System for controlling an electric parking brake by pulse width modulation

Abstract

To provide a method for controlling an electric brake system.SOLUTION: There is provided a control system for controlling an electric parking brake system of an automobile having at least two brakes 12 designed so as to be mounted near wheels 8 of the automobile, wherein each of the brakes 12 includes parking brake actuators 14 operated by electric motors 16, the control system includes a control unit (UC), the control unit (UC) has means for controlling the electric motors 16 to supply power to the electric motors 16 so as to operate the actuators and apply the brakes, when an instruction for applying the parking brake is issued. The control means is designed so as to control the electric motors by pulse width modulation in at least some operation stages of the parking brake actuators.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a system for controlling an electric parking brake system for a motor vehicle and to a method for controlling an electric parking brake system. [Background technology]

[0002] In most modern automobiles, service braking is provided by disc brakes and / or drum brakes.

[0003] These provide the service braking function, intended to slow or even stop the motor vehicle. The parking brake function, intended to bring a stationary vehicle to a stop, and the emergency braking function are generally achieved by a system providing a parking brake.

[0004] Applying the parking brakes can be accomplished by pulling a lever located inside the passenger compartment that is connected via a brake cable to a mechanism found in drum brakes or disc brakes. The mechanism acts to separate the brake shoes of drum brakes and bring them against the drum. In the case of disc brakes, the pads are applied against the brake disc.

[0005] To improve driver comfort and safety when applying the parking brake, the automotive industry is developing electronic parking brakes.

[0006] There are two types of systems: the first type of electric parking brake includes an electric motor that pulls a cable, and is called a cable puller system, and the second type of electric parking brake includes an actuator for at least each rear brake, which is provided with an electric motor, thereby applying the brakes.

[0007] The driver, for example, activates the electric parking brake by pressing a button located on the dashboard.

[0008] Electric parking brake systems include a DC electric motor and the implementation of such a motor is recommended, for example, in the "VDA Recommendation" of the German Association of the Automotive Industry.

[0009] The brake application speed, for example the speed at which the brake pads are applied to the disc in the case of disc brakes, is obtained as a relatively fast constant speed. This is an all-or-nothing control. This all-or-nothing control is satisfactory for the normal operation of a parking brake, and indeed when it is desired to apply the parking brake, most of the time the vehicle is stationary and the pads are applied to the disc at a constant speed.

[0010] However, in some situations, this all-or-nothing control can be damaging.

[0011] For example, when changing pads in the case of disc brakes, the brakes are released to their maximum, i.e. the pistons are moved to their maximum rearward position until they rest against abutments, and contact with these abutments makes it possible to detect that the brakes have reached their maximum release position. However, contact with the abutments can be abrupt.

[0012] In another example, to perform a particular test or validation, a vehicle is mounted on a roller bench, which applies several different driving torques to the wheels of the vehicle, which in turn activates the parking brake. This requires that the applied torque be increased as a function of the torque applied by the rollers. However, in the case of a DC motor, the parking brake is applied all-or-nothing, which results in vibration of the vehicle. This vibration can lead to distortion of the measurement results. Summary of the Invention [Problem to be solved by the invention]

[0013] It is therefore an object of the present invention to provide a system for controlling an electric parking brake system and a method for controlling an electric parking brake system, which make it possible to reduce or even overcome the aforementioned disadvantages. [Means for solving the problem]

[0014] The object of the invention is achieved by a system for controlling an electric parking brake system including at least one direct current electric motor implementing a Pulse Width Modulation (PWM) control in order to make it possible to adjust the motor speed and thus the speed of movement of the brake pads in the case of disc brakes or the brake linings in the case of drum brakes, whereby the electric motor is controlled by pulse width modulation in some phases and at full power in other phases.

[0015] Thus, for example in the case of a pad change, the control system can plan a slower rearward movement of the piston. Contact with the abutment is less abrupt. The risk of damage is reduced. A smaller size of the abutment is also planned, so that it does not have to be dimensioned to withstand a sudden impact.

[0016] When testing vehicles on a roller bench, the control system gradually increases the applied force, which reduces the generation of vibrations.

[0017] The control system according to the invention can also control the parking brake during normal operation when applying it, for example the control system can ensure a fast movement of the pads to the disc during a first phase and then a slower movement of the pads during a second phase, thereby reducing the power required.

