FR2855611A1 - Electromechanical brake caliper application force estimating method for e.g. lorry, involves detecting switching of switch around threshold, and estimating application force from motor shaft displacement using calibration relation - Google Patents

Abstract

The method involves detecting switching of an application force switch around a threshold, and acquiring one rotation angle as a reference angle. A relative displacement of a motor shaft is calculated as a difference between the reference angle and another rotation angle, by a controller (50). An application force is estimated from the displacement, using calibration relation linking the force and displacement. The switch is in a lower state when the application force is lower to the threshold, and is in an upper state when the application force is higher than that of the threshold. Independent claims are also included for the following: (a) a device for implementing a method of estimating force applied on an electromechanical brake caliper (b) a motor vehicle having a hybrid braking system constituted of multiple hydraulic calipers.

Description

i

  The present invention relates to electromechanical brake calipers intended to equip vehicles such as automobiles, trucks or the equivalent. More particularly, the invention relates to a method for estimating the clamping force applied to the brake disc using a switch sensitive to the clamping force. The present invention also relates to a device for implementing this method.

  In order to know the clamping force applied by pads on the brake disc, it is known to provide an electromechanical brake caliper 10 with a continuous clamping force sensor.

  The electromechanical stirrup can be in different actuation modes: - a rest mode in which the plates are in a rest position, placed at a predefined distance from the disc so that the stirrup responds quickly to an actuation ; - a clamping mode, during which the stirrup is actuated so that the plates move towards one another and come into contact with the disc, to apply a clamping force; and, - a loosening mode, during which the stirrup is actuated so that the plates move one away from the other and leave contact with the disc, so as not to apply any more clamping force.

  The function of the continuous clamping force sensor is then to supply the brake caliper control system with information on the clamping force actually exerted by the brake pads on the brake disc. This information allows the control system to perform three types of operation: a first operation consists, in the application mode, of regulating the application force actually exerted by the brake pads in order to reach the target application force required when activating the caliper; - A second operation consists, during the release mode and when the measured clamping force decreases to cross a predefined level, to initiate a procedure for repositioning the 35 brake pads with respect to the disc at the predefined distance; - A third operation consists in allowing the implementation of a procedure for checking and diagnosing caliper faults.

  The continuous clamping force sensors currently used have a large number of drawbacks.

  These are first of all new sensors which are difficult to produce, to maintain in working order and which have a high cost. In addition, such sensors must be able to be used over a wide clamping force range since they must make it possible to measure forces going from a few newtons to several tens of kilo newtons.

  High quality requirements are required on the measurement signal generated by these sensors. For example, the continuous clamping force sensors must be able to measure the clamping force with sufficient precision to allow the management of the wheel slip by an anti-lock braking system (ABS). However, the continuous clamping force sensors currently used produce, among other things, a highly noisy signal, exhibiting hysteresis effects. All this makes the use of continuous clamping force sensors incompatible with the needs of braking systems.

  Finally, such sensors are fragile parts whose lifespan is short. In the event of deterioration of the continuous clamping force sensor, the only change of the sensor proves to be delicate and it is more convenient to replace the entire electromechanical brake caliper.

  Consequently, there is a need to overcome the drawbacks associated with continuous clamping force sensors.

  For this, the present invention relates to a method for estimating the clamping force in an electromechanical brake caliper, the caliper comprising two brake pads placed respectively on either side of a brake disc, a motor electric rotary, a speed reduction and conversion mechanism coupling a shaft of the electric motor to at least one of the brake pads, the mechanism being capable of converting the rotational movement of the shaft into a translational movement of the brake pads. brake one towards or away from the other according to the direction of rotation of the shaft, an angular position sensor 35 capable of measuring an angle of rotation of the shaft, characterized in that the caliper comprises a switch responsive to a clamping force exerted by the brake pads on the brake disc, the switch having a predetermined switching threshold, the switch being in a low state when the clamping force is less lower than the switching threshold and being in a high state when the clamping force is greater than the switching threshold, and in that the method comprises the steps consisting in: - detecting a switching of the switch around the switching threshold of a low state to a high state, or vice versa; - Acquire, at the time of the detected switching, an instantaneous value of angle of rotation measured by the angular position sensor as a reference angle; - acquire, after the reference angle has been acquired, another instantaneous value of angle of rotation measured by the angular position sensor; - calculate a relative displacement value by difference between the other instantaneous value of angle of rotation and the reference angle; and, - determining an estimated value of the clamping force from the relative displacement value, using a calibration relationship relating the clamping force to the relative displacement.

  The Applicant is interested in other types of force sensors which will be referred to in the following as discontinuous clamping force sensors with discrete states or alternatively switches.

  Such a switch has at least one switching threshold, dividing all of the clamping force values into at least two intervals: either the clamping force is greater than the switching threshold and the switch is in a high state; either the clamping force is below the switching threshold and the switch is in a low state. The switch switches from one state to another when the clamping force crosses the switching threshold. When the value of the clamping force is within a particular range, the switch is placed in a state corresponding to that range.

  The use of such a switch makes it possible to determine a reference position of the shaft of the electric motor of the caliper, at the moment when the switch switches. Once an angular reference 35 is defined, the method uses a calibration curve in order to estimate the clamping force applied by the pads for any angular displacement of the motor shaft.

  Advantageously, the continuous clamping force sensor is replaced by a clamping force switch. This, simpler 5 and more robust, must be precise only around the switching threshold.

  Preferably, the step of acquiring the reference angle is carried out during a phase of tightening the brake pads, during which the brake pads are moved towards each other to apply or increase the clamping force, the detected switching then being a switching from a low state to a high state.

  Preferably, the step of acquiring the reference angle is carried out during a phase of spacing of the brake pads, during which the brake pads are separated from each other to reduce or eliminating the clamping force, the detected switching then being a switching from a high state to a low state.

