Unconventional Screen Components Ride the Wave to Future Convenience

March 1999
By Fred V. Reed

Consumers say goodbye to the mouse and keyboard, and let their fingers do the walking across computer screens.

Touch-screen technologies based on surface waves and improved resistive screen systems promise to increase touch-display durability, making these devices more useful for both military and general public applications. Although several current offerings provide users with the convenience of entering mouse-free computer commands, many have drawbacks that have limited their consistent, effective use. Two new approaches address these problems, offering additional options to current users and opening up potential applications in a variety of markets.

Until recently, three touch-screen technologies dominated the marketplace: capacitive, infrared and four-wire resistive. Capacitive screens detect a change in capacitance when a finger touches the screen; however, they cannot be used with a gloved hand. In addition, the user’s body size and electrical path to ground can cause differences in the screen’s response. Frequent recalibration can be necessary.

For infrared screens, an infrared signal is transmitted across the screen from emitters located along the bottom of the screen to photodetectors along the top. When a finger breaks the beam, the detector above it senses this and provides the x-coordinate. A similar array on the sides of the screen furnishes the y-coordinate. However, in this method, dirt, insects crawling on the surface of the screen, or a finger that moves near the screen can produce false responses. In addition, there is no way to gather data reflecting the third dimension—the z-axis.

In the third approach, four-wire resistive screens use two slightly separated layers of material that complete a circuit when forced together by a touch. These screens have had problems with durability and readability.

Products introduced by Elo TouchSystems Incorporated, Fremont, California, address the obstacles presented by these earlier technologies.

Intellitouch, which employs technology patented by the company, uses a different approach to solving problems with capacitive and infrared screens. The product uses glass to form the screen rather than layers of different materials. In this method, there is no problem with the display being dimmed through light absorption. Also, piezoelectric transducers convert mechanical or acoustical signals to electrical signals. To determine the x-coordinate of a touch, a piezoelectric transducer in the lower right-hand corner of the screen emits a brief pulse of extremely high frequency sound at 5 megahertz directed along the bottom of the screen, and later, the pulse is reconverted by a receiving transducer to an electrical signal.

Several reflecting strips are located at a 45-degree angle to the pulse’s travel path along the bottom of the screen. These reflect the pulse upward toward the top of the screen. Each strip reflects only a part of the pulse energy so that the rest of the pulse continues to be reflected by successive strips. To compensate for the attenuation of the signal by preceding strips, the spacing of the reflecting strips is tighter as distance from the sending transducer increases.

“You’re peeling off part of your initial send as it goes along that reflector so you have to be more efficient toward the end,” according to Duane Viano, product manager, Elo TouchSystems Incorporated. “The strips are made of a material developed by us that is screened on. It’s a glass adhesive material that is very hard to scratch off.”

Along the top edge of the screen, other reflecting strips direct the pulse back to a receiving transducer that is also piezoelectric. The pulse follows a path from the sending transducer along the bottom of the screen, up across the face of the screen, and back along the top of the screen to the receiver. The wave is attenuated at the point where a finger touches the screen. A microprocessor can determine where along the x-axis a particular received signal came from by measuring the time it takes for the pulse to make the trip to the receiving transducer. The microprocessor constantly compares the received signal to a stored reference signal of the untouched screen. When it detects a dip in signal strength at a particular point on the time line of the received signal, it knows where on the x-axis the touch occurred.

“Basically you’re getting a serial representation of all the little fragments of signal coming back. You plot that over time. It looks like a square wave, and when you touch the screen, there’s a dip in the square wave. You correlate that time delay to a distance,” Viano explains.

The greater the pressure from the touching finger, the greater the attenuation of the pulse, giving 256 levels of z-axis information. Because the screen does not bend, this is not actually z-axis data, but it is pressure information. This is valuable because it allows software being operated through the screen to do such things as scroll text faster when a user presses harder on the screen.

“You need surface wave to make it work. A wave in the interior of the glass wouldn’t be affected by a touch. We get a wave that propagates across the surface because of a combination of the frequency and the transducer. The transducer is actually a wedge shape made out of an acrylic material, which propagate waves differently from glass. The wedge, being at a specific angle and having propagation characteristics different from those of glass, deflects these bulk waves onto the surface,” Viano says.

The resolution of touch screens is determined by the size of the finger, but Viano offers that a soft-pointed stylus can give touch resolution of about a millimeter.

Elo’s surface-wave screens do not drift, and they automatically accommodate dirt on the screen. Any dip in the received signal that lasts more than approximately 30 seconds is made part of the reference wave by the microprocessor, and thereafter is ignored. In addition, the technology is impervious to external noise, and surface-wave touch screens can be used with flat-screen displays or can be manufactured to fit various types of screens, Viano asserts.

The company’s Accutouch  product addresses difficulties experienced in the third type of touch-screen technology, resistive screen systems. Here, four-wire resistive screens use two slightly separated layers of materials that complete a circuit when pushed together by a touch. All resistive screens consist of an overlay of two layers of plastic, both coated with a nearly transparent conducting material and separated slightly by small protrusions of plastic. An electrical potential is supplied by four wires, two for the x-axis and two for the y-axis. When the screen is touched, the layers are forced together, completing a circuit. The electrical path is longer or shorter depending on where the screen is touched, and the distance to the point of touch is calculated using a voltage divider. This technology has exhibited some problems with durability. Also, because with any resistive technology overlays must be placed on top of the underlying screen, only about 75 percent of light emitted by the screen gets through the overlays to illuminate the display.

Each time users touch resistive screens, the top cover sheet is being bent. With enough use, microfractures develop in the conductive coating. The microfractures change the path of the voltage and thus its coordinate, Viano explains. To address this problem, Elo uses indium tin oxide, one of the few conductive substances that are transparent in a thin layer.

The company also offers a second option. “In five-wire technology, which we patented, we made the glass layer the bottom substrate, to have both the x and y directions and drive from all four sides, so you have four wires, left and right, top and bottom, of the bottom sheet. The top sheet is conductive, but it’s really like a big probe. When it touches down, we look at both the x and y on the bottom sheet. The advantage is having that fifth wire that goes to the top sheet. When you start getting microfractures, they don’t matter because all the measurement is being done on the solid substrate underneath,” Viano says.

Elo officials say that the five-wire screen has been tested to 35 million touches, while four-wire has only been tested to about two million. The product also works with gloved hands or a stylus and does not drift.

The company has written driver software for both Intellitouch and Accutouch that turns the coordinate information into the equivalent of that produced with a standard mouse. This enables use with Windows 95 and Window NT as well as UNIX, IBM OS/2, MS DOS and Macintosh operating systems.

Many applications exist for touch screens, according to Viano. For example, public displays and kiosks use them. Screens available in museums include menu items that, when pressed, play a video about different artifacts. They also reduce training time for new employees in fast food restaurants. Workers can press a specific item key without knowing the product’s price or how to key numbers into a cash register.

The screens also have been used for military applications in cockpits and submarines, Viano adds.