Method for determining motor constants, fault state and / or wear state and contact point, control device, friction brake

Abstract

The invention relates to a method for determining a motor constant (Km) of an electric machine (7), in particular of an actuator assembly (6), in which at least one excitation signal (Im) is specified with at least one direct current portion (I0) and an alternating current portion (Ik), the machine (7) being actuated by means of the excitation signal (Im), at least an actual value of a motor current of the machine (7) and an actual value of a rotational speed (omega) of a rotor shaft of the machine (7) are each determined, and wherein a motor constant (Km) is determined as a function of the excitation signal (Im) and the actual values.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for determining motor constants of an electric machine, and to a method for determining a fault state and / or a wear state of a friction brake, respectively, on the basis of motor constants thus determined, and to a method for determining a contact point of two friction pairs of a friction brake, respectively, on the basis of motor constants thus determined.

[0002] Furthermore, the invention also relates to a control device which is designed specifically for carrying out at least one of the above-mentioned methods, and to a friction brake having such a control device. BACKGROUND

[0003] From the prior art, a hydraulic, friction brake for a motor vehicle is known. In such a hydraulic brake, a pressure sensor in the hydraulic system is usually used to determine the stress of the respective brake, whereby, in addition, the braking torque can also be estimated and adjusted. Here, the uncertainties in the friction pairing (variations in the friction value and, in turn, in the brake characteristic value) represent a decisive limit to the accuracy of the braking torque estimation.

[0004] Varying boundary conditions, for example in an electric vehicle, can also make it attractive to use a brake with electromechanical actuation (EMB). The problem presented here is a suitable sensor system on the actuator level, similar to the pressure sensor described above. In comparison with hydraulic brakes, the expense for a sensor system for direct measurement of operating parameters and actuating forces increases greatly, in particular because the measurement must be made in each wheel actuator, and the prevailing environmental conditions (heat, dust, humidity, vibrations, etc.) dominate. SUMMARY

[0005] The method for determining motor constants of an electric machine according to the invention is characterized in that at least one excitation signal is predefinable with at least one direct current component and an alternating current component, the machine is operated with the excitation signal, at least the actual value of the motor current of the machine and the actual value of the rotational speed of the rotor shaft of the machine are determined, and the motor constants are determined from the excitation signal and the actual values. Due to manufacturing tolerances and degradation effects over the service life of components, the one-time determination of system parameters, such as motor constants or viscous friction coefficients, is only of limited benefit. The idea underlying the invention is therefore to provide a method with which at least the motor constants can be determined in a simple manner as important system parameters as required, in particular periodically. It is advantageously ensured that the accuracy and wear of the respective components within the system, which have an influence on the motor constants, are taken into account. The core of the invention therefore consists in providing a special excitation signal with which the motor current to be regulated is defined as a target current profile, and with which the machine is operated. According to the invention, the excitation signal has at least one, in particular constant, direct current component and an, in particular sinusoidal, alternating current component. By means of the respective direct current component, it is advantageously ensured that the excitation signal has no zero crossings and that the direction of rotation of the machine does not change thereby. Subsequently, the motor constants and, in particular, further parameters, such as friction coefficients, are determined in a simple manner from the respective system response of the system, characterized at least by the actual values of the motor current and the rotational speed determined according to the invention using signal processing methods. The rotational speed is understood to mean the rotational speed or angular velocity of the rotor shaft of the machine. The electric machine is in particular a component of an actuator assembly, for example a friction brake of a motor vehicle. The actuator assembly is in particular configured to move a respective friction pair of the friction brake, for example by means of a transmission assembly. The friction brake is in particular configured as an electromechanical drum brake or disc brake. The friction pair movable by the actuator assembly is in particular a brake pad, for example arranged on a brake block or brake caliper, which is moved onto a brake drum or brake disc also having a brake pad in order to generate a braking torque. It is therefore absolutely necessary for the reliable operation of the machine as part of the respective actuator assembly, for example the friction brake, that the uncertain and / or unknown values of the relevant system parameters are determined accordingly periodically. By means of the method according to the invention, a particularly advantageous, efficient and stable method for repeatedly determining the motor constants as important system parameters is provided. Preferably, at least one further system parameter, in particular the Coulomb friction coefficient or the viscous friction coefficient of the system, is determined from the motor constants thus determined. Thus, an advantageous multi-parameter identification is provided for the system by means of the method according to the invention. On the basis of the parameters determined by means of the method according to the invention, functions such as fault recognition, damage recognition and wear recognition can be advantageously implemented, or the regulation circuit can be recalibrated. The method is in particular carried out as a periodic self-checking program.The method has the further advantage here that no additional sensors are required, rather, in order to ascertain the actual value of the rotational speed, in particular a motor position sensor (rotor position sensor) is used, which is always present anyway, and / or in order to ascertain the actual value of the motor current, in particular an information / current sensor system is used, which is always present anyway. The cost and technical complexity of the actuator assembly are therefore not further increased. The method is of course not limited to the described application case of a friction brake, but can be used in any actuator assembly with an electric motor.