  Preferably, the method comprises a step consisting in calculating a wear of said brake pads as a function of the difference between the acquired reference angle and an initial reference angle, in order to correct a drift from the reference angle.

  Preferably, the step consisting in choosing said calibration relationship as a function of said wear from a list of predetermined calibration relationships connecting, each for a given wear, said clamping force to said relative displacement, to allow correction of the force. tightening according to wear.

  Preferably, the method includes the steps of estimating a temperature of said brake pads, calculating a thermal coefficient as a function of said temperature, and estimating a clamping force reconstructed by weighting the value of clamping force 30 given by the calibration relationship by said thermal coefficient, to allow correction of the clamping force as a function of temperature.

  Preferably, the switching threshold of said switch is a first switching threshold, and that said switch has a second predetermined switching threshold, corresponding to a clamping force greater than said first threshold, said method comprising the steps consisting in: - detecting switching of said switch around said second threshold; - acquire an instantaneous value of angle of rotation at the time of said switching; - determining an estimated value of said clamping force at the time of said switching; and, - estimating a temperature of the brake pads as a function of said estimated value of clamping force and of the second switching threshold, for the calculation of said thermal coefficient (K).

  Preferably, the method comprises a step consisting in resetting the caliper by actuating the electric motor, so that the shaft rotates by a predetermined reset angle 15 in a direction of spacing of the brake pads to starting from a detected switching of the switch around the first switching threshold from a high state to a low state, to move the brake pads from the brake disc and place the caliper in a state of rest.

  Preferably, the caliper includes a continuous clamping force sensor able to measure an instantaneous value of the clamping force exerted by the brake pads on the brake disc, and in that the method comprises the steps consisting in: - acquire an instantaneous value of clamping force measured by the clamping force sensor, as measured clamping force; and, - compare the measured clamping force with the estimated clamping force, to diagnose caliper failures.

  The present invention also relates to a device for implementing the above-mentioned method, comprising an electromechanical brake caliper having two brake pads placed respectively on either side of a brake disc, a rotary electric motor, a speed reduction and conversion mechanism coupling a shaft of the electric motor to at least one of the brake pads, the mechanism being capable of converting the rotational movement of the shaft into a translational movement of the brake pads one towards or away from each other according to the direction of rotation of the shaft, and an angular position sensor capable of measuring an angle of rotation of the shaft, characterized in that the bracket includes a sensitive switch to a clamping force exerted by the brake pads on the brake disc, the switch having a predetermined switching threshold, the switch being in a low state when the clamping force is less than the switching threshold and being in a high state when the clamping force is greater than the switching threshold, and in that the device comprises: - a switching detector, connected to the switch, for detecting a switching of the switch around the threshold for switching from a low state to a high state, or vice versa; a rotation angle acquisition means, connected to the angular position sensor, for acquiring instantaneous rotation angle values; - at least one memory; the angle of rotation acquisition means being adapted to acquire an instantaneous value of angle of rotation in response to a switching detection by the switching detector, and to write the instantaneous value of angle of rotation as 'reference angle in the at least one memory; a means for calculating the clamping force capable of calculating a value of relative displacement, by difference between another instantaneous value of angle of rotation acquired by the means of acquisition of angle of rotation and the reference angle, and able to determine an estimated clamping force value from the relative displacement value using a calibration relationship stored in the memory and relating the clamping force to the relative displacement.

  Preferably, the device comprises a wear calculation means, capable of reading the reference angle, then subtracting an initial reference angle from the reference angle, and writing the result as wear on the brake pads. in the at least one memory.

  Preferably, the clamping force calculation means is able to choose the calibration relationship as a function of wear from a list of calibration relationships stored in the at least one memory, connecting each for a given wear value. , the clamping force at relative movement.

  Preferably, the device comprises reset means capable, in response to a detection of switching between a high state and a low state by the switching detector, to actuate the electric motor so that the shaft performs a rotation d 'a predetermined reset angle stored in the at least one memory to separate the brake pads from the brake disc and place the caliper in a state of rest.

  Preferably, the device comprises means for estimating temperature capable of evaluating a temperature of said brake pads, means for evaluating a thermal coefficient capable of calculating the value of a thermal coefficient from said given temperature by said temperature estimation means, and weighting means able to calculate a clamping force reconstructed by weighting the value of the clamping force, given by said clamping force calculation means, by said thermal coefficient.

  Preferably, the switching threshold of said switch is a first switching threshold, and that said switch has a second predetermined switching threshold, corresponding to a clamping force greater than said first threshold, said device comprising a second switching detector connected to said 20 switch for detecting a switching of said switch around said second switching threshold, from a low state to a high state, or vice versa; and in that said means for estimating the temperature estimating by a thermal coefficient capable of calculating the temperature of the brake pads as a function of the estimated value of clamping force and of said second switching threshold.

  Preferably, the caliper also includes a continuous clamping force sensor, capable of measuring an instantaneous value of the clamping force exerted by the brake pads on the brake disc, and in that the device comprises: - a clamping force acquisition means, connected to the clamping force sensor for acquiring instantaneous clamping force values as a measured clamping force; and, - clamping force comparison means, capable of comparing the measured clamping force with the estimated clamping force or with the reconstructed clamping force, to diagnose failures of said stirrup.

  Finally, the invention also relates to a motor vehicle comprising a so-called hybrid braking system consisting of a plurality of hydraulic calipers at the front and a plurality of electromechanical calipers at the rear, said vehicle work, for the operation of each of the electromechanical stirrups, the device cited above.

  The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of a particular embodiment of the invention, given solely by way of illustration and not limiting, with reference to the accompanying drawings. In these drawings - Figure 1 shows a section through an electromechanical brake caliper usable in various embodiments of the device according to the invention; - Figure 2 shows a schematic view of a control system of the caliper of Figure 1, in the embodiment of the device according to the invention; FIG. 3A shows a curve giving the angle of rotation of the motor shaft as a function of time, during a procedure for actuating the stirrup of FIG. 1; - Figure 3B shows a curve giving the clamping force as a function of time, during the same stirrup actuation procedure as in Figure 3A; FIG. 4 is a block diagram of an algorithm executed by the system of FIG. 2 allowing the implementation of the method for estimating the clamping force according to the invention; and, - Figure 5 shows calibration curves connecting the clamping force to the relative movement.

