Capacitive Touch Manufacturing

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By Paolo Pedrazzoli, Marketing Operations Manager 3M Touch Systems.
 
Capacitive touch screens are very popular for self-service applications, as they are durable, reliable and optically clear to allow the full brightness and colour of the display graphics to shine through. A clear glass sensor is attached to the front of the machine’s display, typically an LCD panel in one of the standard sizes offered by display vendors.
 
For technical reasons, capacitive touch screens are usually offered in the range of 6.4” up to a maximum of 32” in standard- and wide-aspect ratios. The retail application generates the images of touch input controls, such as menu buttons to confirm choices or a numeric keypad to enter a product code. When the user touches the glass sensor at the position of one of these features, an electronic controller calculates the position and communicates the co-ordinates to the computer program. With this information the program is then able to decode the user’s command and react accordingly.
 
The construction of the touch sensor begins with a glass ‘mothersheet’ of high optical quality (approximately 3mm thick). Individual panels are then cut out of this sheet, to match the required display sizes. Of course efficient planning is important to enable the number of individual panels that can be cut from one mothersheet to be optimised. Diversified technology company 3M has its own in-house cutting facilities, which helps to even out lead-time fluctuations.
 
To make the glass touch sensitive, it is first coated with a thin, conductive layer which is transparent to the user. Touching the screen creates a “pathway to ground”, drawing small currents from electrodes at each corner of the panel. By measuring these currents the controller calculates the position of the touch point, and hence the location of the touch. As the user’s finger moves across the screen, the controller recalculates the position quickly, allowing programmers to design actions such as ‘drag and drop.’

 
The currents flowing to the touch point are extremely small, so are safe to use and use less power consumption. The thickness of the conductive layer must be uniform to minimize measurement errors of the small currents.
 
3M applies a proprietary hard coating to enhance durability, usability and optical performance. Protection against wear and scratching of the conductive layer is particularly important in high use applications such as public-access retail terminals. Since the natural surface of the glass sheet is far from perfectly smooth, 3M’s top hard coatlayer and applied anti-scratch top coatlayer  fill in the surface irregularities in the glass. The top coat layer also reduces the surface energy of the sensor, allowing the user’s finger to slide easily and smoothly over the surface.
 
Following the coating processes, a rigorous visual inspection is performed. A flexible ‘tail’ is attached to the glass using a conductive adhesive, which takes electrical connections to the sensor’s electronic controller. Attaching the tail helps eliminate performance risks associated with using a soldering process, thereby increasing production yield and avoiding the costs of down-time and maintenance.
 
Following the tail-bonding process, 3M also applies tail strain relief to the top, bottom and side of the sensor to maximise mechanical strength. This combination promotes reliability by preventing separation of the bonded contacts between the touch screen and the connector tail.
 
After final assembly of the touchscreen and controller, rigorous functional testing is then followed by a linearisation process to ensure accuracy and consistent quality. 3M, for example, references 25 test co-ordinates across the touch screen surface to correlate actual touch positions with calculated positions.
 
Although addressing all of these points in the touch screen itself delivers the essential robustness, reliability and repeatability that suppliers of retail equipment need, the process remains incomplete without high-quality software tools to help developers configure preferences and customize features. These may include deciding whether the touch is detected upon contact or when the user’s finger is removed from the screen, generating a sound when the screen is touched, or adjusting settings such as doubleclick speed for mouse-type controls. Dependable software drivers are also required, to integrate the touch screen with the host computer program.
 
The MicroTouch MT7 software suite from 3M includes drivers for touch screens operating with major operating systems, including Windows variants and Linux-based systems. The MT7 control panel allows developers to quickly optimise settings and configure various touch screen options such as edge acceleration. This value-added feature detects when the user’s finger is moving toward a corner or edge and ensures that the cursor is able to reach features at the extremities of the screen. Other facilities of MT7 software include calibration to ensure the co-ordinates calculated by the controller correspond exactly with the touch-sensitive areas presented by the display.
 
The key three components of a touch screen – the sensor, the electronics and the software – must work together to fulfil all the requirements for the retail self-service kiosk being built. In-house control of all constituent processes from glass cutting onwards enables close management of aspects such as quality, delivery schedules, customer support, and new product development. In practice, these factors also determine the performance and the cost of ownership for retailers using touch screens to deliver higher levels of customer satisfaction and thereby to help increase their revenues and market share.