Technique for characterizing an electromechanical actuator unit for a vehicle brake

Abstract

A method for characterizing an electromechanical actuator unit for a vehicle brake, the electromechanical actuator unit comprising an electric motor and an actuator coupled to the electric motor. The actuator can be moved over a first area of movement without generation of a brake force and over a second area of movement with modification of a brake force. The method is carried out when the actuator moves within the first area of movement, and comprises the following steps: a) a voltage applied to the electric motor is interrupted, b) at least one parameter is determined while the electric motor runs in the generator mode, and c) at least one value is determined for a motor constant of the electric motor on the basis of the at least one parameter. The invention also relates to a vehicle brake, as well as to a computer program and a control unit for implementing the method.

Claims

1. A method for characterizing an electromechanical actuator unit for a vehicle brake, wherein the electromechanical actuator unit comprises an electric motor and an actuator coupled to the electric motor, wherein the actuator is movable over a first movement range without generating a brake force and over a second movement range with modification of a brake force, and wherein the method is implemented when the actuator is moved within the first movement range and comprises the steps of: determining a no-load current of the electric motor; switching off a voltage applied to the electric motor; determining a first value for a voltage induced in the electric motor at a first time while the electric motor is running in generator mode; determining a second value for a voltage induced in the electric motor at a second time while the electric motor is running in generator mode; and calculating a motor constant of the electric motor by means of at least multiplying the difference between the first value and the second value by a mass inertia of the electromechanical actuator unit to obtain a multiplication product, and then dividing the multiplication product by the difference between the second time and the first time.

2. The method according to claim 1, wherein the method further comprises the step of: switching on the voltage applied to the electric motor, prior to performing the step of calculating the motor constant of the electric motor.

3. The method according to claim 1, wherein the method further the step of: determining a value for a loss torque of the electromechanical actuator unit by means of the no-load current, the difference between the first value and the second value for the voltage induced in the electric motor and the mass inertia of the electromechanical actuator unit.

4. The method according to claim 1, wherein the method further comprises the step of: determining a temperature of the electric motor by comparing the determined value for the motor constant with previously known reference values for the motor constant, which are each associated with a certain temperature.

5. The method according to claim 1, wherein the method further comprising the step of: determining a motor current, at which a previously determined brake force is attained, by means of the determined value for the motor constant.

6. The method according to claim 1, wherein the method can be implemented on movement of the actuator from the first movement range to the second movement range or vice versa.

7. The method according to claim 1, wherein the method is further implemented on each actuation of the vehicle brake.

8. A method for characterizing an electromechanical actuator unit for a vehicle brake, wherein the electromechanical actuator unit comprises an electric motor and an actuator coupled to the electric motor, wherein the actuator is movable over a first movement range without generating a brake force and over a second movement range with modification of a brake force, and wherein the method is implemented when the actuator is moved within the first movement range and comprises the steps of: determining a no-load current of the electric motor; switching off a voltage applied to the electric motor; determining a first value for a voltage induced in the electric motor at a first time while the electric motor is running in generator mode; determining a second value for a voltage induced in the electric motor at a second time while the electric motor is running in generator mode; and calculating a value for a loss torque of the electromechanical actuator unit by at least multiplying a mass inertia of the electromechanical actuator by both the no-load current and the difference between the first value and the second value to obtain a multiplication product, and then dividing the multiplication product by the difference between the second time and the first time.

9. The method according to claim 8, wherein the method further comprises the step of: determining a temperature of the electric motor by comparing the determined value for the motor constant with previously known reference values for the motor constant, which are each associated with a certain temperature.

10. The method according to claim 8, wherein the method further comprises the step of: determining a motor current, at which a previously determined brake force is attained, by means of the determined value for the motor constant.

11. The method according to claim 8, wherein the method can be implemented on movement of the actuator from the first movement range to the second movement range or vice versa.

