METHOD FOR CONTROLLING A BRAKE

Abstract

Method of controlling a motor vehicle brake using an estimated clamping force exerted by the brake, said value of the estimated clamping force being obtained from an electric current consumed by said brake, comprising: a) Monitoring the voltage across the brake terminals, b) Detecting a voltage variation likely to distort the estimated clamping force, c) Calculating a corrected estimated clamping force that at least partially eliminates the voltage variation.

Description

Title of the invention: METHOD FOR CONTROLLING A BRAKE TECHNICAL FIELD AND PREVIOUS ART

[0001] The present invention relates to a method of controlling a brake offering increased safety and to a braking system offering increased operational safety.

[0002] A motor vehicle is equipped with a brake at each wheel. This can be a disc brake or a drum brake.

[0003] The brake can be a hydraulic brake or an electromechanical brake designated EMB (“Electromechanical Brake” in Anglo-Saxon terminology).

[0004] In addition, parking brakes are increasingly electrically activated. For example, a screw-nut system operated by an electric motor causes the brake pads to be applied against the disc in the case of a disc brake, and the brake linings against the drum in the case of a drum brake.

[0005] The clamping force exerted by the brake pads or linings is not measured but estimated, notably from the value of the electrical current consumed by the electric motor. Such an estimation eliminates the need for a force sensor, thereby simplifying the braking system. The motor is interrupted when the clamping force reaches a predetermined threshold value.

[0006] This mode of operation is satisfactory.

[0007] The inventors observed that the estimation of the clamping force applied by the parking brake could be inaccurate, in particular could be significantly lower than the actual value and therefore lead to a premature shutdown of the electric motor of the parking brake.

[0008] This error in the estimation results from the measurement of the current drawn by the motor. Indeed, the inventors discovered that a rapid change in voltage in the vehicle's electrical system, particularly a voltage increase, caused a current spike. The electric brake controller interprets this current spike as a clamping force that exceeds the set threshold and commands the electric motor to stop. Thus, the electric brake is considered sufficiently engaged when it is not.

[0009] The voltage variation results from external phenomena that occur simultaneously with the activation of the parking brake. Such phenomena include, for example, the starting of the internal combustion engine, the switching off of the heater, and the disengagement of one of the parking brakes. Indeed, the two parking brakes are activated sequentially to avoid overloading the vehicle's electrical system; when the first parking brake is activated stops when it reaches its engaged state, the voltage increases in the on-board circuit which can affect the electrical current of the second parking brake.

[0010] The premature stopping of the electric motor of a parking brake results in a weaker clamping force, and therefore immobilization of the vehicle, particularly on a slope, which may be less effective. Description of the invention

[0011] It is therefore an object of the present invention to provide a method of controlling at least one brake offering improved operation, and to a braking system exhibiting improved operational safety.

[0012] The stated objective above is achieved by a method of controlling a brake comprising, during an actuation phase of said brake, a step of monitoring the variation of voltage at the level of at least one parking brake, and a step of taking into account this variation of voltage in the control of the brake.

[0013] The monitoring step includes, for example, determining a voltage gradient, comparing the gradient to a high value and establishing a disturbance period where appropriate, during which the clamping force estimation method is modified to at least partially compensate for the effect of the voltage variation on the clamping force estimation.

[0014] In one embodiment, the step of taking into account the voltage variation includes, during the disturbance period, the estimation of the clamping force by a method not using the current consumed by the parking brake.

[0015] In another embodiment, the step of taking into account the voltage variation includes, during the disturbance period, the estimation of the clamping force using the estimation of a clamping force gradient using at least one value prior to the start of the estimation period.

[0016] In the event of a sudden increase in tension, the clamping force may be overestimated, thus leading to premature interruption of the parking brake actuation. In the event of a sudden decrease in tension, the clamping force may be underestimated, which may lead to excessive clamping that could damage the brake.

[0017] Thanks to the invention, the risk of an estimation of the electric brake's clamping force leading to a decrease in the level of safety or damaging the brake is reduced, or even eliminated. The operational safety of the electric brake and, more generally, of the braking system is improved.

[0018] In other words, the electric brake control method determines a period in which the estimation of the clamping force may be distorted by a voltage variation, and takes corrective measures during this period.

[0019] The present invention then relates to a method of controlling an electric brake for a motor vehicle using an estimated clamping force exerted by the brake, said value of the estimated clamping force being obtained from an electric current consumed by said brake, comprising: a) Monitoring the voltage across the terminals of the brake, b) Detection of a voltage variation likely to distort the estimated clamping force, c) Calculation of a corrected estimated clamping force that eliminates at least part of the voltage variation, d) Use of the corrected estimated clamping force instead of the estimated clamping force for controlling the brake.

[0020] Preferably, during step b), the absolute value of a tension gradient IGradUI is compared to a high threshold value UH and if IGradUI > UH, then the estimated clamping force value is considered to be inaccurate.

[0021] For example, after step b), the absolute value of the voltage gradient IGradUI is compared to a low value UL. If IGradUI < UL for a given time t, the estimated clamping force is obtained from the electrical current consumed by the brake.

[0022] In one embodiment, step c) implements Hooke's law to determine the estimated clamping force, where the brake stiffness has been previously determined, for example, on a test bench.

[0023] In another embodiment, step c) implements Hooke's law to determine the estimated clamping force, where the brake stiffness is determined from the estimated clamping force before IGradUI > UH.

[0024] In another embodiment, step c) uses a clamping force gradient at a time tO determined when IGradUI <UH. Dans le cas où le frein comporte un moteur électrique, l’étape c) calcule un gradient de force de serrage à l’instant t et utilise également un rapport entre la vitesse de rotation du moteur électrique à l’instant t et la vitesse de rotation du moteur électrique à l’instant tO.

[0025] The present invention also relates to an electric brake microcontroller for a motor vehicle configured to control said brake, said microcontroller controlling the electric brake based on the clamping force, said microcontroller being configured to: - estimate the clamping force from the electrical current consumed by said brake,

[0026] - monitor the voltage across the brake terminals, - detect a voltage variation likely to distort the estimated clamping force, - calculate a corrected estimated clamping force that at least partially eliminates the voltage variation.

[0027] The present invention also relates to a braking system comprising at least an electric brake and a microcontroller according to the invention.

[0028] The brake advantageously comprises an electric motor.

[0029] The electric brake is, for example, a parking brake. BRIEF DESCRIPTION OF THE FIGURES

[0030] The following description will be better understood with the aid of the attached drawings, in which: - [Fig. 1] is a schematic representation of an example of a parking braking system to which the present invention can be applied, - [Fig.2] represents a flowchart of a method for controlling a parking brake according to the invention, - [Fig.3] is a representation of the variation in brake stiffness in kN / m measured on a test bench as a function of stroke in mm, - [Fig.4] is a graphical representation of the variation of tension as a function of time, the actual clamping force Fr, the estimated clamping force and the corrected estimated clamping force obtained according to a first example of a first embodiment, - [Fig.5] is a graphical representation of the sinusoidal variation of tension as a function of time, the actual clamping force Fr, the estimated clamping force and the corrected estimated clamping force obtained according to the first example of a first embodiment, - [Fig.6] is a graphical representation of the variation of tension as a function of time, the actual clamping force Fr, the estimated clamping force and the corrected estimated clamping force obtained according to a first example of a first embodiment, - [Fig.7] is a graphical representation of the sinusoidal variation of tension as a function of time, the actual clamping force Fr, the estimated clamping force and the corrected estimated clamping force obtained according to the first example of a first embodiment, - [Fig.8] is a graphical representation of the variation of tension as a function of time, of a gradient of actual clamping force Fr, of a gradient of the clamping force estimated according to a first example of a second embodiment, - [Fig. 9] is a graphical representation of the tension variation as a function of time, the actual clamping force Fr, the estimated clamping force, and the corrected estimated clamping force obtained from the estimated force gradient of [Fig. 8]. - [Fig. 10] is a graphical representation of the sinusoidal tension variation as a function of time, the actual clamping force Fr, the estimated clamping force, and... the corrected estimated clamping force obtained from the estimated force gradient of [Fig.8], - [Fig. 1 1] is a graphical representation of the variation of tension as a function of time, of a gradient of actual clamping force Fr, of a gradient of estimated clamping force and of a gradient of corrected estimated force obtained according to a second example of a second embodiment, - [Fig. 12] is a graphical representation of the variation of tension as a function of time, the actual clamping force Fr, the estimated clamping force and the corrected estimated clamping force obtained from the estimated force gradient of [Fig. 11], - [Fig. 13] is a graphical representation of the sinusoidal variation of tension as a function of time, the actual clamping force Fr, the estimated clamping force and the corrected estimated clamping force obtained from the estimated force gradient of [Fig.11]. DETAILED DESCRIPTION OF PRODUCTION METHODS

[0031] In [Fig.1], we can see a vehicle V, represented schematically, comprising a braking system S including brakes F equipping wheels.

[0032] Service braking is provided either by hydraulic brakes or by electric brakes.

[0033] The braking system also includes a parking braking device comprising at least a first parking brake FP1 at the left rear wheel and a second parking brake FP2 at the right rear wheel.

[0034] The FP1 and FP2 parking brakes are electric parking brakes.

[0035] Advantageously the parking brake is integrated into the service brake.

[0036] Each electric parking brake includes an actuator equipped with an electric motor and means for converting the rotational movement of the electric motor into a translational movement applying the brake pads against the brake disc or the brake linings against the drum.

[0037] The braking system advantageously includes a control unit for the anti-lock braking system (ABS) and / or the vehicle stability system (ESP).

[0038] The braking system includes an electronic control unit or ECU (Electronic control unit in Anglo-Saxon terminology), incorporating software, also referred to as a microcontroller MC, for controlling the parking brakes FP1 and FP2. The parking brakes are controlled, for example, by activating a button B located in the passenger compartment.

[0039] The motor vehicle includes an electrical circuit, also referred to as the on-board circuit, comprising at least one electric battery, and to which various electrically consuming systems are connected. These consuming systems are, for example, a temperature control system in the vehicle, the internal combustion engine starting system, and the two electric brakes FP1, FP2.

[0040] The microcontroller includes estimation means 2 for estimating the clamping force exerted by each of the electric brakes.

[0041] In the description that follows, we will describe the control of an electric brake FP1 by the microcontroller, but it will be understood that the microcontroller controls both electric brakes in a similar manner.

[0042] The estimation means 2 are configured to estimate the clamping force applied by the brake FP1 from the current consumed by the electric motor of the brake FPL

[0043] According to the invention, the microcontroller is configured to: - Detect a voltage variation likely to generate a current spike that would distort the estimation of the clamping force, - Establish a correction period during which the clamping force is corrected relative to the estimated force; this value is designated as the corrected estimated force. - Determine the estimated corrected clamping force during this correction period.

[0044] To this end, the microcontroller is associated with detection means 4 configured to determine a voltage variation or a voltage gradient across the parking brake terminals, and to detect whether the voltage variation is likely to cause a current spike in the electric motor of the FPL motor

[0045] This determination and detection takes place during a phase of application of the parking brake, in particular at the end of the phase of application of the parking brake as will be explained below.

[0046] The voltage used is either the voltage applied to the terminals of the parking brake's electric motor or the microcontroller's supply voltage. These two voltages exhibit the same variations. They will be referred to hereafter as the "voltage across the parking brake terminals".

[0047] Determining the voltage variation involves calculating a voltage gradient GradU, for example in V / s. This calculation takes place, for example, every 10 ms from the voltage values ​​across the brake terminals provided by the control unit.

[0048] We seek to detect either sudden increases in tension which can cause an overestimation of the clamping force, or sudden decreases in tension which can cause an underestimation of the clamping force.

[0049] To this end, the absolute value of the gradient GradU IGradUI is compared to a high threshold value UH beyond which the voltage variation is considered likely to cause a current peak. For example, UH is equal to 20 V / s. When the gradient IGradUI <UH, le microcontrôleur passe en phase de correction Te, lorsque la valeur de Te augmente la correction est activée.

[0050] The control unit then establishes a correction period Te of the estimation of the clamping force.

[0051] If IGradUI falls below a low threshold value UL, it is assumed that the voltage variation is unlikely to cause a current spike. Preferably, it is verified that IGradUI remains below UL for a sufficient time to ensure that a current spike is no longer likely to occur. If IGradUI < UL for a given time t, for example, a few tens of milliseconds, for example, 20 ms, the correction period is considered complete, and the clamping force can be estimated conventionally by means 2.

[0052] Furthermore, the microcontroller is configured to provide a corrected estimated clamping force value.

[0053] The method for controlling the parking brake by the microcontroller is represented by the flowchart in [Fig. 2].

[0054] A command to activate the parking brake is issued.

[0055] During a step 100, the voltage gradient IGradUI across the terminals of the parking brake is determined.

[0056] During step 200, it is checked whether IGradUI>UH; if the answer is negative N, it proceeds to step 300 in which the clamping force is estimated from the current consumed by the parking brake motor. If the answer is positive Y, it proceeds to step 600.

[0057] Following step 300, we proceed to step 400, where we check if the clamping force has reached a clamping force value above which we consider that the parking brake ensures secure immobilization. If the answer is positive (Y), we proceed to step 500, during which a stop command for the electric motor is issued. If the answer is negative (N), we return to step 100.

[0058] In step 600, the correction phase Te begins, during which a corrected estimated clamping force is calculated. In step 700, it is checked whether the corrected estimated clamping force has reached a clamping force value above which the parking brake is considered to provide secure immobilization. If the answer is positive Y, the process proceeds to step 800, during which a stop command for the electric motor is issued. If the answer is negative N, the process proceeds to step 900, during which the voltage gradient GradU across the parking brake terminals is measured.

[0059] During step 1000, we check if IGradUI <UL et si IGradUI reste inférieur à UL pendant un temps t donné. Si la réponse est positive Y, on passe à l’étape 300, si la réponse est négative N, on retourne à l’étape 600.

[0060] According to a first embodiment, the microcontroller is configured to estimate the clamping force at time t without using the current consumed by the motor at time t.

[0061] In this first embodiment, the microcontroller uses a spring model to estimate the clamping force. To do this, it estimates the increase in clamping force AFc using Hooke's law, modeling the brake as a spring.

[0062] Hooke's law is written as follows

[0063] [Math.l] AF = kx AStr (Q

[0064] With k the spring constant of the brake or stiffness, and

[0065] AStr is the stroke of the moving element, for example the piston in a disc brake, which is estimated from the motor's rotation. The motor speed can be measured using a sensor or estimated by measuring voltage and current, and estimating the motor parameters R: resistance, K: motor constant, and L: motor inductance. Advantageously, the motor speed is estimated, which eliminates the need for a sensor and thus simplifies the brake.

[0066] In a first embodiment, the value of k is determined beforehand, for example on a test bench, and is stored in the microcontroller. In one example, k is a curve stored in the microcontroller. Indeed, k varies according to the deformation of the brake, and the value of k is selected according to the stroke length. Figure 3 shows an example of the variation of the stiffness k in kN / mm as a function of the stroke in mm, this variation being obtained on a test bench. In another example, k is a constant; the value of the constant is preferably chosen so as to avoid overestimating the clamping force. For example, k is chosen to be 30 kN / mm.

[0067] Figure 4 shows a graphical representation of the variation of the voltage U across the brake terminals as a function of time in seconds, the estimated value Fe of the clamping force in Newtons as a function of time in seconds, and the corrected estimated value Fec of the clamping force in Newtons as a function of time in seconds. The actual value Fr of the clamping force, for example measured by means of a sensor, is also shown. The actual value is obtained, for example, using a test bench.

[0068] We observe the appearance of a strong variation in the estimated clamping force Fe following the variation in tension, IgradUI is then greater than UH.

[0069] Thanks to the invention, the estimated clamping force value Fec, corrected by Hooke's law using a constant stiffness, is relatively close to Fr and is virtually unaffected by tension variations. When a large tension variation occurs, the actual clamping force is less than Fe and greater than Fec; thus, by using the force Fec to control the parking brake, a safety margin is ensured.

[0070] Figure 5 shows another graphical representation of the corrected clamping force estimate for a sinusoidal voltage U. It can be seen that the corrected estimated clamping force is lower than the actual clamping force, thus making the parking brake operation safer.

[0071] In a second embodiment according to the first embodiment, the value of the stiffness k used in the model is not a constant but is calculated in real time using an estimated value of the variation of the clamping force and the stroke.

[0072] Indeed, from formula (I)

[0073] [Math.2] k = AF / AStr

[0074] As long as IGradUk UH, the clamping force is estimated from the current consumed and the stroke is estimated from the motor rotation. AFe can then be calculated as long as IGradUI < UH, which allows a value of k to be calculated. For example, a value is calculated and recorded every calculation cycle, i.e., every 10 ms.

[0075] As soon as IGradUI > UH, the last calculated value of k designated k' is stored and is used to calculate the estimated clamping force corrected from formula (I).

[0076] [Math.3] AFer' = U x AStr (I)

[0077] Figure 6 shows the graphical representation of the corrected estimated clamping force Fec' as a function of time in the case of a step voltage variation. When the large voltage variation occurs, the actual clamping force is less than Fe and greater than Fec'. Thus, by using the force Fec' to control the parking brake, a safety margin is ensured.

[0078] On [Fig.7], we can see the graphical representation of the estimated corrected clamping force Fec' as a function of time in the case of a variation of the tension in the form of a sinusoid.

[0079] According to another embodiment, the microcontroller is configured to calculate the estimated clamping force corrected from an estimated clamping force gradient GradFe(t) at time t.

[0080] The gradient GradFe(t) is calculated for example at all calculation cycles, i.e. every 10ms from the values ​​of clamping forces estimated at all calculation cycles, i.e. every 10ms.

[0081] As soon as IGradUI>UH, the value of GradFe is fixed to the last calculated value designated GradFe(tO).

[0082] By having the value at GradFec(tO) it is possible to go back to the estimated corrected clamping force Fec(t).

[0083] Figure 8 shows the variation of GradFe(t), GradFe(t0), GradFr(t), and the voltage U across the motor terminals. It can be seen that the value of GradFe(t0) takes the value of GradFe from the previous cycle when voltage variation detection is activated. It should be noted that in this representation, the value of GradFe(t0) does not follow the value of GradFe(t) when IGradUlcUH is used for the purpose of comparing GradFe(t0) with GradFr.

[0084] It is observed that the value of GradFec(t) is close to the real value compared to the estimated value when GradU>UH.

[0085] On [Fig.9], we can see the variation of Fe(t), of Fec(t) calculated from GradFe(to), of Fr(t) and of the voltage U across the terminals of the motor.

[0086] On [Fig. 10], we can see the variation of Fe(t), of Fec(t) calculated from GradFe(to), of Fr(t) when the voltage U varies sinusoidally.

[0087] According to another embodiment of the second embodiment, the estimated clamping force gradient is used, which is corrected for t>t0 by the following formula:

[0088] [Math.4] G ra d Fec(t) •=-Gra d Fe ( tO) -*■ w (t) / -w (tO J ■ ( Il}

[0089] With co(t) the rotational speed of the motor at time t.

[0090] It should be noted that the rotational speed of the motor co(t) is estimated using the formula

[0091] [Math.5]

[0092] with U(t) the voltage across the motor terminals, R the resistance of the motor and i(t) the electric current consumed by the motor at time t and K the constant of the motor.

[0093] But, due to the subtraction U(t) - Rxi(t), the influence of the voltage variation is substantially reduced.

[0094] From the knowledge GradFec(t) it is possible to go back to the estimated corrected clamping force Fec(t).

[0095] Figure 11 shows the variation of GradFe(t), GradFec(t), the measured Gradf(t), and the voltage across the motor terminals. It can be seen that the value of GradFe(tO) takes the value of GradFe from the previous cycle when voltage variation detection is activated. It should be noted that in this representation, the value of GradFe(tO) does not follow the value of GradFe(t) when IGradUlcUH is used for comparison of GradFe(tO) with GradFr.

[0096] It is observed that the value of GradFec(t) is close to the real value compared to the estimated value when IGradUI>UH.

[0097] In [Fig. 12], we can see the variation of Fe(t), of Fec(t) calculated from GradFec(t), of Fr(t) and of the voltage U across the motor terminals.

[0098] In [Fig. 13], we can see the variation of Fe(t), of Fec(t) calculated from GradFec(t), of Fr(t) when the voltage U varies sinusoidally.

[0099] The implementation of the corrected estimated clamping force determination is all the more effective when the voltage variation occurs close to the end of the parking brake actuation phase. Indeed, if the voltage variation occurs at the beginning of the actuation phase, it is unlikely that the current peak will result in an estimated clamping force exceeding the threshold clamping force required to stop the parking brake actuation. Conversely, if the voltage variation occurs at the end of the braking phase, the estimated clamping force will be high, and the resulting overestimation due to the current peak is more likely to cause the motor to stop prematurely.

[0100] In the examples described in detail above, the invention makes it possible to reduce the risks of an overestimated clamping force. The present invention also applies to an underestimated clamping force, thereby reducing the risk of brake damage.

[0101] The present invention applies to the control of electric service and parking brakes, more generally to the control of any brake in which the control of the braking force uses the estimation of the clamping force. References

[0102] 2: means for estimating the clamping force 4: means of detecting a voltage variation 100, 200, 300, 600, 400, 500, 700, 800, 900, 1000: Control method steps; S: Braking system; F: Brake FP1: First parking brake FP2: Second parking brake B: Button GradU: voltage gradient GradU: absolute value of voltage gradient; IGradUI: absolute value of voltage gradient; UH: high threshold value; TC: correction phase; UL: low threshold value N: negative answer Y: positive response Fe: estimated clamping force Fec: estimated corrected clamping force Fec': estimated corrected clamping force Fr: actual clamping force R: engine V: vehicle

Claims

Demands

1. A method for controlling an electric brake for a motor vehicle using an estimated clamping force exerted by the brake, said value of the estimated clamping force being obtained from an electric current consumed by said brake, comprising: a) Monitoring the voltage across the brake terminals, b) Detecting a voltage variation likely to distort the estimated clamping force, c) Calculating a corrected estimated clamping force that is at least partially independent of the voltage variation, d) Using the corrected estimated clamping force instead of the estimated clamping force for controlling the brake, wherein in step b), the absolute value of a voltage gradient IGradUI is compared to a high threshold value UH, if 1 GradUI > UH, then the value of the estimated clamping force is considered to be distorted.

2. A control method according to claim 1, wherein after step b) the absolute value of the voltage gradient IGradUI is compared to a low value UL, if IGradUI < UL for a given time t, the value of the estimated clamping force is obtained from an electrical current consumed by said brake.

3. A control method according to claim 1 or 2, wherein step c) implements Hooke's law to determine the estimated clamping force and wherein the brake stiffness has been determined beforehand, for example on a test bench.

4. A control method according to claim 1 or 2, wherein step c) implements Hooke's law to determine the estimated clamping force and wherein the brake stiffness is determined from the estimated clamping force before IGradUI > UL.

5. Control method according to any one of claims 1 or 2, wherein step c) uses a clamping force gradient at a determined time tO when IGradUlcUH.

6. Control method according to claim 5, the brake comprising an electric motor, wherein step c) calculates a clamping force gradient at time t and also uses a ratio between the rotational speed of the electric motor at time t and the rotational speed of the electric motor at time t0.

7. Motor vehicle electric brake microcontroller configured to control said brake, said microcontroller controlling the electric brake from the clamping force, said microcontroller being configured to: - estimate the clamping force from an electric current consumed by said brake, - monitor the voltage across the brake terminals, - detect a voltage variation likely to distort the estimated clamping force, - calculate a corrected estimated clamping force at least partially free from the voltage variation.

8. Braking system comprising at least one electric brake and a microcontroller according to claim 7.

9. Braking system according to the preceding claim, wherein the brake is a brake comprising an electric motor.

10. Braking system according to claim 8 or 9, wherein the electric brake is a parking brake.

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