EP0848844A1

EP0848844A1 - Process for automatically controlling a servo-valve with controllable delivery and pressure

Info

Publication number: EP0848844A1
Application number: EP96930228A
Authority: EP
Inventors: Christian Menard; Patrick Lorinet; Philippe Chabbert
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1995-09-08
Filing date: 1996-09-06
Publication date: 1998-06-24
Also published as: FR2738613B1; WO1997009663A1; US5915401A; CA2231340A1; FR2738613A1

Abstract

The invention concerns a process for automatically controlling a hydraulic servo-valve with a hydraulic slide valve (1) which is displaced by a rotary actuator (12). The process involves a first loop (2, 5, 11, 12, 4) for automatically controlling the delivery as a function of the measurement of the angular position of the rotary actuator, and a second automatic control loop (1, 3, 6, 21, 22, 23, 11, 12, 4, 2) with variable control as a function of the pressure measurement. The invention is characterized in that the first automatic control loop is a proportional action loop whose increase (Kpa) is constant for values of the absolute pressure difference value (εp) which are less than at least a threshold value, then varies in an increasing manner for values of the absolute pressure difference value (εp) which are greater than this first threshold.

Description

PROCESS FOR CONTROLLING A SERVO VALVE THAT CAN BE SERVED BY FLOW AND PRESSURE

The present invention relates to a method for controlling a servo valve which can be controlled by flow or by pressure. It applies in particular to hydraulic servovalves used in brake systems or automatic vehicle guidance.

Such servovalves comprise a hydraulic slide provided with return means and set in motion by a rotary actuator such as an electric motor; the return means maintain the drawer in a neutral position from which the drawer can be driven in translation in two opposite directions to obtain two working positions, corresponding for example to the pressurization or to the covering of the circuit.

Such servovalves have two control loops: one, which is called the flow control loop, operates when the servovalve is intended to dose a flow, and the other, which is called a pressure loop, operates when the servovalve is intended to control a pressure in a volume, a brake, a jack, etc.

Such servovalves are described in particular in French patent applications No. 92 02843 of March 10, 1992 and No. 94 07571 of June 21, 1994 of the applicant.

In this automation, the loops implement: a sensor which measures the position of the motor linked to the valve of the servovalve, in the case of flow control; one or two pressure sensors which measure the pressure in one or two volumes, in the case of pressure control. The measurements are compared with position setpoints for the flow and pressure loop for the pressure loop (s). To deal with the subject in an optimal way, that is to say by ensuring a compromise between gain, phase and stability of the controls, we use the know-how of techniques in the field of automation, which are generally loops of servo control, in which PID (Proportional Integral Derivative) functions are implemented: processing of the linear type.

In the case of servo valve loop control, it is not always easy to use a derived proportional control. The reason is that for small signals it would be necessary to give the loops a first gain and, for large signals, another gain. We then come up against a particular difficulty, which consists in adding to the commands of the linear type, a particular type of control command.

The mechanical elements which push to consider the control command in a particular way are linked to the jet effect, but also to the fact that the electric control motor has a significant self and an inertia; this originates from the power it must develop to counter the forces generated by the jet effect.

In addition, when it is necessary to work on large signal deviations, the regulation loops determine power control levels for the motor which are insufficient for optimal transitions to be obtained on transient speeds. In this case, it is therefore necessary to add a state control module, which determines the current to be applied to the motor and, consequently, the torque to be imposed on the motor. The power is controlled in the part of the equipment constituted by the transistor bridge controlling the motor and the duty cycle of this bridge. The duty cycle varies in relation to the software which imposes the status command. When the drawer is actuated, for example from its cover setting position, towards its pressurization position, it passes through a dead zone for which there is no connection in the circuits to be controlled and, by therefore, no action of the hydraulic circuit takes place. This dead zone leads to hysteresis which reduces performance, in particular in the case of actuation of devices such as brakes.

The object of the invention is to alleviate the aforementioned drawbacks, in particular to improve the speed of response of the servovalve for transient situations and to eliminate the drawbacks due to the passage of the dead zone.

To this end, the subject of the invention is a method for controlling a servovalve comprising a drawer provided with return means and set in motion by a rotary actuator such as an electric motor, the control device comprising two loops nested, a first flow control loop as a function of the measurement of the angular position of the rotary actuator and a second control loop with variable pressure control as a function of the pressure measurement, characterized in that the first control loop is a proportional action loop, the gain of which is constant for values of the absolute value of the position deviation which are less than at least one threshold value, then varies increasingly for the values of the absolute value of the pressure difference which are greater than said first threshold.

According to another characteristic of the invention, in the case of a pressure control, the gain value (Kpa) is an increasing function of the volume capacity of the chamber whose pressure must be controlled, the difference between the gains for two given volumes being constant when the pressure difference varies. According to yet another characteristic of the invention, in the case of a pressure control, for increasing values of the position (x ^) of the rotary actuator, a fixed value is added to the value of the position setpoint supplied to the first control loop from the start of the passage in the dead zone and, for decreasing values of the position

(x _t ) of the rotary actuator, the position setpoint returns to its initial value from the start of passage into the dead zone.

