Multitouch sensor technology has the potential to revolutionize the way we connect with all manner of electronics hardware, giving touch-screen-based Graphical User Interfaces (GUIs) the ability to recognize complex gestures using several fingers such as rotating, two-digit scrolling, three-digit dragging, and pinch zoom, as well as allowing multiple users to collaborate. Analyst firm Markets & Markets predicts that the global multitouch business will reach $5.5 billion by 2016 (constituting more than 30 percent of the total touch panel market by this stage). The multitouch segment is currently exhibiting a compound annual growth rate of more than 18 percent, with the portable consumer sector driving the vast majority of this growth.

Moving forward, the problem for design engineers is knowing how to bring the multitouch capabilities that are already becoming commonplace in smartphones and tablet PCs to other areas that could also derive benefit from them. Digital signage, Point-Of-Sale (POS), public information, and industrial control systems could profit greatly from this sort of functionality. However, certain obstacles are inhibiting the adoption of multitouch in these nonconsumer sectors.

The larger format multitouch sensor options currently on the market, though acceptable for personal use such as all-in-one touch PCs, have serious shortcomings when applied to more demanding application scenarios. Both infrared and camera-based systems require an exposed bezel for housing sensor elements. This means that, in addition to increasing vulnerability to damage from external forces, the buildup of dust or dirt in the bezel recesses can hamper operational performance over time. These systems also suffer from sensor drift and need regular recalibration to rectify this.

Certain forms of projective capacitance such as self-capacitive types, which by their nature are extremely sensitive in the Z-axis, have proved to be well suited for rugged touch-screen implementations and can measure two independent touch points simultaneously. Another form of projected capacitive sensing, mutual capacitive, which measures charge/discharge across a crossover or node between adjacent cells created by an X-Y grid, tends to be less sensitive in the Z-axis and thus typically only works well with thin glass. However, mutual capacitive sensing offers the ability to detect more than two independent touch points when mated with the appropriate control electronics and software. As a result, this technology has been chosen in recent years as the principal method of bringing multitouch functionality to consumer applications.

Pros and cons of mutual capacitive sensing

The current breed of mutual capacitive touch screens usually relies on Indium Tin Oxide (ITO) as the conductive sensing medium. ITO is already widely used throughout the display industry and provides the benefit of being near-transparent.

Though ITO has been a successful choice in touch screens, it has certain limitations when applied outside the consumer arena. First of all, although conductive, ITO has a relatively high electrical resistance. This means it generally has fairly weak through-glass performance, only able to detect touch through a front overlay thicknesses of ~2 mm. Secondly, ITO is only suitable for use with smaller display formats, as the impedance builds up over the length of the conductive track. This means that sensor systems’ signal integrity levels will not be acceptable once the displays involved have diagonals that are much beyond 22", unless high input power, complex tiling arrangements, or other elaborate approaches are utilized. Finally, conventional ITO-based sensors do not permit flexibility in their production, as each new sensor design or size requires a separate set of photolithographic tools to be created (see Figure 1). This calls for considerable upfront investment and can only be justified if a large enough number of units will be manufactured to cover the initial outlay, which can be anywhere from $5,000 to $30,000 depending on size and complexity.

Consider the example of an interactive digital signage or low-volume, customized POS system. The display formats required for this system would probably be too large for an ITO implementation, plus the limited front glass thickness (typically 1-2 mm at most) is unlikely to be strong enough for the demanding, public-facing, high-use environments in which it would be deployed. Furthermore, in many cases, the specialized nature of an interactive digital signage or POS system means that the number of units produced might not justify the high initial outlay for tooling associated with conventional ITO constructions. As a result, hardware designers wishing to create attention-grabbing, unique-looking user interfaces could be forced to settle for generic touch-screen designs.

Engineers also face technical and economic challenges when trying to incorporate projective capacitive-based multitouch functionality into designs where either large form factors or a relatively small number of units is involved, both of which are possible scenarios in nonconsumer design projects. However, projective capacitance sensing still proves to be the best way to ensure the longevity of touch screens in demanding environments.

A new approach to projective capacitive sensing

The engineering team at Zytronic has developed a mutual capacitive sensing approach that overcomes several significant hurdles, delivering a durable projective capacitive sensing mechanism that can simultaneously support at least 10 independent touch points and be realized on display sizes above 70". This multitouch system is based on the company’s patented Projected Capacitive Technology (PCT), where an intricate sensor matrix comprising copper capacitors 10 mm in diameter is embedded into a laminated substrate.

This substrate can be placed behind a thick protective overlay of glass or polycarbonate to protect it from various forms of potential damage. It can detect touch events through up to 6 mm of toughened glass, effectively doubling the projective overlay thickness that can be specified and thereby providing increased protection from impact, scratches, vibration, and exposure to harsh chemicals or extreme temperatures. Furthermore, this mechanism can be operated by gloved hands, making it highly suited to uncompromising industrial environments, or via a conductive stylus, allowing users to write directly onto the screen.

By utilizing the same proven, maskless plotting process used to produce self-capacitive PCT screens for more than a decade, sensor formats can be scaled up as required, without accruing nonrecurring engineering costs for photolithograph masks. This means that the volume of units does not negatively affect the commercial viability of the project employing multitouch operation, allowing small-volume business to take advantage of this functionality in the same way as it is being sported in high-volume, consumer-oriented products.

The multitouch PCT sensor works in combination with Zytronic’s ZXY200 touch controller. This device processes all touch event data being captured by the proprietary design copper array laminated to the rear of the glass sensor. Using a mutual capacitive approach means that each of the intersecting nodes created in the pattern is individually monitored by the controller running proprietary firmware, which is optimized for the use of copper and lower resistance (hundreds of Ohms/m compared to ITO’s thousands of Ohms/m), plus the resulting improvement in Z-axis sensitivity and ultra-large-size capability. Furthermore, because the copper wires are coated in a dielectric, it is possible to deposit the electrodes in a single process/layer, resulting in a simplified cross-sectional structure to the sensor (see Figure 2). This is not possible with ITO, as multiple layers need to be deposited to create the more complex diamond electrode pattern that this requires, with the X and Y electrodes being isolated from one another.

Energy transfer is localized to the intersection where the X and Y electrodes of the multitouch PCT touch sensor cross. An image map of the energy received is then generated from the measurements, and the position of each touch point can be determined (see Figure 3). With multiple touch points being detected on the screen, true “palm rejection” functionality can be incorporated, with touch performance in no way hampered by users resting their hands, arms, or elbows on the screen. The ability to operate with gloved hands makes it particularly appropriate for use in outdoor environments such as retail and public information applications, as well as in medical and industrial deployments where users need to wear protective clothing.

Innovative, versatile human-interface technology

Projective capacitive multitouch technology has already seen widespread uptake in the portable consumer space, where high-volume and small form factor designs are endemic. The emergence of innovative single-layer technology is now presenting the industry with an economically viable way of implementing multitouch functionality that can be supported both on large format displays and in harsh environments. This sophisticated human-machine interaction could effectively become ubiquitous and as a result, no longer restrict complex gesture recognition or simultaneous manipulation by several different users to portable gadgets.

Andrew Morrison, PhD, is technical director at Zytronic.

Ian Crosby is sales and marketing director at Zytronic.

Zytronic [email protected] www.zytronic.co.uk

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