FR2834324A1 - ELECTRICALLY CONTROLLED VALVE, MEMBRANE COMPRISING SUCH VALVES AND BRAKE SUPPORT SERVOMOTOR COMPRISING SUCH A MEMBRANE - Google Patents

Abstract

Valve, in particular pneumatic, electrically controlled, which has a flattened structure and comprises a plate (P) made up of several parallel layers, namely an electrically conductive layer (1) forming a first electrode (El), at least an intermediate dielectric layer (11, 12) applied against one face of the first electrode, a second electrode (E2) formed by another electrically conductive layer (2) applied against the dielectric layer (12) on the side opposite to the first electrode, and at least one support layer (3) located on the side of the second electrode opposite the dielectric layer, at least one passage (4) passing through the support layer and the second electrode, while a flexible tab (9) is provided in the first electrode to close the passage, or to be moved away from it and allow a fluid to circulate, the movement of the tab being controlled by applying a sufficient electrical voltage between the electrodes (E1, E2).

Description

parking (6, 7).

  ELECTRICALLY CONTROLLED VALVE, MEMBRANE COMPRISING

  SUCH VALVES AND BRAKE SUPPORT SERVOMOTOR

INCLUDING SUCH A MEMBER.

  The invention relates to a valve, in particular a pneumatic valve,

s electrically operated.

  There are many types of electrically operated valves provided by at least one electromagnetic coil. Such valves generally operate satisfactorily but are relatively heavy and bulky due to the electromagnetic coil, with a response time which needs to be reduced. Their consumption of electric current is not negligible, and electromagnetic waves

  parasites may appear during the operation of these valves.

  The object of the invention is, above all, to provide an electrically operated pneumatic valve whose size and weight are reduced and whose electrical current consumption is low, in particular less than 100 mA (milliamps). It is also desirable that the response time be very short, in particular less than 5 ms (milliseconds). According to the invention, an electrically controlled pneumatic valve is characterized in that it has a flattened structure and that it comprises a plate made up of several parallel layers, namely an electrically conductive layer forming a first electrode, at least one intermediate dielectric layer applied against one face of the first electrode, a second electrode formed 2s by another electrically conductive layer applied against the dielectric layer on the side opposite to the first electrode, and at least one support layer located on the side of the second electrode opposite the dielectric layer, at least one passage passing through the support layer and the second electrode, while a flexible tab is provided in the first electrode to close the passage or to be separated from it and. allow a fluid, in particular air, to circulate, the movement of the tongue being controlled by application of an electrical voltage

sufficient between the electrodes.

  Preferably, in the absence of electrical voltage between the electrodes, the tongue is moved away from the outlet of the passage and the valve is open, the valve being closed by application of the

  electrical voltage between the electrodes.

  Advantageously, the first electrode, located towards the outside of the plate, is connected to ground while the high voltage is applied.

  to the electrode located within the plate.

  The tongue may have a rectangular shape, adjoining the layer by a small side serving as a hinge, and cut according to its

  two long sides and the other short side.

  The invention also relates to a flexible membrane comprising a set of valves as defined above, characterized in that it comprises several layers corresponding to those of the valves, and that in the membrane are cut a plurality of tabs corresponding to as many passages, each tab and

  associated passage constituting an elementary valve.

  Preferably, the valves of the membrane are arranged in a matrix in rows and columns. Addressing means can be provided to control each valve individually

  of the membrane, or by groups of valves.

  A particularly interesting application of such a membrane relates to a pneumatic assistance servomotor for

  motor vehicle braking system.

  The assistance booster comprises a rigid casing traversed by a rod constituting a manual control member, the casing being separated internally into two chambers by a movable partition in the direction of the axis of the rod, the partition comprising a rigid skirt fixed to the rod and connected along its outer contour to the inner wall of the envelope by a bellows of flexible deformable material allowing the axial displacements of the skirt, and is characterized apart from the fact that the rigid skirt is provided with openings, and formed for example by a grid, and that a membrane such as defined

  previously applied and held against the rigid skirt.

  The actuator may have a second membrane

  disposed against an end wall.

