EP0159949B1 - Control device with servo valves - Google Patents

Description

The invention relates to the control, by means of servo-valves, of a hydraulically actuated member.

A member of this type, for example a distributor drawer in the case of a two-stage system, or a jack, a metering device, in the case of a single-stage system, is associated with at least one subject chamber at a servo pressure, controlled by a servo-valve, ensuring its positioning.

We know servo valve control devices comprising a pallet mounted at the end of a rod housed inside a spring tube fixed on a base, supporting at its free end a movable transverse element in the air gap excitation coils; according to the current applied to the coils, the pallet moves between two nozzles connecting each of the organ control chambers at low pressure and thus determines its position.

Such devices have the disadvantage of being very sensitive to pollution; this is because of the short stroke of the pallet between the two nozzles which is of the order of only a few hundredths of a millimeter. If, in fact, a particle of this order of magnitude, transported by the servo fluid, is inserted between the nozzle and the pallet, it tends to get caught there and paralyze the pallet in an off-center position.

These devices also have the disadvantage of being sensitive to vibrations due to the fragility of the spring tube, the rupture of which leads to the total loss of control.

A remedy to these drawbacks has been to produce an arrangement providing for redundancy of the control means.

Because most servo valves have a double winding, it is easy to achieve electrical redundancy. It is more difficult to double the servo-valves for controlling the same member, because in such a system, it is necessary to provide a means of hydraulic isolation in order to put out the defective servo-valve. Without this insulation, the faulty servo valve prevents the second from fully fulfilling its role. Patent GB-A-1,369,441, for example, clearly shows an electro-hydraulic control system where two servo-valves in parallel receive the same electrical control signals and act simultaneously on the same pressure chambers of an operating member; a device is provided which makes it possible to detect a difference in operation between the two servo-valves and to generate an error signal. However, with such a system, the only possible actions in the event of failure of one of the servo-valves are either the locking of the operating member in position or the implementation of an emergency control bringing the member into a determined position, ie the isolation of one of the servo-valves by means of a hydraulic switching slide.

Thus, US Pat. No. 3,437,312 describes a redundant control system comprising three calculation chains associated with three comparators and two servo-valves, one of the chains being active and the other neutral, the two servos valves being separated by a hydraulic selector which admits on the actuated cylinder sometimes the fluid of the first servo-valve sometimes that of the second.

An object of the invention is therefore to provide a hydraulic control device capable of withstanding the first failure of the pilot servo-valve without loss of servo-control.

According to the invention, the device for controlling a hydraulically actuated member is of the type comprising a piston movable in a cylinder delimiting at least one chamber subjected to a control pressure signal obtained between a high pressure zone and a low zone pressure by a hydraulic potentiometer constituted by a fixed throttle and a variable-section throttle means, the latter being defined by two electro-hydraulic servo-valves, the device also comprising means for producing the control signals in response to signals of sets so that only one servo valve is pilot, the other remaining neutral, and being capable of reacting to a means of detecting malfunction of the pilot servo valve to switch the command in order to make the servo valve pilot neutral and neutral the pilot servo valve. According to a main characteristic of the invention, the servo-valves offer a predetermined passage section in the neutral position and the electrical control signals of the two servo-valves alone ensure the switching of these, the active servo-valve and the servo -Neutral valve cooperating to permanently deliver the servo pressure signal.

According to a preferred embodiment, the servo-valves are of the single-nozzle and vane type which in the neutral position offer a predetermined and adjustable passage section.

• la figure 1 représente un dispositif de commande à servo-valve, connu,
• la figure 2 représente un premier mode de réalisation de l'invention
• la figure 3 représente un exemple de réalisation du système électronique de commande des servo-valves.
• la figure 4 représente un deuxième mode de réalisation.

Other characteristics and advantages of the invention will emerge from the description referring to the attached drawings, given by way of example:

• FIG. 1 represents a known servo-valve control device,
• FIG. 2 represents a first embodiment of the invention
• FIG. 3 represents an exemplary embodiment of the electronic system for controlling the servo-valves.
• FIG. 4 represents a second embodiment.

In Figure 1, there is shown a block diagram of a known embodiment of the control of a dispenser drawer 1 by a servo-valve 2 with pallet 5 and double nozzle 6 and 6 '.

The two free ends of the drawer form pistons of equal sections delimiting two chambers 3 and 3 'with the cylinder in which the drawer is housed. These chambers are supplied with fluid by means of a hydraulic potentiometer constituted between a high pressure source HP and a low pressure cover BP by two fixed diaphragms 4 and 4 'in parallel and two nozzles with variable section 6 and 6' each in series with one of the diaphragms 4 and 4 '. The fluid supplying the chambers 3 and 3 'is taken between a diaphragm and a nozzle so that the pressure prevailing in the chambers is an intermediate pressure between the values of HP and BP. The two nozzles are mounted opposite one another on either side of the movable pallet parallel to their axis so as to reduce, respectively increase the flow section of the fluid towards the low pressure; thus a differential pressure is created between the chambers 3 and 3 'causing the slide to move.

