ELECTRO-HYDRAULIC ACTUATOR SYSTEM

Abstract: The system includes a first pair of actuators (20A, 20B) and control arrangements (21, 22) for the respective actuators (20A, 20B). Each control arrangement includes a control circuit (31) for generating an actuator position error signal for a control valve (37), and a circuit (30) responsive to operating positions (X1A, X2A) of the control valves (37) in the respective arrangements (21, 22) for providing a trimming signal to the control circuit (31) indicating the divergence of the respective valve operating positons from a mean of those positions. Each control circuit (31) is responsive to the trimming signal and the operating positions of the valve (37) and hence the forces on the actuators (20A, 20B) are maintained equal. Each of the control circuits (31) includes means for limiting the magnitude of input signals to the respective valve (37), thereby limiting overdrive of the valves (37) in the event of minor malfunctions thereof, and permitting a malfunction indication to be provided.

Description: This invention relates to hydraulic actuator systems in which fluid pressures for positioning an actuator are controlled by an electro-hydraulic valve whose flow control element is operated by electrical input signals which are dependent on selected and sensed positions of the actuator. It is known, for example from GB-A-2119966, to provide such a system having duplicated hydraulic actuators which are coupled for movement in unison and which provide an output force which is the sum of the forces from the separate actuators. Since each of the aforesaid electrical input signals depends on a difference between a selected and a sensed position of an actuator, and corresponds to a position of the electro-hydraulic valve control element required to effect actuator movement to its selected position, a deficiency or minor malfunction in the valve may result in its control element failing to move immediately to the required position. In such a case the electrical input signal will be maintained, overdriving the control element to cause the selected actuator position to be reached. It is a disadvantage of known systems that such overdrive is likely to mask malfunction of the valve and in addition to drive the valve control element substantially past its required position, giving rise to additional malfunctions in the system. It is an object of the invention to provide an actuator system in which the foregoing problem is overcome. It is known from DE-C-1049239 to provide an electro-hydraulic actuator system comprising a hydraulic actuator, a circuit for generating an error signal which is dependent on a difference between desired and sensed positions of said actuator, an electro-hydraulic valve having a flow control element responsive to an electrical input signal for regulating a supply of pressurised fluid to said actuator, and means for deriving said input signal from said error signal, and means responsive to said error signal and to a signal dependent on the sensed position of said valve control element, for limiting the magnitude of a change in said input signal in response to a difference between said error and sensed valve position signals. According to the invention the system further includes means responsive to said error signal for generating a signal corresponding to a calculated position of said valve control element, and a circuit responsive to a difference between the calculated and sensed positions of the valve control element for generating an indicating signal if this difference exceeds a predetermined amount for more than a predetermined time. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:- Figure 1 is a diagram of a plurality of electro-hydraulic actuating systems as applied to the control surfaces of an aircraft, Figure 2 is a more detailed diagram of one of the actuator systems of Figure 1, Figure 3 is a block diagram of an electrical control circuit forming part of the system of Figure 2, Figure 4 is a block diagram of a circuit forming part of the system of Figure 2, Figure 5 is a diagram of part of the circuit of Figure 3, and Figure 6 shows an electro-hydraulic device and a hydraulic actuator forming part of the system of Figure 2. As shown in Figure 1, an aircraft 10 has a plurality of control surfaces including a rudder 11, four spoilers 12 and two ailerons 13 on a main plane 14, a tail plane 15 and two elevators 16 on the tail plane 15. Pitch of the aircraft is controlled by the tail plane 15 and elevators 16, roll by the spoilers 12 and ailerons 13 and yaw by the rudder 11. The rudder 11 is operated by a tandem hydraulic actuator 20 which is responsive