Design of a Thermal Operational Amplifier : Thermics Applied to Heat Signal Control

A thermal differential operational amplifier was developed through an experimentally verified analysis of design parameters. In general, an operational amplifier (or Op-Amp) is a device which amplifies a signal by a constant factor, called the amplifier's gain. A thermal differential Op-Amp takes a small temperature difference between its negative and positive inputs and produces a much larger output temperature difference. Conceived as the building block for linear thermal logic, or thermics, the amplifier is an active control element operating totally in the thermal-energy domain, using temperatures for input and output, and heat for power. A detailed seventh-order analytical model of the amplifier used physical dimensions and material properties to predict step-input response (open and closed loop). Predictions agreed with open-and-closed loop dynamic-response tests performed on experimental prototypes. The selection of physical dimensions to produce desired dynamic behavior was accomplished with an optimal-design technique. The amplifier was constructed from two effort (temperature) controlled resistors (ECR) which were series connected in a push-pull configuration. The trade of gain for range is identified and expressions for each constructed in terms of thermal-resistor parameters. The prototype utilized planar-film ECR's, in which the level of a conducting fluid in an air gap varies thermal resistance. The fluid level is modulated by the temperature difference between negative and positive sensors, which creates an output temperature proportional to the input temperature difference. When tested open loop, the ratio of amplifier output to input had a constant value of twenty over the amplifier's output operating range, typically + 30F from a reference. Configured closed loop as an effort follower the 90% rise time was less than three minutes. Closed loop operation proved that an amplifier building block for analog thermic circuits was feasible. Thesis Supervisor: B. Shawn Buckley Title: Associate Professor of Mechanical Engineering -2-