The pallet 5 is integral with a rod 7 mounted inside a flexible tube 8 made of a non-magnetic spring metal of small thickness. The tube is secured to the support, not shown, by its base. It also supports a movable soft iron bar 9 inside the excitation coils B1, B2. Thus, depending on the excitation current, the pallet moves in one direction or the other or is brought back to an intermediate position by the spring tube 8.

A device like this has the disadvantage of being very sensitive to pollution because the distance separating the nozzles is very small, of the order of only a few hundredths of a millimeter. A simple particle inserted between a nozzle and the pallet blocks any movement of the latter and renders the entire control inoperative. A second cause of failure is the rupture of the spring tube due to its great fragility due to the small thickness of the wall.

The invention overcomes these drawbacks; Figure 2 shows a diagram of a first embodiment. We recognize at 15 a distributor drawer terminated by two pistons with differential sections; this drawer itself controls the supply of working fluid to a jack driving a body to be positioned. This is given as an example. the invention also applies to the control of a single-stage system such as a metering device or a jack or any other member with equivalent operation. Likewise, no mechanical return of the slide position is shown, it is always possible to introduce counter-force springs in the control chambers. Bedroom. 16, the section of which is the smallest, communicates directly with the source of fluid 18 at a pressure maintained constant by a pressure regulator. Only the pressure in the opposite chamber 17 is modulated, it is taken between the fixed diaphragm 19 and the variable section diaphragm constituted in the example represented by two identical single-nozzle servo-valves arranged in parallel 20, 30, leading to the low pressure. . The two servo valves are constructed on substantially the same principle as the servo valve described above, but each pallet 21, 31 now controls the passage section of a single nozzle 22, 32 and its opening movement is limited by a mechanical stop 23, 33. The position of the stop is adjusted so that the pallet is tangent to it under the action of the spring tube only, in the absence of excitation current from the torque motor, thus determining a maximum passage cross section. The constituent elements are dimensioned so that in the event of a rupture of the spring tube, the pallet also comes to bear against the stop.

The winding of the torque motor of each of the servo valves is connected to an electronic control system 100. This system is designed to control only one of the two servo valves at a time leaving the other at rest in the full open position. . Thus, in normal operation, the pressure inside the chamber 17 is modulated by controlling the section of the single nozzle 22 for example, the nozzle 32 delivering a constant and predetermined leak rate. In the event of a breakdown which may be due to a failure of the electric servo valve control chain, the rupture of the spring tube, the insertion of a particle between nozzle and pallet, the servo valve can no longer perform its function; the controlled member moves away from its set position whatever the order given to the servo-valve. The electronic system then detects this deviation and operates a switch; the pilot servo valve being no longer supplied becomes neutral and the initially neutral servo valve becomes pilot. In this new mode of operation where the control of the servo-valves is reversed, nothing is changed for the drawer because the nozzle 22 whose section was modulated now delivers a constant leak rate equal to the previous rate of the nozzle 32 and the control is provided by the nozzle 32 as it was previously by the nozzle 22. In fact, the device according to the invention does not include a hydraulic selector which would isolate one of the servo-valves, the drawer permanently receives the fluids coming of the two servo-valves 22, 32: FIG. 3 shows a nonlimiting example of an electronic control system capable of ensuring this switching function in the event of a fault. We find the hydraulic circuit with the pressure source 18, the servo-valves 20, 30, and the fixed diaphragm 19, determining the servo pressure for a distributor 15. The latter controls the supply of a drive cylinder of the controlled body X whose displacement is measured by means of detectors. The control system comprises two entirely separate electrical lines 200, 300 each comprising inputs 201, 301 respectively for the setpoint signals, an electronic circuit 202, 302 and a line connected to the winding of the torque motor of the corresponding servo-valve 20, 30. Each of the lines also receives the position detector signals from the controlled member 203, 204 and 303, 304.

Each circuit includes two computers identical, 205. 206 for circuit 202 and 305, 306 for circuit 302, defining the position to be reached according to the data received in 201 and 301. In each circuit the 1st computer 205, 305 is used for the command, it provides a signal which is compared at 211, 311 respectively to a signal corresponding to the actual position of the body, produced by the first detector 203, 303; the difference value signal is amplified in an amplifier 207, 307, the output of which is connected to the winding of the corresponding torque motor by means of a switch 208, 308: the latter reacts to a signal supplied by an adder amplifier 209 , 309 which comprises as input the output signal of the amplifier 309, 209 from the other line and the signal corresponding to the difference value, produced at 210 and 310 between the set value calculated by the second computer 206, 306 and the value of the position of the controlled body supplied by a second detector 204, 304.

