Some Sensing Methods of Touch Panel

Some typical sensing methods of touch panel will be introduced below.

Ⅰ. Resistive film touch panel

Before 2010, resistive film was the most widely used sensing method in the market of touch panel. Touch panels based on such a method are called pressure-sensitive or analog resistive film touch panels. In addition to independent LCD displays, this technology is also used in a variety of devices from small-sized to medium-sized, including smartphones, mobile phones, PDAs, car navigation systems, and Nintendo DS.

Using this method, pressure changes can be used to detect where a finger, stylus, or other objects is touching the screen. This kind of display has a relatively simple internal structure: a layer of a glass screen and a layer of the film screen, with a very small gap in between, and a transparent electrode film (electrode layer) attached to each screen. Pressing the surface of the screen will cause the film to contact the electrodes of the glass layer, thereby generating current. Contact points can be identified by detecting voltage changes.

The advantages of this system include low manufacturing costs due to its simple structure. The power consumption of this system is also lower than other methods. Due to the surface coating, its structure also has a good waterproof and dustproof performance. Input is performed by applying pressure to the film, so it can be operated not only with bare fingers, but also with gloves or a stylus. These screens can also be used to enter handwritten text.

Disadvantages include low light transmittance (reducing display quality) due to the film and two electrode layers; relatively low durability and impact resistance; and a larger screen size which will reduce detection accuracy. (The accuracy can be maintained by other methods, such as dividing the screen into multiple areas for detection.)

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Ⅱ. Capacitive touch panel

Capacitive touch panels are the second most widely used sensing method after resistive film touch panels. Corresponding to the aforementioned analog resistive touch panels, these are also called simulate capacitive touch panels. In addition to independent LCD displays, these are usually used in the same devices as resistive touch panels, such as smartphones and mobile phones.

In this method, the sensor determines the point where the touch occurs by sensing a small current change caused by a finger contact or a change in electrostatic capacity (load). Since the sensors can react to the electrostatic capacity of the human body when a finger approaches the screen, they can also perform similar operations on contacts that move in an area on the screen.

There are two types of touch panels that use this method: surface capacitive touch panels and projected capacitive touch panels. The internal structures of the two types are different.

1. Surface capacitive touch panel

Surface capacitive touch panels are generally used on larger panels. Inside these panels, a transparent electrode film (electrode layer) is covered on the glass substrate, and a protective film is covered on it. Voltage is applied to the electrodes on the four corners of the glass substrate to generate a uniform low-voltage electric field across the entire panel.

The coordinates of the position where the finger touches the screen are determined by measuring the changes in the electrostatic capacity of the four corners of the panel. Although this type of capacitive touch panel has a simpler structure than a projected capacitive touch panel, and thus its cost is lower, it is structurally unable to detect two or more points at the same time (multi-point touch).

2. Projected capacitive touch panel

Projected capacitive touch panels are generally used for screens that are smaller in size than surface capacitive touch panels. They are particularly noticed by mobile devices. iPhone, iPod Touch, and iPad are all using this method to achieve high-precision multi-touch function and high response speed.

The internal structure of these touch panels includes a substrate with an IC chip for processing calculations. The substrate is covered with numerous transparent electrodes to form a special pattern. The surface is covered with insulating glass or plastic film. When a finger approaches the surface, the electrostatic capacity between multiple electrodes changes at the same time, and the contact location can be accurately identified by measuring the ratio between these currents.

A unique feature of the projected capacitive touch panel is that a large number of electrodes can accurately detect the contact of multiple points (multi-point touch). However, the indium tin oxide (ITO) projected capacitive touch panel used in smartphones and similar devices is not suitable for large screens, because larger screen causes increased resistance (i.e., slower current transmission speed), thereby increasing the error and noise of touch point detection.

Larger touch panels use centerline projected capacitive touch panels, laying very thin wires in a grid to serve as a transparent electrode layer. The low resistance makes centerline projection capacitive touch panels extremely sensitive, and they are more suitable for mass production than ITO etching.

