WO1996027051A1

WO1996027051A1 - Electrohydraulic proportional control valve assemblies

Info

Publication number: WO1996027051A1
Application number: PCT/GB1996/000393
Authority: WO
Inventors: Stephen Brian Turner; David Franz Lakin
Original assignee: Ultra Hydraulics Limited
Priority date: 1995-02-25
Filing date: 1996-02-22
Publication date: 1996-09-06
Also published as: GB2298291B; GB9503854D0; CN1175988A; DE69602923T2; DK0809737T3; GB2298291A; CN1070974C; WO1996027051B1; DK0809737T4; DE69602923T3; KR100432381B1; GB2298291C; JPH11501106A; JP2006177561A; DE69602923D1; EP0809737B1; KR19980702483A; EP0809737B2; EP0809737A1; GB9603811D0

Abstract

An electrohydraulic proportional control valve assembly (1) for controlling a bidirectional fluid actuated device (7) has a first actuating port (4) for bidirectional fluid flow between the assembly and a first port of the device (7), a second actuating port (5) for bidirectional fluid flow between the assembly and a secondport of the device (7), a pump port (15, 16) for input fluid flow from a pump (17), and a tank port (18, 19) for output fluid flow from a tank (20). The assembly (1) comprises a first spool valve (2) connected to the first actuating port (4), the pump port (15, 16) and the tank port (18, 19) for controlling the direction and rate of fluid flow to and from the first port of the device (7), and a second spool valve (3) connected to the second actuating port (5), the pump port (15, 16) and the tank port (18, 19) and operable independently of the first pool valve 2 for controlling the direction and rate of fluid flow to and from the second port of the device (7). A position sensing arrangement (23, 24) is provided for supplying electrical position signals indicative of the actual positions of the spools (12, 13) of the first and second spool valves (2, 3) and a pressure sensing arrangement (26, 27, 28 and 29) is provided for supplying electrical pressure signals indicative of the fluid pressures in the first and second actuating ports (4, 5), the pump port (15, 16) and the tank port (18, 19). A servo control (not shown) controls the positions of the first and second spools (2, 3) in dependence on the electrical position and pressure signals and in response to an electrical demand signal provided in response to operator actuation, in order to set the throughflow cross sections for fluid flow to effect the required control of the device (7).

Description

"Electrohvdraulic Proportional Control Valve Assemblies"

This invention relates to electrohydraulic proportional control valve

assemblies for controlling fluid actuated devices.

It is known to utilise a proportional control valve assembly for controlling a fluid actuated device, such as a control ram for a lifting arm of an earth moving

vehicle for example, in response to a demand signal supplied by an operator actuated

joystick. Such a control valve assembly typically incorporates a main spool valve

having a first actuating port for bidirectional fluid flow between the spool valve and

a first port of the fluid actuated device, a second actuating port for bidirectional fluid flow between the spool valve and a second port of the fluid actuated device, a pump

port for input fluid flow to the spool valve from a hydraulic pump, a tank port for

output fluid flow from the spool valve to a hydraulic tank, and a spool for controlling

the direction and rate of fluid flow between the first actuating port and the pump or

tank port and the direction and rate of fluid flow between the second actuating port and

the pump or tank port.

This control valve assembly may also incorporate a pressure compensator in

the form of an auxiliary spool valve which is controlled so as to maintain a constant

pressure drop across the spool of the main spool valve. Such a control valve assembly

enables the fluid actuated device to be controlled independently of the load, so that, in

the case of a control ram for a lifting arm of an earth moving vehicle for example, the arm is lifted or lowered at a substantially uniform rate regardless of the size of the load

lifted by the arm or of variation in loading during lifting due to the structure of the arm itself. However such control valve assemblies are of complex mechanical construction,

and this can render the control valve assemblies costly and difficult to manufacture.

Furthermore such control valve assemblies are capable of only limited control

functions, and in particular are prone to malfunction in an over-running load condition,

that is when external forces acting on the fluid actuated device due to gravity act in the same direction as the moving part of the fluid actuated device is being moved under

hydraulic control.

