Method for measuring deceleration of a motor vehicle wheel

Abstract

The invention relates to a method 1 for controlling a braking system of a vehicle, consisting in providing at least one wheel speed sensor WSS and an algorithm which uses data supplied by this sensor in order to estimate an instantaneous speed value and a deceleration of the wheel, calculated from said algorithm, the algorithm comprising a step 102 of polynomial fitting based on the sensor data.

Description

Method for measuring the deceleration of a wheel of a motor vehicle

[0001] The invention relates to a method for measuring the deceleration of a wheel of a motor vehicle.

[0002] Currently, deceleration measurements are obtained from data via the system bus (CAN: Controller Area Network); however, this measurement is highly filtered and suffers from a significant delay. Therefore, it cannot be used effectively to control the functions of a braking system.

[0003] The aim of the invention is to provide a method for a smoothed measurement of deceleration, with a lower delay than reading from the system bus.

[0004] According to the invention, a control method for a vehicle braking system includes providing at least one wheel speed sensor (WSS) and data processing implementing an algorithm that uses data provided by this sensor to estimate an instantaneous speed value and a deceleration of the wheel, this algorithm including a polynomial fitting step from the sensor data.

[0005] The process advantageously comprises: - an emission, by the sensor, of successive pulses corresponding to a distance traveled; - a time-stamping step, i.e. a recording of an instant corresponding to each pulse, and the recording of successive instants in a memory and the storage in this memory of the last N instants thus recorded; and, - the polynomial adjustment step being carried out from the last N instants recorded in the memory.

[0006] Advantageously, during the polynomial adjustment step, we deduce a curve having a formula of the type:

[0007] Where: - P(t) is the instantaneous position of the wheel at time t; - a, b and c are coefficients from the polynomial adjustment calculation.

[0008] Then, from the formula P(t), we deduce: an instantaneous speed of the wheel: S(t) = at + bet an instantaneous acceleration of the wheel: A(t) = a

[0009] Preferably, when a measured duration between two successive moments becomes greater than a threshold value, an immobilization beacon is activated, then a model of progressive decrease to zero of the values ​​of speed and then acceleration is applied.

[0010] Several embodiments of the invention will be described below, by way of non-limiting examples, with reference to the accompanying drawings in which:

[0011] represents a logic diagram schematically illustrating a process according to the invention;

[0012] illustrates a timestamp in the process of the;

[0013] illustrates a polynomial adjustment step in the process of the ;

[0014] illustrates a stage of immobilization in the process of the;

[0015] illustrates linear wheel speed measurements, during acceleration phase, according to the method of the, in comparison with measurements according to a prior art method;

[0016] illustrates wheel acceleration measurements, during acceleration phase, according to the method of the, in comparison with measurements according to the same method of the prior art;

[0017] illustrates linear wheel speed measurements during deceleration, according to the method of the, in comparison with measurements according to the same method of the prior art; and,

[0018] illustrates wheel acceleration measurements, during deceleration, according to the method of the, in comparison with measurements according to the same method of the prior art.

[0019] In this description, the term acceleration is used interchangeably, whether its value is positive, i.e., corresponding to an increase in speed, or negative, i.e., corresponding to a decrease in speed. The term deceleration is used when one specifically wants to designate an acceleration value that is indeed negative.

[0020] This is a logic diagram illustrating a method 1 for determining the deceleration of a vehicle wheel, specifically for controlling an intelligent braking system and a vehicle dynamic control (VDC) system. This method comprises steps 101-105, namely: - a first step 101, to time-stamp successive pulses M emitted by a wheel speed sensor 2 of the vehicle, with reference to a clock H, and to record each instant t of each pulse M in a memory 3; then, - a second step 102, to establish a polynomial fit from the last N recorded instants t and deduce speed and acceleration values; - a third step 103, to apply a model of progressive reduction to zero of the speed and then deceleration values, when a difference between two successive instants t n , t n+1becomes greater than a threshold DT; then,- a fourth step 104, to apply a Kalman filter, taking into account a possible alert F(ABS) from the anti-lock braking system (ABS: Anti-lock Braking System); then,- a fifth step 105 to transmit the calculated data to an intelligent braking system SB and a vehicle dynamic control system VDC (Vehicle Dynamic Control).

