GB2043961A - Servovalve assembly - Google Patents

Abstract

A servovalve assembly comprising a valve 11 having a movable element 14 such as a jet pipe positionable in response to an input signal, the valve supplying an output fluid the pressure of which is dependent upon the position of the movable element 14, and a feedback mechanism 12 acting as one side only of a feedback lever 23 to move the feedback member against a biasing force, such as spring 24, in response to the output fluid, movement of the lever 23 providing a corresponding feedback movement of the element 14. Valve 11 may be in the form of a pilot valve operating in conjunction with a boost valve 13. <IMAGE>

Description

SPECIFICATION Servovalve assembly This invention relates to a servovalve assembly.

According to the invention, there is provided a servo-valve assembly comprising valve means having a movable element positionable in response to an input signal, the valve means supplying an output fluid the pressure of which is dependent upon the position of the movable element; and a feedback mechanism acting on one side only of a feedback member to move the feedback member against a biasing force in response to the output fluid, said movement of the feedback member causing corresponding feedback movement of the movable element.

Embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is part sectional view of a servovalve assembly according to the present invention, Figure 2 is a schematic diagram of the servovalve of Figure 1, Figure 3 shows the piston used in the feedback motor of the assembly of Figure 1, and Figure 4 shows an alternative arrangement for repositioning the jet pipe of the assembly of Figure 1.

Referring to the drawings, the servovalve assembly 10 includes pilot stage 11 , feedback motor 12 and boost valve 13. Pilot stage 11 is shown in more detail in Figure 2 and is comprised of a jet pipe 14 driven by an electrical actuating coil 1 5 supplied with current from terminals 1 6.

Coil 1 5 cooperates with magnet and pole piece arrangements 1 7 and 1 8 and issues a jet of fluid to nozzles 21 and 22 for providing a control fluid to boost valve 1 3. A feedback lever 23 is attached to jet pipe 14 by spring 24. Fluid is supplied to jet pipe 14 through supply line 25 and returned by line 26 as shown in both Figures 1 and 2.

Centering spring 20 is also included for insuring that nozzles 21 and 22 receive equal amounts of fluid where a predetermined current is supplied to coil 1 5. A more detailed description of pilot stage valve 11 can be found in our co-pending application 79.09688.

The control fluid received by ports 21 and 22 (as shown in Figure 2) is connected to boost valve 13 by lines 31 and 32. As shown in Figure 1, line 32 is behind line 31 through feedback motor 12.

Line 31 is connected to control chamber 33 of boost valve 1 3 and line 32 is connected to control chamber 34.

Boost valve 13 comprises a central bore 35 within which is located a spool having lands 36, 37 and 38. The spool is centred by centering springs 41 within control chamber 33 and spring 42 within control chamber 34. Plug 43 biases spring 41 against land 36 and may be attached to boost valve body 1 3 by any suitable means such as screw threads and spring 42 is biased against land 38 by plug 44 which again may be attached to valve body 1 3 by any suitable means. Return chambers 51 and 52 and supply chamber 53 are provided having larger diameters than bore 35, return chamber 51 and 52 being connected to a pressure return port 54 and supply chamber 53 being connected to a supply port 55. Between lands 36 and 37 in bore 35 is defined an output chamber 56 and between lands 37 and 38 is defined an output chamber 57.Output chamber 56 supplies an output fluid to output port 61 and output chamber 57 supplies an output fluid to port 62. As the spool is moved one way or the other from its center position, one of the output ports 61 and 62 is connected to supply port 55 and the other is connected to return port 54. O-rings 93 and 94 are provided for sealing the fluid within control chambers 33 and 34 of boost valve 1 3.

Feedback motor 1 2 has a slot 71 as shown in to which pilot valve feedback lever 23 extends.

Lever 23 is L-shaped and rotates about pivot 72 at the junction of its two legs. Feedback linkage 23 extends into slot 71 and further into slot 47 in feedback piston 45 which is positioned within the bore 40 forming part of the feedback motor 1 2.

Within bore 40, and on either side of feedback piston 45, are balls 73 and 74. Bore 40 opens into control chamber 76 at one end and control chamber 77 at the other, with control chambers 76 and 77 having a larger diameter than bore 40.

Within control chamber 76 is a spring support plug 78 held in by plate 81 which may be attached to the body of feedback motor 12 by any suitable means. Plug 78 operates against spring 83 for providing a centering bias to piston 45 through ball 73. An O-ring 84 is provided to seal the fluid within control chamber 76.

Within chamber 77 is a spring support plug 85 held within chamber 77 by plate 86 fastened to the body of feedback motor 12 by any suitable means. Through plate 86 is a biasing screw 87 for adjusting the force on centering spring 88 biasing ball 74 and piston 45 against feedback lever 23. A suitable O-ring 89 may be provided for sealing the fluid within chamber 77.

Balls 73 and 74 perform a useful function.

Within ball 74, adjustment of screw 87 tends to rotate spring 88 which rotation is transferred to piston 45 which may be of a circular cross section.

If piston 45 rotates, it tends to engage lever 23 inhibiting, because of friction, the operation of the feedback element. Rotation of ball 74 by spring 88, however, has minimai impact upon piston 45.

Ball 73 may be included if a set screw is used for adjusting the biased force of spring 83 but may be eliminated if no set screw is used on the other side of feedback motor 12.

As shown in Figure 3, piston 45 has a slot 47 therein through which extends feedback lever 23.

Feedback lever 23 is contacted by a member having at least a part spherical surface, such as ball 46, provided within piston 45 by any suitable method such as pressing. Ball 46 provides a low friction contact between piston 45 and feedback lever 23. In this case, the low friction contact is provided by a point contact between ball 46 and feedback lever 23. It is to be noted that a part cylindrical surface could be used in place of ball 46 provided that feedback lever 23 is arranged to rotate around the curvature of the cylindrical surface. This contact would be in the form of a line contact between piston 45 and feedback lever 23 but yet would be of a low friction type.

