STATE-BASED CONTROL IN AN AIR HANDLING UNIT

• US 20160161139A1
• Filed: 12/08/2014
• Published: 06/09/2016
• Est. Priority Date: 12/08/2014
• Status: Active Grant

• Current Assignee: Tyco Fire & Security GmbH (Johnson Controls International plc)
• Original Assignee: Johnson Controls Technology Company (Johnson Controls International plc)
• Inventors: Asmus, Matthew J., Kinsella, Peter A.
• Granted Patent: US 9,835,347 B2
• Time in Patent Office: Days
• 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 11/54 using one central controller connected to several sub-controllers
• CPC Class Codes: F24F 11/56 Remote control
• CPC Class Codes: F24F 11/57 using telephone networks
• CPC Class Codes: F24F 11/58 using Internet communication
• 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 11/63 Electronic processing
• CPC Class Codes: F24F 11/64 using pre-stored data
• CPC Class Codes: F24F 11/65 for selecting an operating mode
• CPC Class Codes: F24F 11/74 for controlling air flow rate or air velocity
• CPC Class Codes: F24F 11/76 by means responsive to temperature, e.g. bimetal springs
• CPC Class Codes: F24F 11/77 by controlling the speed of ventilators
• CPC Class Codes: F24F 11/84 using valves
• CPC Class Codes: F24F 2110/10 Temperature
• CPC Class Codes: F24F 2140/40 Damper positions, e.g. open or closed
• CPC Class Codes: F24F 2140/50 Load
• CPC Class Codes: G05D 23/1928 sensing the temperature of one space
• CPC Class Codes: Y02B 30/70 Efficient control or regulation technologies, e.g. for control of refrigerant flow, motor or heating

Abstract

A state-based control system for an air handling unit (AHU) includes a finite state machine configured to transition between a high cooling load state and a low cooling load state. In the high cooling load state, the system maintains the temperature of a supply airstream provided by the AHU at a fixed setpoint and controls the temperature of a building zone by modulating the speed of a supply air fan. In the low cooling load state, the system operates the supply air fan at a fixed speed and controls the zone temperature by modulating an amount of cooling applied to the supply airstream by one or more cooling stages. A feed-forward module manages disturbances caused by adding or shedding cooling stages by applying a feed-forward gain to the supply air fan setpoint.

Claims

1. A control system for an air handling unit (AHU) in a building HVAC system, the control system comprising:

a supply air fan configured to provide a supply airstream to a building zone;

one or more cooling stages configured to chill the supply airstream;

a supply air temperature sensor configured to measure a temperature of the supply airstream downstream of the cooling stages;

a zone temperature sensor configured to measure a temperature of the building zone; and

a controller configured to operate the supply air fan and the cooling stages based on input from the supply air temperature sensor and the zone temperature sensor, the controller comprising a finite state module configured to cause the controller to transition between;

a high cooling load state in which the controller maintains the supply air temperature at a fixed setpoint and controls the zone temperature by modulating a speed of the supply air fan, and

a low cooling load state in which the controller operates the supply air fan at a fixed speed and controls the zone temperature by modulating an amount of cooling provided to the supply air stream by the cooling stages.

2. The control system of claim 1, wherein the controller comprises:

a zone temperature control module configured to determine a setpoint for the supply air temperature based on the temperature of the building zone when the controller is operating in the low cooling load state; and

a cooling control module configured to modulate the amount of cooling provided to the supply airstream by the cooling stages to achieve the setpoint for the supply air temperature.

3. The control system of claim 2, wherein the finite state module is configured to:

identify a saturation status for the zone temperature control module when the controller is operating in the low cooling load state; and

cause the controller to transition from the low cooling load state into the high cooling load state in response to the saturation status for the zone temperature control module being greater than or equal to a threshold value.

4. The control system of claim 1, wherein the controller comprises a fan control module configured to modulate the speed of the supply air fan based on the temperature of the building zone when the controller is operating in the high cooling load state.

5. The control system of claim 4, wherein the finite state module is configured to:

identify a saturation status for the fan control module when the controller is operating in the high cooling load state; and

cause the controller to transition from the high cooling load state into the low cooling load state in response to the saturation status for the fan control module being less than or equal to a threshold value.

6. The control system of claim 1, wherein the controller comprises a feed-forward module configured to:

detect a change in a number of active cooling stages;

calculate a feed-forward gain for the speed of the supply air fan in response to detecting the change in the number of active cooling stages; and

adjust the speed of the supply air fan in accordance with the calculated feed-forward gain.

7. The control system of claim 6, wherein calculating the feed-forward gain comprises determining a gain for the speed of the supply air fan that causes an amount of cooling provided to the building zone after the change in the number of active stages to be equivalent to an amount of cooling provided to the building zone before the change in the number of active stages.

8. The control system of claim 6, wherein calculating the feed-forward gain comprises:

determining a first difference between a temperature of the supply air before the change in the number of active stages and a setpoint temperature for the building zone;

determining a second difference between a temperature of the supply air after the change in the number of active stages and the setpoint temperature for the building zone; and

using a ratio between the first difference and the second difference as the feed-forward gain.

