A system based on ultrasonic sensors has been developed as a means of tracking moving objects with centimeter accuracy. The system is intended especially for tracking pertinent parts of the body of a human subject engaged in control of a remote anthropomorphic robot or immersed in a virtual environment. The system could also be used to track a mobile robot.

There are increasing demands for more sophisticated methods of characterizing the motions of human subjects for the aforementioned purposes. Prior motion-tracking systems have been fairly crude, costly, and tailored for such highly specialized applications as tracking movements of the head or of the eyes only. Whole-body-tracking systems now on the market utilize, variously, expensive optical sensors or magnetic sensors that are susceptible to errors in the presence of nearby metallic objects. To achieve realistic virtual reality, it will be necessary to measure complete body motions by use of systems that are acceptable to human subjects, that interfere minimally with the subjects’ motions, and that resist environmental interference. The present system satisfies these requirements.

The present system includes several stationary receiving ultrasonic transducers positioned about the region within which the human subject moves. One or more transmitting ultrasonic transducers are positioned on the parts of the human subject’s body that are to be tracked. Putting the transmitters on the human subject minimizes the weight that the subject must carry: the equipment that processes the ultrasonic-signal information is stationary because all such processing is performed in the receivers and in a stationary motion-measurement unit (MMU).

Each transmitter emits a phase-coded waveform at the inaudible frequency of 40 kHz. The outputs of the receivers are sent to the MMU, which performs correlation processing analogous to that performed on microwave signals in the highly successful Global Positioning System. The outputs of the correlation processor are the receiver/transmitter distances; the three-dimensional coordinates of the transmitters are computed from these distances and the known positions of the receivers. No synchronization is needed to enable the receivers and the MMU to distinguish among the signals received from different transmitters because each transmitter uses a unique phase code orthogonal to the phase codes of all the other transmitters.

This work was done by Robert E. Bozek of Genisys Research & Development, Inc., for Johnson Space Center.

Refer to MSC-23028

NASA Tech Briefs Magazine

This article first appeared in the October, 2002 issue of NASA Tech Briefs Magazine.

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