by July 4, 2002 0 comments

Touch technology is about using your fingers, or some other pointer, to view and manipulate information on a screen. On a conventional system, with every mouse click, the OS registers a mouse event. With a touch-screen system, every time your finger touches the screen, a touch event is registered. Before we get into how it works, let’s look at the uses of this technology. 

Touch technology is most effective where there’s no time to learn how to operate a PC using a mouse and keyboard. In a fast-paced restaurant, for example, customers can place their orders on a touch-screen enabled machine. ATM outlets, information counters airports, public-information systems, customer self-service counters, tradeshow displays, electronic
catalogs and computerized gaming are other areas deploying touch technology. 

A basic touch-screen system
A basic touch-screen system is made up of three components: a touch sensor, controller, and software driver. The sensor is a clear panel, which when touched, registers a voltage change that is sent to the controller. The controller processes this signal and passes the touch event data to the PC through a bus interface, be it a bus-card, serial, or USB. The software driver takes this data and translates the touch events into mouse events.

A touch-screen sensor can use one of five mechanisms: resistance, capacitance, acoustics, optics and mechanical force. Understanding these will help you match the technology to your application. Some factors that would affect the choice of sensors are roughness of handling, resolution, precision of touch, activation force, response time, type of stylus used and touch life.

Resistance-based sensors
A resistant sensor uses a thin, flexible membrane separated from a glass or plastic substrate by insulating spacers. Both layers are coated with ITO (Indium-tin-oxide). These metallic coatings meet when a finger or stylus presses against the screen, thus closing an electrical circuit. The advantages of this mechanism are high-touch resolution (you can use your finger or any stylus) and resistance to dirt, dust, water or light. The disadvantages are 75 percent clarity and a sharp object can damage the resistive layers.

Capacitance-based sensors
Here voltage is applied to the corners of the screen with electrodes spread uniformly across the field. When a finger touches the screen, it draws current from each side proportionately. The frequency changes are measured to determine the X and Y coordinates of the touch event. Advantages to using this system include high clarity, good resistance to dirt, grease and moisture and high touch resolution.

Acoustic sensors
These sensors detect a touch event when a finger touches the screen resulting in absorption of sound energy. Bursts of high frequency (5-MHz) acoustic energy are launched from the edges of the screen. Arrays of reflectors at the edges divert the acoustic energy across the screen and redirect the energy to sensors. Because the speed of sound in glass is constant, the energy’s arrival time identifies its path. A touch causes a dip in the received energy waveform for both axes. The timing of dips indicates the X and Y touch-point coordinates. The advantages of this system are high-touch resolution, high clarity and durability with no drift operation thus negating the need for recalibration. On the other hand, this sensor is susceptible to dust, dirt and moisture since it can’t be completely sealed. Further one must use a finger, gloved hand or a soft tip stylus to touch the screen.

These are infrared touch screens that use an array of photodiodes on two adjacent screen edges with corresponding photo sensors on the opposite edges. These diode/sensor pairs establish an optical grid across the screen. Any object that touches the screen disturbs this grid, causing drops in the signals. This then indicates the coordinates. Advantage of IR sensors is in good clarity and immunity to drift. On the down side, it is susceptible to dust, needs special bezels for daylight use, and has parallax problems on curved screens. Since the optical grid floats above the screen, a touch event can be registered before the user’s fingers reach the screen.

This can use two types of force-sensing technologies: strain gauge or platform. When you touch the strain gauge screen, the stresses produced are measured at each corner. The ratio of the four readings indicates the touch-point coordinates. The platform, on the other hand, does not use a screen. The monitor or display device rests on a platform with force measurement sensors at the corners. A touch to the display device translates to forces at the platform’s corners. The platforms controller performs the vector calculations that determine the touch point from the four force measurements through rigid-body mechanics. The controller tracks out static forces, such as gravity and repetitive forces such as vibration. The platform sensor is the easiest to install. On the downside, it needs frequent recalibration. False readings are also possible because of shock or vibration of the pedestal. 

Technology Resistive SAW Capacitive Infrared NFI
Stylus Any object Energy
Conductive Any object Conductive
Resolution 4096 X 4096
900 pts
1024 pts
per axis
1024 X 1024
3 — 4
2 — 3
Proximity Electrical
Transmissivity 57 — 75 % 90% or
85% or
100% 88% or
35 million
at  same spot
50 million
at same spot
25 million
at same spot
100 million
at same spot
Accuracy 0.080
1% over
1% of
Response 13 — 18
18-50 ms 15-25  
18-40 ms < 20ms

Future touch applications
Constant innovation is happening to improve the performance in all sensor technologies. Some of the players in this field are Elo Touchsystems and MicroTouch. The latter has produced a touch screen that only the user can view. This restricted-view angle adds a measure of privacy and security to transactions on a touch screen system. Pen capability is an additional draw that allows for a denser touch point, do annotations, drawings and checklists. 

The immediate future has more in store. The touch system here will address medical, geophysical, design, engineering and other 3D applications. Such applications indicate a design focus on the potentials of touch technology. The end product allows the User to perform more tasks with his hands. Imagine sketching with charcoal on Adobe Photoshop, with your hands applying various pressures on a screen. Touch technology is advancing towards this and much more at a rapid pace.

Priya Ramachandra

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