Patent Publication Number: US-9835347-B2

Title: State-based control in an air handling unit

Description:
BACKGROUND 
     The present invention relates generally to heating, ventilating, and air conditioning (HVAC) systems and more specifically to a state-based control system for an air handling unit (AHU) in a building HVAC system. 
     HVAC systems are used to monitor and control temperature, humidity, air flow, air quality, and other conditions in a building or building system. HVAC systems often include an AHU which functions intake outside air and/or return air from inside the building and to provide a supply airstream to the building at setpoint conditions. Some AHUs use a constant volume fan to provide a constant airflow directly to one or more building zones. Other AHUs use a variable volume fan and/or provide airflow to downstream variable air volume (VAV) boxes which control airflow into the building zone. 
     Typically, AHUs are designed to serve a heating or cooling load within a predetermined load range and must sacrifice energy efficiency to provide heating or cooling outside the predetermined range. Many AHUs also rely on downstream pressure sensors (e.g., static pressure sensors, velocity pressure sensors, etc.) or input from other control loops to achieve setpoint conditions. It would be desirable to provide an AHU that is adaptable to multiple different load conditions without sacrificing efficiency. 
     SUMMARY 
     One implementation of the present disclosure is a control system for an air handling unit (AHU) in a building HVAC system. The control system includes 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 includes a finite state module configured to cause the controller to transition between a high cooling load state and a low cooling load state. In the high cooling load state, 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. In the low cooling load state, 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. 
     In some embodiments, the controller includes 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. The controller may further include 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. In some embodiments, the zone temperature control module is part of an outer cascaded control loop and the cooling control module is part of an inner cascaded control loop. The finite state module may be configured to identify a saturation status for the zone temperature control module when the controller is operating in the low cooling load state. The finite state module may 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. 
     In some embodiments, the controller includes 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. The finite state module may be configured to identify a saturation status for the fan control module when the controller is operating in the high cooling load state. The finite state module may 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. 
     In some embodiments, the controller includes 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. In some embodiments, calculating the feed-forward gain includes 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. 
     In some embodiments, calculating the feed-forward gain includes 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. 
     Another implementation of the present disclosure is a control system for an air handling unit (AHU) in a building HVAC system. The control system includes a fan control loop and a cooling control loop. The fan control loop includes 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. The cooling control loop includes 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. In some embodiments, the cooling control loop is a cascaded control loop. 
     In some embodiments, the control system includes a finite state controller configured to cause the control system to transition between a high cooling load state and a low cooling load state. In the high cooling load state, the cooling control loop may maintain the temperature of the supply airstream at a fixed setpoint and the fan control loop may control the temperature of the building zone by modulating the speed of the supply air fan. In the low cooling load state, the fan control loop may operate the supply air fan at a fixed speed and the cooling control loop may control the temperature of the building zone by modulating an amount of cooling provided to the supply air stream by the cooling stages. 
     In some embodiments, 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. In some embodiments, 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. 
     In some embodiments, the control system includes 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. Calculating the feed-forward gain may include 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. 
     Another implementation of the present disclosure is a method for operating an air handling unit (AHU) in a building HVAC system. The method includes using a supply air fan to provide a supply airstream to a building zone and using one or more cooling stages to chill the supply airstream. The method further includes receiving, at a controller, a measured temperature of the supply airstream downstream of the cooling stages and a measured temperature of the building zone. The method further includes 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. The method further includes 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. The method further includes 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. 
     In some embodiments, operating the AHU in the low cooling load state includes 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. In some embodiments, the cooling control loop is a cascaded control loop. Causing the transition between the high cooling load state and the low cooling load state may include 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. 
     In some embodiments, operating the AHU in the high cooling load state includes using a fan control loop to modulate the speed of the supply air fan based on the temperature of the building zone. Causing the transition between the high cooling load state and the low cooling load state may include 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. 
     In some embodiments, the method includes 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. Calculating the feed-forward gain may include 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. 
     Those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a building equipped with a heating, ventilating, and air conditioning (HVAC) system including an air handling unit (AHU) that provides air to one or more zones of the building, according to an exemplary embodiment. 
         FIG. 2  is a block diagram illustrating the AHU of  FIG. 1  in greater detail, according to an exemplary embodiment. 
         FIG. 3  is a block diagram of a constant volume control system which may be used in conjunction with the AHU of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 4  is a block diagram of a variable volume control system which may be used in conjunction with the AHU of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 5  is a block diagram of a state-based control system which may be used in conjunction with the AHU of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 6  is a state transition diagram illustrating several operating states which may be used by the state-based control system of  FIG. 5  to control the AHU of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 7  is a chart illustrating the functions performed by a zone temperature controller, a cooling controller, and a fan controller of the state-based control system of  FIG. 5  in several of the operating states shown in  FIG. 6 , according to an exemplary embodiment. 
         FIG. 8  is a block diagram illustrating the state-based control system of  FIG. 5  in greater detail, according to an exemplary embodiment. 
         FIG. 9  is a flowchart of a process which may be performed by the state-based control system of  FIG. 5  for controlling an AHU such as the AHU of  FIG. 1  in a HVAC system, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring generally to the FIGURES, systems and methods for operating an air handling unit (AHU) in a building heating, ventilating, and air conditioning (HVAC) system are shown, according to various exemplary embodiments. The systems and methods described herein implement a state-based technique to control the temperature of a building zone T zone  by modulating a supply fan and one or more states of heating or cooling. One implementation of the present disclosure is a state-based control system that has multiple operating states or modes and can transition between the various operating states based on the heating or cooling demand from the building zone. Although the systems and methods of the present disclosure are described primarily with reference to cooling systems, it is understood that the same or similar control techniques can readily be applied to heating systems, humidity control systems, air quality control systems, or other types of control systems for use in controlling any variable state or condition in a building or other controlled environment. 
     In some embodiments, the state-based control system includes a finite state machine configured to cause a transition between a high cooling load state and a low cooling load state. In the high cooling load state, the system may maintain the temperature of a supply airstream T sa  at a fixed setpoint and control the temperature of the building zone T zone  by modulating the speed of a supply air fan. In the low cooling load state, the system may operate the supply air fan at a fixed speed and control the temperature of the building zone T zone  by modulating an amount of cooling applied to the supply airstream by one or more cooling stages. 
     The state-based control system may include a fan control loop configured to modulate the speed of the supply air fan and a cooling control loop configured to modulate the amount of cooling applied by the one or more cooling stages. In some embodiments, the cooling control loop is a cascaded control loop. An outer loop of the cascaded control loop may determine a setpoint supply air temperature T sa,sp  based on a measured temperature of the building zone T zone  and a zone temperature setpoint T zone,sp . An inner loop of the cascaded control loop may use the setpoint supply air temperature T sa,sp  from the outer loop to modulate the amount of cooling applied to the supply airstream. 