[0018] In other words, the electric parking brake is controlled in such a way that it can have a variable actuation speed during the actuation phase, whether or not it is in normal actuation, e.g. during maintenance of the brake and / or during its testing.

[0019] The invention is particularly advantageous in the case of an electric parking brake associated with a hydraulic service brake, and even more advantageous in the case of braking systems further including an Electronic Stability Program (ESP™) that includes a pressure generator.

[0020] In such cases, the PWM control makes it possible to regulate the electric partial power possibly combined with the hydraulic power provided by the braking system, resulting from depression of the brake pedal and / or from the pressure generator of the ESP system.

[0021] One subject of the invention is therefore a system for controlling an electric parking brake system for a motor vehicle comprising at least two brakes arranged on the wheels of the vehicle, each brake comprising a parking brake actuator actuated by an electric motor, said control system comprising a control unit comprising means for controlling the electric motor such that, when a control command to apply the parking brakes is given, the electric motor is powered to actuate the actuator and allow application of the brakes, said control means being arranged to control the electric motor by pulse width modulation during some operating phases of the parking brake actuators and to control the electric motor at full power during other operating phases of the parking brake actuators.

[0022] The control unit is advantageously capable of receiving external information informing the control unit to control the electric motor by pulse width modulation.

[0023] For example, the control means includes at least one power switching device, such as a power transistor or a triac.

[0024] Another subject of the invention is a braking system comprising a control system according to the invention and an electric parking brake system comprising at least two brakes arranged on the wheels of the motor vehicle, each brake comprising a parking brake actuator actuated by an electric motor.

[0025] The braking system preferably includes at least two service brakes.

[0026] Another subject of the invention is a motor vehicle including a braking system according to the invention, the electric parking brake system acting on at least two of the four brakes, whereby the control system is configured to control the electric parking brake system during normal application of the electric parking brake.

[0027] Another subject of the invention is a method for controlling an electric parking brake system including a control unit and at least two brakes arranged on the wheels of a motor vehicle, each brake including a parking brake actuator actuated by an electric motor, said method comprising at least - controlling the electric motor by pulse width modulation when operation of the electric parking brake system is deemed to require speed modulation of the electric motor; - Steps to control the electric motor at full power and Includes.

[0028] The PWM control step can be performed before or after the full power control step.

[0029] Furthermore, the control steps may occur during the same braking action or during different braking actions.

[0030] In one exemplary embodiment, when brake pads of a disc brake are replaced, the electric motor is controlled in the brake release direction by pulse width modulation such that its rotational speed is slower than its rotational speed during at least one part of the application phase of the parking brake in normal operation.

[0031] In another exemplary embodiment, when the vehicle is mounted on a test bench, the electric motor is controlled by pulse width modulation so that its rotational speed is continuously varied and the brake application force is continuously varied.

[0032] In another exemplary embodiment, when the vehicle is mounted on a test bench, the electric motor is controlled by pulse width modulation such that its rotational speed changes in steps, switching progressively from one step to another, and the brake application force changes continuously in steps, switching progressively from one step to another.

[0033] Advantageously, the electric motor is controlled by pulse width modulation such that a normal operating phase of the electric parking brake system comprises a first part at a first speed and a second part at a second speed, the second part corresponding to a compression phase of the brake linings, the second speed being slower than the first speed.

[0034] In another exemplary embodiment, during the step of determining the position of the at least one piston, the electric motor is controlled in the brake release direction by pulse width modulation such that its rotational speed is slower than its rotational speed during a portion of at least one application phase of the parking brake in normal operation.

[0035] The present invention will be better understood on the basis of the following description and the accompanying drawings. [Brief description of the drawings]

[0036] [Figure 1]FIG. 1 is a schematic diagram of a bottom view of a vehicle including an electric parking brake system. [Diagram 2] 4 is a graph showing an example of actuator speed change in an electric parking brake system using a control system according to the present invention and in an electric parking brake system of the prior art; [Diagram 3] 4 is a graph of an example of applied force change as a function of time for a brake of an electric parking brake system using a control system according to the present invention and for a brake of a prior art electric parking brake system. [Figure 4] 4 is a graph of an example of actuator speed change during normal application of the parking brake in an electric parking brake system using a control system according to the present invention. [Diagram 5] 1 is a graph of an example of a square wave voltage that may be used to power a motor with PWM control. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] In the following description, the brakes of the electric parking brake system are disc brakes, but of course an implementation of drum brakes does not depart from the scope of the invention. In Figure 1, a schematic diagram of a motor vehicle equipped with an electric parking brake system S is shown.

