SCHURTER White Paper: Multitouch Systems for Industrial Applications0 pages

PCI

White Paper
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Projected Capacitive Input
System (PCI)
Multitouch Systems for Industrial Applications
Multitouch technology is the focus of many new developments in the human machine
interface (HMI) and medical fields. Based on projected capacitive technology, multitouch
panels have undergone further development into an industry-compatible input system.
The high electromagnetic compatibility (EMC) interference immunity requirements, adequate water tolerance and the glove operation option qualify it for industrial applications.

SCHURTER Multitouch (source: SIEMENS)
Projected-capacitive input systems with a
sealed glass panel without dirt-collecting
edges are easy to clean and are scratch-proof,
abrasion- and chemical-resistant. They
therefore allow multi-purpose use in industrial
automation, machine construction and
medical technology. The sensor positions are
protected behind the glass panel and, for that
reason, are not affected by mechanical stress.
Multitouch allows more intuitive, faster and
safer operation of machines, equipment and
systems. New visualization and operating
concepts have allowed a major leap in
multitouch input system design innovation.
Configuration and Function
Projected-capacitive touch sensors consist of
at least two electrically separate sensor
surfaces, glass or polyester films that are
coated with conductive, highly transparent
indium tin oxide (ITO). Using an etching or laser
process, the ITO surfaces are segmented into
many small individual fields, mapped as x/y
intersections into rows and columns. The
design potentials of these sensor fields are
wide-ranging and follow the different guidelines
of the PCAP basic chip manufacturers.
The structures are outside the viewing area
with printed or laser-etched silver conductive
ink circuitry contacted and conducting to the
connector contacts. Film-based sensors are
laminated so they are highly transparent and

electrically insulating. As a result, a matrix
network of individually addressable sensors
with quiescent reference capacitances is
generated over the entire active surface. With
the approach of a finger its position is detected
by the change in the capacitance of the
individual sensor. Through the interpolation of
the adjacent sensor capacitances the
controller can calculate the exact positions of
the activations and convert them into the
corresponding x/y-coordinates. The matrix
arrangement of the individual capacitive
sensors requires no calibration. As a result,
industry-compatible projected capacitive
multitouch systems always work with position
precision even under harsh environmental
conditions.
The electronic evaluation of the touch
detection works using two principal methods.
In both types a capacitive sensor field is
projected through non-conductive media such
as glass. The change in the self-capacitance or
the mutual capacitance of the sensors is
determined in the process. In the selfcapacitance method the increased charge flow
of the x- and y-sensors due to the approach is
determined at the ground level. The activation
position is the point where the sensors display
an increased charge flow. The mutual
capacitance method detects a change in the
sensor matrix capacitance due to a parallel
coupling of the finger at the intersections. Both
evaluation methods have advantages and
disadvantages. The electronics of an industrycompatible PCI touch controller ideally uses a
combination of both methods.
Industrial requirement: interference immu­
nity, water tolerance and glove operation
One of the most important factors for achieving
industry-compatibility is immunity to EMC
interferences with the system. During EMC
radiation the touch panel must be operated
with at least two actuators. That is the only way
absolute EMC immunity can be qualified.

Input Systems

To achieve an EMC immune multitouch panel,
two main sources of interference are
considered. The first sources of interference
behind the touch panel are integrated displays
and switch mode power supplies. The EMC
standard according to IEC- 61000-4-3 refers
to these interferences. Furthermore, the IEC61000-4-6 EMC standard defines the gridlinked coupling of voltage peaks and
frequencies. Elimination of interference signals
so that the multitouch panel works with
position precision without causing a deviation
of the touch function and no false activations is
the prerequisite for the achieving EMC class A
conformity.
This EMC immunity is achieved with optimized
AD converters, integrated RC filters, enhanced
drive voltages and complex algorithms, e.g.,
the frequency-hopping technique. In addition,
an enhanced signal-to-noise ratio is obtained
with a sensor design adapted perfectly to the
electronics.
This optimization of interference-immune
sensor sensitivity allows finger activation
through several layers of medical gloves and
thicker leather construction gloves. Water
must never lead to false activations of the
multitouch panel. An additional requirement in
the medical field particularly is resistance to
saline solution. The possibility of achieving
complete water resistance consists in the
selection of optimal controllers and their
measurement methodology. Thus PCI input
systems are possible, even for use under
running water with simultaneous detection of
the finger activation.
Absolute industry-compatibility however will
only be achieved if all requirements, i.e., EMC
interference immunity, water tolerance and
glove operability can be guaranteed with a
single software setting. In any event,
conventional,
standard
design
PCAP
controllers and sensors from the consumer
sector do not satisfy these high industry
requirements.

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