Consider
the different methods when adding touch sensing to displays.
Touch-sensing technologies
 |
|
Public access terminals made by Touch4 using Zytouch
technology.
|
|
Common capacitive or resistive touch- sensing technologies
have some limitations that can restrict the potential for designers to create
bold new styles. The major inhibitor is that the sensors are surface-active,
and must be mounted on the outside of the glass front panel. In this position, the
touch sensor is not only vulnerable to damage, but also requires cutting an
aperture in the panel to connect the sensor to the processing electronics
inside the appliance. The size of aperture required may vary, from a small
wiring conduit up the size of a TFT-LCD display.
This increases the cost of the solution by adding an extra
cutting process to produce the aperture and by introducing the need for sealing
against ingress of moisture, dirt or dust into the equipment. There is also an
increased risk of damage to the panel during manufacture, leading to expensive
scrap. In the case of an appliance for hot or cold storage applications, these
apertures will also complicate the design of the front panel by demanding a
secondary sealed panel to effectively isolate the environment within the unit.
There are touch-sensitive technologies that are able to
sense through glass to support an aperture-free front panel in glass-front
appliance designs. An example is digital charge transfer, which combines an
array of electrodes implemented as switches on a PCB managed by a mixed-signal
IC to detect the presence of a finger over any switch in the matrix. However,
some drawbacks include the time required to design and fabricate the PCB, as
well as the need to re-spin the board in the event that the layout of the
switch matrix needs to be changed.
Projected Capacitive Sensing (PCT™) provides an alternative
that is more suited to the particular demands of these modern kiosk designs. By
projecting the sensing field forward, in front of the sensor array, PCT is able
to sense touch when the sensor is located behind up to 20 mm of glass. This
allows the control electronics and touch sensor implementation to be
self-contained inside the cabinet, without requiring apertures to be cut into
the glass or exposing the sensor to increased wear and tear and accidental or
malicious damage.
Because the PCT sensor array is embedded inside the machine,
it cannot become damaged during cleaning. Resistive touch sensors, on the other
hand, are especially vulnerable to damage by a variety of cleaning solvents,
and can be destroyed by sharp fingernails or repeated use with a harsh pointing
object such as a pen. PCT sensing, by contrast, remains fully functional even
if the front panel becomes scratched or chipped.
By incorporating the touch sensor directly into the front
panel, PCT enables attractive glass-front appliances that are economical to
build and deliver robust performance in tough environments.
Sensing through glass
|
|
|
Fig. 1. Two sets of wire electrodes are used to create
sensing arrays for X and Y axes.
|
|
PCT sensor technology is implemented using an array of
microfine capacitors that can be embedded as an integral part of the glass
front panel of an appliance such as a food chiller or vending machine.
Depending on the screen resolution and size, between 16 and 64 individual
circuits are created, effectively subdividing the screen area into pixel-sized
sensing cells. Fig. 1 illustrates how two sets of wire electrodes are used to
create sensing arrays operating in both X and Y axes. The diagram also
illustrates how the pattern of the wires is skewed to prevent optical fringing.
The sensor wires are connected to an integrated electronic
controller board, which establishes an oscillation frequency for each wire.
When a conducting stylus touches the glass surface of the sensor, a change in
capacitance occurs. This results in a measurable oscillation frequency change
in the capacitors surrounding the contact point. The integrated controller then
calculates new capacitive values and this data is transferred to the host
controller.
Since each capacitor is in the form of a wire that has a
diameter of around 25 percent of the thickness of a human hair, the array is
invisible to the human eye when viewed against a powered display. The effect on
light transmission through the screen is also negligible, so that the PCT
sensor does not compromise the brightness or clarity of the display. This
contrasts with resistive or conventional capacitive sensors, which absorb a
considerable proportion of the light from the screen.
Also, unlike a conventional capacitive sensing array, the
PCT sensor is not subject to drift, which produces a progressive loss of
sensing accuracy. The drift-free operation of PCT eliminates the need for
periodic recalibration throughout the operating lifetime of the machine. This
can significantly reduce ownership costs for kiosk operators as well as
servicing overheads for appliance vendors.
