DE102017209877B4 - Method for operating a braking system for a motor vehicle - Google Patents

Abstract

Method (10) for operating a braking system of a motor vehicle, wherein a brake actuator receives time-discrete braking requests from a brake controller at time intervals (20), the brake actuator adjusts a corresponding braking torque of a brake depending on the braking request received (22), an actuator control of the brake actuator by means of at least one received braking request determines an intermediate braking request (26), and the brake actuator sets a braking torque corresponding to the intermediate braking request at a point in time within a waiting time until receipt of a further time-discrete braking request (28).

Description

The invention relates to a method for operating a brake system, with a brake actuator receiving time-discrete braking requests from a brake controller at time intervals and the brake actuator adjusting a corresponding braking torque of a brake depending on the braking request received. Furthermore, the invention relates to a braking system for a motor vehicle.

A method for operating a braking system and a braking system for a motor vehicle with the features of the preambles of the independent claims are from EP 0 908 364 A2 known, which relates to an anti-lock braking system with control valves that can only assume the open and closed states per se. The control valves are operated according to a cyclic flow control strategy such as e.g. B. Pulse width modulation is excited, with which the control valves can operate in a stable, partially open position, whereby a desired fluid flow, which corresponds to the last braking request generated by the control unit, can be adjusted more precisely and with smoother changes.

A network for the bidirectional transmission of digital data between a controller and individual components of the motor vehicle is increasingly being used in motor vehicles. An example of such a network is a bus system based on the CAN (Controlled Area Network) standard. Commands can be transmitted from the controller to individual components connected to the bus and vice versa via the network. This leads to a significant reduction in cable harnesses and thus in costs and weight. Use of a data bus is also known for braking systems in motor vehicles. In this case, for example, a driver of the motor vehicle uses a brake pedal to specify braking requirements, which are transmitted from a brake controller as digital signals to a brake via the data bus. The brake then generates a corresponding braking torque with the help of an actuator.

For example, in the German Offenlegungsschrift DE 10 2008 029 895 A1 describes a braking system for commercial vehicles with a service brake and a permanent brake, which are connected to a brake controller via a CAN bus. The brake controller receives a braking request from a brake pedal via the CAN bus and uses this to determine a continuous braking braking request. The permanent brake braking request is transmitted via the CAN bus as a digital signal to the permanent brake, which then generates a braking torque. The braking torque generated by the permanent brake is recorded by a sensor and sent as a status message from the permanent brake to the brake controller via the CAN bus. The brake controller determines a service brake braking request from the status message received from the permanent brake, the braking request from the brake pedal and the mass of the commercial vehicle. The service brake braking request is transmitted via the CAN bus as a digital signal to the service brake, which then brakes the wheels with a corresponding braking torque. In addition, wheel sensors detect a rotational speed of the wheels. The brake control determines an actual braking torque from the rotational speed of the wheels and compares this with a target braking torque, which results from the status message from the permanent brake and the service brake braking request. If the actual braking torque deviates from the setpoint braking torque, the brake controller transmits a correspondingly corrected service brake braking request to the service brake via the CAN bus.

A disadvantage of using a network with a data bus for transmitting braking requests from a brake controller to the brakes of a motor vehicle results from the time intervals between two consecutive braking requests in a transmission. In the case of a CAN bus, for example, this time interval is 20 ms. Modern brake actuators set a specified braking torque much more quickly and require 2 ms, for example. In other words, the sampling intervals specified by the transmission rate of the network for a braking request by a brake pedal can be significantly larger than the sampling intervals of a brake actuator determined by the setting time. This leads to a stepwise setting of the braking torque by the brake actuator, particularly when a brake pedal is operated relatively quickly with a steep ramp-like progression of braking requests. Such a braking process feels jerky and gradual for a user of the motor vehicle and is therefore undesirable. In particular, the braking behavior does not correspond to the user depressing the brake pedal evenly and thus leads to irritation and a negative driving experience.

One object of the present invention is to specify a method for operating a braking system of a motor vehicle and a braking system for a motor vehicle, the disadvantages mentioned being avoided or at least reduced.

This object is achieved by a method and a braking system as specified in the independent claims.

