Position transducer using a sweeping field null

Abstract

Abstract THE POSITION OF A CAPACITIVE PROBE ON A WRITING TABLET IS CONVERTED INTO DIGITAL AND ANALOGUE SIGNALED BY DETECTING THE TIME AT WHICH A VOLTAGE NULL PASSES THE CAPACITIVE PROBE. A VOLTAGE NULL IS SWEPT ALONG ONE DIMENSION OF THE WRITING TABLET BY APPLYING A PAIR OF EQUAL FREQUENCY OUT-OF-PHASE SIGNALS, WHICH ARE AMPLITUDE MODULATED TO PROVIDE EQUAL RATES OF INCREASE AND DECREASE, RESPECTIVELY, TO THE OPPOSITE ENDS OF A RESISTOR PLACED ALONG THE DIMENSION TO BE MEASURED. THE NULL MOVES LINEARLY ACROSS THE TABLET ALONG THE DIMENSION TO BE MEASURED. THE OUTPUT FROM THE CAPACITIVE PROBE IS A WAVEFORM HAVING AN ENVELOPE WHICH REDUCES TO A NULL AT ONE POINT DURING EACH SWEEPING PERIOD. THE TIME DIFFERENCE BETWEEN THE START OF THE SWEEP AND THE OCCURRENCE OF THE NULL IS A MEASURE OF THE POSITION OF THE CAPACITIVE PROBE ALONG THE DIMENSION TO BE MEASURED. TWO DIMENSIONAL MEASUREMENTS CAN BE ACCOMPLISHED BY SWEEPING THE NULL ALONG THE SURFACE IN A PAIR OF ORTHOGONAL DIRECTIONS, USING EITHER A TIME MUL- TIPLEXING TECHNIQUE, WHEREIN THE NULL IS SWEPT ALONG A SINGLE DIMENSION AT A TIME, OR A FREQUENCY MULTIPLEXING TECHNIQUE, WHEREIN IN CARRIER FREQUENCIES USED TO SWEEP THE NULL ALONG THE ORTHOGONAL DIMENSIONS ARE DIFFERENT FREQUENCIES.

Inventors

• 👤 Herbert Dym

Bibliographic Details