Pattern tracer using digital logic

A circular scanning photoelectric pattern tracer including a scanning head having scanning means to repetitively scan a circular path about a centre of scan including a pattern distinguished by a change in optical reflectance, and produce signals of a first kind whenever an edge of said pattern is crossed by said scan and signals of a second kind whenever said scan passes a reference point within said scanning head, means to derive numerical values representative of the time between occurrence of said signal of said second kind and the occurrence of said signals of said first kind, means to store said numerical values in order of their occurrence and to distinguish by means of first and second symbols numerical values representing signals of said first kind in accordance with whether the scan is entering or leaving the pattern, means to derive a first count representative of the time duration of one complete scan, means to derive a second count representative of the angular offset required to produce the desired kerf, means to derive a third count representative of the predicted limits of angular direction in which the pattern will appear, means to select from said numerical values those occurring during said third count and bearing either first or second symbols as selected, means to combine said selective numerical value with said second count to produce a fourth count representative of the required direction of travel of the said centre of scan to maintain said centre of scan the desired offset distance from said pattern, means to convert said fourth count to a pair of coordinate values representative of the sine and cosine of the angle contained between the centre of scan, the reference point and the point on the scan represented by said fourth count, means to combine each of said coordinate values with a number representative of the desired tangential tracing velocity and produce speed signals representative of the coordinate velocities necessary to cause said tracer to trace said pattern and means to apply said speed signal to a pair of coordinate drive motors to cause said tracer to move with the said coordinate velocities.