It is proposed in International Published Application No. WO 93/01417 to provide such a control valve assembly with a change-over valve incorporating a position sensor which determines which of the two actuating ports connected to the

fluid actuated device is at the higher pressure, and which supplies an electrical position

signal indicative of which port is at the higher pressure to a processor which also

receives an electrical pressure signal indicative of the higher pressure from a pressure sensor, as well as a directional signal indicative of the direction in which the spool is

to be displaced from its neutral position by manual movement of an operator-actuated

lever. The processor incorporates a comparator which establishes whether the input

port as indicated by the directional signal is at the higher pressure, and provides an output signal to a positioning device for controlling the output of the pump in

dependence on the result of this comparison and in accordance with the requirement

of the load. Although such an assembly incorporates special control measures responsive to an over-running load condition, these measures operate only in response

to actual movement of the spool on operator actuation, so that there is still a risk of malfunction in such an over-running load condition.

It is an object of the invention to provide a novel proportional control valve

assembly which can be produced in a straightforward manner and which provides a large number of control functions.

According to the present invention there is provided an electrohydraulic

proportional control valve assembly as defined in the accompanying claims.

Such a control valve assembly utilises adjustment of the position of the two

valve members to control the flow rate and/or pressure at the ports of a fluid actuated

device, such as a hydraulic cylinder or hydraulic motor, in dependence on the sensed

valve member position, the sensed pressures in the ports and the sensed pump pressure

and in response to the operator actuated demand signal, produced by operator actuation

of a joystick for example. The valve members are continuously controlled by the servo

control means in response to a continually updated actuating signal adapted to drive the

valve members to positions corresponding to throughflow cross-sections appropriate

to the required flow and pressure conditions and the desired operating mode of the fluid

actuated device, and a large number of control functions can be provided by

appropriate programming of the control circuitry. For example, the flow rate and/or

pressure at the ports of the fluid actuated device may be controlled so that the device is adjustable at a uniform rate which is independent of the load, that is so that the rate

of movement of the moving part of the device is not affected by variation of the applied load or supply pressure, either in a passive load condition or in an over-running load condition. Furthermore servo control of valve member position in dependence on the

feedback position signals ensures highly accurate valve member positioning, without

requiring either detailed analysis of valve characteristics or adjustment to take account of wear.

The provision of two separate valve means having separately movable valve members is advantageous as it enables differential opening and closing of the first and

second actuating ports to effect control of the fluid actuated device. Independent control of the flow rate and/or pressure at the two ports of the fluid actuated device by

such valve means thus enables operation of the fluid actuated device at a higher level

of efficiency and safety than is possible in prior control arrangements in which

efficiency losses are incurred as a result of the need to displace the moving part of the device against a back pressure. Furthermore, in the event of pressure overload, for

example due to movement of the moving part of the device being blocked by external

overloading, or where free floating of the load is required, one or both of the valve

members may be opened to tank in order to vent the two sides of the fluid actuated

device separately or simultaneously.

In order that the invention may be more fully understood, a preferred

embodiment of control valve assembly in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a hydraulic circuit diagram of the assembly; Figure 2 is a diagrammatic sectional view through a part of the assembly;

Figure 3 is a block diagram showing the electrical interconnections between

various parts of the assembly; and

Figure 4 is a logic diagram illustrating control functions of the assembly.

Referring to Figure 1 the illustrated electrohydraulic proportional control

valve assembly 1 comprises first and second spool valves 2 and 3 connected to first and

second actuating ports 4 and 5 for controlling fluid flow to opposite sides of a movable piston 6 of a fluid actuated device 7 in the form of a hydraulic cylinder or motor. The

first and second spool valves 2 and 3 have spools 12 and 13 which are axially movable

by pilot fluid flows controlled by electrically operated pilot actuator valves 44 and 45

(described more fully below with reference to Figure 2) between end positions in which the spool 12 or 13 places the corresponding actuating port 4 or 5 in communication

with either a pump port 15 or 16 connected to the output of a pump 17 or a tank port

18 or 19 connected to a tank 20. The fluid supplied to the pilot actuator valves 44 and 45 is regulated by a pilot pressure regulator 14.

The spool 12 or 13 or each spool valve 2 or 3 is movable to effect opening

of the spool valve 2 or 3 either to the pump port 15 or 16 or the tank port 18 or 19

over a throughflow cross-section which may be varied proportionately between a

minimum opening value and a maximum opening value in dependence on the position of the spool 12 or 13. Furthermore both spools 12 and 13 are spring biased towards their neutral positions (in which they are shown in Figure 1), and position sensors 23 and 24 are provided for supplying electrical position signals indicative of the positions

of the spools 12 and 13. In addition a pressure relief valve 25 is provided for venting the output of the pump 17 to the tank 20 in a manner which will be described in more

detail below. Four pressure sensors 26, 27, 28 and 29 are provided for supplying electrical pressure signals P_A, P_B, P_s and P_τ indicative of the fluid pressures in the first

and second actuating ports 4 and 5, the pump port 15 or 16 and the tank port 18 or 19.

