Eliminating the ITO coating improves touch screen performance, durability, longevity and … it’s cheaper.
Making use of our innate ability to point and touch, the touch screen has become the most user friendly human interface available. It has found widespread use in all levels of consumer and industrial applications including ATM’s, kiosks, games, retail systems, factory automation, processing plants, and military and aerospace applications. Of the different touch screen technologies available (four-wire resistive, capacitive, and infrared), the four-wire resistive analog touch screen has gained wide acceptance because of its low cost, durability, and reliability, and it is the type we will be discussing in this article.
The touch screen consists of a controller, normally occupying a single PCB slot in the system hardware, and a transparent sensor positioned over a liquid crystal display (LCD), or other flat panel display, functioning as the operator interface. Typical controller functions include switching voltages between sensor layers, performing voltage measurements, and analog-to-digital (A/D) conversions. The sensor consists of two opposing substrates, each coated with a transparent electrically conductive material, separated by a thin gap. When the operator presses the touch screen with a finger or stylus, the top layer flexes and comes into electrical contact with the lower layer. The controller then obtains two voltage measurements representing the point of contact’s X and Y coordinates. System programming associates this position with a specific function graphically depicted on the flat panel display (e.g., start, stop, abort, etc.) and responds accordingly.
In the conventional four-wire configuration, each substrate is sputtered on one side with Indium Tin Oxide (ITO), a uniform transparent coating that functions as a voltage divider and provides a linear voltage drop over the active area of the screen. This is the region of the sensor that the operator touches. Two bus bars formed of conductive material, such as silver, are coated on each substrate outside the active area and are coupled to the opposite ends of the ITO. The two layers are positioned with their ITO coatings facing and their conduction lines running perpendicular to each other. Tiny, transparent, insulating dots maintain a gap between the two substrates.
Touch screen operation entails applying a voltage gradient across the resistive surface of one layer and measuring the voltage on the other layer which acts as a wiper arm on a potentiometer. After completing the first voltage measurement, the roles of the two layers are reversed by controller switching and a second measurement is taken. When the operator touches a point on the screen, the two measured voltages represent the point’s X and Y coordinates. In Figure 1, we see a standard four-wire configuration where the two layers are designated as X substrate and Y substrate. The controller is applying a voltage and ground to the opposite bus bars of the Y substrate and measuring the voltage on one of the bus bars on the X substrate. The ITO coating is shown as an infinite number of resistors across the active area. The voltage applied is typically 5 volts, and the switching speed between the two layers is about 5 microseconds.
The touch screen’s life and positional accuracy are dependent upon two key factors?maintaining the uniformity of the ITO coating on each substrate and minimizing positional drift due to property changes. It is the integrity of the ITO coating that determines the linearity of the voltage drop across the active area. Excessive use or abuse, slight scratches in the ITO’s resistive surface during manufacture, or a cut with a thin blade will change the resistance of the affected area and ITO linearity. Touch screen stability is thereby compromised resulting in incorrect association of measured voltages with graphical switch representations on the display. Touching an area associated with a Stop or Abort switch, for example, could conceivably activate a Start or Continue function instead.
Because the properties of the silver and ITO coatings on the substrate vary at different rates, positional drift occurs near the edges of the touch screen creating a constant need for recalibration. Referring to Figure 2, when 5 volts are applied across a substrate, we would see 2.5 volts when making a voltage measurement anywhere along the middle of the active area. When measurements are taken at the top and bottom of the active area, we would see voltages that are just below 5 volts and just above zero volts, respectively. This is attributable to the voltage drops that occur over the bus bars. As environmental conditions change, such as an increase in temperature, the resistance of the individual coatings will change at different rates resulting in a change in the voltage distribution across all of the coatings. If we repeat the above measurements, we would find that the voltages at the top and bottom of the active area have changed. In fact, had we originally taken voltage measurements at several points going from the center to the top or bottom of the substrate, we would now observe different voltage readings for most of these points.
To minimize the drift and linearity problems inherent in the conventional four-wire touch screen, manufacturers have developed alternative designs including five-wire and eight-wire configurations. In the five-wire configuration, only the bottom substrate uses the ITO coating as a voltage divider, and it is bounded on the perimeter by a frame of four resistive patterns. The top layer serves as a pickup. It has a conductive coating that is positioned facing the ITO on the bottom layer and a fifth wire for voltage measurements. Pressing the top layer with a finger causes it to flex at the point of contact and bring its conductive coating into electrical contact with the ITO beneath it. As the controller switches the voltage and ground between the resistive patterns, voltages are obtained from the fifth wire representative of the X-Y coordinates.
