Touch screen for piezoelectric ceramic applications

Touch screen (also known as "touch screen", "touch panel") is an inductive liquid crystal display device that can receive input signals such as contacts. When a graphic button on the screen is touched, the The haptic feedback system can drive various connected devices according to a pre-programmed program, which can be used to replace mechanical button panels, and use liquid crystal display screens to create dynamic audio and video effects.

The touch screen gives the multimedia a new look and it is a very attractive new multimedia interactive device. It is mainly used for public information query, leadership office, industrial control, military command, video games, song and order, multimedia teaching, real estate pre-sale, etc.

Touch screens are distinguished from technical principles of piezo ceramic disc and can be divided into five basic types: vector pressure sensing technology touch screens, resistive technology touch screens, capacitive technology touch screens, infrared technology touch screens, and surface acoustic wave technology touch screens. Among them, the vector pressure sensing technology touch screen has withdrawn from the historical stage;
Resistive technology touch screen positioning is accurate, but its price is quite high, and it is afraid of scratching and damage;

Capacitive technology touch screen design is reasonable, but its image distortion problem is difficult to be solved fundamentally;Infrared technology of touch screen is cheap, but its frame is fragile, it is easy to produce light interference, and it is distorted under curved conditions;

The surface acoustic wave touch screen solves various defects of the previous touch screens, and is not easy to be damaged. It is suitable for various occasions. The disadvantage is that if water droplets and dust on the screen surface make the touch screen dull or even not work. The following briefly introduces the above types of touch screens:

Resistive touch screen

This touch screen uses pressure sensing for control. The main part of a resistive touch screen is a resistive thin film screen that closely matches the display surface. This is a multi-layer composite film. It uses a layer of glass or hard plastic flat plate as the base layer, and the surface is coated with a layer of transparent oxide metal (transparent conductive (Resistance) conductive layer, which is covered with an outer surface harden smooth and scratch-resistant plastic layer, its inner surface is also coated with a coating, and there are many small (less than 1/1000 inch) transparent between them Isolation points separate the two conductive layers from each other. The key of the resistive touch screen is material technology. Commonly used transparent conductive coating materials are ITO, indium oxide, light transmittance is 80%, and then it decreases when it is thinner, and it rises to 80% when the thickness is 300 angstroms.

Limitations of resistive screens

The common disadvantage of resistive touch screens is that because the outer layer of the composite film is made of plastic material, people who don’t know too hard or use a sharp touch may scratch the entire touch screen and cause scrap. However, within the limits, the scratch will only hurt the outer conductive layer. The scratch of the outer conductive layer is irrelevant to the five-wire resistive touch screen, and it is fatal to the four-wire resistive touch screen.

Capacitive touch screen

It works by using human body's current induction. The capacitive touch screen is a four-layer composite glass screen. The inner surface and the interlayer of the glass screen are each coated with a layer of ITO. The outermost layer is a thin layer of silica glass protective layer. The interlayer ITO coating is used as working surface. Four electrodes, the inner layer of ITO is a shielding layer to ensure a good working environment. When a finger touches the metal layer, due to the electric field of the human body, the user and the touch screen surface form a coupling capacitor. For high-frequency current, the capacitor is a direct conductor, so the finger draws a small current from the contact point. This current is flowing from the electrodes on the four corners of the touch screen, and the current flowing through the four electrodes is proportional to the distance from the finger to the four corners. The controller calculates the position of the touch point through accurate calculation of the four current ratios. 

Defects of capacitive touch screens

Capacitive touch screens have better light transmittance and clarity than four-wire resistive screens. Of course, they cannot be compared with surface acoustic wave screens and five-wire resistive screens. Capacitive screens have serious reflections. In addition, the four-layer composite touch screen of capacitive technology has non-uniform transmittance of light at various wavelengths, and there is a problem of color distortion. Due to the reflection of light between layers, the image characters are blurred.

Infrared touch screen

In the early concepts, infrared touch screens had technical limitations such as low resolution, limited touch methods, and susceptible to environmental interference and malfunction, once they faded out of the market. After that, the second generation of infrared screens partially solved the problem of anti-light interference. The third and fourth generations also improved the resolution and stability performance of piezo disc transducer, but they did not make a qualitative leap in key indicators or comprehensive performance. However, anyone who knows touch screen technology knows that infrared touch screens are not affected by current, voltage and static electricity and are suitable for harsh environmental conditions. Infrared technology is the ultimate development trend of touch screen products. Touch screens using acoustic and other materials technologies have their insurmountable barriers, such as damage and aging of a single sensor, fear of contamination of the touch interface, destructive use, and complicated maintenance. As long as the infrared touch screen truly achieves high stability and high resolution, it will certainly replace other technical products and become the mainstream of the touch screen market.

Surface acoustic wave touch screen

1.Surface Acoustic Wave

Surface acoustic wave, a type of ultrasonic wave, a mechanical energy wave that propagates shallowly on the surface of a medium (such as a rigid material such as glass or metal). Through the wedge-shaped triangular base (strictly designed according to the wavelength of the surface wave), directional, small-angle surface acoustic wave energy emission can be achieved. Surface acoustic wave performance is stable, easy to analyze, and has very sharp frequency characteristics in the process of shear wave transmission. In 2013, the application of non-destructive testing, imaging and waver has developed rapidly. Technology such as conductive materials and detection technology have been quite mature. The touch screen part of the surface acoustic wave touch screen can be a flat, spherical or cylindrical glass flat plate, which is installed in front of the CRT, LED, LCD or plasma display screen. The upper left corner and the lower right corner of the glass screen are respectively fixed with vertical and horizontal ultrasonic transmitting transducers, and the upper right corner is fixed with two corresponding ultrasonic receiving transducers. The ultrasonic transmitting and receiving transducers here are made of piezoelectric ceramics. The four perimeters of the glass screen are engraved with very precise reflection stripes at a 45 ° angle from sparse to densely spaced.

2. Working principle of surface acoustic wave touch screen

Piezoelectric ceramic sheets placed on the four corners or sides of the touch screen are used as supporting points and sensing points of the touch screen. When an external force touches the touch screen, a force is generated at the contact point, and the positions of the contact points are different, resulting in different voltage ratios generated by the four piezo ceramic pieces. The principle of the surface charge or voltage generation of the piezoelectric ceramic transducers are determined based on the piezoelectric effect of the piezoelectric ceramic material. By measuring the voltage generated by the four piezo ceramic pieces, the specific position of the contact point can be calculated, and the magnitude of the force can be known at the same time. By changing the rising and falling edges of the voltage waveform formed on the piezo ceramic, the speed of the force change or the acceleration of the force sensor can be obtained. By determining the position and intensity of the contact point, the purpose of touch is finally achieved. Moreover, the effect of this new type of touch is different from traditional touch. It can not only accurately sense the position of the touch point, but also the magnitude and acceleration of the force. The tactile dimension increases. 

3. Surface acoustic wave touch screen features

4. 

High definition and good light transmittance. High durability, good scratch resistance (compared with surface film of resistance, capacitor, etc.). Responsive. Not affected by environmental factors such as temperature and humidity, high resolution, long life (50 million times under good maintenance); high light transmittance (92%), can maintain clear and transparent image quality; no drift, only need to be installed One-time calibration; there is a response from the third axis (ie pressure axis), and it is used more in public places. Surface acoustic wave screens need to be maintained frequently, because dust, oil, or even liquids of beverages contaminate the surface of the screen, which will block the wave guide groove on the surface of the touch screen, making waves not transmit properly, or making the waveform change and the controller unable to recognize it, which affects normal use of the touch screen, users need to pay strict attention to environmental hygiene.