  FIG. 1 of the drawings placed in the annex represents an electromechanical brake caliper 3 usable in various embodiments of the device according to the invention. Caliper 3 can equip the braking system of a vehicle. The left part of Figure 1 corresponds to the side of the caliper 3 placed opposite the wheel. By convention, we will designate this side as the wheel side, as opposed to the body side corresponding to the right part of FIG. 1. The axis A represents the axis of the wheel. The caliper 3 comprises a housing 8 inside which are housed an electric motor 10, a ball screw 20 and external 6 and internal brake pads 7 having respective friction surfaces 6a and 7a arranged on either side another of a brake disc 4 linked to the wheel (not shown) of the vehicle. The housing 8 is slidably mounted in the direction perpendicular to the brake disc 4 on a yoke, not shown, which is integral with the body of the vehicle.

  The housing 8 comprises, on the wheel side, a claw 18 made in one piece with the housing 8 and having an internal surface 18a against which the external brake pad 6 is fixed. The housing 8 comprises, on the body side, a shoulder 17. A holding element 14, placed against the shoulder 17 and to which the electric motor 10 is fixed, makes it possible to secure the latter to the housing 8.

  The electric motor used in an electromechanical brake caliper can be any electric motor. In Figure 1, the electric motor 10 is a permanent magnet motor. The electric motor 10 comprises a wound stator 11 and a rotor 12, consisting of a permanent magnet and able to rotate around the axis B when an electric power is supplied to the windings of the stator 11. The electric motor 10 comprises a shaft motor 13 secured to the rotor 12. When the latter is set in motion, it transmits its rotational movement to the drive shaft 13.

  In order to control the operation of the electric motor 10 with permanent magnet, an angular position sensor 31 is positioned on the motor shaft 13 at the outlet of the electric motor 10. The angular position sensor makes it possible to measure the angle of rotation of the 'shaft 13 and therefore of the rotor 12, relative to the stator 11 fixed. The angular sensor 31 is typically, for example, a Hall effect sensor. Furthermore, an intensity sensor (not shown) makes it possible to measure the intensity of the electric current passing through the winding of the stator 11. The intensity sensor is typically a resistor mounted in series with the battery between the terminals of the power electronics essentially playing the role of an inverter. The potential difference generated between the terminals of the resistor is proportional to the intensity which passes through the winding of the stator 11. As a variant, and for example in the case of an electric motor 35 having three phases, two Hall effect sensors could be used to access the Imes intensity value.

  At the outlet of the electric motor 10, the shaft 13 is coupled with a reduction box 15 having an output shaft 16. The reduction box 15 makes it possible to obtain a high ratio between the speed of rotation of the shaft 13 and the speed of rotation of the output shaft 16, for example 5 typically from thirty to one. For a rotation of a given angle of the shaft 13, the output shaft 16 will rotate by an angle thirty times smaller.

  On the other hand, the intensity of the torque available on the output shaft 16 is thirty times greater than the intensity of the torque available on the shaft 13.

  The ball screw 20 is coupled to the output shaft 16. This ball screw 20 comprises a screw part 21, balls 26 and a nut part 27. The screw part 21 is of hollow cylindrical shape with axis B and comprises a bore in which the electric motor 10 and the reduction box are housed 15. The screw part 21 is linked to the output shaft 16 by means of a flange 23. The rotational movement of the shaft output 16 is thus transmitted to the screw part 21 of the ball screw 20.

  The outer surface of the screw 21 and the inner surface of the nut part 27 are threaded so as to define between them a helical channel receiving the balls 26. The nut part 27 is guided in the housing 8 by guide means (not shown) so that it can move only in translation, parallel to the axis B. This arrangement allows the screw part 21, driven in a rotational movement relative to the housing 8, to drive the part nut 27 in a translational movement along the axis B relative to the housing 8.

  On the wheel side, the nut part 27 is linked to a piston 29 which carries the internal brake pad 7. When the nut part 27 is moved in translation towards the wheel side, the internal brake pad 7 approaches the external brake pad 6. Once the surfaces 6a and 7a come into contact with the brake disc 4, the caliper 3 applies a clamping force to the brake disc 4. This clamping force is directed along the axis B and is therefore perpendicular to surfaces 6a and 7a.

  The brake pads 6 and 7 are made of a material with a high coefficient of friction. The clamping force creates a friction force directed perpendicular to the axis B, tangentially to the surfaces 6a and 7a. The friction force opposes the rotation of the brake disc 4 and has the consequence of slowing down the rotational movement of the wheel around its axis A and of braking the vehicle.

  The bracket 3, in the preferred embodiment of the device, 5 comprises a clamping force switch 1 of annular shape, placed between the screw portion 21 and the holding element 14. This switch 1 can, for example, be a piezoelectric sensor which, when deformed under the effect of an external stress, emits an electrical signal correlated to the intensity of the stress, that is to say to the intensity of the applied clamping force by the brake pads 6 and 7 on the brake disc 4. The electrical signal is then compared to a reference level, to produce a binary electrical signal. Switch 1 therefore has a single switching threshold Fo equal, for example, to 400 N. We will speak of a high state when the clamping force is greater than the switching threshold F0, and a low state when the clamping force is below the switching threshold F0. The electrical signal emitted by switch 1 is a binary signal. Bit 0 is for example assigned to the low state and bit 1 to the high state.

  Referring now to Figure 2, the part of the device 20 for actuating the electromechanical brake caliper 3 will be described.

