Use of an electromagnetic eye movement monitor for easy measurement of arm movements

This paper proposes a new technique for measuring arm movements, which is an adaptation of the electromagnetic method for measuring eye movements. Two small coils of wire are mounted onto the subject's elbow, moving rigidly with the humerus, and two more coils are mounted onto the subject's wrist, moving rigidly with the radius-ulna. The subject is placed inside three alternating magnetic fields at different frequencies in the X, Y, and Z directions. As the arm moves, the voltages induced into the coils vary with angle. For each coil, the voltages are demodulated to give three dc voltages giving the vector direction of that coil's axis. Corrections for the nonuniformity of the magnetic fields are computed. The method works unambiguously for all possible arm movements within the physiological ranges of the joints. Measurement of finger angles is just as easily done. The five angles describing arm movements can be measured at millisecond intervals with noise as small as 2 arcsec rms and drift of about 1 arcmin over a day, i.e., the electronics is more stable than the attachment of the coils to the arm.