WO1996022905A1

WO1996022905A1 - Process for optimizing the grip of vehicle wheels on application of torque

Info

Publication number: WO1996022905A1
Application number: PCT/FR1996/000138
Authority: WO
Inventors: Jean-Pierre Baudet; Stéfano Bianchi
Original assignee: Baudet Jean Pierre; Bianchi Stefano
Priority date: 1995-01-26
Filing date: 1996-01-26
Publication date: 1996-08-01
Also published as: FR2729908A1; FR2729908B1; EP0804357A1

Abstract

A process for optimizing the use of the grip available when a braking or acceleration torque is applied to the wheels of a motor vehicle or trailer, the torque applied to the wheel no longer being increased when the grip reaches a value very close to, but less than, the maximum value. The invention also concerns an anti-lock control system for hydraulic braking systems in motor vehicles, a process for improving the performance of a braking system and/or the grip available.

Description

PROCESS FOR OP T IM I SAT ION OF

THE TORQUE BELT OF VEHICLE WHEELS.

The present invention relates to a method for optimizing the use of the grip available when a torque - braking or acceleration - is applied to the wheels of a vehicle, motorcycle, automobile, or trailer.

When rolling a wheel subjected to driving and braking forces, the grip of the wheel on the road surface depends on several parameters: the state of the tire, the state of the road surface, the slip of the wheel. .

The coefficient of adhesion can be represented as a function of the slip by a curve represented in FIG. 1. This representation shows that the coefficient of adhesion increases with the slip until reaching a maximum value, for a slip value called μ critical. Beyond this maximum value of the coefficient of adhesion, the wheel enters an unstable zone which quickly leads to blocking of the wheel, and which corresponds to a reduction in the coefficient of adhesion (increase in the braking distance), and in a decrease in the maneuverability and the directional stability of the vehicle, in the case of a torque resulting from braking - or either at total slip, or to a loss of traction and maneuverability in the case of a torque resulting from the engine.

The regulation of the torque applied to the wheels of a vehicle, in order to optimize both acceleration traction and braking safety, should therefore ensure that the slip which results from the application of the torque is close to the critical value μ, without this slip value is exceeded.

Several patents are known in the state of the art relating to anti-lock processes and systems for motor vehicle wheels. By way of example, French patent FR9113119 describes a method and device for braking vehicles by controlling the braking torque applied to a wheel. The grip of a wheel on a braking track is defined by a variable operating point (P) on a grip curve as a function of the slip (g) of the wheel. In the method according to the invention, the braking torque control parameter is the sign of the variation in the slope of the grip curve at the abovementioned operating point (P). The braking torque is controlled so that this operating point (P) coincides with the maximum grip point (M) of the grip curve.

The methods of the prior art are based on the principle that the torque applied to the wheel is limited when the maximum of the coefficient of adhesion and the critical slip are exceeded. We therefore enter the unstable part of the grip / slip curve. This results in an oscillation behavior of the braking pressure, and therefore a loss of efficiency. The maximum grip corresponds exactly to the start of the unstable (and decreasing) part of the curve shown in Figure 1, part for which the natural balance corresponds to the complete locking of the wheel (in the case of braking - and to total slippage in the case of an engine torque): it would therefore be necessary, to optimize grip, to detect in advance when this maximum will be reached: now, for all previous systems, it is the detection of the effective crossing of this maximum that is performed, hence the need to immediately reduce the brake pressure and regulate it around this optimal value.

The systems of the prior art, which make it possible to avoid complete locking of the wheel and the risks of loss of control which result therefrom, therefore have the main drawbacks and limits not being optimal, the fluctuating braking force not being permanently maintained at the maximum possible value,

- to require, to regulate the braking pressure, the presence of an independent external energy source (hydraulic pump, valves, etc ...), this mistletoe increases their complexity and their cost and limits their application to the most expensive,

- not to be due to this complexity totally immune to breakdowns, or untimely operations.

In order to remedy these drawbacks, it would be necessary, in order to optimize the regulation of the torque applied to the wheel and improve the grip, when braking or accelerating, to detect in advance when the maximum of the grip coefficient and the critical slip will be and limit the torque before reaching the unstable zone.

This is the object of the present invention, which overcomes these drawbacks by providing for the optimal use of the available grip.

- perfectly safe operation (fail safe system and redundancy without common modes) - a very economical implementation (possibility of operation without autonomous pressure source), accessible to "low-end" vehicles

- full compatibility with existing A.B.S systems, where it can be integrated directly, without additional components, as a new braking optimization function (maximum emergency braking)

- an application for both braking and non-slip control.

- the possibility, due to its moderate cost, to consider the reconditioning of existing vehicles not equipped, (or equipped, to make their ABS more efficient). The invention relates more particularly to the early detection of the maximum adhesion.

This detection can be obtained according to a first variant by detecting the maximum braking power C.Ω, which slightly precedes the maximum braking force during emergency braking (Ω = actual speed of rotation of the wheel );

According to a second variant, detection by anticipation of the maximum adhesion is obtained by measuring / calculating the signal δ = | Ω'av / Ωav - Ω'ar / Ωar | or :

Ω denotes the true rotation speed of the front wheel,

Ω 'denotes the first derivative of the true rotational speed of the front wheel, Ω 3.2T denotes the true rotational speed of the rear wheel,

Ω 'denotes the first derivative of the true rotational speed of the rear wheel.

The two terms characterize the dynamics of the front and rear wheels: this term δ, substantially constant over the normal grip range, increases suddenly when one of the two wheels comes close to its maximum grip. Near-maximum detection is obtained when δ crosses a predetermined threshold, which can be self-generated from previous values (for example 150% or 200% of this substantially constant previous level).

For braking, the rear wheel used in the calculation of δ can be fixed a priori (eg same side), or the faster of the two rear wheels. For skating, one can advantageously use two wheels on the same side.

