Resistive LCD

Resistive touchscreens are used in more applications than any other touch technology for example, PDAs, point-of-sale, industrial, medical, and office automation, as well as consumer electronics.

Specification

Construction=They are all constructed similarly in layers: a back layer such as glass with a uniform resistive coating plus a polyester coversheet, with the layers separated by tiny insulating dots. When the screen is touched, it pushes the conductive coating on the coversheet against the coating on the glass, making electrical contact. The voltages produced are the analog representation of the position touched. An electronic controller converts these voltages into digital X and Y coordinates which are then transmitted to the host computer.

Force activated=Because resistive touchscreens are force activated, all kinds of touch input devices can activate the screen, including fingers, fingernails, styluses, gloved hands, and credit cards.

Properties=All have similar optical properties, resistance to chemicals and abuse.

Simplicity=Both the touchscreen and its electronics are simple to integrate into imbedded systems, thereby providing one of the most practical and cost-effective touchscreen solutions.

Four-Wire=Four-wire resistive technology is the simplest to understand and manufacture. It uses both the upper and lower layers in the touchscreen “sandwich” to determine the X and Y coordinates. Typically constructed with uniform resistive coatings of indium tin oxide (ITO on the inner sides of the layers and silver buss bars along the edges, the combination sets up lines of equal potential in both X and Y.

Drawback=The primary drawback of four-wire technology is that one coordinate axis (usually the Y axis), uses the outer layer, the flexible coversheet, as a uniform voltage gradient. The constant flexing that occurs on the outer coversheet with use will eventually cause microscopic cracks in the ITO coating, changing its electrical characteristics (resistance), degrading the linearity and accuracy of this axis.

Durability=Unsurprisingly, four-wire touchscreens are not known for their durability. Typically, they test only to about 1 million touches with a finger-far less when activated by a pointed stylus which speeds the degradation process. Some four-wire products even specify 100,000 activations within a rather large, 20 mm x 20 mm area. In the real world of point-of-sale applications, a level of 100,000 activations with hard, pointed styluses (including fingernails, credit cards, ballpoint pens, etc.) is considered normal usage in just a few months’ time.

Accuracy=Also, accuracy can drift with environmental changes. The polyester coversheet expands and contracts with temperature and humidity changes, thereby causing long-term degradation to the coatings as well as drift in the touch location.
While all of these drawbacks can be insignificant in smaller sizes, they become increasingly apparent the larger the touchscreen. Therefore, Elo normally recommends four-wire touchscreens in applications with a display size of 6.4″ or smaller.?

Pros=However, the relative low cost, inherent low power consumption, and common availability of chipset controllers with support from imbedded operating systems, makes Elo AT4 four-wire touchscreens ideal for hand-held devices such as PDAs, wearable computers, and many consumer devices.

Five-Wire Resistive=As we have seen, four-wire touchscreens, while having a simple and elegant design, have a major drawback in terms of durability in that the flexing coversheet is used to determine one of the axes. Field usage proves that the other axis rarely fails. Could it be possible to construct a touchscreen where all the position sensing was on the stable glass layer? Then the coversheet would serve only as a voltage probe for X and Y. Microscopic cracks in the coversheet coating might still occur, but they would no longer cause non-linearities. The simple buss bar design is not sufficient and a more complex linearization pattern on the edges is required.

Solution=In the five-wire design, one wire goes to the coversheet (E) which serves as the voltage probe for X and Y. Four wires go to corners of the back glass layer (A, B, C, and D). The controller first applies 5V to corners A and B and grounds C and D, causing voltage to flow uniformly across the screen from the top to the bottom. Upon touch, it reads the Y voltage from the coversheet at E. Then the controller applies 5V to corners A and C and grounds B and D, and reads the X voltage from E again.

Stability=So, a five-wire touchscreen uses the stable bottom layer for both X- and Y-axis measurements. The flexible coversheet acts only as a voltage-measuring probe. This means the touchscreen continues working properly even with non-uniformity in the coversheet’s conductive coating. The result is an accurate, durable and more reliable touchscreen over four-wire designs.