Stability in Multi-Position Controlled Thermal Processes

• Main Authors: William K. Roots MSc, PhD, WhF, FIEE, Sen Mem IEEE, MIE Aust, Malayappan Shridhar BSc, DMIT, MS (EE)
• Format: Article
• Language: English
• Published: SAGE Publishing 1969-09-01
• Series: Measurement + Control
• Online Access: https://doi.org/10.1177/002029406900200903

The common distributed parameter thermal processes that can be modelled by the cascade of a time constant and a transit delay (ie: boilers without superheaters, furnaces, ovens, vats, torches, fluidised beds, etc) are discussed. These are often multiposition controlled by solenoid valves, tapped transformers and similar discontinuous control elements. It is well known that if controller sensitivity, process gain, process parameters, and transducer locatons are arbitrarily combined these processes can exhibit instability. Over the years empirical design rules have been developed to determine such critical parameters as process gain, controller sensitivity, transducer type and location and the like. This paper uses a describing function approach to establish a facile technique for determining the stability criteria of such processes in terms of the controller parameters. This is then developed into a graphical display that gratly simplifies the design procedures associated with thermal processes and their control and instrumentation.