     Transitions between the low cooling load state and the high cooling load state may be based on the saturation status of the fan control loop and/or the cooling control loop. For example, when the system is operating in the low cooling load state, the finite state machine may monitor a saturation status of the cooling control loop. If the saturation status of the control loop is greater than or equal to a threshold value, the finite state machine may cause a transition into the high cooling load state. When the system is operating in the high cooling load state, the finite state machine may monitor a saturation status of the fan control loop. If the saturation status of the fan loop is less than or equal to a threshold value, the finite state machine may cause a transition into the low cooling load state. 
     In some embodiments, the fan control loop includes a feed-forward module configured to calculate and apply a feed-forward gain to the supply air fan setpoint S fan . Advantageously, the feed-forward gain allows the state-based control system to anticipate and manage disturbances caused by adding or shedding cooling stages before such disturbances are detected as fluctuations in the building zone temperature T zone . For example, the feed-forward module may calculate a feed-forward gain that causes an amount of cooling provided to the building zone after the change in the number of active cooling stages to be equivalent or substantially equivalent to the amount of cooling provided to the building zone before the change in the active number of cooling stages. The feed-forward gain may be applied to the supply air fan setpoint S fan  to calculate an adjusted setpoint S fan,adj  for the supply air fan. These and other advantages of the systems and methods of the present disclosure are described in greater detail in the following paragraphs. 
     Referring now to  FIG. 1 , a perspective view of a building  10  is shown. Building  10  is serviced by HVAC system  20 . HVAC system  20  is shown to include a chiller  22 , a boiler  24 , and a rooftop air handling unit (AHU)  26 . HVAC system  20  uses a fluid circulation system to provide heating and/or cooling for building  10 . The circulated fluid (e.g., water, glycol, etc.) may be cooled in chiller  22  or heated in boiler  24 , depending on whether cooling or heating is required in building  10 . Boiler  24  may add heat to the circulated fluid, for example, by burning a combustible material (e.g., natural gas). Chiller  22  may place the circulated fluid in a heat exchange relationship with another fluid (e.g., a refrigerant) in a heat exchanger (e.g., an evaporator) to absorb heat from the circulated fluid. The circulated fluid from chiller  22  or boiler  24  may be transported to AHU  26  via piping  28 . AHU  26  may place the circulated fluid in a heat exchange relationship with an airflow passing through AHU  26  (e.g., via one or more stages of cooling coils and/or heating coils). The airflow may be, for example, outside air, return air from within building  10 , or a combination of both. AHU  26  may transfer heat between the airflow and the circulated fluid to provide heating or cooling for the airflow. For example, AHU  26  may include one or more fans or blowers configured to pass the airflow over or through a heat exchanger containing the circulated fluid. The circulated fluid may then return to chiller  22  or boiler  24  via piping  30 . 
     The airflow supplied by AHU  26  (i.e., the supply airflow) may be delivered to building  10  via an air distribution system including air supply ducts  38  and may return to AHU  26  from building  10  via air return ducts  40 . In some embodiments, building  10  includes a plurality variable air volume (VAV) units  27 . For example, HVAC system  20  is shown to include a separate VAV unit  27  on each floor or zone of building  10 . VAV units  27  may include dampers or other flow control elements which can be operated to control an amount of the supply airflow provided to individual zones of building  10 . In other embodiments, AHU  26  delivers the supply airflow into one or more zones of building  10  (e.g., via supply ducts  38 ) without requiring intermediate flow control elements. AHU  26  may include various sensors (e.g., temperature sensors, pressure sensors, etc.) configured to measure attributes of the supply airflow. AHU  26  may also receive input from sensors located within the building zone and may adjust the flow rate and/or temperature of the supply airflow through AHU  26  to achieve setpoint conditions for the building zone. 
     Referring now to  FIG. 2 , a block diagram illustrating AHU  26  in greater detail is shown, according to an exemplary embodiment. AHU  26  is shown as an economizer-type air handling unit. Economizer-type air handling units vary the amount of outside air and return air used by the air handling unit for heating or cooling. For example, AHU  26  may receive return air  42  from building zone  12  via return air duct  40  and may deliver supply air  44  to building zone  12  via supply air duct  38 . In some embodiments, AHU  26  is a rooftop unit and may be located on the roof of building  10  (e.g., as shown in  FIG. 1 ) or otherwise positioned to receive return air  42  and outside air  46 . AHU  26  may be configured to operate exhaust air damper  50 , mixing damper  52 , and outside air damper  54  to control an amount of outside air  46  and return air  42  that combine to form supply air  44 . Any return air  42  that does not pass through mixing damper  52  may be exhausted from AHU  26  through exhaust damper  50  as exhaust air  48 . 
     Each of dampers  50 - 54  may be operated by an actuator. As shown in  FIG. 2 , exhaust air damper  50  may be operated by actuator  60 , mixing damper  52  may be operated by actuator  62 , and outside air damper  54  may be operated by actuator  64 . Actuators  60 - 64  may communicate with an AHU controller  70  via a communications link  80 . Actuators  60 - 64  may receive control signals from AHU controller  70  and may provide feedback signals to AHU controller  70 . Feedback signals may include, for example, an indication of a current actuator or damper position, an amount of torque or force exerted by the actuator, diagnostic information (e.g., results of diagnostic tests performed by actuators  60 - 64 ), status information, commissioning information, configuration settings, calibration data, and/or other types of information or data that may be collected, stored, or used by actuators  60 - 64 . AHU controller  70  may be an economizer controller configured to use one or more control algorithms (e.g., state-based algorithms, ESC algorithms, PID control algorithms, model predictive control algorithms, feedback control algorithms, etc.) to control actuators  60 - 64 . Several exemplary controllers that may be used as AHU controller  70  are described in greater detail with reference to  FIGS. 3-6 . 
     Still referring to  FIG. 2 , AHU  26  is shown to include a cooling coil  82 , a heating coil  84 , and a fan  86  positioned within supply air duct  38 . Fan  86  may be configured to force supply air  44  through cooling coil  82  and/or heating coil  84  and provide supply air  44  to building zone  12 . AHU controller  70  may communicate with fan  86  via communications link  88  to control a flow rate of supply air  44 . In some embodiments, AHU controller  70  controls an amount of heating or cooling applied to supply air  44  by modulating a speed of fan  86 . Cooling coil  82  may receive a chilled fluid from chiller  22  via piping  28  and may return the chilled fluid to chiller  22  via piping  30 . Valve  94  may be positioned along piping  28  or piping  30  to control an amount of the chilled fluid provided to cooling coil  82 . In some embodiments, cooling coil  82  includes multiple stages of cooling coils that can be independently activated and deactivated (e.g., by AHU controller  70 ) to modulate an amount of cooling applied to supply air  44 . Heating coil  84  may receive a heated fluid from boiler  24  via piping  28  and may return the heated fluid to boiler  24  via piping  30 . Valve  96  may be positioned along piping  28  or piping  30  to control an amount of the heated fluid provided to heating coil  84 . In some embodiments, heating coil  84  includes multiple stages of heating coils that can be independently activated and deactivated to modulate an amount of heating applied to supply air  44 . 
     Each of valves  94 - 96  may be controlled by an actuator. As shown in  FIG. 2 , valve  94  may be controlled by actuator  97  and valve  96  may be controlled by actuator  99 . Actuators  97 - 99  may communicate with AHU controller  70  via communications links  90 - 92 . Actuators  97 - 99  may receive control signals from AHU controller  70  and may provide feedback signals to controller  70 . In some embodiments, AHU controller  70  receives a measurement of the supply air temperature from a temperature sensor  45  positioned in supply air duct  38  (e.g., downstream of cooling coil  82  and/or heating coil  84 ). AHU controller  70  may also receive a measurement of the temperature of building zone  12  from a temperature sensor  47  located in building zone  12 . 
     In some embodiments, AHU controller  70  operates valves  94 - 96  via actuators  97 - 99  to modulate an amount of heating or cooling provided to supply air  44  (e.g., to achieve a setpoint temperature for supply air  44  or to maintain the temperature of supply air  102  within a setpoint temperature range). The positions of valves  97 - 99  affect the amount of heating or cooling provided to supply air  44  by cooling coil  82  or heating coil  84  and may correlate with the amount of energy consumed to achieve a desired supply air temperature. AHU  70  may control the temperature of supply air  44  and/or building zone  12  by activating or deactivating coils  82 - 84 , adjusting a speed of fan  86 , or a combination of both. 
     In some embodiments, AHU controller  70  executes a state-based control algorithm to control the temperature of building zone  12 . For example, AHU controller  70  may include a finite state machine configured to cause AHU controller  70  to transition between a high cooling load state and a low cooling load state. In the high cooling load state, AHU controller  70  may maintain the temperature of supply air  44  at a fixed setpoint and control the temperature of building zone  12  by modulating a speed of supply air fan  86 . In the low cooling load state, AHU controller  70  may operate supply air fan  86  at a fixed speed and control the temperature of building zone  12  by modulating an amount of cooling provided to supply air  44  by the cooling coils  82 . The state-based control algorithm is described in greater detail with reference to  FIGS. 5-7 . 
     Still referring to  FIG. 2 , HVAC system  20  is shown to include a supervisory controller  72  and a client device  74 . Supervisory controller  72  may include one or more computer systems (e.g., servers, BAS controllers, etc.) that serve as system level controllers, application or data servers, head nodes, master controllers, or field controllers for HVAC system  20 . Supervisory controller  72  may communicate with multiple downstream building systems or subsystems (e.g., an HVAC system, a security system, etc.) via a communications link  76  according to like or disparate protocols (e.g., LON, BACnet, etc.). 
