Fluid machine and method of controlling such a machine.

Abstract

Method for controlling a fluid-driven machine (2) operated by an actuator (1), - the machine (2) comprising a cylinder liner (21) and a piston (22) together forming a chamber (23) and configured to move relative to each other, - the actuator (1) comprising in series: a motor (11) with a rotating shaft, a gearbox (12) for reducing the speed of the motor shaft (11), a mechanical coupling (13) and a connecting rod (14), , the method (200) comprising the following successive steps: - determination (S1) of the relative position of the piston (22) or the cylinder liner (21) with respect to a critical position (Pos_crit) when the shaft of the motor (11) rotates at a first speed (v_nom), - pointwise increase (S2) of the speed of the shaft of the motor (11) as a function of the relative position of the piston (22) or the cylinder liner (21) with respect to the critical position (Post-critical). Figure from the summary: Fig. 7

Description

Title of the invention: Fluid machine and method of controlling such a machine.

[0001] The invention relates to a fluid machine such as a compressor or a pump, and in particular a cryogenic pump. It relates specifically to a machine connected to an actuator.

[0002] The invention also relates to a method of controlling a fluid machine.

[0003] In a known manner, a fluid machine comprises a liner and a piston, for example, inside the liner. The piston and the liner form a chamber for the expansion and compression of a fluid. Furthermore, the piston and the liner are configured to move back and forth relative to each other between a bottom dead center of fluid expansion and a top dead center of fluid compression.

[0004] The relative movement between the piston and the cylinder liner is ensured by an actuator. This generally comprises a motor, which may be of the asynchronous type, a speed reduction gearbox, a mechanical coupling (for example a cardan joint) and a crank connecting rod.

[0005] In particular, the connecting rod is configured to drive the piston in a forward relative movement of the piston with respect to the cylinder liner, for the purpose of admitting and expanding the fluid in the chamber. Furthermore, the connecting rod is configured to drive the piston in a reverse relative movement during the compression of the fluid in the chamber and the expulsion of the fluid from the chamber.

[0006] The action of the connecting rod on the piston induces a positive nominal load on the latter during the compression movement of the fluid, then a zero load during the expansion movement. However, at the transition between the return and forward movements of the piston, the shape of the load recorded on the piston shows that the machine becomes a generator of forces.

[0007] The load recorded on the piston at the transition between the compression stroke and the upstroke is largely due to residual pressure in the expansion and compression chamber, and to a lesser extent to the piston's inertia. This load on the piston results in a backlash at the gearbox, accompanied by some noise.

[0008] The backlash in the gearbox can generate premature wear of the gearbox and vibrations in the structure of the fluid machine.

[0009] One object of the invention is to limit this backlash and to eliminate the level of noise recorded during the operation of the actuator.

[0010] To this end, according to a first aspect, the invention proposes a method of controlling a fluid machine such as a pump or a compressor, the machine being operated by an actuator.

[0011] The machine comprises a sleeve and a piston which together form a chamber for the expansion and compression of a fluid. The piston and the sleeve are configured to move back and forth relative to each other between a bottom dead center of fluid expansion and a top dead center of fluid compression.

[0012] The actuator comprises the following elements arranged in series: a shaft motor The rotary unit consists of a speed reduction gearbox connected to the motor shaft, a mechanical coupling, and a connecting rod. The connecting rod is connected to the fluid machine and configured to alternately drive the piston or the cylinder liner in a first movement of fluid compression and a second movement of fluid expansion.

[0013] The method comprises the following successive steps: i) a step of determining the relative position of the piston or the liner with respect to a critical reference position when the engine shaft rotates at a first speed, ii) a step of punctually increasing the speed of the engine shaft as a function of the relative position of the piston or the liner with respect to the critical position, so as to reduce a mechanical shock between the gearbox and the connecting rod, at the transition between the expansion movement of the fluid and the compression movement of the fluid.

