METHOD FOR CONTROLLING THE BRAKING OPERATION OF A MOTOR VEHICLE

Abstract

A method for controlling the braking operation of a motor vehicle including at least one electric drive machine, which is switchable into a generator mode effectuating a deceleration of the motor vehicle, as well as a friction braking device having multiple friction brakes, each assigned to a wheel, having brake elements, which are movable toward a brake disc via an actuator. A first braking is performed using the drive machine switched into the generator mode until a first threshold value of the regenerative power or deceleration is reached, after which additionally required braking power is provided by the friction braking device, which is also actuated upon reaching the threshold value.

Claims

1. A method for controlling the braking operation of a motor vehicle comprising: at least one electric drive machine, which is switchable into a generator operation effectuating a deceleration of the motor vehicle, as well as a friction braking device having multiple friction brakes, each assigned to a wheel, having brake elements, which are movable toward a brake disc via an actuator, wherein in case of braking, a first braking is performed using the drive machine switched into the generator operation until a first threshold value of the regenerative power or deceleration is reached, after which additionally required braking power is provided by the friction braking device, which is also actuated upon reaching the threshold value, wherein the friction braking device is already activated when a second threshold value of the regenerative power or the deceleration is reached, which is lower than the first threshold value, in such a way that the brake elements are moved toward the brake discs, wherein this state is maintained until reaching the first threshold value.

2. The method as claimed in claim 1, wherein the second threshold value is between 10-40% of the maximum regenerative power or between 0.1-0.4 g deceleration.

3. The method as claimed in claim 1, wherein when the second threshold value is reached, in the scope of the movement of the brake elements, only the distance of the brake elements to the brake disc is reduced, or in that the brake elements are brought into friction contact with the brake discs.

4. The method as claimed in claim 1, wherein a hydraulically operating friction braking device is provided with a brake fluid, wherein the pressure of the brake fluid is increased to move the brake elements when the second threshold value is reached.

5. The method as claimed in claim 1, wherein an electro-mechanical braking device having electrical actuating elements for moving the braking elements is provided, wherein the actuating elements are activated to move the brake elements when the second threshold value is reached.

6. A motor vehicle, comprising: at least one electric drive machine, which is switchable into a generator operation effectuating a deceleration of the motor vehicle, as well as a friction braking device having multiple friction brakes, each assigned to a wheel, having brake elements, which are movable toward a brake disc via an actuator, wherein in case of braking, a first braking is performed using the drive machine switched into the generator operation until a first threshold value of the regenerative power or deceleration is reached, after which additionally required braking power is provided by the friction braking device, which is also actuated when the threshold value is reached, designed to carry out the method as claimed in any one of the preceding claims.

7. The motor vehicle as claimed in claim 6, wherein the friction braking device has at least one control unit, which is designed to control the friction brakes as a function of the regenerative power or the deceleration.

8. The motor vehicle as claimed in claim 7, wherein the brake elements can be moved, when a second threshold value is reached, in such a way that either only the distance between the brake elements and the brake disc is reduced, or in that the brake elements can be brought into friction contact with the brake discs.

9. The motor vehicle as claimed in claim 8, wherein a hydraulically operating friction braking device is provided with a brake fluid, wherein the control unit is designed to increase the pressure of the brake fluid when the second threshold value is reached.

10. The motor vehicle as claimed in claim 8, wherein an electro-mechanical braking device having electrical actuating elements for moving the braking elements is provided, wherein the control unit is designed to activate the actuating elements when the second threshold value is reached.

11. The motor vehicle as claimed in claim 8, wherein the second threshold value is between 10-40% of the maximum regenerative power or between 0.1-0.4 g deceleration.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0021] Further advantages and details of the present disclosure will be apparent from the exemplary embodiments described below and in reference to the drawings. In the figures:

[0022] FIG. 1 shows a schematic illustration of a motor vehicle according to the disclosure;

[0023] FIG. 2 shows a schematic illustration of a properly running braking process in the form of a diagram, and

[0024] FIG. 3 shows a schematic illustration of a faulty braking process in the form of a diagram.

DETAILED DESCRIPTION

[0025] FIG. 1 shows a motor vehicle according to the disclosure, which is designed to carry out the method according to the disclosure. It comprises an electric drive machine 2, that is to say an electric motor, which is fed from a battery 3. In addition to the motor mode that drives the vehicle, this drive machine 2 can also be switched into a generator mode used to brake or decelerate the motor vehicle, in the context of which the drive machine 2 produces current, which is fed into the battery 3, for example, to charge it. The motor vehicle 1 can therefore be decelerated in a regenerative way and the current obtained can be fed back.

