An Inexpensive Low-Power Ultrasonic 3-Dimensional Air Velocity Sensor

The ability to monitor the direction and speed of the air within buildings would enable closed-loop control of systems, improving occupants' comfort, health, and safety, and reducing building energy use. Leveraging inexpensive and low-power Micro-Electro-Mechanical Systems (MEMS) technology used in ultrasonic range-finding, we have developed a unique anemometer (i.e., air velocity sensor) that measures 3-dimensional air velocity for indoor environmental applications. The anemometer uses a novel tetrahedral arrangement of four ultrasonic transceivers, the minimum number of transducers required to capture a 3-dimensional flow. We describe in this paper the hardware and software of the anemometer, and the algorithms that eliminate or compensate for the shielding effects of its support struts and base. The angular and speed accuracies for the anemometer are unprecedented for an inexpensive instrument. In a speed range of 0 - 6 m/s, error averages are ±3% for air speed, 2.9° for yaw (azimuth) angle, and 2.3° for pitch (vertical declination) angle. The anemometer is simple to assemble and needs no calibration.