     In some embodiments, AHU controller  70  receives information (e.g., commands, setpoints, operating boundaries, etc.) from supervisory controller  72 . For example, supervisory controller  72  may provide AHU controller  70  with a high fan speed limit and a low fan speed limit. A low limit may avoid frequent component and power taxing fan start-ups while a high limit may avoid operation near the mechanical or thermal limits of the fan system. In various embodiments, AHU controller  70  and supervisory controller  72  may be separate (as shown in  FIG. 2 ) or integrated. In an integrated implementation, AHU controller  70  may be a software module configured for execution by a processor of supervisory controller  72 . 
     Client device  74  may include one or more human-machine interfaces or client interfaces (e.g., graphical user interfaces, reporting interfaces, text-based computer interfaces, client-facing web services, web servers that provide pages to web clients, etc.) for controlling, viewing, or otherwise interacting with HVAC system  20 , its subsystems, and/or devices. Client device  74  may be a computer workstation, a client terminal, a remote or local interface, or any other type of user interface device. Client device  74  may be a stationary terminal or a mobile device. For example, client device  74  may be a desktop computer, a computer server with a user interface, a laptop computer, a tablet, a smartphone, a PDA, or any other type of mobile or non-mobile device. Client device  74  may communicate with supervisory controller  72  and/or AHU controller  70  via communications link  78 . 
     Referring now to  FIG. 3 , a block diagram of a constant volume (CV) control system  300  that may be used in conjunction with AHU  26  is shown, according to an exemplary embodiment. CV control system  300  is shown to include supply air fan  86 , a plurality of cooling stages  83 , and a constant volume AHU controller  302 . In CV control system  300 , supply air fan  86  may be operated at a constant speed such that the flow rate of supply air  44  is constant or substantially constant (i.e., a constant flow volume). Constant volume AHU controller  302  may be configured to control the temperature of supply air  44  (and consequently the temperature of building zone  12 ) by activating or deactivating various stages of cooling stages  83 . Cooling stages  83  may include, for example, one or more stages of cooling devices (e.g., cooling coils, evaporators, chillers, etc.) that can be independently activated and deactivated by cooling controller  304  to modulate an amount of cooling applied to supply air  44 . In some embodiments, CV control system  300  includes one or more heating stages in addition to or in place of cooling stages  83 . 
     Still referring to  FIG. 3 , cooling controller  304  is shown receiving a zone temperature setpoint T zone,sp  indicating a desired temperature or acceptable temperature range for building zone  12 . The zone temperature setpoint T zone,sp  may be received, for example, from a supervisory controller, from a client device, or any other data source. Cooling controller  304  is also shown receiving a temperature input T zone  from a temperature sensor  47  positioned to measure the temperature within building zone  12 . Cooling controller  304  may compare the measured temperature T zone  with the setpoint temperature T zone,sp  to generate an error signal e (e.g., e=T zone,sp −T zone ). Based on the value of error signal e, cooling controller  304  may activate or deactivate various stages of cooling stages  83  such that error signal e is minimized. Cooling controller  304  may use any type of control methodology (e.g., proportional control, proportional-integral (PI) control, proportional-integral-derivative (PID) control, model predictive control, other types of feedback control, etc.) to determine a control signal u for cooling stages  83  based on the value of error signal e. 
     In CV control system  300 , cooling controller  304  may activate a lesser number of cooling stages  83  during low load conditions and a greater number of cooling stages  83  during high load conditions. When fewer of cooling stages  83  are active, the temperature of supply air  44  may increase, thereby providing less latent cooling to building zone  12 . Supply air fan  86  may continuously move the same volume of supply air  44  in CV control system  300 . Accordingly, the control methodology used in CV control system  300  may cause supply air fan  86  to consume the same amount of energy regardless of load conditions. 
     Referring now to  FIG. 4 , a block diagram of a variable volume (VV) control system  400  that may be used in conjunction with AHU  26  is shown, according to an exemplary embodiment. In VV control system  400 , the flow of supply air  44  to building zone  12  is controlled by both a variable volume AHU controller  402  and a variable air volume (VAV) box controller  404 . Variable volume AHU controller  402  may be used to control operation of AHU  26  and provide supply air  44  to a downstream VAV box  85 . VAV box  85  may include, for example, one or more dampers or other flow control elements configured to control the flow of supply air  44  into building zone  12 . VAV box controller  404  may operate VAV box  85  to modulate the flow of supply air  44  into building zone  12 . 
     Variable volume AHU controller  402  is shown to include two separate control loops. In the first control loop, cooling controller  406  receives a supply air temperature setpoint T sa,sp  indicating the desired temperature or acceptable temperature range for the temperature of supply air  44 . The supply air temperature setpoint T sa,sp  may be received, for example, from a supervisory controller, from a client device, or any other data source. Cooling controller  406  may also receive a temperature input T sa  from a temperature sensor  45  positioned to measure the temperature of supply air  44 . Cooling controller  406  may compare the measured temperature T sa  with the setpoint temperature T sa,sp  to generate a control signal for cooling stages  83 . For example, cooling controller  404  may activate or deactivate various stages of cooling stages  83  to control the supply air temperature T sa  to the supply air temperature setpoint T sa,sp . 
     In the second control loop, fan controller  408  receives a duct static pressure setpoint P static,sp  indicating the desired static pressure of supply air  44  in supply air duct  38 . The duct static pressure setpoint P static,sp  may be received, for example, from a supervisory controller, from a client device, or any other data source. Fan controller  408  may also receive a pressure input P static  from a pressure sensor  49  positioned to measure the static pressure of supply air  44  in duct  38 . Fan controller  408  may compare the measured pressure P static  with the static pressure setpoint P static,sp  to generate a control signal for supply air fan  86 . For example, fan controller  408  may increase or decrease the speed of fan  86  to control the supply air static pressure P static  to the supply air pressure setpoint P static,sp . Supply air  44  is then delivered via supply air duct  38  to VAV box  85  at the temperature and pressure conditions controlled by variable volume AHU controller  402 . 
     Still referring to  FIG. 4 , VAV box controller  404  may operate VAV box  85  to modulate the flow of supply air  44  into building zone  12 . VAV box controller  404  may use a cascaded control scheme to control the temperature of building zone  12  and to ensure a minimum volume of supply air  44  entering building zone  12 . The outer loop of the cascaded control scheme is shown to include a flow controller  414  that receives a zone temperature setpoint T zone,sp  indicating a desired temperature or acceptable temperature range for building zone  12 . The zone temperature setpoint T zone,sp  may be received, for example, from a supervisory controller, from a client device, or any other data source. Flow controller  414  may also receive a temperature input T zone  from a temperature sensor  47  positioned to measure the temperature within building zone  12 . Flow controller  414  compares the zone temperature setpoint T zone,sp  with the temperature of building zone  12  T zone  to determine a flow setpoint Flow sp  for VAV controller  412 . For example, if the measured zone temperature T zone  is greater than the zone temperature setpoint T zone,sp , flow controller  414  may increase the flow setpoint Flow sp  to cause more supply air  44  to enter building zone  12 , thereby increasing the cooling provided to building zone  12  and decreasing the measured temperature T zone . 
     The inner loop of the cascaded control scheme is shown to include a pressure-to-flow converter  410  and a VAV controller  412 . Pressure-to-flow converter  410  may be configured to receive velocity pressure input P vel  from a pressure sensor  51  positioned to measure the velocity pressure of supply air  44  received at VAV box  85 . Pressure-to-flow converter  410  may convert the measured velocity pressure P vel  into an airflow rate Flow and provide the flow rate Flow to VAV controller  412 . VAV controller  412  may compare the flow setpoint Flow sp  with the actual flow rate Flow of supply air  44  to generate a control signal for VAV box  85  such that the actual flow rate Flow is controlled to the flow rate setpoint Flow sp . 
     Still referring to  FIG. 4 , the control operations performed by variable volume AHU controller  402  and VAV box controller  404  may be coupled together in VV control system  400 . For example, as VAV box  85  opens to allow more airflow into building zone  12 , the static pressure in duct  38  may decrease. Such a decrease in static pressure represents an increase in load. Variable volume AHU controller  402  may respond to the increase in load by increasing the speed of supply air fan  86  to maintain the measured duct static pressure P static  at the duct static pressure setpoint P static,sp  and/or activating additional stages of cooling stages  83  to maintain the measured temperature T sa  of supply air  44  at the supply air temperature setpoint T sa,sp . 
     Unlike CV control system  300 , VV control system  400  modulates the speed of supply air fan  86  as the load changes and the temperature of supply air  44  is controlled to a relatively constant temperature (i.e., the supply air temperature setpoint T sa,sp ). The performance of VV control system  400  in controlling the supply air temperature T sa  may depend on the particular configuration of variable volume AHU controller  402  (e.g., staged cooling or proportional control, the number of cooling stages  83 , etc.). In some embodiments, VV control system  400  controls the supply air temperature T sa  to a relatively lower setpoint than CV control system  300 , resulting in more latent cooling. 
