Challenges with integrating a PCAP touch screen


touch display retailTouchscreen technology is used for an array of applications, from industrial equipment, through point-of-sale (POS) terminals, vending machines, automated teller machines and high-end security panels, to medical equipment, automotive and aerospace systems.

Touch panels are an essential part of the human-machine interface and users quite rightly expect a similar experience to their personal smartphones and tablets. Yet the technology that enables one device to have so many functions goes largely unseen. Though there are several types of touch technology, the most common in today’s mobile technology is Projected Capacitive (PCAP) providing the multi-touch capability users expect.  It is fast becoming the touch of choice in industrial applications too.

What we'll cover in this Whitepaper:

The basics: How a touchscreen us made up
Getting started with a new capacitive touch design
Outdoor environments
Industrial operational environments
Electrical environments
Vulnerable environments
Conclusion 

The basics: How a touchscreen is made up

At the heart of PCAP touch sensors are multiple glass or film layers printed with conductive material; typically, Indium Tin Oxide (ITO). These are printed in the shape of interlocking diamonds or other specially-designed patterns serving as X and Y channels in two separated electrode layers. An electric charge is held in the gap between these, so any conductor like a human finger will alter the electrical charge at the node. These electrical changes can be detected by a touch controller that is continuously driving the X channels and receiving from the Y channels. Software in the controller computes the coordinates of all simultaneous touches, providing multi-touch capability.

Figure 1: How a Capacitive Touch Screen works

Getting started with a new capacitive touch design

The development of a new touch panel starts with a thorough understanding of the requirements and technological constraints in terms of capability, performance and cost over the life-cycle. Once these parameters are understood, design experts can look in detail at integrating individual parts of the capacitive touch panel – controller and firmware, display, touch sensor, touch keys and cover lens. Initial design considerations that feed into this process include:

  • Screen dimensions – custom touch panels need not be limited to standard sizes: PCAP touchscreens are viable from 3.5” to 80”!
  • Single touch/multitouch - features like pinch to zoom, rotate, gestures, two-finger scroll, “crumple” and multi-user interaction can all be accommodated, but their practicality will depend on many other constraints. The ideal touchscreen may be 10-touch capable but does the application really need it?
  • EMC conformance – can be costly if left as an afterthought. Susceptibility to conducted current disturbances induced by RF fields and its impact on touchscreen performance needs to be factored-in at the outset.

These initial considerations will inevitably lead to some compromises. Nevertheless, understanding the trade-offs throughout the process should lead to the most satisfactory solution. A key part of custom touch-panel integration is understanding the environment in which it operates. Let’s look at four major challenges; outdoor environments; the operational environment; electrical environment; and vulnerable environments.

Outdoor environment

As well as the obvious challenge of preventing water ingress to the innards of the panel, PCAP touch panels need special attention because water on the screen changes the capacitance signal. Drops can be interpreted as finger touches, so careful firmware tuning to adjust sensitivity for water rejection needs to be applied to reject false touches at the same time as correctly interpreting true touches. Similarly, when liquid is on the screen, wet finger tracking provides the ability to accurately track a finger on the screen in the presence of a liquid.

false touches on a touchscreen

Figure 2: Electrical intereferences - such as water - can cause false touches.

Outdoor applications require both, which can add extra cost and complexity to the PCAP design. Such requirements are accommodated by selecting appropriately designed sensors and the ICs that support them.

It not just rain that’s in the air to challenge outdoor applications. Coastal locations can bring salt spray. Most standard anti-reflective coatings, used on outdoor displays, would not stand up to the corrosive effects of salt, meaning the touch performance would be affected. Salt resistive AR coatings can be selected and should always be used for marine applications.

One challenging new application for PCAPs in outdoor environments is electric vehicle chargers. With the majority of new cars built in the UK by 2030 having to be electric, the need for fast charging stations will be imperative. Fast charging involves delivering several hundred kW per station and connecting the units with payment and monitoring means there are wired and wireless networks to consider, too. So, the touchscreens need good EMC performance as well as ability to operate in rain and bright sunlight. In addition, the PCAP screen should be integrated in a housing protected to IP54 and IK08 specifications.

Industrial operational environment

industrial operator using display device whilst wearing gloves The key consideration for touchscreens for industrial control operations is the operator.  Careful thought needs to go into how they interact with the screen. In particular, there is the issue of gloved operation. Screen response when wearing gloves depends on achieving a suitable combination of ITO pattern, touch controller IC performance, firmware optimisation and coverlens material properties including thickness.

The ITO pattern influences the signal-to-noise ratio and, together with the touch-controller firmware, determines the touch threshold. Both factors influence the permissible cover-glass thickness, and potential performance if the user is wearing gloves.  It is even possible to create touchscreens tuned for optimum performance with a certain type of glove, such as application-specific industrial gloves.

For touch panels operated indoors, the environment is still a factor. For example, liquid spills, condensation and wet finger touches as well as thermal extremes can all affect performance. Ghost touches can be eliminated by tweaking the sensitivity setting in firmware. Identifying false touches is a more complex process, but the algorithms to do this are constantly improving,

Successful integration therefore relies on a thorough understanding and knowledge of the parameters of the touch control IC being used and its firmware. For example, when AndersDX worked with espresso machine pioneer Saeco, integrating a wide viewing angle 7” TFT PCAP touchscreen on their machines. One of the many challenges of display integration within coffee machines is the presence of steam which can cause touch screen dysfunction. A touch controller IC upgrade with complimentary firmware was integrated with the touch sensor enabling the screen to work reliably in the presence of water condensation on the touch surface.

Electrical environment

electromagnetic interference motherboard testingEMC is often forgotten at the initial stages, especially conducted EMI, but achieving the required conformance can be costly if this is left as an afterthought. Electrical interference, called noise, can cause inaccurate readings, known as ‘ghost’ touches. Touch screens can be a source of noise, but noise can also be anything from ambient light to electrical noise from a display or charger.

Interference levels within a factory environment may be especially high, with industrial machinery itself proving a significant source of interference, potentially causing equipment malfunctions and failure. Medical devices, too, need to be immune to very high levels of EMI, necessitating optimum selection of IC and fine tuning of firmware. With suitable optimisation it is still possible to fulfil the other key requirements for medical applications, for example tweaking the firmware to enable use with surgical gloves. Infection spread through use of the touchscreen can be minimised by applying anti- bacterial coatings as a safety precaution.

Vulnerable environments

vending machine display vandalismFrom kiosks and ATMs to vending machines, petrol pumps, ticket machines, many touchscreen systems today risk vandalism or abuse. Door entry systems are another example of systems that benefit from touch panels but need careful protection.

They all need to cope with harsh environments as well as frequent public use necessitating a highly reliable and rugged design. Vandal proofing solutions can be applied to vulnerable touchscreens using high mechanical impact protection rating (IK08, IK09) ensuring a design to withstand the most challenging situations. Further protection can be provided by combining the touch sensor with a thick cover lens made from chemically strengthened or toughened glass. Integration involves understanding the trade-offs between lack of multitouch performance, increased overall physical weight and cost.

Conclusion

A detailed understanding of the application’s physical, electrical and human environments all play their part in bringing a touch panel design from concept to successful completion. Working with design experts from the start gives the benefits of up-to-date technical know-how, the broadest possible product knowledge and practical experience of successful integrations using the best possible touch IC products fit for your application.

If you would like to dicuss your touchscreen project with one of our skilled engineers, please contact us today.



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