CMOS Compatible MEMS Air Velocity Sensor With Improved Sensitivity and Linearity for Human Thermal Comfort Sensing Applications

Abstract

In this paper, we report an optimized CMOS compatible MEMS thermoresistive calorimetric air velocity (TMCV) sensor to measure small air velocity in an indoor environment for the application of human thermal comfort sensing. The sensor is comprised of two upstream thermistors, two downstream thermistors, and a micro-heater placed in between the upstream thermistors and downstream thermistors. The dimensions of the sensor were optimized to achieve better linearity and accuracy in the tiny air velocity (−1 m/s to 1 m/s) range based on the CFD simulation. Furthermore, to reduce the heating power and increase the normalized sensitivity, the sensor was completely released from the substrate using the DRIE process. The sensor was successfully tested both in a lab in a wind tunnel and a real indoor environment. The sensor achieved a maximum sensitivity of 340.2 mV/(m/s), and normalized sensitivity, with respect to heating power and gain, of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$354.37~\mu \text{V}$ </tex-math></inline-formula> /(m/s)/mW within −5 m/s to 5 m/s air velocity range. The normalized sensitivity achieved by our sensor is much higher than most of the reported TMCV sensors. Besides, the sensor obtained a good linearity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}^{2}=0.9996$ </tex-math></inline-formula> ) and accuracy (±0.0236 m/s) within the small air velocity range meeting the standard accuracy requirement (< ±0.05) of ASHRAE-55 and ISO-7726 standards. Moreover, the packaged TMCV sensor was successfully tested in an office to measure very small air velocity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0~\sim ~0.5$ </tex-math></inline-formula> m/s) produced by the HVAC system. The experimental results indicated that our sensor is quite suitable for smart HVAC system integration in the era of the Internet of Things (IoT).