[0006] According to a preferred development of the application, the actual curves of the motor current and the rotational speed, which are each derived from the excitation signal over time, are ascertained, and the mean values are ascertained from the actual curves as the respective actual values. By taking into account the mean values, a particularly advantageous possibility for ascertaining the actual values is provided.

[0007] It is particularly preferred that the actual curves are ascertained over at least two cycles of the alternating current component. By ascertaining over a plurality of cycles, the stability of the method according to the application is advantageously further improved.

[0008] According to a preferred development of the application, the alternating current component has at least one harmonic frequency, in particular a plurality of different harmonic frequencies. By using harmonic frequencies, the stability and the accuracy in ascertaining the motor constants are advantageously further improved.

[0009] It is particularly preferred that the mean values of the motor constants are ascertained using a plurality of different excitation signals. By taking into account a plurality of excitation signals, the following advantages arise, namely that the amount of measurement data is increased, and the stability of the method according to the application is therefore further improved.

[0010] According to a preferred development of the application, the motor constants are ascertained by means of a Fourier transformation. By using a Fourier transformation, it is advantageously ensured that the motor constants are ascertained particularly effectively.

[0011] It is particularly preferred that the machine is an integral part of an actuator assembly, and the motor constants are ascertained from at least one Coulomb friction coefficient and / or viscous friction coefficient of the actuator assembly. By taking into account the respective friction coefficients, a particularly advantageous and simple correlation is provided, by means of which the motor constants can be ascertained. To this end, in particular an equation system or a matrix is formed, which has the respective friction coefficients and the motor constants to be ascertained as unknowns. The motor constants can then be ascertained particularly effectively by means of the Fourier transformation described above.

[0012] According to a preferred refinement of the application, the machine is an integral part of an actuator assembly, and the motor constant is determined in dependence on a prestress and / or a return torque of a spring element of the actuator assembly. By taking into account the prestress and / or the return torque, a particularly simple possibility for determining the motor constant is provided. In particular, an existing mathematical relationship is established between the return torque of the return spring and the above-mentioned quantities according to a simplified model for the above-mentioned specific application case in a drum brake, and the mathematical relationship is solved in terms of the motor constant. For this purpose, the prestress and / or the return torque of the return spring are required as additional known input quantities. If the respective quantities are known, the accuracy of the determination of the motor constant is advantageously further improved.

[0013] It is particularly preferred that the machine is an integral part of an actuator assembly of a friction brake of a motor vehicle, and that the machine is actuated with the excitation signal only in the region of an air gap of the friction brake. By actuation only in the region of the air gap, the advantage arises that the basic model can be maintained particularly simply, since actuation takes place only in a region which is at least almost unloaded, and in particular it is not necessary to take into account the respective forces between the friction partners and the respective friction pair of the friction brake.

[0014] The method according to the application for determining a fault state and / or a wear state of a friction brake of a motor vehicle having at least two friction pairs is characterized in that, in order to move one of the friction pairs towards the other friction pair, the friction brake is assigned an actuator assembly having an electric machine, and the fault state and / or the wear state is determined in dependence on a motor constant of the machine, which is determined by means of the above-mentioned method according to the application. In particular, the fault state and / or the wear state is determined here by comparison of the motor constant and / or a friction coefficient determined in dependence on the motor constant with a predetermined threshold value and / or a tolerance band. The friction brake is in particular configured as an electromechanical drum brake or disc brake. The respective method represents a particularly advantageous application possibility of the determined motor constant. Thereby, in particular an advantageously simple damage recognition is achieved, in the case of which the determined motor constant (torque / current) can be an indicator of a damaged machine, for example, below a predetermined limit.