12. The method according to claim 8, wherein the method is further implemented on each actuation of the vehicle brake.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic view of a vehicle brake for implementing a method for characterizing an electromechanical actuator unit;

(2) FIG. 2 shows schematically the method sequence of the disclosed method for characterizing an electromechanical actuator unit;

(3) FIG. 3 shows schematically the method sequence of an embodiment of the method shown in FIG. 2;

(4) FIG. 4 shows schematically the method sequence of a variant of the embodiment of the disclosed method shown in FIG. 3;

(5) FIG. 5 shows the curve of the electric motor current, of the speed of the electric motor and of the induced voltage as a function of the time during the implementation of the method; and

(6) FIG. 6 shows schematically the method sequence of another embodiment of the method shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

(7) In FIG. 1 there is shown a vehicle brake 10 for implementing the disclosed method. The vehicle brake 10 is formed in a mechanical respect as a generally known sliding caliper brake, wherein only selected components of the vehicle brake 10 are depicted.

(8) The vehicle brake 10 comprises a brake housing 12 in the form of a known brake caliper and a brake disc 14 coupled in a torque-proof manner to a vehicle wheel (not depicted). Lying opposite the brake disc 14 on both sides are friction linings 16, which can be brought into abutment with the brake disc 14 to achieve a brake force.

(9) The vehicle brake 10 further comprises a service brake 11 with a displaceable actuating piston 20 taken up in a bore 18 in the brake housing 12. This piston is formed as a hollow piston and together with the bore 18 delimits a hydraulic chamber 22. By the introduction and draining of hydraulic fluid in the hydraulic chamber 22, a hydraulic pressure can be built up and dissipated in the chamber 22 and the actuating piston 20 can be moved along a displacement axis V. A movement along the displacement axis V in FIG. 1 to the left corresponds in this case to a movement in an application direction Z. Due to movement in the application direction Z, the friction linings 16 can be brought into abutment with the brake disc 14 to achieve a brake force and on dissipation of the hydraulic pressure are released from this again, in order to guarantee the service brake function.

(10) To achieve the desired return movement of the actuating piston 20 into its starting position following dissipation of the hydraulic pressure, the vehicle brake 12 further comprises a seal 24, which is depicted in a schematic manner. The seal 24 is taken up in a groove 26 starting out from the bore 18 and abuts an outer wall of the actuating piston 20. The seal 24 provides in a generally known manner a so-called rollback function, which acts in support to push the actuating piston 20 back into its starting position on dissipation of the hydraulic pressure.

(11) The vehicle brake 10 further has in the hydraulic chamber 22 a parking brake unit 30, which can likewise move along the displacement axis V. The parking brake unit 30 comprises an actuator 32, which is formed as a nut/spindle arrangement. More precisely, the actuator 32 comprises a spindle nut 34, which can be moved by rotation of a spindle 36 in a translatory manner along the displacement axis V. In this process the spindle nut 34 can also be brought into abutment with a piston base 28, which is formed as an inner end wall region of the actuating piston 20 lying opposite the spindle nut 34 and delimiting the hydraulic chamber 22.

(12) The actuator 32 is further connected via a coupling region 38 to the brake housing 12, wherein an electromotive drive and transmission unit, which is not depicted separately, is flanged from the outside onto the brake housing 12 at the coupling region 38. The electromotive drive and transmission unit has an electric motor, which is coupled by means of a transmission unit to the actuator 32. In particular, the transmission unit is coupled on the drive side to a drive shaft of the electric motor and on the output side to a pin coupled to the spindle 36 of the actuator 32. The electromotive drive unit drives the spindle 36 rotationally, in order to achieve the desired translatory displacement movement of the spindle nut 34 along the axis V. The electromotive drive and transmission unit and the actuator 32 form an electromechanical actuator unit.

(13) The disclosed vehicle brake 10 further comprises a control unit, not depicted, which comprises a processor and a computer program product, wherein the control unit is formed to execute a method as described further below.

