Coordinate input system with input pen

Claim 1

A coordinate input system comprising: a tablet (10) composed of an X-direction loop coil assembly (11) having a multiplicity of loop coils (11-1,...,11-48) arranged in a side-by-side fashion in an X-direction, and a Y-direction loop coil assembly (12) having a multiplicity of loop coils (12-1,...,12-48) arranged in a side-by-side fashion in a Y-direction; an input pen (20) having a tuning circuit (29) designed such that the tuning frequency of said tuning circuit is varied around a predetermined central frequency (f0) in accor­dance with the pressure with which said input pen contacts said tablet; X-direction selection means (32) for successively selecting one of the loop coils of said X-direction loop coil assembly; Y-direction selection means (33) for successively selecting one of the loop coils of said X-direction loop coil assembly; signal generating means (31) for generating an A.C. signal of a frequency equal to said predetermined frequency; signal detection means (38) for detecting the A.C. signal of said predetermined frequency; connection change-over means (34,35,36,37) for alternately connecting said signal generating means and said signal detection means successively to said loop coils of X- and Y-direction loop coil assemblies selected by said X- and Y-­direction selection means; coordinate detection means (30,39) for determining the X- and Y-coordinate values of the point designated by said input pen in accordance with the A.C. signals detected from said X- and Y-direction loop coils by said signal detection means; phase detection means (41,42,43,44) for detecting,as a change in the phase with respect to the phase of the tuning frequency, the change in the tuning frequency of said tuning circuit of said input pen in accordance with the A.C. signals detected by said signal detection means; and information computing means (49) for computing information concerning the coordinate point and proportional to the pressure of the input pen.