[0015] The method according to the application for determining the contact point of two friction pairs of a friction brake of a motor vehicle is characterized in that, in order to move one of the friction pairs towards the other friction pair, the friction brake is assigned an actuator assembly with an electric motor, and the contact point of the friction pairs is determined from the motor constant of the system determined by the method according to the application described above, in particular by means of a load torque estimator. The contact point, also referred to as touch point, is to be understood as the point after overcoming the air gap from which the friction pairs come into contact with one another. The friction brake is in particular configured as an electromechanical drum brake or disc brake. The corresponding method represents a particularly advantageous application possibility of the motor constant determined. In this specific application case, an advantageously precise contact point recognition is ensured by the system parameters determined, in particular the motor position at which the brake pad comes into contact with the brake disc / brake drum.

[0016] The control device according to the application is characterized in that it is designed exclusively for carrying out at least one of the methods according to the application. The advantages already mentioned thereby arise. In particular, the control device is configured as a computer device assigned to the friction brake of a motor vehicle, preferably arranged in the motor vehicle.

[0017] The friction brake according to the application has at least two friction pairs, wherein, in order to move one of the friction pairs towards the other friction pair, the friction brake is assigned an actuator assembly with an electric motor, and the friction brake is characterized by a control device according to the application. The advantages already mentioned thereby also arise. The friction brake is in particular configured as an electromechanical drum brake or disc brake, as described above. BRIEF DESCRIPTION OF DRAWINGS

[0018] Further preferred features and feature combinations can be derived from the above description. The application is subsequently explained in detail with the aid of the drawings. To this end,

[0019] Figure 1 An electromechanical drum brake is shown,

[0020] Figure 2 A rotational speed-motor current diagram of a drum brake is shown;

[0021] Figure 3 A method for determining parameters of a drum brake is shown. DETAILED DESCRIPTION

[0022] Figure 1An exemplary structure of an electromechanical friction brake 1 of a motor vehicle, which is not further shown, is shown in simplified form. The friction brake 1 is currently configured as a Simplex drum brake and currently has two first friction pairs 2 in the form of brake blocks 3 and brake linings 4 arranged thereon and a second friction pair 5 in the form of a brake drum 5. The first friction pairs 2 are respectively assigned to the second friction pair 5.

[0023] Here, in order to move the respective first friction pairs 2 toward the second friction pair 5, the friction brake 1 is assigned an actuator assembly 6 with an electric motor 7. Here, the electric motor 7 is respectively connected in force transmission with a first end of the first friction pairs 2, for example by means of a not shown transmission assembly, in order to apply a respective actuating force F thereto. The friction pairs 2 are rotatably supported on a carrier 8 about a second end facing away from the first end.

[0024] In order to actually also generate a braking torque, the friction pairs 2 must first overcome an air gap I until the respective brake linings 4 are applied against the brake drum 5. In order to ensure that the brake linings 4 do not slide on the brake drum 5 in an unactuated state, a spring element 9 is provided as a return spring with a certain prestress, which is respectively fixed at one end on one of the two brake blocks 3 and presses the two brake blocks 3 against one another.

[0025] Subsequently, reference is made to Figure 3 An advantageous method for determining various parameters of the described actuator assembly 6, in particular the motor constants of the electric motor 7, is described. For this purpose, Figure 3 The method is illustrated by means of a flowchart. By means of the method it is ensured, inter alia, that parameters which can change over the service life, for example due to component dispersion and wear, are always determined with high accuracy. In particular, at least one of the following described methods is carried out by means of a control device specially designed for this purpose.

[0026] The method is based here 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. The non-linearities are mainly due to the load of the stiffness characteristic curve of the friction brake 1 and the friction effects within the actuator assembly 6. The load can be neglected in the case of the assumption that the method is carried out only in the region of the air gap I.

[0027] The air gap is here characterized by two motor positions θ L,min and θ L,max which limit the permissible motor position range in the air gap These motor positions can be preset according to the design or determined using a conservative method.

[0028] For the friction working against the motor torque, the approximation common in practice and in the literature is subsequently performed. The friction model is described here in three parameters in the form of a friction coefficient, namely a viscous friction factor D and two Coulomb factors C + and C - which are respectively applicable to the actuation and opening of the friction brake 1.

[0029] Since the electrical time constant is usually much smaller than the mechanical time constant, an exact modeling of the motor 6 can be dispensed with and instead the actuation current I m is used as a measure for the torque τ m ≈ I m K m between the motor constant K m .

[0030] The differential equation thus approximately describing the system behavior (change in the rotational motor position θ) is therefore as follows:

[0031]

[0032] Here, J is the total inertial mass of the actuator (usually known) of the rotation and it applies that:

[0033]

[0034] The positive scale ∈ can be understood as a Karnopp factor and describes a Nullgeschwindigkeits-Band. It is assumed that the corresponding rotational position sensor of the motor 6 is sufficiently precise in order to calculate the rotational speed

[0035] In step S1, the method begins with the excitation signal being preset with at least one direct current component and an alternating current component and the machine 7 being actuated, in particular only in the region of the air gap I of the friction brake 1, with the excitation signal. The alternating current component preferably has at least one harmonic frequency, in particular a plurality of different harmonic frequencies.

