Southampton-based Quantum Research has developed a capacitive
touch screen technology that eliminates many traditional drawbacks,
making it suitable for mobile phone use, claims the firm.
“We have got designs on-going with the top three mobile phone
companies,” Quantum CEO Hal Philip told Electronics
Weekly.
The firm already has capacitance measuring intellectual
property. Its patented charge transfer technique is used in its
Qprox range of chips for touch buttons and touch sliders.
However, said Philip, being able to measure capacitance is only
a small part of the story.
Capacitive touch screens are corner driven conductive films and
suffer from pin cushion distortion. “you touch the middle of an
edge and the point is reported about 20 per cent into the display,”
said Philips. “You can calculate out the error, but it is a
sixth-order calculation.”
This means considerable processing power is required for
accurate operation.
Instead, the firm has moved to an anisotropic conductor,
essentially a set of horizontal conductors separated by gaps, all
joined together at the edges. “This totally eliminates distortion
[in one axis], you get a straight line at the top and the bottom,”
said Philip. “The left and the right are still distorted, but you
can correct it with a second-order calculation.”
Touch position is determined by changing the potential on all
four corners equally and simultaneously, said Philip, and measuring
the amount of charge that passes through each corner.
The other way to remove pin cushion distortion is to have a
low-impedance conductive frame around the transparent indium tin
oxide [ITO] sensing sheet. But this means current drive has to be
increased to get a detectable output, said Philip: “You need a
voltage gradient. No gradient, no output.”
By correcting mathematically “we don’t need bus bars. [Corner]
current is three orders of magnitude lower than what is currently
accepted,” said Philip.
Power, claims the firm, in a phone-size display is
sub-100µW.
Another issue for capacitive touch screens is ‘hand shadow’,
which forces manufacturers to put the conductive film on the front
of a display panel, behind a thin (~10µm) insulator. “You get
50-100pf from the sensor to the finger, and maybe less than 1pF to
the hand,” said Philip. “If you put the sensor behind the glass,
the finger is only a couple of pF and it reports the average
position of the finger and the hand.”
By placing one or two extra electrodes along the display sides,
vertical hand shadows can be calculated out. Which is what Quantum
is doing, allowing the ITO layer to be protected behind 3mm of
front glass. “We assume vertical [hand] displacement. It is the
normal way of operation. Horizontal displacement is not a problem,”
said Philips.
A chip implementing the technique, dubbed QField, is expected
before the end of the year with 256x256 resolution, gloved hand
operation and 20ms response time. A 100mm display touch screen
solution is expected to cost about $3.00.
www.qprox.com