  Certain actions of the driver of the vehicle relating to braking are detected by sensors. For example, sensor 42 beeps when using the parking brake, sensor 43 beeps when the brake pedal is depressed, and sensor 44 beeps when the ignition key is used to start the vehicle. The signals generated by these various sensors are transmitted along the links 42a, 43a and 44a connected to a braking computer 41.

  The purpose of the braking computer 41 is, as a function of the information on the state of the vehicle which reaches it, to calculate the values of the target clamping forces which must be applied to each of the wheels of the vehicle. For this, the brake computer 41 executes programs such as dynamic stability control (ESP), anti-lock braking system (ABS) or other equivalent programs.

  At the output, the braking computer 41 is connected by links 46 to 35 each of the remote controllers 50 actuating the electromechanical brake calipers 3 fitted to the vehicle. At a defined frequency, the braking computer 41 emits a target clamping force signal corresponding to the value of the target clamping force to be applied by the caliper 3.

  The controller 50, placed near the stirrup 3 which it controls, comprises at least one memory and one processor capable of executing, in real time, programs stored in said memory.

  The controller 50 regulates the clamping force applied by the stirrup 3 by controlling the electric power delivered to the stirrup 3. The purpose of the regulation is that the clamping force applied by the stirrup 3 corresponds to the target clamping force required by the brake computer 41.

  Furthermore, the vehicle includes a direct current source, such as a battery 36, so as to generate electrical power. An electronic power system 35, connected to the battery 36, supplies electric current to the electric motor 10 of the caliper 3. As a function of the instantaneous value of the intensity Imes of the current flowing in the windings of the stator 11 of the motor. electric 10, measured by an intensity sensor, of the instantaneous value of the angle measured 0ref by the angular sensor 31, and as a function of a voltage control signal emitted by the controller 50 on the link 51, the 20 electronic power system 35 supplies the electric motor with an electric current adapted in frequency and amplitude.

  In order to regulate the clamping force applied by the stirrup 3, the actuation chain of the stirrup 3 comprises a regulation loop which will now be described.

  In the embodiment of the device according to the invention, the controller 50 is at least connected to the angular sensor 31 and to the switch 1 through an interface 53. Periodically, the controller 50 executes the task 61 in order to acquire the value instantaneous angle of rotation of the shaft measured by the angular sensor 31. This instantaneous value of angle of rotation is stored in the memory space corresponding to the variable 0mes The controller 50 also executes the task 62 in order to acquire the instantaneous value of the state of switch 1. This instantaneous value of the state of switch 1 is stored in the memory space corresponding to the variable Emes.

  An algorithm 55, which will be described later in detail, stored in the form of a program in the memory of the controller 50 and executed by the processor of the controller 50, reconstructs the clamping force Freco applied by the caliper 3, from instantaneous values of angle 0ref and state of the Emes switch, as well as a Mode variable indicating the mode of actuation of the stirrup 3.

  The controller 50 then compares the reconstructed clamping force Freco with the target clamping force. Comparator 51 calculates a regulation variable f by difference between the target clamping force and the reconstructed clamping force Freco, the purpose of regulation being to actuate the caliper 3 so as to cancel the value of the regulation variable f.

  As a function of the value of the regulation variable, an algorithm 52 calculates the value of a control voltage. By means of the interface 53, a control signal corresponding to the value of the control voltage is transmitted on the link 51 in the direction of the electronic power system 35.

  The algorithm 55 for reconstructing the clamping force will be described in detail with reference to FIGS. 3 to 5.

  At the end of the assembly of the stirrup 3, at the outlet of the production line for example, a calibration operation of the stirrup 3 and of initialization of the controller 50 is carried out. For this, the caliper 3 is placed on a measuring bench comprising a continuous reliable clamping force sensor making it possible to measure the clamping force actually applied by the brake pads 6 and 7. The electric motor 10 of the caliper 3 is actuated. In FIGS. 3A and 3B, the curves represent concomitant changes in the angle of rotation 0mes (FIG. 3A) measured by the angular position sensor 31, and in the clamping force Fmes (FIG. 3B) measured by the continuous sensor clamping force, during a typical tightening / loosening cycle. At time C o the switch 1 switches from the low state to the high state, the tightening force is known and 30 is for example equal to 400 N. The angle of rotation measured at time C, being equal here 1500, is then taken as the initial reference angle 0ref 0_.

  For each value of the measured angle 0mes, a relative angular displacement D, expressed in degrees, is calculated by difference between the angle 0mes and the initial reference angle 0ref. The clamping force Fmes is then measured for different values of the relative displacement D. A calibration curve connecting the clamping force and the relative displacement is obtained step by step. Such a calibration curve is represented by the curve C1 in FIG. 5. The calibration curve thus obtained and the initial reference angle 0ref 0 are associated with the stirrup 3, by storing them in the memory of the controller 50.

  During normal use, the actuation of the stirrup 3 is divided into four modes respectively identified by the values of the discrete variable Mode: a rest mode, a tightening actuation mode, a loosening actuation mode and a reset mode.

  During a typical tightening / loosening cycle represented by FIGS. 3A and 3B, the stirrup 3 is first in a rest mode between the initial instant and the instant A (FIG. 3A). In idle mode, no electric power is applied to the electric motor 10.

  The angle 0mes is constant and is worth, for example, an initial angle of 500. In the rest mode, the brake pads 6 and 7 remain separated from the brake disc 4 so that a substantially zero clamping force is applied (FIG. 3B).

  Caliper 3 goes into clamping actuation mode, for example, when the driver of the vehicle depresses the brake pedal, and the sensor 43 emits a signal. The braking computer 41 sends a request corresponding to a target clamping force Fmax of 2400 N. The actuation of the electric motor 10 is then carried out in a first direction of rotation so as to bring the piston 29 closer to the surface 18a of the claw 18a. As a result, the angle 0mes increases. At first, between instants A and B, the brake pads 6 and 7 approach each other and the space between the surfaces 6a and 7a and the brake disc 4 is reduced. This space becomes zero at time B o the brake pads 6 and 7 come into contact with the brake disc 4 and o a clamping force begins to be applied. Then, the electric motor 10 continues its movement so as to increase the angle 0mes and so that the brake pads 6 and 7 are pressed against the brake disc 4 so that the clamping force increases.

