GB2141262A - Servo valve - Google Patents

Abstract

A servo valve 1 comprising a casing 2, a motor 20 carried by the casing and having a rotary output member 21, a first-stage valve element 7, a second-stage valve element 8 housed in the casing, and a rotatable transmission element 23 screw-threadedly engaged at 24 in an opening 25 in a wall 12 of the casing. One end portion of element 23 is connected to member 21. Element 7 is urged by spring means 14 into engagement with the end of element 23 remote from motor 20. The engaging portions of element 7 and element 23 are such that the end surface area of one is substantially larger than the contacted area of the other. <IMAGE>

Description

SPECIFICATION Servo valve This invention relates to servo valves suitable for controlling the supply of fluid under pressure to fluid-pressure-operable actuator means or the like.

Hitherto servo valves of the two-stage type have been proposed in which the first-stage valve element, for example a spool, has been housed in part in the hollow interior of the second-stage valve element, also for example a spool, and has been movable with respect to the second-stage valve element by a motor, for example an electrical stepper motor. In certain cases the output member of the motor has been connected to the first-stage valve element by a device which translates rotary output motion of the motor into an axial movement of the first-stage valve element. Such a device has comprised a sleeve which was carried by the output member and which, at its end portion remote from the motor, was internally screw-threaded for cooperation with an externally screw-threaded end portion of the first-stage valve element.

For satisfactory operation of a servo valve having such a device it is vital that the longitudinal axes of the output member, sleeve and first-stage valve element are maintained accurately coincident.

However, some difficulty could well be experienced in maintaining such coincidence of these axes, any loss of coincidence resulting in possible jamming of the first-stage valve element and consequent malfunctioning of the second-stage valve element which would seriously impair the operation of the apparatus which the servo valve controls.

The invention as claimed is intended to provide a remedy. It solves the problem of how to design a servo valve in which the means for transmitting drive from a motor to the first-stage valve element does not require to be disposed with its axis of rotation necessarily coincident with the longitudinal axis of the first-stage valve element.

According to this invention a servo valve comprising a casing, a motor carried by the casing and having a rotary output member, first-stage valve element and a second-stage valve element housed in said casing, and a rotatable transmission element screw-threadedly engaged in an opening in a wall of said casing, one end portion of said transmission element being connected to said rotary output member, the first-stage valve element being urged by spring means into engagement with the end of said transmission element remote from said motor, and, the engaging portions of the first-stage valve element and transmission element being such that the end surface area of one is substantially larger than the contacted area of the other.

Preferably the first-stage valve element is housed at least partly within the second-stage valve element.

The first-stage valve element and the secondstage valve element may each comprise a spool, the first element then being axially displaceable in, and with respect to, the second element by said motor through the intermediary of said transmission element, Alternatively the first-stage valve element may comprise a ball engageable with a suitable seating provided in a spool which forms said second-stage valve element.

Preferably also, said motor is an electrical stepper motor.

The advantages offered by the invention are mainly that since the engaging portions of the first-stage valve element and transmission element are such that the end surface area of one is substantially larger than the contacted area of the other, the longitudinal axes of those elements need not be maintained coincident so that a slight degree of misalignment of those axes can be tolerated without detrimental effect to the functioning of the servo valve. Since accuracy in alignment is thus not required, assembly of the components of the servo valve during manufacture and servicing is facilitated.

Two ways of carrying out the invention are described in detail below with reference to the accompanying drawings which illustrate two specific embodiments, in which: Figure lisa cross-section of a two-stage electrohydraulic servo valve in accordance with the first embodiment, and Figure 2 is a cross-section of a two-stage electrohydraulic servo valve in accordance with the second embodiment.

With reference to Figure 1 of the drawings the two-stage electro-hydraulic servo valve 1 of the first embodiment comprises a casing 2 having a pressure port 3, service ports 4, 5 which are connectible to a liquid-pressure-operable service (not shown) and drain port 6. The casing houses a first-stage valve element 7 and a second-stage valve element 8.

Element 7 in the form of a spool which is housed in the bore 9 of elementS. Element 8 is also in the form of a spool which is housed in bore 10 and element 7 projects from bore 9 into a chamber 11 formed between element8 and end wall 12 of a block 13 suitably secured to casing 2. A coil spring 14 is provided in bore 9 to the left of element 7. Axial movement of element 8 is limited by stop members 15,16 provided in bore 10.

A passageway 17 having a restrictor 17a is branched from port 3 and is taken to chamber 18 formed by bore 10 to the left of element 8.