As shown diagrammatically on the right hand side of Figure 1, the pilot pressure regulator 14 may also serve to regulate pilot fluid supply to the pilot actuator

valves of a further pair of spool valves, identical to the spool valves 2 and 3, for controlling supply of fluid to a further fluid actuated device 30. The two devices 7 and

30 may be two rams for controlling different linkage axes of an earth moving vehicle

for example, and may be controlled by two valve slices in the assembly as described

in more detail below.

Figure 2 shows a section through a valve slice part incorporating one of the

first and second spool valves 2 and 3 and one of the associated pilot actuator valves 44

and 45, two such parts being provided in each valve slice. The pilot actuator valve 44

or 45 comprises a moving coil 35 fixed to a pilot spool 36 which is centred by two springs 37 and 38, the coil 35 being displaceable in an annular air gap 39 within a

magnetic former 40 when a current is supplied to the coil 35 so as to provide magnetic interaction between the magnetic field associated with the current flow and the magnetic flux produced in the air gap 39 by the former 40. The pilot actuator valve

44 or 45 has two actuating ports 46 and 47 connected to the ends of the spool valve 2

or 3 by connecting conduits 48 and 49 respectively, as well as a tank port 70 connected to the tank and two pump ports 71 and 72 connected to pump either directly or by way of the pilot pressure regulator 14. The spool 12 or 13 of the spool valve 2 or 3 is

centred by two springs 73 and 74 and has an extension 75 at one end enabling a

position feedback signal dependant on the position of the spool 12 or 13 to be outputted

by the position sensor 23 or 24.

With the spool 36 of the pilot actuator valve 44 or 45 in the neutral position

as shown in Figure 2, only slight fluid leakage will take place through the pilot actuator

valve, and hence the spool 12 or 13 of the main spool valve 2 or 3 will be held in its neutral position by the springs 73 and 74, as also shown in the figure. When a

position control current is supplied to the coil 35, a force acts on the spool 36 so as to

move it in one or other direction (dependant on the sense of the current) until an

equilibrium position is reached in which the force is balanced by the forces exerted by

the springs 37 and 38. If the spool 36 moves to the right as shown in the figure, this results in passages of a throughflow cross-section determined by the magnitude of the

current being opened between the pump port 71 and the actuating port 46 and between

the tank port 70 and the actuating port 47, with the result that a controlled displacement

flow of pilot fluid is applied along the conduit 48 to the left hand end of the spool 12

or 13 of the main spool valve 2 or 3, and at the same time controlled venting of pilot fluid from the right hand end of the spool 12 or 13 takes place by way of the conduit 49 to tank. This causes the main spool 12 or 13 to be driven to the right as shown in

the figure, with the speed of movement being determined by the degree of opening of the pilot actuator valve 44 or 45, until the position feedback signal outputted by the

position sensor 23 or 24 indicates that the spool has been driven to the required position

at which time the current to the coil 35 is cut off and the spool 36 of the pilot actuator

valve 44 or 45 is returned to its neutral position by the springs 37 and 38. This results in movement of the main spool 12 or 13 being stopped so that the spool is held in the required position to which it has been driven by virtue of the fluid pressures acting on

the two ends of the spool.

In practice the pilot actuator valve current is controlled in a complex way by the control circuitry in order to achieve optimum dynamic and position control

characteristics, that is in order to rapidly drive the main spool 12 or 13 to the required position and in order to accurately retain the spool in that position for as long as

necessary. This may in practice require energisation of the coil 35 by a small current under servo control even when movement of the main spool 12 or 13 is not required,

so as to provide small fluid flows through the pilot actuator valve 44 or 45 to

compensate for fluid leakage so as to retain the main spool 12 or 13 in the position to

which it has been driven. However any current required to maintain the main spool in position will be very low, and will not adversely affect the generally low current

consumption of the control circuitry which accurately monitors the position of the main

spool valve 12 or 13 by means of the position sensor 23 or 24 at all times and controls the current to the coil 35 continuously so as to provide the required feedback control

of the main spool position.

