EP0074254A1

EP0074254A1 - Hydraulic control valve

Info

Publication number: EP0074254A1
Application number: EP82304629A
Authority: EP
Inventors: John Richard Archer; Stephen Owen
Original assignee: Prutec Ltd
Current assignee: Prutec Ltd
Priority date: 1981-09-02
Filing date: 1982-09-02
Publication date: 1983-03-16
Also published as: JPS5877905A

Abstract

The invention relates to hydraulic control valve controlled by means of a solenoid (24) for the purpose of regulating the supply of fluid to a load such as hydraulic motor. The valve has a valve spool (120) moved by the solenoid (24) and additionally acted upon by the differential pressure across a throttle (42) in the hydraulic circuit so as to provide compensation for the effects of varying viscosity and varying load on the motor. Additionally, to enable a simple design of solenoid (24) to be adopted, a transducer (54) electrically measures the flow rate of hydraulic fluid and varies the current supplied to the solenoid (24) accordingly.

Description

FIELD OF THE INVENTION

The invention relates to hydraulic valve and is particularly concerned with a pressure transducer monitored valve for open loop electrical control of flow in hydraulic circuits.

DESCRIPTION OF PRIOR ART

It is known to control the speed of a hydraulic actuator by regulating the flow of hydraulic fluid to the actuator. Accurate speed control has been achieved by means of a feedback transducer on the actuator or a precise internal metering system in the regulation valve to compensate for variation of viscosity and pressure in the fluid. Electrically controlled flow control valves have tended to be complex and expensive because of the need to provide compensation for viscosity and pressure variation by hydromechanical methods.
When a spool valve having a solenoid actuated spool is used to control the flow a hydraulic motor, it is possible to provide some degree of compensation for variation in viscosity and pressure by providing a throttle in series with the flow to the motor and applying the pressure developed across the throttle to chambers acting on the opposite end of the valve spool. Thus, as the flow rate changes the pressure across the throttle changes and this in turn moves the valve spool in a direction to counteract the change in pressure across the throttle.
In the latter form of valve, it is necessary to use an expensive form of solenoid to operate the valve spool since a solenoid is required in which the force exerted by the solenoid on the valve spool is related only to the current and is independent of the stroke of the solenoid.
The present invention seeks to provide a valve in which the flow rate may be accurately controlled without placing severe demands on the design of the actuating solenoid.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a hydraulic valve comprising :

a housing,
inlet ports in the housing for connection to fluid supply and return lines,
outlet ports in the housing for connection to a hydraulic load,
a flow control element movable within the housing for regulating the flow between the inlet ports and the outlet ports,
an electromagnetic actuator for moving the flow control element to vary the fluid supply rate to the load in dependence upon an electrical control current,
throttling means in at least one port for developing thereacross a differential pressure dependent upon the rate of flow of fluid through the valve, and
means for applying a hydraulic force to the flow control element in proportion to the differential pressure developed across the throttling means so as to vary the force acting on the flow control element in dependence upon flow rate;

