Magnetoacoustic position sensor employing pulse code sequence generators

Abstract

Using a sonic delay line approach, the distance of a point from the ends of a wire is measured by the time required for an acoustic pulse to move along a wire. A magnetostrictive wire extends between its endpoints with several magnetic pickup coils connected in series located at each end of the wire which are adapted to pick up an encoded train of magnetoacoustic signals. A position signalling electromagnetic drive head can be moved along the coil to any desired position. The head can be pulsed electrically to produce an appropriate sequence of acoustic pulses in the magnetostrictive wire. When those pulses are received by an appropriate sequence of coils connected in series which are properly connected (plus and minus), a large pulse is produced at one time position of the waves as they pass through the sequence of coils. Otherwise, a small signal is produced while the pulses pass by. The drive head comprises a Barker sequence coil which produces 3, 7, or 11 pulses in a Barker sequence, which gives a maximum positive autocorrelation, with an output of less than zero for all other positions of the sequence of pulses as they pass through a series of coils. This large output signal is used in conjunction with a timer to provide position measurement. By placing detectors at both ends of the wire, it is possible to provide self-calibration against thermal and other drifts in accuracy and to provide immediate warning of noisy or invalid readings.