METHOD FOR ASCERTAINING A MOTOR CONSTANT, A FAULT STATE AND/OR WEAR STATE, AS WELL AS A CONTACT POINT, CONTROL UNIT, FRICTION BRAKE

Abstract

A method for ascertaining a motor constant of an electric machine, in particular an actuator assembly. At least one excitation signal having at least one direct current component and one alternating current component is specified. The machine is controlled using the excitation signal. At least one actual value of a motor current of the machine and one actual value of a rotation rate of a rotor shaft of the machine are ascertained in each case. The motor constant is ascertained as a function of the excitation signal and the actual values.

Claims

1. A method for ascertaining a motor constant of an electric machine, the method comprising the following steps: specifying at least one excitation signal having at least one direct current component and one alternating current component; controlling the machine using the excitation signal; ascertaining at least one actual value of a motor current of the machine and one actual value of a rotation rate of a rotor shaft of the machine; and ascertaining the motor constant as a function of the excitation signal and the actual value of the motor current and the actual value of the rotation rate.

2. The method according to claim 1, wherein actual temporal profiles of the motor current and of the rotation rate that result from the excitation signal are ascertained in each case, and average values are ascertained as respective actual values as a function of the actual temporal profiles of the motor current and of the rotation rate.

3. The method according to claim 2, wherein the actual profiles of the motor current and of the rotation rate are ascertained over at least two periods of the alternating current component.

4. The method according to claim 1, wherein the alternating current component has a plurality of different harmonic frequencies.

5. The method according to claim 1, wherein an average value of the motor constant is ascertained using a plurality of different excitation signals.

6. The method according to claim 1, wherein the motor constant is ascertained using a Fourier transform.

7. The method according to claim 1, wherein the machine is part of an actuator assembly, and the motor constant is ascertained as a function of at least one Coulomb friction coefficient and/or a viscous friction coefficient of the actuator assembly.

8. The method according to claim 1, wherein the machine is part of an actuator assembly, and the motor constant is ascertained as a function of a preload and/or a restoring moment of a spring element of the actuator assembly.

9. The method according to claim 1, wherein the machine is part of an actuator assembly of a friction brake of a motor vehicle, and the machine is controlled using the excitation signal only in a range of an air gap of the friction brake.

10. A method for ascertaining a fault state and/or wear state of a friction brake including at least two friction partners, of a motor vehicle, wherein the friction brake is assigned an actuator assembly with an electric machine, for displacing one of the friction partners to the other of the friction partners, the method comprising: specifying at least one excitation signal having at least one direct current component and one alternating current component; controlling the machine using the excitation signal; ascertaining at least one actual value of a motor current of the machine and one actual value of a rotation rate of a rotor shaft of the machine; ascertaining a motor constant as a function of the excitation signal and the actual value of the motor current and the actual value of the rotation rate; and ascertaining the fault state and/or wear state is ascertained as a function of the ascertained motor constant of the machine by comparing: (i) the ascertained motor constant and/or a friction coefficient ascertained as a function of the motor constant, with (ii) a specified threshold value and/or tolerance band.

11. A method for ascertaining a contact point of two friction partners of a friction brake of a motor vehicle, wherein the friction brake is assigned an actuator assembly with an electric machine, for displacing one of the friction partners (2) to the other of the friction partners, the method comprising: specifying at least one excitation signal having at least one direct current component and one alternating current component; controlling the machine using the excitation signal; ascertaining at least one actual value of a motor current of the machine and one actual value of a rotation rate of a rotor shaft of the machine; ascertaining a motor constant as a function of the excitation signal and the actual value of the motor current and the actual value of the rotation rate; and ascertaining the contact point of the friction partners is ascertained, using a load torque estimator, as a function of the ascertained motor constant.

12. A control unit specifically configured to ascertain a motor constant of an electric machine, the control unit configured to: specify at least one excitation signal having at least one direct current component and one alternating current component; control the machine using the excitation signal; ascertain at least one actual value of a motor current of the machine and one actual value of a rotation rate of a rotor shaft of the machine; and ascertain the motor constant as a function of the excitation signal and the actual value of the motor current and the actual value of the rotation rate.

13. A friction brake of a motor vehicle, with at least two friction partners, wherein the friction brake is assigned an actuator assembly with an electric machine, for displacing one of the friction partners to the other of the friction partners, the friction brake comprising: a control unit specifically configured to ascertain a motor constant of the electric machine, the control unit configured to: specify at least one excitation signal having at least one direct current component and one alternating current component, control the machine using the excitation signal, ascertain at least one actual value of a motor current of the machine and one actual value of a rotation rate of a rotor shaft of the machine, and ascertain a motor constant as a function of the excitation signal and the actual value of the motor current and the actual value of the rotation rate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows an electromechanical drum brake, according ot an example embodiment of the present invention.

