The temperature control system investigated is commonly called a component oven. A component oven is a small temperature-controlled chamber which encloses temperature-sensitive components or circuits. The system consists of a metallic oven enclosed with insulation material, a power transistor for the heating element, a thermistor for the temperature sensor, and an analog controller circuit.
The temperature error signal is derived from the thermistor in a bridge configuration. The loop gain of the system is a factor in the stability, steady state error, and transient response characteristics. The current industry practice relies on a gain margin calculation to stabilize the system. For example, the loop gain of the prototype system is adjusted by a variable resistor to increase the gain until the controller just starts to oscillate. The loop gain is then decreased by an amount that depends on individual and product experience to stabilize the system.
The purpose of the thesis is to derive an approximation of the system transfer function so that the loop gain may be set for desired design specifications. Also with the transfer function known, a compensation circuit may be designed to improve the controller characteristics. The major emphasis of the thesis is the analysis of the power transistor/thermistor interface. A simulation program for the IBM personal computer was designed to calculate numerically the approximate time delay and time constant properties of the interface.
Analysis of experimental and simulation data indicated that the system could be accurately modelled as first order with a time delay component. The time delay was found to increase with an oven temperature increase. The time constant of the interface was found to be independent of the oven temperature and thermistor locations in the plate. With the transfer function, the system can be designed for gain and phase margin specifications and steady-state error and transient response characteristics can be predicted.