Digitizer pen tilt correction employing wires near the data point

• US 4,939,318 A
• Filed: 11/06/1989
• Issued: 07/03/1990
• Est. Priority Date: 11/06/1989
• Status: Expired due to Fees

• Current Assignee: CalComp, Inc. (Martin Marietta Corporation)
• Original Assignee: CalComp, Inc. (Martin Marietta Corporation)
• Inventors: Watson, James S., Doubrava, Dana
• Primary Examiner(s): SCHREYER, STAFFORD
• Application Number:
  US07/431,886
• Time in Patent Office: 239 Days
• Field of Search: 178/18, 178/19
• US Class Current: 178/18.02
• CPC Class Codes: G06F 3/046 by electromagnetic means

Abstract

In a digitizer tablet system employing a tablet having equally spaced parallel grid wires disposed in a plane along an axis parallel to the tablet'"'"'s upper surface and a pen cursor having a sensing coil disposed concentric with and perpendicular to a longitudinal axis of the pen cursor to develop a characteristic output waveform from electromagnetic inductance between the grid wires and the sensing coil, this invention is the method of compensating a calculation of the position of the pen'"'"'s tip on the tablet'"'"'s upper surface and along the axis determined from an interpolation of the characteristic output waveform to find its zero voltage crossing point for tilt of the pen cursor longitudinal axis from perpendicular to the tablet'"'"'s upper surface comprising using voltage values from grid wires next on opposite sides of the zero crossing point to calculate a pen tilt compensation factor.

Claims

1. In a digitizer tablet system employing a tablet having equally spaced parallel grid wires disposed in a plane along an axis parallel to the tablet'"'"'s upper surface and a pen cursor having a sensing coil disposed concentric with and perpendicular to a longitudinal axis of the pen cursor to develop a characteristic output waveform from electromagnetic inductance between the grid wires and the sensing coil, the method of compensating a calculation of the position of the pen'"'"'s tip on the tablet'"'"'s upper surface and along the axis determined from an interpolation of the characteristic output waveform to find its zero voltage crossing point for tilt of the pen cursor longitudinal axis from perpendicular to the tablet'"'"'s upper surface comprising the steps of:

(a) using voltage values from two grid wires next on opposite sides of the zero crossing point to calculate the position of the zero crossing point along the axis by interpolation;

(b) determining a value "r" which is the fraction of the distance between the two grid wires of the zero crossing point;

(c) using voltage values from a grid wire B next adjacent the zero crossing point on one side and a grid wire A next following B on the one side as well as voltage values from a grid wire C next adjacent the zero crossing point opposite the one side and a grid wire D next following C, determining values for Δ

P and Δ

N according to the following formulas Δ

P=(1-r)A+rB and Δ

N=(1-r)C+rD;

(d) using the values of Δ

P and Δ

N from step (c) to calculate ##EQU3## where the result "T" is a number roughly proportional to the pen tilt; and

,

(d) adding a pre-established constant time T to the original "r" value in calculations of the pen tip position as a tilt correction.

2. In a digitizer tablet system employing a tablet having equally spaced parallel grid wires disposed in a plane along an axis parallel to the tablet'"'"'s upper surface and a pen cursor having a sensing coil disposed concentric with and perpendicular to a longitudinal axis of the pen cursor to develop a chracteristic output waveform from electromagnetic inductance between the grid wires and the sensing coil, the method of compensating a calculation of the position of the pen'"'"'s tip on the tablet'"'"'s upper surface and along the axis determined from an interpolation of the characteristic output waveform to find its zero voltage crossing point for tilt of the pen cursor longitudinal axis from perpendicular to the tablet'"'"'s upper surface comprising the steps of:

(a) using voltage values from two grid wires next on opposite sides of the zero crossing point to calculate the position of the zero crossing point along the axis by interpolation;

(b) determining a value "r" which is the fraction of the distance between the two grid wires of the zero crossing point;

(c) using voltage values from a grid wire B next adjacent the zero crossing point on one side and a grid wire A next following B on the one side as well as voltage values from a grid wire C next adjacent the zero crossing point opposite the one side and a grid wire D next following C, determining values for Δ

P and Δ

N according to the following formulas Δ

P=(1-r)A+rB and Δ

N =(1-r)C+rD;

(d) using the values of Δ

P and Δ

N from step (c), passing a straight line through the Δ

P and Δ

N points on straight line segments of interpolation;

(e) determining where the straight line passes through the zero voltage line;

(f) using the difference between the point where the straight line passes through the zero voltage line and the original "r" value as a compensating number "T" roughly proportional to the pen tilt; and

,

(g) adding a pre-established constant times T to the original "r" value in calculations of the pen tip position as a tilt correction.