to hydraulic pressures from each of two identical control arrangements 21, 22. Each of the control arrangements 21, 22 is responsive to electrical input signals from pilot's control pedals 23, from a control column 24, from a trim signal generator 25, from a pivot pressure sensor 26 and from rate gyros 27, 28, 29 for yaw, roll and pitch respectively. Each of the four spoilers 12 is operated by a separate hydraulic actuator, the outboard spoilers 12 being under control of the arrangement 21, and the inboard spoilers being under control of the arrangement 22. The tail plane 15 is acted upon by two separate hydraulic actuator halves 20A, 20B (Figure 2) which are under control of respective ones of the arrangements 21, 22, the outputs of these actuator halves being mechanically summed by a common shaft. The ailerons 13 and elevator 16 are separately controlled by the control column 24. Figures 2 to 6 show in more detail a part of the system of Figure 1, as it relates to control of the rudder 11. The arrangements 21, 22 are, as shown in Figure 2, generally similar. Only the arrangement 21 will therefore be described in detail. The arrangement 21 includes a control and monitoring circuit 31 which is responsive to signals on line 32 from the yaw rate gyro 27, on line 33 from a first transducer operated by the pedals 23, on a line 34 from a second, monitor transducer operated by the pedals 23 and on a line 35 from the trim signal generator 25. The signals on lines 33, 34 are nominally identical and are used as will be explained hereinafter, to provide monitoring of the input signals to the arrangements 21, 22. The circuit 31 provides an electrical operating signal on a plurality of lines 36 to an electro-hydraulic valve arrangement 37 which, together with the actuator 20 is shown in more detail in Figure 6. A hydraulic supply pressure P1 is applied to the valve arrangement 37 which regulates the hydraulic pressure applied to one half 20A of the tandem actuator 20. As shown in Figure 3, the control and monitoring circuit 31 includes a first summing device 40 responsive to signals on the lines 32, 34, 35 from the gyro 27, pedal monitor transducer and the trim signal generator 25 respectively. A further summing device 41 is responsive to signals on the lines 32, 33, 35. Output signals from the devices 40, 41 are thus respectively dependent on the signals from the monitor and active transducers of the pedals 23, and are applied to respective limiting circuits 42, 43. The signals from the limiting circuits 42, 43 are supplied on respective lines 38, 39, to a trimming circuit 44, shown in more detail in Figure 5. The trimming circuit 44 is responsive to the signal on line 39 to provide operating signals on the lines 36 to the valve arrangement 37, and on a line 46 to a circuit 47 which provides an output signal X1M on a line 48, this signal X1M being an analog model of the position of a control element 49 (Figure 6) of an electrohydraulic servo valve in the valve arrangement 37. The circuit 44 is also responsive to the signal on line 38 to provide a signal on a line 45. The signals on lines 46 and 45 are applied as inputs to a comparator circuit 58 which provides a signal on a line 59 is there is an unacceptable discrepancy between these input signals for more than a predetermined time. A sensed positon X1A of the position of the element 49 is obtained from a suitable transducer 51 and supplied on a line 52 to the control and monitoring circuit 31. The signals X1A on line 52 and X1M on line 48 are also supplied to a comparator circuit 53 which provides an output signal on a line 54 if the difference between signals X1A and X1M exceeds ± 3% of the signal X1M for more than a predetermined time. The trimming circuit 44 is also responsive to a positon feedback signal on a line 55 from a displacement transducer 56 (Figure 6), this signal being responsive to the position of the output element 57 of the actuator 20 and hence to the position of the rudder 11. The equalising circuit 30 is shown in more detail in Figure 4 and is responsive to the aforementioned signals X1M in line 48 and X1A on line 52. The equalising circuit 30 is also responsive to signals X2M and X2A from the control arrangement 22 (Figure 2) and respectively corresponding to the model and sensed positions of an element of the electro-hydraulic servo valve in the arrangement 22. The equalising circuit 30 comprises two identical circuit arrangements 60, 61, only one of which will be described in detail. The arrangement 60 includes a circuit 62, to which the two model signals X1M and X2M are applied. The circuit 62 provides an output signal which corresponds to [(X1M + X2M)/2]-X1M, that