A 2D thermal flow sensor with sub-mW power consumption

Autonomous navigation of a flying object requires the measurement of velocity and position in absolute or relative means. A possible approach is to measure the air flow velocity on the object. Measurement on a flying object requires low-power and low-weight sensors. Since several decades, flow velocity of fluids in two dimensions can be measured by microsystems, which are inherently low-weight sensors.Here, we report the design of a low-power 2D flow sensor and present the characteristics of a variety of sensor designs to investigate the effects of several design considerations. We further discuss the error sources in angle measurement with a 2D thermal flow sensor and propose design and measurement approaches to reduce the error. Highly sensitive amorphous germanium thermistors are used in our design to provide a high temperature resolution and to achieve low velocity measurement limits at low power consumption. Using an optimized design for thermistor locations, diaphragm dimensions, and choice of materials, power consumption of our micromachined thermal flow sensors is reduced to the sub-milliwatt level, which has not been shown by the state-of-the-art devices with keeping high resolution. Initial results in flow velocity characterizations show a resolution below 1 cm/s at a power of 0.177 mWrms. The maximum angle error is determined to be 3.5° in wind channel tests. Theoretical limits indicate that further reduction of the measurement limit to the order of 0.1 mm/s is possible using germanium thermistors.