Slope predictive control and digital PID control

• US 6,715,690 B2
• Filed: 04/28/2003
• Issued: 04/06/2004
• Est. Priority Date: 05/05/2000
• Status: Expired due to Term

• 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
• Primary Examiner(s): WAYNER, WILLIAM E
• Application Number:
  US10/425,136
• Publication Number:
  US 20030197066A1
• Time in Patent Office: 344 Days
• Field of Search: 236/49.3, 236/78.D, 165/217, 700/276, 700/282, 318/610
• US Class Current: 236/49.3
• CPC Class Codes: F24F 11/30 for purposes related to the operation of the system, e.g. for safety or monitoring
• CPC Class Codes: F24F 11/62 characterised by the type of control or by internal processing, e.g. using fuzzy logic, adaptive control or estimation of values
• CPC Class Codes: F24F 11/72 for controlling the supply of treated air, e.g. its pressure
• CPC Class Codes: F24F 13/08 Air-flow control members, e.g. louvres, grilles, flaps or guide plates F24F7/013, F24F13/06 take precedence
• CPC Class Codes: G05D 23/1904 variable in time
• CPC Class Codes: G05D 7/0635 by action on throttling means G05D7/0688, G05D7/0694 take precedence

Abstract

The present invention provides an airflow control loop that uses averaged airflow measurements without the problems that are normally encountered with averaging measurements, such as the delay introduced into the airflow control loop. This is accomplished, in the present invention, through a predictive control scheme. The predictive control scheme of the airflow control loop calculates the damper sensitivity, calculates the damper runtime needed to achieve setpoint, and then runs the damper for the determined period of time. In addition to the unique airflow control loop used to maintain the constant airflow into the room, the present invention also implements a digital form of proportional, integral, and derivative (PID) control to maintain the room temperature. The digital form of PID control uses non-linear gains which vary according to how far the measured temperature is from setpoint.

Claims

1. A method for controlling a Variable Air Volume (VAV) box in a VAV temperature control system, wherein the VAV box includes an airflow control damper, comprising:

receiving a new airflow setpoint indication;

determining if the new airflow setpoint is within a predetermined range of a current airflow setpoint;

if the new airflow setpoint is within the predetermined range, then moving the damper for a first predicted damper runtime that is based at least in part on a sensitivity of the damper; and

if the new airflow setpoint is not within the predetermined range, then

moving the damper while taking substantially instantaneous airflow measurements until a measured airflow is within a second predetermined range of the new airflow setpoint, and

moving the damper for a second predicted damper runtime that is based at least in part on a sensitivity of the damper.

2. The method of claim 1, wherein the damper'"'"'s sensitivity comprises a slope of a damper response curve of change in airflow per unit time.

3. The method of claim 2, wherein the predicted damper runtime is determined by dividing the error between the airflow setpoint and the measured airflow by a slope of the damper response curve.

4. The method of claim 1, further comprising determining if the damper runtime is less than a predetermined minimum time period, and if it is, then making no corrective movement to the damper, and if it is not, then moving the damper for the predicted damper runtime.

5. A Variable Air Volume (VAV) temperature control system, comprising;

a VAV box having an airflow control damper for controlling an airflow delivered to a room an airflow sensor for measuring the airflow;

a temperature system within the room for measuring a temperature of the room; and

a micro controller configured for executing computer-executable instructions for receiving a room temperature measurement from the temperature sensor and using the temperature measurement to calculate an airflow setpoint;

determining if the new airflow setpoint is within a predetermined range of a current airflow setpoint;

if the new airflow setpoint is within the predetermined range, then moving the damper for a first predicted damper runtime that is based at least in part on a sensitivity of the damper; and

if the new airflow setpoint is not within the predetermined range, then

moving the damper while taking substantially instantaneous airflow measurements until a measured airflow is within a second predetermined range of the new airflow setpoint, and

moving the damper for a second predicted damper runtime that is based at least in part on a sensitivity of the damper.

6. A method of claim 5, wherein the damper'"'"'s sensitivity comprises a slope of a damper response curve of change in airflow per unit time.

7. A method of claim 5, wherein the predicted damper runtime is determined by dividing the error between the airflow setpoint and the measured airflow by a slope of the damper response curve.

8. A method of claim 5, further comprising determining if the damper runtime is less than a predetermined minimum time period, and if it is, then making no corrective movement to the damper, and if it is not, then moving the damper for the predicted damper runtime.

9. A method for controlling a Variable Air Volume (VAV) box in a VAV temperature control system, the method relying on a PID control having a proportional term, an integral term, and a derivative term, the method comprising:

receiving a room temperature measurement and calculating a temperature error between the room temperature and a temperature setpoint;

in response to determining that the temperature error is greater than a temperature threshold, calculating a PID output using a first gain set and then calculating the airflow setpoint using the calculated PID output;

after the expiration in the time increment, re-measuring the room temperature and re-calculating the temperature error;

in response to determining that the temperature error is less than the temperature threshold, determining whether the temperature error is within a deadband of the temperature setpoint;

in response to determining that the temperature error is within the deadband of the temperature set point, re-calculating the PID output using the second gain set and then calculating the airflow setpoint using the re-calculated PID output; and

in response to determining that the temperature error is not within the deadband of the temperature set point, re-calculating the PID output using a third gain set and then calculating the airflow setpoint using the re-calculated PID output;

wherein the second gain set is relatively lower than a third gain set, and the third gain set is relatively lower than the first gain set.

10. A method of claim 9, wherein the first gain set comprises significantly increasing the PID output and accumulating the integral term.

11. A method of claim 9, wherein the second gain set comprises updating the proportional term but not updating the integral term.

12. A method of claim 9, wherein the third gain set comprises updating the proportional term and the integral term.

13. A Volume Air Volume (VAV) temperature control system, comprising:

a VAV box having an airflow control damper for controlling an airflow delivered to a room and an airflow sensor for measuring the airflow;

a temperature sensor within the room measuring the temperature of the room; and

a micro controller configured for executed computer-executable instructions for performing a PID control having a proportional term, an integral term, and a derivative term, the computer-executable comprising;

in response to determining that the temperature error is greater than a temperature threshold, calculating a PID output using a first gain set and calculating the airflow setpoint using the calculated PID output;

after the expiration in the time increment, re-measuring the room temperature and re-calculating the temperature error;

in response to determining that the temperature error is less than the temperature threshold, determining whether the temperature error is within a deadband of the temperature setpoint;

in response to determining that the temperature error is within the deadband of the temperature set point, re-calculating the PID output using the second gain set and then calculating the airflow setpoint using the re-calculated PID output; and

in response to determining that the temperature error is not within the deadband of the temperature set point, re-calculating the PID output using a third gain set and then calculating the airflow setpoint using the re-calculated PID output;

wherein the second gain set is relatively lower than a third gain set, and the third gain set is relatively lower than the first gain set.

14. A system of claim 13, wherein the first gain set comprises significantly increasing the PID output and accumulating the integral term.

15. A system of claim 13, wherein the second gain set comprises updating the proportional term but not updating the integral term.

16. A system of claim 13, wherein the third gain set comprises updating the proportional term and the integral term.