Electronic Design
Self-Capacitive Sensing Brings Touch To Large Screens

Self-Capacitive Sensing Brings Touch To Large Screens

Projected-capacitive (p-cap) touch sensing has experienced widespread adoption, contributing to one of the largest consumer electronics revolutions in recent years. Thanks to the proliferation of devices such as smartphones and tablets, product designers across all fields insist on a durable, sensitive touch-enabled user interface.

Most of these devices are now based around a p-cap sensor, thus creating phenomenal growth within this part of the touchscreen market. Figures from industry-analyst DisplaySearch show that, though still relatively new, p-cap has now displaced long-established (and increasingly commoditized) resistive sensing as the most widely used touch-sensing technology on a global scale.

P-cap’s fast uptake is driven by a compelling feature set, including an unlimited lifespan conferred by a resistant all-glass surface, edge-to-edge design capability (with no requirement for bezels), and high levels of sensitivity. However, as OEMs seek to incorporate touch interactivity with a similar style and performance outside the portable consumer domain, it requires touchscreens that satisfy different sets of criteria.

Two Technology Choices

OEMs can choose between two distinct types of p-cap touch-sensing methodologies. Currently, the most common is mutual capacitance. It uses two separate conductive layers: one contains the sensing cells that can identify the position of the touch event; the other has the driving cells through which an electrical signal passes. Each cell, which usually interlocks with other cells, is connected to the control electronics.

Touching the screen causes an alteration of the charge held within the local electric field, reducing the mutualcapacitance built up between the two layers. This alteration is picked up by the cells in the sensing layer. Detection algorithms within the controller electronics determine the individual cells with the greatest change in charge, and output a corresponding X-Y coordinate to the host system.

The second “flavor” of p-cap sensing uses the self-capacitance principle. In contrast to mutual capacitance, this technique employs a separated X-Y grid of open-ended conductive lines connected to a controller that contains the detection algorithms. The charge held on the lines is altered by human body capacitance as the user’s finger comes closer to the touchscreen surface. The X and Y lines with the peak change in charge are detected and the touch coordinate is output to the PC.

Mutual capacitance’s adoption in consumer electronics can be linked to a number of factors. In particular, the technology can provide multi-touch functionality, assuming the availability of sufficient cell density and controller IC power. The high density of individually connected cells makes it possible to gather and interpret the large amounts of touch data required to separate multiple independent touches.

However, conventional mutual capacitive screens can suffer major drawbacks when moving to larger form factors. To accurately track multiple touch points, the controller must capture data from each of the small individual cells. As the screen gets bigger, more information must be captured. Eventually, the size of the data set becomes overwhelming.

In practical terms, once the touch display size reaches 15 inches (approximately 380 mm), the number of cell intersections that must be connected to, and monitored by, the controller becomes a major challenge. Greater complexity in the control electronics and connectivity also adds to the bill of materials and extends integration time and effort.

For those deciding between mutual- and self-capacitive techniques, they also must consider practical manufacturing issues when it comes to larger displays. Mutual-capacitance solutions are generally based on a matrix of cells consisting of indium tin oxide (ITO).

ITO is a conductive, near-transparent material that’s deposited and patterned on glass or film using a semiconductor-style photolithographic manufacturing process. It’s widely used in applications requiring mass-produced, small touch displays (like portable consumer electronic devices), where the volume-friendly production process is a plus. However, if volumes are lower (and this often goes hand-in-hand with larger screen sizes, e.g., such as those used in public self-service applications), the relative inflexibility of the ITO process and the high one-off cost of photomasks become more problematic.

Finally, in addition to manufacturing complexity and cost, one needs to consider the question of touch performance. For all its benefits, ITO has a relatively high resistivity. This means that as the display area increases, creating greater distance between cell and controller, signal-to-noise ratio decreases rapidly. This results in progressively lower touch sensitivity and, in the worst case, an inoperable device.

A Self-Capacitive Alternative

To address these issues, Zytronic developed what it terms “Projected Capacitive Technology,” or PCT.  The self-capacitive system is based on an X-Y matrix of micro-fine capacitors, embedded within a laminated glass substrate. It uses frequency modulation to detect minute capacitance changes within the conductive tracks.