[0014] Embodiments of the invention according to this first aspect may include one or more of the following features: - The step of a temporary increase in the rotational speed of the engine shaft occurs when the relative position of the piston or the cylinder liner coincides with or is close to the critical reference position, - the step of a temporary increase in the rotational speed of the engine shaft extends over a predetermined duration (i.e., a fixed duration) or a variable duration depending on the relative position of the piston or the cylinder liner with respect to the critical reference position, - The step of determining the relative position of the piston or liner with respect to the critical reference position includes an operation of tracking the stroke of the piston or liner during the compression movement of the fluid, - the step of intermittently increasing the rotational speed of the motor shaft accelerates the rotation of the motor shaft from the first speed to a maximum speed higher than the first speed, according to a predetermined acceleration, - the method comprising a step of decreasing the rotational speed of the motor shaft, - The step of decreasing the rotational speed of the motor shaft is consecutive to the step of briefly increasing the rotational speed of the motor shaft; - The step of decreasing the rotational speed of the motor shaft decelerates the rotation of the motor shaft from the maximum speed to the first speed, according to a predetermined deceleration. - The critical reference position is a position of the piston or cylinder liner during the fluid compression movement, beyond which the inertia of the fluid machine becomes greater than the inertia of the actuator. - The step of determining the relative position of the piston or liner with respect to the critical reference position includes an operation to evaluate the distance of the piston or liner from top dead center, for example via a position sensor located at the fluid machine - The step of determining the relative position of the piston or cylinder liner with respect to the critical reference position includes an operation of evaluating the position of the connecting rod with respect to the mechanical coupling, for example via a position sensor located at the connecting rod, - The step of determining the relative position of the piston or cylinder liner with respect to the critical reference position includes an operation of evaluating the position of the engine shaft with respect to an engine frame, for example via a position sensor, - the speed reduction gearbox includes at least one gear train formed of at least two toothed parts, - The step of determining the relative position of the piston or the cylinder liner with respect to the critical reference position includes an operation of evaluating the position of one of the toothed parts with respect to the other toothed parts of the gear train, - The step of intermittently increasing the speed of the motor shaft is performed by a frequency inverter, - the step of decreasing the rotational speed of the motor shaft is initiated when top dead center is reached.

[0015] According to a second aspect, the invention relates to a fluid machine.

[0016] The fluid machine comprises a liner and a piston which together form a chamber for the expansion and compression of a fluid. In particular, the piston and the liner are configured to move back and forth relative to each other between a bottom dead center of fluid expansion and a top dead center of fluid compression.

[0017] The fluid machine also includes an electronic component and an actuator configured to receive a setpoint from the electronic component. The electronic component It is equipped with a microprocessor and configured to acquire and process data. The actuator is configured to intermittently increase the speed of the piston or cylinder stroke based on a signal representing the relative position of the piston or cylinder with respect to a critical reference position.

[0018] Other features and advantages will become apparent upon reading the following description, made with reference to the following figures in which:

[0019] [Fig-1] is a schematic view of a fluid machine connected to an actuator, the actuator comprising a motor, a gearbox, a mechanical coupling and a crank connecting rod, the fluid machine comprising a liner and a piston configured to be in relative motion with respect to each other, the piston being in a top dead center position.

[0020] [Fig.2] is a schematic view of the machine and actuator illustrated in [Fig.1], the piston being in a bottom dead center position.

[0021] [Fig.3] is a partial sectional view of a detail of the speed reduction gearbox illustrated in [Fig.1], said gearbox comprising a gear train formed of at least two gear wheels with an illustration of a functional clearance between two respective teeth of the gear wheels.

[0022] [Fig.4] illustrates an evolution of the load recorded on the piston during a operation of the controlled fluid machine according to a prior art method.

[0023] [Fig.5] illustrates an evolution of the acceleration at the output of the reduction gearbox speed during operation of the machine controlled according to a prior art method.

[0024] [Fig.6] illustrates an evolution of the speed at the output of the reduction gearbox speed during operation of the machine controlled according to a prior art method.

[0025] [Fig.7] illustrates the steps of the piloting method according to the invention.

[0026] [Fig.8] illustrates an evolution of the load recorded on the piston during a operation of the controlled fluid machine according to the method according to the invention.

[0027] [Fig.9] illustrates an evolution of the speed at the output of the speed reduction gearbox during operation of the fluid machine controlled according to the method according to the invention.

[0028] [Fig. 10] illustrates an evolution of the acceleration at the output of the speed reduction gearbox during operation of the controlled fluid machine according to the method according to the invention.

[0029] Figures [Fig.1] and [Fig.2] illustrate a fluid machine 2 connected to an actuator 1.