  The clamping force is first of all below the switching threshold F0 of switch 1. Switch 1 is in a low state. The state of the Emes switch is equal to 0. The clamping force gradually increases and there comes an instant, C, where the intensity of the clamping force exceeds the switching threshold F0. Switch 1 then switches to go from its low state to its high state. The state of the Emes switch is then 1.

  In parallel, as soon as the Mode variable corresponds to the 5 mode of actuation of clamping of the stirrup 3, the task 63 (FIG. 4) is executed periodically by the controller 50. The task 63 begins with a test on the value of the state of the Emes switch. Task 63 includes a time loop, during which the value of the variable Emes is compared with the value of this same variable in the previous period. When a change in the value of the state of the Emes switch is detected, one exits the time loop. At this instant, which corresponds to the detection of the switching of switch 1, the clamping force is equal to the switching threshold F0. At this instant C, the value of the angle 0mes is then written in a memory space 15 corresponding to the reference angle 0ref. The value of a binary indicator Ref takes the value 1, signifying that the determination of a reference angle has been carried out. On curve 3A, the reference angle 0ref is equal to 1500.

  The electric motor 10 continues its movement so as to increase the intensity of the clamping force to reach the target clamping force Fmax.

  Periodically, at any time subsequent to time C which has enabled the definition of the reference angle 0ref, a relative displacement D is calculated. The controller 50 executes the task 64 which subtracts the reference angle Oref from the angle 0mes, and stores the result in a memory space corresponding to the relative displacement D. The sign of the relative displacement D makes it possible to know whether the tree 13 has rotated in one direction or the other with respect to the reference position characterized by the reference angle 0ref. As defined here, when the relative displacement D is positive, the shaft 13 has been rotated such that the brake pads 6 and 7 are closer to each other than in the reference position ; when the relative movement D is negative, the shaft 13 has been rotated such that the brake pads are more distant from each other than in the reference position.

  Then, the controller 50 executes task 65 to calculate the instantaneous value of the estimated clamping force Fest. The processor associates a value of the force with the value of the relative displacement D using the calibration curve stored in the storage means of the controller 50, for example by interpolation between the stored calibration points. The value of the force thus obtained is assigned to the variable Fest. For example in FIG. 5, for a relative displacement D1 equal to 1800, the value F2 of the estimated clamping force Fest by means of the calibration curve CI is 1400 N. The estimated clamping force Fest can be used as a reconstructed force in the regulation loop. However, the presence of a clamping force switch 1 provides other functionalities which will be described.

  Referring to Figures 3A and 3B, after the clamping force has reached the target clamping force Fmax, the stirrup 3 is then placed in a release actuation mode. This is for example the case when the driver releases the pressure which he exerted on the brake pedal. The electric motor 10 is actuated in a second direction of rotation, opposite to the first, so as to separate the piston 29 from the surface 18a. The 0omes angle decreases. Under these conditions, the brake pads 6 and 7 gradually release the clamping and the clamping force gradually decreases.

  At time F, the intensity of the clamping force returns to the switching threshold F0. However, the angle of rotation measured at this time is slightly different from the reference angle 0ref. A difference appears which is a sign of wear of the brake pads 6 and 7 or of an expansion by heating of the pads 6 and 4. In the event of long braking, wear is preponderant; in the event of short and intense braking, thermal expansion is predominant.

  The correction of the error made when estimating the clamping force due to wear will now be described.

  During the application of the brake pads against the brake disc 30, material was removed from the contact surfaces 6a and 7a.

  The thickness of the brake pads has therefore been reduced. In FIG. 3A, an elementary wear u is represented as the difference between the angle of rotation measured at time F and the reference angle 0ref evaluated at time C. It is therefore necessary to regularly update the reference angle 0ref to take into account the wear of the brake pads 6 and 7.

  For example, the reference angle of rotation 0ref will be redefined during the subsequent tightening / loosening cycle, at the instant equivalent to instant C at the moment when the switch 1 switches from its low state to its high state.

  The step leading to the definition of the reference angle 0ref 5 can be carried out each time that the stirrup 3 is in clamping actuation mode, when the switch 1 goes from its low state to its high state around of the first switching threshold. As a variant, the reference rotation angle 0ref can be defined each time that the stirrup 3 is in the release actuation mode, when the switch 1 goes from its high state to its low state around the first threshold. switch F0. In another preferred variant, the reference rotation angle 0ref is determined when the parking brake of the vehicle is released. In this case, the switch 1 switches from its high state to its low state around the switching threshold Fo. This variant 15 has the advantage that the controller 50 defines once the reference angle of rotation 0ref for the entire period of movement of the vehicle. No task relating to the definition of the reference angle comes to use the computation capacities of the controller 50 during the actuation of the stirrup 3, improving the response time of the control chain 20.

  Each time that the reference angle of rotation 0ref is redefined, the controller 50 determines a wear U, expressed in degrees, by executing task 66. Task 66 subtracts the reference angle Oref newly defined from the angle of initial reference 0ref 0 determined during the calibration operation of the caliper 3. The result of this calculation is stored in a wear variable U. This information can be read by the braking computer 41 which is capable of compare with a wear threshold below which an alarm is triggered which may warn the driver of the need to change the brake pads 6 and 7 of caliper 3. When new brake pads are installed on the bracket 3, the initial reference angle 0ref 0 is updated.

  The wear of the brake pads 6 and 7 has a different impact on the clamping force than the simple drift from the reference angular position. A brake pad is made of a relatively deformable material. This property is used to create a friction force from the clamping force. For a given clamping force, if the brake pads can deform and be pressed on the brake disc, the friction force will be significant. On the other hand, if the brake pads are rigid, the friction force 5 will be lower. However, worn brake pads are rigid because the thickness of the deformable material is reduced. Thus, the efficiency of the conversion of the clamping force into a friction force depends, among other things, on the wear U of the brake pads. This phenomenon is all the more marked the greater the clamping force to be applied.

