SYSTEMS AND METHODS FOR ADAPTIVE SAMPLING RATE ADJUSTMENT

• US 20140257528A1
• Filed: 03/11/2013
• Published: 09/11/2014
• Est. Priority Date: 03/11/2013
• Status: Active Grant

• Current Assignee: Tyco Fire & Security GmbH (Johnson Controls International plc)
• Original Assignee: Johnson Controls Technology Company (Johnson Controls International plc)
• Inventors: Salsbury, Timothy I., Perez, Carlos Felipe Alcala
• Granted Patent: US 9,395,708 B2
• Time in Patent Office: Days
• US Class Current: 700/46
• CPC Class Codes: G05B 13/042 in which a parameter or coefficient is automatically adjusted to optimise the performance
• CPC Class Codes: G05B 13/047 the criterion being a time optimal performance criterion

Abstract

A method for adaptively and automatically adjusting a sampling rate in a feedback control system includes monitoring an error signal for a disturbance event, evaluating the error signal in response to the disturbance event to estimate a time constant of a control process, and determining a sampling rate for use in the feedback control system based on the estimated time constant. The error signal may be based on a difference between a setpoint and a feedback signal and the feedback signal may be received from the control process.

Claims

1. A method for adaptively and automatically adjusting a sampling rate in a feedback control system, the method comprising:

monitoring an error signal for a disturbance event, wherein the error signal is based on a difference between a setpoint and a feedback signal, wherein the feedback signal is received from a control process;

evaluating the error signal to estimate a time constant of the control process in response to a detected disturbance event;

determining and using a sampling rate for the feedback control system, wherein the sampling rate is determined based on the estimated time constant.

2. The method of claim 1, wherein monitoring the error signal for the disturbance event includes classifying the disturbance event as at least one of:

a setpoint change and a load disturbance.

3. The method of claim 2, wherein if the disturbance event is classified as a setpoint change, evaluating the error signal in response to the disturbance event includes:

identifying a portion of the error signal estimated to be associated with the disturbance event;

tracking an area bounded by the identified portion error signal; and

dividing the area bounded by the identified portion of the error signal by a magnitude of the setpoint change to determine the estimated time constant of the control process.

4. The method of claim 3, wherein identifying a portion of the error signal estimated to be associated with the disturbance event includes:

determining a time at which the setpoint change occurs;

determining a time at which a magnitude of the error signal crosses a magnitude threshold;

defining a tracking interval beginning at a time based on the time at which the setpoint change occurs and ending at a time based on the time at which the magnitude of the error signal crosses the magnitude threshold; and

identifying a portion of the error signal which occurs during the tracking interval.

5. The method of claim 2, wherein tracking an area bounded by the identified portion of the error signal includes:

determining a magnitude of the error signal at a plurality of discrete time steps along the identified portion of the error signal;

determining an integrated error for each time step by multiplying the magnitude of the error signal at each time step by a duration of an interval between the time steps; and

summing the integrated errors associated with each of the time steps along the identified portion of the error signal.

6. The method of claim 2, wherein if the disturbance event is classified as a load disturbance, evaluating the error signal in response to the disturbance event includes:

identifying a portion of the error signal estimated to be associated with the disturbance event;

tracking a magnitude of the error signal during the identified portion to determine a time at which the magnitude of the error signal reaches a maximum in response to the load disturbance;

subtracting a time at which the magnitude of the error signal begins to increase in response to the load disturbance from the time at which the magnitude of the error signal reaches the maximum to determine the estimated time constant of the control process.

7. The method of claim 1, wherein evaluating the error signal to estimate a time constant is not conducted during steady-state operation of the feedback control system.

8. The method of claim 1, wherein determining a sampling rate for use in the feedback control system includes selecting a sampling interval which is a fraction of the estimated time constant.

9. The method of claim 1, wherein determining a sampling rate for use in the feedback control system includes selecting a sampling interval between one-fortieth of the estimated time constant and the estimated time constant.

10. The method of claim 1, further comprising:

comparing the determined sampling rate with a previous sampling rate used by the feedback control system; and

updating the previous sampling rate to the determined sampling rate if the comparison reveals that one or more updating criteria are met.

11. The method of claim 10, wherein the updating criteria include updating the previous sampling rate to the determined sampling rate if the determined sampling rate is at least twice the previous sampling rate.

12. The method of claim 10, wherein the updating criteria include updating the previous sampling rate to the determined sampling rate if the determined sampling rate is no greater than half the previous sampling rate.

13. A feedback control system for operating a control process, the system comprising:

a process controller having a processing circuit configured to

monitor an error signal for a disturbance event, wherein the error signal is based on a difference between a setpoint and a feedback signal, wherein the feedback signal is received from the control process;

evaluate the error signal in response to the disturbance event to estimate a time constant of the control process; and

determine and use a sampling rate for the feedback control system, wherein the sampling rate is determined based on the estimated time constant.

14. The system of claim 13, wherein monitoring an error signal for a disturbance event includes classifying the disturbance event as at least one of:

a setpoint change and a load disturbance.

15. The system of claim 14, wherein if the disturbance event is classified as a setpoint change, evaluating the error signal in response to the disturbance event includes:

identifying a portion of the error signal estimated to be associated with the disturbance event;

tracking an area bounded by the identified portion error signal; and

dividing the area bounded by the identified portion of the error signal by a magnitude of the setpoint change to determine the estimated time constant of the control process.

16. The system of claim 14, wherein if the disturbance event is classified as a load disturbance, evaluating the error signal in response to the disturbance event includes:

identifying a portion of the error signal estimated to be associated with the disturbance event;

tracking a magnitude of the error signal during the identified portion to determine a time at which the magnitude of the error signal reaches a maximum in response to the load disturbance;

subtracting a time at which the magnitude of the error signal begins to increase in response to the load disturbance from the time at which the magnitude of the error signal reaches the maximum to determine the estimated time constant of the control process.

17. The system of claim 13, wherein determining a sampling rate for use in the feedback control system includes selecting a sampling interval which no greater than the estimated time constant.

18. The system of claim 13, wherein the processing circuit is further configured to:

compare the determined sampling rate with a previous sampling rate used by the feedback control system; and

update the previous sampling rate to the determined sampling rate if the comparison reveals that one or more updating criteria are met,

wherein the updating criteria relate to one or more of;

a value of the determined sampling rate, a difference between the determined sampling rate and the previous sampling rate, and a ratio of the determined sampling rate to the previous sampling rate.

19. The system of claim 18, wherein the updating criteria include updating the previous sampling rate to the determined sampling rate if the determined sampling rate is at least twice the previous sampling rate or no greater than half the previous sampling rate.

20. One or more non-transitory computer-readable media having instructions for adaptively and automatically adjusting a sampling rate in a control system stored therein, the instructions being executable by one or more processors to cause the one or more processors to perform operations comprising:

monitoring a control signal for a disturbance event, wherein the control signal is provided to a control process;

evaluating the control signal to estimate a time constant of the control process in response to a detected disturbance event;

determining and using a sampling rate for the control system, wherein the sampling rate is determined based on the estimated time constant.