Touch Screen Technology: Choosing the Right Touch Type for Industrial Displays

Touch screen technology is the sensor and controller that let a display detect where a finger, glove or stylus touches it and pass that position to the computer as input. Five technologies cover almost every industrial touch display: resistive, surface capacitive, projected capacitive (PCAP), surface acoustic wave (SAW) and infrared (IR). Each senses touch in a different way, so each behaves differently with gloves, water, scratches, sunlight and electrical noise.

You have probably used several of them today. Self-service kiosks, ticket machines and point-of-sale terminals use touch to speed up transactions, and on the factory floor the touch screen on a human-machine interface (HMI) is how operators run and monitor a process. The right choice depends on who touches the screen and with what, where it is installed, how often it is used and how hard it will be treated.

This guide compares the five types side by side, explains how each one works and where it struggles, then covers gloves and water, cover glass and impact ratings, electrical noise and touch controllers, and a selection table by application.

The table compares the five technologies on the points that matter most for industrial displays. Figures are typical; a particular sensor, controller and cover glass can move them either way.

Behind every one of them sits a touch controller, a small board or chip that turns the raw sensor signal into coordinates and sends them to the computer, usually over USB. Our explainer on how touch screens work follows that signal path from the sensor to the operating system.

Resistive touch was the most widely used touch technology for many years and is still common on industrial HMIs. The sensor is a flexible transparent plastic front film over a rear glass panel. The facing surfaces carry a thin conductive coating, usually indium tin oxide (ITO), and a slim air gap, kept open by tiny spacer dots, separates them. One layer carries a small measured voltage. When a finger, glove or stylus presses the film, the two layers touch, the voltage at the point of contact changes, and the touch controller calculates the position from it.

Because it senses pressure rather than charge, resistive touch works with anything: a bare finger, a thick work glove, a pen or a tool. It is unaffected by water on the surface or by most contaminants, draws little power and gives off negligible electromagnetic interference, which is why it has long been used in military and medical equipment. The flexible front can be sealed to front IP65 for wet areas.

4-wire, 5-wire and 8-wire resistive

The wire count describes how the controller drives and reads the layers, and it decides how the screen ages.

For industrial use, 5-wire is the usual choice: the stable rear-glass measurement keeps the screen accurate long after the front film has picked up wear.

Strengths and limits

• Works with any glove, stylus or tool, and ignores water and most surface contamination
• Low power and negligible electromagnetic emissions
• Can be sealed to front IP65 for wet and dusty areas
• Lower light transmission than glass-fronted technologies, because of the film and conductive coatings
• The plastic front can be scratched, cut or punctured by sharp objects, and a damaged area stops working
• Generally single or dual touch, so no pinch or swipe gestures
• Long-term wear can cause touch drift, especially on 4-wire sensors

We supply resistive touch in two forms: resistive touch panel PCs, with the computer built in, and resistive touch monitors for an existing controller or PC.

A surface capacitive sensor is a glass panel with an ultra-thin transparent conductive layer, again usually ITO, under a protective coating. A small, controlled voltage is applied to the layer from electrodes at the four corners. When a bare finger or a conductive stylus touches the screen, it draws a tiny current, and the controller works out the position from how much current flows from each corner.

Surface capacitive gave better image quality than resistive at a modest cost, and it suited supervised indoor uses where durability was not critical. It has largely been replaced by projected capacitive, which does the same job through thicker glass and adds multi-touch, so it is now rare in new industrial designs.

• Better image clarity than resistive, and not affected by dust or grime on the surface
• Sizes up to about 32 inch (32″)
• Needs a bare finger or a conductive stylus; gloves and ordinary pens do not work
• Liquids and damage to the coating affect operation, and wear causes touch drift
• Single touch only, and each sensor is calibrated to its own controller, which makes replacement costly

Projected capacitance has overtaken resistive as the leading touch technology worldwide. Like surface capacitive, it relies on a conductive object changing the electrical charge of the screen, but it senses through the cover glass rather than on its surface. Behind the front glass sits a grid of transparent electrodes arranged in rows and columns. When they are charged, they create an electrostatic field that projects through the glass. A finger or conductive stylus near the screen changes the capacitance at the nearby electrode crossings, and the controller finds the touch at the point of greatest change.

