Kiosk touchscreens - lighen up

The rapid growth in indoor and outdoor public access kiosk applications is fuelling an increased demand for robust and reliable touchscreens that give high levels of performance while withstanding a wide range of environmental, accidental and malicious physical threats.

Unfortunately, using conventional touchscreen technologies in such applications often requires kiosk designers to compromise in key areas such as light transmission, operating lifetime or cost.

Now, however, the advent of touchscreens that use "projected capacitive' touch sensors embedded within a laminated glass or polycarbonate material is delivering screens that provide the high levels of protection required without the need to make these compromises.

The kiosk has become an increasingly popular phenomenon in recent years as companies across a variety of industries seek to deliver low cost, self service to an ever more technically proficient public.

Under the broadest definition, the term "public access kiosk' can be applied to almost any automated, continuously available terminal in which a user is provided with some sort of service without any human interaction with the service provider.

As a result, public access kiosk applications cover applications ranging from ticketing systems, vending machines and gaming equipment to webphones, ATMs and photographic booths.

And whether they are sited indoors or outdoors, the majority of these kiosk applications require some form of touchscreen interface.

Touchscreen requirements
There are a number of important factors that the kiosk designer will need to carefully consider before selecting a touchscreen.

The size and the resolution of the screen are often the first considerations, while the environment in which the screen is expected to operate will also impact on the type of technology that can be used.

Many outdoor screens, for instance, may be exposed to harsh weather and variable ambient light. Performance "” including speed of response "” will need to be evaluated, and it will be important to consider how users are likely to interface with the screen "” for example, is there a possibility they will be wearing gloves rather than using an uncovered finger or some other object to tap the screen?

And, of course, a touchscreen built into any terminal that is freely available to the public must offer adequate levels of protection against everything from spilled drinks, scratches and cleaning agents to vandalism.

In addition to the technical specifications, the majority of kiosk designers will also be under significant commercial pressures to minimise both cost and time-to-market. As a result, selection of the touchscreen technology will also be governed by both ease of implementation and cost.

A touchscreen that requires ongoing calibration, for example, or that has a comparatively low mean time between failure rating, could leave the kiosk manufacturer or customer with significant ongoing maintenance costs throughout the kiosk's lifetime.

Touchscreen technologies
Conventional touchscreen technologies for public access kiosk applications can be divided into three main categories "” resistive, capacitive and surface acoustic wave. The following is intended as a brief summary of the technologies.

Resistive
Resistive touchscreens combine a flexible top layer overlay with a rigid resistive bottom layer that is separated from the top layer by insulated spacer dots. Pressing the flexible top layer creates a contact with the resistive bottom layer and the control electronics identify the point at which that contact is made. These screens are available in four, five and eight wire formats, and the greater the number of wires, the better the resolution.

Capacitive
Capacitive touchscreens use glass with a conductive coating over the surface of the display screen, and this coating is connected to four lectrodes at the edge of the screen.

When a user touches the glass overlay, a capacitive coupling draws current from each of the electrodes to the touch point.

Current drawn is proportional to the distance of the contact point from each electrode, allowing the X-Y location of the contact point to be determined.

Surface acoustic wave (SAW)
SAW touchscreens work by transmission inaudible sound waves across the screen surface and using sensors in the X and Y axes that receive these waves. When the screen is touched, some of the wave energy will be absorbed. The level of attenuation of the sound waves can then be used to determine the touch location.

Conventional technologies and kiosk applications
While there are pros and cons to each, the conventional technologies summarised above all force the kiosk designer to make some important compromises.

Resistive technologies, for example, provide the benefits of high resolution and the fact that any type of pointing device can be used. However, the need for an overlay and spacer dots mean that resistive touchscreens can suffer from reduced brightness and picture clarity. In addition, the flexible top layer can be prone to surface damage from scratches or chemicals.

Capacitive alternatives also offer good resolutions and the advantage that they will continue to operate with surface contamination, but, because they require screen contact via a conducting material, are not suitable for operation with gloved hands.

Capacitive solutions also suffer from drift, meaning that periodic recalibration is required.

SAW touchscreens may seem ideal because they combine high resolutions with zero drift and they have no limitations in terms of objects that can be used for screen contact.

They are not, however, well suited to applications where there is the possibility of contaminants getting onto the screen, as these will absorb the acoustic waves and create "dead' spots. They are also very difficult to seal.

Projected capacitive touch sensors
Now, however, Zytronic's "projected capacitive' Zytouch touch sensor technology is providing kiosk designers with an alternative to conventional touchscreens.

This technology is suitable for public access kiosk applications because it delivers all the benefits associated with conventional touchscreen solutions without any of the trade-offs.

In particular, the screens offer improved levels of accuracy and reliability without compromising light transmission.

Because the screens use rugged, laminated glass or polycarbonate materials, they give high levels of protection against accidental and malicious damage without the need for additional protective screens.

The Zytouch touch sensor is based on the principle of embedding microfine wires within a toughened laminate material.

Depending on the screen resolution and size, between 25 and 32 individual wires are deposited so as to effectively divide up the screen area into pixel-sized sensing cells.

A representation of the wire electrode X and Y array appears in Figure 1, which shows how pattern skew is used to ensure that optical fringing does not occur.

Each wire has a diameter of around a quarter of a human hair, meaning that they become invisible to the human eye when viewed against a powered display.

The 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 wires surrounding the contact point.

The controller then calculates the new capacitive values, this data is transferred to a host controller, and software is used to translate the sensor contact point to an absolute screen position.

The fact that the technology employed is "projected capacitive' has a number of key advantages.

Firstly, the screen can be operated by both bare and gloved fingers, and secondly, the touch sensor itself is protected as it is embedded in a composite matrix.

The standard Zytouch sensor, for example, is constructed from a combination of two plies of soda lime annealed glass with a total thickness of 7 mm (4 mm for the viewing face and 3 mm at the rear).

In between the glass layers is a polyurethane layer that incorporates the Zytouch sensor array.

Because of this, the sensor is impervious to accidental and malicious damage and wear and tear and even severe scratching on the glass surface, or complete physical failure of the outer glass face of the sensor, will not prevent operation of the sensing technology.

This method of touchscreen manufacture also ensures that no additional sealing is needed to prevent the sensor being affected by moisture and rain, solvents, harsh cleaning fluids, grease and dirt.

Finally, because the electric field generated by the microfine wires does not change with time or temperature, there is no possibility of sensor accuracy drifting with time.

The microfine wires deliver uncoated light transmission ratings of up to 92%, which is significantly higher than the transmission characteristics of conventional resistive and capacitive screens.

Resolutions are below 1 mm, and the positional accuracy of the sensor is rated at less than 1% error within the recommended viewing area. Response times of 13 ms are comparable with the best conventional technologies.

In addition, by adjusting the sensitivity, the sensor can be embedded up to a distance of 25 mm from the touch surface for truly robust anti-vandal protection.

Zytouch answer
A range of standard sensor sizes is available from Zytronic, combining the laminated glass touch sensor with a controller card and driver software to ensure ease of implementation and minimised development times.

The driver software is fully compatible with Windows (9x, NT, 2000 and XP) and Linux and simplifies calibration and set-up by translating taps on the touchscreen surface to computer mouse clicks.

The sensors are available with all standard optical filter options including anti-glare and anti-reflection coatings, tinted or neutral density substrates, circular polarisers to improve and enhance contrast, and infrared coatings that eliminates solar gain. Louvred privacy filters and filters for EMI suppression are also available.

As well as standard sensors, the company can tailor custom devices including the thermal or chemical tempering of one or more of the glass plies and the addition of different plastic substrates or surface treatments.