Table possibilities for touchscreen technology on shared surfaces

Table possibilities for touchscreen technology on shared surfaces

Touch tables that enable control of the computing device and associated information exchange are on the cusp of major growth

BY ANDREW MORRISON and IAN CROSBY
Zytronic
www.zytronic.co.uk/

In many spheres of our everyday life, touchscreens have become the chosen method of interacting with electronic devices, providing a far more direct and intuitive interface than mice, keyboards, buttons, switches, or dials. Touchscreens are becoming ubiquitous within the consumer electronics space, through products such as smartphones, MP3 players, and tablet PCs. Building on this success, there is now an expectation by users for the same sophisticated touch functionality used in such personal devices (both portable and desk-top based) to be employed within a broader range of commercial self-service applications. Kiosks are already widely deployed for one-to-one information sharing and transactions, but another family of publically accessible surface computing devices on the cusp of major growth is touch tables — which enable control of the computing device and associated information exchange to become a shared and even social experience. The following article looks at OEMs’ specific requirements when trying to bring to market touch-enabled table-based solutions capable of dealing with these more demanding public environments.

To date the most well-known example of a touch table product is probably Microsoft’s original Surface table (now sold in its second iteration together with Samsung under the name PixelSense). Although products such as this can cope with operation in benign or supervised environments such as homes, offices, or shops, more robust alternatives will be needed to provide continued operation in unattended or lightly supervised, self-service applications such as schools, casinos, shopping malls, transport hubs and other places where the general public gather. Key design considerations to be addressed when implementing a table-based touchscreen human machine interface (HMI) into such applications include:

• Resilience — Horizontal surfaces take abuse — accidental and malicious. So a touchscreen’s ability to continue to perform despite the high degree of wear and tear to be expected in a public environment is important criteria in terms of ROI and user satisfaction.

• Performance — In addition to speed and accuracy, for a large table surface this also includes immunity to “false” touches, the ability to perform even when people are leaning on the touch surface, plus the ability to react to simultaneous multiple user touches.• Customization — People are drawn to the novel. Therefore, rather than providing a generic “black box” solution, the potential to create a stand-out interface design or to optimize the touch active surface to meet a specific application can have real value.

Microsoft’s PixelSense

Touch-sensing methods

Since touchscreens first entered the mainstream during the 1990s, a variety of sensing technologies have been developed, each having different characteristics making them more or less suitable for a specific application’s requirements; such as low unit cost, size range, ease of integration, responsiveness, robustness, and more recently, the ability to detect multiple touches. An oft-cited truism in the touchscreen industry is “there is no perfect touch technology,” and it’s fair to say that each has its advantages and disadvantages depending upon the intended use.

Resistive and surface capacitive systems are relatively inexpensive to purchase, but as the touch detection takes place via a very thin conductive surface coating, these sensors are prone to failure through scratches and adverse conditions. Optical touch technologies such as infrared (IR) or camera based detection, are relatively easy to integrate, but the external bezels in which the transmitters/receivers are mounted can be activated by a shirt cuff or tie accidentally touching the screen, as well as being easily damaged. Finally, acoustic touch detection methods, most commonly surface acoustic wave (SAW), have the advantage of a thick durable pure glass surface, but typically require an open area around the perimeter of the screen to allow sound waves propagated to move unhindered over the surface — thereby making them prone to problems related to dust and liquid ingress.

P-cap sensor technology

The rapid growth in use of projected capacitive (p-cap) touch screens (as seen on Apple’s iPad and other consumer electronic devices) demonstrates that this technology is currently the most well rounded in terms of overall performance, particularly in small, personal devices. However, what many observers have not realized is that p-cap touch sensing is also increasingly being deployed in heavy duty applications, where the ability of the technology to detect touch through thicker protective overlays is a prerequisite. There are two main types of p-cap sensing method — self-capacitive and mutual capacitive.

Mutual capacitive p-cap technology measures the charge-discharge of nodes between adjacent densely packed cells and overlying layers in a conductive sensing grid, typically laminated between thin layers of glass. Energy transfer is localized to the intersection were the x and y electrodes cross in the touch sensor. The array is connected to control electronics which measure and map the energy received and determine touch points accordingly. As each node is being measured, the principle advantage of mutual capacitive sensing is its capability to detect and distinguish between a numbers of individual touch points, that is, multitouch. It is for this reason that it has become the technology of choice in handheld consumer electronic devices over the last five years.

The self-capacitive p-cap sensing technique is less well known. Here the sensor couples human body capacitance to the capacitor within an RC oscillator arrangement. It is thereby possible to detect touch events via the introduction of a capacitance, from glass to finger to ground, parallel with the oscillator’s capacitive element and its natural parasitic capacitance to ground. As a finger approaches the active area, an increase in the circuit capacitance and subsequent decrease the oscillator’s frequency take place. By measuring where peaks in the frequency occur on the x and y axes of the sensor matrix it is possible to pinpoint the location of the touch event. Measuring pF of capacitive change, self-capacitance offers far stronger z-axis sensitivity than mutual capacitance. As a result sensors using it can be placed behind protective overlays that are considerably thicker and less exposed to possible sources of damage. This enhanced z-axis sensitivity also means that this sensing methodology easily supports gloved hand operation. Both of these factors mean that this technology is particularly suited to demanding unattended self-service applications.