In this mode, the value δi of the signal δ represents the relative variation of the adhesion μ '/ μ at the instant ti of the i-th measurement. Its numerical integration ∑δi therefore represents μ (t), by its logarithm ∑δi = k.Ln (μ (t)) where k is a constant substantially equal to 1. From this expression are deduced from other detection variants: In a first variant of this detection by δ, the detection by anticipation of the maximum adhesion is obtained when the calculated term C = δi. exp (-∑δi / K), representing the variation in the coefficient of adhesion (C ~ dμ / dt), crosses a certain threshold S - or what amounts to the same when ln (C) = ln (δi). (-∑δi) / K crosses a certain threshold If [Si = ln (S)] or when Φ (C) crosses the threshold Φ (S), Φ being a monotonic guelcongue function -, where δi is the value of δ = | Ω ' ^av / Ω ^av -Ω' ^r / Ω ^ar j, at ti (i-th measure) and K the measurement frequency (K = l / δt)

Advantageously, the threshold S - or the threshold Si = ln (S)], or the threshold S '= Φ (S) - depends on the gradient β = pi - ρi-1 of braking recession, and increases with it. According to another implementation variant, detection by anticipation of the maximum adhesion is obtained when the calculated term M = exp (-Σδi / K) representing the coefficient of adhesion (μ) crosses a certain threshold S or when the term L = (-Σδi) / K [L = ln (M)] crosses a certain threshold Si [Si = ln (S)], or when the term Φ (M) reaches Φ (S), Φ being a monotonic function guelcongue -, where: δi is the value of δ = | Ω ' ^av / Ω ^av - Ω' ^ar / Ω ^ar | at the instant ti of the i-th measurement, and for example:

K the measurement frequency (K = l / δt), and μo ~ l • This threshold S can be predetermined, or calculated from the measured parameters. It depends on the instantaneous adhesion conditions and these conditions are determined from the pair of values [β.δj, ∑δ], for example at the end of the No = j first measurements, where β = pj-pj-1 is the braking pressure gradient dp / dt, dt = l / K being the measurement interval δj is the value of δ = | Ω ' ^av / Ω ^av -Ω' ^r / Ω ^ar | at tj

C = μO.δ .exp (-Σδj / K) -dμ / dt, (μ0 ~ i), being the real adhesion is determined by comparison of the couple of calculated values Cl = ∑δj, C2 = β. dj. exp (-∑δj / K) [respectively proportional to dF / dμ and μ] to the various adhesion curves F = y (μ) modeled a priori, for example in the form μ, y '(μ) [y' (μ) = dF / dμ] - or any other equivalent form. A variant advantageously making it possible to eliminate certain artefacts from the data coming from the pressure and / or speed sensors consists in applying to the function δ determining the activation threshold for closing the valve a filtering function. This filtering function can be a function proportional to time, a function proportional to the braking pressure applied to the wheel, a function inversely proportional to the braking pressure gradient dP / dt, or a combination of these functions or the association a thresholding of the pressure signal eliminating the data corresponding to pressures lower than a threshold value and corresponding to measurement noise.

In this variant, the signal taken into account for the detection of the valve closing threshold will be a signal of type: dμ / dt.l / (dp / dt) .f (p, t, dt / dp).

The closing of the supply valve of the slave cylinder of the braking system of the wheel concerned will occur when dμ / dt.l / (dp / dt) .f (p, t, dt / dp)> Vs where

Vs designates a predetermined threshold value f (p, t, dt / dp) designates the filtering function. are applied when braking,

In all cases, when these various variants are applied to braking, the detection of the guasi-maximum causes the closure of a valve which "freezes" the pressure, then optimal, for the wheel concerned. [For mode # 2 and its derivatives, Ω '/ Ω for this wheel, not locked (μ constant and optimal), then serves as the reference dynamic for the other wheel: detection by δ unchanged]. This valve is only kept closed as long as the pressure in the upstream circuits (pilot) is greater than or equal to the downstream pressure (brake cylinder). These two detection variants can be used in combination according to a mode which depends on the desired priorities:

In a first mode, the valve will be closed when it detects: - the maximum braking power C.Ω, U

- the rapid growth of the signal δ = | Ω'av / Ωav

- Ω 'ar / Ωar | .

This mode of combination by an "OR" logic gives anti-lock priority, and is particularly suitable for emergency braking.

The second combination mode consists in closing the valve when it is detected:

- the maximum braking power C.Ω, EL

- the rapid growth of the signal δ = | Ω'av / Ωav

- Ω 'ar / Ωar | .

This second combination mode is more particularly suited to moderate braking situations and avoids untimely closing of the valve.

It is possible to automate the transition from the "OR" to the "AND" log by slaving to the gradient δp / δt of the pressure exerted on the brake pedal. When this is less than a threshold value, the "OR" log is activated, when it reaches a threshold value, the "AND" log is activated.

At the end of the first phase (pressure build-up) the braking pressure and force are "locked" to the maximum guasi - if the pressure exerted by the pilot has reached this limit. Then, for a constant μ adhesion, Ω 'remains constant (Ω decreases linearly). When the condition of the road surface and the coefficient of grip vary during the application of the torque on the wheel (eg braking which starts on dry road, and passing over an oil patch or vice versa), the method according to the invention allows torque regulation according to the variation of road conditions:

Improved detection adhesion via Ω'1

: If the grip increases (e.g. passage from greasy to dry pavement) Ω varies proportionally to μ, resulting in a sudden variation of | Ω '|

This detection controls the reopening of the isolation valve, and an increase (if any) of the pressure, depending on the braking pressure exerted by the pilot: the cycle then begins again. In an anti-skid device, this detection will command a reduction in the torque reduction (progressive suppression of the braking of the wheel concerned for example)

Decrease in grip [detection via Ω'1: A decrease in grip will tend to cause the wheel to lock up, resulting in a sudden change in | Ω '|

To prevent blocking it is necessary to reduce the braking pressure:

Minimum passive system (or: "intelligent" limiter): An optical / acoustic signal warns the pilot, who must "pump" (release, then brake again);

Minimum active global system (economic): the rapid temporary opening of a valve or by-pass valve between low and high pressure of the braking assistance momentarily removes the assistance, hence the temporary drop in braking pressure, which goes back up immediately: the cycle begins again.

Optimized active system (separate action for each wheel): The braking pressure is transmitted, for each wheel, by a mobile differential piston - in equilibrium under two forces, F = ps on the pilot side and -ps on the brake side - piston which can also be subjected to an external force - electromagnetic (Figure 3), or other (Figure 8 and 9). The application of a force f <F (eg -0.5.F) opposing the pilot pressure proportionally reduces the downstream pressure and prevents the blocking of the wheel concerned (the cycle then begins again), without modifying the upstream pressure, therefore transparently for the pilot who will not feel any reaction or vibration.

Maximum active system, (ABS + new optimal pressure function): In this case, the pressure drop applied to return to the interior of the available grip (stable area before the maximum) - and the pressure rise which follows (the cycle starts again) are obtained by the usual means of ABS.

As the pilot's decision has priority, the pressure in the downstream circuits (brake cylinder) must never be higher than the pressure in the upstream circuits. - an upstream pressure greater than or equal to the downstream pressure controls the opening of the valve d 'insulation.

