Brake system with pressure-drop compensation and aircraft equipped therewith

Abstract

The invention relates to a braking system comprising at least one brake (1A, 1B) provided with a fluidic actuator (2A, 2B) and a fluid circuit connected to the actuator (2A, 2B), the fluid circuit comprising a pressurized fluid source (12, 13) and a control distributor (3) which is placed between the actuator (2A, 2B) and the fluid source (12, 13) for controlling the actuator (2A, 2B) and which is controlled by pulse width modulation (PWM) by an electronic control unit (5) designed to determine, as a function of a braking setpoint, a duty cycle for controlling the distributor (3). The system comprises a pressure sensor (14) placed in the fluid circuit upstream of the distributor (3) and connected to the control unit (5) to provide it with a pressure signal and the control unit (5) is designed to correct the duty cycle as a function of the pressure signal to at least partially compensate for a drop in pressure. The invention also relates to a landing gear and an aircraft comprising such a system.

Description

[0001]Description Title of the invention: Pressure drop compensation braking system and aircraft equipped therewith. The invention relates to the field of vehicle braking, and in particular aircraft braking. BACKGROUND OF THE INVENTION In the aeronautical field, braking systems are known that comprise brakes, each equipped with a hydraulic actuator and a hydraulic circuit connected to the hydraulic actuator of each brake. The hydraulic circuit includes a pressurized hydraulic fluid source and a two-state pilot valve located between the actuator and the hydraulic fluid source to pilot the actuator. This valve is controlled by an electronic control unit (see, for example, document EP-A-3581446). The pilot valve is generally a 3-way / 2-position solenoid valve (Pilot Solenoid Valve), controlled by a pulsed electrical signal supplied by the control unit.The pilot valve has only two positions: a closed position (actuator on the exhaust side) and an open position (actuator subjected to upstream pressure). Depending on the valve type, the rest position (non-electrically controlled valve) can be either the closed or open position. Brake pressure control can then be achieved by modulating the pulse width (PWM) of the electrical signal supplied by the control unit. The general principle of pulse width modulation is to apply a succession of discrete ON / OFF states for predetermined durations and at a fixed frequency, thus allowing any intermediate pressure value to be obtained on average over a certain period.Modulation is defined by the ratio of the duration in the ON state to the total duration of a cycle (signal period). Thus, a ratio of 0% corresponds to a permanently closed state, a ratio of 100% to a permanently open state, and a ratio of 50% to a valve that is open 50% of the time during a cycle. This ratio is called the duty cycle or PWM ratio. PWM control allows the two-state pilot valve to be transformed into a proportional valve by implementing electrical control. To implement this control mode, the control unit is configured to determine the pilot valve's duty cycle based on a braking command issued by the aircraft pilot. This control mode is particularly effective when the pressure supplied by the hydraulic pressure source is constant, which is the case when the pressure source is a hydraulic pump.However, it is common practice to pair a pressure accumulator with the hydraulic pump to compensate for a hydraulic pump failure. The pressure supplied by such an accumulator tends to decrease as the actuator empties, and aircraft pilots have developed the habit of partially compensating for this pressure drop by increasing the pressure setpoint to the control unit. Nevertheless, braking distances increase when the pressure source is the accumulator, and this increase depends on the pilot's experience and reaction time. This is especially true since a partial failure of the braking system, resulting in the use of the accumulator as a pressure source, causes stress for the pilot. OBJECT OF THE INVENTION The invention aims to provide a more efficient braking system.PRESENTATION OF THE INVENTION To this end, the invention proposes a braking system comprising at least one brake equipped with a fluidic actuator and a fluid circuit connected to the actuator. The fluid circuit includes a pressurized fluid source and a pilot valve located between the actuator and the fluid source to control the actuator. This valve is controlled by pulse-width modulation by an electronic control unit arranged to determine, based on a braking setpoint, a duty cycle for the valve. The system includes a pressure sensor located in the fluid circuit upstream of the valve and connected to the control unit to provide it with a pressure signal. The control unit is arranged to correct the duty cycle based on the pressure signal to at least partially compensate for a pressure drop.Thus, the control unit is designed to compensate for a pressure drop in the pressurized fluid circuit upstream of the servo valve's supply port, regardless of the cause, without pilot intervention. This results in improved braking efficiency in the event of a pressure drop in the pressurized fluid circuit, while also reducing the pilot's mental workload. Depending on optional features, used individually or in whole or in combination: - the control unit is designed to determine a correction factor based on the pressure signal and also the duty cycle; - the correction implements the following formula: ^^ ^ = ^^^( ^(^^, ^^^ × ^^ ; 1^with Min the minimum function, K the correction factor, Ps the pressure obtained from the pressure signal, ^ ^ the duty cycle calculated from the setpoint, ^ ^ ^the corrected duty cycle; - the control unit has a memory containing a table relating reference values ​​for the correction factor, duty cycle, and pressure; - the control unit is configured to determine the value of the correction factor to be applied by interpolation from the reference values ​​contained in the table; - the control unit is configured to apply the correction in the form of a stored mathematical equation; - the mathematical equation is of the