Since the moving coil 35 and the spool 36 to which it is fixed are of light construction, the coil 35 has low power consumption and the control circuitry requires

only low power, low cost components. Furthermore high speed movement of the spool

36 is possible in response to the applied current under servo control, and rapid reversal

of spool movement can be effected by reversal of the current in the coil 35 to cause the

spool 36 to move in the opposite direction. Thus not only can the supply of fluid from

the pump to the main spool 12 or 13, and corresponding venting of fluid from the main

spool 12 or 13 to tank, be controlled rapidly so as to provide for accurate dynamic

control of the main spool position in response to the position feedback signal, but also

the control of the piston 6 of the fluid actuated device 7 can be effected with response

times sufficient to enable highly advantageous control of the load in a manner which

has not previously been possible with known control arrangements. For example, when the movement of the load is blocked, such as when the bucket of an excavator

meets an obstruction, an appropriate pressure relief signal triggered by sensing of a

pressure overload by one of the pressure sensors 26 and 27 can cause operation of the

appropriate pilot actuator valve 44 or 45 so as to rapidly open one of the main spools

12 and 13 to tank, in order to reduce the pressure in the fluid actuated device in such

a manner as to avoid damage due to over pressure. Such pressure relief occurs particularly rapidly due to the high speed dynamic response of the pilot actuator valves

44 and 45. Other control features enabled by the high speed dynamic response of the pilot actuator valves 44 and 45 under servo control will be discussed below.

Referring to Figure 3 the complete control valve assembly comprises, for

example, a bank of two valve slices 50 and 51 of the general form described, each of which has a first actuating port 4 and a second actuating port 5 for connection to a

respective fluid pressure actuated device (not shown), and an end slice 52 connected to the valve slices 50 and 51 and having a pump port 53 and a tank port 54. The end

slice 52 serves to control the supply pressure of hydraulic fluid from a fixed displacement pump (not shown) connected to the pump port 53 in dependence on

demand signals indicative of the demand for fluid to be supplied to the valve slices 50 and 51 , in order to ensure that fluid is supplied only when required and in order to place the pump on standby if there is no requirement for fluid supply to the valve slices

50 and 51. During operator actuation the pressure relief valve 25 shown in Figure 1

is controlled in dependence on the load pressures sensed by the pressure sensors 26 and

27 to control the pressure of fluid supplied by the pump so that it exceeds the highest load pressure sensed by a predetermined amount. When no pressure load is sensed, the pressure relief valve 25 routes the fluid back to the tank at a nominal low pressure.

Alternatively, where a variable displacement pump is provided, the valve 25 may be

configured to pilot the displacement control of the pump in such a way as to ensure

supply of fluid in accordance with the requirements of the system. Although only two

valve slices 50 and 51 are shown in Figure 3 for simplicity, it should be appreciated

that a bank of four to ten valve slices is more likely to be provided in a practical

embodiment. Furthermore a control computer 55 is electrically connected to the valve

slices 50 and 51 and to a joystick 56 by a serial communications network, so as to monitor operator actuation of the joystick 56, and so as to supply to the valve slices 50

and 51 pressure (P) or flow (Q) demand signals, and pressure-flow (P-Q) select

signals. In addition the control computer 55 serves to supply initial set up data to the

valve slices 50 and 51 on initial set up programming utilising a plug-in programmer 57,

and also to provide error monitoring of the valve slices 50 and 51. If required,

provision may be made for temporary connection to the valve slices 50 and 51 of a plug-in driver 58 for emergency operation of the valve slices 50 and 51. Also, if

required, a health monitor display 59 may be connected to the control unit 55 to

indicate correct operation of the valve slices 50 and 51.

The manner in which the control computer 55 is used to control the valve

slices 50 and 51 in order to effect the required control of the fluid pressure actuated

devices will now be briefly described with reference to Figure 4, it being understood

that the control logic for carrying out the control functions described with reference to

Figure 4 is incorporated in the valve slices 50 and 51 themselves and not in the control

computer 55 which serves to provide overall system control. The control computer 55

supplies a pressure-flow (P-Q) select signal to each valve slice and a selection is made

by a selector in each valve slice in dependence on this signal as to whether pressure

control or flow control is to be effected.