In a method according to the invention for operating a brake system of a motor vehicle, a brake actuator receives time-discrete braking requests from a brake controller at time intervals. The brake actuator includes, for example, a hydraulic pump, a pressure valve or a linear actuator and is designed to set a predetermined braking torque acting on one or more wheels of the motor vehicle. The brake controller is in particular a device which is arranged and configured to control brakes of the motor vehicle via one or more brake actuators. For this purpose, the brake control can be provided as an integral part of another control, for example a central control unit, or as a separate unit.

The braking requests transmitted by the brake controller to one or more brake actuators represent current braking torque setpoint values. These values are transmitted at time intervals, in particular at equidistant points in time, via a suitable signal connection and are therefore time-discrete braking requests. The braking requests are preferably transmitted as digital signals. Alternatively, time-discrete analogue signals can also be transmitted. Each brake actuator is designed to receive the transmitted braking requests and includes a suitable interface for this purpose, for example.

Depending on the braking request received as the setpoint braking torque, the brake actuator sets a corresponding braking torque as the actual braking torque of a brake. The brake actuator is designed in particular in such a way that the duration of a braking torque setting is shorter than a time interval between two braking requests that are received one after the other. The duration of an adjustment is preferably less than half or less than a tenth of the time interval between two braking requests received. A brake is understood to be a device that generates a braking torque for one or more wheels of the motor vehicle. The braking system can contain several brakes of the same or different design, for example friction brakes, hydraulically acting retarders or eddy current brakes. In particular, a friction brake can be provided for each wheel. Correspondingly, the braking system comprises one or more brake actuators, each of which sets a predetermined braking torque for one or more of these brakes.

Furthermore, an actuator control of the brake actuator uses at least one received braking request to determine an intermediate braking request. The brake actuator then adjusts a braking torque corresponding to the intermediate braking request at a point in time within a waiting time until receipt of a further time-discrete braking request. An intermediate braking request thus represents a target braking torque for a point in time between the last braking request already received and a braking request yet to be received by the brake controller.

According to the invention, the actuator control takes into account a history of two or more braking requests received last or a change between these braking requests for determining an intermediate braking request. For example, based on a difference between the two braking requests received last and the time difference between them for a specific point in time after the last reception, the actuator control determines a corresponding intermediate braking request in the sense of a linear interpolation. Furthermore, several consecutive intermediate braking requests are preferably determined until a further braking request is received and adjusted by the brake actuator.

With the determination and use of intermediate braking requests, two or more small braking torque changes are carried out in the time interval from a received braking request to the next received braking request during a braking operation instead of a large change in braking torque. This leads to a significantly smoother progression of braking torque changes. Stepped or jerky braking is effectively avoided.

According to a preferred embodiment of the invention, at least three time-discrete braking requests received one after the other are taken into account by the actuator control when determining an intermediate braking request. In particular, the three most recently received time-discrete braking requests are taken into account when determining an intermediate braking request for a point in time after receipt of the last time-discrete braking request. A quadratic interpolation is used for this purpose, for example. When using more than three received braking requests, cubic or higher order interpolation can also be used.

Furthermore, some braking requests can also be selected from a specific set of braking requests received. For example, only every second braking request received can be taken into account for determining an intermediate braking request. The more braking requests received are used in the determination, the more precisely a profile or a change in these braking requests over time can be described. Intermediate braking requests thus correspond more precisely to the actual course of the braking requests received.

In a further embodiment, the actuator control advantageously determines an intermediate braking request in each case at equidistant points in time within the waiting time until receipt of a further time-discrete braking request. The brake actuator sets a braking torque that corresponds to the respective intermediate braking request. For example, the period of time between two equidistant points in time corresponds approximately to an adjustment time of a predetermined braking torque by the brake actuator. With the use of equidistant points in time, an intermediate braking request can be determined quickly and easily. In addition, due to the same time intervals, there is a particularly regular increase or decrease in a braking torque after a time-discrete braking request last received from the brake controller. Braking is very smooth without jerking or annoying steps.

