GB2057718A

GB2057718A - Servo system

Info

Publication number: GB2057718A
Application number: GB8024902A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1979-08-03
Filing date: 1980-07-30
Publication date: 1981-04-01
Also published as: FR2462599A1; DE2931533A1; GB2057718B; FR2462599B1; DE2931533C2

Abstract

A servo system has an output servo motor 4 (104) controllable through one or other of two identical servo control circuits 16A, 16B (116A, 116B) each of which includes its own source of pressure fluid (PA, PB) and a servo control valve 20A, 20B (120A, 120B) for the motor to thereby provide duplicity. Selection of one of the control circuits is made by a two-position switching valve 22 (122). The control valves are coupled together to receive one or more control inputs, the valve in the non-selected circuit following the position of that in the selected circuit. The switching valve is responsive to the pressures of the respective sources by way of two pistons of differential areas 56; 62 (156; 162) so that normally the switching valve is biased to select a particular control circuit but can switch automatically to select the other control circuit. A safety valve 80 (200) is operable by a failure, e.g. jamming of the servo control valve in the normally selected circuit, to release the pressure on the piston associated with that circuit so that again the switching valve switches automatically to select the other control circuit. Upon switching the switching valve cannot revert to its initial position. <IMAGE>

Description

SPECIFICATION Servo system This invention relates to a servo system.

The invention is concerned with the provision in a servo system of a means whereby the action of one control section of the system can be taken over by another section should the first fail. The invention is applicable in hydraulic or pneumatic servo systems. The invention will be discussed in relation to hydraulic systems.

The use of redundancy in systems is well known particularly where safety is at stake.

Duplicate equipment can be provided. One equipment is normally operative. The other takes over in the event of failure of the first.

Redundancy of this kind finds application in aircraft and space vehicles for example.

A hydraulic servo control system as used particularly for controlling aircraft and space vehicles and having a redundant type of construction is described in German Auslegeschrft 2108545. The construction there proposed has two output servo motors, each with associated control means, the motors being coupled to each other, usually in tandem, only one servo motor being hydraulically activated in normal operation whilst the other servo motor is on pressureless operation and together with its associated servo control valve and pressure medium source it runs idly in readiness. Only in the case of a fault is the hydraulic control of the second servo motor activated and the first servo motor is disengaged from the hydraulic system and its associated control circuitry is switched off.However, servo control systems of this kind in which the main components are duplicated or multiplied require a plurality of individual members which are hydraulically or mechanically adapted to one another and have a complicated construction taking up more than twice as much space and weighing more than twice as much as a non-redundant system, and this is a serious disadvantage particularly in aircraft and space vehicles.

The present invention enables us to provide a servo system in which a redundant construction is retained but with the duplication of fewer parts than before. Such a system may be practically realized in a smaller size and particularly with less weight than with the prior proposal.

According to the present invention there is provided a servo system including a servo motor, first and second servo control sections each capable of separately controlling the actuation of the motor upon connection thereto, and each section comprising a respective source of a pressure medium and a control valve, the control valves being coupled together to respond to an applied control input such that each control valve is operable to control the motor in accord with the control input, and a switching valve connected between the control valves and the servo motor and actuable between first and second positions selecting said first and second control sections respectively to control the actuation of the motor and isolating the second and first sections respectively from the motor.

The first and second switching valve positions may be referred to in the following description as the rest and working positions respectively. The control valves of the first and second control sections may be referred to as the first and second control valves respectively.

In implementing a servo system according to the invention, a single output servo motor may be used avoiding the duplication of this major component and its attendant weight.

Duplication is provided in the areas which are more prone to breakdown, i.e. the source of pressure medium and the servo control valve.

