News and New Products

FROM EDN EUROPE: Tough touch screens need no seals

by Graham Prophet -- EDN Europe, 10/13/2005

If you have not been involved with their design, you might assume that the technology of touch screens is mature and robust, and that you can readily specify screens that offer accurate sensing of a fingertip, with good transparency, and that are robust and cost-effective. According to touch-sensor IC-supplier Quantum Research Group, that combination of features is in fact very difficult to achieve-a situation the company aims to rectify with a new series of ICs and reference designs.

Quantum designs chips for touch sensing using a capacitive technology it terms "charge transfer". The technique requires no electrical connection between an activating finger touch and the detection circuitry-as with other capacitive methods, the presence of a finger alters the circuit capacitance between two plates. Quantum's approach is to sense the level shift in a pulsed signal through the capacitor-its chip design digitises the signals, makes comparisons in the digital domain, and stores reference levels in on-chip EEPROM to guard against phenomena such as long-term drift and component aging. This, the company says, is an important differentiator against other sensing techniques, such as oscillator frequency-shift.

In opaque sensors, the capacitor plates can be printed-circuit patterns on low-cost PCB, hidden behind a front panel; in touch-screen applications, the plates are printed patterns of (transparent) indium-tin-oxide. Designers can route connections to both sides of a capacitor plate for multiple keypad sensing patterns relatively easily in PCB, where they can design with double-side boards or jumper connections. Transparent ITO layers present more of a challenge in routing all the connections, in a single layer, to multiple keypad patterns. More complex still is the problem of addressing a grid of capacitor sense plates to make an X-Y position sensor array.

Mass-market touch screens frequently employ resistive technologies, where a finger touch compresses a gap-normally separated by spacers forming a tiny air gap-between two resistor-pattern layers. This, Quantum says, carries disadvantages such as the need to place the sensing element on the "public" face of the screen, making it vulnerable to damage; and calling for a bezel and seal around the edge of the sensing area. It can, however, readily provide individual keypad sites or X-Y position sensing.

Capacitive techniques can address these issues-at the cost of complexity and a certain amount of loss of transparency-using a two-layer structure with one set of plates printed on each of two ITO layers. Quantum addresses the robustness problem in one of its technology solutions by printing the ITO on the reverse of a PET (polyethylene-terephthalate) layer that can then be bonded to the screen. PET (the soda-bottle plastic) is a very tough material that stands up to heavy usage. However, designers still need to seal the sensing area on the front panel. An ideal solution would be to place the sensing array behind the front panel, which they can then make continuous, with no need for seals or bezels. When they use capacitive techniques on the reverse face of display panels, they encounter the added complications of a reduced field disturbance, due to the greater distance and thickness of dielectric mat between finger and sensor. Also, the signal detected from a finger is less distinguishable from that of the hand to which that finger is attached. These problems manifest themselves as signal/noise issues, and problems of determining the exact location of the touch.

Today, Quantum is to introduce a series of chips (the company supplies chips, not complete modules) that it says will solve all of these problems. They will operate with a single layer ITO pattern printed on the reverse side of a transparent fascia up to 3 mm thick, providing good signal/noise ration and detecting finger touch with a resolution of 256×256 points. Making this possible is a labyrinth pattern that Quantum's researcher Luben Hristov devised. The new geometry allows a single-layer, edge-addressable, ITO pattern to form two capacitor plates on an X-Y grid that can be up to 10 cm across. It also projects the capacitor's electric field pattern through the dielectric so that its field lines project into the detection space with the necessary shape.

Using the chips, you will be able to build extremely robust front panels with (for example) LCD screens behind them, that will offer 90% light transmission. On the surface you can define-with software interpreting the reported position and direction of a finger touch-buttons, touch wheels, sliders and 2D track-pads. Removing the need for seals and bezels will suit the technique to medical and industrial environments. Response time is 20 msec, and for battery operation, standby power demand is 30 μA. Quantum says that a 10-cm diagonal ITO film should cost around $1.50, making for a $3.00 overall BOM cost, the chips selling for around $1.50 (100,000).

Quantum Research Group, +44 2380 565600, www.qprox.com.