The resistance of the coatings on a touch screen will vary slightly from unit to unit due to the manufacturing process and can change under some environmental conditions and over time. Environmental factors such as temperature and humidity can alter the resistance characteristics of the coating which will affect the position measurements from the touch screen.
Changes over time in the touch screen coating and in the drift in calibration of the touch screen controller can also affect measurement accuracy. The physical position of the touch screen alignment to the display may also vary slightly from unit to unit.
Because of these factors an initial calibration of the touch screen is usually necessary, and in certain applications subsequent calibrations may be required periodically to maintain accuracy. The type of application will dictate the calibration requirements.
Common 4-wire/6-wire/8-wire resistive touch screens may require re-calibration due to the stress on the film side. 5 wire film does not use coordinates on the film side and hence forms and maintains more stable coordinates.
For example, a touch screen application that requires finger touch accuracy on a small LCD may require only about 10% accuracy of the touch measurement relative to a screen location. The combined amount of drift over time and initial variations in characteristics of the touch screen system typically amount to a total of about 5%. In this application calibration would not be necessary.
In a second example, an application such as signature capture on a 5” LCD with a resolution of 320 x 240, an accuracy of better than one pixel is necessary over the life of the product. This amounts an accuracy of the position measurement relative to the LCD screen of better than 0.3%. In this application calibration would be required. A typical calibration procedure would be to first display a single pixel near one corner of the display and request the user to place a stylus at that point. A measurement is taken and stored. Next, the procedure is repeated near the opposite corner of the display. These two readings now represent the accurate positions of two points on the LCD display. A scaling factor can now be computed for each axis and all subsequent positional measurements can be adjusted using this factor.
Initial calibration is still required with an 8-wire touch screen is critical applications but periodic re-calibration is usually not required.