     Referring now to  FIG. 5 , a block diagram of a state-based control system  500  that may be used in conjunction with AHU  26  is shown, according to an exemplary embodiment. In state-based control system  500 , a state-based AHU controller  502  may be used to control both the speed of supply air fan  86  and the amount of cooling provided by cooling stages  83 . Like VV control system  400 , state-based control system  500  may modulate the speed of supply air fan  86  as the zone load changes to control the supply air temperature T sa  to a supply air temperature setpoint T sa,sp . However, state-based control system  500  advantageously does not rely on pressure measurements and does not require a downstream VAV controller to control the flow of supply air  44  into building zone  12 . 
     State-based control system  500  is configured to operate in multiple different states or modes. For example, state-based AHU controller  502  is shown to include a finite state machine  510  configured to cause state-based AHU controller  502  to transition between a high cooling load state and a low cooling load state. In the high cooling load state, state-based AHU controller  502  may maintain the temperature of supply air  44  at a fixed setpoint and control the temperature of building zone  12  by modulating a speed of supply air fan  86 . In the low cooling load state, state-based AHU controller  502  may operate supply air fan  86  at a fixed speed and control the temperature of building zone  12  by modulating an amount of cooling provided to supply air  44  by cooling stages  83 . 
     State-based AHU controller  502  is shown to include a fan control loop and a cooling control loop. The fan control loop is shown to include a building zone temperature sensor  47 , a fan controller  512 , and supply air fan  86 . Building zone temperature sensor  47  may be configured to measure a temperature T zone  of building zone  12 . Fan controller  512  may use the difference between the zone temperature T zone  and a setpoint temperature T zone,sp  for building zone  12  to determine a speed setpoint S fan  for supply fan  86 . In some embodiments, the fan control loop further includes a feed-forward controller  514  and/or a switch  516 . When a change in the number of active cooling stages  83  is detected, feed-forward controller  514  may adjust the fan speed setpoint S fan  to generate an adjusted fan speed setpoint S fan,adj  and provide the adjusted fan speed setpoint S fan,adj  to switch  516 . Switch  516  selects whether to use the adjusted fan speed setpoint S fan,adj  or a fixed fan speed, depending on the current operating state of state-based control system  500  (described in greater detail below). 
     Still referring to  FIG. 5 , the cooling control loop is shown as a cascaded control loop having an outer control loop and an inner control loop. The outer cascaded control loop is shown to include building zone temperature sensor  47 , a zone temperature controller  504 , and a switch  506 . Zone temperature controller  504  may use the difference between the zone temperature T zone  and a setpoint temperature T zone,sp  for building zone  12  to determine a setpoint T sa,sp  for the temperature of supply air  44 . Zone temperature controller  504  provides the setpoint T sa,sp  to switch  506 . Switch  506  selects whether to use the supply air temperature setpoint determined by zone temperature controller  504  or a fixed temperature setpoint to control the temperature of supply air  44 , depending on the current operating state of state-based control system  500 . 
     The inner cascaded control loop is shown to include supply air temperature sensor  45 , cooling controller  508 , and cooling stages  83 . Supply air temperature sensor  45  measures the temperature T sa  of supply air  44  at a location downstream of cooling stages  83 . Cooling controller  508  may use the difference between the supply air temperature T sa  and the supply air temperature setpoint provided by switch  506  (e.g., the supply air temperature setpoint determined by zone temperature controller  504  or a fixed temperature setpoint) to determine an output for cooling stages  83 . For example, cooling controller  508  may activate or deactivate various stages of cooling stages  83  to control the supply air temperature T sa  to the supply air temperature setpoint. 
     Still referring to  FIG. 5 , finite state machine  510  may be configured to set the current operating state for state-based AHU controller  502 . In some embodiments, finite state machine  510  transitions between a high cooling load state (i.e., “State  1 ” in  FIG. 5 ) and a low cooling load state (i.e., “State  2  in  FIG. 5 ) based on the saturation status of zone temperature controller  504  and/or fan controller  512 . For example, finite state machine  510  is shown receiving saturation status inputs from zone temperature controller  504  and fan controller  512 . The saturation status of a controller may indicate whether the control loop in which the controller is located has any further capacity to affect a change in the controlled variable. For example, the saturation status of zone temperature controller  504  may indicate whether zone temperature controller  504  can further decrease the temperature of building zone  12  by increasing the amount of cooling provided by cooling stages  83 . The saturation status of fan controller  512  may indicate whether fan controller  512  can further decrease the temperature of building zone  12  by increasing the speed of supply air fan  86 . 
     In some embodiments, saturation status is represented as a percentage (e.g., 0% saturated, 50% saturated, 100% saturated, etc.) or normalized value (e.g., 0.0, 0.5. 1.0, etc.). Higher saturation status values indicate that the corresponding control loop is closer to its maximum capacity and lower saturation status values indicating that the corresponding control loop is further from its maximum capacity. For example, if the current saturation status of zone temperature controller  504  is 100%, any further decrease in the supply air temperature setpoint T sa,sp  set by zone temperature controller  504  may not translate into a decrease in the measured zone temperature T zone  or the temperature of supply air  44  because the cooling control loop is at maximum capacity (e.g., all of the cooling stages are already active). Similarly, if the current saturation status of fan controller  512  is 100%, any further increase in the fan speed setpoint S fan  set by fan controller  512  may not translate into a decrease in the measured zone temperature T zone  or the temperature of supply air  44  because the fan control loop is at maximum capacity (e.g., fan  86  is already at its maximum speed). 
     Finite state machine  510  may use the saturation status of zone temperature controller  504  and/or fan controller  512  to determine whether to transition between the high cooling load state and the low cooling load state. For example, when state-based control system  500  is operating in the high cooling load state, finite state machine  510  may be configured to identify the saturation status of fan controller  512 . Finite state machine  510  may compare the saturation status of fan controller  512  with a lower threshold and cause a transition from the high cooling load state into the low cooling load state in response to the saturation status of fan controller  512  being less than or equal to the lower threshold value (e.g., 0%, less than 10%, less than 20%, etc.). 
     When state-based control system  500  is operating in the low cooling load state, finite state machine  510  may be configured to identify the saturation status of zone temperature controller  504 . Finite state machine  510  may compare the saturation status of zone temperature controller  504  with an upper threshold and cause a transition from the low cooling load state into the high cooling load state in response to the saturation status of zone temperature controller being greater than or equal to the upper threshold value (e.g., 100%, greater than 90%, greater than 80%, etc.). 
     Finite state machine  510  may output a state to switches  506  and  516  indicating the current operating state for state-based control system  500 . For example, upon transitioning into the high cooling load state, finite state machine  510  may generate and provide a state output which causes switches  506  and  516  to switch to “State  1 ,” as shown in  FIG. 5 . In the high cooling load state, switch  506  provides cooling controller  508  with a fixed temperature setpoint and switch  516  provides supply air fan  86  with the adjusted speed setpoint S fan,adj  set by feed-forward controller  514 . Thus, in the high load cooling state, state-based AHU controller  502  may maintain the temperature of supply air  44  at a fixed temperature setpoint and control the temperature of building zone  12  by modulating a speed of supply air fan  86  based on the speed setpoint S fan  determined by fan controller  512 . 
     Upon transitioning into the low cooling load state, finite state machine  510  may generate and provide a state output which causes switches  506  and  516  to switch to “State  2 ,” as shown in  FIG. 5 . In the low cooling load state, switch  506  provides cooling controller  508  with the supply air temperature setpoint T sa,sp  set by zone temperature controller  504  and switch  516  provides supply air fan  86  with a fixed speed setpoint. Thus, in the low load cooling state, state-based AHU controller  502  may operate supply air fan  86  at a fixed speed and control the temperature of building zone  12  by modulating an amount of cooling provided to supply air  44  by cooling stages  83  based on the supply air temperature setpoint T sa,sp  set by zone temperature controller  504 . 
     Still referring to  FIG. 5 , state-based AHU controller  502  is shown to include a feed-forward controller  514 . Feed-forward controller  514  may be configured to adjust the fan speed setpoint S fan  from fan controller  512  and provide an adjusted fan speed setpoint S fan,adj  to supply air fan  86  (e.g., via switch  516 ). Advantageously, feed-forward controller  514  may be configured to manage disturbances caused by activating or deactivating one or more discrete stages of cooling stages  83 . For example, feed-forward controller  514  may be configured to increase the speed of fan  86  when a cooling stage is deactivated and to decrease the speed of fan  86  when a cooling stage is activated such that the amount of cooling provided to building zone  12  remains substantially constant throughout the transition. 
     The amount of cooling provided to building zone  12  can be expressed using the equation:
 