Referring to Figure 4, it will be appreciated that the particular control mode in which the fluid actuated device is to be controlled is determined by the form of the select signal supplied by the control computer in dependence on the demand signal supplied by operator actuation of the joystick and/or control mode selector buttons or

switches, as indicated by the control mode iteration loop 80 in the figure. If the flow

control mode is selected, a flow demand signal Q_DEM is supplied to a selector 81 which

determines the required direction of fluid flow to the fluid actuated device, that is whether the flow is to port A or port B. In the event of zero flow being required, the

control is effected so that both main spools are held in their neutral positions. In the

event of flow to port A being required, a further selector 82 determines whether the

pressure in the port A is greater than or less than the pressure in the port B, that is

whether the load is to be treated as a passive load or an over running load. In the event of a passive load, the required throughflow cross-section a of the spool valve for controlling the flow to the port A is calculated at 83 by dividing the flow demand signal Q_DEM by the value V(P_S - PJ and a constant of proportionality. A nominal

downstream back pressure to be applied at the port B is set at 84, and the required

positions of the two spools are then controlled at 85 by supplying control signals to the

pilot actuator valve of the upstream spool valve in order to set the required throughflow

cross-section a and by supplying control signals to the pilot actuator valve of the

downstream spool valve so as to set the downstream back pressure at a predetermined

level.

In the event of an overrunning load the required throughflow cross-section

a of the spool valve for controlling the flow through the port B is calculated at 86 by dividing the flow demand signal Q_DEM by the value V (P_B - P_τ) and the constant of proportionality (where P_τ is the sensed tank pressure or an assumed tank pressure

where a tank sensor is not provided), and control of the filling of the upstream side of

the piston of the fluid actuated device by way of the port A is set at 87, so that control

of the required positions of the two spools at 88 provides for controlled metering out of fluid from the port B by appropriate setting of the throughflow cross-section a of the

downstream valve and controlled filling by way of the port A under control of the upstream valve. In view of the ability of the pilot actuator valves to switch rapidly

between supply of fluid in one direction to the main spool valves and supply of fluid

in the opposite direction, such a control arrangement enables discontinuous switching

from a passive load condition to an overrunning load condition, as when a lifting arm

of an earth moving vehicle is swung by the fluid actuated device through an over centre position so that the direction in which gravity acts on the load is in the same direction as piston movement, rather than in the opposite direction as it is prior to the over

centre position being reached. The provision of a tank sensor enables more accurate

control in the event of an overrunning load, and avoids any control discontinuities.

If the selector 81 determines that the required direction of fluid flow is to the

port B of the fluid actuated device, then a similar series of control steps are carried out

to the steps already described except that the control in relation to the ports A and B

is reversed so that the calculations utilise the sensed pressure signal P_B in place of P_A

and vice versa. In each case the spool positions are continuously monitored by the

position sensors, and the signals supplied to the pilot actuator valves are varied as required in dependence on the position feedback signals from the position sensors.

In the event that pressure control is selected, the pressures applied at the two ports A and B of the fluid actuated device are controlled in dependence on movement

of the joystick by the operator such that the joystick movement determines the rate of

change of pressure (magnitude and sense) applied to the load and, in the event of

movement of the joystick being stopped, no further pressure change is applied to the load. Initially the pressure demand is calculated at 89 from the joystick input signal.

A selector 90 then determines whether the pressure demand requires the application of pressure to the port A or the port B. If the pressure demand is zero both port pressures are set to a nominal value. If the pressure demand requires application of pressure to

the port A, a selector 91 first determines whether or not oscillating pressure is to be

applied to the piston, for example in order to vibrate the load when a compaction mode

has been selected. Depending on the result of this selection the required pressure at the

port A is set to the demand pressure and the required pressure at the port B is set to a nominal value at 92, and the required control signals to the pilot actuator valves of the

two spool valves are applied at 93 in order to control the positions of the main spools

incrementally in dependence on the position feedback signals in order to set the

required pressures in the ports A and B.

If the pressure demand requires application of pressure to the port B, a

similar sequence of control steps is effected except that the demand pressure is applied

to the port B and the pressure in the port A is set to a nominal value, that is a predetermined pressure above the sensed or assumed tank pressure. In the event that the compaction mode is selected, a sine wave varying cyclic demand pressure is added

to the basic demand pressure so that the load is vibrated by the resulting pressure

control.

The pressure control mode can be utilised with advantage in various

operating conditions. For example, when lifting a load, the pressure control mode can

be initiated so as to provide continuous pressure counterbalancing of the load and so

as to allow the load to be manipulated manually with the application of only small pressures. Furthermore, if the load is an excavating arm carrying a bucket for digging through the ground for example, the applied pressure can be controlled so that, in the

event of the bucket hitting an obstruction such an underground utility, a predetermined

pressure limit will not be exceeded, and there is no danger of damage to the underground utility by the application of excess pressure.