Aflanged tubular member 19 is mounted on block 13 and an electrical stepper motor 20 is secured to the right-hand end of member 19. The rotary output member 21 of motor 20 is connected through a coupling device 22 to a transmission element 23.

This element is screw-threadedly engaged at 24 in an opening 25 in block 13 and projects through wall 12 into chamber 11. The coupling device 22 is of the kind which is flexible in the axial sense but nonflexible in the rotary, driving, sense.

The extreme left-hand end portion of element 23 is reduced in diameter at 26 and its end face 27 is in engagement with the right-hand end face 28 of element 7. The surface area of face 28 is, as shown, substantially larger than area 27 which is contacted by face 28.

A sealing assembly 29 is provided between element 23 and block 13 outwardly of the screw threaded portion 24.

Element 7 is provided with an annular groove 30 while bore 9 is provided with two axially-spaced annuli 31,32. A passageway 33 places annular groove 30 in communication with chamber 11.

Element 8 is provided with three axially-spaced lands 34,35,36, land 35 having a port 37. A passageway 38 is taken from port 37 to annulus 31.

Land 35 is co-operable with annulus 39 which is connected to port 3 by a passageway 40. Lands 34, 36 co-operate with annuli 41,42 which are open, respectively through ports 43, to drain passageway 45 which connects with port 6. A passageway 46 connects annulus 32 to annulus 42.

A port 47, incorporating a restrictor 48, is provided in land 34to place annulus 41 in communication with bore 49 of element 8 and thus with chamber 18.

A passageway 50 places chamber 51 to the left of sealing assembly 29 in communication with cham beryl.

In operation of the servo valve 1 liquid under pressure supplied to port 3 passes to annulus 39 in readiness for direction into one or other of the service ports 4, 5 in dependence upon the direction of movement of element 8 away from its neutral position. Liquid under pressure passes also byway of passageway 17 to chamber 18, being allowed to bleed through restrictor 48 to passageway 45 and thus to drain port 6, it being intended to thereby maintain a substantially constant pressure in chamber 18 to provide a substantially constant bias of element 8 to the right in Figure 1.

Movement of element 8 in either direction away from its neutral position is effected by appropriate change in pressure in chamber 11 by movement of element 7 in its bore 9. Movement of element 7 to the left away from its neutral position admits liquid under pressure to chamber 11 by way of port 37, passageway 38, annular groove 30 and passageway 33. Conversely of element 7 to the right away from its neutral position permits liquid in chamber 11 to pass out to drain port 6 by way of passageway 33, annular groove 32, passageway 46 and passageway 45.Such movement of element 7 is by operation of stepper motor 20, rotation of output member 21 in the appropriate direction causing rotation of transmission element 23 which by virtue of its screwthreaded engagement in block 13 moves axially to the left in Figure 1 to move element 7 further in the direction inwardly of the bore 9 against the effort of spring 14, or, moves axially to the right to allow element 7 to move in the direction outwardly of bore 9 under the influence of spring 14. The coupling device 22 has sufficient flexibility in the axial sense to permit such axial movement of element 23 but at the same time provides a direct positive drive connection from member 21 to element 23 in the rotary sense.

Since element 23 is separate from element 7 and since the area of surface 28 is substantially larger than the area of surface 27 contacted by surface 28 the longitudinal axis 52 of element 23 and the longitudinal axis 53 of element 7 need not be maintained coincident and thus a slight degree of misalignment of those axes can be tolerated without detrimental effect to the functioning of the servo valve.

With reference now to Figure 2 of the drawings, the two-stage electro-hydraulic servo valve 60 of the second embodiment comprises a casing 61 basically similar two that of Figure 1 in that it has a pressure port 62, service ports 63,64 and a drain port 65.

However, the first-stage valve element 66 is in the form of a ball which is co-operable with a seating 67 formed in a flanged member 68 carried in the right-hand end portion 69 of a second-stage valve element 70. This element is in the form of a spool having three axially-spaced lands 71,72,73. Stops 74,75 limit axial movement of element 70 and a coil spring 76 biasses the element to the right in the drawing.

A passageway 77 is taken from the port 62 to an annulus 78 over which land 72 closes when element 70 is in its neutral position. A port 79 in land 72, and provided with a restrictor 80, opens into bore 81 of element 70, the member 68 being screw-threadedly fitted at the right-hand end of this bore. A retaining element 82 is provided in member 68 to limit travel of element 66 away from its seating 67.

The chamber 83 to the right of element 70 is in communication with drain port 65 by way of an adjustable restrictor 84 and drain passageway 85.

