Increasingly, this is being done by system integrators who can design and attach touch screens to a manufacturer's standard LCD, eliminating the need for customers to have a clean room. A completely integrated touch kit includes the controller, interface cables and LCD. Customisation is typically available for precise fit and function per the customer's specifications. This is particularly useful if the system designer is new to or inexperienced with touch panels (Figure 1). This article outline the development of a touch-integrated flat-panel-display system.

Define the application

The first step is to define the application. One needs to ask oneself where the touch-integrated flat-panel-display system is going to be used: Is it an industrial-control or machine-automation system? Is it a medical application? Is it a Point-of-Sale (PoS) or Point-of-Information (PoI) system? Is it an information or self-service kiosk? Is it a digitalsignage application? Is the display system located indoor or outdoor? Is it being used in a rugged environment? What is the operatingtemperature range required? Will it be used in a wide range of ambient environments? How will the touch capability be integrated?Touch-screen panels can be directly bonded to the front surface of the LCD, affixed to the display's bezel, or be installed via a mechanical mounting scheme for easy replacement if damaged. A direct-bond or bezel-mounted touch sensor requires a special clean-room environment, and - in the case of the bonded sensor - specialised application equipment and highly trained installation personnel. A mechanically mounted touch sensor is a touch-screen-input device that is designed to be mounted on the outside of a display device and is held in place by physical devices such as brackets or pressure-gasketing material. The external touch screen is less invasive and is used when replacing the sensor in the field or at a repair depot. The touch-screen controller's power requirements also need to be taken into consideration, as many will need a 5 or 12VDC voltage to operate properly. Figure 2 shows the major components of a touch-screen system using an externally mounted touch panel.

Establish touch-panel criteria

Important performance specifications to consider when integrating touch-screen displays are the number of touch points, actuation force, LCD pixel pitch, desired response time, operating life (number of touches), and touch resolution. Complete LCD display and bezel dimensions are required, as are the type of LCD that will be used (for example passive matrix or active matrix TFT LCD). Several other criteria also need to be taken into account: Does it require anti-glare or anti-reflective properties? Are high brightness or transflective properties required for daylight readability? Brightness required for daylight readability is generally 600cd/m2 (nits) or higher - depending on what other properties are involved, such as anti-reflective surface treatments, which are commonly supplied by the manufacturers or value-added integrators of most LCDs today. Does the display require glass bonding for vandal-proof outdoor public environments? What is the relative importance of price, performance, quality and long-term availability? Other parameters to consider include external connections, mounting options and environmental operating parameters.