In the above, we summarized the differences between the two types of capacitive touch panels. The common feature of these panels is that, unlike resistive touch panels, they do not respond to the touch of clothes or a standard stylus. They have excellent dust and drip resistance, high durability and scratch resistance. In addition, its light transmittance is also higher than that of resistive touch panels.

On the other hand, these touch panels require the use of fingers or a dedicated stylus to operate. They cannot be operated while wearing gloves, and they are easily affected by nearby metal structures.

Ⅲ. Surface acoustic wave (SAW) touch panel

The surface acoustic wave (SAW) touch panel is mainly to solve the shortcoming of low light transmittance of the resistive film touch panel, that is, to realize a bright touch panel with high visibility. This type of panel is also called a surface wave or acoustic wave touch panel. In addition to independent LCD displays, these products are also widely used in equipment in public places, such as sale terminals, ATMs, and electronic kiosks.

These panels use the attenuation of ultrasonic elastic waves on the surface to detect the contact position of a finger or other object on the screen. The internal structure of these panels is that multiple piezoelectric transducers are installed on the corners of the glass substrate. The transducers emit ultrasonic surface elastic waves, vibrate on the surface of the panel, and the ultrasonic waves are received by the relatively installed transducers. When the screen is touched, ultrasonic waves are absorbed by fingers or other objects and attenuate. The location can be determined by detecting these changes. Of course, the users will not feel this vibration when touching the screen. These panels are very easy to use.

Since this structure does not require a cover film or transparent electrode on the screen, this type of touch panel has the advantages of high light transmittance and visibility. In addition, the surface glass provides better durability and scratch resistance than capacitive touch panels. Another advantage is that even if the surface is scratched, the panel can remain sensitive to touch. On capacitive touch panels, surface scratches sometimes interrupt the signal. Structurally, this type of panel ensures high stability and long service life without changes or deviations in position over time.

The disadvantages include that it can only be operated with fingers and soft objects such as gloves that can absorb the elastic waves of an ultrasonic surface. This type of panel requires a dedicated stylus and may react to water droplets and other substances or small insects on the panel.

However, the shortcomings of this touch panel are still relatively few. Recent technological developments, such as improvements in production technology, are constantly improving its cost performance.

Ⅳ. Optical touch panel (infrared optical imaging touch panel)

The optical touch panel includes various sensing methods. In recent years, the number of products using infrared optical imaging touch panels is increasing, especially for larger-sized panels. This touch panel is based on an infrared image sensor to sense position through triangulation.

This kind of touch panel is equipped with an infrared LED and an image sensor (camera) on the left and right ends of the top of the panel. Reflective strips are attached to the remaining positions on the left and right sides and on the bottom side to reflect incident light along the incident axis. When a finger or other object touches the screen, the image sensor captures the shadow formed by the blocking of infrared rays. The coordinates of the contact position are obtained by triangulation.

Ⅴ. Electromagnetic induction touch panel

However, this type is different from the touch panel mentioned above. Let's take a look at the electromagnetic induction touch panel. This method is suitable for LCD drawing board, tablet computer, and photo-booth equipment.

This input method was originally used for drawing boards without a display, and a high-precision touch panel was realized by combining a sensor with a liquid crystal panel. When the user touches the screen with a special stylus that can generate a magnetic field, the sensor on the panel receives electromagnetic energy, thereby sensing the position of the pen.

The use of this method is limited because it requires the use of a dedicated stylus, rather than the ability to use a finger or a universal stylus for input. However, this is a mixed bag. It avoids input errors caused by the surrounding environment or unintentionally touching the screen. Since this technology was originally suitable for graphics tablets, it can provide excellent sensor accuracy. For example, when the stylus is pressed onto the screen (electrostatic capacity), it can be changed smoothly by accurately sensing the pressure line width. This design approach also gives the screen high light transmittance and durability.