  Consequently, it is necessary to take into account the wear U of the brake pads 6 and 7 in the actuation chain of the caliper 3 and, for a given braking torque, to actuate the caliper 3 with a clamping force, the higher the wear U is. In the method according to the invention, the effect of the wear of the brake pads is taken into account during the reconstruction of the clamping force, by applying a gain making it possible to underestimate the value of the clamping force. rebuilt. In this way, the value of the regulation variable f is artificially increased, which causes the stirrup to be actuated in the direction of increasing the clamping force.

  To do this, during the caliper calibration operation, a list of calibration curves indexed on the wear of the brake pads is derived from the main calibration curve obtained as described above. For example, the behavior of the brake pads as a function of their wear being known and tabulated, it is easy to derive a list of relations relating the clamping force to the relative displacement as a function of wear from the calibration curve main Ci. This relation list is stored in the storage means of the controller 50. During the use of the stirrup 3, the execution of the task 65 begins with the reading of the value of the variable d ' wear U then by selecting the corresponding calibration relationship. The estimated clamping force Fest is obtained for a relative displacement D using this selected calibration relationship.

  The correction of the error made when estimating the clamping force due to the temperature of the brake pads will now be described.

  The brake pads 6 and 7 do not apply the same friction force 5 to the brake disc 4 whether the material of which they are made is hot or cold. It is known that after a series of successive brakes the brake pads heat up and lose their effectiveness. The value of the coefficient of friction depends on the temperature.

  This effect of temperature on the friction force actually applied by the brake pads is known on hydraulic brake calipers. It is compensated by the driver of the vehicle who presses the brake pedal further.

  When the vehicle is fitted with hydraulic brake calipers at the front and electromechanical calipers at the rear (hybrid braking system), it is not necessary, in the regulation of the clamping force, to correct the error on the braking force estimated for electromechanical calipers. This error is due not only to the error in the clamping force due to the thermal expansion of the pads but also to the variation in the friction index of the pad on the disc. If this error were corrected, the loss of efficiency of the front hydraulic callipers would be entirely compensated by the rear electromechanical callipers. The brake pads of the latter would then heat up dramatically. On the contrary, there is a kind of self-compensation: the error in the estimation of the clamping force results in less braking, which goes in the right direction.

  On the other hand, on braking systems comprising only electromechanical brake calipers, it may be advantageous to compensate for the error in the estimation of the clamping force due to heating to regulate the operation of the electromechanical caliper. .

  In one embodiment of the method according to the invention, the estimated clamping force Fest is corrected so as to obtain a reconstructed clamping force Freco taking account of the temperature of the brake pads 6 and 7 measured for example by a sensor of 35 temperature. When the inserts heat up, the value of the reconstructed clamping force Freco is underestimated compared to the value of the estimated clamping force Fest. By closing the regulation loop on the reconstructed clamping force Freco, the controller 50 actuates the electric motor 10 so that the clamping force actually applied by the brake pads is greater. This has the effect that the frictional force actually applied by the brake pads increases and that the loss of braking efficiency due to the temperature of the brake pads is effectively compensated.

  In a second preferred embodiment of the device according to the invention, the bracket 3 comprises, all other things being equal, a switch 1 having a second predefined switching threshold F1. The switching threshold F1 is greater than the first switching threshold F0 which has been discussed so far. When the switch 1 switches around the second switching threshold F1, the intensity of the clamping force is equal to the second threshold F, for example 900 N in FIG. 5. The state variable Emes of the switch 1 is then coded on two bits, the values 0, 1 and 2 corresponding respectively to the low state, the intermediate state, which is a high state with respect to the threshold F0, and the high state with respect to the threshold Fi.

  The controller 50 periodically performs the task 67, for example, when the stirrup 3 is in the release actuation mode.

  This begins with a test on the value of the state of the Emes switch. This test compares the instantaneous value of the state of the Emes switch with the value of the state corresponding to the high state Emes = 2. Once this condition is fulfilled, task 67 enters a time loop, during which the value of the state of the current Emes switch is compared with the value of this same variable during the previous period. When a change in the state of the switch is detected, a sign of the switching of switch 1 around the second switching threshold FI from the high state to the intermediate state, task 67 calculates the value of a thermal coefficient K The thermal coefficient K is the ratio of the second switching threshold F1 to the value of the estimated clamping force Fest at this time.

  Periodically, at any time subsequent to the definition of the thermal coefficient, the controller 50 weights the estimated clamping force 35 Fest using the thermal coefficient K, and stores the result in a reconstructed clamping force variable Freco. This is for example carried out by task 68 in FIG. 4 which reads the values of the estimated clamping force Fest and of the thermal coefficient K, then performs a simple product of each other to calculate the instantaneous value of the Freco reconstructed clamping force variable.

  In FIG. 4, the curve C2 represents the reconstructed clamping force Freco as a function of the relative displacement D at a temperature of the pads higher than that which prevailed during the acquisition of the main calibration curve C1. The CI curve is weighted by a temperature gain. For the same relative displacement D1 of 1700, the clamping force is reconstructed so as to appear reduced. Thus for brake pads which apply a certain fixed real clamping force, the controller 50 calculates an estimated force F2 of 1350 N at 20 C, and an estimated force F1 of 900 N at a temperature of 300 C. This artefact of calculation avoids having to modify the target force as a function of temperature. Other means could be used to estimate the temperature of the brake pads. In particular, temperature sensors could be used.