[0026] Furthermore, a friction braking device 4 is provided, comprising four friction brakes 5 assigned to the individual wheels, each comprising a brake disc 6 and brake elements 7 assigned thereto, which are movable via an actuating means (not shown in greater detail) toward the rotating brake disc 6 connected to the wheel for active braking. The friction braking device 4 can operate electro-hydraulically, that is to say that a corresponding brake fluid is provided as the braking means, which is put under corresponding pressure in order to move the brake elements 7. Alternatively, it can also be an electrical-mechanical friction braking device 4, that is to say corresponding electric motors are provided as actuating means which move the respective brake elements 7.

[0027] Independently of how the friction braking device 4 is designed, it comprises at least one control unit 8 which controls the operation of the friction braking device 4. For example, the generation of the corresponding brake pressure can be incorporated or the corresponding electric motors can be activated, etc. via the control unit. Such a motor vehicle usually has a brake pedal, the position of which can be detected, wherein the control of the friction braking device 4 can take place, for example, as a function of the brake pedal position, the braking force, the brake pressure, or the like.

[0028] The friction braking device 4 is a second, active brake system, which is provided in addition to the generator brake system, implemented via the drive machine 2 switchable to generator mode. The friction braking device 4 is used, on the one hand, to assist the regenerative brake 2 during stronger braking processes or to provide the additional braking power required. On the other hand, it is also used as an emergency system, via which the entire braking operation can take place, should a problem arise within the regenerative braking system for whatever reason, for example a failure of the drive machine 2.

[0029] The control unit 8 is now able to bring the friction brakes 5 into different operating states within the friction braking device 4, specifically depending on how the currently running regenerative braking operation takes place. Because as described, of course, the attempt is primarily only to use regenerative braking to be able to exhaust the energy generation potential of this braking system and thus to ensure efficient vehicle operation. However, there are limits to this regenerative braking system up to where it can be operated, wherein the friction braking device 4 is then additionally activated.

[0030] FIG. 2 shows, in the form of a solely schematic diagram, the basic mode of operation of the motor vehicle according to the disclosure or the method according to the disclosure.

[0031] Along the ordinate, the regenerative power Rk, that is to say the braking or generator power that is supplied via the drive machine 2 switched into generator mode, is plotted very generally, without any measured value to be assigned. In addition, the friction braking power Rb is plotted, that is to say the power which the friction braking device 4 provides. The time is plotted along the ordinate, also without a measured value.

[0032] Basically, the regenerative braking occurs only up to a predetermined, first threshold value S1 for the regenerative power. If the brake pedal is thus actuated, if it is thus assumed that increasingly stronger braking is required, the regenerative braking power thus rises increasingly strongly with increasing duration of the braking process, up to the maximum permissible first threshold value S1, from which the regenerative power or generator power can no longer be increased. From this time on, the additional braking power which is required as the brake pedal is depressed further is provided by the friction braking device 4, that is to say in addition to the regenerative brake, the friction brake now also effectuates the increasingly stronger overall deceleration. In the diagram according to FIG. 2, this is shown on the basis of the curve K1, which represents the curve of the regenerative power, and the curve K2, which shows the curve of the friction braking power. It can be seen that the curve K1 firstly rises (linearly in the example shown, but this does not necessarily have to be the case), until it reaches the first threshold value S1 at the time t.sub.S1 and then extends constantly from then on, i.e. the regenerative power does not increase further. In return, when the first threshold value S1 is reached at time t.sub.S1, the friction braking power obviously increases, which is additive to the regenerative power that is then constant.

[0033] FIG. 2 furthermore shows a specific curve profile with respect to the curve K2, which indicates that the friction braking device 4 is not in an entirely inactive state up to the time t.sub.S1. Rather, the friction braking device 4 is prepared for a possible complete braking takeover, as will be described below with reference to FIG. 3.

