Capacitive Touch Screens

Capacitive touch screens is also called Projected Capacitive touch screens or in brief writing PCAP or CTP Touch screens.

Capacitive Touch Screens

Capacitive Touch Screens

Shop now

To realize multi-touch on capacitive screen, it depends on adding electrodes of mutual capacitance. Simply put, it is to divide the screen into blocks and set up a set of mutual capacitance modules in each area to work independently, so the capacitive screen can work independently. The touch situation of each area is detected, and after processing, multi-touch is simply realized.

Capacitive technology touch panel CTP (Capacity Touch Panel) is to use the human body’s current induction to work. The capacitive screen is a four-layer composite glass screen. The inner surface of the glass screen and the interlayer are each coated with a layer of ITO (nano-indium tin metal oxide ). The outermost layer is a silica glass protective layer with a thickness of only 0.0015mm, and the interlayer ITO coating As a working surface, four electrodes are drawn from the four corners, and the inner layer of ITO is the screen layer to ensure the working environment.
When the user touches the capacitive screen, due to the electric field of the human body, a coupling capacitor is formed between the user’s finger and the work surface. Because the work surface is connected to a high-frequency signal , the finger absorbs a small current, which flows from the four corners of the screen respectively. In theory, the current flowing through the four electrodes is proportional to the distance from the finger to the four corners. The controller obtains the position through precise calculation of the ratio of the four currents. It can reach 99% accuracy and has a response speed of less than 3ms.

Projected Capacitive Panel
Touch Technology of Projected Capacitive Panel Projected capacitive touch panel is to etch different ITO conductive circuit modules on two layers of ITO conductive glass coating. The patterns etched on the two modules are perpendicular to each other, and they can be regarded as sliders that change continuously in the X and Y directions. Since the X and Y structures are on different surfaces, a capacitive node is formed at the intersection. One slider can be used as the drive line, and the other slider can be used as the detection line. When the current passes through one wire in the driving line , if there is a signal of capacitance change in the outside world, it will cause the change of the capacitance node on the other layer of wire. The change of the detected capacitance value can be measured by the electronic circuit connected to it, and then converted into a digital signal by the A/D controller to allow the computer to perform arithmetic processing to obtain the (X, Y) axis position, and then achieve the purpose of positioning.

In operation, the controller successively supplies current to the drive lines, thereby forming a specific electric field between each node and the line. Then, the sensing lines are scanned column by column to measure the capacitance change between the electrodes to achieve multi-point positioning. When a finger or a touch medium approaches, the controller quickly detects the change in the capacitance value between the touch node and the wire, and then confirms the touch position. This one axis is driven by a set of AC signals, and the response across the touch screen is sensed by electrodes on the other axis. Users refer to this as ‘transverse’ sensing, also known as projected sensing. The sensor is plated with ITO patterns of X and Y axes. When a finger touches the touch screen surface, the capacitance value below the touch point increases according to the distance of the touch point. Continuous scanning on the sensor detects the change in capacitance value. The control chip calculates the touch point and reports it to the processor.

Component classification

Principle of capacitive screen
There are two types of capacitive touch screens: surface capacitive touch screens and projected capacitive touch screens.

Surface Capacitive Touch Screen
The commonly used surface capacitive touch screen has simple working principle, low price and simple circuit design, but it is difficult to realize multi-touch.

