A digitally programmable temperature controller based on a phase-locked loop

A digitally programmable temperature controller is developed based on a phase-locked loop. Temperature under control is converted to frequency form and then compared, in a phase detector, with the reference frequency which corresponds to the target temperature. The detected phase drives the actuator. The frequency difference is also detected by a one-chip microcomputer which changes the reference frequency according to a proportional-integral-derivative operation. A prototype controller achieved 0.1° C stability in the temperature control of a water bath. It is concluded that the principles of control are compatible with intelligent sensors now being developed and are applicable to the control of other quantities, such as humidity, pressure, and flow