type ^(^^, ^^^ = i, j, m, and n are positive integers, Bij is a real number; - the mathematical equation is of the form ^ = ^ × ^^ + ^ with K the correction value, Ps the pressure from the pressure signal, and A and B constants. The invention also relates to an aircraft equipped with such a braking system. The invention will be better understood in light of the following description of a particular embodiment of the invention. DESCRIPTION OF FIGURES Reference will be made to the figures in the accompanying drawings, among which: [Fig. 1] is a schematic front view of an aircraft equipped with a braking system according to the invention; [Fig. 2] is a diagram of the hydraulic system according to the invention; [Fig. 3] is a diagram showing the operation of the electronic control of the braking system by pulse-width modulation; [Fig. 4] is a diagram showing the pressure in the brakes as a function of the control and in the absence of duty cycle correction; [Fig.[Fig. 5] is a diagram showing theoretical values ​​of the corrector according to the invention for different hydraulic circuit pressures of the braking system; [Fig. 6] is a diagram showing the pressure in the brakes as a function of the control input and with duty cycle correction according to a particular embodiment of the invention; [Fig. 7] is a diagram showing the pressure in the brakes as a function of the control input and with duty cycle correction according to another particular embodiment; [Fig. 8] is a diagram showing the distance traveled by the aircraft as a function of time during braking, with and without duty cycle correction. DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1, the invention is described herein with application to an aircraft 100 comprising main landing gear 101 having ends provided with an axle 102 on which are mounted wheels 103 pivoting about a central axis X of the axle 102.Figure 2 illustrates a hydraulic diagram of the braking system according to the invention. The braking system comprises two brakes 1A and 1B associated with braked wheels 103A and 103B of each landing gear 101. The brakes 1A and 1B are known in themselves and each comprises a stack of brake discs consisting of alternating stator discs rotatably connected to the axle 102 and rotor discs rotatably connected to a rim of the wheel 103A, 103B, and a hydraulic actuator 2A, 2B of the cylinder type arranged to press the discs against each other so as to generate, by friction, a braking torque. The hydraulic actuators 2A, 2B are connected to the service port 3.1 of a two-position valve 3, which is connected to an electronic control unit, itself connected to a control instrument 4.The distributor 3 has a movable spool in two positions, one corresponding to a stable rest state (illustrated in Figure 1) in which the spool of the distributor 3 connects the service port 3.1 to a return port 3.2, and, for the other position, to an unstable active state in which the spool of the distributor 3 connects the service port 3.1 to a supply port 3.3. An electromagnetic actuator 7 forces the spool of the distributor 3 to move into the active state when the coil 7 is energized by a control voltage, and a spring 8 returns the spool of the distributor 3 to the rest state when the coil 7 is no longer energized. The return port 3.2 is connected to a fluid reservoir 9 via a non-return valve 10. The supply port 3.3 is connected on the one hand, via a non-return valve 11, to a discharge port of a pump 12 whose suction port is connected to the fluid reservoir 9 and, on the other hand, to a fluid accumulator 13.During operation, the control unit 5 energizes the coil 7 so that the pressure Pc delivered to the brakes 1A, 1B via the service port 3.1 is a function of the position of the instrument (which can be a pedal or a lever) operated by the aircraft pilot. To achieve this, the position of the instrument 4, which represents a braking command, is measured here by means of a potentiometer. The output of this potentiometer is transmitted to the electronic control unit 5, which includes signal processing means adapted to generate a command for a first selector switch that selectively connects one terminal of the coil 7 either to ground or to a DC voltage source (here, 28VDC). This is known in itself. Here, the control unit 5 is adapted to implement a pulse-width modulation (PWM) method illustrated in Figure 3.When instrument 4 is moved by the pilot from an initial position, designated 0%, the selector energizes coil 7 for a predetermined initial time. This facilitates the filling of actuators 2B and 2A of brakes 1A and 1B with hydraulic fluid. Then, for each subsequent period, the control unit 5 determines: - an opening time, which is a function of the instrument 4's position between 0% and 100% (corresponding to instrument 4's maximum stroke), for example, according to a proportional law; - a free exhaust time complementary to the opening time; - a pulsed electrical command C with a duty cycle τs representing the ratio of opening time to total time. The distributor 3 then connects service port 3.1 to supply port 3.3 during the opening time and connects service port 3.1 to return port 3.3.2. During the complementary exhaust time of the opening time: the average pressure Pc seen by the actuators 2A, 2B of the brakes 1A, 1B is a function of the duty cycle τs and increases with this duty cycle τs between the return pressure and the supply pressure. The pulse width modulation frequency naturally depends on the bandwidth of the aircraft's hydraulic circuit, and therefore on the length of the hydraulic circuit, the volume of the cavities of the actuators 2A, 2B of the brakes 1A, 1B… Typically, a frequency of a few hertz is sufficient to ensure proportional control, taking into account the natural filtering induced by the hydraulic circuit. Thus, using a simple two-state valve, we obtain control proportional to the position of the control instrument 4. The pressure Pc in the actuators 2A, 2B is calculated as follows: ^^. ^ ! ^^ = ^^ × ^ " [1]With: ^# the bulk modulus of the fluid, which depends in particular on the fluid pressure, $ the volume of all the pressurized cavities of actuators 2A, 2B downstream of distributor 3, ^ ^ The average flow rate