Select the right touch-screen technology

Once you make the decision to go with a touch-screen interface, the next step is to determine which touch-screen technology is best. Technologies include resistive overlay, capacitive overlay, scanning infrared, or SAW (Surface Acoustic Wave). A comparison chart is shown in Table 1. A resistive touch panel is produced by sandwiching together ITO (Indium Tin Oxide) coated glass and PET (Poly Ethylene Terephthalate) film. The glass provides mechanical stability and the PET provides a flexible medium through which the two parts connect. Microdot spacers, printed onto the glass, separate the layers and enable precise feature control via dot size, height and density. (Dot density determines the operation method from low-density finger- to higher density pen-operation touch panels.)In applications ranging from industrial to consumer, resistive is the most popular form of touch-screen technology. This pressure-sensitive technology is multi-functional and one of the most cost-effective and easy-to-use of the touch technologies. Resistive touch panels are generally more affordable, but the light-transmission levels they provide are typically limited to a high end of 86% (although higher is available), and the front surface of the resistive layer can be damaged by sharp objects and harsh chemicals. Resistive touch panels are not affected by outside elements such as dust or water and - because they can be sealed to NEMA 4/4X standards - most of the major human-machine-interface (HMI) manufacturers have adopted this technology. An example is a 12.1" diagonal active matrix LCD with 400cd nits brightness, SVGA resolution, LVDS interface and high contrast ratio used in a medical application involving patient monitoring and medical-device control. For this application, a 4-wire resistive touch sensor was integrated to the display to meet the requirement that one can operate the touch interface with a gloved hand, without an external pointing device.LCDs using surface-acouststic-wave technology - such as resistive panels - can use any type of pointing device, such as a finger or stylus. SAW provides excellent scratch and damage resistance and superior drift-free calibration stability, as well as a high level of light transmission (92%). It's nearly impossible to physically wear out this touch screen. SAW touch-screen technology is widely used in gaming, office automation and indoor self-service kiosk applications. A downside to SAW technology is that it is highly susceptible to contamination from dirt, dust and other particulates. In a kiosk-type application open to the public, there are additional contamination threats such as rain, snow or some external object stuck to the screen. This can negatively impact the performance of the touch operation by interrupting the acoustic wave pattern on the front of the touch sensor.Capacitive technology makes use of a glass substrate with a tin-oxide coating that is charged with a slight electrical current. When a conductive stylus or finger touches the surface, that contact creates a capacitive coupling that causes a current draw at that point. The X and Y coordinates can then be determined by the touch-screen controller. The glass substrate of a capacitive touch screen is highly resistant to scratching and highly transmissive, and the touch-screen system can be built to NEMA 4/4X standards. One drawback with this technology for industrial applications and clean-room environments is that, because it requires a conductive pointing device of some sort, gloved fingers or non-conductive pointing devices will not activate the system. Scanning Infrared (also known as I/R Touch) technology uses infrared emitter-collector pairs to project an invisible grid of light a small distance above the surface of the screen. When a beam is interrupted, the absence of the signal at the collector is detected and converted to an X/Y touch coordinate (Figure 3). Scanning infrared touch technology is commonly used in kiosk, gaming, retail, healthcare and industrial human-machine-interface (HMI) applications. As it is very rugged and unaffected by dirt, water and other contaminants, it is ideal for kiosk displays that are outdoors and open to the public; moreover, it has no calibration drift. However, it is limited as to how small a point area it can detect, which poses a problem in applications such as PoS that require signature capture, which demands a very high resolution. The resolution provided by scanning infrared touch technology doesn't have the accuracy required to prevent pixilation or distortion of the signature as it switches from one beam to the next.

Select the controller board

In order to select the controller board, one needs to know what kind of interface is being used to connect the touch controller to the CPU on the computer motherboard: serial (RS-232), USB or PS/2? If the touch controller and touch sensor are going to be some distance away from the host computer, this will affect the choice of interface. For example, a serial interface can accommodate distances up to 1.20m, whereas USB interfaces are generally limited to a distance of approximately 0.4m. However, touch controllers with serial interfaces have to be powered externally, requiring a 5 or 12VDC power supply. Controllers with USB interfaces, on the other hand, are self-powered directly from the USB port on the host-computer system.

Select the enclosure

There are a number of factors to consider when packaging the touch panel and touch controller together with the LCD, LCD controller, and associated components. The most important factor is selecting the right enclosure. Does it need to be sealed? Does it need to be NEMA- or IP-rated? What about possible contaminants such as chemicals, or extremes of heat and cold?If you are designing a touch-integrated flat-panel display system yourself, this article should be both insightful and useful. Often short on engineering resources to accomplish such a project in-house, most companies choose to call upon a value-added solutions integrator with many years of experience in displays and associated sub-systems who has the resources and expertise to put all these components together into a perfectly functioning system. Working closely with your design engineers, the end result should be a touch-integrated solution that meets or exceeds your product specifications. If you need a display system with touch controller, serial communication, matching backlight inverter and all associated interface cabling for direct mounting to the LCD display, a systems integrator who specialises in display systems can help you choose the right LCD and LCD controller, as well as making sure that all the proper components are integrated into one seamless touch-integrated display system.