  Advantageously, the actuator comprises a control assembly comprising: - a circuit in which is stored the law characteristic of the deceleration of the vehicle as a function of the pressure in the master cylinder and providing from an input value corresponding to the pressure measured in the master cylinder a signal representing the desired deceleration; a circuit in which the law of deceleration of the vehicle is stored as a function of the stroke of the pedal, this circuit providing at output a desired deceleration value established according to the measured stroke of the brake pedal; - And a comparator circuit having two inputs respectively connected to the outputs of the above circuits and which provides, on an output line, a target deceleration corresponding to the absolute maximum value chosen between the input signals; - and means for determining an error in the stroke of the rod

  of the servomotor from the comparator output signal.

  The control assembly may include a subtractor circuit providing an error signal equal to the difference between the target deceleration and a calculated deceleration, and a circuit which converts the deceleration error signal into an error signal of traînce

  by dividing the deceleration error signal by the mass of the vehicle.

  Preferably, the assembly includes circuits for converting the drag error signal into a target stroke signal of the actuator rod, and a subtractor circuit, an input of which receives the measured value of the stroke of the actuator rod and another enters the target stroke value, this subtracting circuit providing on its output a rod stroke error signal for controlling the diaphragm valves and ensuring the correction of the position of the bulkhead

servomotor and rod.

  The invention consists, apart from the provisions set out below

  above, in a number of other provisions which will be more explicitly discussed below in connection with exemplary embodiments described in detail with reference to the accompanying drawings, but which are in no way limiting. In these drawings: Fig. 1 is a schematic perspective view, with parts

  torn off, from a pneumatic valve according to the invention.

  Fig.2 is a vertical section of the valve of Fig. 1 in the open state.

  Fig.3 shows, similarly to Fig.2, the valve in the closed state.

  Fig.4 is a partial schematic plan view of a membrane

  comprising a plurality of valves according to the invention.

  Fig.5 is a schematic section of a booster

  braking system using at least one membrane of the type in Fig. 4.

  Fig. 6, finally, is the diagram of a control assembly for a pneumatic brake booster for a motor vehicle

  comprising in its movable partition a membrane with valves.

  Referring to Fig. 1, we can see a pneumatic valve V, electrically operated, having a flattened structure. The representation of Fig. 1 is made on an enlarged scale. The thickness of the valve is less than 10 mm and preferably less than 5 mm. The valve V comprises a plate P formed from several parallel layers. An electrically conductive layer 1 forms a first electrode E1 which, in the example shown, constitutes the upper wall of the plate P. The layer 1 is generally metallic. E1 will hereinafter be designated as the upper electrode. Of course, the plate P could have an arrangement other than horizontal, according to which the electrode E1 would no longer constitute the upper face of P. At least one electrically insulating intermediate layer,

  forming a dielectric, is arranged on one side of the electrode E1.

  Advantageously, the dielectric intermediate layer comprises a first layer I1 of reduced thickness applied directly against the face of the electrode E1 and a second dielectric layer I2 of significantly greater thickness, preferably three times that of the

  layer I1, applied against the face of I1 remote from the electrode E1.

  Another electrically conductive layer 2, in particular metallic, forms a second electrode E2 separate from the electrode E1

  by at least part of the dielectric layers I1, I2.

  Advantageously, the electrode E2 is embedded in the layer I2 which comprises a part I2a located between E2 and I1, and a part I2b located

the other side of E2.

  The distance h (Fig. 3) between the facing faces of the electrodes E1

  and E2 is reduced, in particular equal to, or of the order of 0.2 mm.

  At least one support layer 3, thicker than the dielectric layer I2 is. disposed on the side of the second electrode E2 opposite to

the first electrode E1.

  The dielectric layers I1, I2 and the support layer 3 can be produced with sheets of suitable plastic. A passage 4 passes through the support layer 3, the second electrode E2 and the dielectric layer I2. The passage 4 may include a frustoconical entry 5 provided in the support 3 with its large base located on the side s

opposite electrode E2.

  The passage 4 passes through the second electrode E2, and the parts I2b, I2a of the layer I2 surrounding this electrode, following a circular hole 6 which opens into a recess 7, for example with rectangular contour s, provided in the layer I2 and s' opening on layer I1. The diameter D of the hole 6 is small, in particular equal to, or of the order of

0.5mm.

  The recess 7 is made only over a fraction of the thickness of the part I2a so that a dielectric coating 8 remains

  in the bottom of the recess 7 on the second electrode 2.

