Displays

Don't resist the resilient touch option

24th July 2017
Enaie Azambuja

Touchscreens seem to have taken over the world. They are on phones, tablets, and laptops and have now pushed into industrial control and home appliances. Capacitive technology has played a significant role in making touch technology so ubiquitous. But it's in those applications outside computing and communications that we recall why resistive touchscreen technology still has an important role to play.

Capacitive touch has the advantage of supporting gestural user interfaces such as the multi-finger swipe, pinch and zoom movements we now know well. But many systems just need to monitor simple button presses and maybe follow the motion of a finger or stylus to set a value on an animated dial or slider. And those systems need something that is both cost effective and works under a wide range of conditions.

The big problem with capacitive touch is its sensitivity to water and contaminants. The capacitive touchscreen works well where you can expect the surface to stay clean – who wants a dirty smartphone? But what happens if you have a coffee-making machine that shows the user what it is making and lets them select different options from an easy-to-understand menu?

Condensation can easily collect on the touchscreen from the steamy nature of the coffee-making process, making it hard to order a second cup.

In the world of industrial control, the ability to screen out 'false' touches caused by surface contamination is essential to reliable operation. There are ways to reduce the problem of false touches with capacitive technology but they increase design complexity. A better approach is to opt for a different kind of touchscreen construction.

Resistive touch technology overcomes the problem of reliability by making a protective plastic layer part of the interface itself. The resistive technology has decades of experience behind it. The fundamental idea was developed in the 1970s with commercial exploitation taking hold in the 1980s, as graphical interfaces for embedded computers began to become more common.

The resistive touchscreen is a sandwich of transparent layers that are mounted over the display itself. Two of the transparent sheets are coated with conductive material in an x-y pattern. This material is usually indium tin oxide (ITO) as that is also practically transparent in thin layers.

One sheet contains lines that run along the x-axis. The sheet that lies on top has all its lines painted with lines along the y-axis. Tiny transparent spacers sit between the lines to prevent the layers making permanent contact with each other.

When a user presses down to activate a virtual button on the screen, the conductive lines meet in a small region of the display, completing a circuit between at least one pair of x and y lines.

Resistive touchscreen technology offers resilience through its use of multiple layers that sit on top of the display. The outermost plastic covering can act as a protective surface, guarding against chemical attack. As dirt collects on the outer surface, it does not affect touchscreen operation at all – it just makes the screen harder to read if not cleaned off.

As a result, resistive touchscreens have found use in systems that need not just touch control, but to be sealed and easy to clean – bedside monitors and other medical systems provide good examples of why resistive technology can be the better option.

It is also particularly well suited to industrial environments where operators are often required to wear gloves but where the application need’s fast and accurate detection, ruling out the possibility of capacitive touch.

As well as resilience, a big advantage of resistive technology is its relative low cost. A controller that recognises which strips of conductor have been connected by a finger pressing down can be extremely simple, compared with the advanced signal processing that needs to happen inside a capacitive multitouch interface device.

The relatively simple controller architecture fits well into low-energy wireless modules, making it easier to separate the user interface from the device itself and enable remote control. Using wireless communication, you can move the user interface for a system to a more convenient location – and one that is easy for the user to adjust.

By adopting the ideas of the IoT, you can pull together information from many different sensors and systems around the room. Or take advantage of the ability to seal a unit and move it outside.

With the combination of low-power and resilient touch control, designers can explore new types of products such as easy-to-program irrigators for the home greenhouse that respond to information from humidity and temperature sensors.

Alternatively, consider a weather station for the garden outside that sets up alarms after, say, several days of dry and sunny weather, to let the owner know maybe they should water some of the more sensitive plants.

Inside the home or office, the same kind of touchscreen technology can be used to create more intuitive security control panels or to enable more attractive vending machines and coffee makers. As you can see, there are still plenty of applications for resistive touch and complementary technologies such as the IoT are creating even more every day.

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