A NOVEL THEORY OF PI CONTROL FOR HVAC-AND-R SYSTEMS

A novel theory for determining coefficients for proportional integral (PI) control of heating, ventilating, air-conditioning and refrigerati on (HVAC&R) systems is presented. The theory is novel because it focus es on explicitly digital systems whereas conventional theory focuses o n digital that use relatively long sampling intervals. A long sampling interval is considered one that permits a complete response of the sy stem to a change in control signal within the sampling interval. The t heory is also novel because it is developed from a thermo-fluid engine ering perspective, rather than from a control engineering perspective. The theory is applicable to systems with and without storage, i.e., s econd and first order systems. Process sensitivities for systems with and without storage are identified and defined. The sensitivities are based on open loop system operating characteristics. Implementation of a PI control system requires the evaluation of two PI coefficients. T he theory enables the evaluation of the PI coefficients using the proc ess sensitivity and sampling interval to obtain a desired target respo nse characteristic. A modification to the basic PI algorithm for windu p prevention, compatible with the novel theory, is defined.