The risk of total breakdown remains small, since the risk of the main servo motor failing is small as against the risk of a breakdown of a redundantly provided part, i.e. jamming or blocking of a servo control valve member, in particular, or failure of the supply of pressure medium. The use of a switching valve enables the system to be switched to the non-defective second servo control section which has until then been running idly in readiness. The first servo control valve and the first source of pressure medium are isolated from the servo motor and therefore the defect does not affect the continued operation of the main servo motor. It is preferred that the servo control valves should be coupled together so that the valve in the idle section follows the position of that in the active control section.

In order that a pressure failure in the source of pressure medium of the first control section can directly initiate switching to the second control section, the switching valve is preferably pressure-controlled so as to be maintained in the first position by the pressure of the source of the first control section. In a preferred construction the switching valve comprises a movable switching member having first and second portions located in first and second chambers receiving fluid from the pressure sources of the first and second control sections respectively, the pressure on the first and second portions acting to move the switching member to the first and second switching positions respectively.The switching valve may have its switching member in the form of a valve spool or piston, the first and second portions constituting piston surfaces of which the first is of greater area than the second to provide a force difference biasing the switching valve to the first position.

This assumes the two pressure sources to be of equal pressure.

For safety reasons, the switching valve is preferably resiliently biased into the rest position though not so strongly as to prevent movement to the working position under pressure in the second chamber and in its working position breaks the connection between the source of pressure of the first control section and the first chamber. To this end the switching valve may comprise a section connecting the first chamber to the pressure source of the first control section in the first, rest, position of the switching valve and disconnecting the first chamber from this source in the second, working, position.This ensures that, if the source of pressure medium in the first control section is operating irregularly, once the switching valve is brought into the working position as soon as the pressure failure occurs, the valve remains in this position even if the source of pressure medium in the first control section subsequently delivers full operating pressure again for a short time.

In addition a normally closed safety valve may be connected to the first chamber, the valve being operable to release the pressure in the first chamber thereby allowing the switching valve to be set into its working position.

For this purpose a restrictor is located between the pressure source of the first control section and the first chamber and, in the case mentioned where the switching valve comprises a section for isolating the first chamber from the source of the working position, the restrictor is preferably between this valve section and the first chamber.

The safety valve is arranged to be operated by means responsive to a failure of the first servo control valve, such as the valve jamming. Consequently the switching valve goes to the working position to activate the second control section if the servo control valve fails as well as if the pressure source in the first section fails with the consequences described above. Again by employing the means breaking the connection between the first chamber and the associated pressure source, the switching valve remains in the working position even if the first servo control valve resumes working and causes the safety valve to close once again, assuming the safety valve to be connected so as to have its closed condition restored.

If the first servo valve is actuated by at least one valve-controlled auxiliary servo motor responsive to applied input signals, the magnitude of the pressure difference at the auxiliary servo motor may be used as a simple criterion for determining the satisfactory operation of the servo valve, in that, if a predetermined pressure difference at the auxiliary servo motor, which is not attained as long as the servo control valve can easily move (e.g. the spool or piston easily slide in the associated valve bore), is then exceeded, the safety valve is operated.

If the servo system is constructed as a hydraulic system, it is preferably accommodated in a single housing which forms a hydraulic return. This construction aids in a simplification in structure and a saving in space and, in particular, a reduction in weight.

The invention and its practice will be further' described with reference to two embodiments illustrated in the accompanying drawings, in which: Figure 1 shows a schematic diagram of one embodiment of a servo system according to the invention; and Figure 2 shows a diagram similar to Fig. 1 of an embodiment which is modified as regards the servo valves and the associated fault signal path.

In the drawings, parts in Fig. 2 like to those in Fig. 1 are given the same reference numeral increased by one hundred. Return drains for hydraulic fluid are generally designated by R.