  Finally, in FIG. 4, the program 55 includes a task 69 allowing the controller 50 to carry out various consistency tests 20 on the value of the reconstructed clamping force Freco. For example, if the reference angle 0ref is not determined, the binary indicator Ref being equal to 0, task 69 indicates that the reconstructed tightening force must not be taken into account in the regulation loop, and triggers an acquisition from the reference angle 0ref. Alternatively, task 69 follows the value of the state of the Emes switch. Each time switch 1 switches around a given switching threshold, the value of the reconstructed clamping force Freco is compared with the value of the given switching threshold.

  When the controller 50 detects the switching of the switch 1 around the threshold Fo from the high state to the low state, the stirrup 3 switches from the release actuation mode to the reset mode.

  The controller 50 then performs a task of resetting the stirrup 3 aimed at replacing the latter in the optimal rest position. This position is defined by a spacing between the surfaces 6a and 7a and the brake disc 4 such that the brake pads are very close to the brake disc but only apply a residual clamping force of a few newton on the latter. . The optimal position represents a compromise between residual friction and response time. The reset task executed by the controller 50 controls the actuation of the electric motor 10 so that the shaft 13 5 rotates, in the direction of loosening, by a predetermined reset angle X read in the memory of the controller 50. At the end of the execution of this reset task, the stirrup 3 is returned to its rest mode.

  In another embodiment, the stirrup 3 comprises a continuous clamping force sensor making it possible to measure an instantaneous value of the clamping force Fmes. In this case, the program 55 includes a task making it possible to choose either the measured clamping force Fmes or the reconstructed clamping force Freco to close the regulation loop. For example, in the event of deterioration of the continuous clamping force sensor, the measured force no longer makes sense and it is necessary to choose the reconstructed clamping force Freco to close the regulation loop. Furthermore, the comparison of the reconstructed clamping force Freco and the measured clamping force Fmes makes it possible to diagnose possible failures of the continuous clamping force sensor or, more generally, of the caliper 3.

  It should be noted that the clamping force switch can be achieved not by means of a physical sensor, as described above, but by means of a computer routine, executed by the controller 50. The input of such a routine is, for example, constituted by the value of the clamping force measured by a continuous clamping force sensor. The output of such a routine is a discrete variable reproducing the behavior of the state variable of a physical switch. This embodiment has the advantage that the requirements on the continuous clamping force sensor are reduced. In the range of 30 clamping forces, the continuous clamping force sensor must be reliable only around the switching threshold F0 or the thresholds F0 and FI.

  The clamping force switch can have a series of n predetermined switching thresholds, dividing the axis of the clamping force intensities into n + 1 intervals. Each of these n + 1 intervals 35 corresponds to a state of the switch. Depending on the force applied to it, the switch is in the state corresponding to the interval in which the intensity of the clamping force is situated. The electrical signal emitted by the switch then takes a discrete value characterizing said state. When switching the switch around a considered switching threshold, the intensity of the clamping force is equal to the value of the switching threshold.

  Of course, the numerical values given in the detailed description above, and in particular the numerical values of the switching thresholds, are only indicative and other numerical values can be chosen by the manufacturer according to the particular type of vehicle considered.

  Although the invention has been described in connection with a particular embodiment, it is obvious that it is in no way limited thereto and that it includes all the technical equivalents of the means described as well as their combinations if these are within the scope of the invention.

Claims (17)

