Method and system for detecting integrity of a control loop of a physical system
In a control loop, a command signal is applied to physical components to cause movement of the physical components. The position of the physical components is detected and used to generate subsequent command signals. In an implementation, an integrity detection system causes an excitation signal to be injected onto the command signal to cause movement of the physical components at an excitation frequency. This movement is detected in the physical system control loop and isolated by the integrity detection system. If the integrity detection system fails to detect energy in the excitation frequency band, a failure of at least one component of the control system is inferred.
1. An apparatus for controlling a closed loop control system of a physical system, the apparatus comprising:
a controller configured to output a command signal to control a state of a plant of the physical system and to receive a feedback signal providing information about the state of the plant; and
an integrity detection system, wherein the integrity detection system is configured to:
output an excitation signal to the plant;
receive the feedback signal;
determine whether a component associated with the excitation signal is present in the feedback signal; and
identify whether or not the closed loop control system is experiencing a failure based the determination;
wherein the excitation signal has energy in a frequency spectrum,
wherein the integrity detection system comprises:
a first filter for filtering the feedback signal in a first frequency band encompassing at least a portion of the excitation signal frequency spectrum;
a level detector for measuring the filtered feedback signal level;
a noise level estimator for providing a noise level estimate of the feedback signal in the first frequency band; and
a signal ratio computation block for computing a signal ratio, wherein the signal ratio is the ratio of the filtered feedback signal to the noise estimate.
2. The apparatus for controlling a physical system of claim 1 , further comprising a comparator for comparing the computed signal ratio to a predetermined threshold.
3. The apparatus for controlling a physical system of claim 2 , wherein the integrity detection system is configured to cause the control system to enter a failsafe mode of operation when the computed signal ratio is less than the predetermined threshold.
4. The apparatus for controlling a physical system of claim 1 , wherein the noise level estimator comprises a second filter for filtering the feedback signal in a second frequency band.
5. The apparatus for controlling a physical system of claim 4 , wherein the second frequency band is different than the first frequency band but is sufficiently close to the first frequency band in order to provide an estimate of the noise level of the feedback signal in the first frequency band.
6. The apparatus for controlling a physical system of claim 1 , further comprising a variable gain amplifier for adjusting a gain of the excitation signal based on the noise level estimate.
7. The apparatus for controlling a physical system of claim 1 , wherein the excitation signal output is combined with the command signal.
8. A method of detecting integrity of a closed loop control system of a physical system, the method comprising:
outputting a command signal by a controller to control a state of a plant of the physical system;
receiving, by the controller, a feedback signal providing information about the state of the plant;
outputting an excitation signal into the closed loop control system;
receiving the feedback signal;
determining whether a component associated with the excitation signal is present in the feedback signal;
identifying whether or not the closed loop control system is experiencing a failure based the determination;
filtering the feedback signal in a first frequency band encompassing at least a portion of an excitation signal frequency spectrum;
measuring the filtered feedback signal level;
estimating a noise level of the feedback signal in the first band; and
computing a signal ratio, wherein the signal ratio is the ratio of the filtered feedback signal to the noise estimate.
9. The method of claim 8 , further comprising comparing the computed signal ratio to a predetermined threshold.
10. The method of claim 9 , further comprising causing the control system to enter a failsafe mode of operation when the computed signal ratio is less than the predetermined threshold.
11. The method of claim 8 , wherein estimating the noise level comprises filtering the feedback signal in a second frequency band.
12. The method of claim 11 , wherein the second frequency band is different than the first frequency band but is sufficiently close to the first frequency band in order to provide an estimate of the noise level of the feedback signal in the first frequency band.
13. The method of claim 8 , further comprising adjusting an amplitude of the excitation signal based on the noise level estimate.
14. The method of claim 8 , wherein the excitation signal output is combined with the command signal.