[0038] The vehicle includes a front axle 2 and a rear axle 4 each having two wheels 6, 8. Each wheel is equipped with a disc brake 10, 12 respectively.

[0039] The electric parking brake system S implements at least two brakes, typically brakes 12 mounted on the rear wheels.

[0040] According to another embodiment, a vehicle according to the present invention includes four parking brakes.

[0041] The system also includes an actuator 14 for each brake 12 for applying brake pads against a disc fixed to the wheel.

[0042] For example, the actuator 14 is driven by a DC electric motor 16 via a reducer (not shown).

[0043] A disc brake, for example, includes a piston (not shown) attached to a screw. The screw is rotatably driven by an electric motor to move the piston along the screw. The piston pushes a pad against the disc. In reaction, a caliper slides to apply the other pad against the other face of the disc.

[0044] The system for controlling the electric parking brake system comprises a control unit UC for controlling the application of the parking brake. Furthermore, in the illustrated example, the system comprises a control member 18 arranged in the vehicle interior by means of which the driver can activate the electric parking brake. The control member is, for example, a button.

[0045] When the driver presses the button to apply the parking brake, a signal is sent to a control unit, which activates the actuator by powering an electric motor, which moves the brake pads onto the discs.

[0046] The speed at which the pad moves is proportional to the current supplied to the motor, which is constant and set by the circuitry that powers the vehicle.

[0047] According to the invention, the control unit comprises means arranged to perform a pulse width modulation or PWM control of the electric motor 16 .

[0048] PWM control of the motor 16 allows the rotational speed of the electric motor, and therefore the actuation speed of the brake, to be varied.

[0049] PWM control consists of supplying an electric motor with a square-wave voltage. Figure 5 shows an example of a square-wave voltage U that can be used to power a motor with PWM control. This results in an average voltage that depends on the ratio of the square-wave duration T0 and the signal period T. The speed of the motor then depends on the average voltage and by choosing the characteristics of the square-wave signal the speed of the motor can be varied.

[0050] The PWM control is implemented, for example, by means of at least one power transistor, e.g. an insulated gate field effect transistor, also called a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a triac, or other switching and / or power control device.

[0051] According to a first application example, the PWM control of the motor 16 is performed when changing pads, for example when the control unit is informed that a change of brake pads is advisable.

[0052] When the brake pads reach a level of wear requiring their replacement, the motor is activated to move the piston away from the disc, and the control unit detects that the brake is fully released when it measures a sudden increase in motor supply current due to contact between the piston and an abutment means carried by the brake.

[0053] According to the invention, when the control unit is informed that a pad change is necessary, the control unit controls the movement of the motor by PWM control, so that the rearward movement of the piston of the disc brake is performed at a slow speed relative to the motor speed without PWM control, at least in the vicinity of the abutment, i.e. in the rearward position of the piston relative to the disc.

[0054] Therefore, the contact between the piston and the abutment means is not abrupt. The risk of damage to the brake is reduced. It may also be envisaged to reduce the size of the abutment means, thus reducing the brake mass.

[0055] 2 shows a schematic graph of the movement speed v of the actuator 14 in the brake release direction as a function of time t. The speed v is proportional to the rotational speed of the motor 16. Curve I represents the speed of the system according to the invention and curve II represents the speed of the prior art system.

[0056] The speed vII of curve II is faster and the opening time tII is shorter, but the impact between the piston and the caliper is more abrupt and therefore more likely to cause braking to deteriorate.

[0057] Curve I shows a slower speed vI than vII, and therefore a longer opening time tI, but in fact the contact energy is smaller. The risk of damage is substantially reduced.