In the case of an anti-skid device, this detection of a sudden variation of | Ω '| will command a reduction in torque: either globally a reduction in the engine torque, or locally a reduction in the torque applied by increasing the braking of the wheel concerned for example, or both at the same time. Malfunctions and safety

In the event of a malfunction of one of the detection systems, the braking optimization system remains fully operational.

In the event of a malfunction of its two detection systems, the isolation valve being in the open position at rest, the braking system remains fully functional (safety), the optimization / anti-blocking function no longer being ensured. The goal main of the invention being to obtain an optimal adhesion, the use of only one of these two detection systems, sufficient to obtain this optimal adhesion, remains of the field of the invention. Hydraulic system preferably, when the detection threshold is reached, the brake pressure applied to the wheel is kept constant by closing a valve located upstream of the brake cylinder on the main brake circuit which transmits the hydraulic pressure applied by the pilot, this valve being re-opened as soon as the pressure upstream of the main circuit becomes lower than the pressure downstream of the brake cylinder, and in that the braking device also comprises, in bypass on this main circuit, a regulation circuit intended to adjust this pressure in the event of variation in adhesion during this phase when the valve is closed, by means of a floating piston, permanently upstream subjected to the hydraulic pressure, applied by the pilot, of the main circuit, and hydraulically connected to the brake cylinder either via an open valve, separate or coincident with the valve, or, during limitation-regulated phases lation where the valve is closed, via a non-return valve allowing the depressurization of the brake cylinder; this floating piston can also be subjected during the pressure limitation-regulation phases to an external opposing force opposing the upstream pressure, and thus making it possible to reduce the downstream pressure, therefore the pressure in the brake cylinder.

Advantageously, the floating piston has a chamber normally connected to the low reservoir pressure, and which can be, in the event of a sudden variation in the deceleration Ω '- therefore of the adhesion - while the valve is closed, temporarily put into communication, by a valve or solenoid valve, with the hydraulic braking pressure applied by the pilot, which makes it possible to reduce the downstream pressure, as well as that of the braking cylinder up to a predetermined threshold which controls the re-opening of the valve and initiates a new braking and detection cycle for optimal grip, possible restrictions or making it possible to ensure in this cycle a compatible pressure rise gradient with the system response time.

According to a particular embodiment, the floating piston comprises a chamber normally under low pressure, and which can be, in the event of sudden variation in the deceleration Ω '- therefore of the adhesion - while the valve is closed, temporarily put into communication , by a valve or solenoid valve, with a high hydraulic pressure of external origin, and supplied by a device comprising a jack consisting of a plunger and a chamber, which may be integral with one of the chassis, and the other of the wheel, the force transmitted by the piston during braking generating in the cylinder an internal pressure balancing this force, therefore increasing with the load transfer and having an anti-diving effect, a high pressure accumulator, and a low accumulator pressure, both in communication with this room at rest (except braking), a valve or solenoid valve activated by the braking pressure and isolating the two accumulators during the braking phases swimming, the maximum pressure induced by the force of the piston during this phase being transmitted via a non-return valve to the high pressure accumulator which stores this pressure energy, the high pressure accumulator being temporarily put in communication, in case sudden variation of the deceleration Ω 'therefore of the grip while the valve is closed, by a valve or solenoid valve with the chamber, normally connected to the low pressure, in order to decrease the braking pressure and initiate a new search cycle of optimal grip, the gradient of pressure build-up which can be adjusted by possible restrictions.

According to a particular variant, the floating piston includes a chamber normally under negligible pressure, and which can be in the event of a sudden variation in the deceleration Ω '- therefore of the adhesion - while the valve is closed, put under high pressure by the means of a jack, electric or other, compressing by a plunger the hydraulic oil of a chamber communicating with the chamber, in order to reduce the braking pressure and initiate a new cycle of search for optimal grip, the gradient of pressure build-up in this cycle being controlled by the cylinder return speed.

The invention will be better understood on reading the description which follows, referring to the accompanying drawings in which:

- Figure 1 shows the braking / sliding force coefficient curve;

- Figure 2 shows a schematic view of the system;

- Figure 3 shows a schematic view of an alternative embodiment of the braking circuit;

- Figure 4 shows the algorithm of a first embodiment of the detection; - Figure 5 shows the algorithm of a second embodiment of the detection.

- Figure 6 shows the algorithm for monitoring variations in adhesion.

- Figure 7 shows the algorithm of optimal grip detection applied to one anti-skid. FIG. 8 represents the diagram of a device for adapting the pressure in the event of a fall in the available grip.

- Figure 9 shows an embodiment of this pressure adaptation device. - Figure 10 shows another embodiment of this pressure adaptation device.

- Figure 11 shows another embodiment of r this pressure adaptation device. - Figure 12 shows another embodiment of this pressure adaptation device.

- Figure 13 shows an external pressure supply device.

- Figure 14 shows another embodiment of this pressure adaptation device, in a closed circuit.

- Figure 15 shows another variant of this pressure adaptation device in the closed circuit.

Figure 2 shows the block diagram of the braking circuit and the detection system. It comprises in known manner a brake cylinder (1) supplied by a conduit (2) connected to a brake distributor (3) receiving the brake fluid under pressure from the master cylinder (4) associated with the brake pedal According to the invention, a solenoid valve (6) is interposed between the brake pedal and the brake cylinder (1). The solenoid valve (6) occupies at rest a position in which it allows the passage of the brake fluid between the brake distributor (3) and the brake cylinder (1). The operation of the hydraulic braking circuit is therefore not disturbed. On the other hand, when the solenoid valve (6) is activated, it interrupts this braking circuit, and the hydraulic pressure in the brake cylinder (1) is maintained at the value it had at the time of activation of 1 '' solenoid valve (6). When the solenoid valve (6) reopens, the pressures between the brake distributor (3) on the one hand and the brake cylinder (1) occur on the other. The hydraulic system (5) also includes a regulation circuit for adjusting the braking pressure in the event of variation in grip. The electronic box controls the hydraulic system based on data received from speed and pressure sensors.

The invention relates more precisely to the control of the solenoid valve (6). The object of the invention is to control the activation of the solenoid valve (6) just before the braking force reaches its maximum value, which corresponds to the maximum grip. The invention described below proposes two modes for determining the approach of the maximum value of the braking force. The first method consists in cyclically calculating the braking power by applying the product between the braking torque which can be measured from the braking pressure in the braking cylinder (1) (or from the reaction of the caliper ) on the one hand and the speed of rotation of the wheel associated with this brake cylinder on the other hand. The calculation of the braking power, in the case of a vehicle with several braked wheels, is carried out independently for each of the wheels.

A first example of controlling the solenoid valve (6) consists in determining the moment when the product of the pressure pi in the braking system and the angular speed Ωi decreases. An algorithm of this first control method is shown in FIG. 4.