 Q={dot over (m)}h=ωρc   p ( T   sa   −T   zone )
 
where Q is the cooling load, ω is the flow rate of supply air  44 , ρ is the density of supply air  44 , c p  is the specific heat capacity of supply air  44 , T sa  is the temperature of supply air  44 , and T zone  is the temperature of building zone  12 . A negative value for Q indicates that heat is being removed from building zone  12 . Assuming steady state conditions prior to changing the number of active stages of cooling stages  83 , the zone temperature setpoint T zone,sp  can be substituted for the zone temperature T zone  and the supply air temperature setpoint T sa,sp  can be substituted for the temperature of the supply air T sa .
 
     Prior to changing the number of active cooling stages, the amount of cooling provided to building zone  12  can be expressed using the equation:
 
 Q   1 =ω 1   ρc   p ( T   sa,sp   −T   zone,sp )
 
where ω 1  is the flow rate of supply air  44  prior to changing the number of active cooling stages, T sa,sp  is the temperature setpoint for supply air  44 , and T zone,sp  the temperature setpoint for building zone  12 .
 
     After changing the number of active cooling stages, the amount of cooling provided to building zone  12  can be expressed using the equation:
 
 Q   2 =ω 2   ρc   p ( T   sa   −T   zone,sp )
 
where ω t  is the flow rate of supply air  44  after changing the number of active cooling stages and T sa  is the new measured temperature of supply air  44  after changing the number of active cooling stages.
 