There are two ways to measure that change, and most industrial controllers use both.

Using both lets a controller track several fingers while recognising that a film of water is not a touch. Our explainer on what a capacitive touch screen is covers ghost touches, calibration and the ITO grid in more detail.

Strengths and limits

• High brightness and image clarity, because the front is plain glass
• Very resistant to scratching and impact, especially with thick toughened or chemically strengthened glass
• A flat, bezel-free glass front that is easy to clean and seal to IP65, IP66 or IP69K
• Largely unaffected by dust, grease and water droplets with the right controller settings
• True multi-touch for pinch, zoom and swipe
• Works with thin gloves as standard and with thicker gloves when tuned
• Typically costs more than resistive, is more sensitive to electrical noise and needs more care in integration

PCAP is available both as PCAP touch panel PCs and as PCAP touch monitors, with glove touch modes on a number of models.

A SAW screen sends ultrasonic waves across the surface of a plain glass panel. Piezoelectric transducers at the edges generate the waves, reflector arrays spread them across the glass, and receivers on the opposite edges pick them up. A finger or soft object touching the glass absorbs part of the wave, and the controller locates the touch from where and when the signal drops. Because the size of the drop depends on how firmly the screen is pressed, some SAW controllers also report touch pressure.

With no conductive coating on the glass, SAW gives excellent image clarity and the surface is very hard wearing. It is best suited to indoor use: the waves are absorbed by anything resting on the glass, so water droplets, dirt and adverse weather affect it.

• Works with a finger, a gloved finger or a soft-tipped stylus
• Excellent image clarity, with no coating over the panel
• Unaffected by most scratches, and good endurance to wear
• Does not respond to hard objects such as a pen or fingernail
• Water droplets cause false touches, and residue creates dead areas until it is cleaned off
• Hard to seal, because the wave must travel freely to the screen edges, so dust and liquid can get in
• Can be affected by shock and vibration, and is limited to single or dual touch

For indoor kiosks and control rooms, our SAW touch monitors cover a range of screen sizes.

Unlike most touch technologies, an infrared touch screen needs no sensing overlay on the display. Infrared transmitters and receivers sit in a frame, usually plastic or aluminium, fixed around the display bezel. They create an invisible grid of light beams across the screen, side to side and top to bottom. When a finger or object breaks the beams, the controller finds the touch from the X and Y coordinates of the interrupted beams. A protective glass or plastic sheet is often fitted over the panel to guard the LCD surface.

IR suits monitored, well-kept indoor uses where a large or cost-effective touch screen is needed and the risk of damage to the frame or contamination is low. It is not a good choice outdoors: sunlight contains infrared, which can swamp the receivers, and rain, insects and debris break the beams.

• Display brightness and image clarity unaffected, because nothing covers the panel
• Scales to very large displays, 100 inch (100″) and beyond
• Multi-touch with the right controller
• Low cost, simple integration and easy frame replacement if it fails
• The frame must stand proud of the screen, so a flat front is not possible
• Sunlight and other infrared sources cause interference
• Anything that breaks the beams triggers a touch, and dust on the frame can stop it working

Two other approaches appear occasionally. Optical imaging touch uses cameras in the corners of a large screen, and in-cell or on-cell touch builds PCAP electrodes into the LCD panel itself, as phones do. In-cell panels are rare in industrial displays because they tie the touch sensor to one LCD and limit the choice of cover glass.

Gloves are the most common reason an industrial touch screen disappoints. Resistive and SAW respond to a gloved finger by design. PCAP senses the conductive body through the glove, so the thicker and more insulating the glove, the weaker the signal. The difference between resistive and capacitive touch comes down to this pressure-versus-charge distinction.

