Method for providing the clamping force generated by a parking brake

Abstract

In a method for providing the clamping force generated by a parking brake in a vehicle, current measured values ​​of the motor current are determined during an actuation of an electric brake motor and checked for the presence of measurement errors.

Description

[0001] The invention relates to a method for providing the clamping force generated by a parking brake in a vehicle. State of the art

[0002] DE 10 2006 052 810 A1 describes a method for estimating the clamping force of a parking brake generated by an electric brake motor. The electric brake motor acts on a brake piston, which carries a brake pad, against a brake disc. To determine the currently acting clamping force, the current, the supply voltage of the brake motor, and the motor speed are measured. The clamping force is then calculated from a system of differential equations that describe the electrical and mechanical behavior of the brake motor.

[0003] Measurement errors can occur during current measurement, for example due to the influence of EMC interference (electromagnetic compatibility), which can lead to the clamping force determined on the basis of the current measurements being calculated too high or too low. Disclosure of the invention

[0004] The invention is based on the object of increasing the tolerance to current measurement errors in a method for providing the clamping force generated by a parking brake.

[0005] This object is achieved according to the invention with the features of claim 1. The subclaims specify expedient further developments.

[0006] The method according to the invention relates to electromechanical parking brakes in vehicles, which generate a clamping force that holds the vehicle stationary. The parking brake has an electric brake motor, which electromechanically generates the clamping force. When the brake motor is actuated, a brake piston, which carries a brake pad, is pressed against a brake disc.

[0007] The parking brake can optionally be equipped with an auxiliary braking device to generate an additional clamping force, so that the total clamping force is composed of the electromechanically provided component by the brake motor and the additional clamping force. The auxiliary braking device is preferably a hydraulic braking device, in particular the hydraulic vehicle brake, which generates a braking force that decelerates the vehicle during normal driving operation. The hydraulic pressure acts on the brake piston, which is also acted upon by the electric brake motor.

[0008] To determine the actual clamping force generated by the electric brake motor during the application process, knowledge of the current electrical motor parameters is necessary, namely the motor resistance and the motor constants. These motor parameters can be determined from the current motor current, possibly taking into account the current motor voltage and the current motor speed. The motor current, motor voltage, and motor speed can be determined using sensors or calculated from the sensor data. Taking into account the motor constants and the motor resistance, the clamping force provided by the electric motor can then be calculated.

[0009] To reduce the influence of measurement errors on the accuracy of the determined clamping force or to increase the tolerance for measurement errors, which may be due, for example, to EMC interference (electromagnetic compatibility), a check is carried out during the measurement to determine whether one or more current measurements lie outside permissible limits. If this is the case, correction current values ​​are used instead of these current measurements to calculate the clamping force, in particular for calculating the motor constants and the motor resistance, which form the basis for determining the clamping force. This allows for the detection of short-term disturbances that affect one or more, particularly consecutive, measured values ​​during an actuation of the electric brake motor.The advantage of this approach is that, on the one hand, the quality of the clamping force is significantly improved. On the other hand, by identifying the faulty measured values, the remaining measured values ​​that are not affected by an error can be used to determine the motor parameters or the clamping force. Provided the correct current measured values ​​provide a sufficient database, the sensor-determined current measured values ​​from a current actuation process of the brake motor can generally be used.

[0010] Once the faulty current measurements have been identified, corrective current measurements are used, which represent the correct values ​​or represent a better approximation of the correct values. In addition, an error can also be displayed or stored in the vehicle.

[0011] For example, the non-faulty current measurements that were sensed before or, in particular, after the faulty current measurements are used as correction current values. With this approach, the faulty current measurements are masked out; no current measurements are taken into account in the corresponding time interval.

[0012] According to another approach, the correction current values ​​are determined from a substitute function, which has the advantage that current values ​​are also available for the time interval in which the erroneous measured values ​​occurred. This approach has the further advantage that current values ​​are available over a longer period of time, consisting of measured current values ​​within the permissible tolerances and the substitute current values.