9. A control system for an air handling unit (AHU) in a building HVAC system, the control system comprising:

a fan control loop comprising;

a supply air fan configured to provide a supply airstream to a building zone,

a zone temperature sensor configured to measure a temperature of the building zone, and

a fan controller configured to modulate a speed of the supply air fan based on the measured temperature of the building zone to achieve a temperature setpoint for the building zone; and

a cooling control loop comprising;

one or more cooling stages configured to chill the supply airstream,

a zone temperature controller configured to determine a temperature setpoint for the supply airstream based the measured temperature of the building zone, and

a cooling controller configured to modulate an amount of cooling provided to the supply airstream by the cooling stages to achieve the temperature setpoint for the supply airstream.

10. The control system of claim 9, further comprising a finite state controller configured to cause the control system to transition between:

a high cooling load state in which the cooling control loop maintains the temperature of the supply airstream at a fixed setpoint and the fan control loop controls the temperature of the building zone by modulating the speed of the supply air fan, and

a low cooling load state in which the fan control loop operates the supply air fan at a fixed speed and the cooling control loop controls the temperature of the building zone by modulating an amount of cooling provided to the supply air stream by the cooling stages.

11. The control system of claim 10, wherein the finite state controller is configured to:

identify a saturation status for the cooling control loop when the control system is operating in the low cooling load state; and

cause the control system to transition from the low cooling load state into the high cooling load state in response to the saturation status for the cooling control loop being greater than or equal to a threshold value.

12. The control system of claim 10, wherein the finite state controller is configured to:

identify a saturation status for the fan control loop when the control system is operating in the high cooling load state; and

cause the control system to transition from the high cooling load state into the low cooling load state in response to the saturation status for the fan control loop being less than or equal to a threshold value.

13. The control system of claim 9, further comprising a feed-forward controller configured to:

detect a change in a number of active cooling stages;

calculate a feed-forward gain for the speed of the supply air fan in response to detecting the change in the number of active cooling stages; and

adjust the speed of the supply air fan in accordance with the calculated feed-forward gain.

14. The control system of claim 13, wherein calculating the feed-forward gain comprises:

determining a first difference between a temperature of the supply air before the change in the number of active stages and the setpoint temperature for the building zone;

determining a second difference between a temperature of the supply air after the change in the number of active stages and the setpoint temperature for the building zone; and

using a ratio between the first difference and the second difference as the feed-forward gain.

15. A method for operating an air handling unit (AHU) in a building HVAC system, the method comprising:

using a supply air fan to provide a supply airstream to a building zone;

using one or more cooling stages to chill the supply airstream;

receiving, at a controller, a measured temperature of the supply airstream downstream of the cooling stages and a measured temperature of the building zone;

operating, by the controller, the AHU in a high cooling load state in which the controller maintains the temperature of the supply airstream at a fixed setpoint and controls the temperature of the building zone by modulating a speed of the supply air fan;

operating, by the controller, the AHU in a low cooling load state in which the controller operates the supply air fan at a fixed speed and controls the temperature of the building zone by modulating an amount of cooling provided to the supply air stream by the cooling stages; and

causing, by the controller, a transition between the high cooling load state and the low cooling load state based on a saturation status of the controller.

16. The method of claim 15, wherein operating the AHU in the low cooling load state comprises using a cooling control loop to:

determine a setpoint temperature for the supply airstream based on the temperature of the building zone; and

modulate the amount of cooling provided to the supply airstream by the cooling stages to achieve the setpoint temperature for the supply airstream.

17. The method of claim 16, wherein causing the transition between the high cooling load state and the low cooling load state comprises:

identifying a saturation status of the cooling control loop; and

causing the controller to transition from the low cooling load state into the high cooling load state in response to the saturation status for the cooling control loop being greater than or equal to a threshold value.

18. The method of claim 15, wherein operating the AHU in the high cooling load state comprises using a fan control loop to modulate the speed of the supply air fan based on the temperature of the building zone.

19. The method of claim 18, wherein causing the transition between the high cooling load state and the low cooling load state comprises:

identifying a saturation status of the fan control loop; and

causing the controller to transition from the high cooling load state into the low cooling load state in response to the saturation status for the fan control loop being less than or equal to a threshold value.

20. The method of claim 15, further comprising:

detecting a change in a number of active cooling stages;

calculating a feed-forward gain for the speed of the supply air fan in response to detecting the change in the number of active cooling stages; and

adjusting the speed of the supply air fan in accordance with the calculated feed-forward gain.

21. The method of claim 20, wherein calculating the feed-forward gain comprises:

determining a first difference between a temperature of the supply air before the change in the number of active stages and the setpoint temperature for the building zone;

determining a second difference between a temperature of the supply air after the change in the number of active stages and the setpoint temperature for the building zone; and

using a ratio between the first difference and the second difference as the feed-forward gain.