A method allows combining the functions of a heater and a thermometer in a single device, a thermistor, with minimal temperature read errors. Because thermistors typically have a much smaller thermal mass than the objects they monitor, the thermal time to equilibrate the thermometer to the temperature of the object (τt) is typically much shorter than the thermal time of the object to change its temperature in response to an external perturbation (τO), τt << τO.
It is possible to switch the heater controlling the object’s temperature off for this shorter length of time. While the heater is switched off, the thermistor is allowed the time equal to several τt to reach equilibrium with the object, then its temperature is measured, after which the heater is turned back on. The measured value is provided for any temperature control, and the voltage on the thermistor during the heating potion of the cycle will be determined by that control.
In one tested implementation, the τt was measured to be under 0.5 ms, and τO≈2 s. In the timing sequence in this implementation, the heater circuit is turned off, and the measurement current is turned on. After ≈3 ms, the voltage readout is accomplished in ≈1 ms. A fraction of ms later, the heater is turned back on for ≈95 ms.
In one embodiment of this solution, a sample and hold circuit captures the voltage (resistance) readout during the short measurement interval and provides the capture value for the duration of the cycle. This embodiment can be used with any temperature controller, including off-the-shelf PID controllers.
Another embodiment involves a readout that is synchronized with the multiplexed readout of one sensor in a collection, and the heater is connected to the same sensor while the multiplexer is reading other sensors. This solution requires a PID circuit compatible with the multiplexed readout, typically implemented in an FPGA solution with a clock-driven timing.
This work was done by Hyung J. Cho, Kalyani G. Sukhatme, John C. Mahoney, Konstantin Penanen, and Rudolph Vargas, Jr., of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46900
This Brief includes a Technical Support Package (TSP).
Time-Separating Heating and Sensor Functions of Thermistors in Precision Thermal Control Application
(reference NPO-46900) is currently available for download from the TSP library.
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