Improvements in or relating to hydraulic actuator controls

Abstract

 A device for controlling a hydraulic actuator (100) has a fluid pressure operated main valve (20) controlled by a pilot valve (19). The force of a force motor (52) acting on the pilot spool (40) is opposed by a feedback force produced by the pressure difference applied by the pilot valve (19) to the main valve control chambers (36 and 37) and also applied to feedback chambers (55 and 56) of the pilot valve.

Description

[0001] The present invention relates to hydraulic actuator controls and more particularly to a device for controlling hydraulic actuator means responsively to an electrical input signal comprising a main valve having a fluid pressure operated main spool.for controlling the fluid supply to the actuator means, a pilot valve having a pilot spool for regulating the fluid pressure for operating the main spool, and an electrical force motor for producing a force dependent upon the electrical input signal and for applying such force to the pilot spool in opposition to a fluid pressure acting on the pilot spool.

[0002] Such a device is described in British Patent Specification No. 1,406,326 which discloses the use of a flow sensor for producing a pressure difference dependent upon the rate of fluid flow to the actuator means and for applying such pressure difference to the pilot spool in opposition to said force. By suitably designing the flow sensor as described in British Patent Specification No. 1,135,042, an accurate correlation between the magnitude of the electrical input signal and the speed of the actuator means over a wide range of flow rates can be obtained.

[0003] However there are occasions when it is more important to avoid the risk of instability arising through the presence of a closed servo loop (including the flow dependent feedback from the flow sensor to the pilot spool) than it is to obtain an accurate correlation between input and output.

[0004] An object of the invention is to dispense with the closed servo loop including the flow sensor.

[0005] Another object is to use the position of the main spool as an indication of the rate of fluid flow to the actuator means.

[0006] According to the present invention, the regulated fluid pressure for operating the main spool is also applied to the pilot spool in opposition to said force, whereby the position of the main spool, which is displaced by said regulated fluid pressure against a return spring, is determined by the electrical input signal.

[0007] For a given hydraulic supply pressure to the main valve and a given load on the actuator means, the rate of fluid flow to the actuator means is determined by the position of the main spool. Since the latter is itself determined by the electrical input signal in the device according to the invention, the magnitude of the electrical input signal can be used to obtain a measure of control of the speed of the actuator means without the need to provide a flow sensor and the associated closed servo loop.

[0008] The invention is further described, by way of example, with reference to the accompanying drawing which is a flow diagram of a fluid flow control device in accordance with the invention.

[0009] The fluid flow control device comprises a main control valve 20 and a pilot valve 19 for controlling the main valve 20. The main and pilot valves are arranged in separate valve blocks which are bolted together with the respective fluid ports in communication with one another to provide the desired hydraulic connections as hereinafter described.

[0010] The main control valve 20 has a main spool 21 provided with lands 22, 23 and 24 for controlling communication between three inlet/outlet ports 27, 28 and 29 and two service ports 25 and 26 connected by lines 32 and 33 to a hydraulic actuator 100. A supply line 31 is connected to the inlet port 28 and the outlet ports 27 and 29 are connected by a line 30 to tank. The spool 21 is biassed towards its central neutral or null position by return springs 34 and 35 which are conveniently disposed in control chambers 36 and 37 at opposite ends of the spool 21. The main spool 21 is displaced from its neutral position by the application of a pressure difference between the chambers 36 and 37 by means of the pilot valve 19.

[0011] The pilot valve 19 has a valve spool 40 which is provided with three lands 41,42 and 43 controlling fluid communication between a central inlet port 44 and drain ports 45 and 46 on the one hand, and control ports 47 and 48 on the other hand. The inlet port 44 is connected by a line 49 to the outlet of a pressure reducing valve 50 which serves to maintain a constant pressure in the line 49. The inlet to the pressure reducing valve 50 is connected to a supply line 51 which can, if desired, be connected to the same external supply as the line 31. The pressure reducing valve 50 can be arranged in the same valve block as the pilot valve 19 or in a separate port plate on which the valve blocks are directly or indirectly mounted. The pilot spool 40 can be displaced from its neutral position by means of a linear force motor 52 which is adapted to produce a force directly proportional to the electrical current applied thereto. The armature of the force motor 52 is supported on diaphragms which act as centering springs for the pivot spool 40.. The control ports 47 and 48 are connected by respective control lines 53 and 54 to the control chambers 36 and 37 of the main control valve 20.