[0021] In the illustrated example, the clock H is a microprocessor (CPU) clock of the onboard computer. As shown in Figure 1, the memory 3 used to store the time intervals t is of the FIFO (First In First Out) type, in which eight time intervals t3-t 10 successive are stored, a new instant t 11 record replacing in memory 3 the time t3 which is stored there the oldest.

[0022] The polynomial adjustment is established from the last eight instants t recorded, that is to say the eight instants stored in memory 3, that is to say that N is equal to 8 (N=8), knowing that between two pulses M emitted by the WSS sensor, the wheel travels a given constant distance L; this distance L can be an angular distance or a linear distance traveled by its periphery, this angular distance and this linear distance being proportional to each other.

[0023] Polynomial regression allows us to calculate a curve with the following equation:

[0024] Where: - P(t) is the instantaneous position of the wheel at time t; - a, b and c are coefficients from the polynomial adjustment calculation.

[0025] Then, we deduce from P(t): - an instantaneous velocity of the wheel at time t: S(t) = at + b - and an instantaneous acceleration of the wheel at time t: A(t) = a

[0026] Typically, for each wheel rotation, forty-eight (48) pulses M are emitted by the WSS sensor. This number may be insufficient to allow the braking system SB to calculate the acceleration usable by the acceleration. This deficiency is compensated for by using polynomial fitting and the resulting P(t) curve, which is renewed with each new pulse M received.

[0027] When the wheel rotates too slowly, the number of pulses M per unit of time becomes too low for the calculation to be accurate. This occurs when the vehicle's speed becomes too low, or when the vehicle is stationary, or when the wheel slips, spins, or is locked. Thus, during deceleration, the difference between two successive instants t n , t n+1 becomes greater than a threshold DT, that is to say when the velocity S(t) falls below a corresponding threshold, the curve resulting from the polynomial fitting becomes unusable.

[0028] We then proceed to the third step 103, in which: - if the time difference is less than DT (t n+1 - t n+1 < DT) then, the acceleration is given the value A(t) calculated in the previous step 102; and, - if the time difference is greater than DT (t n+1 - t n+1 > DT) then, we substitute for the curve P(t) a deceleration model, such as the one illustrated in the, by which we perform a standstill correction which gives the acceleration a forced value which brings it back to zero, preferably gradually.

[0029] Thus, in the deceleration model illustrated in the figure, the deceleration A(t) is kept substantially constant, so that the velocity decreases linearly, and then, when the velocity has reached a zero value, the acceleration is rapidly and gradually brought back to zero.

[0030] In step 104, the data obtained in the previous steps are subjected to corrections using a Kalman filter. When an F(ABS) alert is issued by the ABS anti-lock braking system, it is taken into account in this step 104.

[0031] At the fifth stage 105, the calculated and filtered data are transmitted to the SB braking system and the VDC dynamic control system.

[0032] Figures 5 to 8 illustrate the advantages of a process according to the invention.

[0033] A: - a curve C51 illustrates a wheel speed curve calculated according to the method of the invention; - a curve C52 illustrates a wheel speed curve calculated according to the prior art from data from the CAN bus; and - a curve C53 illustrates a vehicle speed curve calculated according to the prior art from data from the CAN bus.

[0034] It is observed that the invention makes it possible to calculate a smoother speed curve with less delay relative to the signals collected via the CAN bus.

[0035] A: - a curve C61 illustrates a wheel acceleration curve calculated according to the method of the invention; and, - a curve C62 illustrates an acceleration curve calculated according to the prior art from data from the CAN bus.

[0036] It is observed that the invention makes it possible to calculate a smoother, more precise acceleration curve with less delay relative to the signals collected via the CAN bus.

[0037] A: - a curve C71 illustrates a wheel speed curve, calculated according to the method of the invention; - a curve C72 illustrates a wheel speed curve, calculated according to the prior art from data from the CAN bus; and - a curve C73 illustrates a vehicle speed curve, calculated according to the prior art from data from the CAN bus.

[0038] The C51, C61, C71 curves, calculated according to the invention, have a refresh rate of approximately one millisecond (1 ms).