  A flexible tongue 9 of rectangular shape is cut along its two long sides and a short side in the first electrode E1 and in the layer I1. The tongue 9 is attached to the electrode E1 by the other short side 10 which is not cut, which serves as a hinge. The tongue 9 is located above the circular hole 6 which is located substantially in line with the center of the tongue. The contour of the tongue 9 is parallel to the contour of the recess 7 but situated slightly inside this contour so that a slot 11 exists between the three cut sides of the tongue and the rest of the electrode E1 . The width of the slot 11 is in

general less than 1 mm.

  In the rest position illustrated in Figs. 1 and 2, the tongue 9 is located in the plane of the electrode E1 and the part of the layer I1 situated under the tongue 9 is spaced from the bottom of the recess 7 so that a gas, in particular air , entering through passage 4 can escape through the recess 7 and the slot 11 on the side of the plate P opposite the entry of the

passage 4.

  The electrodes E1, E2 are connected by electrical conductors 12, 13 schematically shown, and by a switching device not shown, to a source of high voltage DC not shown. Preferably, the electrode E1 which constitutes one of the external faces of the plate P is connected to ground, while the electrode E2,

  electrically isolated, is connected to high direct voltage.

  The operation of the valve, or flap, of Figs. 1 to 3 is as follows. In the absence of voltage applied between the electrodes E1 and E2, the tongue 9 occupies its rest position and a stream of gas entering through the passage 4 can escape through the recess 7 and the slot 11, on the side

  opposite the support 3 as illustrated by arrows in Fig. 2.

  The tongue 9 can bend on the side opposite to the support 3 as illustrated in line miste in Fig. 2 to release a passage section more

important for a high flow.

  The closure of the valve V is obtained by applying a high continuous voltage, in particular of the order of 4500 volts, between the electrodes E1 and E2 which causes an electrostatic attraction between the electrodes and the application of the tongue 9 against the bottom. of the recess 7. The circular hole 6 is then closed, as well as the valve V. It is possible to order at will the closing or opening of the valve V. The response time is very short and can be less than 5 ms, while the current consumption is very low,

  less than 100 mA (100 milliamps).

  In Fig.4 a set of pneumatic valves V is formed in a flexible membrane M having a multilayer structure identical to that shown in Figs. 1-3. The membrane M therefore comprises a first conductive layer corresponding to the electrode E1, one or more dielectric layers, a second conductive layer corresponding to the second electrode E2 preferably embedded in a dielectric layer, and a support layer corresponding to the layer 3 of

Fig.2.

  In the membrane M, on the side of the electrode E1, are cut a plurality of tabs 9 corresponding to as many passages 6 and valves V. Preferably the tabs 9 are arranged

  following a matrix with rows and columns intersecting at right angles.

  The electrodes of the elementary valves V of the membrane M can be placed individually, or in groups, under high voltage by means of a not shown addressing, of known type, by lines or lines

  - columns, in particular with transistors.

  The membrane M thus constitutes an electrically controlled variable porosity membrane. The porosity may be zero, which corresponds to an impermeable membrane, all the V valves being closed. Maximum porosity is obtained with all V valves in the open position. Intermediate porosity values are possible

  according to the address provided for controlling the diaphragm V valves.

  3s An advantageous application of a valve V and of a membrane M according to the invention relates to a pneumatic booster for assisting the

  braking of known type, for example by FR 2 658 466 or EP 0 662 894.

  Fig.S illustrates such a booster 14 using

membranes M1, M2 according to the invention.

  The servomotor 14 comprises a rigid casing 15 delimited by two front walls lSa, 15b. The casing 15 is axially traversed by a rod 16 constituting a member of maouelle corumande. The displacement of the rod 16 can be controlled by the driver of the vehicle, who presses on a brake pedal 17 connected by an articulation

at one end of the rod 16.

  The envelope 15 is internally separated into two chambers 14a, 14b by a partition 18 movable in the direction of the axis of the rod 16, and orthogonal to this rod. The partition 18 comprises a rigid skirt 19 fixed to the rod 16. The rigid skirt 19 is provided with openings O1, and is

formed for example by a grid.

  A membrane M1, similar to the membrane M of Fig.4 with valves V not visible in Fig.5, is fixed against the rigid skirt 19. A bellows 20 is hermetically connected to the contour of the membrane M1 and to the inner wall of the envelope 15. The bellows 20 made of flexible material

  deformable allows axial movements of the Skirt 19.

  The chamber 14a is permanently connected by a nozzle 15c to

  a source of vacuum creating a relative vacuum in the chamber 14a.

  The valves of the membrane M 1 are controlled all together by the simultaneous application of a high voltage U1 between the

  electrodes E1 and E: 2 of each valve.

  At rest, no voltage is applied between the electrodes of the valves of the membrane M1; the valves are therefore open and the chambers 14a, 14b communicate and are both under the same relative depression. No effort to assist in the penatematic assistance is then

exerted on the rod 16.

  The chamber 14b is closed at its end remote from the partition 18 by a wall 21 which comprises a rigid support element 21a provided with openings 02, for example formed by a grid. An M2 membrane similar to that shown in Fig. 4, with valves V (not

  visible in Fig.5), is fixed against the rigid element 21a.

  The rod 16 passes tightly and slidingly through a

  sleeve 22 fixed to the center of element 21a and of membrane M2.

  All the valves of the M2 membrane are controlled simultaneously by applying a high voltage U2 between the electrodes of each valve. The wall 15b has a central closure 23 which surrounds the rod 16 and opens into the atmosphere. An air filter 24 is advantageously provided at the inlet of the tobulure 23 to stop dust and particles

contained in the air.

  In the absence of voltage between the electrodes of the membrane S M2, the valves of this membrane are open and the space 25 included

  between the walls 21 and 15b communicates with the chamber 14b.

  It should be noted that the membrane M1 can be provided alone, without the membrane M2 which would be replaced by one or more conventional valves provided in a full wall 21. In addition, the membrane M1 and the skirt 19 could be replaced by a wall in which at least one controlled solenoid valve would be provided, allowing either to

  to communicate, or to isolate, the rooms located on each side.

  The operation of the servomotor 14 of Fig.5 is as follows.

  At rest the voltage U1 is zero while U2 is a high voltage ensuring the closing of the valves of the membrane M2. The chambers 14a and 14b communicate and are in a relative vacuum, the chamber 14b being isolated from the space 25 and from the atmosphere. No effort

  tire is not exerted on the rigid skirt 19 nor transmitted to the rod 16.

  When the driver controls braking by pressing the peJale 17, the start of movement of the rod 16 is detected by means not shown which bring the voltage U1 to a high voltage value so that the valves of the membrane M1 are close and

  that the chambers 14a, 14b are separated from each other.

  Preferably for a short time the valves of the membrane M2 remain closed by maintaining the high voltage U2 until the closure of the valves of the membrane M1 is r. comlrmee. The voltage U2 is then lowered to zero to ensure the opening of the valves of the membrane M2. The atmospheric pressure is admitted into the chamber 14b while the chamber 14a is still under relative vacuum and the high voltage U1 is maintained on M1. The difference in pressure exerted on the partition 18 generates an effort

  pnenmatic assistance which is transmitted to the rod 16.

  When there is stabilization of the braking with maintenance of the pedal 17 in a determined position, U2 is brought to high voltage to ensure the closing of the valves of the membrane M2 and to stop

  the admission of air at atmospheric pressure into the chamber 14b.

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  So that the braking stops (brake release) when the pedal 17 is released, the voltage U1 is lowered to zero. The valves of M1 open and allow the pressure balance between the chambers 14a and 14b and the recoil of the partition 18 and the rod 16. U2 is maintained at high voltage and the valves of M2 remain closed. Advantageously, the axial position of the partition 18 and therefore of the rod 16 can be detected by sensors, in particular capacitive or impedance sensors, formed between the partition 18 and at least one

  end end walls 15a, 21a, 15b of the booster.

  Conventionally, the rod 16, at its end remote from the pedal 17, transmits the force to a piston 26, generally the primary piston of a tandem master cylinder MCT which supplies liquid of

  brake under pressure the brakes such as 13 of the different wheels of the vehicle.

  In the rest position of the booster, the volume of the chamber 14a is preferably relatively large, in particular at least three times greater than the volume of the chamber 14b, to constitute a

relative vacuum reserve.

  Ein referring to Fig. 6 we can see the diagram of a set

  actuator 14 of Fig.5.

  The servomotor and the master cylinder are represented schematically by a block 27 with two inputs connected to lines 28, 29. Line 28 transmits a signal, obtained from a sensor not shown, representing the force Fv exerted by the driver on pedal 17. Line 29 transmits, from a sensor not shown, a signal

  2s representing the Sv stroke of the pedal 17.

  An output line 30 of block 27 transmits a signal representative of the braking pressure Pm measured in the master cylinder. Another output line 31 of block 27 transmits a signal Sm

  corresponding to the measured stroke of the rod 16.

  The signal Pm from line 30 is supplied to the input of a circuit 32 in which is stored the law characteristic of the deceleration of the vehicle plotted on the ordinate, as a function of the pressure P in the master cylinder plotted on the abscissa. From the input value Pm supplied by the line 30, the circuit 32 provides at output on a line 33 a signal representing the desired deceleration Pc (or target deceleration) of the

  vehicle corresponding to the pressure value Pm.

  Line 31 supplies the signal Sm to an input of a circuit 34 in which the law of deceleration of the vehicle, stored on the ordinate, is stored, as a function of the stroke S of the pedal carried on the abscissa. The circuit 34 supplies at output a desired deceleration value Sc established according to the stroke Sm. Lines 33 and 35 are each connected to an input of a comparator circuit 36 which provides, on an output line 37, a target deceleration c corresponding to the absolute maximum value chosen

between Pc and Sc.

  Line 37 is connected to an input of a subtractor circuit 38.

  Another input of this circuit 38 receives, by a line-39, a signal

  representing the calculated deceleration cal. of the vehicle.

  The calculated deceleration is obtained from speed sensors (not shown) provided on each of the four wheels of the vehicle and connected to a circuit 40 which delivers, on four outputs Vavd, Vavg, Vard and Varg, the values of the respective speeds of the right front wheel 'of the

  left front wheel, right rear wheel and left rear wheel.

  These values are supplied to an evaluation circuit 41 which establishes, from the four wheel speed values, a reference speed value

  Vref. of the vehicle delivered on an output line 42.

  This value Vref. is supplied to the input of a time differentiator circuit 43 whose output, connected to line 39,

  delivers the calculated acceleration calc.

  The subtractor circuit 38 provides on an output line 44 an error signal sy equal to the difference between the target deceleration c and the calculated calculated deceleration:

- sy = target - calc.

  Line 44 is connected to the input of a circuit 45 P.I.D (Proportional Integral Derivative) which provides on an output line 46 the deceleration error signal. This signal is sent to the input of a circuit 47 which converts the deceleration error signal into a trance error signal sT by dividing the deceleration error signal by the mass M of the vehicle. Tranal force corresponds to the sum of

  friction forces against the ground of the tires of the vehicle wheels.

  A line 48 transmits the drag error signal sT to a set 49 of four circuits (not shown in detail), which determine from the signal T the liquid pressure to be established in

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  each wheel brake. A Pcible signal is established by weighting the

  fluid pressure values for each wheel brake.

  The Pcible signal is delivered on a line 50 at the input of a circuit 51 suitable for converting the Pcible signal into a target race signal Scib. of the actuator rod 16. This Scib signal. is supplied on an output line 52 at the input of a subtractor circuit 53, another of which

  input receives by a line 54 the measured value of the stroke Sm.

  The circuit 53 provides on its output 55 an error signal sS of the

  stroke of the rod 16, sent to the input of a P.D.

  (proportional derivative) which provides, on its output 57, the signal for controlling the membrane M2 of the servomotor 14 and ensuring the correction

  of position of the partition 18 and of the rod 16.

Claims (15)

  1. Valve, in particular pneumatic, electrically controlled, characterized in that it has a flattened structure and that it comprises a plate (P) consisting of several parallel layers, namely a layer (1) conductive of the electricity forming a first electrode (E1), at least one intermediate dielectric layer (I1, I2) applied against one face of the first electrode, a second electrode (E2) formed by another electrically conductive layer (2) applied against the dielectric layer (I2) on the side opposite the first electrode, and at least one support layer (3) located on the side of the second electrode opposite to the dielectric layer, at least one passage (4) passing through the support layer and the second electrode, while a flexible tongue (9) is provided in the first electrode to close the passage, or to be separated from it and allow a fluid to circulate, the displacement of the tongue being controlled by applying a voltage   sufficient electrical power between the electrodes.   2. Valve according to claim 1, characterized in that in the absence of electrical voltage between the electrodes (E1, E2), the tongue (9) is spaced from the outlet of the passage (4) and the valve is open , the valve being closed by applying the electrical voltage between the electrodes. 3. Valve according to claim 1 or 2, characterized in that the first electrode (E1), located towards the outside of the plate, is connected to ground while the high voltage is applied to the electrode (E2) located within the plate.   4. Valve according to one of claims 1 to 3, characterized in that   the tongue (9) has a rectangular shape, adjoining the layer (1) by a small side (10) serving as a hinge, and cut along its two large sides and the other small side.   5. Valve according to one of the preceding claims, characterized by   causes the second electrode (E2) to be embedded in a dielectric layer (I2) separated into a part (I2a) between the second electrode (E2) and a dielectric layer (I1) applied against the first - 13 electrode (E1), and a part (I2b) located on the other side of the second electrode (E2).   6. Valve according to one of the preceding claims, characterized by the   causes the distance (h) between the facing faces of the electrodes (E1, E2) is of the order of 0.2 mm.   7. Valve according to one of the preceding claims, characterized by the   the fact that the passage (4) which crosses the second electrode (E2) opens into a recess (7) and that a dielectric coating (8) remains in the bottom of the recess (7) on the second electrode (E2), the contour of the tongue (9) being parallel to the contour of the recess (7) but located at inside this outline.   8. Valve according to claim 7, characterized in that in the rest position the tongue (9) is in the plane of the eleckode (E1) and the part of the dielectric layer (I1) located under the tongue '( 9) is   away from the bottom of the recess (7).   9. Flexible membrane comprising a set of valves according to one of   previous claims, characterized in that it comprises   several layers corresponding to those of the valves, and that in the membrane are cut a plurality of tongues (9) corresponding to as many passages, each tongue and associated passage constituting a elementary valve (V).   10. Membrane according to claim 9, characterized in that the valves (V) of the membrane are arranged in a matrix in rows and columns. 11. Pneumatic assistance booster for motor vehicle braking installation, comprising a rigid casing (15) crossed by a rod (16) constituting a tappet control member, and internally separated into two chambers (14a, 14b) by a partition (18) movable in the direction of the axis of the rod (16), the partition (18) comprising a rigid skirt (19) fixed to the rod (16) and connected along its outer contour to the inner wall of the envelope (15) by a bellows (20) of flexible deformable material allowing axial movements of the skirt (19), in which the rigid skirt (19) is provided with openings (01), and a membrane (M1) according to the claim 9 or 10 is applied and maintained against the rigid skirt (19).   12. Booster according to claim 11, in which a second membrane (M2) according to claim 9 or 10 is disposed against a   end wall (21a) of the actuator provided with openings (02).   13. Booster according to claim 11 or 12, comprising a control assembly comprising: - a circuit (32) in which is stored the law characteristic of the deceleration () of the vehicle as a function of the pressure (P) in the master cylinder and providing from an input value (Pm) corresponding to the pressure measured in the master cylinder a signal representing the desired deceleration (yPc); - a circuit (34) in which the deceleration law () of the vehicle is stored as a function of the stroke (S) of the pedal, this circuit (34) providing as output a desired deceleration value (ySc) established according to the measured stroke (Sm) of the rod (16) of the servomotor; - And a comparator circuit (36) having two inputs connected respectively to the outputs of the above circuits (32, 34) and which provides, on an output line (37), a target deceleration (yc) corresponding to the absolute maximum value chosen between the input signals (yPc, ySc) and means (38, 45, 47, 49, 51, 53) for determining a travel error (: S) of the rod (16) from the output signal of the comparator (36) 14. Servomotor according to claim 13, characterized in that it comprises a subtractor circuit (38) providing at output (44) a deceleration error signal () equal to the difference between the target deceleration (yc) and a calculated deceleration (ycalc.), and a circuit (47) which converts the deceleration error signal into a drag error signal (cT) by dividing the deceleration error signal by the mass (M) of the vehicle .e. 3s 15. Booster according to claim 14, characterized in that it comprises circuits (49, 51) for converting the drag error signal (cT) into a target stroke signal (Scib.) Of the rod (16 ) of the servomotor, and a subtractor circuit (53), one input of which receives the measured value (Sm) of the rod (16) of the servomotor and another input of the target stroke value (Scib.), which subtractor circuit (53) provides on its output (55) a rod stroke error signal (úS) to control the diaphragm valves (M2) and ensure the correction of the