You’ve probably encountered some faulty touchscreens that required multiple touches, applying more pressure each time, just to register an entry. That’s because early resistive touchscreen technologies were environmentally unstable and subject to a variety of wearout mechanisms.
Today’s touchscreens, however, are a joy to use. Their technology underpins attractive and responsive interfaces that are easy to modify for additional functionality. Modifications often are a matter of software changes, and the latest touchscreens operate reliably even in RF-polluted environments.
While the iPhone may be the most high-profile device to adopt a touchscreen, some 60 other cell-phone models will employ the technology in 2008, with more than 100 in 2009. We may see around 500 million units by 2012. Mobile phones are just one application, though, with PDAs, PCs, GPS systems, and home appliances also making rapid inroads.
There are five main touchscreen technologies: resistive, surface capacitive, projected capacitive, surface acoustic wave, and infrared. In terms of cost and size, the first three suit mobile products. In all cases, the system consists of a sensing mechanism, a control circuit, and an interface to the control circuit.
Requiring a degree of force, resistive screens perhaps technically aren’t really touchscreens. They use two layers of conductive indium tin oxide (ITO) printed on a plastic film with an air gap in between. Input occurs when the ITO layers touch via finger pressure and touch location detection is achieved by measuring a voltage ratio in the X- then Y-axis.
Resistive technology is cheap, making it viable for highvolume applications. But wider acceptance is limited due to disadvantages like mechanical weakness, few design options, the need for a bezel, thickness of the screen, poor optical performance, and the need for user calibration (Fig. 1). Also, it can’t sense an approaching finger or multiple fingers, options that are now in high demand.
Surface-capacitive screens use a plain ITO layer with a metallized border pattern (Fig. 2). Requiring no sophisticated ITO pattern, the electric field is approximately linear across the ITO. When a finger touches the screen, it bleeds charge from the panel, and sensing comes from the four corners.
A surface-capacitive screen behind a panel is always afflicted with the “hand-shadow effect,” a phenomenon causing sensing errors due to capacitive coupling of the user’s hand and wrist at random angles and distances. These screens are homogeneous layers convolved in three signal dimensions and cannot suppress erroneous signals. Without structuring the ITO into rows and columns, using surface-capacitive technology on the back of panels guarantees failure.
Projected-capacitive touch technology requires one or more etched ITO layers forming multiple horizontal and vertical electrodes, which derive drive from a sensing chip. This chip can offload data to a processor or route touch locations. Single-ended sensing is the usual method for driving the electrodes (Fig. 3).
With transverse sensing, ac signals drive one axis and the response through the screen cycles back via the other electrodes. In each case, position detection comes by measuring the distribution of the change in signals between the X and Y electrodes. Math algorithms then determine the XY coordinates of the touch by processing signal-level changes.
But with capacitive touch, the LCD is very close to the ITO, if not bonded together. It invariably emits large amounts of electrical noise, ranging up to 20 kHz, due to constant pixel scanning. This requires a shield layer between the ITO electrodes and the LCD. It is usually necessary to have three ITO layers: two for the XY matrix and one as the shield, resulting in higher costs and lower transparency.
Most suppliers employ at least two ITO layers plus a shield to achieve noise-free operation. Of note, Quantum Research Group has developed a single-layer projected XY matrix design that doesn’t require a shield layer.
Another hot topic is multitouch, or the ability to sense more than one touch simultaneously. Surface-capacitive screens cannot discern more than one touch at a time. Two-layer projected capacitive screens can, though single-ended versions cannot accurately differentiate between two touches to be able to track them individually.
Adding a third layer can resolve the remaining ambiguity, but at a higher price. Two-layer projected capacitive screens using transverse sensing can in theory discern two or more touches clearly, tracking each touch independently.
Unlike resistive and surface-capacitive screens, projected-capacitance screens require no user or, often, factory calibration because the structure of the electrodes defines the screen response. As surface resistances degrade and become non-uniform over time, homogenous screen technologies need substantial calibration.
SETTING THE SUPPLY CHAIN
Creating an attractive, reliable, and rugged touchscreen based on projected-capacitance technology involves selecting the right basic technology and a vendor that can deliver it. Some vendors offer turnkey packages including a controller and screen-sensing element, often integrated together. Others offer a chip solution and assist in the design and selection process of the ITO film.
Choosing a supply chain involves many tradeoffs transcending the underlying technology. Key issues include the ability to multi-source films, manufacturability, quality control, and testing. The final process, laminating film into the end product, needs particular attention. Many failures occur in this step due to stresses and inaccuracies in the lamination process.
Here to stay, projected-capacitance technology solves many problems associated with prior methods and is now available from at least two chip vendors. Selecting the right vendor depends on design requirements, cost, and supply-chain management issues, now emerging as significant factors.