Digital position measurement system with stylus tilt error compensation

• US 3,873,770 A
• Filed: 03/21/1974
• Issued: 03/25/1975
• Est. Priority Date: 03/21/1974
• Status: Expired due to Term

• Current Assignee: John T. Ioannou
• Original Assignee: John T. Ioannou
• Inventors: Ioannou, John T.
• Primary Examiner(s): Robinson, Thomas A.
• Application Number:
  US05/453,660
• Time in Patent Office: 369 Days
• Field of Search: 340/347AD,146.3SY 178/18,19,20 346/139C
• US Class Current: 178/18.02
• CPC Class Codes: G06F 3/046 by electromagnetic means

Abstract

A system for digitizing graphic data from a worksheet by tracing out or pointing to curves and points on the worksheet with a stylus. A tablet having a surface to receive the worksheet includes a conductor grid defining two perpendicular axes of measurement. The conductors are sequentially excited and a coil in the stylus picks up an impulse having an envelope which shows positive and negative peaks spaced by a distance 2h, where h is coil height above the grid plane measured along the stylus axis. The conductor grid planes are physically displaced from the tablet surface by some small but finite distance thus giving rise to an apparent position error if the stylus is tilted during use; i.e., the indicated position will be the projected intersection of the stylus axis with the grid plane, not the surface. Means are provided for detecting stylus tilt and variations in h and to compensate for position errors.

Claims

1. In a system for producing digital data representing the position of a stylus relative to a two-dimensional surface for receiving a work sheet, a plurality of closely spaced, parallel and coplanar conductors disposed parallel to said surface and spaced a distance Zpg therefrom, means for energizing the conductors in sequence with a unidirectional current pulse thereby to produce a flux wave which travels across the surface in a direction perpendicular to the conductors, a stylus having an end point freely positionable over the surface, pickup means carried by the stylus and responsive to flux produced by the energization of the conductors to produce an output representing the passage of the wave past the end point of the stylus on the surface, means responsive to the output to produce a signal quantity representing the position of the end point along an axis parallel to the direction of said flux wave, and means for compensating said signal quantity for the apparent position difference between the actual end position of the stylus along said axis and the projected end point in the plane of the conductors taken along the axis of the stylus.

2. Apparatus as defined in claim 1 wherein the means for producing the signal quantity includes a digital count signal source, a register for receiving the signals, means for starting the count at the beginning of the conductor energization sequence and means for stopping the count on occurrence of said output signal.

3. Apparatus as defined in claim 2 wherein said pickup means includes a coil mounted in the stylus perpendicular to the longitudinal axis thereof thereby to generate an impulse voltage the envelope of which passes from a positive peak upon energization of the first conductor within a distance h ( phi ) from the projection of the end point in the conductor plane on one side of the end point to a negative peak upon energization of the last conductor within a distance h ( phi ) on the other side of the end point where h ( phi ) is coil height from the plane of the conductors along the stylus and output means connected to the coil for producing the output signal at a predetermined signal value between the positive and negative peaks.

4. Apparatus as defined in claim 3 including as part of said means for compensating a correction counter connected between the count signal source and the register, and logic means for counting the counter so as to compensate for the apparent position difference.

5. Apparatus as defined in claim 4 including means for detecting the positive and negative peaks, means for determining h as a function of the time between the peaks, means for determining which of the peaks has the greater absolute value, and means for determining the position difference as a function of h.