Abstract:
A method and a device for controlling a wheel brake in open-loop, which is provided with an electrically controllable control device. The control, in particular the clearance adjustment, is effected as a function of the change tendency of a braking-force or braking-torque signal over a path signal which represents a measure for the path covered by the brake pad.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for controlling a wheel brake in open loop. 
     BACKGROUND INFORMATION 
     When controlling a wheel brake, in some applications, it is important to know when the brake pads (or linings) lift off from the brake disc or drum. This holds true, for example, in connection with the adjustment of the clearance (air gap). Another application can be seen in the control of the wheel brake when the brake releases, it being possible to ensure that the vehicle wheel brakes are released essentially simultaneously. In the following, the problem of adjusting the clearance is discussed as the preferred field of application. 
     The adjustment of the clearance at a wheel brake is desirable with a view to the continually existing wear and tear of the brake pad, as well as changes in the marginal conditions such as the brake temperature. This holds true in particular for motor-vehicle wheel-brake systems operated using electric motors, in which the wheel brakes are controlled by driving brake actuators having electric motors. Motor-vehicle wheel brakes operated by electric motors are known, for example, from PCT Publication No. 94/24453 or German Published Patent Application No. 19 526 645, where the application force is produced by an electric motor. In an electrically controlled braking system, a required clearance (distance between the brake pad and the brake disc) should be maintained in all working points in the unbraked state. For example, in the event of a lateral runout of the brake disc, a clearance which is too small leads to friction losses between the brake disc and the brake pad even in the unbraked state. If there is no compensation for the continually existing wear and tear of the brake pad, then a clearance which is too large will occur. This will result in a reduction of the dynamic response of the braking process, since after initiating the braking process, the electric motor would first of all have to run through the clearance before any braking action would come about at the brake disc. In addition, the goal should be for the clearance to be adjusted approximately equally at both wheels of a vehicle axle. A different clearance leads to a different response characteristic of the two wheel brakes of a vehicle axle during a braking process. In this context, one must expect a quicker braking action at the wheel having the smaller clearance than at the wheel having a larger clearance. Accordingly, the clearance has a not inconsiderable influence on the comfort and safety during the braking process of a motor vehicle. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to specify measures which make it possible to determine the release of a wheel brake, in particular, to adjust a predefined clearance at the wheel brake. 
     The following described design approach makes it possible to determine exactly and simply when the wheel brake has released, i.e., the wheel pads have lifted off from the disc or the drum, even when the sensors employed at the wheel brake exhibit drifts. 
     It is particularly advantageous that it allows a defined adjustment of a clearance after the termination of a braking. In so doing, appearances of drift in the sensor signals used for adjusting the clearance are taken into account. In this manner, a zero shift of the sensors used, e.g., of braking-force, braking-torque, and/or angular-position sensors, as the result of temperature influences, etc., are rectified, it being possible to always adjust the same clearance, irrespective of the respective zero shift. 
     Used in particularly advantageous manner to ascertain the clearance is not the absolute value of the braking-force or braking-torque sensor, but rather a relative value of such a sensor. From this relative value can be determined the offset of an angular-position sensor, according to whose signal the optimum clearance is adjusted. 
     The adjustment of the clearance is particularly accurate if the offset determination is repeated at brief time intervals, until the temperature equilibrium between the wheel brake and the ambient temperature has settled. Therefore, to adjust the clearance, in advantageous manner the brake pads are applied for a short period and released again outside of the braking process, as well. 
     Particularly advantageous, particularly because of low costs, is the determination of an angle in the region of a brake actuator having an electric motor, e.g., the rotational angle of the electric motor or a rotational angle of the gear output of the brake actuator using Hall-effect sensors which emit several pulses per revolution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a synoptic block diagram of a braking system having application of the brakes by an electric motor, using a wheel pair as an example. 
     FIG. 2 a  shows a first graph illustrating the characteristics of one of the braking-force and braking-torque signal under a particular condition, as well as the advantageous effects of the following described design approach. 
     FIG. 2 b  shows a second graph illustrating the characteristics of one of the braking-force and braking-torque signal under a particular condition, as well as the advantageous effects of the following described design approach. 
     FIG. 2 c  shows a third graph illustrating the characteristics of one of the braking-force and braking-torque signal under a particular condition, as well as the advantageous effects of the following described design approach. 
     FIG. 2 d  shows a fourth graph illustrating the characteristics of one of the braking-force and braking-torque signal under a particular condition, as well as the advantageous effects of the following described design approach. 
