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Dual Level Pen for Capacitive Sensing of Tablet Signals

IP.com Disclosure Number: IPCOM000081705D
Original Publication Date: 1974-Jul-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Dym, HH: AUTHOR [+2]

Abstract

The capacitive sensing pen in Fig. 1 is used with a graphic input tablet This pen incorporates a specially designed switch 17 and internal capacitor 16, to detect contact of the pen tip T against a tablet or document surface. Pen height and contact with the surface when no switch is used are sensed by electronically sensing signal amplitude. A signal exceeding a predetermined level is taken to indicate surface contact. Within a limited range of conditions this amplitude threshold method is adequate.

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Dual Level Pen for Capacitive Sensing of Tablet Signals

The capacitive sensing pen in Fig. 1 is used with a graphic input tablet This pen incorporates a specially designed switch 17 and internal capacitor 16, to detect contact of the pen tip T against a tablet or document surface. Pen height and contact with the surface when no switch is used are sensed by electronically sensing signal amplitude. A signal exceeding a predetermined level is taken to indicate surface contact. Within a limited range of conditions this amplitude threshold method is adequate.

For more general tablet applications, however, these conditions do not remain constant. Factors which affect signal amplitude when the pen is contacting a surface are type of surface material and its texture, for example, paper, plastic overlay or directly on tablet, as well as material thickness or number of documents. The pressure and angle of the pen also influence detected signal amplitude.

A switch in the pen allows consistent contact detection independent of the signal amplitude. Ideal switch operation occurs when the user is unaware of its existence. This means virtually no pen tip travel and a very low-actuating force. Furthermore, it is necessary that the pen always signals whether the switch is open or closed. This is required for determining pen position when it is above the tablet as well as contacting the surface. A change of signal level of about 10 to 15 percent between the open and closed states is provided, to exceed variations caused by the other external factors.

Electronics for detecting contact measures the amplitude level. The discontinuity in the amplitude at switch closure is sufficient to provide reliable threshold crossing. To eliminate bounce or too rapid switching between pen up and down, two threshold levels are used. When the higher level threshold is exceeded the pen height indicator is latched in the down or contact state. The signal must fall below the lower threshold to release the latch indicating pen up.

To achieve the two signal levels through the pen, a series impedance is inserted in the signal path. In order to keep signal characteristics such as phase and frequency response unchanged, the impedance must be capacitive. A value of the order of 100 pf is desirable. Below is described the internal pen design for conveniently incorporating this series capacitor and the means for switching it in or out.

Fig. 1 shows a capacitive sensing pen consisting of metallic upper and lower shells 11, 11a, which can be a standard metallic ball-point pen body. Upper shell 11 contains a miniature coaxial connector 10, a threaded rod 19, and a switch gap adjustment 18. Lower shell 11a contains an insulator 13. Constrained axially between switch gap adjustment 18 and insulator 13 are a standard metallic ball- point pen refill 12, a protrusion adjustment shim 14, a return spring 15, a sheath capacitor assembly 16...