The hydraulic servo control shown in Fig. 1 is used for displacing a movable control surface 2 of an aircraft, e.g. for controlling the blade angle of the main rotor of a helicopter, and includes, an output servo motor 4 in the form of a double-action hydraulically actuated piston-cylinder unit having operating chamber 6 and 8, the working piston 10 of which is coupled in movement to the control surface via a piston rod 1 2 and a mechanical connection 1 4 (schematically shown) and which applies the output actuating force for setting the control surface 2. The output servo motor 4 is controlled by a first servo control section 1 6A which is paralleled by a second similar servo control section 1 SB. The two sections are in parallel but are not simultaneously active.The motor 4 is normally controlled by the hydraulic circuit 1 so, with the circuit 1 SB providing a redundant back-up to be brought into service should section 1 SA fail. Each servo section 1 SA and 1 SB contains its own source of pressure fluid 1 8A and 1 8B respectively in the form of a hydraulic pump and a servo control valve 20A and 20B respectively. The selection of the servo control section to be in active or working relationship with the servo motor 4 is effected by means of a two position switching valve 22 which is located between the two servo control valves 20A, B and the operating chambers 6, 8 of the servo motor 4. The valve 22 is normally held in a position connecting control section 1 SA to the motor and isolating section 1 6B therefrom.

The means for doing this are described below.

The servo valve 20A contains a control spool or piston 24 which has a plurality of piston lands and is slidably movable in a valve bore 26 co-operating with ports in the bore.

The valve has output ports connected to control lines 28A and 30A which are connected to the operating chambers 6 and 8, respectively, of the servo motor 4 through the switching valve 22 as shown in its rest position as it will be called. The valve 20A has an input port connected to pump 18A. The servo valve 20B is identical in construction with valve 20A and connected in an identical hydraulic circuit but the control lines 28B, 30B associated with it are blocked off from the operating chambers 6, 8 of the servo motor 4 in the rest position of the switching valve 22.

Thus, as long as the switching valve 22 stays in the position shown, the servo motor 4 is completely isolated from the servo portion 1 6B and the latter, including its hydraulic pump P,, merely runs idly in readiness, whilst the particular hydraulic pressure established in the control lines 28B and 30B via the servo valve 20B cannot affect the operation of the servo motor 4.

The servo valves 20A, 20B are coupled together to receive a common control input which in this embodiment is applied through a mechanical linkage. The valve spool positions are mechanically adjusted by means of a longitudinally movable input member 32 which is actuated, for example, by the pilot via the control stick and which is coupled to one end of a two-armed articulated lever 34 pivotally connected at the end of its other arm to a second piston rod 36 of the servo motor 4. The lever 34 is pivotally connected between its ends to a control rod 40 via an intermediate guide rod 38. The rod 40 is guided so as to be movable in a direction parallel to the axes of valves 20A, 20B and to the direction of movement of input member 32.This control rod 40 is coupled to adjust the control pistons 24 of the servo valves 20A and 20B in their associated valve bores 26, by interposed spring members 42A and 42B respectively. The piston settings are a function of the position of the input member 32 and the position of the servo motor 4 which reacts via the piston rod 36 and the articulated lever 34. The spring temper of each of the spring members 42A, 42B is such that it is not deformed as long as the control piston 24 to which it is coupled can move easily, i.e.

without jamming or becoming blocked, in the associated valve bore 26.

Assuming the switching valve 22 remains in the rest position shown, when the control piston 24 of the servo valve 20A is in the neutral central position shown, its control lines 28A and 30A leading to the operating chambers 6, 8, respectively, of the servo motor 4 are blocked by the inner piston lands.