The system is electronically redundant, each of the lines receives the same input signals and produces identical control signals: but in normal operation, the switch 208 is closed, and the switch 308, the control of which is reversed with respect to 208 , is open so that only the servo valve connected to the first line is pilot, the other remaining neutral. in the open position. If a failure occurs, the difference signal between the value calculated by 206 respectively 306 and the value provided by 204 respectively 304 increases until it exceeds a predetermined threshold. Then the amplifiers 209 and 309 produce control signals which will reverse the position of the switches 308 and 208 respectively. The servo-valve 20, no longer supplied, becomes neutral and the pallet is brought into abutment against the mechanical stop. The servo valve 30, in turn, becomes pilot. The switching from one line to the other is bi-stable so that the command does not switch back to the 1st line when the deviation signal returns to a value below the triggering threshold of the circuits 209, 309. We see from this fact that the switching relates only to the electric control signals of the two servo-valves, these contributing together permanently to supply the control pressure signal.

This electronic control circuit is given only by way of example. Instead of a redundant system, it is possible to design a single line with signal switching on the second servo valve in the event of a malfunction if the servo valves are less reliable. than electronics.

Figure 4 shows an alternative mounting of the two servo valves applying the same principle of the invention. We find the same elements as previously arranged differently, the references have been assigned a premium sign. In this case the servo valves are no longer mounted in parallel but in series. The control fluid flows from the pressure HP to the pressure BP in a circuit comprising in series the fixed diaphragm 19 ', the first servo-valve 30' and the second servo-valve 20 '; chamber 17 'of the distributor communicates with the circuit, between 19' and 20 '. In normal operation, the pressure in this chamber is controlled by the single servo valve 20 '. The servo-valve 30 'being neutral, the pallet pressing against the mechanical stop defines a fixed diaphragm corresponding to the section of the nozzle. If a failure occurs. altering the normal functioning of the servo-valve 20 ', the electronic system interrupts its supply and transfers the signal to the second servo-valve.

In summary, thanks to the invention, an electro-hydraulic control device is produced which tolerates the first pollution failure or the first mechanical failure of the vibrating tube rupture type, while retaining 100% of the capacity of the device. This device is at the same time simple, the redundancy is achieved by retaining the same number of nozzles as a usual servo-valve, it does not require a hydraulic switching slide, the latter being obtained electrically by opening the switch. of the electrical control circuit of one of the servo-valves and the simultaneous closing of the corresponding switch of the other circuit. This device is self-repairing to a certain extent. In fact, the mere fact of switching to the second command causes the first command to pass, the increase in the nozzle-pallet clearance which results therefrom may very well allow the possible polluting particle to be eliminated alone; the first order then becomes available again in case of failure of the second order.

On the other hand, it can be seen that the invention, although this has not been described on a drawing board, can also be applied to the redundant control of double-acting cylinders using for the control of each enclosure of the cylinder a set of two servo-valves arranged according to the invention, in series or in parallel.

Claims (5)

1. Control device of a hydraulically operated member, of the type comprising a piston (15, 15') movable in a cylinder defining at least one chamber subject to a servo-control pressure signal obtained from between a high pressure zone and a low pressure zone by a hydraulic potentiometer formed by a fixed throttle (19, 19') and throttling means of variable section, the latter being defined by two electro-hydraulic servo-valves (20, 30 ; 20', 30'), the device further comprising means (100, 100') for producing control signals in response to command signals so that a single servo-valve is master, the other staying neutral, and being capable of reacting to means for detecting malfunction of the master servo-valve to switch over the control in order to render the neutral servo-valve master, and the master servo-valve neutral, characterised in that in the neutral position the servo-valves (20, 30 ; 20', 30') present a predetermined flow section and in that the electrical control signals of the two servo-valves ensure only their switching over, the active servo-valve and the neutral servo-valve cooperating to continue to deliver the servo-control pressure signal.

2. Control device of a hydraulic member according to claim 1, characterised in that the servo-valves (20, 30) are hydraulically disposed in parallel (Figure 2).

3. Control device of a hydraulic member according to claim 1, characterised in that the servo-valves (20'. 30') are hydraulically disposed in series (Figure 4).

4. Control device of a hydraulic member according to claim 2 or claim 3, characterised in that the servo-valves are of the single nozzle (22, 32 ; 22', 32') and plate (21, 31 ; 21', 31') type and in that in the neutral position, the plate is drawn back to a known and adjustable position.

5. Control device of a hydraulic member according to any one of the preceding claims, characterised in that the means producing the control signals comprises two independent electronic circuits connected each to one servo-valve vis a switch (208, 308), the said switches being controlled according to the advent of a breakdown in one of the electro-hydraulic circuits.

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