Case study of hysteresis and pi control as applied to engine coolant temperature control and proposal of a combination control logic

Embedded systems are expected to work without human intervention based on some environment conditions and take required action based on these sensed condition in real-time. The software in an embedded system contains the control logic which decides what action is to be taken based on the sensed parameter. In this paper we try to bring out most important and widely used types of control logic that can be implemented, its pros and cons with the example of an engine coolant fan control based on coolant temperature. Engine coolants are used to absorb the excess heat of the engine, which causes the coolant to get heated. A coolant fan is used to take away the excess heat based on the coolant temperature. The control logic software is used to vary the duty-cycle of the fan appropriately to maintain the engine coolant temperature within certain range. This paper describes in detail two control algorithms - Hysteresis and PI Control logic for this purpose. It is shown that Hysteresis is most suitable when the FAN has only a few well defined set-point duty-cycles, strict memory requirements. PI Control is ideal when precise temperature control is required. In cases where very precise control of at the set-point is not needed, the PI Control exhibits a cycling effect near the set-point. A combination of control logic is proposed to overcome this drawback. Also, the major factors which influence the selection of the control logic are described in detail.