Displays: Adding the Right Touch (May 2007)
May 1, 2007
Glass-front appliances are known to perform better than non-glass-front units in vending machine applications, and also promise greater performance and functionality in commercial and home appliances. A touchscreen technology that is compatible with the glass-front panel will extend the reach of these types of machines and enable further improvements in appearance and styling.
Glass-front vending machines are well-proven merchandisers. As long ago as 2001, the facilities management services company Sodexho demonstrated a six-fold increase in revenue from glass-front vending machines, which allow customers to see items on offer before buying, compared to non-glass-front units. Appliance designers are now seeking the functional advantages of a glass-front design in a wider range of equipment and environments, and are also looking to improve on the appearance of more traditional appliances.
But it is also time for the traditional glass-front design to move forward, to present a more sophisticated appearance, and also to enable savings in equipment costs and assembly time. The most common glass-front layout combines the glass-fronted merchandising section alongside a separate control panel that usually includes a traditional keypad and monochrome alphanumeric display.
Retail, commercial and consumer appliance markets are ready for enhanced user interfaces that incorporate high-tech controls and colourful, dynamic graphics that support a wider variety of choices. The availability of color TFT-LCD screens at competitive prices offers one solution to directly replace the limited display and keypad features of the traditional glass-front design. The TFT-LCD screen can be combined with a touchscreen panel to eliminate the keypad, but such a solution would allow only modest advances in styling and user-convenience. On the other hand, implementing the touchscreen in an area of the glass front panel creates even greater freedom for designers to create performance differentiators as well as futuristic and efficient designs.
Touch-sensing technologiesCommon 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 glassPCT 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.
Flexible stylesIn most touchscreen kiosk applications, the touch sensor is used in conjunction with a CRT or TFT-LCD panel. The sensor array is aligned with the viewable area of the panel, which displays the user controls as graphical objects. This has the advantage of allowing the control panel layout or functions to be reconfigured quickly and at relatively low cost through software changes. On the other hand, extra engineering effort is required to fix the display panel behind the glass front. Emerging OLED technologies may offer a solution in the future, being lighter and thinner than a TFT-LCD panel.
Alternatively, touch features can be implemented using PCT sensing without the use of a graphical display by applying a decal to the front panel, directly over the touch sensitive area. A wide range of styles and effects can be created, limited only by the imagination of graphic designers. For example, the use of period, retro or futuristic graphics and typefaces could be used to quickly customize an appliance design for special applications or environments such as hotels, themed restaurants or special venues. The touch panel design could be incorporated into a decal covering the entire machine, allowing striking and attractive effects. PCT is an especially powerful touch technology in this context, by allowing decals to be applied quickly and easily to the aperture-free front panel.
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