[0030] The fluid machine 2 comprises a sleeve 21, a piston 22 which forms with the The cylinder liner has a chamber 23 for the expansion and compression of a fluid. In particular, the piston 22 is configured to be in relative reciprocating motion with respect to The cylinder liner 21 is located between a first extreme position (see [Fig. 1]) of fluid intake and expansion and a second extreme position (see [Fig. 2]) of fluid discharge and compression. The extreme position of fluid intake and expansion is called bottom dead center. The extreme position of fluid compression and discharge is called top dead center.

[0031] In the illustrated example, the piston 22 is tubular and formed around a fixed part. Furthermore, the piston 22 is disposed inside the sleeve 21.

[0032] The actuator 1 comprises the following elements connected in series: a motor 11, a speed reduction gearbox 12, a mechanical coupling 13 and a connecting rod 14.

[0033] In particular, the motor 11 may be of the asynchronous type. It comprises a shaft rotating about itself. The shaft has one end connected to the speed reduction gearbox 12.

[0034] The speed reduction gearbox 12 includes a gear train comprising at least two toothed parts 121, 122. These parts each have teeth. These parts 121, 122 cooperate in pairs comprising a first toothed part 121 and a second toothed part 122. The teeth of the first toothed part 121 drive the teeth of the second toothed part 122.

[0035] The connecting rod 14 is configured to drive the piston 22 or the liner 21 during a fluid expansion movement in the chamber 23. The connecting rod 14 is also configured to drive the piston 22 or the liner 21 during a fluid compression movement in the chamber 23.

[0036] The action of the connecting rod 14 on the piston 22 in principle induces a positive nominal load on the latter during the compression movement of the fluid, then a zero load during the expansion movement. However, as illustrated in [Fig. 4], between the compression and expansion movements of the fluid, the piston 22 becomes a generator of forces.

[0037] The load recorded on the piston at the transition between the compression stroke and the expansion stroke is largely due to residual pressure in the expansion and compression chamber, which acts on the piston. This load is also due, to a lesser extent, to the inertia of the piston 22.

[0038] At the level of the speed reduction gearbox 12, the load recorded on the piston 22 results in a backlash (mechanical shocks) and noise or vibration exceeding an permissible threshold. This backlash can cause premature wear of the speed reduction gearbox 12.

[0039] An analysis of this backlash shows that it is due, for example, to functional clearances provided between the respective teeth of the toothed parts 121, 122 of a gear train of the gearbox 12. The larger the clearances, the greater the energy of the There will be a significant backlash at the 12-speed gearbox. An example of such play is illustrated in [Fig.3].

[0040] Analysis of the backlash produced in the speed reduction box 12 shows that it is associated with a progressive decrease in the speed v at its output, while a constant speed setpoint is imposed at the motor 11. This decrease in the speed at the output of the speed reduction box is observed during the compression movement of the fluid.

[0041] The evolution of the velocity v at the output of the speed reduction gearbox 12 over time t is illustrated in [Fig. 5] where three compression / expansion cycles, respectively No. 1, No. 2 and No. 3, are shown. Cycles No. 1, No. 2 and No. 3 are separated by vertical dashed lines.

[0042] For each compression / expansion cycle, the circle indicates the transition from the compression phase to the expansion phase. A decrease in velocity v is observed during a large part of the compression phase, followed by an increase in velocity towards the end of the compression phase and the beginning of the expansion phase, before the velocity stabilizes for the remainder of the expansion phase. In other words, considering a given cycle, for example cycle no. 2, we observe that the velocity v at the output of the 12-speed gearbox decreases during the compression phase, after a constant velocity recorded during the expansion phase of a previous cycle, here cycle no. 1.

[0043] The evolution of the speed ven output of the speed reduction gearbox 12 can be represented by an acceleration curve F.

[0044] Figure 6 illustrates an acceleration curve F for three compression / expansion cycles. The cycles are separated by vertical dashed lines. For each cycle, the acceleration F is observed to be zero over a large part of the expansion phase (meaning that the velocity v is constant during this part of the expansion phase). Furthermore, the acceleration Y is observed to be non-zero, and in particular negative, over a large part of the compression phase (meaning that the velocity decreases during this part of the compression phase). Between the end of the expansion phase and the beginning of the compression phase, a change in the acceleration regime is observed, which coincides with the backlash recorded in the speed reduction gearbox 12.

[0045] In order to limit the backlash described above and the associated disadvantages (reduction of the speed at the output of the gearbox), the invention introduces a new method 200 of controlling the fluid machine 2.

[0046] Method 200 includes a first step SI of determining the relative position of the piston 22 or the cylinder liner 21 with respect to a reference critical position Pos_crit. During this step SI, the motor shaft 11 rotates at a first speed. This could be a setpoint speed for the rotation of the motor shaft 11, for example a constant nominal speed v nom.

[0047] Method 200 also includes a step S2 of intermittently increasing the rotational speed of the motor shaft 11 as a function of the relative position of the piston 22 or the cylinder liner 21 with respect to the critical position Pos_crit. This intermittent increase in the rotational speed of the motor shaft 11 makes it possible to reduce mechanical shock in the speed reduction gearbox 12 by maintaining contact between the respective teeth of the gear train teeth 121, 122.

[0048] By reducing the mechanical shock in the speed reduction gearbox 12, method 200 makes it possible to maintain a substantially constant rotational speed at the output of the speed reduction gearbox 12 during the transition between the fluid expansion and compression motions. Furthermore, by reducing the mechanical shock in the speed reduction gearbox 12, method 200 also makes it possible to reduce the shock between the speed reduction gearbox 12 and the connecting rod 14 during the transition between the fluid expansion and compression motions.

[0049] The critical position Pos_crit reference during the fluid compression movement is a position of the piston 22 or the liner 21, beyond which the inertia of the fluid machine 2 becomes greater than the inertia of the actuator 1. In other words, the critical position Pos_crit reference is a position of the piston 22 or the liner 21, beyond which the fluid machine 2 would become a generator of forces on the actuator 1 if step S2 were not implemented.

[0050] Advantageously, the SI step of determining the relative position of the piston 22 or the liner 21 with respect to the reference critical position Pos_crit includes an Sla operation of tracking a stroke of the piston 22 or a stroke of the liner 21 during the compression movement of the fluid.

[0051] Advantageously, the step SI of determining the relative position of the piston 22 or the liner 21 with respect to the reference critical position Pos_crit includes an operation Slb for evaluating the distance of the piston 22 or the liner 21 from top dead center, for example via a position sensor located at the fluid machine 2. Such a sensor can be configured to directly detect the relative position of the piston 22 or the liner 21, or via another element of the fluid machine 2.

[0052] As an alternative or in addition to the operation Slb, the step SI of determining the relative position of the piston 22 or the liner 21 with respect to the critical reference position Pos_crit may include an operation Sic of evaluating a position of the connecting rod 14 with respect to the mechanical coupling 13, for example via a position sensor disposed at the level of the connecting rod 14.

[0053] As an alternative or in addition to the previous operations Slb, Sic, the step SI of determining the relative position of the piston 22 or the liner 21 with respect to a critical reference position Pos_crit may include an operation Sld of evaluating a position of the shaft of the motor 11 with respect to a frame of the motor 11, for example via a position sensor disposed on the frame.

[0054] As an alternative or in addition to the operations Slb, Sic, Sld, the step SI of determining the relative position of the piston 22 or the liner 21 with respect to a critical reference position Pos_crit may include an operation S le of evaluating a position of one 121 of the toothed parts 121, 122 with respect to the other 122 of the toothed parts 121, 122 of the gear train.

[0055] It should be noted that the step S2 of the instantaneous increase in the rotational speed of the motor shaft 11 occurs when the relative position of the piston 22 or the cylinder liner 21 coincides with or is close to the reference critical position Pos_crit. Furthermore, this step S2 of the instantaneous increase in the rotational speed of the motor shaft 11 extends over a predetermined duration (i.e., a fixed duration) or a variable duration depending on the relative position of the piston 22 or the cylinder liner 21 with respect to the reference critical position Pos_crit. Finally, this step S2 of the instantaneous increase in the speed of the motor shaft 11 can be executed by a variable frequency drive (VFD).

[0056] Advantageously, step S2, which involves a temporary increase in the rotational speed of the motor shaft 11, accelerates the rotation of the motor shaft 11 from the initial nominal speed vnom to a maximum speed vmax, which is higher than the initial nominal speed vnom, according to a predetermined acceleration λl. To achieve this, the motor shaft rotational speed control setpoint can, for example, be increased for a specified duration.

[0057] Advantageously, method 200 includes a third step S3 of decreasing the rotational speed of the motor shaft 11. This step S3 is consecutive to step S2 of momentary increase of the rotational speed of the motor shaft 1. In addition, this step S3 can be initiated as soon as top dead center is reached.

[0058] Advantageously, the third step S3 of reducing the rotational speed of the motor shaft 11 includes an operation of decelerating the rotation of the motor shaft 11 from the maximum speed vmax reached at the end of step S2 to the nominal speed v'om, according to a predetermined deceleration y2. To do this, the control setpoint for the rotational speed of the motor shaft 11 can, for example, be reduced for a determined time.

[0059] Advantageously, the actuator 1 may include a controller (not shown) which is configured to receive from any of the position sensors mentioned above- above at least one signal relating to a position of the piston 22 or a position of the cylinder liner 21. The controller can also be configured to send to the VFD frequency converter an acceleration or deceleration command for the rotation of the motor shaft 11.

[0060] The acceleration command can be based on a signal relating to the relative position of the piston 22 or a signal relating to the relative position of the liner 21. The deceleration command can be based on a signal indicating that top dead center has been reached.

[0061] Also, advantageously, the first step SI of determining the relative position of the piston 22 or the liner 21 with respect to the reference critical position Pos_crit can include an operation of transmitting a signal relating to the position of the piston 22 or the liner 21 from any of the position sensors to the controller.

[0062] The S2 step of punctual increase of the rotational speed of the shaft of the motor 11 and the S3 step of decrease of the rotational speed of the shaft of the motor 11 can respectively include an operation of transmitting an acceleration order, respectively an operation of transmitting a deceleration order of the rotation of the shaft of the motor 11. This order is issued from the controller to the VFD frequency converter.

[0063] The step S2 of punctual increase of the speed of the shaft of the motor 11 and the subsequent step S3 of decrease of the rotation speed of the shaft of the motor 11 have an effect on the shape of the load curve recorded on the piston 22 or on the liner 21, on the shape of the speed curve v at the output of the speed reduction gearbox 12, and on the shape of the acceleration curve F at the output of the speed reduction gearbox 12.

[0064] As illustrated in [Fig.8], at the end of the fluid compression movement, the fluid machine 2 no longer generates forces on the actuator 1. The transition from the compression movement to the expansion movement occurs without any backlash at the speed reduction gearbox 12.

[0065] Furthermore, as illustrated in [Fig. 9], the rotational speed v at the output of the speed reduction gearbox 12 is generally constant during both the expansion and compression movements. The only variations in the rotational speed v at the output of the speed reduction gearbox 12 are those induced by increasing the rotational speed of the motor shaft 11 (step S2), and those induced by decreasing the rotational speed of the motor shaft 11 (step S3).

[0066] Finally, as illustrated in [Fig.10], the acceleration F at the output of the speed reduction gearbox 12 remains zero throughout the compression and expansion phases, with the only variations being those introduced in step S2 and step S3.

[0067] Thus, the acceleration Y at the output of the speed reduction gearbox 12 is no longer passively experienced as in the prior art control mode. Thanks to the invention, the acceleration Y is anticipated after an evaluation of the position of the piston 22 or the cylinder liner 21 relative to the critical position Pos_crit

Claims

Demands

1. Method (200) of controlling a fluid machine (2) such as a pump or compressor, for example, for a cryogenic fluid such as hydrogen, the fluid machine (2) being connected to an actuator (1), - the fluid machine (2) comprising a sleeve (21) and a piston (22) together defining a chamber (23) for the expansion and compression of a fluid, the piston (22) and the sleeve being configured to be in reciprocating motion relative to each other between a bottom dead center of fluid expansion and a top dead center of fluid compression, - the actuator (1) comprising the following elements connected in series: a motor (11) with a rotary shaft, a speed reduction gearbox (12) connected to the shaft of the motor (11), a mechanical coupling (13) and a connecting rod (14),the connecting rod (14) being connected to the fluid machine (2) and configured to alternately drive the piston (22) or the liner (21) in a first fluid compression movement and a second fluid expansion movement, the method (200) comprising the following successive steps: - a step (S1) of determining a relative position of the piston (22) or the liner (21) with respect to a reference critical position (Pos_crit) when the engine shaft (11) rotates at a first speed (v„om), - a step (S2) of punctually increasing the rotational speed of the engine shaft (11) as a function of the relative position of the piston (22) or the liner (21) with respect to the reference critical position (Pos_crit), so as to reduce a mechanical shock between the speed reduction gearbox (12) and the connecting rod (14), at the transition between the fluid expansion movement and the fluid compression movement.