Furthermore, it is preferred that a previously determined intermediate braking requirement is taken into account when determining an intermediate braking requirement. For example, a new intermediate braking requirement can be determined iteratively from intermediate braking requirements that have already been determined. A correction amount can be determined using one or more previously determined intermediate braking requests and a new intermediate braking request can be determined as the sum of the correction amount and the last intermediate braking request. In this way, rapid determination of intermediate braking requirements is achieved without great effort.

$$b=\left(y_2-a{T}^2-y_1\right)/T$$

For example, quadratic interpolation is particularly preferred for determining intermediate braking requests, which carries out a determination of two coefficients a, b from the last three braking requests received: $$a=\left(y_3+y_1-2y_2\right)/\left(2T^2\right)$$

$$b=\left(y_2-a{T}^2-y_1\right)/T$$

In these equations, y ₁ , y ₂ and y ₃ are the last, second last and third last brake request received from the brake actuator and T is the settling time of the brake actuator. The adjustment time T can also be referred to as the sampling rate of the brake actuator for incoming braking requests and is approximately 2 ms, for example. With the coefficients a, b, the brake actuator determines a new intermediate braking request y _new from an existing braking request y, for example as follows: $$y_{ne\mathrm{and}}=y+a{T}^2+bT$$

To determine intermediate braking requirements, the actuator control includes a determination module, which contains, for example, an electronic circuit or a computer with appropriate software.

According to one embodiment of the invention, the time-discrete braking request is transmitted as a digital signal via a network of the motor vehicle from the brake controller to the brake actuator. In particular, the network includes a CAN bus system. The structure and mode of operation of a CAN bus are specified internationally and are known to those skilled in the art. Networks with a CAN bus are already widely used in motor vehicles and lead to a reduction in cable harnesses, which saves cost and weight. With the use of an already existing, reliably functioning network, the method according to the invention can be used in a motor vehicle without great effort. Alternatively, time-discrete braking requests can also be transmitted to the brake actuator via another suitably designed digital network.

In an advantageous embodiment, it is provided that the brake requests pass through a filter for smoothing before they are transmitted to the brake actuator during the brake control. The filter is, for example, a first-order filter, such as a first-order low-pass filter. Alternatively, a 2nd or higher order filter can also be used. The filter can be designed as an analog filter for braking requests in the form of analog signals or as a digital filter for braking requests in the form of digital signals. The filter is used to smooth out sudden changes in braking demand over time. For example, a transition from braking requests that increase like a ramp to a constant braking request will be smoother or smoother. Such a transition occurs, among other things, when a brake pedal is actuated to a certain stopping point.

With the smoothing, in particular, intermediate braking requests that are too large or too small and an associated overshooting of a braking torque beyond a desired braking torque in the event of a major change in the course of the braking requests are avoided. Furthermore, intermediate braking requirements and thus also braking torques are effectively prevented from settling to a constant value.

According to one embodiment, a sample and hold module of the braking system preferably generates the time-discrete braking requests. A sample and hold module samples an analog signal at specific, preferably equidistant points in time and holds the sampled value until the next sample. It thus generates time-discrete values from analog braking requests. In addition, the sampled braking requirements can be digitized. Alternatively, the sampling and holding module is designed to sample digitally present braking requests at a predetermined sampling frequency. In this way, a sampling rate of the braking requirements can be changed to a predetermined value. The sample and hold module is arranged, for example, at a transmitter for analog braking requests, such as a brake pedal, or at the brake controller. Finally, a CAN bus system with a certain transmission rate, for example a brake request every 20 ms, can also serve as a sample and hold module.

A braking system according to the invention for a motor vehicle includes a brake controller for sending time-discrete braking requests at time intervals, and a brake actuator for receiving the time-discrete braking requests and for setting a braking torque of a brake after a received braking request. The brake actuator contains an actuator control, which is designed to determine an intermediate braking request based on at least one received braking request. Furthermore, the brake actuator is configured to set a braking torque corresponding to the intermediate braking request at a point in time within a waiting time until receipt of a further time-discrete braking request.

In one embodiment of the braking system according to the invention, the actuator control includes, for example, a determination module which contains an electronic circuit or a computer with appropriate software for determining intermediate braking requirements. Furthermore, the brake system can include a CAN bus system for transmitting time-discrete braking requests to one or more brake actuators, and the brake actuator or actuators can have a correspondingly designed interface for a CAN bus.

Analogously to the method according to the invention, during a braking operation, the braking system carries out two or more small braking torque changes in the time interval from a received braking request to the next received braking request instead of a large change in a braking torque. In this way, a significantly smoother and softer progression of braking torque changes is achieved with a time-discrete transmission of braking requests to a brake actuator.

In particular, a braking system according to the invention is designed to carry out one of the methods described above.