Projected capacitive touch screen
Projected capacitive touch screen has the function of multi-finger touch. Both of these capacitive touch screens have the advantages of high light transmittance, fast response speed, and long life. The disadvantage is: with the change of temperature and humidity, the capacitance value will change, resulting in poor working stability and often drift phenomenon. The screen needs to be calibrated frequently, and ordinary gloves cannot be worn for touch positioning.
Projected capacitive screens can be divided into two types: self-capacitance screens and mutual-capacitance screens. For example, the more common mutual-capacitive screens are composed of driving electrodes and receiving electrodes. The driving electrodes emit low-voltage high-frequency signals and project them to the receiving electrodes to form a stable When the human body touches the capacitive screen, due to the grounding of the human body, the finger and the capacitive screen form an equivalent capacitance, and the high-frequency signal can flow into the ground wire through this equivalent capacitance, so that the amount of charge received by the receiving end is reduced. When the finger is closer to the transmitting end, the charge decreases more obviously, and finally the touched point is determined according to the current intensity received by the receiving end.
The horizontal and vertical electrode arrays are made of ITO on the glass surface. These horizontal and vertical electrodes respectively form capacitors with the ground. When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the capacitance of the screen body, which increases the capacitance of the screen body.
During touch detection, the self-capacitance screen detects the horizontal and vertical electrode arrays in turn, determines the horizontal and vertical coordinates respectively according to the change of capacitance before and after the touch, and then combines them into a plane touch coordinate. The self-capacitance scanning method is equivalent to projecting the touch points 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 touch points. [5]
If it is a single-point touch, the projections in the X-axis and Y-axis directions are unique, and the combined coordinates are also unique. If there are two touches on the touch screen and the two points are not in the same X direction or the same Y direction, then There are two projections in the X and Y directions respectively, then 4 coordinates are combined. Obviously, only two coordinates are real, and the other two are commonly known as “ghost points”. Therefore, self-capacitance screens cannot achieve true multi-touch.
The mutual capacitance screen also uses ITO to make horizontal electrodes and vertical electrodes on the glass surface. The difference between it and the self-capacitance screen is that a capacitor will be formed where the two groups of electrodes intersect, that is, the two groups of electrodes constitute the two poles of the capacitor respectively. When a finger touches the capacitive screen, the coupling between the two electrodes near the touch point is affected, thereby changing the capacitance between the two electrodes. When the mutual capacitance is detected, the horizontal electrodes send out excitation signals in turn, and all the vertical electrodes receive signals simultaneously, so that the capacitance values of all the intersections of the horizontal and vertical electrodes can be obtained, that is, the capacitance of the entire two-dimensional plane of the touch screen. According to the two-dimensional capacitance change data of the touch screen, the coordinates of each touch point can be calculated. Therefore, even if there are multiple touch points on the screen, the real coordinates of each touch point can be calculated. [5]
The advantage of the mutual capacitive screen is that there are fewer wirings, and the difference between multiple contacts can be recognized and distinguished at the same time. The self-capacitive screen can also sense multiple contacts, but because the signal itself is fuzzy, it cannot be distinguished. In addition, the sensing scheme of the mutual capacitance screen has the advantages of high speed and low power consumption, because it can measure all nodes on a driving line at the same time, so the number of acquisition cycles can be reduced by 50%. This two-electrode structure has the function of self-shielding external noise, which can improve the signal stability at a certain power level. [5]
In any case, the touch location is determined by measuring the distribution of signal changes between the X and Y electrodes, and then using mathematical algorithms to process these changed signal levels to determine the XY coordinates of the touch point.

Structure and composition

basic structure
The basic structure of the capacitive touch screen is: the substrate is a single-layer plexiglass, a layer of transparent conductive film is evenly forged on the inner and outer surfaces of the plexiglass, and a long and narrow cone is placed on the four corners of the transparent conductive film on the outer surface. electrode. Its working principle is: when the finger touches the capacitive touch screen, the high-frequency signal is connected to the working surface. At this time, the finger and the working surface of the touch screen form a coupling capacitance, which is equivalent to a conductor. Because there is a high-frequency signal on the working surface, when the finger touches A small current is sucked at the touch point, and this small current flows out from the electrodes on the four corners of the touch screen respectively. The current flowing through the four electrodes is proportional to the straight-line distance from the finger to the four corners. Calculation, the contact point coordinate value can be obtained.
The capacitive touch screen can be simply regarded as a screen composed of four layers of composite screens: the outermost layer is a glass protective layer, followed by a conductive layer, the third layer is a non-conductive glass screen, and the innermost fourth layer Also a conductive layer. The innermost conductive layer is the shielding layer, which plays the role of shielding the internal electrical signals. The middle conductive layer is the key part of the entire touch screen. There are direct leads on the four corners or four sides, which are responsible for the detection of the touch point position. [3]
The top cover is either tempered glass or polyethylene terephthalate (PET). The advantage of PET is that the touch screen can be made thinner, and on the other hand, it is cheaper than the existing plastic and glass materials. The insulating layer is glass (0.4-1mm), organic film (10-100um), adhesive, and air layer. The most important layer is the indium tin oxide (ITO) layer. The typical thickness of ITO is 50-100nm, and its sheet resistance is about 100-300 ohms. The three-dimensional structure of the ITO process has a great influence on the capacitive touch screen, which is directly related to the two important capacitive parameters of the touch screen: the inductive capacitance (finger and the upper layer of ITO) and the parasitic capacitance (between the upper and lower layers of ITO, and between the lower layer of ITO and the display screen). between).
The structure of the capacitive touch screen is mainly to coat a transparent film body layer on the glass screen, and then add a protective glass outside the conductor layer. The double glass design can completely protect the conductor layer and the sensor, and the light transmittance is higher. Better support for multi-touch.

The capacitive touch screen is coated with long and narrow electrodes on the four sides of the touch screen, forming a low-voltage alternating current electric field in the conductor. When touching the screen, due to the electric field of the human body, a coupling capacitor will be formed between the finger and the conductor layer, and the current from the four electrodes will flow to the contact, and the current strength is inversely proportional to the distance from the finger to the electrode, the controller behind the touch screen will It will calculate the proportion and strength of the current, and accurately calculate the position of the touch point. The double glass of the capacitive touch screen can not only protect the conductors and sensors, but also effectively prevent external environmental factors from affecting the touch screen. Even if the screen is stained with dirt, dust or oil, the capacitive touch screen can still accurately calculate the touch position.
Since the capacitance varies with the contact area and the dielectric of the medium, its stability is poor, and drift often occurs. This kind of touch screen is suitable for the debugging stage of system development.

Technical indicators

Accuracy: 99% accuracy. [13] 
Material: Completely scratch-resistant glass material ( Mohs hardness 7H), not easy to be scratched and worn by sharp objects, and not affected by common pollution sources, such as water, fire, radiation , static electricity , dust or oil, etc. It has the eye protection function of goggles .
Sensitivity: less than two ounces of force can be sensed, less than 3ms fast response.
Clarity: Three finishes (Polish, Etch, Industrial) to choose from. The MTBF of the SMT controller is greater than 572,600 hours (per MILHANDBOOK-217-F1).
Touch life: Any point can withstand more than 50 million touches, and the cursor will not drift after one calibration.
Capacitive touch technology is a touch technology that utilizes changes in capacitance generated when a finger approaches a capacitive touch panel. Capacitive touch has two important capacitance parameters, one is the inductive capacitance between the finger and the upper sensing material (such as ITO), and the other is between the sensing material (such as the upper and lower layers of ITO) or between the sensing material and the optical panel Parasitic capacitance between (eg ITO and LCD)

Parasitic capacitance will be generated between conductors, and when the finger conductors are close to the sensing conductors of different voltages, the induced capacitance changes will also be generated. Capacitance sensing effect is how to detect small changes in sensing capacitance of 0.1 to 2 pF units under a large parasitic capacitance value (30 pico Farad; pF). Capacitive touch technology is relatively stable and highly reliable. Due to the characteristic that the human body is a capacitive body, the change in capacitance generated when touching the touch panel achieves the effect of sensing the touch. Christopher Ard, director of marketing at Atmel, points out that sensor designs can be single-sided ITO graphics for minimal functional interfaces, such as a single touch point for applications such as large virtual buttons, sliders , etc., although a more common implementation is a two-layer design (separately X and Y layers), which requires more complex performance and precision.

Data processing

After the capacitive touch screen receives the touch signal, it converts the touch data into electrical pulses and transmits it to the touch screen control IC for processing. The signal is amplified by a low noise amplifier (LNA), then converted and demodulated by analog to digital, and finally sent to a DSP for data processing.
A capacitive touch screen generally has M+N (M columns and N rows) physical capacitive touch sensors. The M+N interlaced sensors form MN capacitive sensing points, and when the user’s finger approaches the touch screen, its capacitance will change accordingly. The spacing of the sensors (that is, the distance between adjacent rows or columns) is usually about a few millimeters, and this spacing determines the physical resolution MN of the touch screen. [5]
The coordinate system between the capacitive touch screen module and the LCD module is completely different. The pixel coordinates of the LCD module are generally determined by its resolution. For example, a WVGA screen has a resolution of 800*480, which means that there are 800 lines, and each line has 480 RGB pixels. Thus, a specific position can be determined by the pixel points (x, y) in the X and Y directions. The capacitive touch screen module determines the coordinate system according to its original physical size in the X and Y directions. There must be a reasonable mapping method between the two coordinate systems to ensure the correctness of input and output operations. [5]
Therefore, the DSP processor of the touch screen control IC has to perform pixel mapping conversion between the capacitive touch screen module and the LCD module on the obtained data, so as to ensure that the touch point sensed by the user on the touch screen is the point pointed by the user.
In addition, in order to maintain the stability of the touch coordinates, the touch screen control IC needs to further process the jitter of the touch point, including the jitter of the finger and the noise of the capacitance data, and change the filter coefficient of the low-pass filter according to the change of the coordinates to realize the smoothing of the coordinates. deal with. [5]
Finally, software is used to analyze the data to determine what function each touch is intended for, before passing it on to the host computer. This process involves determining the size, shape and location of the touched area on the screen. If necessary, the processor organizes similar touches into groups. If the user moves the finger, the processor calculates the difference between the beginning and end of the user’s touch.

Advantages and disadvantages

Advantage
Capacitive touchscreens require only touch, not pressure, to generate a signal.
Capacitive touchscreens require only one or no calibration after production, while resistive technology requires regular calibration.
The lifespan of the capacitive solution is longer because the components in the capacitive touch screen do not require any movement. In resistive touch screens , the upper ITO film needs to be thin enough to be flexible enough to bend down to contact the underlying ITO film.
Capacitive technology is superior to resistive technology in terms of optical loss and system power consumption.
The choice of capacitive or resistive technology depends largely on the object touching the screen. If it is finger touch, capacitive touch screen is a better choice. If you need a stylus , whether plastic or metal, a resistive touchscreen will do the trick. Capacitive touchscreens can also use a stylus, but a special stylus is required.
The surface capacitive type can be used for large-size touch screens, and the compatibility is low, but it cannot support gesture recognition at present: the inductive capacitive type is mainly used for small and medium-sized touch screens, and can support gesture recognition.
Capacitive technology is wear-resistant, has a long life, and has low maintenance costs when used by users, so the overall operating expenses of the manufacturer can be further reduced.
Capacitive touchscreens are multi-touch capable, and are less responsive and less prone to wear than resistive touchscreens.

Shortcoming

The light transmittance and clarity of the capacitive touch screen are better than that of the four-wire resistive screen, and of course it cannot be compared with the surface acoustic wave screen and the five-wire resistive screen. The capacitive screen is seriously reflective, and the four-layer composite touch screen of capacitive technology has uneven light transmittance for each wavelength of light, and there is a problem of color distortion. Due to the reflection of light between the layers, the image characters are also blurred.
Current: In principle, the capacitive screen uses the human body as an electrode of a capacitor element. When there is a conductor close to the interlayer ITO working surface coupled with a capacitor of sufficient capacity, the current flowing away is enough to cause the capacitive screen to fail. Malfunction.
Although the capacitance value is inversely proportional to the distance between the electrodes, it is proportional to the relative area, and is also related to the insulation coefficient of the medium. Therefore, when a larger area of the palm or a hand-held conductor is close to the capacitive screen instead of touching it, it can cause the capacitive screen to malfunction. In wet weather, this situation is particularly serious. Hold the monitor with your hand and your palm is 7 cm close to the monitor. Within 15 cm or within 15 cm of the body close to the display can cause the capacitive screen to malfunction. Another disadvantage of capacitive screens is that they do not respond when touched with a gloved hand or hold a non-conductive object because of the addition of a more insulating medium.
Drift: The main disadvantage of the capacitive screen is drift: when the ambient temperature and humidity change, the ambient electric field changes, which will cause the capacitive screen to drift, resulting in inaccuracy. For example: the temperature rise of the display after the power is turned on will cause drift: when the user touches the screen, the other hand or the side of the body will drift close to the display; the larger object near the capacitive touch screen will drift after being moved, and if the user touches it, if someone comes around to watch it It will also cause drift; the reason for the drift of the capacitive screen belongs to the congenital deficiency of technology. Although the environmental potential surface (including the user’s body) is far away from the capacitive touch screen, it is much larger than the area of the finger, which directly affects the touch position. Determination.
Others: In addition, many relationships that should be linear in theory are actually non-linear, such as: people with different body weights or different degrees of finger wetness draw different total currents, and the changes in total currents and the four component currents The change is a non-linear relationship. The custom polar coordinate system of the four corners used by the capacitive touch screen has no origin on the coordinates, and the controller cannot detect and recover after drifting . The calculation process from the value of the divided flow to the X and Y coordinate values of the touch point on the rectangular coordinate system is complicated. Since there is no origin, the drift of capacitive screens is cumulative and often requires calibration on the job site. The outermost silica protective glass of the capacitive touch screen is very scratch-resistant, but it is afraid of being knocked by nails or hard objects. If a small hole is knocked out, the interlayer ITO will be damaged , whether it is the interlayer ITO or the installation and transportation process. The ITO layer on the inner surface, the capacitive screen will not work properly.

Notes and FAQs

1. If the user is using a capacitive touch screen , it is recommended that when the user uses it for the first time, first install the drivers required by the capacitive touch screen correctly according to the requirements of the relevant instructions , and then click the “Start”/”Program”/”Microtouch Touchware” to run the screen calibration program, after the calibration is completed, the system will automatically store the calibrated data in the controller’s register, and after restarting the system, there is no need to calibrate the screen again.
2. If you change the display resolution or display mode of the touch screen when operating the capacitive touch screen in the middle, or adjust the refresh frequency of the touch screen controller by yourself, if you feel that the cursor and the touch point do not correspond, you must reset the touch screen. The system performs a calibration operation.
3. In order to ensure the normal operation of the touch screen system, in addition to ensuring the correct installation of the system software , it is also necessary to remember not to install two or more touch screen drivers on one host, which will easily cause the system to run. conflict, so that the touch screen system cannot be used normally.
4. When using a resistive touch screen , if the cursor does not move or can only move in a local area, the user can check whether the touch area of the touch screen is always pressed by other touch objects. If it is pressed, it is equivalent to a certain point being touched all the time, so the coordinate position fed back to the controller will be inaccurate.
5. As mentioned earlier, once the system changes the display resolution, adjusts the screen size and the first installation, there will be click inaccuracy or drift, you need to start the positioning program that comes with the application to reposition, but in When positioning, it is best to use a relatively thin pen or fingertip for positioning, which is more accurate.
6. The working environment of the surface acoustic wave touch screen is relatively high. It must work in a clean and dust-free environment, and the dust on the surface of the touch screen should be cleaned regularly. Otherwise, the dust in the air will cover the surrounding area of the touch screen. When reflected on streaks or transducers, the correct positioning of the system can be affected.
7. Do not let water droplets or other soft things stick to the surface of the touch screen, otherwise the touch screen is easy to mistakenly think that the surface acoustic wave screen is inaccurate due to hand touch. In addition, when removing the dirt on the touch screen surface, the user can carefully wipe the touch screen surface from the center of the screen with a soft dry cloth or detergent, or use a dry soft cloth dipped in denatured alcohol or glass cleaning solution to clean the touch screen surface.
8. If you touch the surface acoustic wave touch screen with your hand or other touch objects, the touch screen responds very slowly, which means that the touch screen system is outdated, the internal clock frequency is too low, or there are water droplets moving on the surface of the touch screen. In order to restore the quick response of the touch screen, the system must be replaced or upgraded, or the water droplets on the surface of the touch screen must be wiped off with a rag.
9. The touch screen generally uses the serial port for signal transmission, and takes the signal from the PS/2 port, while the TPS screen directly takes the power from the host power supply. If the indicator light is not on, it means that no signal is received, and the PS/2 cable on the control box may be broken. If the light is on, but still does not flash, the control box is broken, so users must replace the control box. If the replacement of the control box still does not work, it is possible that the screen is pressed too tightly, and the screws around it need to be loosened a little, because the touch screen is made of special materials, and it should not be easily damaged. If the serial port is bad or disabled, the driver cannot be installed because the serial port will be automatically searched for when the driver is installed. Even if it can be installed, the mouse does not move or cannot be positioned. It is best not to use the serial port mouse to judge the quality of the serial port. Maybe the 9 serial ports are used in different ways for them. If the screen is pressed, or the ground wire is not properly connected, it will lead to inability to locate. If some areas cannot be clicked or the response is slow, it may be affected by dust, and the case needs to be disassembled to remove the dust.
10. When a finger touches a certain position of the capacitive touch screen, if the touch screen does not respond, it is very likely that the touch position is not accurate, and of course the cursor cannot be positioned correctly. If the touch area is pressed by the cabinet casing, the user can increase the distance between the cabinet and the monitor screen. If the monitor casing presses the touch area, the user can try to loosen the screws of the monitor casing.