(14) In FIG. 1 there are also shown the gap dimensions S present in the case of non-actuation of the service brake and the parking brake function, which dimensions have to be bridged to achieve a brake force. These relate to (from left to right in FIG. 1): a gap S between the brake housing 12 and the left friction lining 16 in FIG. 1, a gap S between the left friction lining 16 and the brake disc 14, a gap S between the right friction lining 16 in FIG. 1 and the brake disc 14 and a gap S between the actuating piston 20 and the right friction lining 16. To generate brake forces, the parking brake unit 30 must additionally overcome a gap S between the spindle nut 34 and the piston base 28 of the actuating piston 20.

(15) The gaps S between the friction linings 16 and the brake disc 14 are termed clearance or brake clearance, which is why these gaps S are additionally provided with the reference sign L. The clearance L should assume a predetermined minimum value in order to avoid residual torques in the sense of an undesirable abutment of the friction linings 16 on the brake disc 14 when the vehicle brake 10 is not actuated.

(16) The gap S between the spindle nut 34 and the piston base 28 of the actuating piston 20 is a safety clearance, which is why this gap S is additionally provided with the reference sign X. For system safety reasons the safety clearance X assumes a predetermined minimum value in order to guarantee a proper service brake function in the case of non-actuation of the parking brake function.

(17) In normal driver-controlled service braking, a hydraulic pressure is built up in the hydraulic chamber 22 and the actuating piston 20 is moved along the application direction Z into an actuating position generating a brake force. It comes into abutment here with the right friction lining 16, brings this into abutment with the brake disc 14 and applies the vehicle brake 10 in a known manner according to the sliding caliper design. In this process all gap dimensions S including the air gap L are bridged, with the exception of the safety clearance X between the spindle nut 34 and the piston base 28. To remove the brake force, the actuating piston 20 moves as a consequence of reducing the hydraulic pressure and with roll-back support by the seal 24 opposite to the application direction Z, whereupon the gap dimensions S, L previously bridged are reinstated.

(18) The parking brake unit 30 can generally be activated in the presence or absence of a hydraulic pressure to move the actuating piston 20 into its actuation position and/or lock it there mechanically. To do this, the spindle nut 34 is moved in the manner described above along the axis V and is supported in so doing (at least in a movement in the application direction Z) on the piston base 28.

(19) A movement of the spindle nut 34 in a range in which the gap dimensions S are bridged and no brake force is generated is defined as movement in a first movement range, and a movement of the spindle nut 34 in a range in which the friction linings 16 abut the brake disc 14, so that the brake force is modified, is defined as a movement in a second movement range.

(20) The disclosed method is executed upon exercising the parking brake function, to be precise in a movement of the actuator 32 within the first movement range, in which no brake force is yet generated or a brake force is no longer generated, depending on whether the method is implemented when applying the brake or when releasing the brake. The method can thus be implemented both when applying the parking brake, thus in a movement from the first movement range to the second movement range, and when releasing the brake, thus in a movement from the second movement range to the first movement range, within the first movement range.

(21) Quite generally, in the disclosed method the electric motor is briefly switched off and on again in idling, in a movement within the first movement range. While the electric motor is switched off, one or more parameters are determined in generator mode of the electric motor. The parameter or parameters determined are then used to determine a value for a motor constant of the electric motor.

(22) The sequence of the method is depicted schematically by means of FIG. 2. In a first step S1, the voltage applied to the electric motor is switched off on movement of the actuator within the first movement range. Following switching off of the voltage, at least one parameter is determined in step S2, while the electric motor continues to run with the voltage switched off. Finally, in step S3 at least one value for a motor constant of the electric motor is determined on the basis of the at least one parameter. Following step S2 (thus e.g. in step S3 or a step S4, not depicted), the voltage can then be switched on again, so that the electric motor continues running and, for example, an operating cycle that is currently in progress (applying or releasing the brake) can be continued substantially free of interruption.

(23) In FIG. 3 an embodiment of the method from FIG. 2 is depicted schematically. Step S1 corresponds to step S1 in FIG. 2. Step S2 from FIG. 2 is divided into two steps S21 and S23 in the method according to FIG. 3. In step S21, a first value for a voltage U.sub.i induced in the electric motor is determined at a first time. The induced voltage U.sub.i is determined by a measurement. In step S23, furthermore, a first value for a speed w of the electric motor is determined at the first time. The parameters determined in step S2 of FIG. 2 are thus the induced voltage and the speed. The speed can either be calculated by means of a model or determined by a measurement. The measurement can take place by means of a ripple detector, for example. The ripple detector detects the ripple in the motor current signal of the drive motor. The ripple count is a measure of the actuating path covered by the spindle nut. Measurement by means of a motor speed sensor is also possible. A direct measurement of the actuating path of the spindle nut by means of a path sensor is also possible. Starting out from the actuating path, the speed of the electric motor can then also be deduced with knowledge of the gear reduction of the transmission.

(24) In step S31, which corresponds to step S3 of the method in FIG. 2, the value for the motor constant k.sub.m is then determined by means of the first value for the voltage U.sub.i induced in the electric motor and the first value for the speed of the electric motor. The value for the motor constant k.sub.m specifically results from the quotient between the induced voltage U.sub.i and the speed of the electric motor, according to the following equation (1):
k.sub.m=U.sub.i/.

(25) Between step S23 and step S31 the voltage applied to the electric motor is switched on again and the application process of the brake or the release process of the brake is continued. The time between switching on and switching off of the voltage at the electric motor is typically 30 ms to 50 ms.

(26) In a variant of the embodiment shown in FIG. 3, the motor constant k.sub.m is determined by means of the loss torque T.sub.0 of the electromechanical actuator unit and a no-load current. To do this, the no-load current I.sub.0 of the electric motor is measured in a step S0, not depicted, before step S1, thus before switching off the voltage applied to the electric motor. Furthermore, after step S23, in addition to the first value for the voltage U.sub.i induced in the electric motor and to the first value for the speed of the electric motor, which are determined at the first time, a second value for the voltage U.sub.i induced in the electric motor and a second value for the speed of the electric motor are determined at a second time in the steps S25 (induced voltage U.sub.i) and S27 (speed ), which are not depicted. The second time is different from the first time. The second time can be 20 ms after the first time, for example.

(27) After that a difference between the first value and the second value is calculated for the speed of the electric motor in a further step. The calculation takes place typically after the voltage applied to the electric motor was switched back on again.

(28) The loss torque T.sub.0 of the electromechanical actuator unit is then calculated by means of the mass inertia J.sub.Antr. of the electromechanical actuator unit, which is known, the calculated difference between the first value and the second value for the speed of the electric motor and the time difference t between the first time and the second time. The calculation takes place according to the following equation (2):
T.sub.0=J.sub.Antr.*/t,

(29) Following on from this, the motor constant can be determined by means of the loss torque T.sub.0 according to equation (2) and by means of the no-load current determined before step S1. The calculation takes place according to the following equation (3):
k.sub.m=T.sub.0/I.sub.0,

(30) The value for the motor constant can thus arise according to equation (1) and according to equation (3). Here the determination can be implemented according to equation (3) alternatively to the determination according to equation (1). In this case the calculation of the motor constant according to equation (3) replaces step S31 of FIG. 3, and the electric motor is switched on again after determination of the second value for the induced voltage U.sub.i and of the second value for the speed of the electric motor.

(31) However, the determination according to equation (3) can also be carried out additionally following the determination according to equation (1), in order to verify the value determined according to equation (1). In this case the calculation of the motor constant according to equation (3) takes place in addition to the step S31 shown in FIG. 3, in a step S33 (not depicted). Step S31 can be carried out before step S25 or after step S27 together with step S33. The electric motor is switched on again in particular after determination of the second value for the induced voltage (step S25) and of the second value for the speed of the electric motor (step S27) and before step S33.

(32) According to another variant, the value for the motor constant can also be determined in a step S35 by means of the second value for the voltage induced in the electric motor and by means of the second value for the speed of the electric motor at a second time determined in steps S25 and S27, according to equation (1). Step S35 can be alternative or additional both to step S31 and to step S33. If step S35 is carried out additionally to step S31 and/or step S33, the differently calculated values for the motor constant can be used for mutual verification. Step S35 can be carried out in particular even before step S33, to be precise in particular after switching on of the voltage applied to the electric motor.

(33) A possible variant of the method as described by means of FIG. 3 is shown in FIG. 4. In step S0 the no-load current I.sub.0 is determined. After this, the voltage applied to the electric motor is switched off in step S1. Steps S21, S23 and S31 of FIG. 4 correspond respectively to steps S21, S23 and S31 of FIG. 3. As described above, in step S25 a second value for a voltage induced in the electric motor is then determined at a second time, and in step S27 a second value for a speed of the electric motor is determined, which is associated with the voltage induced in the electric motor at the second time. In step S4, the voltage applied to the electric motor is then switched on again, before in step S33 a value for the motor constant is determined by means of the second value for the induced voltage and by means of the second value for the speed. In a step S5, as described above, the loss torque T.sub.0 is determined according to equation (2). Finally, another value for the motor constant can be determined in step S35 by means of T.sub.0 and I.sub.0 according to equation (3) for verification of step S31 and S33.

(34) In FIG. 5, the curve of the motor current I, the induced voltage U.sub.i and the speed is depicted schematically. If the voltage applied to the electric motor is switched off, no further motor current flows after a short time. This is to be seen in FIG. 5 in a time span t.sub.2t.sub.1 between 0.2 s and 0.3 s, in which the motor current is 0 A. At a first time in this time span t.sub.2t.sub.1, the induced voltage U.sub.i is measured, by means of which a value for the motor constant can be calculated according to equation (1). If a second value for the induced voltage is measured, for verifying the value for the motor constant and/or for calculating the loss torque T.sub.0 according to equation (2), this second value is measured at a second time in the time span t.sub.2t.sub.1. After this the motor is switched back on again, so that the motor current rises again to a value different from 0 A, and the braking process or release process is continued.

(35) Another embodiment of the method depicted in FIG. 2 is shown in FIG. 6. In this embodiment of the disclosed method, the motor constant is determined without knowledge of the speed of the electric motor. For this the no-load current I.sub.0 is required for determining the motor constant. Accordingly, in a first step S0 before the voltage applied to the electric motor is switched off (before step S1), the no-load current I.sub.0 of the electric motor is determined in the presence of the idling speed, in particular by measurement. In step S22 a first value is then determined for the induced voltage at a first time, in particular by measurement. In step S24 a second value for the induced voltage is then determined at a second time, in particular by measurement. The first value and the second value are measured after switching off the electric motor voltage, when no further current is flowing, thus in the time span t.sub.2t.sub.1 of FIG. 5. The motor current is then switched on again and the braking or release process resumed. Steps S22 and S24 of FIG. 6 correspond here to step S2 of FIG. 2, wherein the parameter determined in step S2 of FIG. 2 is the induced voltage.

(36) To calculate the motor constant, a difference U.sub.i between the first value and the second value for the voltage induced in the electric motor is calculated in step S32. Step S32 corresponds to step S3 of FIG. 2. The calculation of the motor constant k.sub.m takes place by means of the following equation (4):

(37) $k_m=\left(\sqrt{\mathrm{Ui}}*\sqrt{\mathrm{JAntr}}.\right)/\left(\sqrt{t}*\sqrt{I0}\right),$

(38) wherein I.sub.0 is the no-load current of the electric motor, J.sub.Antr. is the mass inertia of the electromechanical actuator unit and t is the time difference between the second time, at which the second value for the induced voltage was measured, and the first time, at which the first value for the induced voltage was measured.

(39) Equation (4) results from equating from equation (2) and the following equation (5)
k.sub.m=U.sub.i/,

(40) and replacing T.sub.0 from equation (2) by k.sub.m*I.sub.0 according to the relationship known from equation (3) between T.sub.0 and the motor constant k.sub.m.

(41) The loss torque T.sub.0 can be calculated in the method according to the other embodiment even without knowledge of , according to the following equation (6)

(42) $T_0=\sqrt{\frac{U_i*J_{\mathrm{Antr}}*I_0}{{}_t}}$

(43) wherein U.sub.i is again the difference between the first value and the second value for the induced voltage, I.sub.0 is the no-load current, J.sub.Antr. is the mass inertia of the electromechanical actuator unit and t is the time difference between the second time, at which the second value for the induced voltage was measured, and the first time, at which the first value for the induced voltage was measured.

(44) Equation (6) results from equating k.sub.m from equation (3) and equation (5), and replacing in equation (5) by the relationship known from equation (2).

(45) The disclosed method thus provides several possibilities for determining the motor constant of the electric motor by means of at least one parameter, which is determined or are determined shortly after switching off the electric motor, when no further motor current flows. In the method according to one embodiment, the motor constant is determined among other things by means of the determined speed of the electric motor, in the method according to the other embodiment the motor constant is determined without knowledge of the speed of the electric motor.

(46) Furthermore, the axial three of the spindle nut can be calculated by means of the determined value for the motor constant according to the method disclosed. The motor constant is a parameter that determines the torque of the electric motor as a function of the current flowing in the electric motor. Using the no-load current I.sub.0 and the known gear reduction of the transmission, the axial force of the spindle nut can be calculated as a function of the current flowing in the electric motor. It is thus possible to determine the current upon attainment of which the electric motor should be switched off, as the desired clamping force or axial force has been attained. In detail the target switch-off current I.sub.Switch-Off results from the following equation (7)
I.sub.Switch-Off=I.sub.0+TransmissionRatio.sub.Actuator+F.sub.CLtarget+k.sub.m/.sub.Actuator,

(47) wherein I.sub.0 is the no-load current, k.sub.m the motor constant, TransmissionRatio.sub.Actuator is the gear reduction of the transmission unit of the electromechanical actuator unit, F.sub.CLtarget is the axial force of the spindle nut, at which the electric motor is to be switched off, and .sub.Actuator is the efficiency of the electromechanical actuator unit.

(48) The motor constant is a known motor characteristic value, which is, however, temperature-dependent. Furthermore, the motor constant can vary due to manufacturing tolerances in a motor size by +/10% from motor to motor. Inaccuracies of +/10% thus arise in calculation of the target switch-off current at a desired clamping force by means of the known motor characteristic value on account of manufacturing tolerances alone. Added to that is the fact that the exact motor temperature is not known, which leads additionally to inaccuracies when calculating the target switch-off current on attaining a desired clamping force.

(49) According to the disclosed method, the motor constant is determined by means of at least one parameter, which is determined during the idling phase in vehicle operation. The determined value for the motor constant thus contains variations on account of manufacturing tolerances and temperature influences. A more accurate clamping force setting is consequently possible than when using a previously known motor characteristic value. This leads to smaller overloads and thus to a longer transmission service life.

(50) According to the disclosed method, the temperature of the electric motor can also be determined by comparing the motor characteristic values determined according to the method with known reference values for the motor constant as a function of the temperature. The known reference values are stored in a memory unit of the vehicle brake, for example. The temperature of the electric motor can thus be determined in a simple manner by means of the determined motor constant without additional sensor.

(51) The disclosed method can be implemented on each actuation of the vehicle brake in parking mode or regularly at defined time intervals on actuation of the vehicle brake in parking mode. By comparing the measured no-load currents I.sub.0 and the determined loss torques T.sub.0 at known temperatures over time, the efficiency of the electromechanical actuator unit can be inferred. Wear states, for example, can be determined from this. Wear-dependent losses can be compensated for in the switch-off current. The temperatures required for the comparison can have been determined, for example, as described above by comparing the values determined for the motor constant with known reference values for the motor constant.

(52) Since only the voltage is measured when no motor current is flowing, there are no measuring errors either on account of different line resistances and internal motor resistances.

(53) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.