The main advantages of the invention are that it makes it possible to use a proportional derivative control and, consequently, to obtain short response times, by eliminating the effects due to the passage of the drawer in the dead zone.

Other characteristics and advantages of the invention will appear with the aid of the description which follows, given with reference to the appended drawings which represent: FIG. 1, a block diagram of the servo-control device;

- Figure 2, a curve showing the different gain values of the first control loop;

- Figure 3, a flowchart for controlling the first loop;

- Figure 4, a flowchart for controlling the second loop;

- Figure 5, a series of curves illustrating the control of the second loop. The invention is described below with reference to a hydraulic servovalve, but it is understood that it can also be implemented for a pneumatic servovalve, subject to modifications which are within the reach of those skilled in the art. on reading this description. Figure l is a block diagram of a servovalve and its servo device. The servovalve itself is constituted by a hydraulic slide 1, controlled by an electric motor 2 and provided with return devices not shown. The hydraulic slide 1 controls, for example, the pressurization or the covering of a chamber 3.

The motor is controlled by an electronic control device 4 with modulation of the pulse width. A position sensor 5 measures the angular position of the motor and a pressure sensor 6 measures the pressure prevailing in the chamber 3.

A first control loop includes a comparator 11, which receives a position reference value and the actual value of the position supplied by the sensor 5. This comparator provides a position deviation signal to a control circuit 12 proportional control, which provides a control signal to the electronic control device 4, by pulse width modulation. This first loop is completed by the motor 2 and the position sensor 5.

This first control loop is nested in a second control loop, which comprises a use chamber 3 and a pressure sensor 6, the output signal of which is sent to a variable control servo device 21. The latter supplies a corrected pressure signal to a comparator 22 which also receives a pressure reference value and supplies a pressure difference signal to an electronic control circuit 23 supplying a position reference signal to the comparator 11 of the first loop . This second control loop also includes the control device 12, the electronic circuit 4, the motor 2 and the hydraulic slide 1. In accordance with the invention, the proportional action servo-control circuit 12 provides a gain value which is a function of the value of the pressure difference signal ε supplied by the comparator 22. The law for controlling the value of the gain is indicated by the curve in FIG. 2 which represents the value of the gain K as a function of the value of the pressure difference ε which is equal to the difference between the desired pressure, which can go up to 166 bars for example , and the instantaneous pressure measured by the sensor 6. When a pressurization of the chamber 3 is ordered, this difference has firstly a high value, then it decreases as the pressure is established in the operating chamber 3.

When the pressure difference is less than a certain threshold, in absolute value, the gain is maintained at a constant value. Advantageously, the threshold and the value of the constant gain are different for the positive and negative values of the pressure difference ε. For positive values, the threshold S is higher but the value of the gain K is less than the value K of the gain for the negative values, the threshold S 'for the negative values being, in absolute value, lower than the threshold S for positive values.

When the value of the pressure difference ε exceeds, in absolute value, the threshold values indicated above, the value of the gain varies progressively as a function of the absolute value of the position difference, according to a linear function in the example shown.

Advantageously, for the negative values of the position deviation, a second threshold S ′ is provided which is greater than the first threshold S ′ and, from this threshold value S ′ of the absolute value of the position deviation,

2 ^c 'the gain varies linearly with a higher slope than for the values between the two thresholds

S 'and S'. 1 2 Advantageously, the gain values, for the same absolute value of the position deviation, are higher for the negative values of the latter than for its positive values.

In an exemplary embodiment applied to the pressure control of a vehicle brake, the positive threshold S is equal to 40 bars, the thresholds for the negative values S 'and S' are respectively equal to 8 and 14 bars , the absolute value of the slope of the linear function for positive values greater than 40 bars is equal to 1; for negative values between 8 and 14 bars in absolute value, the absolute value of the slope of the linear function is equal to 0.5, and for negative values greater than 14 bars in absolute value, the slope of the linear function is equal to 1 as for positive values greater than 40 bars.

FIG. 3 represents the computer control flow diagram making it possible to obtain this law of variation of the gain of the first control loop of the position of the motor.

In a first step 30, it is determined whether the pressure difference εp is positive or negative. In the case where it is positive, the pressure difference is compared with the first threshold value S in a step 31. As long as this difference is greater than this threshold, the gain is determined according to a linear function in a step 33, the slope of this linear function being equal to 1. When the pressure difference drops below this threshold value, in step 32 the value of the gain K has a constant value K.

If the pressure difference signal ε is negative, the absolute value is first calculated in a step 34 and then compared with the threshold S 'in a step 35 and when the absolute value of the pressure difference is lower than this threshold value, the gain Kpa is fixed at the value K2 in a step 36. Otherwise, the absolute value of the pressure difference _ε is compared with the second threshold S 'in a step 37. If this absolute value is less than this threshold, a first linear function is determined in a step 38 whose absolute value of the slope is equal to 0 , 5 and, in the opposite case, a second linear function with a higher slope is determined in a step 39, the absolute value of which is equal to 1.

