GB2038511A - Servo control valve - Google Patents

Abstract

A servo control valve 58 is slidably disposed in a valve sleeve 42 and is controllable from outside the valve housing via a control signal input 90. The control sleeve (42) is guided for displacement in the valve housing (14) and is formed with the control ports (48, 50), said control sleeve being controllable independently of the spool via another control signal input (70). The valve may be used, for example, to control the pitch of a helicopter rotor, the sleeve being operated through an auxiliary servo motor 72 by a flight controller while the spool is actuated manually by the pilot. <IMAGE>

Description

SPECIFICATION Servo control valve The invention relates to a servo control valve, more particularly but not exclusively to a hydraulic servo control system for an aircraft.

In particular types of servo valve used to control hydraulic servo motors it is known to introduce a control signal for adjustment of the spool via a two-armed lever which is pivotally connected to the spool substantially at the centre of the lever one end of the lever being connected to the piston rod of the hydraulic servo motor and the other end forming, for example, the servo control system input member which is actuated manually by the pilot via the control column.If the spool is moved out of its neutral middle position on actuation of the input member, an hydraulic flow, which is unthrottled to a varying degree according to the deflection of the spool, is released from the working chambers of the servo motor via the servo control valve allowing the servo motor to move, its movement reacting via the pivoted lever on the control spool in the opposite direction to the initial deflection of the input member until the servo control valve resumes its neutral middle position in which it again hydraulically locks the servo motor-now in its new position.In servo control systems of this kind it is frequently required to superimpose automatically on the arbitrary manual control commands given by the pilot other control signals which are generally of short duration and of a rapidly changing nature and which, uninfluenced by the control commands, are produced, for example, by the in-flight controller, e.g. correction signals to correct for gusts of wind, for limiting the acceleration to a maximum permissible value, or for some other flight correction. These additional automatic control signals are usually superimposed on the manual control commands in a mixer linkage which precedes the pivoted lever, and like the control commands they act on the control valve signal input on the spool side.A disadvantage of such a control system for the servo control valve is the increased expenditure for the construction of the signal mixing system and, particularly, the resulting relatively considerable hysteresis (control errors) in control which have a negative effect particularly on the sensitivity of response and accuracy of control of the servo control system in respect of the weak automatic control signals.

The invention has as its object to construct a servo control valve of the type indicated hereinbefore in such a manner that two separate control signals can be mixed in a structurally simple manner in the servo control valve itself without a separate mixing system preceding the valve.

According to this invention therefore we provide a servo control valve comprising a control spool slidably diposed in a valve housing and controllable from outside via a control signal input and which has at least one control edge which co-operates with a control port to open and close it on displacement of the spool, wherein the spool is slidably located in a control sleeve, which is guided for displacement in the valve housing and which is provided with control port or ports, the control sleeve being controllable from outside via another control signal input independently of the control spool.

Because of its special construction, a servo control valve in accordance with the invention has two separate control signal inputs, one of which is allocated to the control spool and the other to the control sleeve. Thus the relative position of the control edges of the control spool to the control ports and hence the state in which the control valve is at any time, can be changed by means of these two separate inputs. Since signal mixing is effected by two movable valve elements of the control valve itself, a signal mixer is unnecessary and the sensitivity of response and accuracy of control is improved with weak automatic correction or control signals. These signals are introduced via a separate control signal path to a separate movable valve element irrespective of and uninfluenced by the signal path for arbitrary control commands.The signals are fed preferably directly to the control sleeve via the control signal input associated with the latter while the mechanical input linkage associated with the control spool, including the pivoted lever, is retained for coarser control commands. In a preferred embodiment, the control sleeve is advantageously controlled hydraulically via an auxilliary servo motor in order to keep control errors in the control signal path as small as possible for the automatic correction signals.The auxiliary servo motor is preferably associated with a monitoring valve which responds above a predetermined pressure differential and delivers a disturbance signal in the event of failure of the servo control valve (e.g. if the spool jams in the sleeve), or when it is switched off and when the control spool should be adjusted by the pivoted lever, but cannot because it is jammed in the control sleeve. In the event of such a disturbance, a pressure difference builds up on the auxiliary servo motor working piston via the control spool and the control sleeve and initiates a disturbance signal via the monitoring valve above the permissible differential pressure.

A preferred embodiment of the invention will now be described in detail with reference to the accompanying drawing showing an hydraulic servo control system with a servo control valve.

