COMPENSATING FOR SETPOINT CHANGES IN A NON-PERIODICALLY UPDATED CONTROLLER

• US 20130184837A1
• Filed: 01/17/2012
• Published: 07/18/2013
• Est. Priority Date: 01/17/2012
• Status: Active Grant

• Current Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Original Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Inventors: Blevins, Terrence L., Nixon, Mark J.
• Granted Patent: US 9,298,176 B2
• Time in Patent Office: Days
• US Class Current: 700/11
• CPC Class Codes: G05B 17/02 electric

Abstract

A technique for controlling a process using non-periodically received process variable measurements enables more robust controller responses to setpoint changes. The control technique implements iterations of a control routine to generate a control signal using a reset or rate contribution component that produces an expected process response to the control signal. When a new measurement of the process variable is unavailable to the controller, the reset or rate contribution component that was generated in response to the receipt of the previous process variable is maintained when generating the control signal. However, the reset contribution component is iteratively recalculated during each controller execution cycle so that the output of the reset contribution component incorporates expected process changes that occur as a result of a setpoint change.

Claims

1. A method of controlling a process, comprising:

implementing, on a computer processing device, multiple iterations of a control routine to generate a control signal including, during each iteration of the control routine;

generating a feedback contribution for use in producing the control signal, including determining a feedback contribution value for the current iteration of the control routine from a feedback contribution value of a preceding iteration of the control routine and a value of the control signal; and

using the feedback contribution to generate the control signal for the current iteration of the control routine when a new process response indication is received, and using the feedback contribution that was generated from an iteration of the control routine at which the preceding communication of a process response indication was received from the process to generate the control signal when a new process response indication to the control signal is not received from the process; and

using the control signal to control the process.

2. The method of claim 1, wherein the feedback contribution is a reset contribution.

3. The method of claim 2, wherein the feedback contribution includes a derivative contribution.

4. The method of claim 1, wherein the process response indication is a measurement of a process parameter being affected by the control signal.

5. The method of claim 4, wherein the process parameter is a process variable controlled by a field device responsive to the control signal.

6. The method of claim 1, wherein:

the control routine is a proportional-integral-derivative (PID) control routine for controlling a process variable in accordance with a setpoint, wherein the PID control routine includes one of an integral component and a derivative component;

the feedback contribution is generated by one of the integral component and the derivative component of PID control routine; and

the process response indication comprises a measurement of either the process variable or a process parameter responsive to the control signal that affects the process variable.

7. The method of claim 1, wherein generating the feedback contribution for the control signal for an iteration of the control routine includes determining the feedback contribution as a summation of the feedback contribution value of a preceding iteration of the control routine and a component based on a difference between the current control signal and the feedback contribution value of the preceding iteration of the control routine.

8. The method of claim 7, wherein determining the feedback contribution includes generating the component based on the difference between the current control signal and the feedback contribution of the preceding iteration of the control routine multiplied by a factor dependent on a reset time and a controller execution period.

9. The method of claim 1, wherein implementing multiple iterations of a control routine to generate a control signal includes, during each iteration of the control routine, generating the control signal based on a setpoint value, a measurement of a process variable and the feedback contribution.

10. A device for controlling a process, comprising:

a processor;

a communication interface coupled to the processor to receive a process variable indication;

a computer readable medium; and

a control routine stored on the computer-readable medium that executes on the processor to produce a control signal based on the process variable indication, wherein the control routine executes during each of a plurality of execution cycles to generate the control signal and wherein during each execution cycle, the control routine;

generates a feedback contribution to the control signal in accordance with the generated feedback contribution of a previous control routine execution cycle and a controller execution period; and

uses the feedback contribution to generate the control signal for the current execution cycle when a new process variable indication is received via the communication interface and uses the feedback contribution that was generated from the execution cycle of the control routine at which the preceding communication of a process variable indication was received via the communication interface to generate the control signal when a new process variable indication is not received via the communication interface.

11. The device of claim 10, where the communication interface includes a wireless communication unit that receives the process variable indication via a wireless transmission.

12. The device of claim 10, wherein the feedback contribution is a reset contribution.

13. The device of claim 10, wherein the feedback contribution includes a derivative contribution in which the feedback contribution is updated on detection of new measurement and based on derivative time constant and the time that has elapsed since the last measurement update.

14. The device of claim 10, wherein the control routine is a proportional-integral-derivative (PID) control routine for controlling a process variable in accordance with a setpoint, wherein the PID control routine includes one of an integral component and a derivative component which generates the feedback component, and wherein the process variable indication comprises a measurement of either the process variable or a process parameter responsive to the control signal that affects a process variable.

15. The device of claim 10, wherein the control routine generates the feedback contribution for the control signal by iteratively determining the feedback contribution as a summation of the feedback contribution of a preceding execution cycle of the control routine and a component based on a difference between the current control signal and the feedback contribution of the preceding execution cycle of the control routine.

16. The device of claim 15, wherein the control routine generates the feedback contribution by determining the component based on the difference between the current control signal and the feedback contribution of the preceding execution cycle of the control routine multiplied by a factor dependent on a reset time and a controller execution period.

17. A controller for producing a control signal to control a process based on a setpoint and a set of non-periodic measurements from the process, the controller comprising:

a setpoint input that receives a setpoint value;

a process variable input that receives non-periodic measurements of a process variable;

a control signal generation unit coupled to the setpoint input and the process variable input that produces a control signal during each of a set of controller iterations based on the setpoint value, a measurement of the process variable and a feedback contribution;

a continuously updated filter that generates a new value of the feedback contribution for each of the plurality of controller iterations, wherein the continuously updated filter generates each new value of the feedback contribution in accordance with the generated feedback contribution value of a previous controller iteration, and the control signal produced by the control signal generation unit; and

a switch coupled between the continuously updated filter and the control signal generation unit, wherein the switch provides a feedback contribution value that was generated by the continuously updated filter during a previous controller iteration as a result of the receipt of a previous communication of the process variable measurement to the control signal generation unit when a new process variable measurement value is unavailable at the process variable input and that provides the feedback contribution value that was generated by the continuously updated filter as a result of a current controller iteration to the control signal generation unit when a new process variable measurement value is available at the process variable input.

18. The controller of claim 17, wherein the continuously updated filter comprises a reset contribution generation unit to develop a reset contribution signal as the feedback contribution.

19. The controller of claim 18, wherein the controller includes a derivative contribution generation unit to develop a derivative contribution signal as the feedback contribution.

20. The controller of claim 17, wherein the controller is a proportional-integral-derivative (PID) control routine for controlling a process variable in accordance with the setpoint, wherein the controller includes one of an integral component and a derivative component which includes the continuously updated filter, and wherein the process variable measurements comprise measurements of either a process variable or a process parameter responsive to the control signal that affects a process variable.

21. The controller of claim 17, wherein the continuously updated filter generates the feedback contribution as a summation of the feedback contribution of a preceding controller iteration and a component based on a difference between the current control signal and the feedback contribution of the preceding controller iteration.

22. The controller of claim 22, wherein the continuously updated filter generates the feedback contribution by determining the component based on the difference between the current control signal and the feedback contribution of the preceding controller iteration multiplied by a factor dependent on a reset time and a controller execution period.