Method for controlling the braking torque of an electromechanical brake

Abstract

The invention relates to a method for controlling the braking torque of an electromechanical brake. The brake comprises an actuator and at least one braking torque sensor. The braking torque sensor is configured to measure a braking torque exerted by the actuator. Oscillation of the braking torque controller due to a disturbance detected by the braking torque sensor is to be reduced. For this purpose, the method includes calculating (100) a first difference between a predetermined target braking torque (M) and the actual braking torque. Soll ) and actual braking torque (M Ist ). Then a setpoint variable (ω) is determined (110). Soll ) of the actuator and a calculation (130) of a second difference from the setpoint variable (ω Soll ) and a previous output setpoint (ω Soll,A ). A first derivative (ω Soll ) the setpoint variable is calculated (140) and using a limiting function (ω̇ Max , ω̇ Min) limited (150). The limited first derivative (ω̇ Soll,lim ) of the setpoint is integrated (160) to obtain an output setpoint (ω Soll,A ) to obtain for the actuator.

Description

[0001] The invention relates to a method for controlling the braking torque of an electromechanically actuated brake for a wheel of a vehicle, wherein the brake comprises an actuator and at least one braking torque sensor, the braking torque sensor being configured to measure a braking torque exerted by the actuator. The invention further relates to a control unit configured to carry out such a method, and to a braking system comprising an electromechanical brake and at least one such control unit.

[0002] Such a brake, in particular a drum brake, is known, for example, from DE 10 2017 217 413 A1. There, an operating parameter of the drum brake is determined by means of a force sensor, for example a clamping force or a braking torque, which can then be used to control the drum brake.

[0003] Drum brakes, known for their low wear and friction, have long been used in vehicles and, despite being largely replaced by more powerful disc brakes, are still employed, particularly in commercial vehicles and lower-powered passenger cars. In recent years, however, drum brakes have seen a resurgence in electric vehicles, especially in conjunction with regenerative braking, as a lower maximum braking torque is sufficient in this application. The longer maintenance intervals of drum brakes also better suit the lower maintenance requirements of electric vehicles.

[0004] An electromechanically operated drum brake typically comprises a rotating drum against which, for example, two brake shoes, movably mounted on a brake anchor plate, are pressed radially inwards by an actuator to generate a braking torque. During operation, the drum and the brake shoes rotate relative to each other during the braking process.

[0005] If the drum exhibits out-of-roundness due to manufacturing tolerances, deformation during operation caused by external forces, or wear, this leads to a malfunction in the brake torque control according to the prior art. The out-of-roundness of the drum induces a malfunction in the brake torque sensor of the drum brake. Due to the out-of-roundness of the brake drum, the brake torque sensor measures a fluctuating brake torque. This results in positive feedback in the brake torque controller, which attempts to compensate for this fluctuation by adjusting the brake torque. Depending on the wheel speed and the available dynamics of the actuator, this leads to a more or less pronounced oscillation of the system consisting of the mechanical components of the drum brake and the controller. This initially results in non-ideal braking behavior of the drum brake, as the actual brake torque fluctuates around the target brake torque requested by the driver or a vehicle control system.Furthermore, this can also increase wear on the drum brake, partly because the maximum braking torque is sometimes higher than necessary, which can accelerate the abrasion of the brake shoes. The electrical and mechanical components (gears, etc.) of the drum brake, such as those in the actuator or any spreading unit, can also experience increased wear due to the unnecessary oscillation. Finally, the energy consumption of the drum brake can increase.

[0006] In other electromechanical brake types, such as electromechanical disc brakes, periodic disturbances can also cause the system to oscillate due to the mechanical components of the brake and the controller when using a controller with a brake force or brake torque sensor. These disturbances can be caused by manufacturing tolerances, deformation during operation due to external forces, or wear, leading to an asymmetry, for example, in the brake disc, which in turn introduces a disturbance into the brake torque sensor. This results in the same disadvantages as with electromechanical drum brakes, namely increased wear and energy consumption, as well as suboptimal braking performance.

[0007] The object of the invention is therefore to provide a method for controlling the braking torque of an electromechanically actuated brake that at least reduces the aforementioned problems.

[0008] According to the invention, a method for controlling a braking torque of an electromechanical brake for a wheel of a vehicle is provided, wherein the brake comprises an actuator and at least one braking torque sensor, wherein the braking torque sensor is configured to measure a braking torque exerted by the actuator. the procedure comprises the following steps: - Calculating an initial difference between a predetermined target braking torque and an actual braking torque measured by the brake torque sensor, - Determining a target actuator value using the first difference, - Calculating a second difference between the target control variable and a previous output target control variable of the actuator, - Calculating a first (time) derivative of the setpoint variable using a time constant and the second difference, - Limiting the first derivative of the setpoint variable using a limiting function to obtain a limited first derivative of the setpoint variable, - Integrating the limited first derivative of the setpoint variable to obtain an output setpoint variable for the actuator.

[0009] This method can significantly reduce the oscillation of the system from the mechanical components of the brake and the controller by limiting the change (first time derivative) of the actuator's setpoint variable.

[0010] Preferably, limiting the first derivative of the target control variable comprises limiting the first derivative of the target control variable between a maximum limiting parameter and a minimum limiting parameter of the limiting function. The term "limiting the first derivative of the target control variable" means that if the first derivative of the target control variable exceeds a maximum limiting parameter of the limiting function, the first derivative of the target control variable is limited to the maximum limiting parameter. Additionally, a second, minimum limiting parameter can be used, such that if the first derivative of the target control variable falls below the (negative) minimum limiting parameter, the first derivative of the target control variable is limited to the minimum limiting parameter.

[0011] Preferably, the limiting function depends on the wheel speed Ω. Preferably, positive and negative first derivatives of the target control variable are limited symmetrically. For example, the maximum limiting parameter of the first derivative ω̇ can be... Max (Ω) of the setpoint variable and the minimum limiting parameter of the first derivative ω̇ Min (Ω) of the setpoint control variable ω̇ Max (Ω) = -ω̇ Min (Ω) apply to all wheel speeds Ω.

[0012] Whenever this application refers to wheel or wheel speed, this refers to the vehicle wheel to which the brake is assigned. Preferably, wheel speed is understood to mean the magnitude of the wheel speed.

[0013] It is preferred that the limiting function be a monotonically decreasing function with the wheel speed. This applies in particular to the maximum limiting parameter ω̇. max (Ω), where the minimum limiting parameter ω̇ Min(Ω) preferably increases monotonically with the wheel speed (e.g. with ω̇) Max (Ω) = -ω̇ Min (Ω)). Since the wheel speed significantly determines the strength and frequency of the disturbance via the out-of-roundness / asymmetry of the drum / brake disc(s), etc., such a choice of the limiting function is advantageous. The limiting function therefore has a higher value at low wheel speeds (larger changes are permitted) than at high wheel speeds (only smaller changes are permitted). This ensures that the actual braking torque can be reduced sufficiently quickly during a braking process once the wheel speed (and thus the frequency of the disturbance) has decreased.

[0014] In one embodiment, the limiting function is calculated from a functional relationship stored in a brake control unit that depends on the wheel speed. This solution is somewhat more computationally intensive than using a lookup table, but the functional relationship can be chosen relatively simply.

[0015] Preferably, the limiting function is read from a lookup table stored in a brake control unit. This solution may be slightly more memory-intensive but less computationally intensive than storing a functional relationship for calculating the limiting function, e.g., in a control unit.

[0016] In a preferred embodiment, the manipulated variable is a target motor speed of an electric motor of the actuator. In this embodiment, the electric motor of the actuator can be continuously active during the braking process in order to maintain a position of the brake shoes / brake pads / brake discs corresponding to a target braking torque.

[0017] Alternatively, the control procedure for determining the manipulated variable can be designed to calculate the actuator position from the actual braking torque and to calculate a target actuator position from the target braking torque using a model, and to form the first difference between the calculated target actuator position and the calculated actuator position.

[0018] Preferably, the step of limiting the first derivative of the target control variable during a new braking request is only performed once the first difference since the start of the braking request has fallen below a threshold value. The limitation is therefore only activated when a (steady-state) operating point is nearly reached. Otherwise, the limitation would slow down the initial brake torque build-up and thus worsen the brake's response time (especially at high wheel speeds). However, if the limitation is only activated once the first difference since the start of the braking request has fallen below a threshold value, this disadvantage is eliminated. The operating point (the target control variable) can be steady-state during the brake torque build-up or change (only slowly) during this process, for example, if the brake request changes during this time.The limit value refers to the current target control variable at any given time.