[0013] The equivalent current values ​​form a substitute function or are determined from a substitute function, which is calculated primarily based on correct current measurements, for which suitable compensation functions such as the least squares method can be considered. In this approach, an extrapolation into the time range of the erroneous current measurements is performed based on the current measurements identified as correct. Thus, current values ​​are available over a longer period for determining the clamping force.

[0014] To verify whether the measured current values ​​are within the permissible limits, it is advantageous to observe the falling branch of the motor current during the initial phase after the brake motor is switched on during a brake application process. Immediately after switching on, the motor current increases sharply and decreases exponentially after exceeding a current maximum; the current measurements for determining the motor parameters and the clamping force are performed in the falling branch. The dynamic change in the falling branch of the current curve enables sufficiently precise determination of the motor parameters, i.e., the motor resistance and the motor constant. Subsequently, the motor current increases again as soon as the brake shoes contact the brake disc and an electromechanical clamping force is generated.

[0015] To assess whether the measured current values ​​during the measurement period under consideration are within the permissible limits, various criteria can be considered cumulatively or alternatively. For example, in the descending branch during the initial phase of the motor current, immediately consecutive measured current values ​​must decrease. If this is not the case, an erroneous measured value can be assumed, and the above-mentioned correction current values ​​are determined or applied.

[0016] According to another criterion, the measured current values ​​must lie within absolute upper and / or lower limits. Another criterion that can be considered is whether the gradient of the measured current value function lies within defined limits. Furthermore, it can be checked whether the course of the measured current value function generally corresponds to a pattern characteristic of the branch of the current function under consideration. For example, it can be checked whether the measured current values ​​lie within a defined tolerance to the pattern.

[0017] Furthermore, in the case of several consecutive invalid current measurements, it is advisable to discard all current measurements determined up to that point, and only consider subsequent current measurements for determining the motor parameters or clamping force. While this approach reduces the time interval considered with permissible current measurements, it results in greater accuracy and reliability with regard to the current measurements used to determine the motor parameters.

[0018] The above criteria are applied alternatively, if necessary cumulatively. When applying the criteria alternatively, a measurement error is already present if the measured value falls outside the permissible limits according to one criterion.

[0019] According to a further advantageous embodiment, the criteria are modified in the event of a significant voltage dip. A voltage dip that exhibits a voltage fluctuation above a defined threshold also results in a change in the motor's current function. The current values ​​measured during a voltage dip can nevertheless be considered for determining the motor parameters or the clamping force, even if the gradient of the current measurement function lies outside the defined limits. In the event of a voltage dip, this criterion, which is applied to the current measurement values, is thus overridden.

[0020] The method according to the invention runs in a control unit in the vehicle, which can be a component of the parking brake.

[0021] Further advantages and practical embodiments can be found in the further claims, the description of the figures, and the drawings. They show: Fig. 1 a section through an electromechanical parking brake for a vehicle, with an electric brake motor for generating a clamping force that immobilizes the vehicle, Fig. 2 a diagram showing the time-dependent course of the motor current and the motor speed during the initial phase of an application process of the electric brake motor, Fig. 3 a flow chart with process steps for determining current measurement values ​​during the initial phase of the application process of the electric brake motor.

[0022] Fig. 1 shows an electromechanical parking brake 1 in a vehicle, wherein a clamping force can be generated via the parking brake to hold the vehicle stationary. The parking brake 1 has a brake caliper 2 with a caliper 9 that engages a brake disc 10. The actuator of the parking brake 1 is a brake motor 3 designed as an electric motor, which drives a spindle 4 in rotation, on which a spindle component 5 is mounted axially displaceably and rotationally fixed relative to the housing. The spindle component 5 is adjusted axially when the spindle 4 rotates. The spindle component 5 moves within a brake piston 6, which supports a brake pad 7 that is pressed against the brake disc 10 by the brake piston 6. On the opposite side of the brake disc 10, another brake pad 8 is arranged, which is held stationary on the caliper 9.