across all these cavities. The average flow rate ^ ^ in the cavities refers to the average flow rates entering and exiting through service port 3.1 of distributor 3, and their relationship is defined as: ^^ = ^% − ^' [2]^ % and ^ ' respectively denote the inflow and outflow of distributor 3 and are defined by the following mathematical relationships (assuming that the flow is constantly turbulent through the service port 3.1 of distributor 3): With : ( ) the pressure loss coefficient across distributor 3, * the cross-sectional area in distributor 3, 2 the density of the fluid flowing through distributor 3, ^ %the supply pressure at the supply port 3.3 of the distributor 3, ^ ' the pressure at the return port 3.2 of distributor 3, ^ %The duty cycle of the control signal emitted by the control unit 5, between 0 and 1. Figure 4 illustrates the pressure curve Pc that can be obtained in actuators 2A, 2B as a function of the pressure at the supply port 3.3 and the duty cycle: - the Pc200 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 200 bar at the supply port 3.3; - the Pc160 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 160 bar at the supply port 3.3; - the Pc120 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 120 bar at the supply port 3.3; - The PC80 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 80 bars at the supply port 3.3; - the Pc60 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 60 bar at the supply port 3.3; - the Pc40 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 40 bar at the supply port 3.3. The pressure that can be obtained in actuators 2A, 2B, and therefore the maximum braking torque that can be obtained, are obviously highly dependent on the pressure at the supply port 3.3. In nominal operating mode, the supply port 3.3 of the distributor 3 is supplied by the pump 12 at a constant pressure of 200 bar. On the other hand, when pump 12 fails, the supply port 3.3 of distributor 3 is supplied by accumulator 13 whose pressure decreases as it empties.It follows that for the same displacement of instrument 4, the pressure that can be obtained in actuators 2A, 2B, and therefore the braking force obtained, depends on the pressure at the supply port 3.3. Figure 6 illustrates the pressure curve Pc that can be obtained in actuators 2A, 2B as a function of the pressure at the supply port 3.3 and the displacement of instrument 4: - the Pc200 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 200 bar at the supply port 3.3; - the Pc180 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 180 bar at the supply port 3.3; - the Pc160 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 160 bars at the supply port 3.3; - the Pc140 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 140 bar at the supply port 3.3; - the Pc120 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 120 bar at the supply port 3.3; - the Pc100 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 100 bar at the supply port 3.3; - the PC80 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 80 bar at the supply port 3.3; - the Pc60 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 60 bars at the supply port 3.3; - the Pc40 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 40 bar at the supply port 3.3. It is therefore understood that the response of the braking system to the movement of instrument 4 is completely different between supply via pump 12 and supply via accumulator 13, and is completely different between the moment when accumulator 13 at the beginning of use provides a pressure of 200 bar and the moment when accumulator 13 at the end of use provides a pressure of 20 bar. According to the invention, a pressure sensor 14 is mounted between accumulator 13 and the supply port 3.3 of distributor 3 (i.e., upstream of distributor 3 with reference to fluid circulation during the supply of distributor 3). The pressure sensor 14 is mounted here just downstream of the accumulator 13 but it could be mounted elsewhere upstream of the power port 3.3 and for example immediately downstream of the check valve 11. According to the invention, the pressure sensor 14 is connected to the control unit 5 to provide it with a pressure signal representative of the supply pressure Ps and the control unit 5 is arranged to correct the duty cycle as a function of the pressure signal to compensate at least partially for a pressure drop. More precisely, the control unit 5 is arranged to determine a correction factor as a function of the pressure signal and also of the duty cycle and the correction implements the following formula: ^. ^ ^ = ^^^( ^(^^, ^^^ × ^^ ; 1^With Min the minimum function, K the correction factor, Ps the pressure obtained from the pressure signal, ^ ^ the duty cycle calculated from the setpoint and a theoretical pressure value of 200 bar, ^ ^ ^The corrected duty cycle. The minimum function prevents the corrected duty cycle from exceeding 1. Figure 5 shows examples of values ​​for the correction factor K as a function of the initial duty cycle. ^ , corresponding to the full supply pressure here 200 bar, and depending on the supply pressure ^ ^ - the curve ^ ^ ^ 3 4^ represents the corrected duty cycle curve, for a pressure of 180 bar at the supply port 3.3; - the curve ^ ^ ^ 3 5^ represents the corrected duty cycle curve, for a pressure of 160 bar at the supply port 3.3; - the curve ^ ^ ^ 3 6^ represents the corrected duty cycle curve, for a pressure of 140 bar at the supply port 3.3; - the curve ^ ^ ^ 3 ,^represents the corrected duty cycle curve, for a pressure of 120 bar at the supply port 3.3; - the curve ^ ^ ^ 3 ^^ represents the corrected duty cycle curve, for a pressure of 100 bar at the supply port 3.3; - the curve ^ ^ ^ 4 ^ represents the corrected duty cycle curve for a pressure of 80 bar at the supply port 3.3; - the curve ^ ^ ^ 5 ^ represents the corrected duty cycle curve, for a pressure of 60 bar at the supply port 3.3; - the curve ^ ^ ^ 6 ^This represents the corrected duty cycle curve for a pressure of 40 bar at the supply port 3.3. It can be seen that the minimum function saturates the correction factor value so that the result of multiplying this factor by the initial duty cycle does not exceed 100%. According to a first, digital