A Feedback System for Control of an Unstable Process

This paper describes a feedback system designed to control the surface temperature of a metallic body immersed in a boiling liquid. The process is inherently unstable in one of its three regions of operation; however, under automatic feedback control, stable and precise operation is obtained in all regions. The surface temperature can be accurately maintained at any desired steadystate level in any region of process operation, and sine-wave or transient variations in temperature can be obtained when the system is driven from an electrical signal generator.