PCAP tuning for gloves and water

Glove and water performance on a PCAP screen is set by the controller firmware and the cover glass, not by the sensor alone. Raising the sensitivity lets the controller detect a gloved finger, but it also makes the screen more likely to react to water, a resting palm or electrical noise. Thicker cover glass protects better against impact but moves the finger further from the electrodes. Every PCAP design is a balance between the two.

Ask for glove and rain modes by name, and test with the gloves operators really wear and the liquids that really reach the screen. Several of the stainless steel panel PCs in our range have PCAP with glove and rain modes; where operators wear heavy gloves all shift, 5-wire resistive remains the dependable choice.

Outdoors and in wet areas the choice narrows quickly. The table summarises what usually works, and the IP rating chart explains the sealing levels it refers to.

For brightness and reflections in direct sun, see our guide to choosing a sunlight readable display. For hosed-down areas, our guide to hygienic design for washdown computers covers seals, stainless grades and cleaning, and IP69K panel PCs are built for high-pressure, high-temperature cleaning. In cleanrooms, the same flat front matters for particles; see clean room computers.

Temperature affects touch too. Resistive films stiffen in the cold and soften in heat, and the adhesives in any touch stack have their own limits, so check the touch screen’s rated range as well as the computer’s. A rugged rugged panel PC rated for wide temperatures states the range for the whole system, touch included.

On PCAP and SAW screens, the cover glass is the part operators touch, so it decides how the screen survives knocks, scratches and cleaning. Most industrial glass is either chemically strengthened, which keeps it thin and very scratch resistant, or thermally toughened, which suits thicker panels and breaks into small fragments if it fails.

Coatings add to it. An anti-glare (AG) etch scatters reflections and hides fingerprints, while an anti-reflective (AR) coating cuts reflected light for better contrast in bright conditions; the difference between anti-glare and anti-reflective explains which suits which setting. Anti-fingerprint and antimicrobial coatings are used on public and healthcare screens.

Optical bonding and touch accuracy

Optical bonding fills the gap between the touch sensor or cover glass and the LCD with a clear resin or optically clear adhesive. It removes internal reflections, so the screen reads better in bright light, and stops condensation forming inside the stack. It also makes the touch screen stiffer and reduces parallax, the offset between where the finger lands and the image underneath, which helps touch accuracy on thick glass.

Impact ratings under IEC 62262

Impact resistance is graded by the IK code in IEC 62262, which sets the energy of a standard impact the enclosure must withstand. Our explainer on IK ratings covers the test itself.

For screens in public places, our explainer on what makes a touch screen vandal-proof covers glass, fixings and the IK level a public kiosk usually needs.

PCAP measures very small changes in capacitance, so it is the touch technology most affected by electrical noise. Switching power supplies, variable-frequency drives, welding equipment and poor grounding can all inject noise that the controller mistakes for touches or that makes the cursor jitter. Electromagnetic interference of this kind is why industrial PCAP designs pay attention to grounding, shielding and controller filtering, and why conducted immunity under IEC 61000-4-6 is part of EMC testing for touch displays. Resistive, SAW and IR are far less sensitive.

Resistive screens usually need a one-off calibration so the touch lines up with the image, and may need recalibrating as the film ages. PCAP is calibrated by the manufacturer and normally needs none. On a separate touch monitor, touch comes back to the computer over its own USB cable alongside the video cable; our guide to monitor connection types covers the video side.

When we supply a touch display, we configure the touch controller settings, such as glove, water and sensitivity profiles, then test the display with the host computer before it ships. On a panel PC, the touch driver and settings are part of the system image we build and test.

Most projects can be placed in one of the rows below before looking at individual models. Choosing the touch screen is one part of choosing an industrial HMI, alongside the computer, mounting and software.

Interactive public screens have their own requirements for content, mounting and vandal resistance; our explainer on interactive kiosks covers them. For built-in displays, our guide to open frame displays explains how the touch screen fits into an OEM enclosure.