This process consists in carrying out the periodic acquisition, at times ti, of the following two parameters:

- the pressure pi in the braking system

- the angular speed Ωi,

- the product Pi of the two previous values, Pi = Ωixpi. The start t0 of the acquisition phase of these two parameters corresponds to the start of braking. This start t0 may correspond to the signal from a detector on the brake control, for example at the brake pedal, or more simply to a certain (minimum) brake pressure threshold. This detection of a braking action initializes the values Ωi and pi and triggers the iteration of the index i.

The values of the three parameters Ωi, pi and Ωixpi and Ωi ₊ i, pi ₊ 1 and Ωi ₊ i pi + i are stored in registers in random access memory. At each iteration of i, a shift register replaces the values corresponding to the index i-1 with the values corresponding to the index i, and stores the new value i + 1. The last n values of Ω, p, pxΩ are kept, as well as their respective sums, and the smoothed value Pi of Pi = Ωixpi (excluding sensor noise) calculated and stored.

A comparator checks if Pi ₊ i is less than Pi. If the response is negative, it iterates i, and the acquisition phase continues. If the response is positive, the solenoid valve (6) is closed.

The reopening can occur at different times, for example by a timed command, or by a signal delivered by a comparator comparing the pressure upstream of the solenoid valve (6) and the pressure downstream of this solenoid valve, and controlling the opening when the downstream pressure is greater than the upstream pressure.

FIG. 5 represents the algorithm of another mode for detecting the adhesion limit, and for controlling the solenoid valve (6).

According to the control mode represented in FIG. 5, the solenoid valve is activated when the difference between Ω '/ Ω of the front wheel and Ω' / Ω of the rear wheel is greater, in absolute value, than a threshold value, for example a value equal to 150 or 200% of the value before braking. For this purpose, starting from the start of braking, the periodic acquisition of the value Ω of the rotation of the front wheel on the one hand and of the rear wheel on the other hand is carried out. We then periodically calculate the derivative Ω 'with respect to time and the ratio Ω' / Ω. According to a variant, 1 ^• solenoid valve is activated when the difference between Ω '/ Ω of the front wheel and Ω' / Ω of the rear wheel is greater, in absolute value, than a threshold value, for example a value equal to 150 or 200% of the average value of the previous values (from tl to ti-k). This is done, from the start of braking, to the periodic acquisition of the value Ω of the rotation of the front wheel on the one hand and of the rear wheel on the other hand.

We then periodically calculate the smoothed derivative Ω 'with respect to time, and the ratio Ω' / Ω, for each wheel.

Then we calculate at each period the term δi = (Ω '/ Ω) front wheel - (Ω' / Ω) rear wheel) and the sum ∑δi, from which we can deduce the average value of δ, £ i = (∑δi ) / i

A comparator checks whether the smoothed value of δi is greater than the predetermined threshold S, which can be a function of previous measurements, or of the pressure gradient dP / dt: for example, 200% of the average value δ_k after k measurements, with k = i-10. If the answer is negative, it iterates i, and the acquisition phase continues.

If the response is positive, the solenoid valve (6) is closed.

The re-opening can take place at different times, for example by a timed command, or by a signal delivered by a comparator comparing the pressure upstream of the solenoid valve (6) and the pressure downstream of this solenoid valve, and controlling the opening when the downstream pressure is higher than the upstream pressure.

According to other variants of this mode, in addition to the terms Ω'AV, Ω'AR, δi = (Ω '/ Ω) AV - (Ω' / Ω) AR and the term ∑δi, an auxiliary variable Φ ( δi, ∑δi).

In a first variant, the term C = δi .exp (-Σδi / K) is calculated, representing the variation in the adhesion coefficient (C ~ dμ / dt), and the solenoid valve is activated when C crosses a certain threshold S - or what amounts to the same when Φ (C) crosses a certain threshold If [Si = Φ (S) j [Φ being any monotonic function]. This threshold S can be predetermined, or depend on the parameters measured.

In another variant, the term M = exp (-Σδi / K) representing the adhesion coefficient μ is calculated, and the solenoid valve is activated when M crosses a certain threshold S - or this mistletoe returns to the same when Φ (C ) crosses a certain threshold If [Si = Φ (S)] [Φ being a monotonic guelcongue function]. This threshold S can be predetermined, or depend on the measured parameters, or on the adhesion conditions, which can be determined from the measured parameters:

According to a particular embodiment of this variant, the adhesion conditions are determined from the pair of values [β.δj, ∑δj] at the end of the No = j first measurements - where β = pj-pj-1 is the braking pressure gradient dp / dt, dt = l / K being the measurement interval - the actual grip being determined by comparison of the couple of calculated values

.exp (-Σδj / K), C2 = ∑dj, [respectively proportional to dF / dμ and μ] to the various adhesion curves F = y (μ) modeled a priori - for example in the form μ, y '( μ)) = dF / dμ], or any other equivalent form.

According to another variant, applicable to an isolated wheel, of this control mode shown in FIG. 5, the solenoid valve is activated when the value of the term | Ω '/ Ω | of the wheel becomes greater than a threshold value (increasing) function of the instantaneous braking pressure.

FIG. 6 shows the algorithm for checking instantaneous grip and adapting the braking pressure while the solenoid valve (6) is closed.

According to the control mode represented in FIG. 6, after detection of the almost optimal adhesion by any of these modes, and closing of the solenoid valve (6), the variations in Ω 'are monitored, which remains constant as long as grip is constant. To this end, after the solenoid valve is closed, the acquisition continues of the value Ω of the rotation of the corresponding wheel. The derivative Ω 'is then periodically calculated with respect to time.

Instead of the derivative of Ω, it is also possible to monitor the derivative of the power, the two variables being directly proportional during the valve closing period.

To this end, after continuing the solenoid valve, the periodic acguisition of the value Ω of the rotation of the corresponding wheel continues. The derivative Ω 'is then periodically calculated with respect to time.

When a sudden or rapid change in Ω 'is detected, compared to previous measurements - for example their average Ω' ni - either it is an acceleration [Ω '>Ω' _m and | Ω '| <| Ω ' _m |], that is a deceleration [Ω'<Ω' _m and | Ω' | > | Ω ' _m |].

In the first case, which corresponds to an improvement in adhesion, the reopening of the solenoid valve (6) is ordered: the cycle begins again.

In the second case, which corresponds to a decrease in adhesion, a reduction in the braking pressure upstream of the solenoid valve (6) is commanded, sufficient to return to the stability domain (domain before the maximum adhesion ): for example, at around 50% of the braking pressure value corresponding to the maximum grip on dry ground. This pressure, lower than the pressure downstream of the solenoid valve (6), causes the solenoid valve to reopen, and the cycle begins again.

