Hydraulic actuator controlled by servovalve

Abstract

 In an hydraulic system in which an hydraulic actuator (1) is controlled by a servovalve (10;50), a feed back signal derived from fluid flow associated with actuator movement is used to operate the servovalve, to ensure accurate control of the hydraulic actuator. The feedback signal can be an hydraulic signal derived from the pressure differential developed across a restriction (30) in the fluid flow, which can be applied to the servovalve directly, or an electrical signal for application to an electrical input to the servovalve, after modification if required, obtained from the pressure differential.

Description

[0001] The invention relates to an hydraulic system in which an hydraulic actuator is controlled by a servovalve.

[0002] In certain applications of such hydraulic systems, for example, where the servovalve is controlling the hydraulic actuator in response to an electrical command signal in an active vehicle suspension system, it is important that the movement controlled by the servovalve be accurately related to the movement required by the command signal.

[0003] The present invention provides, in an hydraulic system having an actuator controlled by a servovalve, means responsive to the actuator movement or to a variable related thereto to provide a feedback signal to the servovalve.

[0004] The feedback signal can be derived from fluid flow associated with the actuator movement and can be either electrical or hydraulic. The invention can thus provide a system in which an electric feedback signal derived from fluid flow between an electrohydraulic servovalve and an hydraulic actuator, which may be a double acting hydraulic actuator, is fed back to the control of the servovalve, through circuitry by which the signal is modified if required. The feedback signal can instead be an hydraulic signal, which may again be derived from fluid flow between a servovalve and the hydraulic actuator, and which can be applied to the servovalve directly. Where the feedback signal is derived from fluid flow, a restriction can be placed in the fluid flow line and the pressure differential across it can be sensed. The feedback signal can be constituted by the pressure differential as such or by an electrical signal developed from it by suitable transducer means.

[0005] The invention thus provides simple and convenient means for linearizing actuator velocity relative to a command current signal. The inherent stabilizing effect obtained is of value particularly but not exclusively in active suspension systems for vehicles for example road vehicles.

[0006] The invention is further described below, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a first servovalve apparatus in accordance with the invention, in which feedback from an hydraulic actuator to a servovalve is effected electrically; and

Figure 2 is a schematic view of a second servovalve apparatus in which feedback is effected hydraulically.

[0007] The servovalve apparatus schematically shown in Figure 1 comprises a double-acting hydraulic actuator 1 controlled by an electrohydraulic servovalve 10. The actuator 1 is indicated simply as a piston 2 movable within a cylinder 4 and having piston rods 5 extending from the piston outwardly through the cylinder end walls to provide a mechanical output. In practice, the hydraulic actuator may be incorporated for example in an active vehicle suspension system as described in WO 84/02886 and may have a structure as described in WO 85/01919, both of which are incorporated herein by reference.

[0008] The servovalve 10 comprises a valve spool 11 slidable axially within a cylinder 12 provided with two pressure fluid inlet ports 14 communicating with a source 16 of oil or other hydraulic fluid and longitudinally spaced apart on opposite sides of a central return port 15 leading back to the source. An outlet port 17 is located between each of the inlet ports 14 and the return port 15 and communicates by way of an hydraulic line 19 with a respective end of the actuator cylinder 4. The servovalve spool 11 has three spaced valve portions connected by stem portions and shaped and dimensioned so that the position of the actuator piston 2 is controlled in dependence on the spool.

[0009] The position of the spool 11 can be adjusted by any suitable means. As shown, the spool position is controlled by fluid pressure from the fluid pressure source 16 or another such source applied to the outer ends of the spool under control of a torque motor 20 with mechanical spool position feedback by way of a spring wire 21. The torque motor 20 is under control of control system circuitry or logic 25 which may be an analogue, digital or hybrid system and which provides a valve drive current proportional to the required actuator velocity. The actual velocity response of the actuator 1 depends on such factors as its loading, upon the fluid pressure within the system, on friction, and compliance between the servovalve 10 and the piston 2. It is desirable to linearise the actuator velocity relative to the demand current and in accordance with the invention, provision is made for a velocity feedback signal to the servovalve 10, realized in the present embodiment in the form of an electric signal applied to the torque motor 20.

[0010] A restriction or orifice 30 is provided in one or other of the lines 19 between the servovalve 10 and the actuator 1, so that a pressure differential is generated across it during movement of the hydraulic fluid, which pressure differential is proportional to the square of the fluid velocity, with a sign dependent on the direction of flow. The pressure differential is sensed by pressure transducers 31, exposed to the fluid pressures at opposed sides of the orifice 30 by way of side tube connections 32. The transducer outputs representing the pressure differential are treated by suitable circuitry or logic, which may be incorporated in the control system circuitry 25, so as to generate a control signal for the torque motor 20 which is proportional to the velocity, whereby an inherently stabilizing control effect for the servovalve is achieved.

[0011] In the second form of apparatus in accordance with the invention shown in Figure 2, parts similar to those of the apparatus of Figure 1 are given the reference numerals used in that Figure. The pressure differential developed across the orifice 30 is here employed to act on the valve spool of the servovalve directly.

[0012] Thus the modified servovalve 50 has chambers 51 formed at respective ends of a servovalve cylinder 52, in which are slidingly received working pistons 56 on end extensions of the valve spool 55. The side tube connections 32 extend from opposed sides of the orifice 30 to these chambers 51. The apparatus of Figure 2 is thus an hydraulic analogue of the electrical apparatus described with reference to Figure 1, and does not need the transducers 31 of that apparatus or the related circuitry.

[0013] It will be evident that the invention can be embodied in a variety of ways other than as specifically described and illustrated.

Claims

1. An hydraulic system comprising an hydraulic actuator (1), and a servovalve (10;50) for controlling movement of the hydraulic actuator, characterized by means (30,31,32;30,32,51) responsive to actuator movement to provide a feedback signal for application to the servovalve.

2. An hydraulic system as claimed in claim 1 wherein the feedback signal is derived from fluid flow between the servovalve (10;50) and the actuator (1).

3. An hydraulic system as claimed in claim 2 wherein the feedback signal is derived from a pressure differential developed by the fluid flow.

4. An hydraulic system as claimed in claim 3 wherein the pressure differential is developed by fluid flow through an orifice (30) in a fluid connection (19) between the servovalve (10) and the hydraulic actuator (1).

5. An hydraulic system as claimed in claim 1, 2, 3 or 4 wherein the means (30,32,51) responsive to actuator movement provides the feedback signal in the form of an hydraulic signal.

6. An hydraulic system as claimed in claim 5 wherein the hydraulic feedback signal is applied directly to the valve member (55) of the servovalve (50).

7. An hydraulic system as claimed in claim 6 wherein the hydraulic feedback signal is applied as a pressure differential to opposed ends of the servovalve valve member (55).

8. An hydraulic system as claimed in claim 1, 2, 3 or 4 wherein the means (30,31,32) responsive to actuator movement provides the feedback signal in the form of an electric signal.

9. An hydraulic system as claimed in claim 3 or 4 wherein the pressure differential is sensed by pressure transducers (31) providing electrical outputs which constitute the feedback signal.

10. An hydraulic system as claimed in claim 8 or 9 wherein the servovalve (10) is hydraulically operated under control of a torque motor (20), the torque motor position being controlled by circuitry (25) responsive to the electric feedback signal.

Drawing