Touching the Future: Projected Capacitive Touch Screens to Lead New Markets
May 08, 2018
There are two main types of touch screens that use capacitive sensing as their primary input method: surface-type and projected. The surface capacitive touch screen employs a layer of indium tin oxide (ITO) and has at least four electrodes on the periphery. When a grounded object approaches, such as a finger, these electrodes can sense changes in surface capacitance. 3M Micro-Touch is one of the most important suppliers of this technology. This method has been used on kiosk touchscreens for a long time.
However, the surface capacitive touch screen has some limitations. It can only recognize one finger or one touch. In addition, taking into account the size of the electrodes, it is impractical for small-sized screens, such as those used on handheld platforms.
The electrostatic field line emitted by the sensor using the touch screen electrode is called a projected capacitive touch screen. There are two types of capacitors commonly used in projective capacitive sensing: self capacitance and mutual capacitance.
Self capacitance, also called absolute capacitance, is the most widely used method. It uses the object being sensed as another plate of capacitance. The object senses a charge between the sensing electrode and the sensed electrode and is thus sensed. The measured charge is stored in the resulting capacitive coupling. Figure 2 shows how the above principles work.
The mutual capacitance is also called the transcapacitance, which is the capacitance generated by the coupling of adjacent electrodes. When the object to be sensed approaches the electric field line from one electrode to the other, a change in the mutual capacitance is sensed to report the position. In automotive applications, mutual capacitive sensors are widely used as conductivity sensors for gasoline regulation.
Millions of self-capacitance methods are used in the position sensing of people's daily life. For example, on today's laptops, touch input boards exist everywhere. The most typical laptop touch input board uses an X×Y sensor electrode array to form a sensing grid. When the finger is close to the touch input pad, a small amount of charge is generated between the finger and the sensing electrode. A specific algorithm is used to process the signals from the row and column sensors to determine the location of the sensed object (in this case, the finger).
In the above two types of projected capacitive sensors, the sensing capacitance can be designed according to a certain method so that the touch can be detected at any given time, and the touch is not limited to one finger or multiple fingers.
Opaque projected capacitive sensors can be used on many devices, such as touch input pads and projected capacitive touch screens, and the basic principle of operation is the same. The difference lies in the sensor electrode material, the sensor substrate, the manufacturing method, and many problems in the method stack. The touch input pad can be made of an opaque material, such as a carbon-based electrode in the field of metal or transfer. Projected capacitive touch screens, which must be transparent, are often made of the same materials as the transparent conductors based on resistive touch screens.
However, unlike resistive touch screens, projected capacitive touch screens do not require air separation between layers, or do not need to deform any layers, so the sensor can use a rigid glass or plastic substrate. The key difference between the projected capacitive and resistive touch screen configurations is the requirement for ITO. The projected capacitive touch screen's ITO is used as the latter layer on the previous layer, instead of continuous film deposition like a resistive touch screen. . Although increasing the complexity of the process, it is still worthwhile considering the benefits of a projected capacitive touch screen.
There is no obvious difference in the choice of glass and plastic substrates. All can be stacked on plastic or glass lenses (screen protection), depending on the original equipment manufacturer's product design. Glass is relatively thick, important, and expensive, but overall it is harder and can potentially reduce the cost of the rest of the equipment. Glass has a higher transmission coefficient than plastic, although both materials are higher than resistive touch screens of the same size. Plastic sensors are relatively thin and relatively easy to laminate on product lenses (because laminating flexible materials onto hard material layers is easier than laminating two layers of hard materials). Since the manufacturing methods are similar, both glass and plastic substrates can be used to make self-capacitive touch screens and interactive capacitive touch screens.
Most sensor suppliers use the same batch of sputtering processes to etch the lTO pattern on the substrate.
In 2007, 3M MicroTouch announced the possibility of roll-to-roll manufacturing of projected capacitive sensors. In the past, although the technique of etching such a pattern has been very mature, it will cause the reflection coefficient of the touch screen surface to be different, which will cause the pattern to be visible when the light is drawn across the surface. The recently obtained reflection coefficient is more matched and the sensor pattern is barely visible.
Surface capacitive touch screens have practical limitations in small-scale production, whereas projected capacitive touch screens have limitations in large sizes.
The sensor electrode must be close enough so that the finger can affect the electric field lines of at least two electrodes to determine the position of the finger. Likewise, the number of sensor electrodes also needs to be increased according to the corresponding geometric ratio as the screen size increases. As the projected capacitive touch screen increases in size, the number of sensor electrodes that need to be sent back to the controller increases rapidly, forcing the sensor's non-active bezel to expand. There are some tips for making large-size projected capacitive touch screens, but these designs have not been verified in real products so far.
