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When using devices located in public spaces, we increasingly interact with them through touch screens. We operate in this manner devices such as ATMs, ticket machines, parking meters, parcel machines, and self-service checkouts. Although touch screens visually look similar, they are technologically different solutions, characterized by different ways of operation and factors such as sensitivity and precision, durability, or light permeability.
Touch Technologies: An Alternative to Buttons
Firstly, we should clarify the meaning of the term touch panel. Simply put it means display, touch sensor, and glass combined. It also includes other essential components, such as a controller responsible for processing and transmitting signals when the screen is touched. All these elements are necessary to produce a properly functioning display module with touch feature.
Capacitive Touch Sensors (CTP)
In capacitive touch screens, the touch location is determined by detecting changes in the electrostatic field. Capacitive sensors are built from ITO (indium tin oxide) electrodes, which can be placed in two arrangements – in the screen’s corners (surface capacitive variant) or distributed across the entire screen surface (projected capacitive variant). When we touch the screen with conductive objects such as fingers or a stylus, their conductivity causes changes in the electrostatic capacitance between electrodes, which is registered as a point of touch.
Resistive Touch Sensors (RTP)
In resistive touch screens, the position is figured out by identifying changes in resistance. Resistive sensors are made of two foils – an upper (elastic, which bends under pressure) and a lower (firm) foil. Both are coated with indium tin oxide from the inside. When the screen is touched, the upper and lower foils are in short-circuit, resulting in two voltage dividers depending on the contact point. Now the voltage measurement is performed by an ADC (analog-to-digital converter), which defines the specific location on the screen, interpreted in the X and Y coordinates.
Infrared Touch Sensors (IR)
On the market there are also other touch technologies available, such as infrared (IR). These touch sensors are built from LED diodes emitting infrared light, arranged at the screen edges, creating a grid of infrared beams. When the screen is touched by a finger or object, the light beams are interrupted, defining the contact point.
There is no one fit-for-all solution – each touch panel must be selected to a given application – this is a matter that should be considered individually, primarily due to the conditions in which the device will be operated, like the target location – is it indoors or outdoors, what is the intensity of use, what risk of contamination or flooding, etc. The table below compares capacitive and resistive technology, referring to example factors to consider when choosing a touch sensor.
| parameter | Capacitive Sensor (CTP) | Resistive Sensor (RTP) |
|---|---|---|
| sensitivity | higher | lower |
| precision | higher | lower |
| reaction time | faster | slower |
| multi-touch | yes | no |
| operating with objects | no (only with dedicated objects, i.e. stylus) | yes |
| operating with gloves | yes, after calibration | yes, without calibration |
| operating with surface contamination | yes, after calibration | yes, without calibration |
| durability | lower | higher |
| readability in bright environment | better | worse |
| price | expensive | cheaper |
Touch Panel Calibration
Touch panel calibration involves adjusting the parameters of the display module to enable additional functions and enhancements:
- Water rejection mode – ensures proper operation of the touch panel with water on the screen surface,
- Palm rejection mode – ensures proper operation of the touch panel when larger objects, such as the entire palm, come into contact with the screen surface,
- Noise detection or frequency hopping functions – adapt the module’s operation to environmental conditions in real-time; these functions are particularly recommended for frequent changes in the level and frequency of noise.
Touch panel calibration allows adapting the touch panel for use with gloves – such as working, latex, nitrile, and rubber gloves. This solution is recommended in industries such as manufacturing and medicine.
Cover Glass Customization
Devices located in public spaces are prone to mechanical damage – resulting from material fatigue due to intensive use, but also from accidental or intentional impacts. Glass covers every touch screen display module to protect delicate electronic components inside. Glass can be modified by adjusting its thickness – it can be up to 15 mm (!) thick, without affecting the operating conditions of the touch screen. Typically, 5 mm thick glass is sufficient to provide adequate protection for the display module, with a mechanical resistance level of IK08 according to PN-EN 62262 guidelines.
There are also other customization options for glass – it can be cut in different shapes, drilled (to create holes for mechanical elements such as buttons, switches, or knobs), and painted.
Coatings
For devices intended for public places, one should pay attention to user comfort and safety. This can be achieved by using coatings. One of the most popular solutions are anti-glare (AG) and anti-reflective (AR) coatings, which both reduce light reflections. These are nowadays standard solutions – increasingly applied by manufacturers even in standard display modules.
Examples of other coatings that work well in devices found in public spaces (and are used by thousands of people) include:
- Anti-fingerprint (AF), which reduces the adherence of contaminants to the screen surface, including grease (which is a component of fingerprints),
- Anti-shatter (AS), which prevents the spread of glass fragments in case of the display damage,
- Antimicrobial (AM), which limits the growth of microorganisms transferred to screens surface by its users; their effectiveness is reviewed at least at a level of 99.9%.
Optical Bonding
Optical bonding is a method of joining displays with touch sensors and protective glass, which involves bonding individual layers using transparent adhesives or films, typically cured with UV light. As a result, the air between those components is removed. This improves the quality of the displayed image by eliminating the phenomenon of light refraction or limiting dust penetrating the display.
Summary
Creating a display with a touch screen feature that meets end-users’ preferences is always a technological challenge. Our expertise gathered over the years, which includes developing touch screen solutions for standard and non-standard applications, allows us – Unisystem Solution Department engineers and project managers – to meet even the most difficult challenges concerning proper application of touch screen technologies.
Jacek Marcinkowski,
Head of Solution Department
