Abstract
Ultrasonic sensors have dominated miniaturized depth measurement applications such as robot collision avoidance and walking cane hazard detection yet have limited spatial resolution. Optical time-of-flight (ToF) depth sensors offer the potential for improved spatial resolution, however, ToF depth-sensing cameras may be too large and power-hungry for hand-held applications. We address this gap by experimentally evaluating an infrared ToF sensor (the ST VL53L1X) that uses a single-photon avalanche photodiode array to provide coarse spatial resolution while remaining miniaturized and low-power, thus allowing the generation of hazard maps in hand-held applications. We develop methods and present characterization results for distance measurement accuracy, noise, error, and tolerable ambient illumination. The IR ToF sensor sustains accuracy better than 2% up to a distance of 3000mm for a 73% reflective target in the presence of zero interfering ambient light. We characterize the spatial resolution enabled by this region-of-interest and find off-axis pointing of up to 15.7° in steps of 2.5°. Many hazard detection systems may be moving, which dynamically changes the position and pointing of the depth sensor. We demonstrate the use of a 9-degree-of-freedom (3-axis accelerometer, gyroscope, and magnetometer) inertial measurement unit (IMU) to track sensor pointing. The ToF sensor combined with an IMU forms the basis for a miniaturized depth mapping solution that consumes 97.5mW when operating at 30Hz, and requires simple serial interfaces to a microcontroller.