As in the first embodiment an electrical stepper motor 86 is provided to operate the first-stage valve element 66 through the intermediary of a transmission element 87 which is screw-threadedly engaged with an opening 88 in block 89 and which projects into the chamber 83. Drive from the rotary output member 90 of motor 86 to element 87 is by way of a coupling device 9I similar to that of Figure 1.

The left-hand end portion 92 of element 87 is stepped down in diameter, but its end face 93 is substantially larger in area than the portion of the ball 66 contacted by face 93, because here only point contact exists between the ball and face 93.

In operation of servo valve 60 liquid under pressure supplied to port 62 passes to annulus 78 in readiness for direction into one or other of the service ports 63, 64, in dependence upon the direction of movement of element 70 away from its neutral position. Movement of element 70 in either direction away from its neutral position is effected by appropriate change in pressure in chamber 83.

Such change in pressure is under the control of stepper motor 86. To maintain element 70 in its neutral position against the effort of coil spring 76 motor 86 positions element 87 so that the element 66 is held away from its seating 67 sufficiently for the necessary pressure to be maintained in chamber 83 having regard to the pressure of liquid delivered to the servo valve and the flow characteristics of restrictors 80, 84. Movement of element 66 further away from its seating results in a higher pressure in chamber 83, whereupon the element 70 moves to the left against spring 76 whereby liquid under pressure passes from annulus 78 to service port 64 and service port 63 is placed in communication with drain port 65.

If however motor 86 is operated to cause element 66 to move to the other side of the position it assumes for equilibrium of element 70 so that it is now much closer to its seating 67 forthe pressure in chamber 83 decays through restrictor 84 to drain.

Thus under the influence of spring 76 element 70 moves to the right whereupon liquid under pressure passes to service port 63 and service port 64 is placed in communication with drain port 65.

Since the area of face 93 is substantially larger than the contacted area of element 66 the longitudinal axis 94 of element 87 need not be maintained coincident with the longitudinal axis 95 of element 70 or with the centre of element 66. Thus a slight degree of misalignment of those axes and that centre can be tolerated.

In both of the above embodiments the stepper motor is adapted for connection to a signal pulse generating digital computer (not shown) so that the electro-hydraulic servo valve can operate in accordance with the low power signals applied by the computer to the motor. The first-stage valve element is therefore operated in accordance with the pulses applied to the motor and the second-stage valve element follows the first-stage valve element. Thus the liquid-pressure-operable service (not shown) being controlled by the servo valve follows the demands of the electrical stepper motor pulse for pulse.

The invention is not limited to the form of coupling device described above between the stepper motor and the transmission element as in other embodiments other suitable forms of coupling device may instead be provided which permit axial movement of the transmission element simultaneously with the rotary driving movement applied by the motor.

Although in the embodiment above described with reference to Figure 2 the second-stage valve element 70 is biased to the right by a coil spring 76, in alternative embodiments element 70 is biassed to the right by fluid pressure conducted to the chamber at the left-hand end of that element by a restricted passageway branched from port 62. As in the embodiment of Figure 1 that chamber is in communication with drain through a suitable restrictor, it being intended that a substantially constant fluid pressure is maintained in that chamber.

Claims (9)

1. A servo valve comprising a casing, a motor carried by the casing and having a rotary output member, a first-stage valve element and a secondstage valve element housed in said casing, and a rotatable transmission element screw-threadedly engaged in an opening in a wall of said casing, one end portion of said transmission element being connected to said rotary output member, the firststage valve element being urged by spring means into engagement with the end of said transmission element remote from said motor, and, the engaging portions of the first-stage valve element and transmission element being such that the end surface area of one is substantially larger than the contacted area of the other.

2. A servo valve as claimed in claim 1, wherein said first-stage valve element is housed at least partly within the second-stage valve element.

3. A servo valve as claimed in claim 2, wherein said first-stage valve element and said second-stage valve element each comprise a spool, said first-stage element then being axially displaceable in, and with respect to, said second-stage valve element by said motor through the intermediary of said transmission element.

4. A servo valve as claimed in either claim 1 or claim 2, wherein said first-stage valve element comprises a ball engageable with a suitable seating provided in a spool which forms said second-stage valve element.

5. A servo valve as claimed in any one of the preceding claims, wherein said one end portion of said transmission element is connected to said rotary output member through the intermediary of a coupling device.

6. A servo valve as claimed in claim 5, wherein said coupling device is of the kind which is flexible in the axial sense but non-flexible in the rotary, driving, sense.

7. A servo valve as claimed in any one of the preceding claims, wherein said motor is an electrical stepper motor.

8. A servo valve substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

9. A servo valve substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.