[0023] The spindle component 5 can move axially forward within the brake piston 6 in the event of a rotational movement of the spindle 4 in the direction of the brake disc, or axially backward in the event of an opposite rotational movement of the spindle 4 until it reaches a stop 11. In order to generate a desired target clamping force, the spindle component 5 acts on the inner end face of the brake piston 6, so that the brake piston 6, which is mounted axially displaceably in the parking brake 1, is pressed with the brake pad 7 against the facing end face of the brake disc 10.

[0024] The brake piston is also subjected to the hydraulic pressure of the regular hydraulic vehicle brake, which is used to slow the vehicle while driving. However, the hydraulic pressure can also provide support when the parking brake is applied while the vehicle is stationary, so that the total clamping force is composed of the electric motor-driven component and the hydraulic component.

[0025] In Fig. 2 is a graph showing the course of the motor current I mot and the course of the engine speed n mot of the electric brake motor is shown as a function of time for an application process.

[0026] At time t1, the brake application process begins by applying an electrical voltage and energizing the brake motor with the circuit closed. The start phase (Phase I) lasts from time t1 to time t2. At time t2, the motor speed n motreaches its maximum. The phase between t2 and t3 represents the no-load phase (phase II), in which the motor current I mot is at a minimum level. This is followed by the force build-up phase (Phase III) from time t3 until time t4, in which the brake pads rest on the brake disc and are pressed against the brake disc with increasing clamping force. At time t4, the electric brake motor is switched off by opening the circuit, so that the speed n mot of the brake motor drops to zero.

[0027] In the starter phase I, the motor current I motIt rises sharply and then drops until a no-load current is reached. The falling branch in Phase I can be used to determine the motor parameters, i.e., the motor resistance and the motor constant. The motor parameters are required later in Phase III to determine the currently effective, electromotively generated clamping force.

[0028] The motor current is available as a measured value in the falling branch of the current curve in Phase I. In order to identify and, if necessary, rule out any corrupting influence of measurement errors or disturbances in the braking current curve, particularly disturbances caused by EMC (electromagnetic compatibility), the current measured values ​​in the falling branch of the current curve in Phase I are checked to see whether they lie within permissible limits. Various criteria can be defined for this purpose, all of which must ideally be met to ensure an error-free current measurement curve. If, however, one of the criteria is not met, i.e. the current measured values ​​lie outside the defined limit values ​​according to one criterion, an error has occurred, after which corrective measures are taken.

[0029] In the event of a fault, correction current values ​​are determined instead of the faulty current measurements, which are used to calculate the motor constants and the motor resistance of the brake motor. The correction current values ​​are either current measurements that have been identified as correct and thus lie within the permissible limits. These current measurements preferably originate from the same brake motor application process, from the falling branch of the current curve in phase I. The current measurements following the faulty measurements are preferably used, so that the faulty current measurements and, if applicable, the current measurements preceding them are not taken into account when determining the motor parameters.However, it is also possible to consider both the current measurements before and after the faulty values, or, according to a further alternative, only the current measurements before the faulty values. In any case, it is advisable to determine the motor parameters only if a sufficiently large and reliable database is available. If this is not the case, standard parameters can be used, or the measured values ​​from a previous or subsequent brake motor application process can be used.

[0030] According to a further alternative, the correction current values ​​can also be determined from a substitute function, which is calculated, for example, using the least squares method based on correct current measurements. This approach has the advantage that correction current values ​​are also available for those times when measured values ​​outside the permissible limits were determined.

[0031] In the flow chart according to Fig.Figure 3 shows the various process steps for handling measured value errors. After starting the process in the first process step 20, a query is made in process step 21 as to whether the current measured value lies within the permissible limits. Several criteria are checked cumulatively, whereby a permissible measured value is only present and the process continues to the next process step 22 following the Yes branch ("Y") if all checked conditions for the relevant current measured value are cumulatively met. Otherwise, i.e., if only one of the checked conditions for the admissibility of the current measured value is not met, the process continues to step 23 following the No branch ("N").

[0032] In step 21, for example, it is queried whether the gradient of the current measurement function lies between zero and a lower limit, whether the current measurement value lies between a lower limit and an upper limit and whether the current current measurement value is smaller than the previous current measurement value.