implementation, the control unit 5 has a memory containing an array relating reference values: the correction factor, the duty cycle, and the pressure. The control unit 5 is configured to determine the value of the correction factor to be applied by interpolation from the reference values ​​contained in the array. A drawback of this implementation is that a large number of reference values ​​are required in the array to avoid non-linearities in the correction function. Therefore, the memory must be relatively large.According to a second implementation using a microcontroller, the control unit 5 is arranged to determine the correction to be applied in the form of a stored mathematical equation established from basic, trigonometric and / or logarithmic mathematical functions. More precisely, the mathematical equation is non-linear and has a polynomial form of the type: i is a positive integer, j is a positive integer, m is a positive integer, n is a positive integer, B ij is a real number. The coefficients B ij define each polynomial function A i (P s The function A i (P s ) determines the value of the coefficients that define the polynomial function K(P s) of degree n. The values ​​i and j are the indices that define the mathematical expression of the sum and denote the coefficients that encompass the terms to be considered in the sum; this mathematical expression is an international representation. On the other hand, m and n (positive integers) define respectively the degree of the polynomial functions A i (P s ) and K. These values ​​m and n are chosen to obtain the best representation of the correction coefficient K. Generally, a degree 2 is sufficient to obtain a suitable representation. It is possible to define distinct polynomial degrees for each function A i For example, with m=2, we will have: This implementation method requires computing resources such as a microcontroller or microprocessor with its associated software. A purely analog solution is also possible. However, using the equation from the second implementation method for this purpose is impractical because it would necessitate the use of specific analog components such as multipliers, which would complicate the control unit, resulting in higher cost and increased mass. On the other hand, a simpler analog solution based on a linear correction function implemented using simple electronic components is possible. In a third, analog implementation method, the mathematical equation is simpler, and the correction coefficient depends only on the pressure Ps.The equation used to determine the value of the correction coefficient is then linear of the form ^= ^ × ^^ + ^ In this equation, K is the correction factor and Ps the pressure from the pressure signal, A and B are constants. Figures 7 and 8 show the pressure curve Pc that can be obtained in actuators 2A, 2B as a function of the pressure at the supply port 3.3 and the displacement of instrument 4, after correction of the duty cycle by the second implementation mode (figure 7) and by the third implementation mode (figure 8): - the Pc200 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 200 bar at the supply port 3.3; - the Pc180 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 180 bars at the supply port 3.3; - the Pc160 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 160 bar at the supply port 3.3; - the Pc140 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 140 bar at the supply port 3.3; - the Pc120 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 120 bar at the supply port 3.3; - the Pc100 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 100 bar at the supply port 3.3; - The PC80 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 80 bars at the supply port 3.3; - the Pc60 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 60 bar at the supply port 3.3; - the Pc40 curve represents the pressure curve Pc that can be obtained in actuators 2A, 2B for a pressure of 40 bar at the supply port 3.3. By comparing the curves in figures 7 and 8 with those in figure 6 (in which the duty cycle is not corrected to compensate for a pressure drop), we observe that the pressure in actuators 2A, 2B increases much more rapidly with the duty cycle correction, so that, for the same braking setpoint, the pressure in actuators 2A, 2B with the duty cycle correction is higher than the pressure in actuators 2A, 2B without the duty cycle correction.Figure 8 shows the distance traveled by aircraft 100 as a function of time during braking: - curve d1 corresponds to nominal braking in which distributor 3 is supplied by pump 12, - curve d2 corresponds to degraded braking in which distributor 3 is supplied by accumulator 13 with duty cycle correction, - curve d3 corresponds to degraded braking in which distributor 3 is supplied by accumulator 13 without duty cycle correction. It can be seen that the duty cycle correction during degraded braking allows the braking system performance to remain close to that obtained during nominal braking, without any corrective action from the pilot (the pilot's command remains the same). The invention is not limited to what has just been described but, on the contrary, encompasses any variant falling within the scope defined by the claims.In particular, although here the distributor 3 is actuated by means of a coil, any other actuator can be used, such as an electric motor. Although the instrument sensor is a potentiometer, any other type of sensor can be used, such as an RVDT inductive sensor. The electronic unit can be analog or digital; it can include a microprocessor, a microcontroller, and / or a programmable logic circuit, for example, an FPGA. The hydraulic circuit can have a different structure than the one described and include, for example, flow restrictors, flow sensors, filters, check valves, and other components. The actuator of distributor 3 can be a monostable actuator, as shown here, or a bistable one (without a spring 8: the transition from the active to the inactive state is achieved via a control signal from the actuator 7).The number of wheels and brakes can vary (for example, two, four, eight, or other). The invention is applicable to any type of wheeled vehicle, including aircraft, land vehicles, etc.

