Method and apparatus for digitizing the location of an instrument

Abstract

A method and apparatus are provided for converting a position of a pen to a digital number representing the pen position with respect to a grid of conductors. A coil disposed above the tip of the pen is driven by a continuous AC input voltage of fixed frequency and amplitude, thereby inducing corresponding signals in each of the conductors of the grid. Scanning circuitry sequentially scans the grid conductors, first in the X direction and then in the Y direction, thereby sequentially coupling the induced signals to the input of a differential amplifier. A position counter is reset at the beginning of scanning in each direction and is incremented at a rate proportional to and greater than the scanning rate. The outputs of the differential amplifier are inputted to a phase sensitive detector producing a characteristic output which changes polarity as grid conductors on opposite sides of the pen tip location are sequentially scanned. The characteristic output signal is filtered by a low pass filter and is then differentiated and inputted into a comparator which produces a pulse having a leading edge indicating when conductors on opposite sides of the pen tip are sequentially scanned. The comparator output is utilized to stop the position counter. The digital number then stored in the position counter represents the location of the pen tip with respect to the direction of the grid being scanned.