[0058] According to one alternative, the PWM control can be such that the speed of the motor changes in steps, more specifically decreases in steps. In the illustrated example, in curve III, the speed is equal to vII in a first stage P1 and then decreases continuously according to a linear function to vI in a second stage P2. At the end of stage P3, the piston contacts the caliper at a time tIII between tII and tI. The opening period tIII is therefore shortened with respect to the opening period tI with a speed of impact time equal to VI and the same energy dissipation.

[0059] The speed of forward or rearward movement of a brake piston of known type is, for example, equal to 1.2 mm / s.

[0060] The application of PWM control according to the invention makes it possible to reduce this speed, for example to a ratio between 1.1 and 100, preferably between 1.5 and 30, and even more preferably between 2 and 5, for example to a factor of 2 or 3 or 4.

[0061] According to another exemplary embodiment, PWM control of the motor is performed when the vehicle is mounted on a roller bench and the brakes are tested. PWM control allows the braking force to be changed more gradually and more slowly. In fact, the braking force depends on the closure of the brake, i.e. on the position of the pads relative to the disc. The way in which the braking force changes over time depends on the way the pads move and therefore on the speed of closure of the brake.

[0062] By finely controlling this change in speed, it is possible to have finer control over the change in braking force than in the prior art.

[0063] When the control unit is informed that the vehicle is mounted on a roller bench, it applies PWM control to the motor in order to increase the braking force more gently and progressively, thus reducing uncontrolled movement / vibration of the vehicle.

[0064] FIG. 3 shows an example of the variation of the braking force F as a function of time t for a braking system according to the invention (curves IV and V) and for a braking system of the prior art (curve VI).

[0065] Looking at curve VI, the force changes in steps: F1, F2, F3, switching almost instantly from one step to the other. The steps are pre-programmed. When the vehicle is on the roller bench, the brakes operate according to a specific program called the roller bench operating program. The activation of this program is obtained either manually by a control sequence or by detecting that the vehicle is attached to the roller bench, thereby performing a specific dynamic behavior.

[0066] Curve IV also shows a stepwise increase in speed: with PWM control according to the invention, the switch from one step to another is made gradually by varying the average supply voltage.

[0067] Curve V shows the gradual increase in braking force achieved by the present invention.

[0068] Thus, sources of vehicle vibration are reduced or even eliminated, reducing the risk of distorting the measurements.

[0069] In this application, the PWM control ensures that the average voltage is constant during a step and that it increases gradually when switching from one step to another.

[0070] The invention makes it possible to avoid false rejections of vehicles due to distorted measurements of the parking brake.

[0071] In practice, the normal operation of a parking brake is generally a static operation, i.e. the wheels are stopped and the brakes are applied.

[0072] However, to test the parking brake on a bench ramp, the ramp is dynamically simulated by rotating the wheels. However, if the electric motor is only controlled at full power, the braking system will detect wheel lock and release the brake. This will cause the parking brake to be considered as faulty.

[0073] According to the invention, the motor is controlled with pulse width modulation during the braking phase, which allows a gradual increase in the braking level: on the one hand, there is no risk of detecting wheel lock and releasing the brakes and thus falsely rejecting the vehicle, and on the other hand, the risk of brake damage is reduced.

[0074] According to another application example, PWM control of the motor is used during normal operation of the parking brake, which means the expected operation when a driver puts his vehicle in park, needs to stop the vehicle and applies the electric parking brake.

[0075] For example, the control unit may control the brakes so that over a first portion of the stroke the piston moves faster before the pads contact the disc, and over a second portion of the stroke the piston moves slower as the pads are compressed against the brake, reducing the power required. Although the period of parking brake application is slightly longer than in conventional systems, this period is generally short and the extended application phase is substantially imperceptible to the user.

[0076] A schematic diagram of this change with velocity V is shown in Figure 4. Curve VII shows a movement at a high velocity Ve followed by a movement at a lower velocity Vf during pad compression.

[0077] It will be appreciated that the velocity may be changed in more than two steps, or even the velocity may be decreased continuously during the compression stage, as shown by curve VIII.

[0078] According to one alternative, the speed may be decreased continuously.

[0079] According to one embodiment, the control unit applies PWM control during the entire operation of the parking brake.

[0080] According to another example, phase A is performed according to conventional control and phase B is performed according to PWM control.

[0081] According to another example, the electric parking brake may be controlled during normal operation as an all-or-nothing control or by PWM control depending on the situation.