[0021] FIG. 2 shows a rotation rate vs. motor current diagram of the drum brake, according to the present invention.

[0022] FIG. 3 shows methods for ascertaining parameters of the drum brake, according to an example embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0023] FIG. 1 shows a simplified form of an exemplary design of an electromechanical friction brake 1 of an otherwise not further illustrated motor vehicle. In the present case, the friction brake 1 is designed as a simplex drum brake and comprises two first friction partners 2 in the form of brake shoes 3 with brake pads 4 arranged thereon, and a second friction partner 5 in the form of a brake drum 5. The first friction partners 2 are each assigned to the second friction partner 5.

[0024] For displacing each first friction partner 2 to the second friction partner 5, the friction brake 1 is assigned an actuator assembly 6 with an electric machine 7. The electric machine 7 is operatively connected to a first end of the first friction partners 2, for example by means of a gear assembly not shown, in order to apply a corresponding actuating force F to them. The friction partners 2 are mounted on an abutment 8 so as to be rotatable about a second end facing away from the first end.

[0025] In order that a braking torque is actually generated, the friction partners 2 must first overcome an air gap 1 until the corresponding brake pad 4 makes contact with the brake drum 5. In order to ensure that the brake pads 4 in the unactuated state do not rub against the brake drum 5, a spring element 9 is provided as a return spring with a certain preload, which spring element is attached at each end to one of the two brake shoes 3 and pushes the two brake shoes 3 toward each other.

[0026] With reference to FIG. 3, an advantageous method for ascertaining various parameters of the actuator assembly 6 described, in particular a motor constant of the electric machine 7, is described below. For this purpose, FIG. 3 shows the method in the form of a flow chart. In particular, the method ensures that the parameters, which may change over the service life due to, for example, component spreading and wear, are always determined with high accuracy. In particular, at least one of the methods described below is performed by means of a control unit specifically configured for this purpose.

[0027] The method is based on a simplified state space model of the actuator assembly 6. The actuator assembly 6 can thus be described sufficiently accurately as a second-order dynamic system. Non-linearities are predominantly the load, which results from the stiffness characteristic curve of the friction brake 1, as well as friction effects within the actuator assembly 6. Assuming that the method is performed exclusively in the range of the air gap 1, the load can be neglected.

[0028] The air gap is characterized by two motor positions .sub.L,min and .sub.L,max, which limit the allowable motor position range in the air gap {|.sub.L,min<<.sub.L,max}. They may be specified for design reasons or ascertained using a conservative method.

[0029] For the friction that works against the motor torque, a common approximation in practice and literature is then carried out.

[0030] The friction model is described with three parameters in the form of friction coefficients, a viscous friction factor D, and two Coloumb factors C.sub.+ and C.sub., each of which applies to the actuation and opening of the friction brake 1.

[0031] Since the electrical time constant is typically much smaller than the mechanical time constant, precise modeling of the electric machine 6 can be dispensed with, and the relationship between actuation current I.sub.m and torque .sub.mI.sub.mK.sub.m via the motor constant K.sub.m can be used instead.

[0032] The differential equation, which approximately describes the system behavior (change in the rotational motor position ), is therefore

[00001]$\stackrel{.Math.}{}{J}^{-1}\left({}_m-\stackrel{}{}D-\mathrm{sign}\left(\stackrel{}{}\right)C_s\right)$

[0033] Here, J is the rotational total inertial mass of the actuator mechanism (usually known) and the following applies:

[00002]$C_s=\{\begin{array}{c}{C}_{+}\mathrm{if}\stackrel{.}{}>\\ C_{-}\mathrm{if}\stackrel{.}{}<-\end{array}$

[0034] The positive scalar is understood to mean a Karnopp factor and describes a zero speed band. It is assumed that a corresponding rotor position sensor of the electric machine 6 is sufficiently accurate to calculate the rotational speed {dot over ()} with acceptable error, for example by means of a finite difference method.

[0035] In a step S1, the method begins with specifying at least one excitation signal having at least one direct current component and one alternating current component, and with driving the machine 7 by means of the excitation signal, in particular only in the range of the air gap 1 of the friction brake 1. The alternating current component preferably has at least one harmonic frequency, in particular a plurality of different harmonic frequencies.

[0036] The excitation signal is thus in particular composed of a direct current component I.sub.0 and one or more harmonic alternating current components I.sub.k(t)=.sub.ksin(.sub.kt+.sub.k):

[00003]$I_m\left(t\right)=I_0+\underset{k=1}{\overset{K}{.Math.}}{I}_k\left(t\right)$

[0037] The use of multiple harmonics (K>1) is advantageous in order to obtain a higher robustness in the identification of the motor constant, which becomes apparent in the following sections.