If a float mode is selected by the operator by actuation of a special switch,

both main spools are controlled at 94 so as to open both sides of the piston of the fluid

actuated device to tank so as to enable free floating movement of the piston and any

load coupled thereto.

Whilst the above described valve assembly utilizes first and second spool valves 2 and 3 for controlling fluid flow to and from the fluid actuated device, an

alternative, non-illustrated valve assembly in accordance with the invention utilizes a pair of poppet valves in place of each such spool valve for controlling respectively the

flow of fluid to the device from the pump by way of the associated actuating port and

the flow of fluid from the device to the tank by way of the actuating port. In each case the pair of poppet valves associated with each actuating port is controlled by the pilot

actuator valves to provide the required fluid flows in the various control modes.

Furthermore each of the pilot actuator valves may itself comprise a pair of poppet

valves for controlling the fluid flows to and from the main valve or valves in response

to current actuation of the moving coil.

Claims

C ΔIMS

1. An electrohydraulic proportional control valve assembly for controlling a

bidirectional fluid actuated device having first and second ports, the valve assembly

having a first actuating port for bidirectional fluid flow between the valve assembly and

the first port of the fluid actuated device, a second actuating port for bidirectional fluid flow between the valve assembly and the second port of the fluid actuated device, a pump port for input fluid flow to the valve assembly from a hydraulic pump, and a

tank port for output fluid flow from the valve assembly to a hydraulic tank, the valve

assembly comprising first valve means connected to the first actuating port, the pump

port and the tank port for controlling the direction and rate of fluid flow between the

first actuating port and the pump port and between the first actuating port and the tank port, and second valve means connected to the second actuating port, the pump port

and the tank port for controlling the direction and rate of fluid flow between the second

actuating port and the pump port and between the second actuating port and the tank

port, the first valve means having a first valve member which is movable to vary the throughflow cross-section for fluid flow between the first actuating port and the pump

or tank port, and the second valve means having a second valve member which is movable, independently of movement of the first valve member, to vary the

throughflow cross-section for fluid flow between the second actuating port and the

pump or tank port, position sensing means for supplying electrical position signals

indicative of the actual positions of the first and second valve members, pressure

sensing means for supplying electrical pressure signals indicative of the fluid pressures in the first and second actuating ports and the pump port, and servo control means for controlling the positions of the first and second valve members in dependence on the

electrical position and pressure signals and in response to an electrical demand signal

provided in response to operator actuation, in order to set the throughflow cross-

sections for fluid flow through the first and second valve means between the first actuating port and the pump or tank port and between the second actuating port and the

pump or tank port to effect the required control of the fluid actuated device.

2. An assembly according to claim 1, wherein the first and second valve

members are spools which are axially displaceable to vary the throughflow cross- section for fluid flow between each actuating port and the pump or tank port.

3. An assembly according to claim 1 or 2, wherein the servo control means

includes electrically operable pilot valve means for controlling the position of each of the valve members by applying a controlled displacement flow of pilot fluid to one part

of the valve member, whilst at the same time applying controlled venting of pilot fluid

from another part of the valve member, sufficient to drive the valve member to a required position in a first operating mode, and by subsequently discontinuing said

displacement flow of pilot fluid to the valve member and said venting of pilot fluid so

as to hold the valve member in said required position in a second operating mode.

4. An assembly according to claim 3, wherein the pilot valve means comprises

a first pilot valve for effecting bidirectional axial movement of the first valve member, and a second pilot valve for effecting bidirectional axial movement of the second valve member independently of movement of the first valve member.

5. An assembly according to claim 4, wherein each pilot valve comprises an

actuating coil movable relative to a magnetic former by the application of an electrical

actuating current to the coil, and a valve element movable by the coil to simultaneously control application of pilot fluid to said one part of the valve member and venting of

pilot fluid from said other part of the valve member.

6. An assembly according to any preceding claim, wherein the servo control

means is operable, in a pressure control mode, to control the positions of the first and

second valve members in response to an operator actuated electrical pressure demand

signal corresponding to a required load pressure, in order to apply controlled fluid flow to one of the actuating ports and controlled venting of fluid from the other actuating

port to produce a pressure difference across the fluid actuated device corresponding to

the required load pressure.