• 1 ein vereinfachtes Flussdiagramm eines Ausführungsbeispiels eines erfindungsgemäßen Verfahrens zum Betreiben eines Bremssystems für ein Kraftfahrzeug, und
• 2 ein vereinfachtes Diagramm mit zeitlichen Änderungen von verschiedenen Bremsmomenten bei dem Verfahren nach 1.

The invention is explained in more detail below by way of example with reference to the drawings. Show it:

• 1 a simplified flowchart of an exemplary embodiment of a method according to the invention for operating a braking system for a motor vehicle, and
• 2 a simplified diagram with changes over time of different braking torques in the method 1 .

In 1 10 designates a method for operating a braking system for a motor vehicle. Method 10 is described below together with an exemplary embodiment of a braking system according to the invention. First, a brake request is received 12 in a brake controller of the brake system from a brake pedal or a driver assistance system. A braking request represents a coded time-dependent target braking torque. During a braking process, a large number of braking requests therefore occur at short time intervals. Braking requests entered by a user of the motor vehicle using the brake pedal are transmitted via a suitable signal line, for example via a CAN bus of the motor vehicle. For this purpose, the brake pedal includes a sample and hold module for converting an analog signal from the brake pedal into a time-discrete digital signal. Braking requests from a driver assistance system can also be transmitted to the brake controller via the CAN bus. Alternatively, the driver assistance system and the brake control can be integral parts of a central vehicle control. In this case, an internal bus can be used to transmit braking requests to the brake controller.

The brake controller carries out a check 14 of the braking requests received and makes corrections if necessary. In particular, if there is a very steep rise in the braking requirements, such as in the case of emergency braking, further processing, for example filtering as described below, can be prevented and the braking requirements can be transmitted to a brake actuator without delay. The braking requests are then filtered by a filter to smooth the braking requests, 16. The filter is a first-order digital filter and is designed to smooth the digital time-discrete braking requests. The filter is used to smooth out sudden changes in braking demand over time. For example, a transition from braking requests that rise in a ramp-like manner to braking requests of constant magnitude, as occurs in many braking processes, is smoothed out by the filter. Alternatively, it is also possible to use a higher-order filter.

Finally, the filtered braking requests are sent 18 to one or more brake actuators via the CAN bus of the motor vehicle. The individual braking requests are transmitted according to a transmission rate of the CAN bus with a time interval of approximately 20 ms as time-discrete digital braking requests. A brake actuator includes, for example, a hydraulic pump, a pressure valve or a linear actuator and is designed to set a braking torque acting on one or more wheels of the motor vehicle in accordance with a predetermined braking requirement.

In the case of the brake actuator, the transmitted time-discrete braking requests are received 20 via an interface to the CAN bus at time intervals of approximately 20 ms. After the receipt 20 of a braking request, a corresponding braking torque is set 22 by the brake actuator. For this purpose, an actuator control of the brake actuator interprets the braking request received and causes the setpoint braking torque to be set. This happens within an adjustment time, which is only a fraction of the transmission rate of the CAN bus. In this exemplary embodiment, the brake actuator activates friction brakes on the wheels of the motor vehicle. Alternatively or additionally, a retarder, for example an eddy current brake or a hydraulic retarder, can also be activated.

Furthermore, the received braking request is stored 24 in a memory of the brake actuator. The memory includes memory locations for three or more received time-discrete braking requests and stores the braking requests received last. In addition, the memory includes memory locations for values of a quadratic interpolation described below.

$$b=\left(y_2-a{T}^2-y_1\right)/T$$

On the basis of the last three braking requests received, the actuator controller carries out a determination 26 of intermediate braking requests based on quadratic interpolation. For this purpose, the actuator control first determines two coefficients a, b from the last braking request y ₁ , the second to last braking request y ₂ and the third to last braking request y ₃ , as well as a predetermined time interval T between two intermediate braking requests. In this exemplary embodiment, T corresponds to the adjustment time of the brake actuator of approximately 2 ms: $$a=\left(y_3+y_1-2y_2\right)/\left(2T^2\right)$$

$$b=\left(y_2-a{T}^2-y_1\right)/T$$

With the coefficients a, b, the brake actuator determines an intermediate braking request y _new from a braking request y that has already been set and the time interval T as follows: $$y_{ne\mathrm{and}}=y+a{T}^2+bT$$

To determine this intermediate braking request, the actuator control includes a determination module, which includes, for example, an electronic circuit or a computer with appropriate software. Subsequently, the determined intermediate braking request 28 is set as the new setpoint braking torque by the brake actuator with the aid of the actuator control. This adjustment 28 takes place approximately 2 ms after the receipt 20 of the last braking request.

