Slope Predictive Control and Digital PID Control

• US 20070090199A1
• Filed: 11/17/2006
• Published: 04/26/2007
• Est. Priority Date: 05/05/2000
• Status: Active Grant

• Current Assignee: Gerry Hull, Raynold Saar, Steven Tom
• Original Assignee: Gerry Hull, Raynold Saar, Steven Tom
• Inventors: Hull, Gerry, Saar, Raynold, Tom, Steven
• Granted Patent: US 7,802,438 B2
• Time in Patent Office: Days
• 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, 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;

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

based on the temperature error,

(i) determining a region of operation,

(ii) calculating a PID output using a gain set associated with that region, and

(iii) calculating an airflow setpoint using the calculated PID output;

after the expiration of a time increment, re-measuring the room temperature, recalculating the temperature 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 1, 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 comprises incrementing the integral term at a fixed rate not varying with the error.

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

7. 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 microcontroller 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 instructions comprising;

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

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

based on the temperature error,

(i) determining a region of operation,

(ii) calculating a PID output using a gain set associated with that region, and

(iii) calculating an airflow setpoint using the calculated PID output;

after the expiration of a time increment, re-measuring the room temperature, recalculating the temperature error and repeating steps (i)-(iii).

8. The system of claim 7, wherein at least one gain set comprises significantly increasing the PID output and accumulating the integral term at an accelerated rate.

9. The system of claim 7, wherein at least one gain set comprises updating the proportional term but not updating the integral term.

10. The system of claim 7, wherein at least one gain set comprises updating the proportional term and the integral term.

11. The system of claim 7, wherein at least one gain set comprises incrementing the integral term at a fixed rate not varying with the error.

12. The system of claim 7, wherein at least one gain set comprises implementing derivative control action by enabling or disabling the integral action.