The illustrated servo control system is used for the adjustment of a movable contr AI sur face 2 of an aircraft, e.g. for controlling the pitch of a main helicopter rotor, the main component of the system being a servo motor 4 and a servo control valve 1 2. The motor 4 comprises a double-acting hydraulically actuated piston and cylinder unit, the working piston 6 of which is coupled to the control surface 2 via a piston rod 8 and a mechanical connection 10 (shown diagrammatically) which supplies the actuating force for adjustment of the surface 2. The servo control valve 12 which hydraulically controls the servo motor 4 has a valve housing 1 4 with a middle port 1 6 connected to a pressure medium supply conduit 20 fed by a hydraulic pump 18, two outer ports 22 and 24 each leading to a return R, and two lateral ports 26 and 28 disposed between the middle port 1 6 and the outer port 22 in the longitudinal direction of the valve housing 14. The ports 26 and 28 are connected via control lines 30, 32 to portions 34, 36 of the servo motor 4 working chambers. Each of the ports 16 and 22 to 28 leads via a widened portion 38 in the form of a groove into a central longitudinal bore 40 of the valve housing 14 in which a control sleeve 42 is axially slidable disposed.Control sleeve 42 co-operates in sliding and sealing relationship with a plurality of O-ring seals fixed in the valve housing 14 to shut off the individual ports 1 6 and 22 to 28 and their widened portions 38 from one another and from the exterior: Control sleeve 42 has a central longitudinal bore 46 into which lead two control ports 48 and 50 associated with the lateral housing ports 26 and 28, and transverse bores 52, 54, 56 associated respectively with the housing ports 16, 22, 24. The positions of these bores in the longitudinal direction of the control sleeve 42 are selected so that they are in unthrottled communication with the associated widened portion 38 and the corresponding housing port over the entire range of movement of the control sleeve 42.

A control spool 58 having four lands 60, 62, 64, 66 spaced apart over the longitudinal direction of the spool and each co-operating with the central sleeve bore 46 in a close sealing and sliding fit is guided for longitudinal displacement in the central bore 46 of the control sleeve 42. The annular chamber between the two middle lands 62 and 64 constantly receives hydraulic medium at operating pressure from the pump 18, while the two outer annular chambers between the lands 60, 62 and 64, 66 respectively are permanently relieved of pressure towards the return R.In the illustrated neutral position of the valve, the two middle lands 62 and 64 each bounded laterally by control edges 62A, 62B and 64A, 64B respectively just cover the control ports 48 and 50 of the control sleeve 42 so that they are shut off from the pressure medium source 1 8 and the return R and the servo motor 4 is hydraulically locked.

The distance between the control edges 52A, 62B, 64A, 64B is dimensioned with great precision to the size and position of the control ports 48, 50 of the control sleeve 42 so that when the spool 58 is moved out of the neutral position the control ports 48 and 50 are increasingly opened so that one working chamber 34 or 36 of the servomotor 4 is brought into increasingly unthrottled communication with the pressure medium supply conduit 20 and the same applies to the other working chamber with respect to the return R.

Under the action of the differential pressure between the working chambers 34 and 36, the magnitude and direction of which is dependent upon the relative displacement of the control spool 58 and the control sleeve 42, the working piston 6 of the servo motor 4 moves at a corresponding speed to pivot the control surface 2.

The relative displacement between the control sleeve 42 and the control spool 58 is controlled via two separate control signal inputs, i.e. a first input in the form of an intermediate link 68, which is coupled for movement to the control spool 58 and which it in the housing 14, and a second input 70 which controls the displacement of the control sleeve 42 in the central housing bore 40 and, in the examplified embodiment illustrated, is in the form of a piston rod of an auxiliary hydraulic servo motor 72.The control signals acting on the spool 58 and the sleeve 42 respectively via the two separate inputs 68, 70 are summated in the servo control valve 1 2 and in respect of direction, magnitude and speed jointly determine the relative displacement of the control edges 62A, 62B and 64A, 64B in relation to the corresponding control ports 48, 50 and hence the pressure control of the servo motor 4 in dependence thereon.

Automatic control or correction signals are advantageously applied via the control sleeve 42, the control signal input 70 and the auxiliary servo motor 72. These signals are produced by a flight controller (not shown), e.g.

for wind gust correction, in order to limit flight acceleration, or for some other automatic correction (usually of short duration and only slight) of the angle of incidence of the control surface 2. The control signals are fed electrically via lines 74 to an auxiliary servo valve 76 which is hydraulically connected via a branch line 78 to the feed line 20 and via an outlet line 80 to the return R. Control lines 82, 84 from the servo valve 76 are connected to the working chambers 86, 88 respectively of the auxiliary servo motor 72 to control the movement of the auxiliary servo motor piston in dependence on the automatic correction signals.The larger-amplitude control commands which usually change more slowly than the automatic correction signals act on an input member 90 which is moved in accordance with the movement of the control column by the pilot, member 90 being disposed at the free end of lever 92 which is pivotally connected between its ends to intermediately link 68 and at its upper end to a second piston rod 94 of the servo motor 4.