The actuator control then carries out a check 30 as to whether a new braking request has been received from the brake control. If this is not the case, the method is continued with a further determination of an intermediate braking torque 26, an adjustment of the determined intermediate braking torque 28 and a check 30 for receipt of a new braking request. A new intermediate braking request is thus set as the new target braking torque approximately every 2 ms until a new braking request is received 20 from the brake controller. The steps already described for the new braking request are then carried out. In this way occurs in the approximately 20 ms between setting two received Braking requests every 2 ms setting a determined intermediate braking request.

2 FIG. 1 shows a simplified diagram with changes over time in different braking torques in method 10. Braking torques M are shown over time t in the diagram. The time profile of setpoint braking torques 40, as specified by a user of the motor vehicle, a driver assistance system, or the brake controller, is shown as a dashed line. It is a ramp-like increase up to a maximum braking torque, which should then be kept constant. The braking requests 42 smoothed after passing through the filter are shown as a dotted line 42 . The smoothing of the kink during the transition from increasing to constant braking torques can be clearly seen. Furthermore, due to the filter, there is a slight delay of a few milliseconds in the braking requests.

By transmitting the braking torques as time-discrete braking requests via the CAN bus, the brake actuator only receives a new braking torque approximately every 20 ms. Received braking torque requests 44 therefore have a graduated profile shown as a dot-dashed line. This would lead to a gradual change over time in the braking torques set by the brake actuator. With the determination and setting of approximately ten intermediate braking requests in the described method 10 as quadratically interpolated values, the brake actuator generates braking torques 46 with a much more finely graded profile. These braking torques 46 generated by the brake actuator are in 2 shown as a solid line.

The smoothing 16 by the filter serves to avoid a brief overshooting or transient of the determined intermediate braking requirements when there is a change in the profile of the setpoint braking torques. Such a change would be, for example, the transition from increasing to constant braking torques in the course of setpoint braking torques 40. A braking process with a transmission of braking requests via a CAN bus is carried out much more evenly and without a disruptive stepwise increase in braking torque using method 10.

Claims (8)

Method (10) for operating a braking system of a motor vehicle, wherein a brake actuator receives time-discrete braking requests from a brake controller at time intervals (20), the brake actuator adjusts a corresponding braking torque of a brake depending on the braking request received (22), an actuator control of the brake actuator by means of at least one received braking request determines an intermediate braking request (26), and the brake actuator sets a braking torque corresponding to the intermediate braking request at a point in time within a waiting time until receipt of a further time-discrete braking request (28). procedure after claim 1 , characterized in that at least three time-discrete braking requests received one after the other are taken into account by the actuator control when determining (26) an intermediate braking request. Method according to one of the preceding claims, characterized in that the actuator control determines an intermediate braking request at equidistant points in time within the waiting time until receipt of a further time-discrete braking request (26) and the brake actuator sets a braking torque corresponding to the respective intermediate braking request (28). Method according to one of the preceding claims, characterized in that when determining (26) an intermediate braking requirement, a previously determined intermediate braking requirement is taken into account. Method according to one of the preceding claims, characterized in that the time-discrete braking request is transmitted (18) as a digital signal via a network of the motor vehicle from the brake controller to the brake actuator. Method according to one of the preceding claims, characterized in that braking requests pass through a filter for smoothing (16) before they are transmitted to the brake actuator during the brake control. Method according to one of the preceding claims, characterized in that a sample and hold module of the braking system generates the time-discrete braking requests. Braking system for a motor vehicle with a brake controller for sending time-discrete braking requests at time intervals, and a brake actuator for receiving the time-discrete braking requests and for adjusting a braking torque of a brake after a received braking request, wherein the brake actuator contains an actuator control which is used to determine an intermediate braking request is formed based on at least one received braking request, and the brake actuator is configured to set a braking torque corresponding to the intermediate braking request at a point in time within a waiting time until receipt of a further time-discrete braking request.

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