In the alternative embodiment of FIG. 3, this pressure reduction is obtained by means of a regulating solenoid valve (7). This solenoid valve (7) comprises a core (8) making it possible to vary the value of the downstream pressure independently of the pressure imposed by the brake distributor. To this end, the displacements of the core (8) cause a variation in the downstream pressure by a value F / S where F denotes the force opposite to the upstream pressure exerted on the core by the winding and S the section of the core. The upstream pressure variation control can also be carried out in a known manner by an ABS type system.

In another variant (FIG. 8), the force F applied to the pressure transmitter piston (8), in order to reduce the downstream braking pressure in the event of a decrease in grip, is obtained by the application of a repelling hydraulic pressure the piston, here in the annular chamber (15). This hydraulic pressure is supplied by a vacuum cylinder (9), analogous to those of the brake assist devices known as vacuum booster, or by an electric cylinder, which compresses the fluid by means of a piston (10). of the high pressure chamber (11) - this hydraulic pressure could also be supplied by a pump -. This vacuum cylinder will for example be activated under each brake. During the initial braking phase, the annular chamber (15) remains in communication, via

1 solenoid valve (14), with a low pressure accumulator

(12) and the back pressure in the chamber (15) remains negligible. If the limit braking conditions have been reached, the solenoid valve (6) has been closed. If during this phase where the solenoid valve (6) remains closed, a decrease in adhesion is detected (by sudden variation of Ω '),

1 solenoid valve (14) is briefly switched, this mistletoe puts the chamber (15) in communication with the high pressure (11), and drops the brake pressure downstream of (8), depending on the dimensioning - for example to 50% or less of the pressure corresponding to optimal braking on dry ground. This drop in downstream pressure - which has thus become lower than the pressure in the brake cylinder - leads to the reopening of the solenoid valve (6), and a very brief reduction in the braking applied to the wheel, which returns to the conditions of adhesion: 1 solenoid valve (14) immediately returning to its initial position, the pressure begins to increase again and the cycle begins again.

At the end of braking, the vacuum servo-cylinder becomes inactive, the force of the piston (10) disappears, the pressure drops in the chamber (11), authorizing the return of the BP fluid by the valve (13).

The diagram in FIG. (9) represents a possible example of an embodiment of the pressure regulation device in the form of a compact circuit. The movable piston (8) can be used as a pressure booster, and include a safety limit switch (19) which communicates the upstream and downstream pressures when the piston (8) comes to a stop - due to abnormal leaks and drift : the active reduction of the braking pressure in the event of a decrease in grip after intervention of the pressure limiting solenoid valve (6) then becomes ineffective (the pressure being however reduced from P downstream to P upstream in the event of amplification) , but all the other braking functions are retained (safety).

In another variant (FIG. 10), a bypass (20) open at rest, and closed during the braking phases by a controlled valve (21), connects upstream and downstream to facilitate filling and purging of the circuit, and allow the rest ( except braking), under the action of the spring (22) slightly tared, the return of the piston (8) in the initial position in case of drift due to abnormal internal leakage. Again, the loss of the valve shutter function results in the only loss of the active reduction of the brake pressure and safety remains assured. With this bypass (20), the drift always being brought to zero, the safety valve would never be stressed, and becomes superfluous.

The low pressure accumulator can be obtained by simple use in the chamber (13) of a compressible foam (17), advantageously insulated by a membrane (18), and possibly swollen (low pressure), or conventionally by swollen membrane.

For better clarity, Figures 9 and 10 show the diagram without scale and for a single wheel, hence a single solenoid valve (14) + piston (8) sub-assembly shown, and the solenoid valve (6) braking, on the downstream pressure P at the outlet of (8), is not shown: Advantageously, the chambers (11) and (12) of this figure 9 will be placed in communication with several solenoid valves of the same type as (14) - one for each wheel - associated with pistons of the same type (8) - for example according to a circular symmetry, of order 4 for 4 wheels. The valves, solenoid valves (14), (6), and pressure adapter piston (8) can be very small. FIG. 12 represents a particularly simple variant of the invention. In this variation,

(figure 12 and diagram 11), the main circuit (11) is normally open, and in communication with upstream and downstream of the floating regulating piston (8), in equilibrium under these two identical pressures, the chamber 15 being unpressurized

(tank pressure). When the detection threshold is reached, the brake pressure applied to the wheel is kept constant, the brake cylinder being isolated from the main circuit by closing the solenoid valve (6), and from the regulation circuit by the valve 16. The detection during this phase of a loss of adhesion, by abrupt variation of Ω ', temporarily activates the solenoid valve 14 which puts the chamber 15 of the floating stage piston in communication with the hydraulic braking pressure, reducing the downstream pressure in proportion to the section reports - at p ~ 0 if S ~ s

(Annular section, end section). A non-return valve 23 functionally in parallel with

1 solenoid valve 16, causes the simultaneous drop in hydraulic pressure in the brake cylinder. When this downstream pressure has reached the reguis threshold, the solenoid valve 14 returns to its initial position, depressurizing the chamber 15, again in communication with the reservoir pressure, and the valve 16 is reopened, letting the downstream pressure rise; the pressure gradient can be adjusted by possible restrictions 24 and 25 to ensure its compatibility with the response time of the system.

Advantageously, in the variant of FIG. 11, the solenoid valves 6 and 16 are replaced by a single solenoid valve. In another variant (fig 14), which creates a closed circuit, the two annular chambers of the floating stage piston are in communication, by means of a 4-way / 2-position solenoid valve, which may be produced by four piloted valves , with those of a second identical or similar stage piston, subjected at one end to the pilot's braking pressure: at rest, the valve communicates the two annular chambers of the regulating piston; When activated (regulation phase), it sends the pressure (pilot) to the downstream annular chamber, and reduces the braking pressure accordingly. Only one such auxiliary piston can be used for all four wheels. In a closed circuit variant of the device of FIG. 14 (fig 15), a non-return valve (30), piloted or not, and a non-return valve (31) are used to isolate the brake cylinder, a third, non-return valve, piloted, allowing the pressure to be sent temporarily to the annular chamber to reduce the braking pressure: an internal or external bypass (33), with restriction, allowing the pressure in the brake cylinder to then rise gradually, until the new optimum.