  1. Method for estimating the clamping force in an electromechanical brake caliper (3), said caliper comprising two brake pads (6, 7) placed respectively on either side of a brake disc 5 (4) , a rotary electric motor (10), a speed reduction and conversion mechanism (15) coupling a shaft (13) of said electric motor to at least one of said brake pads, said mechanism being capable of converting the rotational movement of said shaft in a translational movement of said brake pads towards or away from each other according to the direction of rotation of said shaft, an angular position sensor (31) capable of measuring an angle of rotation of said shaft, characterized in that said caliper includes a switch (1) responsive to a clamping force exerted by said brake pads on said brake disc, said switch having a predetermined switching threshold (F0) 15, said switch being in a state low when said clamping force is lower than said switching threshold and being in a high state when said clamping force is greater than said switching threshold, and in that said method comprises the steps consisting in: - detecting a switching (63) of said switch around said threshold for switching from a low state to a high state, or vice versa; - Acquire (61), at the time of said detected switching, an instantaneous value of angle of rotation (Onmes) measured by said angular position sensor as a reference angle (Oref); - acquire, after the reference angle has been acquired, another instantaneous value of angle of rotation measured by said angular position sensor; calculating (64) a relative displacement value (D) by difference between said other instantaneous value of angle of rotation and said reference angle; and, - determining (65) an estimated value of said clamping force (Fest) from said relative displacement value, using a calibration relation (C1) connecting the clamping force to said relative displacement.   2. Method according to claim 1, characterized in that the step of defining said reference angle (0ref) is carried out during a tightening phase of said brake pads (6, 7), during which the brake pads are moved toward each other to apply or increase said clamping force, said detected switch (63) then being a switch from a low state to a high state. 5 3. Method according to claim 1, characterized in that the step of defining the reference angle (0ref) is carried out during a spacing phase of said brake pads (6, 7), during which the brake pads are spaced from each other to reduce or eliminate said clamping force, said detected switch (63) 10 then being a switch from a high state to a low state.   4. Method according to one of claims 1 to 3, characterized in that it comprises, a step consisting in calculating (66) wear (U) of said brake pads (6, 7) as a function of the difference between l 'reference angle (0ref) acquired and an initial reference angle 15 (0ref_ 0), to correct a drift of the reference angle.   5. Method according to claim 4, characterized by the step (65) consisting in choosing said calibration relationship as a function of said wear (U) from a list of predetermined calibration relationships connecting, each for a given wear, said force of 20 tightening at said relative displacement, to allow correction as a function of the wear of the clamping force.   6. Method according to one of claims 1 to 5, characterized in that it comprises the steps of estimating a temperature of said brake pads, of calculating a thermal coefficient (K) as a function of said temperature , and estimation of a reconstructed clamping force (Freco) by weighting the clamping force value given by the calibration relation (Ci) by said thermal coefficient, to allow correction as a function of the force temperature Clamping.   7. Method according to claim 6, characterized in that said switching threshold (Fo) of said switch (1) is a first switching threshold, and that said switch has a predetermined second switching threshold (F1), corresponding to a force tightening above said first threshold, said method comprising the steps of: - detecting (67) a switching of said switch around said second threshold; - acquire an instantaneous value of angle of rotation (Omes) at the time of said switching; - determining an estimated value of said clamping force (Fest) at the time of said switching; and, - estimate a temperature of the brake pads as a function of said estimated value of clamping force (Fest) and of the second switching threshold (F1), for the calculation of said thermal coefficient (K).   8. Method according to one of claims 1 to 7, characterized in that it comprises a step consisting in resetting said stirrup (3) by actuating said electric motor (10), so that the shaft (13) performs a rotation of a predetermined reset angle (X) in a direction of separation of said brake pads (6, 7) from a detected switching of said switch (1) around said first switching threshold (Fo) from a high state to a low state, to move said brake pads from the brake disc (4) and place said caliper in a state of rest.   9. Method according to one of claims 1 to 8, characterized in that said caliper (3) comprises a continuous clamping force sensor capable of measuring an instantaneous value of the clamping force exerted by said brake pads on said disc brake, and in that the method comprises the steps consisting in: - acquiring an instantaneous value of clamping force measured by said clamping force sensor, as measured clamping force (Fmes); and, - comparing said measured clamping force with said estimated clamping force (Fest), to diagnose failures of said caliper.   10. Device for implementing the method according to one of claims 1 to 9, comprising an electromechanical brake caliper (3) having two brake pads (6, 7) respectively placed on either side of a brake disc (4), a rotary electric motor (10), a speed reduction and conversion mechanism (15) coupling a shaft of said electric motor to at least one of said brake pads, said mechanism being capable of converting the rotational movement of said shaft in a translational movement of said brake pads one towards or away from the other according to the direction of rotation of said shaft, and an angular position sensor (31) capable of measuring an angle of rotation 5 of said shaft, characterized in that said caliper comprises a switch (1) sensitive to a clamping force exerted by said brake pads on said brake disc, said switch having a predetermined switching threshold (F0), said commutat eur being in a low state when said clamping force is less than said switching threshold o10 and being in a high state when said clamping force is greater than said switching threshold, and in that said device comprises: - a switching detector ( 63), connected to said switch, for detecting a switching of said switch around said switching threshold from a low state to a high state, or vice versa; - a rotation angle acquisition means (61), connected to said angular position sensor, for acquiring instantaneous values of rotation angle (Onmes); - at least one memory; said angle of rotation acquisition means being adapted to acquire an instantaneous value of angle of rotation in response to a detection of switching by said switching detector, and to write said instantaneous value of angle of rotation as 'reference angle (Oref) in said at least one memory; a means for calculating the clamping force (64, 65) capable of calculating a value of relative displacement (D), by difference between another instantaneous value of angle of rotation acquired by said means for acquiring angle of rotation and said reference angle, and capable of determining an estimated value of clamping force (Fest) from said value of relative displacement using a calibration relation (C1) stored in said memory and connecting said clamping force to said relative displacement.   11. Device according to claim 10, characterized in that it comprises a wear calculation means (66), capable of reading said reference angle (0ref), then subtracting an initial reference angle (Oref o) from said 35 reference angle, and writing the result as wear of said brake pads (U) in said at least one memory.   12. Device according to claim 11, characterized in that said clamping force calculation means (64, 65) is able to choose said calibration relationship as a function of said wear (U) from a list of calibration relationships stored in said at least one memory, connecting, each for a given wear value, the clamping force to said relative displacement.   13. Device according to one of claims 11 to 12, characterized in that it comprises suitable reset means, in response to a detection of switching between a high state and a low state 10 by said switching detector (1) , actuating said electric motor (10) so that said shaft (13) rotates by a predetermined reset angle (X) stored in said at least one memory to move said brake pads (6, 7) of said brake disc (4) and placing said caliper (3) in a state of rest.   14. Device according to one of claims 10 to 13, characterized in that it comprises means for estimating temperature capable of evaluating a temperature of said brake pads, means for evaluating a thermal coefficient (67) able to calculate the value of a thermal coefficient (K) from said temperature given by said temperature estimation means, and weighting means (68) able to calculate a reconstructed clamping force (Freco) by weighting the value of the clamping force (Fest), given by said clamping force calculating means (64, 65), by said thermal coefficient.   15. Device according to claim 14, characterized in that said switching threshold (Fo) of said switch (1) is a first switching threshold, and that said switch has a second predetermined switching threshold (Fi), corresponding to a force tightening above said first threshold, said device comprising a second switching detector (67) connected to said switch for detecting a switching of said switch around said second switching threshold, from a low state to a high state, or vice versa; and in that said means for estimating the temperature estimating by a thermal coefficient (67) capable of calculating said temperature of the 35 brake pads as a function of said estimated value of clamping force (Fest) and of said second switching threshold (FI).   16. Device according to one of claims 10 to 15, characterized in that said caliper (3) also comprises a continuous clamping force sensor, capable of measuring an instantaneous value of the clamping force exerted by said brake pads ( 6, 7) on said brake disc (4), and in that the device comprises: - a means for acquiring clamping force, connected to said clamping force sensor for acquiring instantaneous values of clamping force in as long as the clamping force measured (Fmes); and, - clamping force comparison means, capable of comparing said measured clamping force with said estimated clamping force (Fmes) or with said reconstructed clamping force (Freco), for diagnosing failures of said caliper.   17. Motor vehicle comprising a so-called hybrid braking system consisting of a plurality of hydraulic calipers at the front and a plurality of electromechanical calipers at the rear, said vehicle employing, for the operation of each electromechanical stirrups, the device according to one of claims 10 to 16.