Velocity Based Control in a Non-Periodically Updated Controller

• US 20160209815A1
• Filed: 03/25/2016
• Published: 07/21/2016
• 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: Stoltenow, Pete, Blevins, Terrence L., Nixon, Mark J.
• Granted Patent: US 10,423,127 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G05B 13/02 electric

Abstract

A technique for controlling a process using slow or non-periodically received process variable measurements enables more robust controller responses to setpoint changes and disturbance changes even when the process variable measurement feedback signals are reviewed at a rate on the order of the rate associated with the response time of the process dynamic or variable being controlled. The control technique implements iterations of a control routine to generate a control signal using a reset or rate contribution component that, in some sense, defines 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 is maintained at zero or at some other previous level when generating the control signal. However, the reset contribution component is iteratively recalculated during each controller execution cycle, even when no new process variable measurement has been received, so that the output of the reset contribution component incorporates expected process changes that occur as a result of a setpoint or a feed-forward change that impacts the process input or control signal between the times that actual process variable measurement values are received at the controller. This technique makes the controller more robust when producing control signals in the presence of setpoint or feed-forward changes received between the times at which non-periodic process variable measurements are received at the controller and makes the controller operate better when the process variable feedback time interval is greater, equal to or on the order of the process response time.

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, using a computer processing device, an integral feedback contribution for use in producing the control signal, including using, during each of the multiple iterations, an iterative filter to determine a current integral feedback contribution value for the current iteration of the control routine from an integral feedback contribution value of a preceding iteration of the control routine and a value of the control signal; and

using the current integral feedback contribution to generate the control signal for the current iteration of the control routine during each controller iteration at which a new process response indication is received, and not using the current integral feedback contribution to generate the control signal during controller iterations at which a new process response indication to the control signal is not received; and

using the control signal to control the process.

2. The method of claim 1, further including generating a proportional contribution during each iteration and using the proportional contribution during each iteration to generate the control signal.

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

4. The method of claim 3, wherein the process parameter is a process variable controlled by a field device that is responsive to the control signal.

5. The method of claim 1, wherein determining the integral feedback contribution includes generating the integral feedback contribution value based on the difference between the control signal for the current iteration of the control routine and the integral feedback contribution value of the preceding iteration of the control routine multiplied by a factor dependent on a reset time and a controller execution period.

6. 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 most recent measurement of a process variable and the integral feedback contribution.

7. 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 a proportional component from a setpoint value, a most recently received process response indication, and a proportional gain value, and generating the control signal using a difference between the proportional component generated during the current iteration and the proportional component generated during a previous iteration.

8. The method of claim 7, wherein generating the integral feedback contribution for use in producing the control signal includes determining a current integral feedback contribution value for the current iteration of the control routine from an integral feedback contribution value of a preceding iteration of the control routine and a summed value of the control signal for all previous iterations since the last iteration at which a new process response indication was received.

9. The method of claim 1, wherein generating the integral feedback contribution for use in producing the control signal includes determining a current integral feedback contribution value for the current iteration of the control routine from an integral feedback contribution value of a preceding iteration of the control routine and a summed value of the control signal for all previous iterations since the last iteration at which a new process response indication was received.

10. A process controller that generates a control signal for controlling a process variable during each of a multiplicity of controller iterations of the process controller, comprising:

a communications unit that receives a new value of the process variable at less than each of the multiplicity of iterations of the process controller;

a proportional control component that generates a proportional control signal value during each of the iterations of the process controller including,

a first summer that determines a difference between a set point value for the process variable and a received value of a process variable, and

a proportional gain unit coupled to the summer;

an integral control component that generates an integral control signal value during each of the iterations of the process controller including,

an iterative filter that determines a preliminary integral control component during each iteration of the process controller based on a previous value of the preliminary integral control component generated during a previous iteration of the process controller and based on the control signal for the current iteration of the process controller; and

a switch coupled to the iterative filter that receives the preliminary integral control component and a further value, wherein the switch operates to provide the preliminary integral control component generated by the iterative filter during process controller iterations associated with the receipt of a new value of the process variable at the communications unit as the integral control signal value and that provides the further value as the integral control signal value during controller iterations that are not associated with the receipt of a new value of the process variable at the communications unit; and

a second summer that sums the proportional control signal value and the integral control signal value during each process controller iteration to produce the control signal.

11. The process controller of claim 10, wherein the integral control component further includes a third summer coupled to the iterative filter, wherein the third summer sums the control signal for previous controller iterations to produce a summed control signal value and wherein the third summer provides the summed control signal value as an input to the iterative filter.

12. The process controller of claim 11, wherein the third summer resets the summed control signal upon receipt of a new value of the process variable at the communications unit.

13. The process controller of claim 12, wherein the third summer resets the summed control signal to zero upon receipt of a new value of the process variable at the communications unit.

14. The process controller of claim 11, wherein the further value is zero.

15. The process controller of claim 11, wherein the proportional control component includes a difference unit coupled to the proportional gain unit.

16. The process controller of claim 15, wherein the difference unit is coupled between the proportional gain unit and the second summer and determines a difference value between the output of the proportional gain unit from a previous controller iteration and the output of the proportional gain unit at the current controller iteration and provides the difference value as the proportional control signal value to the second summer.

17. The process controller of claim 10, further including a control signal conversion unit coupled to the second summer that converts the control signal to an output control signal to be sent to control a device within a process.

18. The process controller of claim 17, wherein the control signal represents a change in the output control signal to be sent to the process.

