Designing a successful projected capacitive touch screen system requires careful consideration of the mechanical design, substrate selection, and user interface of the device. In addition, compromises between cost and technology must not be forgotten at all stages of the design process. Unlike resistive touch screen technology, projected capacitive touch screens are much easier to handle finger movements, especially multi-touch user input. Resistance techniques rely on finger pressure to make electrical contact with multiple mechanical layers of the touch screen. This method of operation affects the smoothness of finger sliding and the dexterity of gesture operations. In addition, the multi-layered mechanical structure of the resistive touch screen is prone to premature wear due to repeated use. Several common multi-touch gestures implemented with a projected touch screen include finger fitting, zooming, two-finger sliding, and rotation. They can handle data, content and user parameters quickly and easily. Portable games and text/email applications can also utilize multi-touch technology. In a multi-finger touch process, the multi-touch APA (full-addressable) mode can accurately determine the position of the coordinate pressed by each finger. No need to first press Shift to change the character set and then enter the actual character, multi-touch can simultaneously click Shift + the actual character. Multi-touch methods are also widely used in GPS navigation. Instead of entering the starting place and destination, APA can select the target location on the screen to allow people to reach their destination faster. Figure 1 illustrates some of the possible scenarios of multi-touch operations. To evaluate the mechanical design of a device, several key issues must be addressed: 1. Is the protective layer (touch surface) flat or curved ? It is generally recommended that the capacitive touch screen be mounted on a flat touch surface. Surfaces increase complexity. To achieve a robust capacitive touch design, a transparent touch sensor must be neatly clamped under the protective layer. Any air bubbles due to uneven pressure can reduce the touch performance and affect the appearance of the product. The surface protective layer can only use PET (polyester) as the base of the touch sensor. Plastic sensors can be bent to adapt to the shape of the protective layer. If you must use a curved protective layer, from the perspective of reflection, it is recommended that the curvature does not exceed 45 degrees. The increase in curvature increases the difficulty of the lamination process and may damage the ITO (Indium Tin Oxide) pattern, which may affect the yield. Using pressure sensitive adhesives (PSAs) to achieve press fit is cheaper, but it cannot be used for curved protective layers. To ensure better press-fit integrity, more expensive UV-curable adhesives may have to be used. UV adhesives are expensive, but they are easy to use, have a thin adhesive layer, and have very high optical qualities (transparency greater than 95%). 2. What is the width of the edge of the non-working area (opaque area) ? For touch screens smaller than 4 inches (10 cm) in size, the edge width of the touch screen should not be narrower than 10 mm on the side of the tail of the touch sensor and not less than 3 mm on either side of the touch sensor. This rim space is used to hide the non-transparent silver foil lines that link the transparent ITO pattern to the control circuit and hide the control circuit itself. For glass-based touch screens, the width of the edges may be made narrower, but the above guidelines are still recommended. Figure 2 describes these guidelines. Figure 2: Requirements for non-working edge areas of the touch screen 3. What material is used for the protective layer ? In the touch screen working area, conductive materials cannot be used for the protective layer and any decorations. Because the use of a conductive material shields the electric field of the capacitive sensor and greatly reduces the sensing performance. The thickness of the protective layer should be 1 mm or less. 4. What is the distance between the bottom of the protective layer and the liquid crystal display module (LCM)? Due to the small size of the portable communication device, the spacing between the liquid crystal module (LCM) and the protective layer needs to be considered. There must be ample room to install a thin touchscreen sensor, and there must also be a large enough air gap to prevent the touch sensor from electromagnetic interference from the LCM. It is recommended to leave at least 0.5mm clearance between the touch sensor substrate and the LCM. 5. How to deal with electrostatic discharge (ESD)? To prevent electrostatic discharge events on the touch surface, a low impedance ground path must be provided throughout the device. The touch sensor should be protected with a grounding ring placed in the non-working boundary area of ​​the protective layer. The grounding ring can be a simple metal foil. It must be ensured that there is a reliable connection between the grounding ring and the system ground of the device. After the mechanical evaluation is completed, a suitable base must be selected for the touch screen. Figure 3 shows a typical ITO pattern of a projected capacitive touch screen. The two main materials for projected capacitive touch screen substrates are glass and PET (polyester). These two materials have their own advantages. If the machine design of the equipment does not have special requirements for substrate selection, you should choose the substrate that best suits your product based on your marketing strategy. Table 1 gives a comparison of the characteristics of the two substrates. Glass substrates are commonly used in applications where high optical performance and environmental resistance are required. In most applications, glass-based touch sensors are used in conjunction with a protective layer of tempered glass with a near-reflectance coefficient. In addition, the protective layer is usually treated with an anti-glare, anti-reflection, and anti-scratch coating to reduce reflections and further increase optical performance. Transparency is used to define the amount of light that passes through a material. The reflection coefficient is used to measure the amount of light reflected. All projection touch screens include a