If the control piston 24 is moved to the left as shown in the drawing, the control line 28A is opened progressively wider to the hydraulic pump PA and the control line 30A is opened progressively wider to an outlet 44 leading to the return R. If the control piston 24 is moved from its neutral central position to the right as shown in tne drawing, the control line 30A is brought more and more into communication with the hydraulic pump PA and at the same time the control line 28A is brought more and more into communication with another outlet opening 46 leading to the return.Thus, when the control piston 24 of the servo valve 20A is moved out of its neutral position by a displacement in one direction of the input member 32, one operating chamber of the servo motor 4 is progressively engaged with the hydraulic pump PA and the other operating chamber is progressively engaged with the return R to move the working piston 10 under the influence of the pressure difference established between the operating chambers 6, 8 and at a corresponding speed. The control surface 2 pivots accordingly. The piston 10 moves in a direction that has a counter effect to that of the movement of the input member 32.The movement of the working piston 10 is applied via the piston rod 36, articulated lever 34, intermediate guide rod 38, control rod 40 and spring member 42A, to the piston of servo valve 20A in a direction counter to the initial deflection caused by the input member 32, and continues until the servo valve 20A returns once again to the neutral position. The servo motor 4 is now hydraulically locked in place by the blocking condition of the servo valve 20A in its new position set by the displacement of input member 32. Thus, throughout the entire range of adjustment, each position of the input member 32 is associated with a specific position of the working piston 10 and hence also of the control surface 2 whilst the load acting on the servo motor 4 or on the control surface 2 is not reflected at the input member 32.Similarly the control lines 288, 30B of the servo valve 20A, which is adjusted via the control rod 40 in synchronism with the servo valve 20A, are opened up to a greater or lesser extent relative to the hydraulic pump Ps or to the return R, respectively so that the position of the valve spool of valve 20B follows that of valve 20A.

However, with the switch valve 22 in the rest position, the valve 20B does not affect the operation of the servo motor 4.

If, however, the switching valve 22 is switched to the right-hand position as seen in Fig. 1, that is from the rest position shown into what may be called the operated or working position, the control lines 28A and 30A of the servo valve 20A are blocked by the piston lands 48, 50 of the piston 52 of the switching valve whilst the control lines 28B, 30B of the servo valve 208 are put in communication with the operating chambers 6, 8 of the servo motor 4, so that the latter comes under the exclusive control of the second control servo section 16B, the section 1 6A now being isolated from the motor.

However, as long as the first servo section 16A is operating satisfactorily, the switching valve 22 is kept in the rest position partly by means of a relatively weak compression spring 54 biasing the valve piston 52 to that position and also by virtue of the fact that a differential force is established on the piston 52 by the pressure sources 18A, 18B. They are assumed to be of equal pressure. To this end one end face 56 of the piston 52 is located in a pressure chamber 60 connected by line 58 to source 1 8B to produce a force acting in the direction of the working position of the switching valve 22. An opposite end face 62 of the piston is located in a pressure chamber 64 that is connected to source 1 8A through line 70 to produce a force acting in the direction of the rest position of the valve.The surface area at 56 is made smaller than that at 62 so that the net force acts to keep the piston in the rest position as long as both pressure sources are operating normally. The connection of the hydraulic pump PA to chamber 64 is through a control section 68 of the piston of valve 22 and a restrictor 66. The control section provides a chamber connecting the restrictor to the pressure line 70 when the switching valve 22 is in the rest position.

Since the operating pressures of the two hydraulic pumps are of the same magnitude during trouble-free operation, the switching valve 22 is thus biased into the rest position in which the servo control section 1 6A is active.

If the operating pressure of the first hydraulic pump PA fails, there is a lowering of pressure in the chamber 64 and under the effect of the operating pressure of the second hydraulic pump P5 in the pressure chamber 60, the control piston 52 of the switching valve 22 moves into the working position, the force on surface 56 overcoming the force of the spring 54. At the same time, the pressure line 70 is closed off from the valve chamber 68 by the piston land 72 and this chamber is brought into engagement with the return R by the additional piston land 74.This ensures that even if the first hydraulic pump PA operates irregularly, i.e. if, after an initial pressure loss, the pump briefly resumes supplying the full operating pressure, the pressure chamber 64 remains free from pressure and thus the switching valve 22 does not return to the rest position but instead maintains the disconnection of the defective servo control section 1 6A and switches to the non-defective servo control section 16B.

Another type of breakdown occurs if the servo valve piston 24 of the normally active servo section 1 6A becomes jammed in the associated valve bore 26 or for some other reason no longer follows the regulating movements of the control rod 40. In this case, the spring member 42A is compressed or extended, but because the control rod 40 is mounted to be movable in parallel manner this does not affect the second servo valve 20B, and consequently this servo valve 20B is still set precisely in sympathy with the longitudinal movements of the intermediate guide rod 38 and control rod 40.Fixed to the upper end of the control rod 40 is an actuating arm 76 which co-operates with the actuator of a microswitch 78 mounted on the servo valve piston 24, and holds the actuator in operated condition as long as the servo valve 20A is operating satisfactorily, i.e. as long as the spring member 42A is not deformed or is only insignificantly deformed. If there is a fault in the servo valve 20A, i.e. relative movement between the control rod 40 and the servo valve piston 24 and a correspondingly high tensile or compressive stress on the spring member 42A, the actuating arm 76 releases the actuator of the microswitch 78, with the result that this switch operates an electrically-actuated, normally-closed, safety valve 80. The valve is opened.This safety valve 80 is mounted in a branch line 82 leading from the pressure chamber 64 to the return R and is separated from the pressure source PA by restrictor 66. Since only a limited amount of hydraulic fluid can flow from the hydraulic pump PA into the pressure chamber 64 via the restrictor 66, the pressure in this chamber is released when safety valve 80 is opened. As a result if the servo valve 20A becomes defective, the safety valve 80 is operated and the valve 22 moves to the working position and remains in this position even if the servo valve 20A briefly returns to correct operation and, therefore, the safety valve 80 ought to close again.In this case, as before, the pressure chamber 64 remains free from pressure via the valve chamber 68 which is sealed off from the pressure line 70 by the piston land 72 and which is opened relative to the return R by the piston land 74. Consequently no pressure can be established in chamber 64 to switch valve 22 back into the resting position counter to the operating pressure of the second hydraulic pump P5 in the pressure chamber 60. Thus, in summary, if there is a fault in the hydraulic pump PA and/or the servo valve 20A of the normally active servo section 16A, the servo motor 4 is completely disconnected from the section 1 6A and switched to the control of servo section 16B.

Both servo sections 16A, 16B together with, the servo motor 4 and all the hydraulic connections are accommodated in a single housing (not shown) which forms the return R and recycles the hydraulic medium, including any leakages, to the hydraulic pumps.

The servo system shown in Fig. 2 includes substantially all the features of the embodiment shown in Fig. 1 and the corresponding parts bear the same reference numerals increased by 100. The main difference is a different construction of the servo control valves enabling further control inputs to them and a modified method of generating fault signals if the normally-operative servo control valve .(120A) fails. In as far as the circuit of Fig. 2 operates similarly to that of Fig. 1, such operation will not be described again.

For the purpose of passing additional control signals to the servo control valve 1 20A via a second control signal input, in addition to and independently of the control commands fed in through the input member 1 32 by the pilot, the valve includes a control sleeve 1 84 that is inserted between the valve bore 1 26 and the servo valve piston 1 24 and that is slidably movable in the axial direction.

The control sleeve 1 84 comprises a plurality of ports that co-operate with the lands on piston 1 24 in the manner that the ports in bore 26 so co-operated in the valve 20A in Fig. 1. The valve bore inlets and outlets are enlarged so as to remain in communication with associated sleeve ports whatever the position of the sleeve in the bore. The outer two ports constantly communicate with the housing outlets 1 44 and 146 respectively whilst the central port is constantly connected to the hydraulic pump PA and the two motor control ports are constantly connected to the control lines 1 28A and 130A, respectively.Thus, the control action of the servo control valve is dependent, on the one hand, on the position of the valve piston 1 24 mechanically controlled via the control rod 1 40 and input member 1 32 and, on the other hand, on the position of the control sleeve 1 84 relative to the valve bore 1 26. Displacement of the control sleeve 1 84 in the valve bore 1 26 is controlled by a second control signal input which is transmitted to the piston rod 1 86A of a hydraulic auxiliary servo motor 1 88A associated with the control sleeve 1 84. The control signals acting on the valve piston 1 24 via the input member 132, on the one hand, and on the control sleeve 1 84 via the piston rod 186A, on the other hand, are combined in the control valve and together determine, in direction, magnitude and velocity, the relative movement between the valve piston 1 24 and control sleeve 1 84 and thus also determine the pressure control of the servo motor 104 dependent thereon. That is control of the motor is dependent on the relative positions of sleeve 1 84 and piston 1 24 which are shown in a relative neutral, central, position in the figure.

Appropriately, automatic control or correction signals are fed into the control sleeve 1 84 via the control signal input 1 86A and the auxiliary servo motor 188A. These signals are generated by an auto-pilot (not shown), i.e.

for correcting squalls, for limiting the flight acceleration or for making some other generally brief, slight, automatic correction to the manual control commands which the pilot feeds in via the input member 132, and are conveyed electrically to an auxiliary servo valve 1 90A which is hydraulically connected to the hydraulic pump PA via a branch line 1 92 and to the return R via an outlet 1 94A and to the operating chambers of the auxiliary servo motor 1 88A via control lines 196A, 198A. Servo valve 1 90A controls the regulating movement of the auxiliary servo motor 1 88A as a function of the automatic correction signals.If the valve piston 1 24 and the control sleeve 1 84 are moved relative to each other out of the neutral central position shown, so that the servo motor 104 is hydraulically actuated and moves under the effect of the pressure difference between its operating chambers, its movement is reflected as before, via the piston rod 136, the articulated lever 134, the intermediate guide rod 138, the control rod 140 and the spring member 142A, to the valve piston 1 24 to move the latter in the opposite direction to the initial actuation of the servo valve 120A.The counter movement continues until the servo valve reaches its relative neutral central position again irrespective of whether the servo control valve 120A was moved out of its neutral central position via the mechanical control signal input (input member 132) or the hydraulic control signal input (auxiliary servo motor 188A) or via both control signal inputs at the same time. The second servo valve 1 20B is identical in construction to the servo valve 1 20A and also contains an auxiliary servo motor 1 88B including an associated auxiliary servo valve 190B, which, during normal operation, operate in sympathy and simultaneously with the electrical control signal input at the servo control valve 120A.

Thus valve 1 20B in the non-active servo section 11 6B follows the a nIve 1 20A in section 116A.

The response to a pressure failure of source 11 8A is as previously described. For the purpose of monitoring the operation of the servo control valve 120A, the auxiliary servo motor 1 88A is associated with a safety valve 200 connected in branch line 1 82 from pressure chamber 1 64. Like valve 80 in Fig. 1, the valve 200 is operable to release the pressure in chamber 1 64 to switch valve 1 22 to the working position. The valve 200 responds when a predetermined pressure difference arises between the operating chambers of the auxiliary servo motor 188A. The valve includes an adjusting piston 202 comprising two piston lands of identical diameter, this piston being resiliently held against movement in both axial directions so as to be normally in the blocking position shown in which its piston lands block off the branch line 1 82 from the return R. However, if a given differential pressure from the operating chambers of the auxiliary servo motor 1 88A acts on the piston lands of the adjusting piston 202, this piston moves out of the blocking position shown against the spring force to the right or left, depending on the direction of this pressure difference.The branch line 1 82 is connected to the return R and switching of the switching valve 1 22 into the working position occurs in the same way as when the safety valve 80 of Fig. 1 is opened.

