Actuator hardover monitor

1. An actuator hardover monitor for a control surface, comprising:

an actuator sensor for detecting an actuator position of a first control-surface actuator configured to control the control surface;

a computer having a processor for executing software instructions stored in non-transitory memory, the software instructions comprising:

a command model of an expected position of the control-surface actuator based on an input command;

a command delay for delaying the input command by a time lag to synchronize the command model with a deflection of the control surface; and

a monitor to determine whether a difference between the actuator position and the expected position exceeds a threshold for a predetermined duration; and

a switch for switching to a second control-surface actuator configured to control the control surface when the difference exceeds the threshold for at least the predetermined duration.

2. The actuator hardover monitor of claim 1 , further comprising a controller to adjust the actuator position of the first control-surface actuator to reduce the difference between the actuator position and the expected position.

3. The actuator hardover monitor of claim 1 , wherein the threshold comprises a softstop limit corresponding to a deflection range based on an aircraft parameter that is within a physical capability of the control surface.

4. The actuator hardover monitor of claim 1 , further comprising a control surface sensor for detecting a control-surface position such that the monitor determines whether a difference between the control-surface position and the expected position exceeds the threshold for the predetermined duration.

5. The actuator hardover monitor of claim 1 , wherein the monitor determines whether a difference between the control-surface position and the actuator position exceeds the threshold for the predetermined duration.

6. A method of mitigating unintended deflection of a control surface, comprising:

providing, from a sensor, a position signal indicative of a position of the control surface;

providing a softstop limit corresponding to a deflection range of the control surface based on an aircraft parameter;

determining when the position signal exceeds the softstop limit;

monitoring the position signal for a first duration; and

switching from a primary control-surface actuator to a backup control-surface actuator for driving the control surface, when the position signal persists for at least the first duration.

7. The method of claim 6 , further comprising, prior to the step of switching, preparing a switch by delaying the switching step to enable the backup control-surface actuator time to establish control over the control surface for providing a smooth transition from the primary control-surface actuator to the backup control-surface actuator.

8. The method of claim 6 , further comprising filtering the position signal based on a first time constant to provide a first filtered position signal.

9. The method of claim 8 , further comprising:

filtering the position signal based on a second time constant, different than the first time constant, to provide a second filtered position signal;

monitoring the second filtered position signal for a second duration, different than the first duration; and

switching from the primary control-surface actuator to the backup control-surface actuator when the first filtered position signal persists for at least the first duration and the second filtered position signal persists for at least the second duration.

10. A method of anticipating and mitigating unintended deflections of a control surface, comprising:

commanding an actuator valve to a commanded position, wherein the actuator valve is configured to control the control surface;

determining continuously when the commanded position exceeds an actuator valve limit to provide an exceedance; and

switching to a backup control-surface actuator when the exceedance exceeds the actuator valve limit for a predetermined duration.

11. The method of claim 10 , further comprising filtering the exceedance based on a time constant to provide a filtered exceedance.

12. The method of claim 10 , further comprising modeling the actuator valve position based on a physics-based computational model of the actuator valve and the commanded position.

13. The method of claim 10 , wherein the step of determining comprises evaluating the commanded position as a binary state such that the exceedance is determined to be any absolute value above the actuator valve limit.

14. The method of claim 10 , wherein the step of determining comprises evaluating the commanded position as a range such that the exceedance is determined to be any value outside a predetermined range.

15. The method of claim 10 , wherein the step of determining comprises evaluating the commanded position as a range with a correct sign such that exceedance is determined to be any value outside the predetermined range in a predetermined direction.

16. The method of claim 10 , wherein the step of determining comprises evaluating the commanded position by dynamically tracking the actuator valve position within a predetermined tolerance using a sensor.

17. The method of claim 10 , further comprising modifying the actuator valve limit by a scaled factor to determine a threshold, and determining continuously when the commanded position exceeds the threshold to provide the exceedance.

18. A method of correcting uncommanded motion of a control surface of an aircraft, comprising:

providing an actual position of the control surface from a position sensor;

modeling an expected position of the control surface based on a control input;

determining a difference between the actual position and the expected position;

providing a limit for the control surface position based on an aircraft parameter;

determining whether the difference exceeds the limit for a predetermined duration; and

switching to a backup control surface actuator when the difference exceeds the limit for at least the predetermined duration.

19. The method of claim 18 , further comprising filtering a signal of the difference based on a time constant.

20. The method of claim 18 , wherein the difference is due to an incorrect direction of motion.

21. The method of claim 18 , wherein the difference is an overshoot of the actual position above the expected position.

22. The method of claim 18 , wherein the difference is an undershoot of the actual position below the expected position.

23. The method of claim 18 , wherein the aircraft parameter is from the group consisting of airspeed, estimated airspeed, stabilizer trim position, and flap position.

24. The method of claim 18 , further comprising resetting the expected position to equal the actual position such that the difference is zero prior to the step of switching to a backup control surface actuator.

25. A method of mitigating unintended deflection of a control surface, comprising:

providing a measured position of the control surface based on a control-surface sensor;

deriving a control surface rate based on the measured position of the control surface;

providing an expected position of the control surface based on a position of an actuator valve from an actuator-valve sensor, wherein the actuator valve is configured to control the control surface;

deriving an actuator rate based on the position of the actuator valve;

determining a difference between the control surface rate and the expected position actuator rate;

determining whether the difference exceeds a predetermined threshold for a predetermined duration; and

switching to a backup actuator valve when the difference is greater than the predetermined threshold for at least the predetermined duration.

26. The method of claim 25 , further comprising filtering a signal of the difference based on a time constant to provide a filtered signal difference.