In another variant of the above device, the high and low pressures for supplying the annular chamber of the floating regulating piston 15 are of external origin and supplied by the device of FIG. 13, which comprises: a cylinder consisting of a plunger 11 and a chamber 11, which can be respectively secured to the chassis and the wheel, the force transmitted by the piston during braking then being proportional to the load transfer, hence in addition to an anti-diving effect, an HP accumulator 26, and a BP accumulator 27 both in communication with this chamber 11 at rest (apart from the braking actions), a valve 28 activated by the braking pressure, which isolates the two accumulators during the braking phases: the maximum pressure then induced in the chamber 11 by the force of the piston 10 during this phase is transmitted via the non-return valve 29 to the HP accumulator which stores this pressure energy, which may, in the event of a fall in grip during optimal braking, be transmitted by the solenoid valve 14 to the chamber 15 of the floating piston, in communication otherwise by this same solenoid valve 14 with the low pressure accumulator.

After braking, the valve 28 returning to its rest position puts the accumulators 26 and 27 back in communication with the chamber 11, depressurizing the HP accumulator 26.

FIG. 7 represents the algorithm for detecting the adhesion limit and controlling the torque applied during the acceleration phases (traction control). According to the control mode represented in FIG. 7, the torque applied is reduced when the difference between Ω '/ Ω of the driving wheel and Ω' / Ω of the driven wheel is greater, in absolute value, than a threshold value, which can be self-generated, for example a value equal to 150 or 200% of the average value observed during the initial phase, before skating (stability zone, before maximum grip).

To this end, from the start of the acceleration, there is a periodic acquisition of the value Ω of the rotation of the driven wheel on the one hand and of the wheel. leading on the other hand. We then periodically calculate the derivative Ω 'with respect to time and the ratio Ω' / Ω.

The invention is described in the foregoing by way of nonlimiting example. Those skilled in the art will be able to produce various variants without departing from the scope of the invention.

Its optimal efficiency, which leads to great comfort of use thanks to the absence of vibrations and jolts, combined with its simplicity, and its low cost, make it applicable to all types of motor vehicles, including two wheels.

Claims

1 - Method for optimizing the use of the available grip when a couple - of braking or acceleration - is applied to the wheels of a vehicle, automobile or re-drive, characterized in that the cease of increasing torque applied to the wheel when the grip reaches a value very close to, but less than, the maximum value.

2 - Method for optimizing the use of the available grip, according to claim 1, when a braking torque is applied to the wheels of motor vehicles, characterized in that a valve (6) is closed ) arranged between the brake pedal and the brake cylinder (1) when the grip reaches a value very close to but less than the maximum value.

3 - Method for improving the performance of a motor vehicle braking system according to claim 2, characterized in that a valve (6) disposed between the brake pedal and the cylinder is closed braking (1) when the maximum braking power p.Ω is detected, where

p designates the braking torque or pressure Ω designates the actual angular speed of the wheel on which the braking is applied,

4 - Method for improving the braking or acceleration performance of motor vehicles according to claim 1, characterized in that it stops increasing the torque applied to the wheel when the sudden growth of d is detected = | Ω ' ^av / Ω ^av -

/ Ω OR: Ω ^av designates the true rotation speed of the front wheel,

Ω ' ^av designates the first derivative of the true rotation speed of the front wheel, Ω designates the true rotation speed of the rear wheel,

5 A method for improving the performance of a motor vehicle braking system according to claim 2 characterized in that one proceeds to the closure of a valve (6) disposed between the brake pedal and the brake cylinder (1) when δ - | Ω'av / Ωav - Ω 'ar / Ωar | reaches a predetermined threshold value ds.

6 - Method for improving the performance of a motor vehicle braking system according to claim 5 characterized in that the threshold value δs is self-generated from previous values, and a predetermined multiplier.

1 method for improving the performance of a motor vehicle braking system according to claim 2 characterized in that one proceeds to the closure of a valve (6) disposed between the brake pedal and the brake cylinder (1) of a wheel when the term | Ω '/ Ω | for this wheel crosses a self-generated threshold from the instantaneous value of the braking pressure (pilot), and increasing with this pressure, and from predetermined coefficients, where:

Ω denotes the true rotation speed of the wheel, Ω 'designates the first derivative of the true rotation speed of the wheel.

8 - A method for improving the performance of a motor vehicle braking system according to one of the preceding claims 2 to 7, characterized in that the controlled valve (6) is not kept closed for as long gue the pressure in the upstream circuits (pilot) is greater than or equal to the downstream pressure (brake cylinder).

9 - Method for improving the performance of a motor vehicle braking system according to claim 2 characterized in that a valve (6) is closed when an emergency braking is detected and :

- the maximum braking power p.Ω,

SU

- the rapid growth of the signal δ = | Ω'av / Ωav - Ω 'ar / Ωar | .

10 - A method for improving the performance of a motor vehicle braking system according to ^{* '} claim 2 characterized in that a valve (6) is closed when moderate braking is detected and :

- the maximum braking power p.Ω,

≤

- the rapid growth of the signal δ = | Ω'av / Ωav - Ω 'ar / Ωar | .

11 - Motor vehicle braking system implementing a method for improving braking performance, according to one of the characteristics 1 to 6, characterized in that it includes sensor devices for detecting parameters of operation of the vehicle as well as an electronic control and adjustment device which, after analysis of the output signals from the sensor devices sent to it, generates braking pressure control signals, and that the sensor devices are made up on the one hand by a sensor of the braking pressure pi and on the other hand by a sensor capable of measuring the speed of rotation Ωi of the wheel on which the braking is applied, the system further comprising a valve (6) interposed between the brake pedal and the brake cylinder, the valve (6) interrupting the hydraulic circuit between the pedal and the brake cylinder when the braking power decreases.

12 - Anti-blocking adjustment system for hydraulic braking systems according to claims 9 or 10, characterized in that the valve (6) interrupts the hydraulic circuit between the pedal and the brake cylinder when the signal corresponding to Pi = pi x Ωi decreases.

13 - Anti-blocking adjustment system for hydraulic braking systems according to claims 9 or 10, characterized in that the valve (6) interrupts the hydraulic circuit between the pedal and the brake cylinder when rapid growth is detected. signal δ = | Ω'av / Ωav - Ω'ar / Ωar | .

14 - Method for improving the performance of a motor vehicle braking system according to any one of claims 2 to 12, characterized in that after closing the valve (6) disposed between the brake pedal and the brake cylinder (1) of the wheel, any variations in the grip conditions for this wheel are detected by detecting the sudden change in | Ω '|, where Ω 'designates the first derivative of the true rotation speed of the wheel.

15 - A method for improving the performance of a motor vehicle braking system according to claim 14, characterized in that a positive variation of Ω '(acceleration), corresponding to a gain in grip, controls the reopening of the valve (6) disposed between the brake pedal and the brake cylinder (1) of the wheel.