As an alternative, the piston 45 may be in the shape of a barbell such as 49 shown in Figure 4. In this case, balls 73 and 74 can be eliminated because the barbell arrangement 49 having cylindrical elements can rotate without interferring with the operation of feedback lever 48. The feedback lever must be modified to provide a part spherical end so that a low friction contact between it and one end of barbell 49 results. Thus, feedback lever 48 is not pinched between balls 73 and 74 as in our earlier referred to co-pending application although barbell 49 is positioned by both the pressures within chambers 76 and 77.

When jet pipe 14 is in its center position, ports 21 and 22 receive equal pressures such that the pressures within control chambers 33 and 34 are equal and the spool of boost valve 1 3 is centered.

If an appropriate input signal is supplied to terminals 1 6 connected to coil 1 5 such that jet pipe 14 is moved to increase the pressure within port 21 and to decrease the pressure within port 22, the pressure within control chamber 34 increases and the pressure within control chamber 33 decreases driving the spool to the left. Land 36 opens return chamber 51 to output chamber 56 such that fluid flows through port 61 into output chamber 56, through return chamber 51 and return line 54 to pressure return. At the same time, land 37 opens supply chamber 53 to output chamber 57 to allow fluid to flow from supply port 55 through supply chamber 53, output chamber 57 and then through outlet port 62. This operation results in a decrease in pressure in output chamber 56 and an increase in pressure in output chamber 57.

Since output chamber 56 is connected to control chamber 76 of feedback motor 12 by line 91, there will be a concomitant decrease in pressure within control chamber 76. At the same time, since control chamber 77 is connected to output chamber 57, there will be a concomitant increase in pressure in control chamber 77. This pressure differential across piston 45 and balls 73 and 74 will move piston 45 to the left rotating feedback lever 23 in a clockwise direction repositioning jet pipe 14 in a direction to equalize the pressure within ports 21 and 22 which will tend to return the spool of boost valve 1 3 to its center position.

On the other hand, if the electrical signal applied to terminal 1 6 and coil 1 5 positions jet pipe 14 to increase pressure within port 22 and decrease the pressure within port 21, the pressure within control chamber 33 of boost valve 1 3 will increase and the pressure within chamber 34 will decrease. The spool of boost valve 1 3 will, therefore, move to the right connecting output port 62 to return port 54 for decreasing the pressure within output chamber 57 and connecting output port 61 to supply port 53 for increasing the pressure within output chamber 56.

This operation results in an increased pressure within control chamber 76 and a decreased pressure within control chamber 77 of feedback motor 12 driving the piston 45 and balls 73 and 74 to the right allowing counter-clockwise rotation of feedback lever 23 due to spring 20 allowing jet pipe 14 to return to its center position which tends to recenter the spool of boost valve 1 3 for maintaining the pressure differential between output lines 61 and 62.

Although the above servovalve assembly has been described as a two stage valve having a pilot valve and boost valve, it should be noted that the present invention is also applicable to a single stage valve with the boost valve omitted.

Claims (9)

1. A servovalve assembly comprising valve means having a movable element positionable in response to an input signal, the valve means supplying an output fluid the pressure of which is dependent upon the position of the movable element; and a feedback mechanism acting on one side only of a feedback member to move the feedback member against a biasing force in response to the output fluid, said movement of the feedback member causing corresponding feedback movement of the movable element.

2. The assembly of Claim 1, wherein said valve means includes a pilot valve for supplying a control fluid in dependence upon the position of the movable element, and a boost valve responsive to the control fluid for supplying said output fluid.

3. The assembly of Claim 2, wherein the feedback mechanism includes a piston positioned in a bore in a body of the assembly and acted upon by the output fluid to be moved in either one of opposite directions, the piston engaging one side only of the feedback member, so that movement of said piston in one direction causes movement of the feedback member in that direction, and movement of the piston in an opposite direction permits the movable member to be moved under the influence of the biasing force.

4. The assembly of Claim 3, wherein the feedback member is a biased lever and wherein one of the contacting surfaces of the piston and lever is of part-spherical or part-cylindrical shape to reduce thereby the frictional contact therebetween.

5. The assembly of Claims 2, 3, or 4, wherein the movable element its a jet pipe for issuing a jet of fluid towards first and second ports which receive different fluid pressures dependent upon the position of the jet pipe relative to the first and second ports, the jet pipe being positioned in response to an electrical signal.

6. The assembly of Claim 5, wherein the boost valve means comprises a bore and a spool located within said bore, said spool having first and second lands and a middle land between said first and second lands, said first and second lands partly defining first and second control chambers, said first control chamber being connected to said first port and said second control chamber being connected to said second port.

7. The assembly of Claim 6 as appendant to Claim 3, wherein said output fluid is derived from a first output chamber defined by said first and middle lands within said boost valve bore and from a second output chamber defined by said second and middle lands within said boost valve bore, said first and second output chambers being connected to said piston for supplying said output fluid to said piston for repositioning said movable element.

8. The assembly of Claim 6 or 7, wherein said boost valve bore further comprises first and second enlarged diameter portions cooperating with said correspondin.g first and second lands and a middle enlarged portion cooperating with said middle land, said first and second enlarged portions connected to a return line and said middle enlarged portion connected to a supply line.

9. The assembly of Claim 3 or any one of Claims 4 to 8 as appendant to Claim 3, wherein two centering springs are provided one of each side of the piston, wherein at least one of the springs is adjustable to adjust the centering force and wherein an at least part-spherical member is located between that one spring and the piston.