decontrol 
takes place at all wheel brakes essentially in synchronism and with a 
similar time-dependent trace. Between the time points t.sub.3 and t.sub.4, 
fluctuations of the torque value about the zero point can take place as a 
consequence of the decay of oscillation of the vehicle suspension. This 
signal too can be applied as an alternative or as supplementary to the 
static friction peak for detecting standstill. 
In a further embodiment, and in the context of the open-loop control, a 
so-called stopping ABS is realized in the region of low wheel rpms. This 
stopping ABS avoids an uncontrolled blocking of the wheels in the region 
of the lowest speeds. As a standstill criterion and therefore as the basis 
for the transition to open-loop control, in this case, and in accordance 
with step 106, the presence of static-friction peaks of the brake torque 
signals are evaluated from two, three or four wheels in addition to the 
road speed threshold. This leads to the situation that, based on the 
control function, the brakes are initially released at the first wheels 
moving into standstill. If the standstill criterion is satisfied, then, at 
wheels with released brakes, braking force is built up synchronously in 
the context of the open-loop control and, at the other wheels, the 
controller actuating variable is replaced (if needed while increasing the 
braking force) by the corresponding control actuating variable. 
For the synchronous braking, the standstill criterion for two, three or 
four wheels must be satisfied dependent upon the embodiment. 
The above solution is made clear in FIGS. 4a to 4e with the aid of 
quantities plotted as a function of time. Thus, in FIGS. 4a to 4d, the 
control variable Yi.sub.S is plotted as a function of time for the four 
wheel brakes of the vehicle; whereas, in FIG. 4e, the trace of the brake 
torque signal for three selected wheels of the vehicle are shown. 
All brakes are in closed-loop control operation before time point t.sub.1. 
At time point t.sub.1, the standstill criterion for the first wheel I is 
satisfied (see FIG. 4e). In accordance with FIG. 4a, this leads to a 
corresponding drive (which releases the brake) in the context of the 
open-loop control of the brake assigned to this wheel. At time point 
t.sub.2 (see FIG. 4e), the standstill criterion for a second wheel II is 
detected as satisfied. Accordingly, and in accordance with FIG. 4b, this 
brake is released at time point t.sub.2. At time point t.sub.3, the 
standstill criterion is satisfied for a third wheel III. According to 
FIGS. 4a to 4d, this leads to the synchronous buildup of the braking force 
at the wheels I and II (see FIGS. 4a and 4b) in the context of the 
open-loop control; whereas, for the wheels III and IV, the braking force, 
which is obtained from the closed-loop control operation, is maintained by 
corresponding control actuating variables. The fourth wheel IV (standstill 
criterion has up to now not been satisfied) is braked, if needed, with 
great intensity. 
At standstill of the vehicle, a parking-brake function is realized by the 
open-loop control of the brake system. This is initiated when the driver 
actuates a corresponding operator-controlled element (hand brake lever, 
foot brake, switch element, et cetera) to activate the parking brake. 
A preferred embodiment is described as a program of a microcomputer in the 
flowchart of FIG. 5. FIG. 5 is a detailed view of the open-loop control of 
step 108 of FIG. 2. 
In the first step 1080, the control actuating variable Yi.sub.S of the 
selected wheel brake is computed from the desired torque of the brake 
pedal Mdes and, if required, from a pregiven time function T. In the next 
step 1081, the control actuating variable Yi.sub.park is determined for 
the parking brake. 
During standstill, it is possible that the axle loads change because of 
loading. For this reason, this control actuating variable in the case of 
the parking brake cannot be derived from the operating brake desired value 
Mdes which was last effectively measured. In addition, if an actual torque 
Mbrake greater than zero is measured, this indicates that the vehicle is 
on an incline at standstill. The gravity drive force on the incline is to 
be countered by means of an increased control voltage. For this reason, in 
step 1081, the control variable of the parking brake is computed in 
dependence upon the desired value Mparkdes of the parking brake actuating 
device and from the measured actual torque (Mbrake). This was described 
above as adaptive parking brake. In the next step 1085, a check is made as 
to whether the actual braking torque is constant. This is only the case 
(caused by the drive Yi.sub.park which changes), when the vehicle is 
standing still. If this is not the case, then, in step 1082, the drive 
quantities computed in steps 1080 and 1081 are compared to each other. The 
brake actuator is driven by the larger of the two control actuating 
quantities in accordance with steps 1083 or 1084. If no brake torque 
change can any longer be determined (step 1085), then the clutch of the 
brake actuator is driven in accordance with steps 1086 and 1087 in the 
sense of closing and maintaining the braking force and the control 
actuating variables are set to zero. With a renewed actuation of the 
parking brake actuating device (release), the clutch is opened and the 
vehicle is released. 
It is understood that the foregoing description is that of the preferred 
embodiments of the invention and that various changes and modifications 
may be made thereto without departing from the spirit and scope of the 
invention as defined in the appended claims.