TechArchive

Original article date: March 1998

Touch screen technology: applications and possibilities

Gerry Harling international marketing director of Danielson explains the developing possibilities available for industrial touch screen applications.

The use of touch screen technology is growing rapidly both in consumer and industrial applications. The world touch screen market is estimated to be increasing by a third every year. However while many people may now have used touch screens in cash points or consumer information systems there appears to be less appreciation of the potential in the industrial arena. However typical industrial applications do already range from machine tools and process control to medical and navigation instruments. Touch screens are also used in notebook computers where they provide a simple-to-use alternative to mouse and keyboard operation. In many circumstances they minimise the time taken for operator training and reduce confusion in multilingual situations.

There are four principal types of touch screens available: Infra-red (IR) surface acoustic wave capacitive and resistive. Traditionally considered to be the simplest of the four resistive touch screens are now emerging as perhaps the most popular device for use in a range of industrial applications. This is largely due to their simplicity which results in relatively low costs and a long operating life. Combined with excellent resolution good optical clarity and resistance and the ability to operate with all types of stylus these advantages provide the flexibility and resilience required to adapt to a range of industrial environments.

In essence resistive touch screens consist of two polyester layers coated on their inner surfaces with a thin transparent conductive material such as indium tin oxide and separated by a series of non conductive spacer dots. When the front of the screen is touched the layers come into contact. This alters the voltage applied across the two layers allowing a controller to detect the change and calculate the position in which the screen has been touched.

This thin flexible assembly can be mounted onto a rigid glass panel for clarity and rigidity or for applications where glass is unsuitable on a polycarbonate back panel. The outer surface of each touch screen typically has a tough coating of silicon dioxide which can be supplied in a choice of anti-reflective and gloss finishes.

Two forms of resistant touch screens – matrix and analogue – are generally available. While the mechanical construction of the screens differs little the electronic construction considerably influences the applications appropriate to each resistive screen.

The inner surface of the polyester layers of matrix touch screens consist of an etched array typically arranged in rows and columns. Touching the screen effectively closes a circuit indicating which row and column has been activated.

Simple and reliable matrix touch screens are ideal for high volume applications and are particularly suitable where menu selection or a maximum resolution of 25 x 25 (dependent on mechanical dimensions) is required.

Standard four-wire analogue resistive touch screens have two continuous planes of conductive material one with horizontally opposed busbars the other with vertically opposed busbars. Each plane is alternatively driven with a constant 5 DC voltage. A drop in the voltage indicates that the two layers have touched under pressure from the operator allowing the touch point coordinates to be determined.

Analogue screens give exceptionally high resolution and accuracy. For example a 10 bit A-to-D converter gives a resolution of 1024 x 1024. By processing the signals produced in conjunction with a microprocessor it is possible to produce data suitable for interpretation by application software. Consequently analogue screens are essential for driving software written for hand-held pointing devices such as mice or pen plotters or for fine resolution work.

A range of standard PC interfaces including driver software is also available with analogue touch screens allowing each system to be set up and operated as quickly and as simply as possible.

One of the main factors restricting the widespread use of both forms of resistive touch screens has been that until recently they could only be incorporated into a front panel as a separate component with the use of stepped mouldings and non-visible joints. While satisfactory for many applications this has prevented use in applications where hygiene is of extreme importance for example in the medical and food industries. However some companies are now addressing this problem by providing integrated touch screen solutions for industrial applications.

As the integration of touch screen technology into customised systems is simplified the use of the screens as an easy to learn and use man/machine interface is likely to grow. The increased flexibility offered by screen customisation is likely to ensure that touch screens become a more accepted part of industrial environments. Danielson incorporates with its touch screen panels in a sealed unit membrane keyboards force sensing switches electroluminescent lamps and displays together with a choice of polarisation filters antiglare finishes and EMC shielding.

A recent integrated design combined a matrix touch screen with two non-tactile membrane keys to produce an operator panel for an X-ray generator. The resulting panel allows the excellent levels of hygiene and functionality required in a medical environment to be achieved. An additional benefit of the unit is that the design incorporates universally recognised operating notation and graphics. This has obviated the legal requirement for all X-ray controls to be indicated in the user’s language with the result that only a single version of the touch screen need be produced allowing more economic production.

The same company also commissioned the design of a universal operator module for multiple process control applications specifying a high level of cleanliness sealing and quality of design as significant parameters. The resulting panel consisted of a fully integrated touch screen and glass support complete with an aluminium front panel and polyester overlay. The touch screen assembly is accredited to IP65 and can be utilised in a temperature range of -30degC to +70degC and can quickly be customised to take up different displays according to the process in question.

The alternative technologies

Infra-red

Infra-red (IR) touch screens are based on the high speed projection of a series of IR beams in a grid pattern across the screen surface. Interruption of these beams when a finger or other stylus touches the screen triggers a photo transistor which in turn transmits a signal to a screen controller allowing the position of the stylus to be established.

Although IR touch screens offer excellent optical clarity typical resolution is poor and environmental resistance lower than for both resistive and capacitive types.

Surface Acoustic Wave

This technique relies upon the transmission of acoustic waves over a glass overlay to series of receivers. A stylus touching the screen absorbs wave energy disturbing the pattern reaching the receiver. The controller then measures the effect on the wave to determine the x and y co-ordinates at the point of contact.

These touch screens offer very good optical clarity with medium resolution and are fairly robust. However the number and type of stylus that can be used is restricted.

Capacitive

A typical capacitive screen features a single layer substrate usually glass with a thin transparent metallic coating such as indium tin oxide applied to the outer surface. Electrodes positioned around the screen are connected to wires located at each corner linked directly to tuned oscillators on the controller. When the screen is touched the oscillations of the tuned circuits change. By comparing these oscillations with earlier readings the controller is able to establish point of contact.

Capacitive screens are extremely robust offer good optical clarity and resolution. While the choice of stylus is limited the screens are ideally suited to use in public applications such as vending machines and the provision of tourist information.

• Danielson
• Tel: 01296 424478
• Fax: 01296 392141

March 1998