Conveniently, the transducer may comprise a pressure sensitive transducer across which is applied the differential pressure developed across the throttling means.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a section through a first embodiment of hydraulic valve in accordance with the invention, and
Figure 2 is a detail of the inlet port in Figure 1 showing an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In Figure 1, there is shown a hydraulic valve which comprises a housing 10 formed with a bore in which a valve spool 12 is slidable. The valve housing 10 is provided with five ports of which the two outlet ports 14 and 16 are shown in solid lines being in the plane of the section of the drawing and the remaining three ports designated 18, 20 and 22 are shown in dotted lines. The outer ones of the inlet ports 18 and 20 are connected to a common return line, the inlet port 22 is connected to a supply line, and the outlet ports are connected to a load such as a hydraulic motor.
The spool 12 has three lands which in the closed position of the valve block off all inlet ports 18, 20 and 22. When the valve spool 12 is moved to the left, as shown, the outlet port 16 is connected to the supply line 22 and the outlet port 14 is connected to the return line 18. The valve spool 12 is moved to the left by means of a solenoid 24 which acts by way of a connection rod 26 on the spool 12.
The spool is urged against the action of a return spring 28 contained in a chamber 30 lying to the left of the valve spool, as shown, and closed by a plate 32. When a current is passed through the coil of the solenoid, a force is developed related to the current and the armature of the solenoid moves to a position of equilibrium in which the electromagnetic force is balanced by the force of the compressed spring 28.
If desired, the plate 32 can be replaced by a second solenoid for urging the valve spool to the right, as shown, whereupon the valve can be used to establish bi-directional flow to the hydraulic load connected to the outlet ports 14 and 16. The symmetrical design of the valve body and.the valve spool enables such an alternative construction without further modification.
The chamber 34 formed in the body to the right of the end of the valve spool 12 is permanently connected to the return line 20 by means of a groove 36 formed in a sleeve 40 surrounding the valve spool 12. A similar groove 38 connects the chamber 30 to the left of the valve spool to the other return line 18. Furthermore, as shown in dotted lines, a throttle, shown diagramatically as a plug 42 with a restriction screwed into the return line, is provided for restricting the flow in each of the return lines 18 and 20.
As so far described, the valve is generally similar to valves to be found in the prior art. The purpose of the throttle plug 42 in the return line is to develop a differential pressure thereacross which varies with the flow rate. By virtue of the grooves 36 and 38, this pressure is applied to the ends of the valve spool 12 and the resultant force is superimposed on the force exerted by the solenoid 24 so as additionally to vary the opening cross-section of the valve in dependence upon flow rate.
The disadvantage in relying exclusively on such a hydraulic negative feedback pressure from the throttle to correct for variation in flow rate resulting, for example, from changes in viscosity is that it is assumed that the same electromagnetic force is applied to the valve spool 12 by the solenoid regardless of the stroke of the solenoid 24, that is to say regardless of the position of the valve spool 12 in the housing 10. This places excessive demands on the design of the solenoid 24 resulting in a far more expensive construction. In this respect, it should be pointed out that the overall cost of the solenoid controlled valve is dictated to a large extent by the cost of the solenoid as the mechanical parts are generally considerably less expensive.
The further components of the valve now described are intended to enable an inexpensive solenoid to be used without adversely affecting the quality of control.
The chambers 30 and 34 are connected by means of bores 50 and 52 in the housing 10 to the opposite ends of a pressure transducer 54. The pressure transducer 54 produces an electrical output signal which is applied by way of a lead 56 to an electrical circuit 58 which is shown only schematically in the drawing as being a printed circuit housed beneath a removable cover 60 held in place by screws 62. The circuit 58 has a further input lead 64 which carries the control current normally fed directly to the solenoid and has output leads 66 connected to the solenoid 24. 1
The circuit 58 may conveniently comprise a differential amplifier the two inputs of which receive signals derived respectively from the control current and the transducer feedback signal and the output of which drives the solenoid 24. By virtue of the electrical feedback which enables the force exerted by the solenoid to be varied regardless of stroke as a function of the pressure developed across the throttle in the return line it is possible to use a relatively inexpensive solenoid without reducing the performance of the valve. Thus, the flow rate and hence the speed of the hydraulic motor controlled by the valve will vary only as a function of the control current and will be substantially independent of fluid viscosity and pressure differences caused by differences in the load on the hydraulic motor.
It should be mentioned that in a throttle the pressure developed across the throttle is not linearly related to . the flow rate but is proportional to the square of the flow rate. Consequently, in the known flow control valve previously described, the rate of flow of hydraulic fluid is also not linearly related to the current supplied to the solenoid but follows a square law relationship. If in an embodiment in accordance with the invention it is desired to provide linear control, then the electrical circuit may incorporate a squaring circuit for squaring the applied control current in an input circuit leading to the differential amplifier. In this way a more linear relationship between the applied current and the speed of actuation of the hydraulic motor may be achieved.
Pressure transducers have a limit as to the ratio of the maximum to minimum pressures that they can measure with accuracy. However, because the pressure developed across the throttle is related to the square of the flow rate, the ratio of maximum to minimum flow rates that can be measured by the transducer is equal to the square root of the maximum to minimum pressure ratio. If, for example, the maximum and minimum pressures that can be measured by the transducer are in the ratio of 9:1 then the maximum and minimum measured flow rates will be in the ratio of 3:1. Thus, the measurement range in a valve provided with a fixed throttle is limited.
In order to mitigate the above problems, the insert plug 42 to be introduced into the return port may be a variable throttle in place of a fixed throttle. The variable throttle 42' shown in Figure 2 comprises a conical closure member 43 urged by means of a spring 44 into a maximum throttling position. However, as the flow rate increases, the closure member 43 moves to increase the throttling cross section with the result that the pressure across the throttle is no longer related to the square of the flow rate but some lesser power depending upon the configuration of the variable throttle.
An alternative form of variable throttle that may be employed comprises a disc with a central aperture from which radiate a plurality of radial grooves. The disc is made of a flexible material and is placed in a plane perpendicular to the flow direction with the object of deflecting the radial fingers to increase the effective flow cross section as the flow rate increases. The use of a variable throttle in the return line enables the range of flow rates monitored by a given pressure transducer to be extended.
It should be clear that various modifications may be made to the invention as described without departing from the scope of the claims. Thus, it is not essential that the throttle be provided in a return line as it need only be in any part of the hydraulic circuit of the controlled load.
Furthermore, though a pressure sensitive transducer has been described acting in conjunction with the throttle in order to measure the flow rate, it is alternatively possible to measure the flow rate more directly. For example, in an embodiment incorporating the alternative form of variable throttle described above, it is possible to measure the flow rate by measuring the extent of deflection of the radial fingers which bend to vary the throttling cross-section. This may be achieved by the use of a strain gauge acting directly on the flexible fingers.

