Bert Moore

A version of this article was originally published in the June 1995 issue of Automatic ID News. Company names and product offerings may have changed since this article was written but the technology issues remain the same.

Introduction

There are five basic types of touch screen technology: resistive, capacitive, infrared, surface acoustic wave (SAW) and strain gauge. Resistive and capacitive are the most commonly implemented although all the technologies are viable and suited to different applications. At the time this article was written none of the vendors mentionedoffer a strain gauge device and only IBM, of those listed, offers infrared.

The major difference between products (other than those noted below) is that some offer "continuous" touch areas while others offer "discrete" touch areas. With continuous screens, buttons can be placed in any location and made any shape and size. With discrete touch screens, the shape and size of touch-sensitive areas are pre-determined. In practical terms, however, many of these discrete touch areas are about one-quarter the size of a finger, allowing relatively flexible positioning of buttons.

How They Work

Resistive -- this technology uses two layers of conductive material to create an X/Y grid (vertical lines on one surface, horizontal the other). When pressure is applied to the flexible top layer (typically Mylar), the grid surfaces come in contact and allow current to flow. The presence of current in a vertical and horizontal line gives the X/Y coordinates of the touch.

Because the screen itself supplies all current, workers with heavy gloves can work this type of screen.

The drawback to resistive screens is that they are subject to scratches, burns and other types of damage. To protect against damage, companies such as DynaPro apply an additional Mylar overlaminate which can be replaced if damaged

Capacitive -- a conductive laminate is applied to a glass screen. Current is applied to all four corners of the screen. When a conductive surface (such as a finger or special stylus) is applied to the surface, the current flows through the conductive surface. (The electrical current is so low that a user doesn't feel it.) By calculating the relative draw from all four corners, the controller can determine the location of the touch.

Because capacitive screens are glass, they are resistant to abuse and damage. These screens are used in gaming applications where burning cigarettes are often left on the screen.

The disadvantage is that workers wearing thick gloves don't register on these screens (although thin surgical gloves do register). Grounded styluses can be used in applications where gloves would interfere with operation.

Infrared -- is similar to resistive, infrared screens project horizontal and vertical beams of infrared light over the surface of the screen. When a finger or other object breaks those beams, the X/Y coordinates are calculated.

These screens can also be used by workers with gloves and are relatively impervious to damage.

Dirt, flies and other objects, however, do offer the possibility for registering stray touches or blocking the light emitting or receiving apertures. Workers who "cruise" with their fingers before touching the screen can also trigger a false response.

SAW -- high frequency sound waves are transmitted through vertical and horizontal axes on the screen. When a finger touches the surface, it shortens the time for the "echoes" to reach receivers, giving the location of the touch. SAW technology can also register on the Z axis, determining the amount of force exerted on the screen.

Gloves are not a hindrance to operation and these screens are relatively impervious to stray touches. In addition, if an application requires measurement on the Z axis, SAW is a good choice.

A new variation, guided acoustical wave (GAW), directs the signal through the panel itself, not over its surface. This allows the screen to be completely sealed.

SAW and GAW provide the brightest images since there are no overlays to diminish the image.

The disadvantage to SAW is that it's not as well supported and is more expensive than the three more popular technologies.

Strain Gauge -- the screen is spring mounted on the four corners and strain gauges are used to determine deflection when the screen is touched. This technology can also measure the Z axis.

Advantages and disadvantages are the same as SAW with the additional consideration that, since the screen must be free to move, it is much more difficult to completely seal the unit.