2. Method (200) according to claim 1 wherein the step (S2) of punctual increase of the rotational speed of the motor shaft (11) occurs when the relative position of the piston (22) or the liner (21) coincides with or is close to the critical reference position (Pos_crit).

3. Method (200) according to any one of claims 1 or 2, wherein the step (S2) of pointwise increase in speed of rotation of the motor shaft (11) extends over a predetermined time or a variable time depending on the relative position of the piston (22) or the liner (21) with respect to the critical position (Pos_crit) of reference.

4. Method (200) according to any one of the preceding claims, wherein the step (SI) of determining the relative position of the piston (22) or the liner (21) with respect to the critical position (Pos_crit) of reference includes an operation (Sla) of tracking a stroke of the piston (22) or the liner (21) during the compression movement of the fluid.

5. Method (200) according to any one of the preceding claims, wherein the step (S2) of pointwise increase of the rotational speed of the motor shaft (11) accelerates the rotation of the motor shaft (11) from the first speed (v nom) to a maximum speed (v max) greater than the first speed (v nom ), according to a predetermined acceleration (yl).

6. Method (200) according to any one of the preceding claims, comprising a step (S3) of decreasing the rotational speed of the motor shaft (11), the step (S3) being consecutive to the step of momentarily increasing the rotational speed of the motor shaft (1).

7. Method (200) according to the preceding claim taken in its connection with claim 5, wherein the step (S3) of decreasing the rotational speed of the motor shaft (11) decelerates the rotation of the motor shaft (11) from the maximum speed (v max ) to the first speed (v „om), according to a predetermined deceleration (y2).

8. Method (200) according to any one of the preceding claims, wherein the step (SI) of determining the relative position of the piston (22) or the liner (21) with respect to the critical position (Pos_crit) of reference includes an operation (Sla) of evaluating the distance of the piston (22) or the liner (21) with respect to top dead center, for example via a position sensor disposed at the level of the fluid machine (2).

9. Method (200) according to any one of the preceding claims, wherein the step (SI) of determining the relative position of the piston (22) or the cylinder liner (21) with respect to the critical reference position (Pos_crit) includes an operation (Slb) for evaluating a position of the connecting rod (13) relative to the mechanical coupling (13), for example via a position sensor disposed at the connecting rod (13).

10. Method (200) according to any one of the preceding claims, wherein the step (SI) of determining the relative position of the piston (22) or the liner (21) with respect to the critical reference position (Pos_crit) includes an operation (Sic) of evaluating a position of the engine shaft (11) with respect to an engine frame (11), for example via a position sensor.

11. Method (200) according to any one of the preceding claims, wherein the speed reduction gearbox (12) comprises at least one gear train formed of at least two toothed parts (121, 122).

12. Method (200) according to the preceding claim, wherein the step (SI) of determining the relative position of the piston (22) or the liner (21) with respect to the critical position (Pos_crit) of reference includes an operation (Sld) of evaluating a position of one (121) of the toothed parts (121, 122) with respect to the other (122) of the toothed parts (121, 122) of the gear train.

13. Method (200) according to any one of the preceding claims, wherein the step (S2) of punctual increase of the speed of the motor shaft (11) is performed by a variable frequency drive (“VFD”).

14. Method (200) according to any one of claims 6 to 13, wherein the third step (S3) of reducing the rotational speed of the motor shaft (11) is initiated when top dead center is reached.

15. A fluid machine (2) comprising a sleeve (21) and a piston (22) together defining a chamber (23) for the expansion and compression of a fluid, the sleeve (21) and the piston (22) being configured to move reciprocally relative to each other between a bottom dead center of fluid expansion in the chamber (23) and a top dead center of fluid compression in the chamber (23), the machine (2) further comprising an electronic component equipped with a microprocessor and configured to acquire and process data, and an actuator (1) configured to receive a setpoint from the electronic component and to incrementally increase the speed of the stroke of the piston (22) or of the liner (21) as a function of a signal representative of a relative position of the piston (22) or of the liner (21) with respect to a critical position (Pos_crit) of reference.

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