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Browse Prior Art Database

Pressure-Sensing Touch Panel

IP.com Disclosure Number: IPCOM000061325D
Original Publication Date: 1986-Jul-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 20K

Publishing Venue

IBM

Related People

Balliet, L: AUTHOR [+2]

Abstract

This article describes a pressure-sensing touch panel for computer display entry utilizing a specially constructed overlay over the face of the display. Touch-sensitive screens are increasingly important as input devices to digital computers. Implementation techniques presently employed include the use of infrared (IR) emission and detection, conductive and capacitive overlays and strain gauges. Present approaches are generally either unreliable, expensive or difficult to use without special mounting. The panel disclosed herein is a potentially low cost alternative to the present art that does not exhibit these undesirable characteristics. A specially constructed overlay is placed over the face of the display and detects the pressure caused by a finger (or other object) when it touches the surface. Fig.

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Pressure-Sensing Touch Panel

This article describes a pressure-sensing touch panel for computer display entry utilizing a specially constructed overlay over the face of the display. Touch- sensitive screens are increasingly important as input devices to digital computers. Implementation techniques presently employed include the use of infrared (IR) emission and detection, conductive and capacitive overlays and strain gauges. Present approaches are generally either unreliable, expensive or difficult to use without special mounting. The panel disclosed herein is a potentially low cost alternative to the present art that does not exhibit these undesirable characteristics. A specially constructed overlay is placed over the face of the display and detects the pressure caused by a finger (or other object) when it touches the surface. Fig. 1 illustrates the concept and structure of the touch panel. Fig. 2 is a sectional view of the touch panel through A-A. The overlay 1 is constructed of transparent plastic with a continuously hollow channel 2 embedded and built into it. The channel forms a tube that is filled with air or similar gas. Two pressure sensors 3 and 4 are placed at opposite ends of the channel to detect changes in pressure. When a finger (or other object) is pressed against the overlay, the channel is collapsed, thereby reducing the volume formed by the channel and initiating a pressure wave toward both ends of the channel. The pressure fronts travel similar to that of a sound wave and arrive at the two ends, typically at different times. The pressure sensors detect changes in pressure and convert it to electrical signals. The difference in arrival times, along with knowledge of which pressure front arrived first, is used to calculate the location of the initializing action. Assuming a tube with constant cross-sectional area, from the law of ideal gases it can be shown that

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where P = pressure change P = normal atmospheric pressure (approximately
14.7 lbs/sq. inch) L = length of the tube WL = change in length caused by finger or other object The concept can be used with any size display. However, for purpose of illustration, assume a display overlay of one foot wide and 1/2 foot high, a channel spacing of 1/4 inch, and a finger displacement of 1/2 inch, the following parameters would be representative. L - 288 inches (24 feet) WP =
.025 lb. per square inch worst case (finger furthest from sensor) WP = .05 lb. per square inch average Pressure changes of this magnitude are readily detectable by conventional pressure transducers. Since the pressure front travels at the speed of sound, a difference in arrival time could be as great as 21.4 milliseconds when contact is made adjacent to a sensor. When contact is in the center of the display, arrival time at each sensor will be equal and half this amount. Fig. 3 shows the circuitry used to convert detected changes in pr...