Miniature proportional + integral + derivative (PID) temperature controllers that would be implemented as hybrid microcircuits have been proposed to satisfy special requirements to minimize size and mass and to maintain g-ray-detector modules at temperatures of 10±0.5 °C. These temperature controller would be significantly smaller and less massive than are commercial PID temperature controllers, and would control temperatures more accurately than do commercial hybrid solid-state on/off-switching thermostats.

A Miniature PID Temperature Controller would generate an output in the form of (1) a proportional current or proportional voltage supplied to a heater, or (2) switching of a solid-state relay for on/off control of the heater.
The basic idea is to sense the temperature of a module and to regulate the electric power supplied to a film heater that is bonded to the module. The major drawback of on/off switching of heater power is that it causes the temperature to oscillate about the set-point temperature and can give rise to significant overshoot during warmup or load changes. In the intended application, the oscillations and overshoots, as well as the temperature gradients that occur when heater power is on, make it impossible to satisfy the requirement to keep the temperature within ±0.5 C° of the set point.

A controller as proposed would supply full power to a heater until the temperature entered a specified proportional-control band that would contain the set point. The controller would then reduce the heater power gradually until the temperature became stabilized at the set point. The advantage of the PID control characteristic is that it would prevent overshoot and compensate for droop, thereby increasing the accuracy of temperature control.

The controller would have dimensions of about 0.25 by 1 by 1.5 in. (about 0.6 by 2.5 by 3.8 cm). Its mass would be about 0.5 oz (about 14 g). The figure is a simplified schematic diagram of the controller in relation to the temperature sensor and the heater. The temperature sensor would be a platinum resistance temperature detector (RTD) or epoxy-encapsulated thermistor, which could be either separate from the controller or laminated inside the body of the controller. Hybrid microcircuits inside the body would generate the PID response.

Due to the use of hybrid microcircuits, each controller could consist of multiple temperature control units. This reduces the number of temperature controllers required significantly. Also, the temperature set point of the controller could be changed in flight by using a temperature-versus-voltage calibration curve.

This work was done by Michael K. Choi of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Components category.

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NASA Tech Briefs Magazine

This article first appeared in the December, 2001 issue of NASA Tech Briefs Magazine.

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