[0033] If all conditions from process step 21 are met, the "Yes" branch is followed by process step 22, where the values ​​are classified as valid and stored in an indexed format. In the following process step 24, a query is made as to whether a sufficient number of valid measured values ​​have already been determined, for example, ten measured values. If this is the case, the "Yes" branch is followed by process step 25; otherwise, the program returns to the beginning of process step 21 to check the next current measured value.

[0034] In process step 25, a follow-up is activated, during which additional current measurements are recorded to verify the current curve. Subsequently, in process step 26, a query is made as to whether the current measurements determined in the follow-up according to process step 25 are still plausible. If this is the case, the system proceeds to process step 27 following the "Yes" branch; otherwise, the system proceeds to process step 30 following the "No" branch. In process step 27, a substitute function is generated using the least squares method on the basis of the current measurements identified as correct or plausible. This also allows for filling in gaps that have arisen due to inadmissible measured values ​​in the current measurement curve. The current electrical motor parameters R M (motor resistance) and K M(engine constant) which are used to calculate the actual clamping force generated during the application process.

[0035] If it is detected in process step 21 that one of the conditions is not met, i.e., the current measurement value under consideration lies outside the permissible limits with regard to at least one checked criterion, the "No" branch is followed by process step 23, in which a further query is made as to whether the gradient of the current measurement function is positive or whether the current measurement value is greater than the previous measured value. If this is not the case, the "No" branch is followed by a return to the beginning of process step 21 and the next current measurement value is checked. Otherwise, the "Yes" branch is followed by process step 28, and an entry is made in an error memory.

[0036] In the subsequent method step 29, a further check is performed to determine whether the number of error entries made in step 28 exceeds a threshold value. If this is not the case, the system returns to the beginning of method step 21 following the "No" branch. If, on the other hand, the number of errors already registered exceeds the assigned threshold value, the system advances to step 30 following the "Yes" branch, and all recorded data is discarded. The measured values ​​determined from the current brake motor application process cannot therefore be used. Instead, measured values ​​from a previous or subsequent application process must be considered to determine the motor parameters. The system then returns to the beginning of method step 21.

Claims

[1] Method for providing the clamping force generated by a parking brake (1) in a vehicle, which is generated at least partially by an electromechanical braking device with an electric brake motor (3) which acts on a brake piston (6) against a brake disc (10), wherein the clamping force is determined from current measured values of the motor current (I mot ) which are measured during operation of the brake motor (3), characterized by that in order to identify measurement errors, it is checked whether one or more current measured values are outside permissible limits, whereby instead of these current measured values, correction current values are used to calculate the clamping force. [2] Method according to claim 1, characterized by that the correction current values are the current measured values following the erroneous current measured values. [3] Method according to claim 1 or 2, characterized bythat the correction current values are determined from a substitute function. [4] Method according to claim 3, characterized by that the replacement function is calculated on the basis of correct current measurements. [5] Method according to one of claims 1 to 4, characterized by that the current measured values after switching on the brake motor (3) in the falling branch in the initial phase of the motor current (I mot ) can be determined. [6] Method according to claim 5, characterized by that checks whether consecutive current measurements become smaller. [7] Method according to one of claims 1 to 6, characterized by that checks whether the current measurements are within absolute limits. [8] Method according to one of claims 1 to 7, characterized by that checks whether the gradient of the current measurement function lies within defined limits. [9] Method according to claim 8, characterized bythat in the event of a voltage dip, the current measured values are used even if the gradient of the current measured value function lies outside the defined limits. [10] Method according to one of claims 1 to 9, characterized by that checks whether the course of the current measurement function matches a pattern course. [11] Method according to one of claims 1 to 10, characterized by that in the case of several consecutive inadmissible current measurements, all current measurements determined up to that point are discarded and only subsequent current measurements are taken into account. [12] Method according to one of claims 1 to 11, characterized by that the current measurement values are determined during an application process of the brake motor (3) to generate a clamping force. [13] Control device for carrying out the method according to one of claims 1 to 12. [14] Parking brake in a vehicle with a control device according to claim 13.

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