ASYMMETRIC TORQUE SWITCHING FOR AIRCRAFT AUTOPILOT

Abstract: Apparatus for providing asymmetrical torque-switching for an aircraft autopilot. The invention provides an increased amount of available pitch servo torque at high altitude. The direction of pitch of the aircraft is sensed (23, 25). An increased amount of torque is provided for the pitch-up direction, thereby providing more positive control of the aircraft.

Description: This invention relates to the automatic control of the pitch axis of an aircraft. Specifically, it applies to safe, enhanced pitch axis control of an aircraft at high altitude. In prior art torque switching aircraft autopilots, automatic pitch control of the aircraft has typically included circuitry for providing a high servo torque level at low altitude and a low servo torque level at high altitude. This dual state torque level is incorporated so that the aircraft will not be overcontrolled and become unsafe should a malfunction cause the aircraft suddenly to experience a maximum pitch-down condition. For high altitude operation, the U.S. Federal Aviation Administration (FAA) permits a higher level of pitch servo torque in the pitch-up direction than in the pitch-down direction if a G-monitor is incorporated into the autopilot. However, previous autopilots provided the same low servo torque in both pitch directions at high altitude thus providing a safe level of operation in the pitch-down direction, but less than optimal autopilot operation in the pitch-up direction. The present invention is defined in the appended claims and comprises an asymmetric torque switching circuit which provides an increased amount of available pitch servo torque in the pitch-up direction at high altitude. A G-Monitor disengages the autopilot during a pitch malfunction before FAA established limits are exceeded. The present invention is also an asymmetrical torque switching circuit which provides an increased amount of available pitch servo torque at high altitude which comprises two voltage sense buffers each coupled to a polarity detector. Each voltage sense buffer/polarity detector determines whether the aircraft is in a pitch-up or pitch-down state. If the aircraft is pitching up and in the high altitude state, the first voltage sense buffer outputs a signal placing an autopilot current limiter and current monitor in the high current position, thereby increasing the available pitch servo torque when the aircraft is in a pitch-up condition. In the pitch-down condition while in the high altitude state, no signal is output from the voltage sense buffer/detector and the autopilot current limiter and current monitor remains in the low current state. If the aircraft is pitching up and in the high altitude state, the second buffer/polarity detector outputs a signal to disable the warning annunciator. The present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:- Figure 1 illustrates a typical two-state torque switching circuit with the asymmetrical torque switching apparatus of the present invention enclosed in a broken line, and Figure 2 illustrates a G-Monitor circuit used in conjunction with the present invention. Figure 1 illustrates the structure of the asymmetric torque switching circuit 1 for a typical autopilot in which a standard pitch axis control signal 2 is sent to a dual-state current limiter 2a. The output of the limiter 2a is applied to an elevator servo amplifier 3 which drives a servo actuator 4. The output of the actuator 4 drives an aircraft elevator control surface (not shown) through a clutch 5. The current limiter 2a limits the amount of torque the actuator 4 can apply through the clutch 5 to the control surface (not shown) should a malfunction occur in the autopilot's pitch control law command signal 2. A non-limited malfunction of the pitch control law signal 2 to the servo amplifier 3 would result in an unsafe aircraft manoeuvre. The state of the current limiter 2a is set by a switch 6. The high current or low current position of the switch 6 is selected through an OR circuit 27 by a comparator 7 which compares the actual aircraft altitude 8 and a predetermined altitude switch point 9. If actual altitude 8 is above the predetermined altitude switch point 9, the output of OR circuit 27 places the switch 6 in the low current position. If actual altitude 8 is below the predetermined altitude switch point 9, the switch 6 is placed in the high current position. Thus, two maximum torque levels are available from the servo actuator 4 to drive the elevator control surfaces; a lower current level at altitudes above the switch point, and a higher current level at altitudes below the switch point. The actual current limit and switch point altitude are selected from flight test data to limit a pitch malfunction to a safe aircraft manoeuvre. In addition to the current limiter 2a, a current monitor comparator 10 is incorporated to ensure that the output of the servo amplifier 3 is not excessive due to a current limiter 2a or a servo amplifer 3 malfunction. Should an excessive current condition occur, it is detected by the current monitor comparator 10. An output signal is sent through an OR circuit 32 to disengage circuitry 11 which will cause the servo clutch 5 to disengage the control surfaces. The current monitor 10 compares the actual servo current sensed from a resistor 12 and the high or low current level signal from a switch 13 the position of which is controlled by an altitude comparator 7 through an OR circuit 30. To ensure that the current limiter switch 6 has not erroneously switched to an incorrect state, the output signal 14a of a barometric switch 14 and current limiter control signal 15 are compared by an AND circuit 16. If the current limiter switch 6 is set to the high current state (corresponding to low altitude) and the barometric switch 14 is in the high altitude position, the AND circuit 16 will output a signal to illuminate a torque warning annunciator 17. As a check that the barometric switch 14 has not failed, an additional comparator 18 compares the barometric switch output 14a with the actual aircraft altitude signal 8. The torque warning annunciator 17 is controlled by an OR circuit 19 which will illuminate the annunciator 17 if a barometric switch miscomparison signal 20 or a torque state miscomparison signal 21 is present at the input of the OR circuit 19. Asymmetrical torque switching of the present invention is accomplished as follows: The polarity of a servo amplifier output signal 22 is sensed by a voltage sense buffer 23 and voltage sense buffer/detector 24. The output voltage sense buffer 23 drives a polarity detector 25 that determines which direction the servo actuator 4 is pitching the aircraft. If a pitch-up condition exists, a polarity detector output signal 26 is provided to an OR circuit 27 which will set the current limiter switch 6 into the high current position, regardless of whether the actual altitude 8 is above or below the altitude switch point 9. The pitch-up signal 26 is also supplied to a driver circuit 28, the output signal 29 of which is supplied to an OR circuit 30 which sets the current monitor switch 13 to the high current state if the elevator servo actuator 4 is providing a pitch-up signal to the servo clutch 5. In accordance with the present invention, a pitching up command to the servo actuator while switches 6 and 13 are in the low current position will be sensed by the voltage sense buffer 23 and automatically cause switches 6 and 13 to be switched to the high current position as previously described. However, this would normally cause illumination of the torque warning annunciator 17. To inhibit the annunciator 17, an output signal 31 from the voltage sense buffer/detector 24 is inverted by an invertor 23 and supplied to an AND circuit 34 which combines the signal from the torque state comparison and circuit 16 and the invertor 33. The output of the AND circuit 34 drives the torque warning annunciator 17 through the OR circuit 19. Thus, a pitch-up condition will automatically place the torque switching circuitry into the high torque state and will prevent the torque warning annunciator 17 from illuminating (unless a barometric switch fault is detected). The additional torque obtained by this method in the pitch up direction greatly enhances the performance capabilities of the autopilot in controlling the aircraft. When the control law attempts to control with more torque authority than is available, poor performance, for example oscillations in the flight path is usually the result. A G-monitor circuit 50 shown in Figure 2 generates a disengage signal 46 if excessive aircraft G-loading is detected. The G-monitor as used with the asymmetric torque-switching circuit in Figure 1 assures that there will be a low probability of a pitch malfunction causing an unsafe pitch manoeuvre. The G-monitor 50 is required with the asymmetrical switching feature since a larger pitch malfunction manoeuvre is possible in the pitch-up direction than with the standard torque-switching. The G-monitor 50 operates as follows: The output signal from a normal accelerometer 36 and roll attitude compensation signal 36a from the autopilot (not shown) are subtracted in a summer 37. A roll normalised signal 38 is filtered in a noise filter 39 and applied to an upper G-level comparator 40, and a lower G-level comparator 41. The upper level comparator 40 compares the filtered normalised acceleration signal 42 with a predetermined upper G-level limit 43. The lower level comparator 41 compares the filtered, normalised acceleration signal 42 with a predetemined lower G-level limit 44. The outputs of the comparators 40 and 41 are applied to delay circuit 45. The delay circuit output 46 will only activate if either the upper level comparator output signal 47 or the lower level comparator output signal 48 activates for a predetermined time interval (typically 200 ms.). If either of the comparator signals 47 or 48 reset, then the delay resets, and the comparators 40 and 41 must again activate for the full delay time to get a disengage signal 46. This resetting delay prevents turbulence from tripping the G-monitor 50. The activation of the delay causes a signal 46 to be applied through an OR circuit 32 shown in Figure 1 to the disengage circuitry.