[0039] The speed curve of the C72 wheel, calculated according to the prior art, is established with a refresh rate of only twenty milliseconds (20 ms) and with a delay of about twenty-five milliseconds (25 ms) on the C71 curve established according to the invention.

[0040] The speed curve of the C73 vehicle calculated according to the prior art, is established with an even lower refresh rate of only forty (40 ms) milliseconds and with an even greater delay of about one hundred and sixty milliseconds (160 ms) on the C71 curve established according to the invention.

[0041] A: - a curve C81 illustrates a realistic deceleration curve calculated according to the method of the invention; and, - a curve C82 illustrates a deceleration curve calculated according to the prior art from data from the CAN bus.

[0042] In the illustrated example, the Kalman filter used in step 102 for WSS signal processing is defined by the following equations.

[0043] The prediction equations are:

[0044] The update equations are:

[0045] Equations in which: defines the state of the system is the control input is the measured output is the state transition matrix is the input transition matrix is the output matrix is the noise matrix of the process with if ABS is activated

[0046] Or in other cases is the measurement noise matrix

[0047] Matrices in which: is the sampling period; is the variance of the process; and, , , And are the variances of the measurements of position, velocity and acceleration.

[0048] As shown in Figures 5 to 8, the notable advantages of a method according to the invention for deceleration control, for a closed-loop intelligent braking system (SmartBrake, in English).

[0049] It will indeed become apparent to the man of the art that various modifications can be made to the methods of implementation described above, in the light of the teaching which has just been disclosed to him.

[0050] In particular, the Kalman filter may be different from the one presented in this description.

[0051] List of references 1: method according to the invention 2: wheel 3: memory 101: first step of the method 1102: second step of the method 1103: third step of the method 1104: fourth step of the method 1105: fifth step of the method 1a, b, c: polynomial adjustment coefficients A(t): instantaneous acceleration C51, C61, C71, C81: curves established by a method according to the invention C52, C53, C62, C72, C73, C82: curves established according to the prior art DT: threshold delay F(ABS): anti-lock braking system alert H: clock L: distance traveled between two successive M pulses M: pulse emitted by the wheel speed sensor N: number of the last instants recorded in memory 3 P(t): instantaneous position S(t): instantaneous speed SB: intelligent braking system t: instant of a pulse MVDC: system WSS dynamic vehicle control: wheel speed sensor

Claims

Method (1) for controlling a braking system (SB) of a vehicle comprising an algorithm that uses data (M) provided by a wheel speed sensor (WSS) (2) to estimate an instantaneous speed value (V(t)) and a deceleration (A(t)) of the wheel, said algorithm comprising a polynomial fitting step (102) from said sensor data and a step (105) for transmitting said speed (V(t)) and said acceleration (A(t)) thus estimated to a vehicle dynamic control system (VDC). A method according to claim 1, characterized in that it comprises: - an emission, by the sensor (WSS), of successive pulses (M) corresponding to a distance (L) traveled; - a step (101) for a timestamp (101), that is to say a reading by a clock (H) of an instant (t) corresponding to each pulse (M), and the recording of said successive instants (t) in a memory (3) and the storage in said memory of the last N instants (t) thus recorded; and, - the step (102) of polynomial adjustment being carried out from said last N instants (t) recorded in said memory (3). A method according to claim 2, characterized in that at the polynomial fitting step (102), a curve is derived having a formula of the type: Where: - P(t) is an instantaneous position of the wheel; and - a, b and c are coefficients, then, we deduce from said formula the instantaneous velocity (S(t)) of the wheel (2) and the instantaneous acceleration (A(t)) of said wheel. A method according to any one of claims 1 to 3, characterized in that when a difference between two successive instants (t n , t n+1 ) becomes greater than a threshold value (DT) an immobilization beacon is activated, and a model of progressive decrease to zero of the speed and acceleration values ​​is applied (103). A method according to any one of claims 1 to 3, characterized in that a Kalman filter defined as follows is applied. The prediction equations are: The update equations are: Equations in which: defined the state of the system, is the control input, is the measured output, is the state transition matrix, is the input transition matrix, is the output matrix, is the noise matrix of the process with if ABS is activated or in other cases, is the measurement noise matrix; Matrices in which: is the sampling period; is the variance of the process; and , , And are respectively the variances of the measurements of position, velocity and acceleration.

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