     FIG. 2 e  shows a fifth graph illustrating the characteristics of one of the braking-force and braking-torque signal under a particular condition, as well as the advantageous effects of the following described design approach. 
     FIG. 3 shows the framework of a computer program implementing the design approach described. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a synoptic block diagram of a braking system having application of the brakes by an electric motor, using a wheel pair as an example. This wheel pair could be allocated to one axle or to a diagonal of the vehicle. The vehicle brake pedal is represented by  10 . The driver&#39;s braking input is detected using sensor system  12  by measuring the angle, path and/or force, and is fed via lines  14  to an electronic control system  16 . In one advantageous layout, this control system  16  is composed of decentrally distributed control units. Sensor system  12 , as well as, at least partially, electronic control system  16 , are redundantly designed. The electronic control system  16  actuates electric motors  22  and  24  via output lines  16  and  20 , for example, by a pulse-width-modulated voltage signal, using an H-bridge output stage. In one advantageous exemplary embodiment, commutator direct-current motors are used. The electric motors  22  and  24  are part of brake actuators  26  and  28 . The rotatory movements of these motors  22  and  24  are converted in the downstream gear stages  58  and  60  into translatory movements which lead to displacements of brake pads  30  and  32 . The brake pads  30  and  32  are guided in brake calipers  34  and  36  and act on brake discs  38  and  40  of wheels  1  and  2 . Moreover, in one preferred exemplary embodiment, provision is made for an electrically operable spring-powered brake, with whose aid, the associated brake actuator  26 ,  28  can be retained in the prevailing position, so that the electric motor  22 ,  24  can be switched into the currentless state. The position of the brake actuator  26 ,  28  is then retained without energy consumption. 
     Employed at each wheel  1  and  2  are force or torque sensors  42  and  44 , whose signals are fed to electronic control system  16  via measuring lines  46  and  48 . In one design variant, the axial supporting forces of the actuators  26  and  28  during a braking process are measured by sensors  42  and  44 , and thus form a measure for the normal forces acting on the brake discs  38  and  40 . In the following, this variant is named force measurement. Therefore, the braking force is the force with which the brake shoes  30  and  32  press against the brake discs or drums  38  and  40 . In another design variant, the tangential supporting forces of the brake pads  30  and  32  are measured, and thus form a measure for the friction forces or their friction moments occurring in the brake discs  38  and  40 . This measurement—as well as the use of a direct torque sensor—is designated in the following as torque measurement. In addition, the wheel speeds are detected using sensors  50  and  52 , and are transmitted to control system  16  via input lines  54  and  56 . Provision is further made for angular-position sensors  62  and  64 , whose signals are fed to control system  16  via lines  66  and  68 . In one preferred exemplary embodiment, these angular-position sensors  62  and  64  are Hall-effect sensors which, for example, detect the revolution of the electric motor of the associated brake actuator  26 ,  28  and deliver several pulses per revolution, the number of pulses being a measure for the angle covered, and thus for the path covered. In other exemplary embodiments, other sensors (e.g. variable-inductance sensing elements, potentiometers, etc.) are used for measuring the path or angle. 
     Setpoint values for the individual wheel brakes or groups of wheel brakes are determined in electronic control system  16  from the detected braking input provided by the driver, in accordance with pre-programmed families of characteristics. For example, these setpoint values correspond to the braking torques or braking forces to be adjusted at a wheel or a wheel pair  1 ,  2 , whose quantities are a function, inter alia, of the axle load distribution of the vehicle. From the ascertained, possibly wheel-individual setpoint values, system deviations are determined by comparison with the actual values of the braking forces or braking torques measured in sensors  42  and  44 , the system deviations being fed to controller algorithms, for example, in the form of time-discrete PID controllers. The controlled variable of this controller is used to drive the electric motors  22  and  24 , corresponding drive signals being output via lines  16  and  20 . At the end of a braking process, in each case the reference input variables for the braking forces or the braking torques to be adjusted become zero. This is the instant for the beginning of the clearance adjustment, the actuator position being locked by driving the spring-powered brake after the conclusion of the clearance adjustment. 
     According to the following design approach, the clearance of a brake disc  38 ,  40  is adjusted utilizing the path or angle signal on the basis of the force signal or torque signal. In this context, the change tendency of the braking force or the braking torque over the path or angle is ascertained. If this change tendency is zero, then the brake pad is no longer contacting the brake disc  38 ,  40 . The zero point or the offset value of the path or angle measurement and of the force or torque sensor  42 ,  44  is ascertained at this instant. This value is the starting point of the clearance adjustment by a closed-loop path or angle control to a pre-defined setpoint clearance calculated from the zero point. 
