GB845109A - Improvements in or relating to electrically operated servo valves - Google Patents

Abstract

845,109. Fluid-pressure servomotor control systems. MOOG SERVOCONTROLS Inc. Oct. 8, 1956, No. 30610/56. Class 135. An electro-hydraulic servo valve is controlled by an electrical signal fed into an electromagnetic device which deflects a magnetically permeable plate positioned between the opposed nozzles of two hydraulic amplifiers to induce a differential pressure between the amplifiers, which pressure differential is used to actuate a balanced movable valve member that is proportionately controlled in response to the magnitude of the electrical control signal. The servo valve comprises a valve body 1 with a pressure fluid supply port 2, pressure fluid control ports 3 and 4 leading to a power device, and a fluid drain port (not shown). A bore 19 in the body contains a cylindrical bushing 18 with a number of annular grooves isolated from one another by sealing rings 25<SP>1</SP>, groove 17 communicates with supply port 2, grooves 21 and 24 with drain port 5 (not shown) and grooves 22 and 23 with control ports 3 and 4 respectively. The grooves also communicate through respective radial passages 17<SP>1</SP>, 21<SP>1</SP>, 22<SP>1</SP>, 231, 241, with the bore of the bushing in which is an axially slidable valve spool 25 that has annular grooves 26 and 27 arranged about its mid point controlling the power device by placing supply port 2 in communication with control port 4, and control port 3 in communication with drain port or vice versa depending on the direction of the displacement of the valve spool. The valve spool is spring urged to its neutral position (shown) by springs 29 bearing against each end of the spool through abutment assemblies and its central position may be adjusted by means of screw 37. A portion of the pressure fluid in passage 17 is diverted through a short vertical passage 40 to a transverse passage 41 which communicates with cavities 43 containing filter assemblies 44; where it passes to the interior of a hollow porous metal filter 45 through an orifice 461, past a permanent magnet 49 and outwardly through a radial passage 50 to a passage 51 which intersects a diagonal passage 52. Passages 52 extend between nozzle assemblies 53 and passages 52<SP>1</SP> extending around and through the ends of bushing 18 and communicating with opposite ends of valve spool 25. The nozzle assemblies 53 arranged in opposed relation to each other comprise a hollow outer member 54 having an annular passage 55 communicating with passage 52, and radial passages connecting passage 55 with the interior of member 54, a plug 56 also formed with radial passages connects these to a longitudinal passage extending into a hollow filter 57 of wire cloth. Pressure fluid passes through filter 57 into chamber 53<SP>1</SP> and through an orifice 57<SP>1</SP> and impinges on a cantilever mounted reaction plate 58 arranged between the orifices 57<SP>1</SP> to form an annular orifice. The reaction plate 58 is constructed of low hysteresis, magnetic, resilient material and forms an armature in an electric motor device 59, which is provided with pole pieces 63 and 64, and a coil 65 connected to terminals 61 which lead to a plug. Permanent magnets 66 and 67 (not shown) are arranged to maintain dissimilar poles at the pole pieces so that the spring rate of plate 58 is balanced against the magnetic field force of the pole pieces when there is no external signal, and the fluid jets are balanced on opposite sides of the plate 58 to centre the plate; also the pressures in the nozzle assemblies are substantially identical and act to maintain the valve spool in a centred position together with the action of springs 29. When an electrical signal energizes the coil 65 the plate 58 is moved relative to the nozzles producing differential changes in the areas of annular orifices resulting in a differential pressure drop across the orifices which is transmitted through the filters to the ends of the valve spool and moves the valve spool a distance proportional to the pressure difference. Nozzles 53 and motor device 59 are enclosed by a cap 69 which forms a sump for the pressure fluid which drains through a passage 72, with a restriction 73 so that the sump is slightly pressurized to reduce pressure surges to a drain cavity which leads to drain port.