Claims

CLAIMS 1.A braking system comprising at least one brake (1A, 1B) equipped with a fluidic actuator (2A, 2B) and a fluid circuit connected to the actuator (2A, 2B), the fluid circuit comprising a pressurized fluid source (12, 13) and a pilot valve (3) which is placed between the actuator (2A, 2B) and the fluid source (12, 13) to actuate the actuator (2A, 2B) and which is controlled by pulse-width modulation (PWM) by an electronic control unit (5) arranged to determine, as a function of a braking setpoint, a duty cycle for controlling the valve (3), characterized in that the system comprises a pressure sensor (14) placed in the fluid circuit upstream of the valve (3) and connected to the control unit (5) to provide it with a pressure signal, and in that the control unit (5) is arranged to correct the duty cycle as a function of the pressure signal. to at least partially compensate for a drop in pressure. 2.A system according to claim 1, wherein the control unit (5) is arranged to determine a correction factor as a function of the pressure signal and also of the duty cycle.

3. A system according to claim 2, wherein the correction implements the following formula: ^. ^ ^ = ^^^( ^(^^, ^^^ × ^^ ; 1^With Min the minimum function, K the correction factor, Ps the pressure obtained from the pressure signal, ^ ^ the duty cycle calculated from the setpoint, ^ ^ ^ the corrected duty cycle.

4. System according to any one of claims 2 and 3, wherein the control unit (5) has a memory containing A table relating reference values ​​for correction factor, duty cycle, and pressure.

5. A system according to claim 4, wherein the control unit (5) is arranged to determine the value of the correction factor to be applied by interpolation from the reference values ​​contained in the table.

6. A system according to any one of claims 2 to 5, wherein the control unit (5) is arranged to apply the correction in the form of a stored mathematical equation.

7. A system according to claim 6, wherein the mathematical equation is of the form ^(^^, ^^^ = ∑^^^^ ^^(^^^ × ^^^ with K the correction factor, Ps the pressure obtained from the pressure signal, ^ ^The duty cycle calculated from the setpoint, i, j, m, and n are positive integers, and Bij is a real number.

8. System according to claim 6, wherein the mathematical equation is of the form ^ = ^ × ^^ + ^ with K the correction value, Ps the pressure from the pressure signal, and A and B constants.

9. Lander (101) provided with at least one wheel (103) equipped with a brake (1A, 1B) belonging to a braking system according to any one of the preceding claims.

10. Aircraft (100) comprising at least one lander (101) according to claim 9.

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