     For clarification, the problem in the use of force or torque sensors  42  and  44  is set forth in the following paragraphs with reference to FIGS. 2 a  through  2   d . In so doing, braking force F or braking torque M is in each case plotted over the travel path of brake pads x. 
     The situation shown in FIG. 2 a  corresponds to the situation at the beginning of a braking. The braking force is increased from the zero point (force zero, path zero) with increasing path. Correspondingly, the torque signal over the path behaves according to FIG. 2 c.    
     After a strong braking, the disc  38 ,  40  heats up and becomes thicker because of the thermal expansion. Therefore, a zero shift dx results in the path according to FIG. 2 b . The new path zero point indicates the path which is necessary to lift the brake pads  30 ,  32  off from the brake discs  38 ,  40 . In the same manner, the associated force sensors  42  and  44  heat up, which likewise leads to a corresponding zero shift dF as shown in FIG. 2 b.    
     The same holds true for when sensors  42 ,  44  are embodied as torque sensors. The problem is the same as in the case of the force sensor described above, but it is even more intensified, because when the wheel is stationary, the torque sensor delivers a completely false signal. As above, in the case of the torque sensor  42 ,  44 , shifts also occur which are designated in FIG. 2 d  by dx and dM 3 . If, given a wheel coming to a standstill, on the basis of the mechanical hysteresis, the residual torque in the brake shoe  30 ,  32  is dM 1 , if there would be no thermal expansion of the disc  38 ,  40 . Due to the thermal expansion of the disc  38 ,  40 , the sensor  42 ,  44  indicates dM 3 , although the residual torque of the hysteresis dM 1  is in the brake shoe  30 ,  31 . The situation is corresponding when the vehicle is on a downgrade. In this case, the sensor  42 ,  44  indicates the downgrade force dM 2 , although the residual torque of the hysteresis dM 1  is in the brake shoe  30 ,  32 . 
     In addition, neither the force sensor nor the torque sensor  42 ,  44  has an exactly reproducible characteristic curve, which likewise contributes to the zero shift. Particularly in the case of forward travel and reverse travel, great differences occur in the zero point. The result of this can be, for example, that on the basis of the force-sensor signal, the zero point is no longer reached after the braking, and residual forces are present in the wheel brake, which lead to heating and wear and tear. 
     Because of these conditions, an additional signal is used to exactly adjust the clearance, in order to unequivocally and reliably eliminate the residual forces. 
     Used as an additional signal is a path or angle signal, e.g., by detecting the angle of the electric motor  22 ,  24  of the actuator  26 ,  28  using Hall-effect sensors  62 ,  64  or other sensors. In so doing, first of all, the path which the brake pad  30 ,  32  covers can be measured; on the other hand, a corresponding signal can be determined from an angle measurement of the revolutions of the rotor of the electric motor  22 ,  24 . In the preferred exemplary embodiment, this angle is measured by Hall-effect sensors  62 ,  64 , and the path which the brake pad  30 ,  32  covers is calculated within the framework of the following equation: 
     
       
         x′=S*PHI/(I*2*3.14) 
       
     
     S signifies the rise of the actuator spindle, 
     I signifies the transmission ratio of the actuator gear, 
     PHI signifies the measured angle of the rotor of the motor, 
     x′ signifies the sought path of the brake shoe or of the brake pad. 
     Problematic in this case is that the same holds true for the path sensor  62 ,  64  as for the other sensors. It is likewise not free from faults, and can deliver a faulty signal after the braking because of temperature drift or due to missing pulses. This means that this sensor signal moves out of its zero point as well, and thus the adjustment of the clearance is not always exact on the basis of this signal alone. 
     With the aid of FIG. 2 e , in which measured force F or measured path x′ is plotted over actual path x′, a procedure is shown which, in spite of the drift-affected signals, makes it possible to compensate the residual forces and to adjust an exact clearance. 