     In some embodiments, it is desirable to have the same Q entering building zone  12  before and after the number of cooling stages changes (i.e., Q 1 =Q 2 ). Accordingly, Q 1  can be set equal to Q 2  and the resulting equation can be solved for the airflow ratio ω 2 /ω 1  that that results in Q 1 =Q 2 . For example: 
     
       
         
           
             
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     Feed-forward controller  514  may receive a signal from cooling controller  508  indicating when the number of active cooling stages changes. In response to a change in the number of active cooling stages, feed-forward controller  514  may calculate the ratio ω 2 /ω 1  using the preceding equation and apply the calculated ratio as a feed-forward gain to the fan speed setpoint S fan . Feed-forward controller  514  may calculate the adjusted fan speed S fan,adj  by multiplying S fan  by the feed forward gain. For example: 
     
       
         
           
             
               S 
               
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     Advantageously, the feed-forward compensation technique applied by feed-forward controller  514  enables state-based AHU controller to anticipate and handle disturbances caused by changing the number of active cooling stages before such disturbances have an effect on the measured building zone temperature T zone . 
     Referring now to  FIG. 6 , a state transition diagram  600  illustrating several operating states  602 - 616  of state-based control system  500  are shown, according to an exemplary embodiment. Transitions between operating states  602 - 616  may be controlled by finite state machine  510  based on the value of T zone  and the saturation status of zone temperature controller  504  and/or fan controller  512 , as described with reference to  FIG. 5 . State transition diagram  600  is shown to include a start state  602 . In start state  602 , finite state machine  510  may determine whether the measured temperature of T zone  is reliable. If T zone  is reliable, finite state machine  510  may cause state-based control system  500  to transition into a temperature reliable state  604  (transition  620 ). However, if T zone  is unreliable, finite state machine  510  may cause state-based control system to transition into a temperature unreliable state  606  (transition  622 ). State-based control system  500  may transition from temperature reliable state  604  into temperature unreliable state  606  at any time if the value of T zone  is determined to be unreliable (transition  624 ). Similarly, state-based control system  500  may transition from temperature unreliable state  606  into temperature reliable state  604  if the value of T zone  is determined to be reliable (transition  626 ). 
     Temperature reliable state  604  is shown to include a heating required state  608 , a cooling required state  612 , and a no heating or cooling required state  610 . Finite state machine  510  may cause state-based control system  500  to transition into heating required state  608  if the value of T zone  is less than a heating setpoint (transition  628 ) and out of heating required state  608  if the value of T zone  is greater than or equal to the heating setpoint (transition  630 ). Finite state machine  510  may cause state-based control system  500  to transition into cooling required state  612  if the value of T zone  is greater than a cooling setpoint (transition  632 ) and out of cooling required state  612  if the value of T zone  is less than or equal to the cooling setpoint (transition  634 ). Finite state machine  510  may cause state-based control system  500  to transition into no cooling or heating required state  610  if the value of T zone  is greater than or equal to the heating setpoint (transition  630 ) or less than or equal to the cooling setpoint (transition  634 ) and out of no cooling or heating required state  610  if the value of T zone  is less than the heating setpoint (transition  628 ) or greater than the cooling setpoint (transition  632 ). 
     Cooling required state  612  is shown to include a low cooling load state  614  and a high cooling load state  616 . Finite state machine  510  may cause state-based control system  500  to transition into low cooling load state  614  if the saturation status of fan controller  512  is less than or equal to a lower threshold value (i.e., Sat 1 ≦Thresh low ). Finite state machine  510  may cause state-based control system  500  to transition into high cooling load state  616  if the saturation status of zone temperature controller  504  is greater than or equal to an upper threshold value (i.e., Sat 2 ≦Thresh high ). 
     Referring now to  FIG. 7 , a chart  700  of the state outputs in a selection of the operating states  602 - 616  shown in state transition diagram  600  are shown, according to an exemplary embodiment. In heating required state  608 , no heating or cooling required state  610 , and temperature unreliable state  606 , several of the controllers shown in  FIG. 5  may be deactivated or not used. For example, chart  700  shows zone temperature controller  504 , fan controller  512 , cooling controller  508 , and cooling stages  83  with values of “off” in states  606 - 610 . In states  608 - 610  supply air fan  86  may be fixed at a maximum speed. 
     In low cooling load state  614 , zone temperature controller  504  may be used to modulate the supply air setpoint T sa,sp  based on the value of T zone . Cooling controller  508  may receive the value of T sa,sp  from zone temperature controller  504  (e.g., via switch  506 ) and use the value of T sa,sp  to modulate cooling stages  83 . In low cooling load state  614 , fan controller  512  may be turned off or not used and supply air fan  86  may receive a fixed speed setpoint via switch  516 . 
     In high cooling load state  616 , zone temperature controller  504  may be turned off or not used. Cooling controller  508  may receive a fixed supply air setpoint via switch  506  and use the fixed supply air setpoint to modulate cooling stages  83 . In high cooling load state  616 , fan controller  512  may modulate the fan speed setpoint S fan  based on the value of T zone.  The fan speed setpoint S fan  may be adjusted by feed-forward controller  514  and the adjusted value S fan,adj  may be passed through switch  616  to supply air fan  86 . 
     Referring now to  FIG. 8 , a block diagram illustrating state-based AHU controller  502  in greater detail is shown, according to an exemplary embodiment. State-based AHU controller  502  is shown to include a communications interface  802  and a processing circuit  804 . Communications interface  802  may include wired or wireless interfaces (e.g., jacks, antennas, transmitters, receivers, transceivers, wire terminals, etc.) for conducting data communications with various systems, devices, or networks. For example, communications interface  802  may include an Ethernet card and/or port for sending and receiving data via an Ethernet-based communications network. In some embodiments, communications interface  802  includes a wireless transceiver (e.g., a WiFi transceiver, a Bluetooth transceiver, a NFC transceiver, etc.) for communicating via a wireless communications network. Communications interface  802  may be configured to communicate via local area networks (e.g., a building LAN) and/or wide area networks (e.g., the Internet, a cellular network, a radio communication network, etc.) and may use a variety of communications protocols (e.g., BACnet, TCP/IP, point-to-point, etc.). 
     In some embodiments, communications interface  802  receives measurement inputs from sensors  840 . Sensors  840  may include, for example, temperature sensor  45  configured to measure the temperature T sa  of supply air  44  in supply air duct  38  and temperature sensor  47  configured to measure the temperature T zone  of the air in building zone  12 . Communications interface  802  may receive sensor inputs directly from sensors  840 , via a local or remote communications network, and/or via an intermediary downstream controller  842 . For example, if state-based AHU controller is implemented in a supervisory controller or enterprise controller, sensor inputs may be collected by a downstream controller  842  (e.g., a local controller, a device controller, etc.) and forwarded to state-based AHU controller  502 . In other embodiments, state-based AHU controller  502  is implemented in AHU  26  and receives sensor inputs directly from sensors  840 . 
