UK capacitive touch controller works in tough industrial environments

Claimed noise immunity is far higher than competing controllers – 80dB against typically 50-55dB, company MD Chris Ard told Electronics Weekly.

And TouchNetix should know about about capacitive sensing, as the team has its roots in Quantum Research Group, which was bought by Atmel for its QTouch capacitive sensing intellectual property and expertise. As the Touch Technology Products Group at Atmel, what became the TouchNetix team, was responsible for the definition and delivery of the Atmel maXTouch touchscreen driver ICs.

Ard emphasises that sensitivity has not been gained by simply turning up drive power. “Some competitors drive at up to 40V to improve signal to noise ratio, which has the side-effects of enormous emissions and potential damage to touch screen conductors and the glue,” he said. “Our technology uses only 2.5Vp-p. Actually, it is +/- 1.25V, which is very important as there is no net voltage stress on the touch screen materials.”

Instead of high drive power, the chip, dubbed aXiom AX310, uses a narrow-band signal technique and includes a digital signal processor followed by a CPU to pull the sensed signals out of return signals. “To do it properly, a hell of a lot of maths is involved,” said Ard. “The signal is narrow-band sinewave and there is a competent front-end followed by a custom DSP to get the signal-to-noise ratio. At the back-end, the microprocessor core gets high-confidence from the DSP. We have minimised analogue content on the chip, and it is all based on an RTOS, so we can change operation for customers.”

Design of the chip was shared with Norwegian company MyWo – another firm with a strong lineage, as its founders can trace their roots through Atmel to the original AVR design team. According to Ard, the DSP is a completely custom design, created in the UK, and the microcontroller is off-the-shelf, although he will not reveal from whom. When they met, “MyWo and TouchNetix had overlapping ideas for touch chip for non-consumer applications”, said Ard.

There is enough processing on-board for the chip to continuously scan its electrical environment and automatically change its operating frequency to avoid interferers “without dropping any frames”, said Ard. It can also be programmed to not output certain sensing frequencies.

Performance is such that multi-touch operation is available through gloves, damp, thick glass, plastic 1cm thick, and 3D variable thickness screens – the latter of which, according to Ard, are being investigated to guide fingers in automotive and industrial applications – the part is AEC-Q100 Grade 2 qualified for automotive use.

Proximity sensing is available at up to 10cm, and coarse finger position sensing at as much as 6cm to support hover and proximity functions.

Another advantage of high sensitivity, he said, is that the design of the associated transparent sensing conductors can sometimes be relaxed, reducing the skill-level needed by a customer’s design team, and broadening manufacturing tolerance.

For users that work at arms-length, where first finger-tip contact can be in the wrong place, AX310 supports touch-reposition-and-press operation using the firm’s patented force-sensing intellectual property that capacitively senses a few microns of compression in an elastomer gasket around the edge of the display between glass and enclosure. Suitable hardware and firmware is included in the chip to decode gasket signals, and the whole system is compatible with fully-bonded screens, said Ard.

Power consumption will typically be a few hundred milliwatts in industrial application.

Touch screens with up to 56 x 56 sensing channels can be supported on displays as large at 15.6″, as is touch and force-sensing sampling above 200Hz.

Features at a glance:

• Touchscreens can have a thick, varying-thickness or contoured overlay while maintaining uniform sensitivity across the whole screen surface
• Hover, proximity, touch and concurrent force-sensing functions possible with a single capacitive touchscreen controller
• Integrates haptics control – customers can implement 3D touch effects.
• Handle arbitrary and non-standard display formats, including ultra-wide aspect ratio and non-rectangular shapes
• Reliable touch-sensing performance amongst multiple high-power noise sources
• Meets CISPR25 Level 3 specifications for radiated emission without added noise counter-measures
• Reduced risk of early touchscreen failure due to materials degradation in hot and humid environments