[0036] The excitation signal thus consists in particular of a direct current component I0and an alternating current component I k (t) = a k sin(ω k t + φ k ) of one or more harmonics:

[0037]

[0038] It is advantageous to use a plurality of harmonic factors (K > 1) in order to achieve a higher stability in the identification of the motor constant, as will be seen in the following passages.

[0039] In step S2, the actual values of the motor current of the machine 7 and of the rotational speed of the rotor shaft of the machine 7 are now each determined. To this end, the actual curves of the motor current and of the rotational speed over time, which are each derived from the excitation signal (I m ) are determined, and the average values are determined from the actual curves as the respective actual values. In particular, the actual curves are determined over at least two periods of the alternating current component (I k ). It is particularly preferred to determine the average values of the motor constant using a plurality of different excitation signals.

[0040] Depending on the selected direct current component, an average rotational speed is formed after the transient process (t > T e ) of the rotational speed ω(t):

[0041]

[0042] It is noted here only that a stick-slip effect, which is not taken into account in the simplified model, does not occur. For a series of measurements with different I0, the Coulomb factor can be determined very simply by linear interpolation and the viscous friction from the as yet unknown motor constant K m .

[0043] Figure 2 A corresponding rotational speed-motor current diagram is shown, which has a plurality of measurement points for the motor current I and the rotational speed ω. As described above, the state space model used has different friction coefficients as further unknown parameters. It is therefore advantageous to determine the motor constant K m from the Coulomb friction coefficients C + , C - and the viscous friction coefficient D of the actuator assembly 6.

[0044] The corresponding determination of the friction coefficients is preferably effected by linear matching of the data points. The slope of the respective straight line G + for the friction coefficient C + or G - for the friction coefficient C - corresponds to

[0045] The y-axis intersection point corresponds to In the present example, the Coulomb friction is different for positive and negative rotational speeds (the shown dashed line corresponds to

[0046] The resulting relationship is:

[0047]

[0048] The system of equations can be solved using N ≥ 2 independent measurements:

[0049]

[0050] In step S3, the motor constants are now determined based on the excitation signal and the actual values. One possibility is to solve the system of equations using a Fourier transform for this purpose.

[0051] Due to the assumptions of a linear model and the superposition principle, the system and excitation signal are specifically divided into multiple subsystems:

[0052]

[0053] Subsystems ω′1,ω′2,...,ω′ K The transfer function within the Laplace range is

[0054]

[0055] From this, we can derive the following system of equations.

[0056]

[0057] By using frequencies ω1, ω2, ..., ω K The absolute values ​​of the Fourier coefficients of the rotational speed signal ω(t) can be easily obtained with respect to |H k (jω k The value of | is determined, and all uncertain parameters D and C are determined using the previously calculated A. + C - and K m When multiple excitation frequencies (K>1) are used, the obtained values ​​become more stable, and the parameters used for each frequency can then be averaged, for example:

[0058]

[0059] In the first test, the frequency was between 10Hz and 20Hz, and the amplitude was approximately 1A. k It has been proven to be advantageous.

[0060] The second alternative is to determine the motor constant K based on the prestress and / or reset torque of the spring element 9 in actuator assembly 6. m .

[0061] As in Figure 2As can be seen in the middle, the Coulomb friction in the electromechanical drum brake with return spring can depend on the sign of the rotational speed (different friction when actuated than when open). The difference is caused by the pre-stress of the return spring and the friction in the suspension of the brake block.

[0062] If the pre-stress of the return spring and the resulting return torque τ F is known and the friction of the brake block suspension is negligible, the motor constant K m :

[0063]

[0064] The method now ends in step S4. Optionally, the now determined motor constant and / or at least one of the friction coefficients can now be used to determine further parameters or as input quantities for a method constructed therefrom.

[0065] On the one hand, this can be a method for determining a fault state and / or a wear state of a friction brake. Here, the determined motor constant K m of the machine 7, in particular by means of the motor constant K m and / or at least one of the friction coefficients D, C m , C + , C - determined as described above is compared with a predetermined threshold value and / or a tolerance band to determine the fault state and / or the wear state.