     Communications interface  802  may enable communications between state-based AHU controller  502 , downstream controller  842 , an upstream controller  844  and/or a client device  846 . For example, state-based AHU controller  502  may receive sensor inputs from downstream controller  842  via communications interface  802 . State-based AHU controller  502  may use the sensor inputs to generate control signals for supply air fan  86  and cooling stages  83  and output the control signals via communications interface  802 . Communications interface  802  may facilitate user interaction with state-based AHU controller  502  via client device  846 . For example, state-based AHU controller  502  may receive a setpoint temperature for building zone  12  T zone,sp  from client device  846  (e.g., a computer terminal, a wall-mounted interface, etc.) and use the setpoint temperature T zone,sp  to generate control signals for supply air fan  86  and cooling stages  83  as described above. 
     Still referring to  FIG. 8 , processing circuit  804  is shown to include a processor  806  and memory  808 . Processor  806  may be a general purpose or specific purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGAs), a group of processing components, or other suitable processing components. Processor  806  may be configured to execute computer code or instructions stored in memory  808  or received from other computer readable media (e.g., CDROM, network storage, a remote server, etc.) to perform one or more of the FDD processes described herein. 
     Memory  808  may include one or more data storage devices (e.g., memory units, memory devices, computer-readable storage media, etc.) configured to store data, computer code, executable instructions, or other forms of computer-readable information. Memory  808  may include random access memory (RAM), read-only memory (ROM), hard drive storage, temporary storage, non-volatile memory, flash memory, optical memory, or any other suitable memory for storing software objects and/or computer instructions. Memory  808  may include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described in the present disclosure. Memory  808  may be communicably connected to processor  806  via processing circuit  804  and may include computer code for executing (e.g., by processor  806 ) one or more of the control processes described herein. Memory  208  is shown to include a zone temperature control module  810 , a cooling control module  812 , a fan control module  814 , a feed-forward module  816 , a finite state module  818 , a supply air setpoint switching module  820 , a fan speed setpoint switching module  822 , a low cooling load control module  824 , and a high cooling load control module  826 . 
     Still referring to  FIG. 8 , memory  808  is shown to include a zone temperature control module  810 . Zone temperature control module  810  may be configured to perform the functions of zone temperature controller  504 , as described with reference to  FIG. 5 . For example, zone temperature control module  810  may use a difference between a measured zone temperature T zone  and a setpoint temperature T zone,sp  for building zone  12  to determine a setpoint T sa,sp  for the temperature of supply air  44 . Zone temperature control module  810  may provide the setpoint supply air temperature T sa,sp  to supply air setpoint switching module  820 . In some embodiments, zone temperature control module  810  provides a saturation status to finite state module  818 . The saturation status provided by zone temperature control module  810  may indicate whether the building zone temperature T zone  can be further decreased by modulating the supply air temperature setpoint T sa,sp . Finite state module  818  may use the saturation status from zone temperature control module  810  to determine whether to transition from a low cooling load state to a high cooling load state. 
     Still referring to  FIG. 8 , memory  808  is shown to include a supply air setpoint switching module  820 . Supply air setpoint switching module  820  may be configured to perform the functions of switch  506 , as described with reference to  FIG. 5 . For example, supply air setpoint switching module  820  may receive a state input from finite state module  818  indicating the current operating state of state-based AHU controller  502 . If the state input indicates a high cooling load state (e.g., state  616 ), supply air setpoint switching module  820  may provide cooling control module  812  with a fixed temperature setpoint for use as the supply air temperature setpoint T sa,sp . However, if the state input indicates a low cooling load state (e.g., state  614 ), supply air setpoint switching module  820  may provide cooling control module  812  with the supply air setpoint determined by zone temperature control module  810 . 
     Still referring to  FIG. 8 , memory  808  is shown to include a cooling control module  812 . Cooling control module  812  may be configured to perform the functions of cooling controller  508 , as described with reference to  FIG. 5 . For example, cooling control module  812  may use the difference between the supply air temperature T sa  and the supply air temperature setpoint provided by supply air setpoint switching module  820  (e.g., the supply air temperature setpoint determined by zone temperature control module  810  or a fixed temperature setpoint) to determine an output for cooling stages  83 . For example, cooling control module  812  may activate or deactivate various stages of cooling stages  83  to control the supply air temperature T sa  to the supply air temperature setpoint. In some embodiments, cooling control module  812  provides a signal to feed forward module  816  when the number of active cooling stages changes. 
     Still referring to  FIG. 8 , memory  808  is shown to include a fan control module  814 . Fan control module  814  may be configured to perform the functions of fan controller  512 , as described with reference to  FIG. 5 . For example, fan control module  814  may use a difference between a measured zone temperature T zone  and a setpoint temperature T zone,sp  for building zone  12  to determine a setpoint S fan  for the speed of supply air fan  86 . Fan control module  814  may provide the setpoint fan speed S fan  to feed forward module  816 . In some embodiments, fan control module  814  provides a saturation status to finite state module  818 . The saturation status provided by fan control module  814  may indicate whether the building zone temperature T zone  can be further decreased by modulating the setpoint S fan . Finite state module  818  may use the saturation status from fan control module  814  to determine whether to transition from a high cooling load state to a low cooling load state. 
     Still referring to  FIG. 8 , memory  808  is shown to include a feed-forward module  816 . Feed-forward module  816  may be configured to perform the functions of feed-forward controller  514 , as described with reference to  FIG. 5 . For example, feed-forward module  816  may use a signal from cooling control module  812  to detect a change in the number of active cooling stages. When a change in the number of active cooling stages is detected, feed-forward module  816  may adjust the fan speed setpoint S fan  to generate an adjusted fan speed setpoint S fan,adj  and provide the adjusted fan speed setpoint S fan,adj  to fan speed setpoint switching module  822 . Advantageously, feed-forward module  816  may be configured to manage disturbances caused by activating or deactivating one or more discrete stages of cooling. For example, feed-forward module  816  may be configured to increase the speed of fan  86  when a cooling stage is deactivated and to decrease the speed of fan  86  when a cooling stage is activated such that the amount of cooling provided to building zone  12  remains substantially constant throughout the transition. 
     In response to a change in the number of active cooling stages, feed-forward module  816  may calculate a feed-forward gain to apply to the fan speed setpoint S fan . In some embodiments, feed-forward module  816  calculates the feed-forward gain using the following equation: 
                 ω   2       ω   1       =         T     sa   ,   sp       -     T     zone   ,   sp             T   sa     -     T     zone   ,   sp                 
where ω 1  is the flow rate of supply air  44  prior to changing the number of active cooling stages, ω 2  is the flow rate of supply air  44  after changing the number of active cooling stages, T sa,sp  is the temperature setpoint for supply air  44  (or the temperature of supply air  44  prior to changing the number of active cooling stages), T sa  is the new measured temperature of supply air  44  after changing the number of active cooling stages, and T zone,sp  the temperature setpoint for building zone  12  (or the measured temperature of building zone  12  prior to changing the number of cooling stages).
 