16 - A method for improving the performance of a motor vehicle braking system according to claim 14, characterized in that a pressure proportional to the upstream braking pressure (pilot) is transmitted downstream (brake cylinder) by a piston floating in equilibrium under these two pressures, which can also be subjected to a possible external force.

17 - A method for improving the performance of a motor vehicle braking system according to claim 16, characterized in that the floating piston can be subjected to an external force of electromagnetic origin.

18 - A method for improving the performance of a motor vehicle braking system according to claim 16, characterized in that the floating piston comprises at least a third chamber capable of receiving hydraulic pressure of external origin, independently and in addition upstream and downstream pressures, and can thus be subjected to an external force of hydraulic origin. 19 - A method for improving the performance of a motor vehicle braking system according to claim 18, characterized in that the external hydraulic pressure is supplied by a vacuum cylinder compressing a piston in a chamber, and activated at least during braking phases.

20 - A method for improving the performance of a motor vehicle braking system according to claim 18, characterized in that the external hydraulic pressure is supplied by an electric actuator.

21 - Method for improving the performance of a motor vehicle braking system according to claim 18, characterized in that the upstream (pilot side) and downstream (brake cylinder) circuits of the floating piston are hydraulically connected by a conduit closed by a non-return safety valve, closed at rest, and open by a pusher when the floating piston comes to a limit stop on the downstream side.

22 - Method for improving the performance of a motor vehicle braking system according to claim 18, characterized in that the upstream (pilot side) and downstream (brake cylinder) circuits of the floating piston are hydraulically connected by a conduit closed by a controlled non-return valve, open at rest to facilitate filling and bleeding of the system, and activated during closing during braking phases.

23 - Method for improving the performance of a motor vehicle braking system according to claim 22, characterized in that a lightly calibrated spring, downstream side (cylinder of brake), systematically returns the floating piston to the neutral position outside the braking phases.

24 - A method for improving the performance of a motor vehicle braking system according to claim 18, characterized in that the chamber for application of an external hydraulic force can be brought into communication by means of a 2-way solenoid valve , either with an external hydraulic pressure source, or with a low pressure return tank.

25 - A method for improving the performance of a motor vehicle braking system according to claim 18, characterized in that the chamber (15) for application of an external hydraulic force can be brought into communication by means of a 2-way solenoid valve, either with the external hydraulic pressure source, or with a return circuit comprising a low-pressure accumulator forming a reservoir of variable volume.

26 - A method for improving the performance of a motor vehicle braking system according to claim 25, characterized in that the low pressure accumulator consists of a tank partially filled with compressible foam insulated by a membrane.

27 - A method for improving the performance of a motor vehicle braking system according to claims 16 and 8, characterized in that an abrupt negative variation of Ω '(deceleration), corresponding to a decrease in grip, control the brief and temporary application to the floating piston of an external force s Opposing the downstream pressure, and sufficient to reduce the upstream pressure to a level lower than that corresponding to the adhesion limit, and thus cause the reopening of the valve (6) disposed between the brake pedal and the brake cylinder (1) of the wheel, and with the disappearance of the external force and the resulting growth for the upstream pressure, initiate a new braking cycle and detection of optimal grip.

28 - A method for improving the performance of a motor vehicle braking system according to claims 25 and 27, characterized in that in the event of an abrupt negative variation of Ω '

(deceleration), corresponding to a decrease in adhesion, the brief and temporary application to the floating piston of an external force is obtained by hydraulic means, by a brief temporary switching of the solenoid valve (14) which sets the chamber (15) - initially connected to the low pressure accumulator, in communication with the high pressure which exerts on the floating piston a hydraulic force opposing the downstream pressure, and sufficient to reduce the upstream pressure to a level lower than that corresponding at the adhesion limit, and thus cause the valve (6) disposed between the brake pedal and the brake cylinder (1) of the wheel to reopen.

29 - Method for optimizing the use of the available grip when a driving torque is applied to the wheels of motor vehicles, according to claim 1, characterized in that one proceeds to the stabilization of the driving torque, during the grip reaches a value very close to, but less than the maximum value.

30 - A method for improving the performance of a motor vehicle anti-skid system according to claim 29, characterized in that the engine torque is stabilized, ie at overall level of the engine, either at the local level of the wheel concerned by applying a braking torque to this wheel - or both at the same time - when the sudden growth of δ = | Ω'av / Ωav - is detected Ω'ar / Ωar |, where: Ωav denotes the true rotation speed of the front wheel,

Ω'av denotes the first derivative of the true rotation speed of the front wheel,

Ωar denotes the true speed of rotation of the rear wheel,

Ω'ar indicates the first derivative of the true rotation speed of the rear wheel.

31 - Method for] 'improving the performance of a motor vehicle anti-skid system according to claim 29, characterized in that the engine torque is stabilized, at the global level of the engine, or at the local level of the wheel concerned by applying a braking torque to this wheel - or both at the same time - when the term | Ω '/ Ω | for this wheel crosses a self-generated threshold from the instantaneous value of the fuel flow rate, and increasing with it, and from predetermined coefficients, where:

Ω denotes the true rotation speed of the wheel,

Ω 'designates the first derivative of the true rotation speed of the wheel.

32 - Process for improving the braking or acceleration performance of motor vehicles according to claims 1 or 4, characterized in that the torque applied to the wheel is ceased to increase during the calculated term C = di .exp ( -Σdi / K) representing the variation μ 'of the adhesion coefficient μ crosses a certain threshold S, which can either be predetermined, or depends on the parameters measured - or when ln (C) = ln (di). (-∑di) / K crosses a certain threshold Si, [Si = ln (S)], or when Φ (C) crosses the threshold Φ (S), Φ being any monotonic function -, where di is the value of d = | Ω ' ^av / Ω ^av -Ω' ^ar / Ω ^ar | , at ti (i-th measurement) and K the measurement frequency (K = l / δt)

33 - Method for improving the performance of a motor vehicle braking system according to claim 32 characterized in that the threshold S - or the threshold Si = ln (S)], or the threshold S '= Φ (S ) - depends on the gradient β = pi - pi-1 of braking recession, and increases with it.