[0034] A second threshold value S2 with respect to the regenerative power, i.e. the curve K1, is shown in the diagram according to FIG. 2. It is obvious that the second threshold value S2 is significantly lower than the first threshold value S1. If the regenerative power increases in the scope of the braking process, the friction braking device 4 is thus also activated when the second threshold value S2 is reached, in such a way that the friction brakes 5 are activated or thus either the brake fluid pressure is somewhat increased or the actuating motors are activated accordingly via the control unit 8, so that the brake elements 7 are moved into minimal friction contact, i.e. friction engagement, with the brake discs 6. This takes place at time t.sub.S2 and is shown in the small jump in the curve branch K2′ in the curve K2, wherein this jump is shown exaggeratedly large here, since actually active generation of a friction braking torque is not linked to merely bringing the brake elements 7 into contact with the brake discs 6, i.e. no friction braking power is linked thereto. Rather, this exclusively involves “pre-tensioning” or “preparing” the friction braking device 4 with respect to a possible complete takeover of the braking operation in the event of failure of the regenerative brake, as will be described below. This means that the jump in the curve K2 at time t.sub.S2 is shown exaggerated and is only intended to illustrate that there is an active activation of the friction braking device 4 in order to “pre-tension” it and only to bring the brake elements 7 into contact with the brake discs 6 in this case. This state assumed in the curve branch K2′ is maintained with the continued braking process until the first threshold value S1 is reached and thus until time t.sub.S1. Since the friction braking device 4 has to supply the additional braking contribution from this state in any case, the additional braking power is supplied via the friction braking device 4 from this time, see the profile of the curve K2.

[0035] Starting from the correct basic functionality, as described with reference to FIG. 2, FIG. 3 shows a curve profile as results in the event of a failure of the regenerative brake via the drive machine 2. It is assumed that an initially regular braking process is also initiated here. The curve K1 of the regenerative power of the drive machine 2 clearly rises again. When the second threshold value is reached, the friction braking device 4 is again pre-tensioned, and the jump (also again shown exaggerated here) occurs at time t.sub.S2 on the part of the curve K2 of the friction braking power, the brake elements are pre-tensioned back into contact or minor friction contact with the brake discs 6. With increasing braking process and rising regenerative power and thus stronger and stronger braking, however, at time t.sub.3, a total failure of the regenerative brake occurs, which means that the drive device 2 can no longer actively brake. Immediately accompanying this or with an extremely minimal time delay, however, the friction braking device 4 takes over the complete braking operation, which means that the friction braking power is suddenly increased strongly in order to be able to completely compensate for the regenerative braking power which has dropped to zero, and also to provide a continuously rising braking power. This can be seen in the profile of the curve K2, which shows a sharp jump immediately after the time t.sub.3, to ultimately quasi-continue the curve K1 with respect to the total braking power.

[0036] What is essential here is that there is only a negligible, if any, time delay in providing the full braking torque or full braking power via the friction braking device 4 after the regenerative brake fails, i.e. the extreme jump in curve K2 only takes place with a minimal delay from the extreme drop of the curve K1 or the two processes only take place delayed from one another in the millisecond range. This is because the fact that the brake elements 7 are already brought into grinding contact application, although minor, with the brake discs 6, enables the control unit 8 to activate the friction brakes 5 immediately after the time Sand thus the failure of the regenerative brake in such a way that they provide the full braking torque. This means that, for example, the servomotors are now activated in such a way that they press the brake elements 7 against the brake discs with the required pressure to generate the curve jump of the curve K2. This minimal time offset is made possible due to the “pre-tensioning” or “preparation” of the friction braking device 4. It does not matter at what time within the times t.sub.S2-t.sub.S1 the regenerative brake fails, since within this time, as described, the friction braking device 4 is permanently “pre-tensioned” and can therefore take over the full regenerative operation at any time.

[0037] A further advantage is that, since the friction braking device is able to provide the full braking power at any time, the first threshold value Si can also be set higher than in previously known systems. Linked to this, however, is better utilization of the regenerative potential of the regenerative brake, that is to say that significantly more current can be regenerated and fed back into the battery 3 than in previously known systems. At the same time, there is complete fallback safety in the event of a problem, since the friction braking device 4 is prepared at any time to instantaneously take over the complete braking operation.

[0038] Although FIG. 3 shows an example with a total failure of the regenerative brake, the system according to the disclosure is similarly usable in the event of a partial failure of the regenerative brake, thus when it can only provide a part of the actually required braking power for whatever reason. This is because due to the “pre-tensioning”, the friction braking device 4 is also capable here at any time, from reaching the threshold value S2, of providing the missing braking power immediately and without delay.