[0019] The method according to the invention can include further limitations within the framework of brake torque control, for example a limitation of the magnitude of the motor speed of the electric motor of the actuator itself or a limitation of the actuator position.

[0020] Preferably, the brake is an electromechanical drum brake, comprising a drum and at least one brake shoe, with the brake torque sensor being configured to measure a braking torque exerted on the drum by the actuator via the brake shoe. For example, a drum out-of-roundness is the cause of the recorded periodic disturbance.

[0021] In another embodiment, the brake is an electromechanical disc brake, wherein the disc brake comprises a brake disc, and the brake torque sensor is configured to measure a braking torque exerted on the brake disc by the actuator. The invention is applicable to both partial disc brakes with a brake caliper and to full disc or multi-plate brakes with one or more brake discs. The actuator can therefore be configured to press brake pads against the brake disc or to press several brake discs against each other.

[0022] The invention further relates to a control unit for an electromechanical brake, wherein the control unit is configured to carry out a method according to one of the preceding embodiments. The control unit is preferably part of a modular brake or a wheel control unit. Alternatively, the control unit can also be a common control unit for several brakes or the entire brake system.

[0023] The invention relates to a vehicle braking system comprising at least one electromechanical brake and at least one control unit. The braking system can, for example, include one or more electromechanical brakes according to the invention and (optionally) one or more brakes of other types, such as regenerative brakes and / or electrohydraulic brakes.

[0024] In the following, embodiments of the invention are explained in more detail with reference to the accompanying figures. The figures show: Fig. 1 an embodiment of a controller circuit of the method according to the invention, Fig. 2 a schematic diagram of the actual braking torque and the target braking torque during brake activation, Fig. 3 a schematic diagram of the limitation function.

[0025] Fig. Figure 1 shows an embodiment of a controller circuit of the method according to the invention. The method is described below by way of example for an electromechanical drum brake, but is also fundamentally applicable to other brake types, such as electromechanical disc brakes.

[0026] The method serves to control the braking torque of a drum brake for a wheel of a vehicle, wherein the drum brake comprises a drum, at least one brake shoe, an actuator, and at least one brake torque sensor. The brake torque sensor is configured to measure a braking torque exerted on the drum by the actuator via the brake shoe.

[0027] The procedure initially comprises step 100 of calculating a first difference from a predetermined target braking torque M. Soll and an actual braking torque M measured by the brake torque sensor Ist Then, in step 110, a target actuator variable is determined using the first difference. Here, the target actuator variable is the target motor speed ω. Soll of an electric motor for the actuator of the drum brake.

[0028] Here, in step 120, an optional limitation of the target motor speed ω is implemented. Sollto maximum possible or maximum permissible engine speeds ω Min and ω Max itself, the output variable is the signal ω Soll,Lim .

[0029] In step 130, a second difference is calculated from the optionally limited target control variable ω. Soll,Lim and a previous output setpoint ω Soll,A of the actuator. This second difference is calculated in the following step 140 together with a time constant T. F to calculate a first time derivative ω̇ Soll the target engine speed ω Soll or ω Soll,Lim (depending on whether without or with optional limiting step 120 of the target engine speed).

[0030] In step 150, the first derivative of the target motor speed ω is calculated. Soll or ω Soll,lim using a limiting function limited by a limited first derivative of the target motor speed ω̇ Soll,lim to obtain, where the limiting parameters ω̇ Max(Ω) and ω̇ Min (Ω) of the limiting function here depends on the rotational speed Ω of the wheel, which is symbolized by the arrow representing the increasing wheel speed Ω. Integrating the limited first derivative of the target motor speed ω̇ Soll,lim This occurs in step 160 to determine an output setpoint ω. Soll,A to obtain for the actuator. This output setpoint ω obtained in this way. Soll,A This value is then specified as the setpoint for the subordinate motor speed controller and fed back in the next sampling step of the controller to form the second difference in step 130.