34 - Process for improving the braking or acceleration performance of motor vehicles according to claims 1 or 4, characterized in that the torque applied to the wheel is stopped increasing when the calculated term M = exp (- Σdi / K) representing the coefficient of adhesion (μ) crosses a certain threshold S - which can be predetermined, or calculated from the measured parameters - or when the term L = (-∑di) / K

[L - ln (M)] crosses a certain threshold If [Si = ln (S)], or when the term Φ (M) reaches Φ (S), Φ being a long monotonic function -, where: di is the value of d = | Ω ' ^av / Ω ^av - Ω' ^ar / Ω ^ar |, at time ti of the i-th measure, and

K the measurement frequency (K = l / δt), and μo ~ l -

35 - Method for improving the performance of a motor vehicle braking system according to claims 32 or 33 or 34, characterized in that the threshold depends on the instantaneous grip conditions and that these conditions are determined from the torque of values [β.dj, ∑dj], for example at the end of the NQ = j first measurements, where β = Pj-Pj ^-1 is the brake pressure gradient δp / dt, dt = l / K being the measurement interval dj is the value of d = | Ω ^{, av} / Ω ^av -Ω ' ^ar / Ω ^ar | at tj C = μO.dj .exp (-Σdj / K) ~ dμ / dt, (μO-l), and in this case the real adhesion is determined by comparison of the couple of calculated values Cl = ∑dj, C2 = β .dj. exp (- ∑dj / K) [respectively proportional to dF / dμ and μ] to the various adhesion curves F = y (μ) modeled a priori, for example in the form μ, y '(μ) [y' ( μ) = dF / dμ] - or any other equivalent form.

36 - Method for improving the performance of a motor vehicle braking system according to one of the preceding claims, characterized in that, when the detection threshold is reached, the braking pressure applied to the wheel is maintained constant by closing a valve (6) located upstream of the brake cylinder (1) on the main brake circuit (2) which transmits the hydraulic pressure applied by the pilot, this valve being re¬ opened as soon as the upstream pressure of the main circuit again becomes lower than the pressure downstream of the brake cylinder, and in that the braking device further comprises, bypassing on this main circuit, a regulation circuit intended to adjust this pressure in the event of variation in adhesion during this phase where the valve (6) is closed, by means of a floating piston (8), permanently subjected upstream to the hydraulic pressure, applied by the pilot, of the cir main fired, and hydraulically connected to the brake cylinder either via an open valve 16, separate or merged with the valve (6), or, during the limitation-regulation phases where the valve (6) is closed, via a non-return valve return allowing depressurization of the brake cylinder; this floating piston can also be subjected during the limitation phases- pressure regulation to an external antagonistic force opposing the upstream pressure, and thus making it possible to reduce the downstream pressure, therefore the pressure in the brake cylinder.

37 - Method for improving the performance of a motor vehicle braking system according to claim 36, characterized in that the floating piston (8) comprises a chamber (15) normally connected to the low tank pressure, and which can be , in the event of a sudden variation in the deceleration Ω '- therefore of the grip - while the valve (6) is closed, temporarily put into communication, by a valve or solenoid valve (14), with the hydraulic braking pressure applied by the pilot, which makes it possible to reduce the downstream pressure, as well as that of the braking cylinder up to a predetermined threshold which controls the reopening of the valve 16 and initiates a new braking cycle and of detection of the optimal grip , any restrictions (24) or (25) making it possible to ensure in this cycle a pressure rise gradient compatible with the response time of the system.

38 - Method for improving the performance of a motor vehicle braking system according to claim 36, characterized in that the floating piston (8) comprises a chamber (15) normally under low pressure, and which can be, in the event sudden variation of the deceleration Ω '- therefore of the adhesion - while the valve (6) is closed, put temporarily in communication, by a valve or solenoid valve (14), with a high hydraulic pressure of external origin, and provided by a device comprising a jack consisting of a plunger (10) and a chamber 11, which can be integral one of the chassis, and the other of the wheel, the force transmitted by the piston during braking generating in the cylinder an internal pressure balancing this effort, therefore increasing with the load transfer and having an anti-diving effect, a high pressure accumulator (26), and a low pressure accumulator (27), in communication all two with this chamber 11 at rest (except braking), a valve or solenoid valve (28) activated by the braking pressure and isolating the two accumulators during the braking phases, the maximum pressure induced by the force of the piston (10) during of this phase being transmitted via a non-return valve (29) to the high pressure accumulator which stores this pressure energy, the high pressure accumulator (26) being temporarily put into communication, in the event of sudden variation in the deceleration Ω '' therefore adhesion while the valve (6) is closed, by a valve or solenoid valve (14) with the chamber (15), normally connected to low pressure, in order to decrease the braking pressure and initialize er a new search cycle for optimal adhesion, the pressure rise gradient can be adjusted by possible restrictions (23) or (24).

39 - Method for improving the performance of a motor vehicle braking system according to claim 36, characterized in that the floating piston (8) comprises a chamber (15) normally under negligible pressure, and which may be in the event of sudden variation in the deceleration Ω '- therefore of the grip - while the valve (6) is closed, put under high pressure by means of a jack, electric or other, compressing by a plunger the hydraulic oil d 'A chamber communicating with the chamber (15), in order to reduce the braking pressure and initiate a new cycle of search for optimal grip, the pressure rise gradient in this cycle being controlled by the speed of return of the jack. 40 - Method for improving the performance of a motor vehicle braking system according to claim 4 or 5 characterized in that one proceeds to the closure of a valve (6) when dμ / dt.1 / ( dp / dt). f reaches a threshold value Vs or passes through a maximum, where

Vs denotes a predetermined threshold value f denotes a filtering function.

41 - Method for improving the performance of a motor vehicle braking system according to claim 40 characterized in that the filtering function f is a function f (p, t, dt / dp) proportional to the temp and / or to the braking pressure applied to the wheel and / or inversely proportional to the braking pressure gradient dP / dt.

42 - A method for improving the performance of a motor vehicle braking system according to claim 36 characterized in that the pressure in the annular chamber of the stage piston is provided by the communication by a valve (34) of the chamber annular piston (14) with the chamber (35), under pressure under the force of the upstream pressure Pp, of a second similar stage piston (36), the two annular non-pressure chambers (38) and (37) being simultaneously put in communication to realize a closed circuit.

43 - A method for improving the performance of a motor vehicle braking system according to claim 37 characterized in that the two annular chambers 15 and 38 of the floating piston stage 8 can be brought into communication by a valve 34 with two chambers respectively under presson and without pressure, of variable volumes and constant total volume, to achieve a firm circuit.

44 - A method for improving the performance of a motor vehicle braking system according to claim 42 characterized in that the two annular chambers 15 and 38 of the floating piston stage 8 can be placed in communication, by means of a 4-way / 2-position solenoid valve 34, which can be achieved by four piloted valves, with those of a second piston identical or similar stage, subjected at one end to the braking pressure of the pilot, the valve 34 putting at rest in communication the two annular chambers 15 and 38 of the regulating piston.