     Feed-forward module  816  may then calculate the adjusted fan speed S fan,adj  by multiplying S fan  by the feed forward gain. For example: 
     
       
         
           
             
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     Still referring to  FIG. 8 , memory  808  is shown to include a fan speed setpoint switching module  822 . Fan speed setpoint switching module  822  may be configured to perform the functions of switch  516 , as described with reference to  FIG. 5 . For example, fan speed setpoint switching module  822  may receive a state input from finite state module  818  indicating the current operating state of state-based AHU controller  502 . If the state input indicates a high cooling load state (e.g., state  616 ), fan speed setpoint switching module  822  may provide supply air fan  86  with the adjusted fan speed setpoint S fan,adj  determined by feed-forward module  816 . However, if the state input indicates a low cooling load state (e.g., state  614 ), fan speed setpoint switching module  822  may provide supply air fan  86  with a fixed speed setpoint. 
     Still referring to  FIG. 8 , memory  808  is shown to include a finite state module  818 . Finite state module  818  may be configured to perform the functions of finite state machine  510 , as described with reference to  FIG. 5 . For example, finite state module  818  may cause state-based AHU controller  502  to transition between the high cooling load state and a low cooling load state. In some embodiments, finite state module  818  receives saturation status inputs from zone temperature control module  810  and/or and fan control module  814 . The saturation status of a control module may indicate whether the corresponding control loop has any further capacity to affect a change in the controlled variable. 
     Finite state module  814  may use the saturation status of zone temperature control module  810  and/or fan control module  814  to determine whether to transition between the high cooling load state and the low cooling load state. For example, when state-based controller  502  is operating in the high cooling load state, finite state module  814  may be configured to identify the saturation status provided by fan control module  814 . Finite state module  814  may compare the saturation status of fan control module  814  with a lower threshold and cause a transition from the high cooling load state into the low cooling load state in response to the saturation status provided by fan control module  814  being less than or equal to the lower threshold value (e.g., 0%, less than 10%, less than 20%, etc.). 
     When state-based controller  502  is operating in the low cooling load state, finite state module  814  may be configured to identify the saturation status provided by zone temperature control module  810 . Finite state module  814  may compare the saturation status of zone temperature control module  810  with an upper threshold and cause a transition from the low cooling load state into the high cooling load state in response to the saturation status of zone temperature control module  810  being greater than or equal to the upper threshold value (e.g., 100%, greater than 90%, greater than 80%, etc.). Finite state module  814  may output a state to supply air setpoint switching module  820  and fan speed setpoint switching module  822  indicating the current operating state for state-based controller  502 . 
     Still referring to  FIG. 8 , memory  808  is shown to include a low cooling load control module  824  and a high cooling load module  826 . Low load cooling control module  824  and high cooling load module  826  may be configured to operate state-based control system  500  in the low cooling load state  614  and high cooling load state  616 , respectively. In low cooling load state  614 , low load cooling control module  824  may operate supply air fan  86  at a fixed speed and control the temperature of building zone  12  by modulating an amount of cooling provided to supply air  44  by the cooling stages  83 . In the high cooling load state, high cooling load module  826  may maintain the temperature of supply air  44  at a fixed setpoint and control the temperature of building zone  12  by modulating a speed of supply air fan  86 . 
     Referring now to  FIG. 9 , a flowchart of a process  900  for operating an air handling unit (AHU) in a building HVAC system is shown, according to an exemplary embodiment. In some embodiments, process  900  is performed by state-based AHU controller  502 , as described with reference to  FIG. 5 . 
     Process  900  is shown to include using a supply air fan to provide a supply airstream to a building zone (step  902 ) and using one or more cooling stages to chill the supply airstream (step  904 ). The supply air fan may be a variable speed fan configured to operate at multiple different speeds based on the value of a control signal provided to the supply air fan. Each of the speeds may correspond to a different flowrate of the supply airstream to the building zone. The cooling stages may be positioned in the supply airstream and may include, for example, one or more stages of cooling devices (e.g., cooling coils, evaporators, chillers, etc.) that can be independently activated and deactivated to modulate an amount of cooling applied to the supply airstream. 
     Still referring to  FIG. 9 , process  900  is shown to include receiving a measured temperature of the supply airstream downstream of the cooling stages and a measured temperature of the building zone (step  906 ). The temperature of the supply airstream T sa  may be measured by a temperature sensor (e.g., temperature sensor  45 ) positioned downstream of the cooling stages in the supply airstream. The temperature of the building zone T zone  may be measured by a temperature sensor (e.g., temperature sensor  47 ) positioned in or near the building zone. The measured temperatures may be received at a communications interface of state-based AHU controller  502  and provided to processing circuit  804 . 
     Process  900  is shown to include operating in a high cooling load state in which the temperature of the supply airstream is maintained at a fixed setpoint and the temperature of the building zone is controlled by modulating a speed of the supply air fan (step  908 ). Operating in the high cooling load state may include using a fan control loop to modulate the speed of the supply air fan based on the temperature of the building zone. Step  908  may include providing a fixed supply air setpoint to a cooling controller (e.g., cooling controller  508 ). The cooling controller may use the fixed supply air setpoint to maintain the supply airstream at a constant or substantially constant temperature. Step  908  may include using a fan controller (e.g., fan controller  512 ) to determine a speed setpoint for the supply air fan based on the current temperature of the building zone. The fan controller may modulate the fan speed setpoint to achieve a setpoint temperature for the building zone. 
     Still referring to  FIG. 9 , process  900  is shown to include operating in a low cooling load state in which the supply air fan is operated at a fixed speed and the temperature of the building zone is controlled by modulating an amount of cooling provided to the supply air stream by the cooling stages (step  910 ). Operating in the low cooling load state may include using a cooling control loop. In some embodiments, the cooling control loop is a cascaded control loop. An outer loop of the cascaded control loop may determine a setpoint temperature for the supply airstream based on the temperature of the building zone. An inner loop of the cascaded control loop may then modulate the amount of cooling provided to the supply airstream by the cooling stages to achieve the setpoint temperature for the supply airstream. 
     Still referring to  FIG. 9 , process  900  is shown to include causing a transition between the high cooling load state and the low cooling load state based on a saturation status of a controller (step  912 ). The controller may be, for example, a zone temperature controller of the cooling control loop (e.g., zone temperature controller  504 ) or a fan controller of the fan control loop (e.g., fan controller  512 ). In some embodiments, step  912  includes identifying a saturation status of the cooling control loop while operating in the low cooling load state. Step  912  may include causing a 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. 
     In some embodiments, step  912  includes identifying a saturation status for the fan control loop while operating in the high cooling load state. Step  912  may include causing a 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. In some embodiments, step  912  includes 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. 
     Calculating the feed-forward gain may include 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. For example, calculating the feed-forward gain may include determining a first difference between a temperature of the supply airstream T sa,sp  before the change in the number of active stages and the setpoint temperature T zone,sp  for the building zone (i.e., T sa,sp −T zone,sp ). Calculating the feed-forward gain may further include determining a second difference between a temperature of the supply airstream T sa  after the change in the number of active stages and the setpoint temperature T zone,sp  for the building zone (i.e., T sa −T zone,sp ). In some embodiments, the zone temperature T zone  can be substituted for the zone temperature setpoint T zone,sp  (assuming steady state conditions prior to changing the number of active stages) and the supply air before the change in the number of active stages can be substituted for the supply air temperature setpoint T sa,sp  Step  912  may include using a ratio between the first difference and the second difference 
             (       e   .   g   .     ,         T     sa   ,   sp       -     T     zone   ,   sp             T   sa     -     T     zone   ,   sp             )         
as the feed-forward gain. The feed-forward gain may be multiplied by the fan speed setpoint S fan  to determine an adjusted value S fan,adj  for the supply fan setpoint
 
     
       
         
           
             
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     The construction and arrangement of the systems and methods as shown in the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.). For example, the position of elements may be reversed or otherwise varied and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present disclosure. 
     The present disclosure contemplates methods, systems and program products on any machine-readable media for accomplishing various operations. The embodiments of the present disclosure may be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the present disclosure include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a machine, the machine properly views the connection as a machine-readable medium. Thus, any such connection is properly termed a machine-readable medium. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. 
     Although the figures show a specific order of method steps, the order of the steps may differ from what is depicted. Also two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps.