Capacitive touch screen technology USES the human body’s current induction to work. The capacitive touch screen is a four-layer composite glass screen. The inner surface of the glass screen and the interlayer are coated with one layer of ITO respectively. The outermost layer is a thin layer of silica soil glass protective layer. When the finger touches the metal layer, the user and the touch screen surface form a coupling capacitor due to the electric field in the human body.
The capacitor is a direct conductor to the high-frequency current, so the finger sucks a small current away from the contact point. The current flows from the electrodes at the four corners of the touch screen respectively, and the current flowing through the four electrodes is proportional to the distance from the fingers to the four corners. The controller calculates the precise proportion of the four currents to get the position of the touchpoint.
Overview of capacitive touch screen principle
In order to realize multi-touch on a capacitive screen, it is necessary to add electrodes of mutual capacitance. In a simple words, it is to divide the screen into blocks and set a group of mutual capacitance modules in each area to work independently. Therefore, the capacitive screen can independently detect the touch situation of each area and simply realize multi-touch after processing.
How capacitive touch screen works
Capacity Touch Panel USES the current induction of the human body to work. The capacitive screen is a four-layer composite glass screen. The inner surface of the glass screen and the interlayer are coated with ITO (indium sikgold oxide nano). The outermost layer is a protective layer of silica glass with a thickness of 0.0015mm.
When a user capacitive touch screen, the electric field due to the human body, your fingers and face form a coupling capacitance, because the working plane to have a high-frequency signal, so the fingers on a very small current, respectively from the current screen in the four corners of the electrode, and theoretically through the four electrodes with the finger to the four corners of the current is proportional to the distance, the controller through the precise calculation of the four current ratio, it is concluded that location. It can achieve 99% accuracy, with a response speed of less than 3ms.
Projective capacitor panel
Projective capacitive touch screens etch different ITO conductive circuit modules on two layers of ITO conductive glass coatings. The etched patterns on the two modules are perpendicular to each other and can be thought of as sliders with continuous changes in X and Y directions. As the X and Y architectures are on different surfaces, their intersection forms a capacitor node. One slider can be used as the drive wire and the other as the detection wire. When a current passes through one of the wires in the drive wire, if there is a signal of a change in the capacitance outside, it will cause a change in the capacitance node on the other layer of wire. The change in capacitance can be detected by measuring the electrical circuit connected to it, and then converted to A digital signal by A/D controller, which can be processed by A computer to obtain the (X, Y) axis position, so as to achieve the positioning target.
3M shows 60 points capacitive touch screen
During operation, the controller successively supplies power to the driving wire, thus forming a specific electric field between each node and the wire. Then scan the sensor line one by one to measure the capacitance change between the electrodes so as to achieve multi-point positioning. When the finger or touch medium is close, the controller can quickly detect the change of capacitance between the touch node and the wire, and then confirm the position of the touch.
One axis is driven by a set of AC signals, and the response across the touch screen is measured by electrodes on the other axis. Users call this’ transversal ‘sensing or projective sensing. The sensor is plated with the ITO pattern of the X and Y-axis. When the finger touches the touch screen surface, the capacitance under the touchpoint increases according to the distance of the touchpoint. The continuous scanning on the sensor detects the change of capacitance value, and the control chip calculates the touchpoint and returns it to the processor.
Capacitive touch element classification
There are two types of capacitive touch screens: surface capacitive touch screens and projective capacitive touch screens.
Surface capacitive touch screen
Commonly used is the surface capacitive touch screen, which works in simple principle, low price, simple circuit design, but difficult to achieve multi-touch.
Projective capacitive touch screen
Projective capacitive touch screens are multi-finger touch. These two capacitive touch screens have the advantages of high light transmittance, fast response speed, and long life, etc. The disadvantages are: with the change of temperature and humidity, the capacitance value will change, resulting in poor work stability, often drift phenomenon, need to frequently proofread the screen, and can not wear ordinary gloves for touch positioning.
The projected capacitive touch screen can be divided into the capacitance and mutual capacitance screen two types, one of the more common mutual capacitance screen as an example, the internal electrode and receiving electrode by the driver, drive electrode signal low voltage high frequency projected onto the receiving electrode form stable current, when human exposure to the capacitance screen, earth due to the human body, fingers and capacitance screen to form an equivalent capacitance, and the high-frequency signal by the equivalent capacitance into the ground, in this way, the receiver receives charge is reduced, when fingers near the transmitter, electric charge, the more significant, according to the receiving end receives the current strength of to determine the touchpoint.
Arrays of transverse and longitudinal electrodes are made from ITO on the surface of the glass. These transverse and longitudinal electrodes form capacitors with the ground respectively. This capacitor is commonly referred to as self-capacitance, that is, the capacitance of the electrode to the ground. When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the capacitance of the screen, thus increasing the capacitance of the screen.
During touch detection, the horizontal and longitudinal electrode arrays are respectively detected from the capacitive screen. According to the changes of capacitance before and aftertouch, the horizontal coordinates and longitudinal coordinates are determined respectively, and then the touch coordinates of the plane are combined. The scanning method of self-capacitance is equivalent to projecting the touchpoints on the touch screen to the X-axis and Y-axis directions respectively, and then calculating the coordinates in the X-axis and Y-axis directions respectively, and finally combining them into the coordinates of the touchpoints.
If it is a single touch, the projection in the X and Y direction is unique, and the combined coordinates are unique. If there are two touches on the touch screen and the two touches are not in the same X direction or the same Y direction, then there are two projections in the X and Y direction respectively, and the combined coordinates are 4.Apparently, only two of the coordinates are real, and the other two are known as ghost points. Therefore, self – the capacitive screen can not achieve true multi-touch.
The mutual capacitor screen also USES ITO to make the transverse electrode and the longitudinal electrode on the glass surface. The difference between it and the self-capacitor screen is that the place where the two groups of electrodes cross will form a capacitor, that is, the two groups of electrodes form the electrodes of the capacitor respectively. When a finger touches a capacitive screen, the coupling between two electrodes near the touchpoint is affected, thus changing the capacitance between the two electrodes.
When the mutual capacitance is detected, the transverse electrode will send out excitation signals successively, and all the longitudinal electrodes will receive signals at the same time. In this way, the capacitance value of all the intersection points of the transverse and longitudinal electrodes can be obtained, that is, the capacitance value of the entire two-dimensional surface of the touch screen. The coordinates of each touchpoint can be calculated according to the two-dimensional capacitance variation data of the touch screen. Therefore, even if there are multiple touchpoints on the screen, the actual coordinates of each touchpoint can be calculated.
The advantage of the mutual capacitive screen is less wiring, and can simultaneously identify and distinguish the difference between multiple contacts since the capacitive screen can also sense multiple contacts, but because the signal itself is fuzzy, so can’t distinguish. In addition, the induction scheme of the mutual capacitive screen has the advantages of fast speed and low power consumption, because it can measure all the nodes in a driveline at the same time, thus reducing the number of acquisition cycles by 50%. The dual-electrode structure has the function of self-shielding external noise and can improve signal stability at a certain power level.
In any case, the touch position is determined by measuring the distribution of signal changes between the X and Y electrodes, and a mathematical algorithm is then used to process the changed signal levels to determine the XY coordinates of the touchpoint.
Advantages and disadvantages of capacitive touch screen
• Capacitive touch screens require only touch, not pressure, to produce signals.
• Capacitive touch screens require only one or no correction after production, whereas resistor technology requires regular correction.
• Capacitive schemes last longer because the components in the capacitive touch screen do not need to move at all. In a resistive touch screen, the top layer of the ITO film needs to be thin enough to be elastic so that it bends down and touches the bottom layer of the ITO film.
• Capacitor technology is superior to resistor technology in optical loss and system power consumption.
• The choice of capacitor or resistor depends largely on the object touching the screen. If it is a finger touch, the capacitive touch screen is a better choice. If a stylus is needed, whether plastic or metal, a resistive touch screen will do. A capacitive touch screen can also use a stylus but requires a special stylus to work with it.
• Surface capacitances can be used for large touch screens and are relatively low, but they currently do not support gesture recognition: inductive capacitances are mainly used for small and medium-sized touch screens and can support gesture recognition.
• Capacitive technology is wear-resistant, has a long service life, and has low maintenance costs when users use it, so the overall operating costs of manufacturers can be further reduced.
• Capacitive touch screens are designed to support multi-touch technology and are less responsive and less prone to wear and tear than resistive touch screens.
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