Ultrasonic two-axis rotation detector

A two-axis rotation monitor which determines the relative displacement between a pair of ultrasonic detectors using the phase difference of a continuous ultrasound wave generated by a single, distant source is described. The monitor has been used to measure heat rotation around the vertical and horizontal axes, but can easily be adapted to other body segment rotations or translations. It produces high-sensitivity recordings of wide spatial and dynamic range. Although the device is quite linear with good isolation between channels, a computer-based linearization/calibration routine which further increases linearity and reduces crosstalk is described. The device is unobtrusive, inexpensive, and has proven reliable and easy to use.