Switched linear control of vapor compression systems under highly transient conditions

Vapor compression systems, while historically prevalent in relatively steady-state thermal transport applications, are recently being investigated for application in modern advanced vehicle systems, where far greater transient operational conditions are expected to occur. Critical to the control of these systems is the regulation of evaporator superheat, which is known to exhibit highly nonlinear behavior, especially at low values. This behavior includes a reversal of signs in the input-to-output gains between operating conditions with zero superheat and those with positive superheat. As an alternative to addressing the system's nonlinear response with nonlinear control, this paper demonstrates the use of a switched linear control approach. This is implemented by switching among linear model-based controllers for distinct modes of the system. This principle of matching modes of the controller to modes of the system is demonstrated in simulation for an evaporator superheat recovery process. The control design is shown to regulate the system on a timescale suitable for the transients expected of application in advanced mobile energy systems.