Claims: Apparatus for providing asymmetrical torque switching at high altitude in an aircraft autopilot of the type having a first and second state current limiter (2a) for limiting maximum pitch manoeuvres of the aircraft, a current monitor (10) for disengaging the autopilot in case of malfunction and a torque warning annunciator (17) for alerting aircraft crew if the autopilot malfunctions characterised in that the apparatus comprises first sensing means (23,25) for determining the polarity of a signal representative of the direction of pitch of the aircraft and for providing a first signal when the aircraft pitch is upward, means (27) coupled to the first sensing means for providing a first switching signal to the autopilot for switching the current limiter (2a) to a first state upon receipt of the first signal, means (30) coupled to the first sensing means for providing a second switching signal to the autopilot for switching the current monitor (10) to the first state upon receipt of the first signal, second sensing means (24) for determining the polarity of said signal representative of direction of pitch of the aircraft and for providing a second signal when the aircraft pitch is upward, means (33, 34, 19) coupled to the second sensing means for providing a third switching signal to the autopilot for inhibiting the output of the annunciator (17) upon receipt of the second signal, and means (50) to monitor G-loading of the aircraft for providing a signal to disengage the autopilot when the aircraft G-load exceeds a predetermined limit. Apparatus according to claim 1, characterised in that the first sensing means includes a voltage sense buffer (23) and a polarity detector (25). Apparatus according to claim 1 or 2, characterised in that the means coupled to the first sensing means (23, 25) for providing a first switching signal comprises an OR logic circuit (27). Apparatus according to any of claims 1 to 3, characterised in that the means coupled to the first sensing means (23, 25) for providing a second switching signal comprises a OR logic circuit (30). Apparatus according to any of the preceding claims characterised in that the means coupled to the second sensing means (24) for providing a third switching signal comprises an AND logic circuitry (34). Apparatus according to any of the preceding claims, characterised in that the means to monitor G-loading of aircraft comprises: summer means (37) for receiving first and second input signals representative of normal acceleration and roll attitude compensation and for providing an output signal, the output signal being the difference of said first and second input signals, first comparator means (40), coupled to receive the summer output signal and a signal representative of a predetermined upper G-level limit, for providing a first comparator output signal when the summer output signal exceeds the signal representative of the upper G-level limit, second comparator means (41), coupled to receive the summer output signal and a signal representative of a predetermined lower G-level limit, for providing a second comparator output signal when the summer output signal is less than the signal representative of the lower G-level limit, delay means (45) coupled to receive the first and second comparator output signals and for providing a disengage output signal when either the first or second comparator output signal is received by the delay means for a selected time period.

Inventors: BIRKEDAHL, BYRON FOSTER
Applicants: HONEYWELL INC.
Application year: 1986
Main classification: G05D1
IPC codes: G05D1