This matrices used in the two methods of p-cap sensing described above are most commonly made of chemically deposited and etched indium tin oxide (ITO) as it is near transparent and already widely used in the associated display industry. However, ITO has a relatively high electrical resistance that limits touch sensitivity and the sensors ability to detect touch in the z-axis (that is, depth). Consequently it can usually only be used in conjunction with a thin overlay. Furthermore the higher resistance of ITO also leads to limitations in terms of how big a display format it can successfully be applied to (being optimal for products with small form factors, usually less than 15 inches). When implemented into displays above ~20 in. the build-up of resistance across the grid normally results in a poor signal to noise ratio (SNR) which impairs touch detection on large screens. The relatively inflexible nature of ITO also has to be considered. Fabrication of each design of sensor will normally require a new set of photolithographic masks costing several thousand dollars. This is not an issue when large numbers of units are manufactured such as for consumer devices, as tooling costs can be amortized over the production run, but for smaller volume designs in large sizes, such as screens for touch tables, the cost associated can be prohibitive.

Unlike ITO, the proprietary projected capacitive technology (PCT), developed by Zytronic, enables self and mutual capacitive touch sensors to be manufactured using a matrix of microscopic copper capacitors (~10 µm diameter). These are directly deposited and laminated to the rear of a 3 to 6-mm thick toughened glass substrate. In the case of the self-capacitive variants, this substrate can further be placed behind additional protective overlays, and in extreme applications the total thickness of protective front glass can exceed 20 mm far thicker than the 1 to 2-mm maximum thickness typically required for good operation of ITO-based mutual capacitive touchscreens. In addition, the direct plotting process employed does not require tooling, and is design flexible, so that hardware developers can create bespoke designs quickly, even in low volumes.

Importance of multitouch

Touch tables will always naturally draw people into collaborative use. Users tend to cluster around tables to exchange information and communicate — just witness a board room, restaurant table or casino game. Therefore, the ability of any touch technology to be used in a surface computing device such as a touch table, to support multitouch operation is certain to be a highly desirable feature moving forward. It will mean that complex gestures using several fingers (rotating, two digit scrolling, three digit dragging, pinch zooms) can be benefited from, as well as allowing simultaneous operation by numerous users. With multiple touch points being detected on the screen, the use of palm rejection is also of significance — so people accidentally resting parts of their body (hands, arms, etc.) on the table’s surface do not affect system performance. Multi-touch functionality can now be added to PCT p-cap screens, in a cost-effective and completely scalable format. This advancement allows the detection of at least 10 independent touch points simultaneously on screens between approximately 20 and 80 in.

Fig. 1: Multi-touch screens benefit many users around a table display with simultaneous operation by many users.

Examples of table-based touchscreens using PCT

Advanced user interface specialist, SunVision has used PCT in interactive dining tables for exclusive Taipei restaurant Mojo. Rather than use an LCD embedded in each surface, a projector is mounted above each table works in combination with the self-capacitive touchscreen (which is hidden from view beneath the wooden surface to place interactive menus on its surface. Diners can scroll through the options available and make selections by simply touching the table — the order being automatically submitted to the kitchen. Additional features integrated into this touch-enabled system allow users to instant message diners on different tables, as well as accessing image libraries and games while waiting meals to arrive.

Fig. 2: Another method for a table-based touchscreen uses a projector mounted above the self-capacitive touchscreen table — allowing diners to scroll through menus.

This technology has also been employed by VitalTouch in its Interactive School Desk. Each desk unit incorporates a full color display with an integrated touchscreen, using a 19-in. PCT sensor along with a high performance touch controller supporting dual touch operation. These desks are now being deployed throughout Taiwan (at a rate of fifty schools/year) — adding new dimensions to the education process, by dispensing with traditional writing equipment and white boards in an attempt to enhance pupils’ learning abilities.

The flexibility to produce large, customized touch table designs in small quantities was demonstrated when PCT sensors were chosen to heighten the visual impact of imaginative exhibition displays at the Silent Heroes Memorial Centre in Berlin, which documents the actions of the German Resistance and its efforts to protect persecuted people during the Nazi dictatorship. By embedding a series of eleven 32-in. sensors, , into a glass-topped media table over 5-m long, the exhibition designers created a massive fully interactive presentation that allows visitors to access to hundreds of texts and biographies.

The installation of touch interactive tables in public settings places tough demands on the technology employed. While developers have a number of options available when creating large format surface computing devices, the rugged p-cap technologies described here are already showing their worth in such applications — and with the addition of true multitouch capability, can only gain wider acceptance. ■