19. The process controller of claim 10, further including a derivative control component that determines a derivative control signal value, the derivative control component including a derivative gain unit coupled to the first summer, and a second difference unit coupled to the derivative gain unit that determines a further difference value between the output of the derivative gain unit from a previous controller iteration and the output of the derivative gain unit at the current controller iteration and provides the further difference to the second summer as the derivative control signal value, and wherein the second summer sums the derivative control signal value with the integral control signal value and the proportional control signal value to produce the control signal.

20. The process controller of claim 10, wherein the further value is an integral control signal value output by the switch in a previous controller iteration.

21. A method of generating a velocity based process control signal during each of a multiplicity of iterations of a control routine, comprising:

receiving, via a computer processing device, a new value of the process variable being controlled at less than each the multiplicity of iterations of the control routine;

generating, using a computer processing device, during each of the multiplicity of iterations of the control routine, a difference based integral feedback contribution for use in producing the control signal, including summing the control signal generated during each iteration of the control routine since the last iteration of the control routine at which a new process variable value was received to produce a summed control signal, providing the summed control signal to an iterative filter, and using, during each of the multiplicity of iterations, the iterative filter to determine a current integral feedback contribution value for the current iteration of the control routine from an integral feedback contribution value of a preceding iteration of the control routine and the summed control signal; and

using the current integral feedback contribution value to generate the control signal for the current iteration of the control routine during each iteration at which a new process variable value is received, and not using the current integral feedback contribution value to generate the control signal during iterations at which a new process variable value is not received from the process; and

using the control signal to control the process variable.

22. The method of generating a velocity based process control signal of claim 21, further including generating, using a computer processing device, a difference based proportional contribution during each of the iterations of the control routine and using the proportional contribution during each of the iterations of the control routine to generate the control signal.

23. The method of generating a velocity based process control signal of claim 21, wherein the new process variable value is a measurement of a process parameter being affected by the control signal.

24. The method of generating a velocity based process control signal of claim 21, wherein determining the integral feedback contribution includes generating the current integral feedback contribution value based on the difference between the summed control signal for the current iteration of the control routine and the integral feedback contribution value of the preceding iteration of the control routine multiplied by a factor dependent on a reset time and a controller execution period.

25. The method of generating a velocity based process control signal of claim 21, wherein not using the current integral feedback contribution value to generate the control signal during iterations at which a new process variable value is not received from the process includes using a fixed value as the current integral feedback contribution to generate the control signal during iterations at which a new process variable value is not received from the process.

26. The method of generating a velocity based process control signal of claim 21, wherein the fixed value is zero.

27. A velocity based process controller that generates a control signal for controlling a process variable during each of a multiplicity of controller iterations of the process controller, comprising:

a communications unit that receives a new value of the process variable at less than each of the multiplicity of iterations of the process controller;

an integral control component that generates an integral control signal value during each of the iterations of the process controller including,

an iterative filter that determines a preliminary integral control component during each iteration of the process controller based on a previous value of the preliminary integral control component generated during a previous iteration of the process controller and based on the control signal for the current iteration of the process controller; and

a switch coupled to the iterative filter that receives the preliminary integral control component and a further value, wherein the switch operates to provide the preliminary integral control component generated by the iterative filter during process controller iterations associated with the receipt of a new value of the process variable at the communications unit as the integral control signal value and that provides the further value as the integral control signal value during controller iterations that are not associated with the receipt of a new value of the process variable at the communications unit; and

a control signal generator coupled to receive the integral control signal value to generate a control signal for use in controlling the process during each of the controller iterations.

28. The velocity based process controller of claim 27, further comprising a proportional control component that generates a proportional control signal value during each of the iterations of the process controller including,

a first summer that determines a difference between a set point value for the process variable and a received value of the process variable, and

a proportional gain unit coupled to the summer;

and

wherein the control signal generator includes a second summer that sums the proportional control signal value and the integral control signal value during each process controller iteration to produce the control signal.

29. The velocity based process controller of claim 28, wherein the proportional control component includes a difference unit coupled to the proportional gain unit.

30. The velocity based process controller of claim 29, wherein the difference unit is coupled between the proportional gain unit and the second summer and determines a difference value between the output of the proportional gain unit from a previous controller iteration and the output of the proportional gain unit at the current controller iteration and provides the difference value as the proportional control signal value to the second summer.

31. The process controller of claim 28, further including a derivative control component that determines a derivative control signal value, the derivative control component including a derivative gain unit coupled to the first summer, and a second difference unit coupled to the derivative gain unit that determines a further difference value between the output of the derivative gain unit from a previous controller iteration and the output of the derivative gain unit at the current controller iteration and provides the further difference to the second summer as the derivative control signal value, and wherein the second summer sums the derivative control signal value with the integral control signal value and the proportional control signal value to produce the control signal.

32. The velocity based process controller of claim 27, wherein the integral control component includes a further summer coupled to the iterative filter, wherein the summer sums the control signal for previous controller iterations to produce a summed control signal value and wherein the summer provides the summed control signal value as an input to the iterative filter.

33. The velocity based process controller of claim 32, wherein the summer resets the summed control signal upon receipt of a new value of the process variable at the communications unit.

34. The velocity based process controller of claim 32, wherein the summer resets the summed control signal to zero upon receipt of a new value of the process variable at the communications unit.

35. The velocity based process controller of claim 27, wherein the further value is zero.

36. The velocity based process controller of claim 27, further including a control signal conversion unit coupled to the control signal generator that converts the control signal produced by the control signal generator to an output control signal to be sent to control a device within a process.

37. The velocity based process controller of claim 36, wherein the control signal represents a change in the output control signal to be sent to the process.