A control system using a low-drift power-feedback signal was implemented applying thermal waves, giving a sensor output independent of resistance drift and thermo-electric offset voltages on interface wires. Kelvin-contact sensing and power control is used on heater resistors, thereby inhibiting the influence of heater resistance drift. The thermal waves are detected with a sensing resistor using a lock-in amplifier and are mutually cancelled by a thermal-wave balancing controller. Offset due to thermal gradient across the chip and resistor drift are eliminated by the lock-in amplifier and power controller, and therefore do not influence the sensor output signal. A microchannel thermal-wave balancing flow sensor with integrated Al resistors has successfully been fabricated. The thermal flow sensor is capable of measuring water flow rates with nl· min−1 precision, up to about 500 nl· min−1 full scale. Measurement results are in good agreement with a dynamic model of the flow sensor. Drift measurements show the sensor output signal to be ompensated for resistance drift and thermal gradient across the chip.
Dijkstra, M., Lammerink, T. S. J., Pjetri, O., de Boer, M. J., Berenschot, J. W., Wiegerink, R. J., & Elwenspoek, M. C. (2014). Thermal-wave balancing flow sensor with low-drift power feedback. Journal of micromechanics and microengineering, 24(5), 055016. https://doi.org/10.1088/0960-1317/24/5/055016