[0012] The pilot valve 19 has annular feedback chambers 55 and 56 at the sides of the lands 41 and 43 facing the respective ends of the spool 40. The chambers 57 and 58 at the ends of the spool 40 are connected to a drain line 59 as are the drain ports 45 to 46. Pressures are applied to the feedback chambers 55 and 56 from the control chambers 36 and 37 of the main valve 20 via lines 70 and 71 which are connected respectively to the lines 53 and 54.

[0013] The drawing shows the main spool 21 displaced to the right from the neutral or null position by the force motor 52 having been energized to displace the pilot spool 40 to the right, as shown, to connect the inlet port 44 to the control port 47 and the control port 48 to the drain port 46. The resulting pressure difference between the control chambers 36 and 37 is balanced by the increased compression of the spring 35, whereby this pressure difference is proportional to the displacement of the main spool 21 from its null position. This pressure difference is also applied to the pilot feedback chambers 55 and 56 in opposition to the force of the force motor 52 which is dependent on the magnitude of the energizing current. The pilot spool 40 is returned to its null position when the feedback force due to this pressure difference balances the applied force from the force motor.

[0014] For a given supply pressure applied to the inlet port 28 and a given load on the actuator 100 the rate of supply of hydraulic fluid to the actuator 100 is dependent on and generally substantially proportional to the displacement of the main spool 21 from neutral and therefore proportional to the magnitude of the energizing current fed to the force motor 52.

[0015] To move the actuator in the other direction, the force motor 52 is energized in the opposite direction to connect the pilot inlet port 44 to the control port 48 and the control port 47 to the drain port 45 whereby the main spool 21 is displaced to the left from its null position.

[0016] It should be mentioned that the forces of the springs 34 and 35 tending to return the main spool 21 to the null position are supplemented by the so-called Bernoulli forces which exerted on the spool 21 by the fluid being controlled and which are themselves dependent on the rate of fluid flow and the valve pressure drop. The Bernoulli forces may or may not be significant, dependent upon the design of the main valve and the forces of the springs 34 and 35.

[0017] The pilot valve 19 can be exactly the same construction as the flow control pilot valve of the control device described and illustrated in British Patent Specification No. 1,406,326. Likewise the main valve 20 can be of identical construction to that of the device of this prior patent. Thus the device of the present invention can be made from the parts designed for the device of the prior patent. The only modifications are the omission of the flow sensor and an associated shuttle valve and the provision of extra drillings to provide the lines 70 and 71.

[0018] The provision of means for applying a feedback pressure difference to the pilot spool 40 of the pivot valve 19 must not be confused with the application of the pressure drop across the actuator 100 to the pivot spool to obtain a pressure feedback for operation in the so-called "pressure control mode" as is also described in the aforementioned British Patent Specification No. 1,406,326. The application of a pressure difference to the feedback chambers 57 and 58 in the device of the present invention provides for position control of the main spool 21.

Claims

1. A device for controlling hydraulic actuator means responsively to an electrical input signal comprising a main valve having a fluid pressure operated main spool for controlling the fluid supply to the actuator means, a pilot valve having a pilot spool for regulating the fluid pressure for operating the main spool, and an electrical force motor for producing a force dependent upon the electrical input signal and for applying such force to the pilot spool in opposition to a fluid pressure acting on the pilot spool, characterised in that the regulated fluid pressure for operating the main spool (21) is also applied to the pilot spool (40) in opposition to said force, whereby the position of the main spool, which is displaced by said regulated fluid pressure against a return spring (34 or 35) is determined by the electrical input signal.

2. A device according to claim 1 characterised (1) in that a pressure reducer (50) providing the pilot fluid fed to the pilot jvalve (19) is disposed )in the same valve block as the pilot valve, the main valve (20) being in a separate valve block.

3. A device according to claim 1 characterised in that the pressure reducer (50) is arranged in a port plate on which the valve blocks are directly or indirectly mounted.

4. A device according to claim 1, 2 or 3 characterised in that the force of the return spring (1) see request to correct of November 30,1978 (34 or 35) is supplemented by Bernoulli forces acting on the main spool (21).

Drawing