Slope predictive control and digital PID control for a variable temperature control system

• US 20080114500A1
• Filed: 06/12/2007
• Published: 05/15/2008
• Est. Priority Date: 05/07/2001
• Status: Active Grant

• Current Assignee: Automated Logic Corporation (Carrier Global Corporation)
• Original Assignee: Automated Logic Corporation (Carrier Global Corporation)
• Inventors: Hull, Gerry Glancy, Tom, Steven Treece, Saar, Raynold Lee
• Granted Patent: US 7,669,777 B2
• Time in Patent Office: Days
• US Class Current: 700/300
• CPC Class Codes: G05D 23/19 characterised by the use of electric means G05D23/1393 takes precedence

Abstract

The present invention applies a slope predictive control method to a Variable Temperature Control (VTC) system, measuring the slope of the discharge temperature as a function of flow regulation device position, and using this slope to predict the position of the flow regulation device needed to achieve the desired discharge temperature. The present invention also monitors the response of the VTC over time and utilizes a self-learning algorithm to predict the response time of the system in order to determine when further control adjustments need to be taken.

Claims

1. A method for implementing PID control having a proportional term, an integral term, and a derivative term, the method comprising:

receiving a controlled variable measurement and calculating an error between the controlled variable measurement and a desired condition;

defining at least two distinct regions of operation based at least in part on the error, wherein each region has a predetermined gain set; and

based on the error, (i) determining a region of operation from the at least two distinct regions, (ii) calculating a PID output using a gain set associated with that region, and (iii) after the expiration of a time increment, re-measuring the controlled variable, re-calculating the error and repeating steps (i)-(iii).

2. The method of claim 1, wherein at least one gain set comprises significantly increasing the PID output and accumulating the integral term at an accelerated rate.

3. The method of claim 1, wherein at least one gain set comprises updating the proportional term but not updating the integral term.

4. The method of claim 2, wherein at least one gain set comprises updating the proportional term and the integral term.

5. The method of claim 1, wherein at least one gain set increments the integral term at a fixed rate not varying with the error.

6. The method of claim 1, wherein at least one gain set implements derivative control action by enabling or disabling the integral action.

7. The method of claim 1, further comprising calculating a setpoint based on the PID output.

8. A method for implementing PID control having a proportional term, an integral term, and a derivative term, the method comprising:

measuring a controlled variable;

calculating an error between the controlled variable and a desired condition;

comparing the error to a threshold;

if the error is within the threshold, setting a PID output to a maximum value and accumulating the integral term;

if the error is within the threshold, determining whether the error is within a deadband;

if the error is within the deadband, calculating the PID output by updating only the proportional term; and

if the error is outside the deadband, calculating the PID output by updating the proportional term and the integral term.

9. The method of claim 8, wherein the step of measuring the controlled variable comprises determining an average of the controlled variable over a period of time.

10. The method of claim 9, wherein the period of time comprises ten seconds.

11. The method of claim 8, further comprising defining the threshold by a user.

12. The method of claim 8, wherein the maximum value comprises 100% or 0%.

13. The method of claim 8, wherein accumulating the integral term comprises:

saving a starting PID output percentage (Starting Output);

setting a previous integral term (I_previous) equal to 0 at startup; and

accumulating the integral term using the formula;

I_previous+(100%−

Starting Output)/12.

14. The method of claim 8, wherein calculating the PID output by updating only the proportional term comprises:

setting the proportional term equal to a percentage of the error; and

adding the proportional term to the integral term.

15. The method of claim 14, wherein the percentage is 20 percent.

16. The method of claim 8, wherein calculating the PID output by updating the proportional term and the integral term comprises:

setting the proportional term equal to a percentage of the error; and

adding a second percent to the integral term if a current error is greater than or equal to a previous error;

subtracting the second percent from the integral term if the current error is less than the previous error; and

adding the proportional term to the integral term.

17. The method of claim 16, wherein the percentage is 20 percent.

18. The method of claim 16, wherein the second percentage is 1 percent.

19. The method of claim 8, further comprising calculating a setpoint using the PID output.