Developing A Touch-Integrated Flat Panel Display System

Step-By-Step Design

Developing A Touch-Integrated Flat Panel Display System

By Richard E. McKay,
Apollo Display Technologies, LLC.

TOUCH SCREEN INTERFACES ARE A POPULAR CHOICE FOR INDUSTRIAL AND COMMERCIAL COMPUTER SYSTEMS. This technology eliminates the need for a keyboard or traditional mouse, offering instead a simple, direct interaction with graphical icons that represent the specific tasks at hand. In industrial applications, this helps keep plant-floor operators focused on the application and can be used by most operators regardless of their computer skills. Key to the successful application of touch screens is selecting the right technology and addressing the steps necessary to integrate them into a flat panel display system. 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 a customer clean room. A completely integrated touch kit includes the controller, interface cables and LCD. Customization 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 (see Figure 1). The following steps outline the development of a touch-integrated flat panel display system.

1. DEFINE THE APPLICATION
Questions to consider are: Where is the flat panel touchintegrated display system being 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 digital signage application? Is the display system indoor or outdoor? Is it being used in a rugged environment? What is the operating temperature 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, specialized application equipment and highly trained installation personnel. A mechanically mounted touch sensor is a touch screen input device that is designed to mount on the outside of a display device and is held in place by a physical device 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 5V or 12V DC voltage to operate properly (see Figure 2).

2. ESTABLISH TOUCH PANEL CRITERIA
Important performance specifications to consider when integrating touch screen displaysare 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 well as the type of LCD that will be used, e.g., passive matrix or active matrix TFT LCD, followed by several additional criteria as follows. Does it require anti-glare or anti-reflective properties? Are high brightness or transflective properties required for daylight readability? Brightnessrequired for daylight readability is generally 600 cd/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 vandalproof 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.

3. SELECT THE TOUCH SCREEN TECHNOLOGY
Once you make a 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. Factors to consider include:

• operating environment
• optical performance
• LCD size
• degree of transmissivity or transparency
• desired lifetime
• degree of scratch and damage resistance
• degree of calibration stability
• choice of finger or stylus touch (will the finger operation be gloved?)
• anti-reflective treatment (for high ambient light conditions)
• cost of the touch panel and controller

A resistive touch panel is produced by sandwiching together ITO (Indium Tin Oxide) coated glass and PET film (Poly Ethylene Terephthalate). This process is illustrated in Figure 3. 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.)

Resistive is the most popular form of touch screen technology in applications ranging from industrial to consumer. 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 400 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 the touch interface work with a gloved hand with no external pointing device.

LCDs utilizing surface acoustic wave technology, like resistive panels, can utilize 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 (e.g., ATM) 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--rain, snow or some external object stuck to the screen (such as a wad of chewing gum)--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, it 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, is 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 nonconductive 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. See Figure 4 for an illustration of how this technology works. Scanning infrared touch technology is commonly used in kiosk, gaming, retail, healthcare and industrial human-machine interface (HMI) applications. It is very rugged and unaffected by dirt, water and other contaminants, making it ideal for kiosk displays that are outdoors and open to the public; and 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 point-of-sale (POS) that require signature capture, which demands a very high resolution. The resolution provided by scanning infrared touch technology doesn't really have the accuracy required to prevent pixilation or distortion of the signature as it switches from one beam to the next.

4. SELECT THE CONTROLLER BOARD
Factors to consider when selecting the controller board are as follows. What kind of interface is being used to connect the touch controller to the CPU on the computer motherboard?

• Serial (RS-232)
• USB
• 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 50', whereas USB interfacesare generally limited to a distance of approximately 16'. However, touch controllers with serial interfaces have to be powered externally, requiring a 5V or 12V DC power supply. Controllers with USB interfaces, on the other hand, are self-powered directly from the USB port on the host computer system.

5. 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?

CONCLUSION
If you are designing a touch-integrated flat panel display system yourself, this article should be both insightful and useful as a resource for future reference. If short on engineering resources to accomplish this in-house, most companies choose to utilize a value-added solutions integrator with many years of experience in displays and associated sub-systems who has the expertise to put all these components together into a perfectly functioning system.

Richard E. McKay is Managing Director at Apollo Display Technologies, LLC. He can be reached by email at [email protected].