PCT’s high sensitivity allows it to detect a touch through very thick overlays, protective glass, and even heavily gloved hands, suiting it for industrial and public access applications, and even for outdoor use. Because the system requires unique detection algorithms running within the control electronics, the company developed touch controller hardware and firmware designed specifically to work with its PCT sensors. The latest controller will output two separate touch coordinates so that it’s able to support most gesture recognition and multi-touch software.

There are two types of PCT sensors. The first is an ITO-based solution, which suits higher-volume applications that require a rugged interface with a relatively small screen size (e.g., white goods and industrial vehicle telematics). Although this senses in the self-capacitive style, it uses the same basic manufacturing processes as mutual-capacitive sensors designed for consumer electronic applications.

The second PCT sensor type, which targets large-format and lower-volume applications, features a capacitive matrix that consists of 10-\\[LC GREEK MU\\]m-diameter copper electrodes. The material’s extremely low resistivity (10 times less than ITO) allows for touch detection without noticeable degradation of sensitivity, even on screens larger than 80 inches. In addition, the copper electrodes can be deposited directly onto the rear glass surface without the need for photomasks. This ultimately helps cut the time for creation, testing, and manufacturing of new designs.

The ductility of copper means it can also be applied onto curved planes. For example, Microsoft Corp. used this capability when developing a touchscreen for its Envisioning Lab (at the corporation’s headquarters in Redmond, Wash.). A wrap-around ZYBRID touch sensor was supplied for its conceptual 10-display multi-monitor workstation, called the Spatial Desk (Fig. 1). The desk, which operates through a single PCT touch-enabled surface, is used to demonstrate the latest Microsoft technologies to customers.

Due to the choice of materials and of mutual- or self-capacitive sensing p-cap methodologies, Zytronic developed a toolkit that helps designers create a touchscreen for any application. Factors under consideration often involve the deployment environment, touch performance, physical screen size, and required volume.

Serving Up A Different App

P-cap sensing can become a solution in a host of applications, from the standard to the exotic. The latter was evidenced when advanced user interface specialist Sunvision Technology was asked to create interactive dining tables for Mojo, an exclusive restaurant located in Taipei, Taiwan.

For this project, it was necessary to make the wooden tables touch-sensitive. This required a technology with extremely high levels of Z-axis sensitivity and the capability to detect touch through the wooden surface of the dining table. To make it happen, Sunvision embedded Zytronic’s 22-in. ZYBRID PCT sensors behind each tabletop, linking each to a computer-controlled projector mounted above the table that presents interactive menus on the touch-enabled surface. With the touch sensors hidden from view, when coupled with software specifically written for Mojo by Sunvision, diners can interact with projected images, scrolling through dining options, placing orders, playing games, and messaging diners at other tables (Fig. 2).

PCT is getting more play versus mutual-capacitance technology when it comes to very large screens. For example, digital-signage specialists such as Infinitus have increasingly turned to PCT. A 65-in. version of the rugged ZYTOUCH product was specified for the Infinitus’ iMotion high-definition digital-signage systems designed for use in outdoor, public environments (such as ski resorts, plazas, and amusement parks) (Fig. 3).

Summary

More design engineers in markets outside of consumer electronics are looking to adopt similar levels of touch interactivity already enjoyed in the latest handheld tablets and smartphones. In the process, the limitations of conventional mutual-capacitance techniques become apparent. The nature of the materials used and their manufacturing processes (with resulting economies of scale) means that mutual-capacitive p-cap screens will probably remain best suited to small-format, high-volume designs.

More ruggedized applications, which require volume flexibility and large form factors, demand alternative approaches. As a result, p-cap sensors derived from advanced self-capacitance sensing, such as PCT, are likely to remain at the forefront of applications involving industrial controls, self-service terminals and medical devices.

Continued improvement in p-cap controller ICs coupled with sensor developments using printable conductive inks and nano-materials are likely to further extend the capability and use of this versatile touch technology family.

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