Normally, the control sleeve 184, like the valve piston 124, is easily displaceable, i.e.

with a force of about 3 N. The pressure difference required to displace the auxiliary servo motor 1 88A is also correspondingly small. However, if the control sleeve 1 84 is impeded, e.g. because it is caught on the valve piston 124, when the auxiliary servo valve 1 90A is opened by an automatic control signal in one or other direction, the pressure difference across auxiliary motor 1 88A increases sharply whereupon the safety valve 200 responds.The valve 200 is still opened if movement between the piston 1 24 and sleeve 1 84 is impeded even if the auxiliary servo valve 1 90A is not actuated and therefore the control lines 1 96A and 19SA are blocked, and only the valve piston 1 24 is sought to be moved by the input member 1 32. In this case a pressure difference above the normal operating range is built up between the operating chambers of the auxiliary servo motor 1 88A via the valve piston 124, the control sleeve 1 84 and the piston rod 186A, and this pressure difference opens the safety valve 200 and moves the switching valve 1 22 into the working position. Again the servo motor 104 is isolated from the servo control section 11 6A -and is brought under the control of the non-defective servo control section 11 6B which has hitherto been running idly in readiness.

Claims

1. A servo system including a servo motor, first and second servo control sections each capable for separately controlling the actuation of the motor upon connection thereto, and each section comprising a respective source of a pressure medium and a control valve, the control valves being coupled together to respond to an applied control input such that each control valve is operable to control the motor in accord with the control input; and a switching valve connected between the control valves and the servo motor and actuable between first and second posi tions-selecting said first and second control sections respectively to control the actuation of the motor and isolating the second and first Sections respectively from the motor.

2. A servo system as claimed in Claim 1 in which said switching valve is normally biased into said first position and the control valves are coupled together such that the position of the control valve of the second control section follows the position of the control valve of the first control section.

3. Servo system as claimed in Claim 2 in which the switching valve comprises means responsive to the pressure of the source of the first control section to normally bias the switching valve into the first position and to set the switching valve to the second position if the pressure fails,

4. A servo system as claimed in Claim 3 comprising means responsive to a failure of the control valve of the first control section to - cause the switching valve to be set to the second position.

5. A servo system as claimed in Claim 1, in which the switching valve comprises a movable switching member having first and second portions located in first and second chambers receiving fluid from the sources of said first and second control sections respectively, the pressures on said first and second portions acting to move said switching member to said first and second positions respectively.

6. A servo system as claimed in Claim 5 in which said switching member comprises a valve spool and said first and second portions provide piston surfaces, the area of the first piston surface being greater than that of the second.

7. A servo system as claimed in Claim 5 or 6 in which said switching valve comprises resilient means biasing the switching member into the first position.

tl. A servo system as claimed in any one of Claims 5 to 7 in which said switching valve comprises a section connecting said first chamber to the pressure source of the first control section in the first position of the switching valve and disconnecting the first chamber from this pressure source in the second position.

9. A servo system as claimed in any one of Claims 5 to 8 in which said first chamber is connected to the pressure source of the first control section via a restrictor, an-d further comprising a further, normally-closed, valve connected to said first chamber and operable to release the pressure in the first chamber thereby allowing the pressure in the second chamber to cause the switching valve to be set into its second position.

1 0. A servo-system as claimed in Claim 8 in which said first chamber is -connected to said valve section via a restrictor, further comprising a further, normatly-closed, valve connected to said first chamber and operable to release the pressure in the first chamber thereby allowing the pressure in the second chamber to cause the switching valve to be set into its second position.

11. A servo system as claimed in Claim 9 or 10 further comprising means responsive to a failure of the control valve of the first control section to operate said further valve.

1 2. A servo system as claimed in Claim 11 in which said control valve of the first control section has mechanical linkage thereto for supplying a control input to the control valve, said mechanical linkage including a resilient coupling element deformable by a failure of the control valve to move in response to a displacement applied to the mechanical linkage, and wherein said failure responsive means is responsive to a deformation of said coupling element.

1 3. A servo system as claimed in Claim 11 in which the control valve of the first control section is coupled to an actuating servo motor therefor and said failure responsive means is connected to the actuating servo motor and is responsive to a pressure differential therein exceeding a predetermined value to operate said further valve.

14. A servo system as claimed in any preceding claim that is constructed as a hydraulic system, the construction including a housing enclosing the hydraulic system and providing a hydraulic return.

1 5. A servo system substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.