7. An assembly according to any preceding claim, wherein the servo control

means is operable, in a float mode, to control the positions of the first and second valve members in response to an operator actuated electrical float demand signal, in order

to vent fluid from both actuating ports so as to allow free floating movement of a load

coupled to the fluid actuated device.

8. An assembly according to any preceding claim, wherein the servo control

means is operable, in a compaction mode, to control the positions of the first and second valve members in response to an operator actuated electrical compaction demand signal, in order to rapidly alternate the direction of fluid flow to the actuating

ports so as to vibrate a load coupled to the fluid actuated device.

9. An assembly according to any preceding claim, wherein the servo control means is operable, in a pressure relief mode, to control the positions of the first and

second valve members in response to an electrical pressure relief signal triggered by sensing of a pressure overload in one of the actuating ports by the pressure sensing

means, in order to provide controlled venting of fluid from said one actuating port to relieve the pressure.

10. An assembly according to any preceding claim, wherein the pressure sensing

means comprises a first pressure sensor for supplying a first electrical pressure signal

indicative of the fluid pressure in the first actuating port, a second pressure sensor for

supplying a second electrical pressure signal indicative of the fluid pressure in the

second actuating port, a third pressure sensor for supplying a third electrical pressure

signal indicative of the fluid pressure in the pump port, and a fourth pressure sensor

for supplying a fourth electrical pressure signal indicative of the fluid pressure in the tank port, and wherein the servo control means is adapted to control the positions of

the first and second valve members in dependence on the first, second, third and fourth

electrical pressure signals.

11. An assembly according to any preceding claim, wherein a control computer

is provided for monitoring the operator actuated electrical demand signals and for providing overall function control of the servo control means in dependence on the

demand signals.

12. An assembly according to any preceding claim, which is of modular

construction and includes a bank of valve slices assembled together and adapted to

control a plurality of fluid pressure actuated devices.

13. An electrohydraulic proportional control valve assembly for controlling a

fluid actuated device, the valve assembly incorporating an actuating port for fluid flow

between the valve assembly and the fluid actuated device, a pump port for input fluid

flow to the valve assembly from a hydraulic pump, a tank port for output fluid flow

from the valve assembly to a hydraulic tank, and valve means for controlling the

direction and rate of fluid flow between the actuating port and the pump port and

between the actuating port and the tank port, the valve means incorporating at least one valve member which is movable to vary the throughflow cross-section for fluid flow

between the actuating port and the pump or tank port, pressure sensing means for

supplying electrical pressure signals indicative of the fluid pressures in the actuating

port and the pump port, and servo control means for controlling the position of said at

least one valve member in dependence on the electrical pressure signals and in response

to an electrical demand signal provided in response to operator actuation, in order to

set the throughflow cross-section for fluid flow through the valve assembly between the actuating port and the pump or tank port to effect the required control of the fluid

actuated device, the servo control means including electrically operable pilot valve means for controlling the position of said at least one valve member by applying a

controlled displacement flow of pilot fluid to the valve member sufficient to drive the

valve member to a required position in a first operating mode, and by subsequently discontinuing said displacement flow of pilot fluid to the valve member so as to hold

the valve member in said required position in a second operating mode.

14. An electrohydraulic proportional control valve assembly for controlling a fluid actuated device, the valve assembly incorporating an actuating port for fluid flow

between the valve assembly and the fluid actuated device, a pump port for input fluid flow to the valve assembly from a hydraulic tank, a tank port for output fluid flow from the valve assembly to a hydraulic tank, and valve means for controlling the direction and rate of fluid flow between the actuating port and the pump port and

between the actuating port and the tank port, the valve means incorporating at least one

valve member which is movable to vary the throughflow cross-section for fluid flow

between the actuating port and the pump or tank port, first pressure sensing means for

supplying a first electrical pressure signal indicative of the fluid pressure in the actuating port, second pressure sensing means for supplying a second electrical pressure

signal indicative of the fluid pressure in the pump port, third pressure sensing means

for supplying a third electrical pressure signal indicative of the fluid pressure in the

tank port, and servo control means for controlling the position of said at least one valve

member in dependence on the first, second and third electrical pressure signals and in response to an electrical demand signal provided in response to operator actuation, in order to set the throughflow cross-section for fluid flow through the valve assembly

between the actuating port and the pump or tank port to effect the required control of

the fluid actuated device.