[0031] In Fig. Figure 2 is a schematic diagram of the actual braking torque M according to the invention. Ist and the target braking torque M Sollas a function of time t during brake activation. This illustrates an embodiment in which the step of limiting the first derivative of the target control variable during a new brake request is only performed when the first difference M Soll - M Ist since the start of the braking request at time t A The limit value Δ has been undercut for the first time. The limitation is only activated at time t0 when the target value for the braking torque has almost reached a (steady-state) operating point and the limit value Δ has been undercut since t. AThe limit is reached the first time it is undershot. Otherwise, the limitation would slow down the initial brake torque build-up and thus worsen the brake's response time (especially at high wheel speeds). The operating point (the target control variable) can be steady during brake torque build-up or change (especially slowly) during this process, for example, if the braking demand changes. The threshold value being undershot refers to the instantaneous target control variable at any given time.

[0032] Fig. 3 an exemplary course of the limitation function ω̇ according to the invention MaxThe limiting function is plotted against the wheel speed Ω. Here, the limiting function is a (strictly) monotonically decreasing function with the wheel speed. Since the wheel speed significantly determines the intensity of the disturbance via the out-of-roundness of the drum / asymmetry of the brake disc(s), such a choice of limiting function is advantageous. The limiting function can, for example, be stored as a functional relationship or as a lookup table in a control unit of the brake or brake system according to the invention. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2017 217 413 A1

[0002]

Claims

[1] Method for controlling a braking torque of an electromechanical brake for a wheel of a vehicle, wherein the brake comprises an actuator and at least one braking torque sensor, wherein the braking torque sensor is arranged to measure a braking torque exerted by the actuator, the method comprising the following steps: - Calculating (100) a first difference from a predetermined target braking torque (M Soll ) and an actual braking torque measured by the braking torque sensor (M Ist ), - Determining (110) a desired manipulated variable (ω Soll ) of the actuator using the first difference, - Calculating (130) a second difference from the target manipulated variable (ω Soll ) and a previous output setpoint control variable (ω Soll,A ) of the actuator, - Calculate (140) a first derivative (ω̇ Soll ) of the target manipulated variable using a time constant (TF ) and the second difference, - Limiting (150) the first derivative (ω̇ Soll ) of the target manipulated variable using a limiting function (ω̇ Max , ω̇ Min ) to obtain a limited first derivative (ω̇ Soll,lim ) of the target manipulated variable, - Integrate (160) the limited first derivative (ω̇ Soll,lim ) of the setpoint manipulated variable to obtain an output setpoint manipulated variable (ω Soll,A ) for the actuator. [2] Method according to claim 1, wherein limiting the first derivative of the desired manipulated variable comprises limiting the first derivative (ω̇ Soll ) of the target manipulated variable between a maximum limiting parameter (ω̇ Max ) and a minimal limiting parameter (ω̇ Min ) of the limiting function (ω̇ Max , ω̇ Min ) includes. [3] Method according to claim 1 or 2, wherein the limiting function (ω̇ Max (Ω), ω̇ Min(Ω) depends on the wheel speed (Ω). [4] Method according to claim 3, wherein the limiting function (ω̇ Max (Ω), ω̇ Min (Ω) is a function that decreases monotonically with the wheel speed (Ω). [5] Method according to one of the preceding claims, wherein the limiting function (ω̇ Max , ω̇ Min ) is calculated from a functional relationship stored in a brake control unit which is dependent on the wheel speed (Ω). [6] Method according to one of the preceding claims, wherein the limiting function (ω̇ Max , ω̇ Min ) is read from a lookup table stored in a brake control unit. [7] Method according to one of the preceding claims, wherein the manipulated variable is a target engine speed (ω̇ Soll ) of an electric motor of the actuator. [8] Method according to one of the preceding claims, wherein the step of limiting the first derivative of the desired manipulated variable (ω̇ Soll ) is only carried out when a new braking request is made to the brake from the time when the first difference M Soll - M Ist has fallen below a limit value for the first time since the start of the braking request. [9] Method according to one of the preceding claims, wherein the brake is an electromechanical drum brake, wherein the drum brake comprises a drum and at least one brake shoe, wherein the braking torque sensor is arranged to measure a braking torque exerted by the actuator via the brake shoe on the drum. [10] Method according to one of claims 1 to 8, wherein the brake is an electromechanical disc brake, wherein the disc brake comprises a brake disc, wherein the braking torque sensor is arranged to measure a braking torque exerted by the actuator on the brake disc. [11] Control unit for an electromechanical brake, wherein the control unit is configured to carry out a method according to one of the preceding claims. [12] Braking system for a vehicle comprising at least one electromechanical brake and at least one control unit according to claim 11.

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