is the amount by which X1M differs from the mean of X1M and X2M. This output signal is supplied to the non-inverting input of a difference amplifier 63. Feedback signals are applied on lines 65, 66 to the inverting inputs of the amplifier 63. The output signal from amplifier 63 is supplied through a rate limiting circuit 67 and through a change-over switch device 68 to an integrating amplifier 69 which provies a trimming output signal on a line 70, this trimming signal also corresponding to the value [(X1M + X2M)/2]-X1M. The feedback signal on line 65 is provided only when the output of the amplifier 69 exceeds the values +A or -A, as determined by a limiting circuit 71. A signal on line 65 causes the input to the rate limiting circuit 67 to be removed, and thereby limits the signal on line 70 to a range corresponding to +A and -A. The feedback signal on line 66 is derived from the signal on line 70 through a delay circuit 72 having a time constant K1, the arrangement being such that a step change in the output signal from the circuit 62 results in a slower change in the signal on line 70. In the event of malfunction of the servo valve in the arrangement 22, a circuit corresponding to the circuit 53 provides, in a manner to be described, a signal on the line 130. This signal on line 130 results, in a manner to be described, in operation of the change-over switch device 68 to isolate the amplifier 69 from the signals X1M, X2M and to connect the amplifier 69 to a delay circuit 73 having a time constant K2 of approximately 20 seconds. The circuit 73 provides a delayed negative feedback signal from the trimming signal on line 70. Thus, operation of the change-over switch 68 in response to a malfunction signal causes the signal on line 70 initially to be maintained at the level at which the malfunction occurred and then slowly to reduce to zero as the negative feedback signal is applied by the circuit 73. The signal on line 70 is applied to the trimming circuit 44 (Figure 5) which forms part of the control and monitoring circuit 31. The function of the trimming circuit is, as will be described, to reduce differences between the signals to the electro-hydraulic valves in the respective arrangements 21, 22, in normal operating conditions. Delay of the signal on line 70 allows the switching device 68 to be operated in the event of a malfunction in the arrangement 22 before signals from the circuit 44 attempt to cause the valve arrangement 37 to match the corresponding valve in the arrangement 22. The circuit arrangement 61 is, as described above, identical with the arrangement 60, and is responsive to the sensed position signal X1A of the control element 49 (Figure 6) and the position X2A of the control element in the corresponding electro-hydraulic servo valve in the control arrangement 22 (Figure 2). The circuit arrangement 61 provides a trimming signal on a line 74 to the trimming circuit 44 (Figure 5). As shown in Figure 5, the trimming circuit 44 includes integrating amplifiers 80, 81, 82, a power amplifier 83 and difference amplifiers 84, 85. The input signals on lines 38, 39 from the limiting circuit 42, 43 (Figure 3) are applied to non-inverting inputs of the respective amplifiers 84, 85. The position feedback signal on line 55 from the transducer 56 (Figure 6) is applied to inverting inputs of the amplifiers 84, 85. The signal on line 70 from the equalising circuit 30 Figure 4) is applied to a further non-inverting input of the amplifier 84, and the signal on line 74 from the equalising circuit 30 is applied to a non-inverting input of the amplifier 85. A signal on a line 86 from the amplifier 85 thus corresponds to a difference between desired and sensed positions of the rudder actuator 20, modified in accordance with a difference between the operating position X1A of the electro-hydraulic valve 50 and the mean of that position and the position X2A of the corresponding valve in the control arrangement 22 (Figure 2). The signal on line 86 from amplifier 85 is applied by way of a limiting circuit 87 to the amplifier 81, the circuit 87 effectively limiting the rate of response of the amplifier 81. The signal on line 52 from the transducer 51 (Figure 6) is applied to an inverting input of the amplifier 82 and the output signal on a line 88 from amplifier 81 is applied to non-inverting inputs of amplifiers 82, 83. The output signal from amplifier 82 is applied to a further non-inverting input of the power amplifier 83. The output signal on line 88 is limited by the amplifier 81 so as to correspond to an output current of 10mA from the power amplifier 83. The output signal from amplifier 82 is limited to correspond to