State-based control in an air handling unit

Patent metadata

Field	Value
Publication number	US-9835347-B2
Application number	US-201414563771-A
Country	US
Kind code	B2
Filing date	Dec 8, 2014
Priority date	Dec 8, 2014
Publication date	Dec 5, 2017
Grant date	Dec 5, 2017

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.

First claim

What is claimed is:

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 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, and
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 airstream by the cooling stages;

the controller further comprising 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, 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 cooling stages to be equivalent to an amount of cooling provided to the building zone before 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.

2. The control system of claim 1 , wherein the controller comprises:
a zone temperature control module configured to determine a setpoint for the temperature of the supply airstream 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 temperature of the supply airstream.

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. 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;

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; and

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;

wherein calculating the feed-forward gain comprises:
determining a first difference between a temperature of the supply airstream before the change in the number of active cooling stages and the temperature setpoint for the building zone;
determining a second difference between a temperature of the supply airstream after the change in the number of active cooling stages and the temperature setpoint for the building zone; and
using a ratio between the first difference and the second difference as the feed-forward gain.

7. The control system of claim 6 , 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 airstream by the cooling stages.

8. The control system of claim 7 , 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.

9. The control system of claim 7 , 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.

10. 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 temperature of the supp

Assignees

• Johnson Controls Tech Co

Inventors

• Asmus Matthew J
• Kinsella Peter A

Classifications

• F24F11/77 Primary

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