Immediate advantages, claimed by proponents of the five-wire touch screen, are the additional flexing and abuse that the upper layer is capable of enduring since ITO linearity concerns and positional drift due to different coating properties do not apply to the conductive material.The eight-wire touch screen is similar to the conventional four-wire configuration with the exception that it adds two sense lines to each substrate?one to each bus bar?at the edges of the touch screen as shown in Figure 3. Using these sense lines to obtain a reference voltage, the controller can ensure that the proper voltage is applied across the ITO by compensating for voltage drop variations in the bus bars. Although manufacturers of the eight-wire design contend that touch screen stability is increased as a result of eliminating positional drift, it seems, however, that the ITO linearity problem remains unaddressed.
A third design has been recently introduced that effectively addresses both problems? positional drift and ITO linearity. It essentially converts the touch screen’s active area from a resistive to a conductive area by eliminating the ITO coating as the voltage divider. Figure 4 provides a functional illustration of the new design that has been named RuggedTouch* by its manufacturer, CAM Touchview Products Inc, located in Amityville, New York. It is a four-wire configuration that uses a proprietary durable resistor, with properties similar to silver, that has been reduced in size to a thin strip and deposited outside the active area of each substrate. A second resistive strip, for reasons we will address later, is also deposited in the inactive area on the opposite side of the substrate; both resistors function as voltage dividers in place of the ITO. Rows of thin conductive ITO strips are formed on each substrate running across the active area and are coupled on either side to the resistors. Since the ITO strips function as conductors only, the resistance of each substrate’s active area is negligible, and the touch screen’s life and positional accuracy are no longer dependent upon maintaining the resistance uniformity of each layer. Furthermore, significantly reducing the size of the resistive material and placing it out of harm’s way in the inactive area of the screen has resulted in extending the screen’s life. Constant flexing and minor damage, which would usually necessitate recalibration or replacement of conventional touch screens, do not degrade the RuggedTouch’s stability and performance.
Positional drift has also been minimized due to the similar properties of the resistor and the silver bus bars. Since they will have similar variations in resistance due to changing electrical and environmental conditions, their voltage distribution remains unaffected.
Although the RuggedTouch employs a unique circuit configuration, it is designed to be a drop-in replacement for standard four-wire resistive analog touch screens. The controller still occupies a single PCB slot with no requirements for hardware or software modifications in the host system. The value of the RuggedTouch resistor can be controlled to individual customers’ specifications. While typical resistive analog touch screens are limited by the availability of standard ITO coatings and the geometry of the touch screen, the deposited resistor can be configured for axis resistance values from a few ohms to 20,000 ohms.
The RuggedTouch operation also remains unchanged. As shown in Figure 4, the two layers representing the X substrate and Y substrate are positioned with their ITO conduction lines facing and running perpendicular to each other. The controller applies a voltage and ground to the left and right resistive strips on the Y substrate and measures the voltage on the X substrate. The controller then switches the voltage and ground to the X substrate and measures the voltage on the Y substrate thereby obtaining the two voltages representing the X-Y coordinates.
The second resistive strip in each substrate, provides an alternate current path in the event any of the ITO conductive strips are severed. In the conventional touch screen, a thin cut would alter the resistance of the ITO coating affecting the positional accuracy of the screen. In the RuggedTouch, however, ITO resistance is not a consideration, and the voltage can be read at either one of the severed ends of the conductive strip. Conceivably, a damaged area on the RuggedTouch that is considerably larger than a razor cut but smaller than the contact area of the operator’s finger on the screen will have no adverse effect on the operation and may even go unnoticed. Even if the damaged area were larger, the operator would just touch another area that lies within the graphical switch.
A key benefit derived from this feature is reduced system down time. This is of particular significance for situations that present hostile environments for touch screens such as kiosks, games, and industrial applications (workers have been known to use screwdrivers and other sharp tools instead of their fingers). With the exception of damages considerably larger than the type we have discussed thus far, the RuggedTouch can continue to function without having to close down an ongoing operation. Repair actions can be postponed until the operation is completed or even until the next scheduled maintenance.
A tradeoff usually accompanies product improvement, and, more often than not, it’s an increase in price. For RuggedTouch, however, it’s a case of better and cheaper. Eliminating the ITO as the resistive element, has resulted in a considerable reduction in the rejection rate during production. The resulting decrease in production cost and increase in production yield has allowed the manufacturer to pass on a 10 to 20 percent savings in price to the customer.
* Patent Pending