an output current of 1.8mA from the power amplifier 83. The power amplifier 83 provides the operating signal on line 36 to servo valve 50 (Figure 6). The signal on line 36 is a demand signal for an operating position of the electrohydraulic valve 50 and is limited by the amplifier 82 to a maximum of 118% of the position demanded by the signal on line 88. Thus even if a large discrepancy arises between the demanded operating position and the sensed position indicated by the feedback signal X1A on line 52, as a result of some deficiency in the valve 50, an overdrive signal applied in an attempt to correct the discrepancy will be limited to the 18% value from the amplifier 82. This has the effect that severe deficiencies in the valve 50 will be detected by the comparator 53 (Figure 3) instead of being masked by a high overdrive signal. Additionally the valve 50 is less likely to be urged substantially past its selected position, and thereby give rise to additional and more severe failures. The output signal on line 46 from the amplifier 81 is responsive to a difference between desired and sensed positions of the actuator 20, as modified by the signal on line 74, and is applied to the analog model circuit 47 and the comparator circuit 58 (Figure 3) as previously described. The effect of the signal on line 74 from the circuit 30 is that the operating signals on line 36 to the servo valve 50 (Figure 6) and on line 46 to the model circuit 47 are increased or decreased so as to render the operating position of the control element 49 in that valve equivalent to the operating position of the corresponding control element in the control arrangement 22. The operating pressures to the two halves of the tandem actuator 20 are thus maintained equal in normal operation of the system. The signal on line 46 to the analog model circuit 47 is similarly responsive to the trimming signal on line 74 from the circuit 30, so that the calculated values X1M and X2M are maintained substantially equal to each other and to the signals X1A and X2A. In normal operating conditions, therefore, correction of the operating signal on line 36 to bring the servo valve 50 in to an operating condition corresponding to that of the servo valve in the arrangement 22 is matched by a corresponding correction to the model circuit 47, whereby the comparator 53 provides no output indication on line 54. The equalising circuit 75 (Figure 2) in the control arrangement 22 is similarly responsive to the values X1M, X2M, X1A and X2A and provides signals on line 76, 77, these signals respectively corresponding to (X1M + X2M)/2]-X2M and [X1A + X2A)/2]-X2A. The servo valves in both arrangements 21 and 22 are thus urged to a mean position. The model circuits in both arrangements are also controlled to supply corresponding output signals. As shown in Figure 6, the electro-hydraulic valve arrangement 37 includes the servo valve 50 and is connected to a first supply pressure P1 by a line 100 and with a first return pressure R1 by a line 101. Pressures P1 and R1 communicate with the servo valve 50 through a combined shut-off and by-pass spool control element 102. The element 102 is urged by springs 103, 104 to a position (shown) in which the servo valve 50 is isolated from the lines 100, 101 and in which there is a by-pass connection between two chambers 105, 106 of the actuator half 20A. In this position a land 107 in the element 102 provides a flow restriction between the chambers 105, 106. The element 102 can be urged rightwardly to its normal operating position by a plunger 108 which is responsive to a pressure in a line 109 from the corresponding valve arrangement in the control arrangement 22. The arrangement is such that pressure is present in the line 109 in the absence of a malfunction in the arrangement 22. In normal operation a solenoid valve 110 is urged by the pressure P1 to a position in which this pressure is admitted to a chamber 111, the greater area of the spool element 102 adjacent the chamber 111 causing the element 102 to be moved leftward against the force exerted by the plunger 108. In this condition of the solenoid valve 110 the pressure P1 is also applied through a line 112 to the corresponding shut-off and by-pass valve in the arrangement 22, the pressure in line 112 being applied to a plunger corresponding to the plunger 108. The pressure in line 109 is derived from a solenoid valve, corresponding to the valve 110, in the arrangement 22. In the event of a malfunction in the arrangement 21 a signal is applied on a line 113 to urge the solenoid valve 110 to the position shown, connecting the chamber 111 and the line 112 to the return pressure R1, whereupon the