Variable air volume control using a finite state machine

Abstract

Air terminals are an important component of most variable air volume air conditioning systems. Most air terminal controllers share the same continuous closed feedback loop control of the room loop whether the controller is pneumatic, analog electronic or digital electronic or other. Unfortunately, the controllers also share the same inability to provide good hybrid control. The use of split range control for hybrid control, or the sequencing of the continuous loops for the controlled devices, may cause cycling between heating and cooling, simultaneous heating and cooling, and does not provide useful diagnostics. Improper VAV terminal unit control is a significant problem because sales exceed half a million new units each year and are increasing by at least 10% per year. An innovative and simple control method called a finite state machine was applied to this problem. A finite state machine is an abstract representation of a reactive system, a classification which includes most controllers. The finite state diagram fully describes the controller's behavior and can be mechanistically implemented. Performance of the air terminal improved significantly, including better temperature control and diagnostics with reduced energy usage. The concepts presented in this paper can probably be applied with equal or greater success to almost any other controller in the HVAC&R industry.