Method for controlling a brake

Abstract

The invention relates to a method for controlling a brake for a motor vehicle using an estimated clamping force exerted by the brake, the value of the estimated clamping force being obtained from an electric current consumed by the brake, the method comprising: a) monitoring the voltage at the terminals of the brake, b) detecting a voltage variation likely to falsify the estimated clamping force, and c) calculating a corrected estimated clamping force by at least partially eliminating the voltage variation.

Description

[0001] control of a brake offering increased safety and a braking system offering increased operational safety.

[0002] A motor vehicle is equipped with a brake on 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 English 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 pressed 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 pads or linings is not measured but estimated, in particular, from the value of the electric current consumed by the electric motor. Such an estimation makes it possible to avoid using a force sensor, which in particular simplifies the braking system. The motor activation is interrupted when the clamping force has reached a previously set 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 distorted, in particular could be significantly lower than the actual value and therefore result in the electric motor of the parking brake stopping prematurely.

[0008] This estimation error results from the measurement of the current consumed by the motor. In fact, the inventors discovered that a rapid voltage variation in the vehicle's electrical system, particularly an increase in voltage, caused a current spike to appear. The electric brake controller translates this current spike into a clamping force that is greater than the set threshold and orders the electric motor to stop. Thus, the electric brake is considered sufficiently applied when this is not the case.

[0009] The voltage variation results from external phenomena that occur simultaneously with the actuation of the parking brake. Such phenomena include, for example, the starting of the combustion engine, the switching off of the heating and the switching off of one of the parking brakes. In fact, the two parking brakes are activated in a delayed manner to avoid overstressing the on-board network; when the first activated parking brake stops when it reaches its applied state, the voltage increases in the on-board circuit which can influence the electrical current of the second parking brake.

[0010] The premature stopping of the electric motor of a parking brake results in a lower application force, and therefore immobilization of the vehicle, particularly on slopes, which may be less effective.

[0011] STATEMENT OF THE INVENTION

[0012] It is therefore an object of the present invention to provide a method of controlling at least one brake offering improved operation, and a braking system having improved operational safety.

[0013] The aim stated above is achieved by a method of controlling a brake comprising, during an actuation phase of said brake, a step of monitoring the voltage variation at the level of at least one parking brake, and a step of taking this voltage variation into account in the control of the brake.

[0014] The monitoring step comprises, for example, determining a voltage gradient, comparing the gradient with a high value and establishing a disturbance period where appropriate, during which the method for estimating the clamping force is modified to at least partially offset the effect of the voltage variation on the estimation of the clamping force.

[0015] In an exemplary embodiment, the step of taking into account the voltage variation comprises, during the disturbance period, the estimation of the clamping force by a method not using the current consumed by the parking brake. In another exemplary embodiment, the step of taking into account the voltage variation comprises, 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 and thus lead to premature interruption of the parking brake actuation. In the event of a sudden decrease in tension, the clamping force may be underestimated, which can lead to excessive clamping, which can damage the brake.

[0017] Thanks to the invention, the risk of an estimate of the application force of the electric brake which could lead to a reduction in the safety level or which could damage 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 clamping force estimate may be distorted by a voltage variation, and takes corrective action during this period.

[0019] The present invention then relates to 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 at 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 at least partly freeing itself from 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 voltage gradient IGradUI is compared to a high threshold value UH and if IGradUI > UH, then it is considered that the value of the estimated clamping force is distorted. 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 value of the estimated clamping force is obtained from an electric current consumed by said brake.

[0021] In an exemplary embodiment, step c) implements Hooke's law to determine the estimated clamping force and in which the stiffness of the brake has been determined beforehand, for example on a test bench.

[0022] In another exemplary embodiment, 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>UH.

[0023] In another exemplary 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.

[0024] 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 from the clamping force, said microcontroller being configured to:

[0025] - estimate the clamping force from an electric current consumed by said brake,

[0026] - monitor the voltage at the brake terminals,

[0027] - detect a variation in voltage likely to distort the estimated clamping force,

[0028] - calculate a corrected estimated clamping force which at least partially frees itself from the tension variation.

[0029] The present invention also relates to a braking system comprising at least one electric brake and a microcontroller according to the invention.

[0030] The brake advantageously includes an electric motor.

[0031] The electric brake is for example a parking brake. BRIEF DESCRIPTION OF THE FIGURES

[0032] The following description will be better understood with the help of the attached drawings, in which:

[0033] - Figure 1 is a schematic representation of an example of a parking braking system to which the present invention can be applied,

[0034] - Figure 2 represents a flowchart of a method for controlling a parking brake according to the invention,

[0035] - Figure 3 is a representation of the variation of the stiffness of a brake in kN / m measured on a test bench as a function of the stroke in mm,

[0036] - Figure 4 is a graphical representation of the variation of tension as a function of time, of the actual clamping force Fr, of the estimated clamping force and of the corrected estimated clamping force obtained according to a first example of a first embodiment,

[0037] - Figure 5 is a graphical representation of the sinusoidal variation of voltage as a function of time, of the actual clamping force Fr, of the estimated clamping force and of the corrected estimated clamping force obtained according to the first example of a first embodiment,

[0038] - Figure 6 is a graphical representation of the variation of tension as a function of time, of the actual clamping force Fr, of the estimated clamping force and of the corrected estimated clamping force obtained according to a first example of a first embodiment,

[0039] - Figure 7 is a graphical representation of the sinusoidal variation of voltage as a function of time, of the actual clamping force Fr, of the estimated clamping force and of the corrected estimated clamping force obtained according to the first example of a first embodiment,

[0040] - Figure 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,

[0041] - Figure 9 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 Figure 8,

[0042] - Figure 10 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 Figure 8,

[0043] - Figure 11 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 estimated clamping force and of a gradient of corrected estimated force obtained according to a second example of a second embodiment,

[0044] - Figure 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 Figure 11,

[0045] - Figure 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 Figure 11.

[0046] DETAILED DESCRIPTION OF EMBODIMENTS

[0047] In Figure 1, we can see a vehicle V, represented schematically, comprising a braking system S comprising brakes F fitted to the wheels.

[0048] Service braking is provided either by hydraulic brakes or by electric brakes.

[0049] The braking system also comprises a parking brake 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.

[0050] The FP1 and FP2 parking brakes are electric parking brakes.

[0051] Advantageously, the parking brake is integrated into the service brake.

[0052] Each electric parking brake comprises 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.

[0053] The braking system advantageously includes a computer for the anti-lock braking system (ABS) and / or the vehicle stability system (ESP).

[0054] The braking system includes an electronic control unit (ECU), which integrates software, also known as a microcontroller (MC), for controlling the FP1 and FP2 parking brakes. The parking brakes are controlled, for example, by pressing a button B located in the passenger compartment.

[0055] The motor vehicle has an electrical circuit, also called an on-board circuit, comprising at least one electric battery, and to which various electricity-consuming systems are connected. Consumer systems include, for example, a temperature control system in the vehicle, the starting system for the combustion engine, and the two electric brakes FP1, FP2.

[0056] The microcontroller includes estimation means 2 for estimating the clamping force exerted by each of the electric brakes.

[0057] In the following description, 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 way.

[0058] 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 FP1.

[0059] According to the invention, the microcontroller is configured to:

[0060] - Detect a voltage variation likely to generate a current peak distorting the estimation of the clamping force,

[0061] - Establish a correction period during which the clamping force is corrected in relation to the estimated force, this value is designated the corrected estimated force.

[0062] - Determine the estimated corrected clamping force during this correction period. For this purpose, the microcontroller is associated with detection means 4 configured to determine a voltage variation or a voltage gradient at the terminals of the parking brake, and to detect whether the voltage variation is likely to cause the appearance of a current peak at the electric motor of the motor FP1.

[0063] This determination and detection take place during a parking brake application phase, in particular at the end of the parking brake application phase as will be explained below.

[0064] 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 vary the same way. They will subsequently be referred to as "parking brake terminal voltage."

[0065] 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 ​​at the brake terminals provided by the control unit.

[0066] The aim is to detect either sudden increases in tension which may cause an overestimation of the clamping force, or sudden decreases in tension which may cause an underestimation of the clamping force.

[0067] To do this, the absolute value of the gradient GradU | GradU | is compared to a high threshold value UH beyond which it is considered that the voltage variation is likely to cause the appearance of a current peak. For example UH is equal to 20 V / s. When the gradient | GradU | <UH, le microcontrôleur passe en phase de correction Te, lorsque la valeur de Te augmente la correction est activée.

[0068] The control unit then establishes a correction period Te for the estimation of the clamping force.

[0069] If IGradUI becomes lower than a low threshold value UL, it is considered that the voltage variation is not likely to cause the appearance of a current peak. Preferably, it is checked that | GradU | remains lower than UL long enough to ensure that a current peak is no longer likely to appear. If | GradU | < UL for a given time t, for example a few tens of ms, for example 20 ms, it is considered that the correction period is over and the clamping force can be estimated in a conventional manner by means 2.

[0070] Additionally the microcontroller is configured to provide a corrected estimated clamping force value.

[0071] The process of controlling the parking brake by the microcontroller is represented by the flowchart in Figure 2.

[0072] A parking brake activation command is issued.

[0073] In a step 100, the voltage gradient IGradUI across the terminals of the parking brake is determined.

[0074] In step 200, we check whether | GradU | >UH, if the answer is negative N, we go to step 300 during which we estimate the clamping force from the current consumed by the parking brake motor. If the answer is positive Y we go to step 600.

[0075] Following step 300, we proceed to step 400, we check whether the clamping force has reached a clamping force value from which we consider that the parking brake ensures safe immobilization. If the response is positive Y, we proceed to step 500 during which an order to stop the electric motor is issued. If the response is negative N, we return to step 100.

[0076] 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 from which it is considered that the parking brake ensures safe immobilization. If the response is positive Y, we proceed to step 800, during which an order to stop the electric motor is issued. If the response is negative N, we proceed to step 900, during which the voltage gradient GradU across the terminals of the parking brake is measured.

[0077] At step 1000, we check if IGradUI <UL et si IGradUI reste inférieur à

[0078] UL for a given time t. If the response is positive Y, we go to step 300, if the response is negative N, we return to step 600. 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.

[0079] In this first embodiment, the microcontroller uses a so-called spring model to estimate the clamping force. To do this, it estimates the increase in the clamping force AFc using Hooke's law by treating the brake as a spring.

[0080] Hooke's law is written as follows

[0081] AF = kx AStr (l)

[0082] With k the brake spring constant or stiffness, and

[0083] AStr the stroke of the moving element, for example the piston in a disc brake, which is estimated from the rotation of the motor. The motor speed can be measured by means of a sensor or estimated using voltage and current measurements, and the estimation of 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.

[0084] In a first exemplary embodiment, the value of k is determined beforehand, for example on a test bench, and is recorded 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 value of the stroke. In Figure 3, we can see 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 equal to 30 kN / mm.

[0085] Figure 4 shows a graphical representation of the voltage variation U across the brake terminals in V as a function of time in seconds, the estimated value Fe of the clamping force in N as a function of time in seconds, and the corrected estimated value Fee of the clamping force in N 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.

[0086] 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.

[0087] Thanks to the invention, the estimated clamping force value Fee corrected by Hooke's law using a constant stiffness is relatively close to Fr and is almost not disturbed by the tension variation. When the strong tension variation occurs, the actual clamping force is less than Fe and greater than Fee, so by using the force Fee for the parking brake control a safety margin is ensured.

[0088] Figure 5 shows another graphical representation of the corrected estimate of the clamping force in the case of a voltage U varying in the form of a sinusoid. It can be seen that the corrected estimated clamping force is lower than the actual clamping force, which makes the parking brake control more reliable.

[0089] In a second exemplary 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 the estimated value of the variation in the clamping force and the stroke.

[0090] Indeed, from the formula (I) k = AF / AStr

[0091] As long as | GradU | < UH, the clamping force is estimated from the value of the current consumed and the stroke is estimated from the motor rotation. AFe can then be calculated as long as | GradU | < 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.

[0092] As soon as | GradU | > UH, the last calculated k value designated k' is stored and is used to calculate the corrected estimated clamping force from formula (I).

[0093] AFer' = k' x AStr (I) Figure 6 shows the graphical representation of the corrected estimated clamping force Fee' as a function of time in the case of a step variation in voltage. When the strong voltage variation occurs, the actual clamping force is less than Fe and greater than Fee'. Thus, by using the force Fee' for the parking brake control, a safety margin is ensured.

[0094] Figure 7 shows the graphical representation of the corrected estimated clamping force Fee' as a function of time in the case of a variation of the voltage in the form of a sinusoid.

[0095] According to another embodiment, the microcontroller is configured to calculate the corrected estimated clamping force from an estimated clamping force gradient GradFe(t) at time t.

[0096] The gradient GradFe(t) is calculated for example at every calculation cycle, i.e. every 10ms from the clamping force values ​​estimated at every calculation cycle, i.e. every 10ms.

[0097] As soon as | GradU | >UH, the value of GradFe is set to the last calculated value designated GradFe(tO).

[0098] By having the value at GradFec(tO) it is possible to go back to the corrected estimated clamping force Fec(t).

[0099] In Figure 8, we can see the variation of GradFe(t); GradFe(tO), of GradFr(t) and the voltage U at the motor terminals. We can see that the value of GradFe(tO) takes the value of GradFe from the previous cycle when the voltage variation detection is activated. It should be noted that in this representation the value of GadFe(tO) does not follow the value of GradFe(t) when | GradU | <UH à des fins de comparaison de GradFe(tO) avec GradFr.

[0100] We observe that the value of GradFec(t) is close to the real value compared to the estimated value when GradU>UH.

[0101] In Figure 9, we can see the variation of Fe(t), Fec(t) calculated from GradFe(to), Fr(t) and the voltage U at the motor terminals.

[0102] In Figure 10, we can see the variation of Fe(t), of Fec(t) calculated from

[0103] GradFe(to), of Fr(t) when the voltage U varies sinusoidally. According to another exemplary embodiment according to the second embodiment, the estimated clamping force gradient is used which is corrected for t>tO by the following formula:

[0104] GradFec(t) = GradFe(tO) x uj(t) / oo(tO) (II)

[0105] With ùü(t) the rotation speed of the motor at time t.

[0106] It should be noted that the rotation speed of the motor oo(t) is estimated using the formula u)(t) = (U(t) - Rxi(t)) / K (III) with U(t) the voltage at 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 motor constant.

[0107] But, due to the subtraction U(t) - Rxi(t), the influence of the voltage variation is significantly reduced.

[0108] From the knowledge GradFec(t) it is possible to go back to the corrected estimated clamping force Fec(t).

[0109] In Figure 11, we can see the variation of GradFe(t); GradFec(t), of Gradf(t) measured and the voltage at the motor terminals. We can see that the value of GradFe(tO) takes the value of GradFe of the previous cycle when the voltage variation detection is activated. It should be noted that in this representation the value of GadFe(tO) does not follow the value of GradFe(t) when IGradUI <UH à des fins de comparaison de GradFe(tO) avec GradFr.

[0110] We observe that the value of GradFec(t) is close to the real value compared to the estimated value when | GradU | >UH.

[0111] In Figure 12, we can see the variation of Fe(t), Fec(t) calculated from GradFec(t), Fr(t) and the voltage U at the motor terminals.

[0112] In Figure 13, we can see the variation of Fe(t), Fec(t) calculated from GradFec(t), Fr(t) when the voltage U varies sinusoidally.

[0113] The implementation of the determination of the corrected estimated clamping force is all the more effective when the voltage variation appears close to the end of the parking brake actuation phase. Indeed, if the voltage variation appears at the beginning of the actuation phase, it is unlikely that the current peak will result in an estimate of the clamping force greater than the threshold clamping force to stop the parking brake actuation. On the contrary, if the voltage variation appears at the end of the braking phase, the estimated clamping force will be high and the overestimation resulting from the current peak is more likely to generate a premature motor stop.

[0114] 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, which makes it possible to reduce the risks of damage to the brake. 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.

[0115] REFERENCES

[0116] 2: means of estimating the clamping force

[0117] 4: means of detecting a voltage variation

[0118] 100, 200, 300, 600, 400, 500, 700, 800, 900, 1000: Ordering method steps

[0119] 5: braking system

[0120] F: brake

[0121] FP1: first parking brake

[0122] P2: second parking brake

[0123] B: button

[0124] GradU: voltage gradient

[0125] |GradU | : absolute value of voltage gradient

[0126] UH: high threshold value

[0127] TC: correction phase

[0128] UL: low threshold value

[0129] N: negative response

[0130] Y: positive response

[0131] Fe: estimated clamping force

[0132] Fee: Corrected estimated clamping force

[0133] Fee': corrected estimated clamping force

[0134] Fr: actual clamping force

[0135] A: engine

[0136] V: vehicle

Claims

CLAIMS 1. Method for controlling an electric brake for a motor vehicle using an estimated clamping force exerted by the brake, said estimated clamping force value being obtained from an electric current consumed by said brake, comprising: a) Monitoring the voltage at the terminals of the brake, b) Detecting a voltage variation likely to distort the estimated clamping force, c) Calculating a corrected estimated clamping force at least partly freeing itself from the voltage variation, d) Using the corrected estimated clamping force instead of the estimated clamping force for controlling the brake, wherein during step b), the absolute value of a voltage gradient IGradUI is compared to a high threshold value UH, if | GradU | > UH, then it is considered that the estimated clamping force value is distorted.

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

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

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

5. Control method according to one of claims 1 or 2, in which step c) uses a clamping force gradient at a time t0 determined when | GradU | <uh.

6. Control method according to claim 5, the brake comprising an electric motor, in which 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. Electric brake microcontroller for a motor vehicle 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 at the brake terminals, - detect a variation in voltage likely to distort the estimated clamping force, - calculate a corrected estimated clamping force which at least partially frees itself from the tension 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, in which the brake is a brake comprising an electric motor.

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

Images