According to another characteristic of the invention, a correction of the dead zone is carried out in order to remove the time constant introduced by the passage of the slide in this dead zone. A dead zone value ZM is defined which corresponds to the travel of the slide until the hydraulic connection is established; this dead zone ZM corresponds for example to 0.5 mm for a total stroke of the drawer of 4 mm.

According to the invention, when the travel of the slide, x, varies in an increasing manner, that is to say in the direction of the pressurized position of the chamber 3, as soon as the slide enters the dead zone, a constant value is added to the position setpoint supplied to the comparator 11 which is preferably equal to the value of the aforementioned dead zone ZM, that is to say 0.5 mm in the example indicated. This increase in the set value is maintained and when the slide stroke, x. , decreases, that is to say that the latter goes to the cover setting position, as soon as the drawer reaches the dead zone, the increase in the position setpoint is deleted and the latter returns to its normal value . This is illustrated in the flow chart of FIG. 4 which indicates the law for determining the position setpoint as a function of the position of the slide or of the position of the motor supplied by the sensor 5, x.

In a first step 40, the sign of variation Δx of the slide stroke is determined, each sign corresponding to a direction of translation of the slide. Yes this variation is positive, it is compared with the value -ZM in a step 41 and if the position x is greater than this value, in a step 42, a fixed value corresponding to the dead zone ZM.

If the variation of the position provided by the sensor 5 is negative, it is compared, in a step

43, at a value equal to + ZM and as long as the value of position x. is greater than this value, the correction of step 42 is carried out.

When the slide has reached the dead zone, that is to say when the position x is less than ZM, the position reference value returns to the value normally produced by circuit 23.

This is illustrated by the curves in FIG. 5a which represent, as a function of time t, for curve 5a, the position setpoint normally supplied by the circuit 23, for curve 5b, the copy of the position of the motor, for the curve 5c, the flow rate of the servovalve without correction of the dead zone, for curve 5d, the correction of the set point produced according to the invention and for curve 5e, the flow rate of the servovalve when the zone correction is carried out dead.

These control processes are performed digitally by a microprocessor located in the device 21 and use software for controlling the servovalve.

The servovalve according to the invention can be used with chambers of different volumes. It is sufficient, for the regulation of the gain K, to determine a basic value which is an increasing function of the volume of the chamber of use. The software is the same and the curves in Figure 2 corresponding to different pressures are simply offset by a fixed value and are therefore obtained by translation parallel to the axis of gains K

Claims

REVENDI CATI ONS

1. Servo-control method for a servovalve comprising a drawer (1) provided with return means and set in motion by a rotary actuator (12) such as an electric motor, the servo-control device comprising two nested loops, a first loop (2, 5, 11, 12, 4) for controlling the flow rate as a function of the measurement of the angular position of the rotary actuator and a second loop (1, 3, 6, 21, 22, 23, 11 , 12, 4, 2) variable control servo depending on the pressure measurement, characterized in that the first servo loop is a proportional action loop whose gain (K _pa ) is constant for values of the absolute value of the pressure difference (£ _p ) which are less than at least one threshold value (S _lf S '^, then varies increasing for the values of the absolute value of the pressure difference ( S _p ) which are greater than said first threshold (S ^, S '!).

2. Method according to claim 1, characterized in that, for the same value of the absolute value of the pressure difference (Ep), the value of the gain (K _pa ) of the control loop is higher for the negative value of said deviation only for its positive value.

3. Method according to claim 1 or 2, characterized in that, when the absolute value of the pressure difference (£ _p ) is greater than said first threshold, the gain varies according to a linear function.

4. Method according to any one of the preceding claims, characterized in that there is provided a first and a second threshold (S _{1 #} S '^) different for the negative values of the pressure difference (£ _p ) and for its positive values, its value (S '!) being lower for negative values.

5. Method according to any one of the preceding claims, characterized in that, for the negative values of the pressure difference (E _p ), the gain (K _pa ) varies according to a first linear function up to a third threshold (S ₂ ) of the absolute value of the pressure difference (S p), then according to a second linear function of greatest slope.

6. Method according to claim 5, characterized in that the absolute value of the slope of the second linear function for the negative values of the pressure difference (£ _p ) is equal to the slope of the linear function for the positive values pressure difference (Ep).

7. Method according to any one of claims 1 to 6, characterized in that the value of the gain (Kpa) is an increasing function of the capacity of the volume of the chamber (3) whose pressure must be controlled, the difference between the gains for two given volumes being constant when the pressure difference varies.

8. Control method according to any one of the preceding claims, characterized in that, in the case of a pressure control, for increasing values of the position (x ^) of the rotary actuator, a fixed value at the position setpoint value supplied to the first control loop from the start of passage into the dead zone and, for decreasing values of the position (xt) of the rotary actuator, the setpoint position returns to its initial value from the start of the passage in the dead zone.

9. A control method according to claim 7, characterized in that said fixed value corresponds to the movement of the slide (1) between its neutral rest position and one of its two working positions.

10. Method according to one of the preceding claims, characterized in that it is applied to a hydraulic servovalve.

11. Method according to one of claims 1 to 9, characterized in that it is applied to a pneumatic servovalve.