What Is a Capacitive Touch Screen?

A capacitive touch screen is a touch sensor that detects the electrical charge of a finger rather than its pressure. The human body conducts electricity, so a fingertip near a charged electrode changes the capacitance, the electrode’s ability to store charge. The sensor measures that change and the touch controller turns it into a position on the screen.

The electrodes are made from a conductor thin enough to be see-through, usually indium tin oxide (ITO). Larger screens often use metal mesh, lines of copper or silver too fine to see, because it conducts better over long distances. The touch is sensed through the cover glass, so nothing on the front of the screen moves or wears.

Because the screen needs a conductive touch, a plastic pen or fingernail does nothing, while a finger, a capacitive stylus or a glove with conductive fingertips works. That single fact explains most of the strengths and limits of the technology.

There are two families of capacitive touch screen, and projected capacitive splits again by how it measures.

Projected capacitive takes its name from the way the electric field projects through the cover glass. Mutual-capacitance PCAP measures every crossing point on the grid separately, so it can tell two or ten fingers apart without the “ghost” positions that self-capacitance sensing produces with more than one touch. Many current controllers combine both methods: mutual capacitance for positions, self-capacitance for gloves and water rejection.

A projected capacitive touch screen is a stack of layers bonded in front of the LCD. Each one affects strength, clarity and cost.

Capacitive sensing is precise, but anything else that is conductive or electrically noisy can affect it. These are the three issues an industrial specification has to deal with.

Gloves. Thin nitrile and latex gloves usually work as standard. Thicker work gloves need a controller with a glove mode, which raises sensitivity, and the cover glass thickness has to be chosen with it in mind.

Water. Droplets and running water are conductive and can register as touches. Controllers with water rejection compare self- and mutual-capacitance readings to ignore them, which matters on screens that are washed down or used outdoors.

Ghost touches. A ghost touch is a touch the user did not make. The usual causes on industrial sites are electromagnetic interference from motor drives, inverters and switching power supplies, poor earthing between the screen and the computer, and water or conductive dust on the glass. Shielding, good earthing and a controller tuned for noise immunity fix most cases, and immunity is commonly checked with the conducted-disturbance test in IEC 61000-4-6.

A passive capacitive stylus has a soft conductive tip, often rubber or mesh, that imitates a fingertip. It works on any capacitive screen but is no more precise than a finger. An active pen contains its own electronics and transmits a signal the controller can track at a much finer point, with pressure levels and palm rejection, using protocols such as Wacom AES or the Microsoft Pen Protocol. The pen and the controller must support the same protocol, so it is chosen with the screen rather than added later.

Projected capacitive is now the default for new operator interfaces. A flat, edge-to-edge glass front is easy to clean and to seal, it suits gestures that operators already know from phones, and it gives a brighter image than a resistive film. Typical uses include machine HMIs, food and pharmaceutical lines, medical carts, kiosks and vehicle displays.

Our PCAP touch panel PCs and PCAP touch monitors cover small HMIs to large operator stations, with sealed fronts, strengthened glass and bonding options. For front impact protection, check the IK rating for impact resistance alongside the touch specification.

We set up the PCAP controller on every touch panel PC and monitor we supply as standard, matching glove, water and noise settings to the installation, and test the touch before the system ships.