Claims

1. An electromagnetically operable hydraulic control valve comprising a housing, inlet ports in the housing for connection to fluid supply and return lines, outlet ports in the housing for connection to a hydraulic load, a flow control element movable within the housing for regulating the flow between the inlet ports and the outlet ports, an electromagnetic actuator for moving the flow control element to vary the fluid supply rate to the load in dependence upon an electrical control current, throttling means in at least one port for developing thereacross a differential pressure dependent upon the rate of flow of fluid through the valve, and means for applying a hydraulic force to the flow control element in proportion to the differential pressure developed across the throttling means so as to vary the force acting on the flow control element in dependence upon flow rate; characterised in that there are additionally provided a transducer (54) for producing an electrical signal representative of the rate of flow of hydraulic fluid to the load, and an electrical circuit (58) for varying the current applied to the electromagnetic actuator (24) in dependence upon the said differential pressure.

2. A hydraulic control valve as claimed in Claim 1, characterised in that the transducer (54) is a pressure sensitive transducer across which is applied the differential pressure developed across the throttling means (42).

3. A hydraulic control valve as claimed in Claim 1, characterised in that the transducer (54) is responsive directly to the rate of flow of hydraulic fluid to the load.

₄. A hydraulic control valve as claimed in any preceding claim, characterised in that the flow control element is a valve spool (12) slidable within the housing (10) of the valve and the electromagnetic actuator is a solenoid (24) having an axially displaceable armature connected to displace the valve spool(12).

5. A hydraulic control valve as claimed in Claim 4, characterised in that the electrical circuit (58) comprises a differential amplifier having a first input coonected to receive a signal derived from the control current and a second input connected to receive a signal derived from the output of the transducer(54).

6. A hydraulic control valve as claimed in Claim 5, characterised in that the first input is connected to receive the signal derived from the control current by way of a squaring circuit whereby to render the rate of fluid flow through the control valve linearly dependent upon the magnitude of the control current.

7. A hydraulic control valve as claimed in any preceding claim, characterised in that the throttling means (42') has a throttling cross-section which increases with increasing rate of flow of hydraulic fluid.

8. A hydraulic control valve as claimed in claim 7, characterised in that the throttling means (42') comprises a seat and a conical closure member (43) resiliently biassed in a direction to minimise the throttling cross-section, the fluid flow being operative to move the conical closure member (43) away from the seat in a direction to increase the throttling cross-section.

9. A hydraulic control valve as claimed in any preceding claim, characterised in that two electromagnetic actuators (24) are provided acting in opposite directions on the flow control element (12), to enable the direction of flow of hydraulic fluid through the load to be reversed.