An ultrasonic position detecting system for motion tracking in three dimensions
Description
A real-time, ultrasonic position detection instrument has been developed, yielding accuracy of the order of millimeters over an operating volume of several cubic meters. The ultrasonic ranging system consists of a transmitter mounted on the tracked target and four or more stationary receivers. An iterative, weighted least squares algorithm was developed to fuse one dimensional distance measurements. Triangulation calculations, to determine the transmitter's coordinates in space, utilize a temperature estimated speed of sound. Position information obtained by sensor fusion was used to provide adjustable windows on time of flight measurements and thus eliminate malfunctioning sensors A novel hardware level method was developed to improve range, robustness, and accuracy of ultrasonic ranging systems. Peak detection is combined with phase measurement to accurately determine the time-of-flight of the transmitted signal. An automatic gain controller taking feedback from the level of the peak of the received signal is used to increase the working range of the unit, and to attenuate signals resulting from undesirable reflections The factors limiting the reliability and accuracy of a three dimensional ultrasonic ranging system were investigated. It was determined that misalignment between transducers contributes the largest position errors. The phase shifts of the acoustic wave were experimentally mapped in three dimensional space. It was also verified that temperature effects on the speed of sound are a major source of ranging errors A prototype was constructed, and a three dimensional experiment was performed on a target moving along a complex path in an un-controlled laboratory environment. The resolution of the instrument was shown to be within $\pm$0.5 mm in each coordinate direction, given a redundant set of seven functional receivers. The absolute accuracy of the unit was approximately 2.5 mm in each coordinate direction. The identification and suppression of malfunctioning or occluded receivers were successfully accomplished. The position update rate was approximately 5 Hz