[0038] In a step S2, at least one actual value of a motor current of the machine 7 and one actual value of a rotation rate of a rotor shaft of the machine 7 are ascertained in each case. Preferably, actual temporal profiles of the motor current and of the rotation rate that result from the excitation signal (I.sub.m) are ascertained for this purpose in each case, and that average values are ascertained as respective actual values as a function of the actual profiles. In particular, the actual profiles are ascertained over at least two periods of the alternating current component (I.sub.k). Particularly preferably, an average value of the motor constant is ascertained by means of a plurality of different excitation signals.

[0039] Depending on the direct current component selected, a measurable average rotation rate arises after the transient response (t>T.sub.e) of the rotation rate (t):

[00004]$\stackrel{}{}=\frac{1}{T-T_e}{}_{T_e}^T\mathrm{dt}$

[0040] Here, it should only be noted that ||>t>T.sub.e so that no stick-slip effects occur, which are not taken into account in the simplified model. For a series of measurements with different I.sub.0, the Coloumb factor as well as the viscous friction as a function of the still unknown motor constant K.sub.m can very simply be determined via linear interpolation.

[0041] FIG. 2 shows a corresponding rotation rate vs. motor current diagram with a plurality of measurement points for motor current I and rotation rate . As already described above, the state space model used has different friction coefficients as further unknowns. It is therefore advantageous if the motor constant K.sub.m is ascertained as a function of the Coulomb friction coefficients C.sub.+, C.sub. and the viscous friction coefficient D of the actuator assembly 6.

[0042] The corresponding ascertainment of the friction coefficients is preferably carried out via a linear fit to the data points. The slope of a corresponding straight line G.sub.+ (for the friction coefficient (C.sub.+) or G.sub. (for the friction coefficient C.sub.+) in each case corresponds to

[00005]$\frac{1}{K_m}D.$

[0043] The y-intercept corresponds to

[00006]$\frac{1}{K_m}{C}_s.$

In this example, the Coloumb friction is different for positive and negative rotation rates (the dashed line shown corresponds to

[00007]$y=\frac{1}{2K_m}\left(C_{+}+C_{-}\right)0).$

[0044] The resulting relationship is:

[00008]$K_m{I}_o=C_s+D\stackrel{}{}.Math.I_0=\frac{1}{K_m}{C}_s+\frac{1}{K_m}D\stackrel{}{}$

[0045] With N2 independent measurements, the equation system can be solved:

[00009]$\left[\begin{array}{cccc}{I}_0^{\left(1\right)}& I_0^{\left(2\right)}& .Math.& I_0^{\left(N\right)}\end{array}\right]=\left[\begin{array}{cc}\frac{1}{K_m}{C}_S& \frac{1}{K_m}D\end{array}\right].Math.\left[\begin{array}{cccc}1& 1& .Math.& 1\\ {\stackrel{}{}}^{\left(1\right)}& {\stackrel{}{}}^{\left(2\right)}& .Math.& {\stackrel{}{}}^{\left(N\right)}\end{array}\right].Math.A:=\left[\begin{array}{cc}{A}_1& A_2\end{array}\right]=\left[\begin{array}{cc}\frac{1}{K_m}{C}_S& \frac{1}{K_m}D\end{array}\right]=\left[\begin{array}{cccc}{I}_0^{\left(1\right)}& I_0^{\left(2\right)}& .Math.& I_0^{\left(N\right)}\end{array}\right].Math.{\left[\begin{array}{cccc}1& 1& .Math.& 1\\ {\stackrel{}{}}^{\left(1\right)}& {\stackrel{}{}}^{\left(2\right)}& .Math.& {\stackrel{}{}}^{\left(N\right)}\end{array}\right]}^{}$

[0046] In a step S3, the motor constant is ascertained as a function of the excitation signal and the actual values. One option is to solve the equation system by means of a Fourier transform.

[0047] Due to the assumption of a linear model and due to the superposition principle, the system as well as the excitation signal are in particular divided into multiple subsystems:

[00010]$\stackrel{}{}:=\stackrel{.Math.}{}={\stackrel{.Math.}{}}_0+{\stackrel{.Math.}{}}_1+.Math.+{\stackrel{.Math.}{}}_K$${\stackrel{}{}}_0^{}:={\stackrel{.Math.}{}}_0=J^{-1}\left(I_0{K}_m-C_s-{\stackrel{}{}}_{0}D\right)$${\stackrel{}{}}_1^{}:={\stackrel{.Math.}{}}_1=J^{-1}\left(a_1\sin \left({}_{1}t\right)K_m-{\stackrel{}{}}_{1}D\right)$$.Math.$${\stackrel{}{}}_K^{}:={\stackrel{.Math.}{}}_K=J^{-1}\left(a_K\sin \left({}_{K}t\right)K_m-{\stackrel{}{}}_{K}D\right)$

[0048] The transfer function in the Laplace domain of the subsystems .sub.1.sub.2, . . . , .sub.K is

[00011]$H_k\left(s\right):=\frac{{}_k^{}\left(s\right)}{I_k\left(s\right)}=\frac{K_m}{J\left(s+{\mathrm{DJ}}^{-1}\right)}$

from which the following equation system can be derived:

[00012]$K_m^2-{.Math.H_k\left(j{}_k\right).Math.}^2{D}^2-{.Math.H_k\left(j{}_k\right).Math.}^2{J}^2{}_k^2=0.Math.K_m^2-{.Math.H_k\left(j{}_k\right).Math.}^2{\left(K_m{A}_2\right)}^2-{.Math.H_k\left(j{}_k\right).Math.}^2{J}^2{}_k^2=0.Math.K_m\left({}_k\right):=\frac{.Math.H_k\left(j{}_k\right).Math.J{}_k}{\left(1-{.Math.H_k\left(j{}_k\right).Math.}^2{A}_2^2\right)}$

[0049] The absolute values of the Fourier coefficients of the rotation rate signal (t) at the frequencies .sub.1.sub.2, . . . , .sub.K can be used to ascertain the values for |H.sub.k(j.sub.k)| simply and to determine all uncertain parameters D, C.sub.+, C.sub. and K.sub.m using the previously calculated A. The ascertained values become more robust if multiple excitation frequencies are used (K>1), the parameters for the individual frequencies can then be averaged, for example:

[00013]${\stackrel{\_}{K}}_m=\frac{1}{K}\underset{k=1}{\overset{K}{.Math.}}{K}_m\left({}_k\right)$

[0050] In first experiments, frequencies between 10 and 20 Hz and an amplitude .sub.k of about 1 A have proven to be advantageous.

[0051] A second, alternative option is that the motor constant K.sub.m is ascertained as a function of a preload and/or a restoring moment of the spring element 9 of the actuator assembly 6.

[0052] As can be seen in FIG. 2, the Coloumb friction in electromechanical drum brakes with return spring may be dependent on the sign of the rotation rate (different friction when actuating than when opening). The difference results from the preload of the return spring and from the friction in the suspension of the brake shoes.

[0053] If the preload and the resulting restoring moment .sub.F of the return spring are known and the friction in the brake shoe suspension is negligible, the motor constant K.sub.m can be ascertained after A has been determined (see previous sections):

[00014]${}_F=\frac{1}{2K_m}\left(C_{+}+C_{-}\right).Math.K_m=\frac{1}{2{}_F}\left(C_{+}+C_{-}\right)$

[0054] The method ends with a step S4. Optionally, the motor constant just ascertained and/or at least one of the friction coefficients may now be used to ascertain further parameters or to serve as an input variable for methods based thereon.

[0055] On the one hand, this may be a method for ascertaining a fault state and/or wear state of the friction brake. Here, the fault state and/or wear state is ascertained as a function of the ascertained motor constant K.sub.m of the machine 7, in particular by comparing the motor constant K.sub.m and/or at least one of the friction coefficient D, C.sub.+, C.sub., ascertained as a function of the motor constant K.sub.m as described above, with a specified threshold value and/or tolerance band.

[0056] For example, damage or the like can be detected if the ascertained system parameters are outside a tolerance band, i.e., for example, the motor constant is below a specified threshold value and/or the viscous friction coefficient exceeds a specified threshold value:

[00015]$K_m<K_{m,\min },D>D_{\max },.Math.$

[0057] On the other hand, this may be a method for ascertaining a contact point of the friction partners 2, 5 of the friction brake 1. Here, the contact point of the friction partners is also ascertained, in particular by means of a load torque estimator, as a function of the ascertained motor constant K.sub.m of the machine 7.

[0058] A load torque estimator is thus implemented in particular based on the simplified linear actuator model described above. Here, the model is extended by a virtual state variable {circumflex over ()}.sub.L, for example as follows:

[00016]$\stackrel{.Math.}{}=J^{-1}\left({}_m-\stackrel{}{}D-\mathrm{sign}\left(\stackrel{}{}\right)C_s+{\stackrel{}{}}_L\right)+$${\stackrel{.}{\stackrel{}{}}}_L=$

[0059] The terms and are assumed, zero-mean system noise as is common with model-based estimators. With this formulation, a Kalman filter can be implemented, for example. A threshold value, for example {circumflex over ()}.sub.L>.sub.thresh, may be used as a criterion for touch point detection. The accuracy of the estimator and of the detection benefits from the ascertained system parameters.