Control system utilizing fault detection

• US 5,949,677 A
• Filed: 01/09/1997
• Issued: 09/07/1999
• Est. Priority Date: 01/09/1997
• Status: Expired due to Fees

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Ho, John K.
• Primary Examiner(s): Elmore, Reba I.
• Assistant Examiner(s): Natnithithadha, Navin
• Application Number:
  US08/780,906
• Time in Patent Office: 971 Days
• Field of Search: 364/184, 364/185, 364/183, 364/187, 364/140.04, 364/148.01, 364/150, 364/153, 364/724.011, 364/737, 701/8, 701/9, 701/58, 701/59, 701/76, 701/99
• US Class Current: 700/80
• CPC Class Codes: G05B 23/0237 based on parallel systems, e.g. comparing signals produced at the same time by same type systems and detect faulty ones by noticing differences among their responses
• CPC Class Codes: G05B 9/03 with multiple-channel loop, i.e. redundant control systems

Abstract

A control architecture having improved fault detection and correction capabilities is disclosed. The system comprises primary and monitor control systems, each having an associated control signal. A fault detector generates an alarm signal based upon differences observed between the primary and monitor control signal. The detector comprises an integrator and a memory means, and alarm signals are generated based upon the total amount of difference observed over a predetermined period of time. In one embodiment of the invention, primary and monitor control signals are averaged to provide a signal that is more fault tolerant than the individual control signals.

Claims

1. An apparatus for controlling a plant comprising:

a primary control system having a primary control signal;

a monitor control system having a monitor control signal;

a plurality of difference signals, each difference signal corresponding to the difference between said primary control signal and said monitor control signal at a fixed point in time; and

a fault detector comprising;

a filter transforming each of said difference signals into absolute difference values corresponding to the absolute difference between each of said difference signals and a magnitude threshold;

a memory storing each of said absolute difference values for at least a pre-determined period of time; and

an integrator in communication with said memory, said integrator maintaining a running accumulation of the absolute difference values observed during only the pre-determined period of time.

2. The apparatus of claim 1 further comprising means for computing an average of said primary control signal and said monitor control signal, wherein said average is provided to the plant.

3. The apparatus of claim 1 wherein said filter comprises a deadband filter.

4. The apparatus of claim 3 wherein said filter further comprises an absolute value function.

5. The apparatus of claim 4 wherein said filter comprises a signal limiter.

6. The apparatus of claim 5 wherein said filter comprises a frequency shaping filter.

7. The apparatus of claim 1 wherein said fault detector is implemented in a digital computer.

8. The apparatus of claim 7 wherein said filter is implemented in a digital computer.

9. The apparatus of claim 1 wherein the integrator computes the total of the absolute difference values observed over only a predetermined period of time by adding each absolute difference value substantially at the fixed point in time and by subtracting each difference signal after the pre-determined period of time has elapsed.

10. A method of detecting faults in a control system, the method comprising the steps of:

determining differences between a primary control signal and a monitor control signal at regular time intervals;

determining absolute differences between the differences and a threshold value;

adding said absolute differences to a total absolute difference signal;

retaining each of said absolute differences in a memory for at least a pre-determined period of time;

subtracting each of said absolute differences from said total absolute difference signal after said predetermined period of time has expired; and

providing an alarm signal when said total absolute difference signal exceeds a maximum allowable difference value.

11. The method of claim 10 further comprising the step of averaging said primary control signal and said monitor control signal to generate an input to a plant.

12. The method of claim 10 further comprising the step of filtering said differences prior to retaining said differences in said memory.

13. The method of claim 11 further comprising the step of filtering said differences prior to retaining said differences in said memory.

14. A method of controlling a plant comprising the steps of:

providing a primary control signal (P) and a monitor control signal (M);

averaging the primary control signal and the monitor control signal to produce an average control signal that is provided to the plant;

subtracting said average control signal from said primary control signal to obtain a resultant signal substantially equal to (P-M)/2; and

multiplying said resultant signal by two to obtain an error signal substantially equal to P-M.

15. The method of claim 14 further comprising the step of providing said error signal to a fault detector.

16. The method of claim 15 wherein said fault detector comprises an integrator and a memory means.

17. The method of claim 16 further comprising the steps of:

determining absolute differences between the error signal and a threshold value at discrete points in time;

adding said absolute differences to a total absolute difference signal at the integrator;

retaining each of said absolute differences in the memory for at least a pre-determined interval;

subtracting each of said absolute differences from said total absolute difference signal after said predetermined interval has expired; and

providing an alarm signal to the plant when said total absolute difference signal exceeds a maximum allowable difference value.

18. An apparatus for controlling a plant comprising:

a primary control system having a primary control signal;

a monitor control system having a monitor control signal;

a plurality of difference signals, each difference signal being related to the difference between the primary control signal and the monitor control signal at a fixed point in time; and

a fault detector having an integrator, the integrator maintaining a representation of the total of the difference signals observed over only a pre-determined interval, the fault detector providing an error signal to the plant when the total of the difference signals exceeds a threshold value.

19. The apparatus of claim 18 wherein the fault detector further comprises a memory in communication with the integrator, wherein the memory retains each difference signal for at least the pre-determined interval.

20. The apparatus of claim 19 wherein the integrator computes the total of the difference signals observed over only a predetermined interval by adding each difference signal substantially at the fixed point in time and by subtracting each difference signal after the pre-determined interval has elapsed.

21. The apparatus of claim 18 wherein the plurality of difference signals are computed by filtering logic.

22. The apparatus of claim 21 wherein each of the plurality of difference signals are absolute difference signals substantially related to the difference between the primary control signal and the monitor control signal minus a second threshold value.

23. The apparatus of claim 18 further comprising a hybrid control signal that is provided to the plant, the hybrid control signal being substantially equal to the average of the primary control signal and the monitor control signal.