If the servo control valve 1 2 is either mechanically or electrically actuated by the input member 90 or the servo valve 76 respectively, or is actuated by both the spool 58 moves out ot its neutral middle position,illustrated so that the control ports 48, 50 are opened to a degree depending on the position of the sleeve 42 to the pressure supply line 20 and to the return R respectively.This causes the piston 6 in servo motor 4 to move the piston rod 94, the pivoted lever 92 and the intermediate link 68 coupled to the control spool 58 in the opposite direction to the initial direction of movement of the spool 58 which opened the ports 48 and 50 until the servo control valve 1 2 resumes its neutral middle position irrespective of whether it was moved out of that position via one or other or, simultaneously, via both of the control signal inputs 68, 70.The servo control system illustrated thus operates after the style of a boosted follow-up control with two separate control signal paths, a mechanical path on the one hand and an electro-hydraulic path on the other hand, which are independent of one another and of which one acts on the valve element 58 having the control edges, while the other acts on the valve element 42 of the servo control valve 1 2 having the associated control ports.

To monitor the serviceability of the servo control valve 12, the auxiliary servo motor 72 is coupled to a monitoring valve 96 which responds if a predetermined differential pressure between the control lines 82 and 84 is exceeded. Valve 96 comprises a spool 102 which has two lands 98, 100 of equal diameters and which is held resiliently in the closed position illustrated, in which its lands 98, 100 shut off a pressure inlet 106 from the outlets 110, 11 2. Pressure inlet 106 is connected via a branch line 104 to the pressure supply line 20 while the outlets 110, 11 2 lead to a disturbance signal line 108.If, however, a predetermined differential pressure from the control lines 82, 84 acts on the lands 98, 100, the spool 102 moves out of the closed position illustrated to the right or left, depending on the direction of the differential pressure in opposition to the spring force so that the pressure signal is transmitted from the inlet 106 via one or other outlet 110 or 11 2 to the disturbance signal line 1 08.

The control sleeve 42 and the spool 58 are normally very easily movable, i.e. with a force of about 3 N. The differential pressure required for actuating the auxiliary servo motor 72 is accordingly also small. If the control sleeve 42 becomes obstructed e.g. because the control sleeve 42 is jammed on the spool 58, the differential pressure rises steeply.

However, if the auxiliary servo valve 76 is opened in one or other direction by an automatic control signal, and reaches as a maximum value the pressure difference between the lines 78 and 80. Even if the auxiliary servo valve 76 is not actuated and only the spool 58 is to be moved by the input member 90, but is jammed in the sleeve 42, the pressure difference building up between the working chambers 86, 88 of the auxiliary servo motor 72 via the intermediate link 68, control spool 58, control sleeve 42 and piston rod 70 is above the normal working range and causes the monitoring valve 96 to respond and deliver a distrubance signal which, for example, then switches off the servo control system and switches on a second stand-by servo control system associated with surface 2.

Claims (5)

1. A servo control valve comprising a control spool slidably disposed in a valve housing and controllable from outside via a control signal input and which has at least one control edge which co-operates with a control port to open and close it on displacement of the spool, wherein the spool is slidably located in a control sleeve which is guided for displacement in the valve housing and which is provided with control port or ports, the control sleeve being controllable from outside via another control signal input independently of the control spool.

2. A servo control valve according to Claim 1, wherein the control sleeve is controllable from outside via a valve-controlled hydraulic auxiliary servo motor.

3. A servo control valve according to Claim 2, including a monitoring valve which responds and delivers a disturbance signal in the event of a predetermined differential pressure being exceeded at the auxiliary servo motor.

4. A servo control valve according to any one of the preceding claims, wherein said valve housing includes first and second ports for coupling to a servo motor, a third port for coupling to an hydraulic pressure fluid source and fourth and fifth ports coupled to a return, said sleeve having five ports each associated with respective ones of the ports in said casing and said spool having lands which control the throttling of the ports in dependence on the movement of said spool and sleeve thereby controlling the amount and pressure of the fluid passing from said source to said first and second ports in said casing.

5. A servo control valve substantially as described herein with reference to the accompanying drawing.