patterned transparent ITO conductor. Ideally, the reflectivity of the ITO pattern is equal to the reflectance of the foil line gap without the ITO pattern, which ensures that the conductive ITO foil lines are not visible. Glass touch sensors and protective layers can also be chemically treated to improve the ability to resist drop shock. Glass-based, projected capacitive systems are generally more expensive than PET. The main benefits of using PET substrates are thin, high pressure composites and lighter structures. Of course, the cost of PET touch screen systems is much lower than similar glass substrate solutions. The thin film substrate is usually fitted with a relatively inexpensive PMMA protective film, which has a low strength and the surface is easily scratched. In order to understand the cost difference between PET and glass touch sensors, it is necessary to examine the yield of the two at different stages of manufacture. In the manufacturing process of the touch sensor, ITO is sprayed on the glass/PET substrate to form a fine ITO deposition layer on the surface of the substrate. Then, a photomask for creating an ITO pattern needs to be fabricated to prepare for the next processing stage. A photomask is used in the etching process to remove unwanted ITO, producing the desired ITO pattern. For a bi-substrate projected capacitive touch screen, the worst case manufacturing yield is given by the following equation. Productivity Projection = Coating Finishing Rate x Etch (X) Finished Rate x Finishing Finished Rate x Etch (Y) Finished Rate + Pressed Combine Rate Production experience shows: Spray Finishing Rate (PET) > Spray Finishing Rate (Glass) In addition, the chemical treatment process performed to increase the mechanical strength of the glass touch sensor will also potentially affect its yield, while the PET-based solution does not involve this issue. Another very important consideration for projected capacitive touch screen designs is the user interface. Capacitive touch screens are not suitable for use with touch pens. People are currently studying two new types of capacitive touch pens, one using a conductive eraser and the other using a conductive dielectric. However, whether or not these two solutions can provide a solid, easy-to-use, and inexpensive touch pen is not yet clear. In addition, the capacitive touch will be affected by the contact area between the finger and the touch screen. A pointed stylus cannot produce a capacitive signal of the finger. Capacitive touch screens are mostly designed for finger operation. Using capacitive touch for web browsing is an application that requires accurate finger selection. In web pages, many links are closely arranged, and it seems difficult to choose the exact link that you want. One way of designing an interface is to allow the user to scroll through the link options one by one. Each link is interleaved in the scrolling process so that touch selection can be easily performed. Users can easily access a link by touching the zoomed-in option. Another method is to zoom in on all the links that are close to the touch point of the finger so that the user can select the desired link from the enlarged set of links. In short, user interface design should take into account the uncertainty associated with finger size, movement, and positioning. The accuracy of repeated hand presses is typically 3-4 mm. Part of the reason for this bias is the difference in the parallax between the eyes and fingers and the dexterity of the person. The diameter of the icon/button is preferably greater than 5mm. The buttons should also be fully separated to improve usability (minimum 5-10mm). If you plan to use the thumb to operate the buttons, the buttons should be larger and the spacing should be further away. In addition, the user should be provided with some form of visual, aural or tactile feedback as appropriate to indicate whether the choice is correct. The lack of feedback will reduce the accuracy of user input. The tactile feedback type touch screen generates tactile feedback on the finger pressing behavior on the solid state touch screen. It uses vibration motors (actuators) to provide feedback to the user. Among mobile devices, the most common actuators are the Eccentric Rotating Mass Actuator and the linear resonant actuator. For one touch behavior, the surface of the projected capacitive touch screen will vibrate indicating that an input has been detected. The intensity and duration of the vibration can be adjusted depending on the type of input feedback. For the user interface, the most basic design requirements are simple, allowing the user to complete common tasks with just a few clicks of the screen. This not only produces a more enjoyable user experience, it also reduces the difficulty of beginners. For projected capacitive touch screen systems, mechanical evaluation, substrate selection, and user interface design are all important considerations. Understanding these mechanical constraints provides the basis for substrate selection and ensures that the touch screen provides the highest performance. Substrate selection is a trade-off between cost, robustness, and optical performance. The simple, intuitive and well-chosen dimensions of the finger selection icons are the basic conditions for ensuring that the user interface is easy to use. Careful consideration of all aspects of touch screen design can ensure the success of the end product and greatly reduce the development risk. Traffic Facilities,Waterproof Traffic Facilities,Outdoor Traffic Facilities,Traffic Control Devices Yangzhou Heli Photoelectric Co., Ltd. , https://www.heli-eee.com
Figure 1. Multi-touch screen can accept a variety of user gestures
Figure 3: Typical ITO pattern (red and blue indicate two different layers)
Table 1: Comparison of Properties of Glass Substrates and PET Substrates
Etch Yield (PET)> Etch Rate (Glass)
Pressed synthetic product rate (PET)> Compressed synthetic product rate (glass)
Practical considerations in the design of capacitive touch screen systems
As consumer mobile communication devices increasingly adopt digital methods and integrate more functions, the development of intuitive and innovative user interface (UI) solutions becomes more important for device design. As part of the design of the user interface, a projected capacitive touch screen helps address this challenge.