[0066] If the determined system parameters lie outside the tolerance band, i.e. for example the motor constant lies below a predetermined threshold value and / or the viscous friction coefficient exceeds a predetermined threshold value, for example a damage or the like can be identified:

[0067] K m <K m,min ,D>D max ,...

[0068] On the other hand, this can be a method for determining the contact point of the friction pair 2, 5 of the friction brake 1. Here, the contact point of the friction pair is also determined from the determined motor constant K m of the machine 7, in particular by means of a load torque estimator.

[0069] The load torque estimator is thus implemented, in particular, on the basis of the simplified linear actuator model described above. Here, the model can be extended with virtual state variables for example as follows:

[0070]

[0071] Item and is an assumed mean value free system noise as is common in model based estimators. With this formulation a Kalman filter can be exemplarily implemented. Limit values can be used as a criterion for contact point identification, e.g. The accuracy of the estimator and the identification benefits from the system parameters that are sought.

Claims

1. A method for determining the motor constant (K) of a motor (7), particularly an actuator assembly (6). m ) method, -in, At least one excitation signal (I m It is preset to have at least one DC current share (I0) and one AC current share (I). k ), - wherein the machine (7) utilizes the excitation signal (I m (Being manipulated) -In this context, the actual value of the motor current of the machine (7) and the actual value of the rotational speed (ω) of the rotor shaft of the machine (7) are respectively determined at least once, and -wherein, the motor constant (K) m According to the excitation signal (I) m The actual value is obtained.

2. The method according to claim 1, characterized in that, From the excitation signal (I) respectively m The actual curves of the motor current and the rotational speed (ω) over time obtained from the data are calculated, and the average value is calculated as the corresponding actual value based on the actual curve.

3. The method according to claim 2, characterized in that, The actual curve is in the alternating current share (I) k It is sought over at least two periods of ).

4. The method according to any one of the preceding claims, characterized in that, The AC current share (I) k It has at least one harmonic frequency, and in particular multiple different harmonic frequencies.

5. The method according to any one of the preceding claims, characterized in that, The motor constant (K) m The average value of ) is obtained using multiple different excitation signals (I m (It is requested.) 6. The method according to any one of the preceding claims, characterized in that, The motor constant (K) m It is obtained using Fourier transform.

7. The method according to any one of the preceding claims, characterized in that, The machine (7) is a component of the actuator assembly (6), and the motor constant (K) m According to at least one Coulomb friction coefficient (C) of the actuator assembly (6) + C - The coefficient of friction (D) and / or the coefficient of viscous friction are determined.

8. The method according to any one of the preceding claims, characterized in that, The machine (7) is a component of the actuator assembly (6), and the motor constant (K) m The prestress and / or reset torque of the spring element (9) of the actuator assembly (6) are determined.

9. The method according to any one of the preceding claims, characterized in that, The machine (7) is part of the actuator assembly (6) of the friction brake (1) of the motor vehicle, and the machine (7) is operated only in the region of the air gap of the friction brake (1) using the excitation signal.

10. A method for determining the fault condition and / or wear condition of a friction brake (1) of a motor vehicle having at least two friction pairs (2, 5), particularly an electromechanical drum brake, wherein, In order to move one friction pair (2) toward the other friction pair (5), an actuator assembly (6) having a motor (7) is associated with the friction brake (1), characterized in that the fault state and / or the wear state are based on the motor constant (K) of the machine (7) obtained by the method according to any one of claims 1 to 9. m ) is obtained, especially by obtaining the motor constant (K) m ) and / or according to the motor constant (K) m The friction coefficients (D, C) were obtained. + C - The comparison with the preset threshold and / or tolerance zone is obtained.

11. A method for determining the contact points of two friction pairs (2, 5) of a friction brake (1) of a motor vehicle, particularly an electromechanical drum brake, wherein, In order to move one friction pair (2) toward the other friction pair (5), an actuator assembly (6) having a motor (7) is associated with the friction brake (1), characterized in that the contact point of the friction pair is determined according to the motor constant (K) of the machine (7) obtained by the method according to any one of claims 1 to 9. m This can be obtained by means of load torque estimators, especially by means of load torque estimators.

12. A control device, characterized in that, The control device is specifically designed to perform the method according to any one of claims 1 to 9, the method according to claim 10, and / or the method according to claim 11.

13. A friction brake (1) for a motor vehicle, particularly an electromechanical drum brake, having at least two friction pairs (2, 5), wherein, In order to move one of the friction pairs (2) toward the other friction pair (5), an actuator assembly (6) having a motor (7) is attached to the friction brake (1), characterized by having a control device according to claim 12.

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