[0082] Other situations may include single PWM control of the parking brake system, or a combination of PWM control and all-or-none control.

[0083] According to another example, PWM control may be applied when the parking brake is used as an emergency brake. Emergency brake control is achieved by depressing the brake button when driving at a significant speed, e.g., if the speed is above 5km / h, actuation of the brake button activates the emergency brake, and if the speed is zero or below 5km / h, the parking brake is applied.

[0084] The PWM control according to the invention may furthermore be performed outside the braking phase, whether during service braking, parking braking and / or emergency braking.

[0085] For example, the calculator moves the piston backwards to the rear abutment each time it needs to know the position of the disc brake piston. This is done, for example, after a pad change, but it can also be done periodically by the calculator of the electronic stability program (ESP®). The PWM control is applied, for example, each time the calculator triggers the stage of determining the piston position. To determine the piston position, the latter moves the piston backwards until it rests against the rear abutment, forming a reference position. From this reference position, the calculator can determine the next position of the piston, for example by counting the number of revolutions of the electric motor of the actuator. By implementing the PWM control, the piston speed is reduced when approaching the rear abutment, thus reducing the risk of damage without disturbing the stage of determining the piston position.

[0086] Advantageously, the PWM control is applied to any operating type of brake, be it parking brake or emergency brake, during and outside the braking phase.

[0087] The means for implementing the PWM control may be specific to the brakes implementing these controls and / or already implemented elsewhere, such as those implemented in a computer for controlling braking modulation, e.g. ESP® (Electronic Stability Program).

[0088] The invention is applicable to both hydraulic service brakes with parking and / or electric emergency brakes and fully electric, i.e. so-called EMB (electromechanical brake), service, parking and / or emergency brakes.

[0089] In one exemplary embodiment, the invention is applicable to an electric service brake with or without a parking brake, where the electric motor is controlled at least by pulse width modulation at some operating stages of the electric motor, e.g. for changing brake pads or for indicating a piston position.

[0090] Additionally, the present invention is applicable to disc brakes that include one or more pistons, each moved by its own actuator, the movement of each actuator being separately controllable. [Explanation of symbols]

[0091] 2 front axle 4 rear axle 6, 8 wheels 10, 12 Brake 14 Actuator 16 Electric motor S Electronic Parking Brake System UC Control Unit

Claims

1. A method for controlling an electric brake system including a control unit (UC) and at least two brakes (12) arranged on wheels (8) of a motor vehicle, each brake (12) including a brake actuator (14) actuated by an electric motor (16), said method comprising at least - during a second operating phase, when the operation of the electromechanical braking system is deemed to require a speed modulation of the electric motor, controlling the electric motor by pulse width modulation; - controlling said electric motor at full power during a first operating phase; Including, the second actuation step occurs after the first actuation step, and the first actuation step and the second actuation step occur during the same braking action, including application of the brakes and release of the brakes; and During the step of determining the position of at least one piston, the electric motor (16) is controlled in the brake release direction by pulse width modulation such that its rotational speed is slower than its rotational speed during a part of at least one application phase of the brake in normal operation, or When brake pads of a disc brake are replaced, the electric motor (16) is controlled in the brake release direction by pulse width modulation during the second operating phase so that its rotation speed is lower than its rotation speed during at least a part of one operating phase of the brake in normal operation, and when the brake pads are replaced, the electric motor (16) is controlled in the brake release direction with full power during the first operating phase. Control methods.

2. 2. The method of claim 1, wherein the electric motor (16) is controlled by pulse width modulation such that its rotational speed varies continuously and the brake application force varies continuously when the vehicle is mounted on a test bench.

3. 2. The method of claim 1, wherein the electric motor (16) is controlled by pulse width modulation such that its rotational speed changes in steps, switching progressively from one step to the other, and such that the brake application force changes continuously in steps, switching progressively from one step to the other, when the vehicle is mounted on a test bench.

4. 2. The method of claim 1, wherein the electric motor (16) is controlled by pulse width modulation such that a normal operating phase of the electric brake system includes a first portion at a first speed during a first operating phase and a second portion at a second speed during a second operating phase, the second portion corresponding to a brake lining compression phase, and the second speed being slower than the first speed.

5. The method of claim 1, wherein the brake (12) is a parking brake.

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