     FIG. 2 e  shows the situation after a strong braking. It is assumed that the sensors  62 ,  64  have drifted away from their zero point in the manner indicated, i.e., the path sensor  62 ,  64  exhibits an offset x′ 0 , the force sensor  42 ,  44  exhibits an offset dF. The characteristic of this situation is that the force sensor  42 ,  44  no longer changes its signal in the zero point when the brake shoe  30 ,  32  lifts off from the disc  38 ,  40 . The corresponding holds true as well for a torque sensor  42 ,  44 , given a rotating wheel  1 ,  2 . Therefore, in response to the brake pad  30 ,  32  lifting off from the disc  38 ,  40 , the change of the force over the path is zero, i.e., the quotient dF/dx′=0. The change tendency of this quotient is monitored, and the zero point is assumed when this change tendency becomes zero. In this context, it is advantageous that not the absolute value of the force sensor  42 ,  44 , but rather merely its relative change plays the dominating role for recognizing the zero point. If the quotient becomes zero, the offset x′ 0  of the path sensor  62 ,  64  is determined at this point. The path measurement is again correct on the basis of this measured offset value, so that the brake pad  30 ,  32  can be run into the pre-defined clearance. In this process, the clearance is pre-defined by a specific path from the zero point away from the brake disc  38 ,  40 . The described process must be repeated at brief time intervals until the temperature equilibrium at the wheel brake has settled at the ambient temperature. The clearance is then adjusted in such a way that the residual forces are completely reduced and the wear and tear of the brake pad  30 ,  32  is compensated. In one preferred exemplary embodiment, the clearance is adjusted within the framework of a closed-loop position or angle control of the electric motor  22 ,  24 . 
     The corresponding procedure is selected in connection with a torque measurement. However, in this case it is necessary to take into account that sensor  42 ,  44 , when serving as a torque sensor, only emits a signal when the wheel  1 ,  2  is moving. Therefore, the most unfavorable case must generally be assumed, namely, the case where the maximum residual torque is retained in the brake caliper  34 ,  36  because of the friction, while the wheel  1  and  2  is at standstill on a level. Therefore, in the procedure in connection with a torque measurement, the brake shoe  30 ,  32  is initially run by the path sensor  62 ,  64  into the predefined zero point. This is not exact, so that a slight residual torque can be retained. The correct adjustment of the clearance is effected when the wheel  1 ,  2  is rotating, and proceeds according to the procedure presented above. 
     To exactly adjust the clearance, the brake pads  30 ,  32 , outside of the braking process, are applied for a brief period repeatedly if necessary within the framework of an adjustment operation, in order to ascertain the zero point according to the procedure presented above. 
     This procedure is implemented within the framework of a computer program whose basic structure is sketched within the framework of the flow chart according to FIG.  3 . 
     The program sketched in FIG. 3 is carried out at the end of each braking process, be it a braking desired by the driver, or be it after an application of the pads  30 ,  32  specially started to ascertain the zero point. In the first step  100 , the variables existing for each wheel i(i=1, 2, 3, 4) are read in, i.e., measured force Fi or measured torque Mi, measured angle PHIi, as well as the respective rotational frequency ni. Thereupon, in step  102 , path x′i is calculated from the respective angle signal PHIi according to the above formula. In the following step  104 , change tendency dFi/dx′i or dMi/dx′i is calculated. In each case, this is done by taking into account two successive values of the force, of the torque or of the path. In step  106 , it is checked whether the change tendency calculated in step  104  is zero (taking into consideration any tolerances). If this is not the case, the program is repeated with step  100  and the change tendency is ascertained for the next run. If step  106  has revealed that the change tendency is zero, then, given the use of a torque sensor, it is checked in step  108  whether rotational frequency ni is greater than rotational frequency n 0 , below which a standstill of the wheel  1 ,  2  must be assumed. If this is not the case, according to step  110 , the clearance is adjusted within the framework of a closed-loop position control on a basis of a predefined value for clearance, as well as possibly a stored predefined offset value x 0 . If the wheel  1 ,  2  is rotating or if a force sensor  42 ,  44  is used, then in step  112 , measured path x′ which exists when the change tendency according to step  106  becomes zero, is stored as offset value x 0  of the angular-position sensor. Thereupon, according to step  110 , the clearance is adjusted according to the above description, taking into consideration the newly ascertained value x 0 . After step  110 , the program is terminated and is started again in response to the next braking or in response to the next test movement to ascertain the zero point. 
     The test movement lies in the fact that first of all, the brake pads  30 ,  32  are somewhat applied, and thereupon the brake is released in order to determine the exact zero point. As described above, such a test movement is repeated at brief time intervals until a temperature equilibrium has settled in, i.e., particularly until the ascertained offset value no longer changes substantially. 
     In this manner, an exact adjustment of the clearance results, with the advantages indicated at the outset. Besides the use for an electro-mechanical brake, this procedure can also be used to adjust the clearance in other braking systems, e.g., for electro-hydraulic or electro-pneumatic brakes which are provided with the appropriate sensors.