spool of the element 102 moves rightward to the position shown, shutting off pressures P1 and R1 from the servo valve 50. The reduced pressure in line 112 ensures that the corresponding shut-off and by-pass valve in the arrangement 22 remains in its leftward, operating condition in the absence of a malfunction in the arrangement 22. If both of the arrangements 21, 22 should malfunction, both of the shut-off and by-pass valves will operate, and the two chambers of the respective halves of the actuator 20 will be interconnected through the flow restriction provided by the land 107 and by the corresponding land in the arrangement 22. The actuator 20 may then be moved by aerodynamic forces on the rudder 11 to a position of no-load. A pressure switch 114 provides an indicating signal on a line 115 when the pressure P1 is at its normal operating level. The transducer 56 comprises two differential transformers which supply position feedback signals to the arrangements 21, 22 respectively. The sum of the output voltages of the secondary windings of a differential transformer will, in correct operation, be substantially constant. These secondary winding voltages are summed, for the separate transformers, in a monitoring circuit 116. In the event that the sum of the voltages from the secondary windings departs by more than a predetermined amount from the aforesaid constant value, a signal is provided on a line 117 or 118, dependent on which of the transformers has malfunctioned. As shown in Figure 2, the arrangements 21, 22 are responsive to separate input hydraulic pressures P1 and P2, and to separate return pressures R1 and R2. As indicated in that figure, moreover, the following signals are applied to an OR gate 120, whose output provides the signal on line 113 to operate the solenoid valve 110 (Figure 6):- (i) a signal on line 59 from the comparator circuit 58 (Figure 3) indicating an unacceptable discrepancy between the signals on lines 45 and 46; (ii) a signal on the line 54 from the comparator circuit 53 indicating an unacceptable discrepancy between the signals on lines 52 and 48 from the servo valve transducer 51 and the modelling circuit 47 respectively; (iii) a signal on line 117 from the monitoring circuit 116 indicating that the differential transformer which provides the signal on line 55 has malfunctioned. Any of these signals will result in a signal on line 113 which renders the arrangement 21 inoperative, energises an indicating device 119, and operates a change-over switch in the equalising circuit 75, corresponding to the switch 68 in the equalising circuit 30 (Figure 4). Any corresponding malfunction in the arrangement 22 will render that arrangement inoperative and provide a signal on the line 130 (Figures 2 and 4) to operate the change-over switch in the equalising circuit 30.

Claims: An electro-hydraulic actuator system comprising a hydraulic actuator (20A), a position sensor (56) on said hydraulic actuator (20A), a circuit (81, 85) for generating an error signal which is dependent on a difference between desired and sensed positions of said actuator (20A), an electro-hydraulic valve (37) having a flow control element (49, 102) responsive to an electrical input signal for regulating a supply of pressurised fluid to said actuator (20A), a position sensor (51) on said flow control element (49, 102), and means (83) for deriving said input signal from said error signal, a device (82) responsive to said error signal and to a signal dependent on the sensed position of said valve control element (49, 102), for limiting the magnitude of a change in said input signal in response to a difference between said error signal and said sensed valve positon signal, characterised in that said system includes means (47) responsive to said error signal, for generating a signal which corresponds to a calculated position of said valve control element (102), and a circuit (53) responsive to a difference between the calculated and sensed positions of the valve control element (49, 102) for generating an indicating signal if that difference exceeds a predetermined amount for more than a predetermined time. A system as claimed in Claim 1 in which said means (82) for limiting the magnitude of a change in said input signal comprises means for generating a control signal whose magnitude is a fixed fraction (18%) of the magnitude of said error signal, said means (83) for deriving said input signal being responsive to the sum of said error and control signals.

Inventors: BURRAGE, ROBERT GRAHAM, GLAZE, STANLEY GEORGE
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1985
Main classification: F15B9
IPC codes: B64C13, F15B9, G05D1, F15B18