What is a touch panel? Unveiling the Mystery of Touchscreens
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Technical points of touch panel:
From a technical principle perspective, a touch screen is a transparent absolute positioning system, and first of all, it must be ensured to be transparent; Secondly, it is an absolute coordinate system where the finger can touch it, unlike a mouse that requires a cursor for relative positioning, making it easy to distract attention as you need to constantly monitor where the cursor is.
The structure is usually covered with a pressure plate on a semi reflective liquid crystal panel (ITO transparent conductive electrode), which has high sensitivity to pressure. When an object is pressed on it, a current signal is generated and the pressure source position is determined, and it can be dynamically tracked. This is the on cell technology reported by our media. Nowadays, In cell Touch touch components are also integrated into display panels, allowing the panels to have touch functionality without the need for additional bonding and assembly with the touch panel to achieve touch effects and applications. This is mainly being researched by Apple.
Next, we will mainly introduce the differences and principles of different touch technologies from transparency and positioning methods.
Classification and principles of touch panels:
From the perspective of technical principles, touch screens 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 exited the historical stage; Infrared technology touch screens are inexpensive, but their outer frames are fragile and prone to light interference, resulting in distortion on curved surfaces; Surface acoustic wave touch screens almost solve all the various defects of touch screens, are clear and not easily damaged, suitable for various occasions. The fatal disadvantage is that if there are water droplets and dust on the screen surface, the touch screen will become sluggish or even not work, so it is difficult to popularize and use. Let's mainly talk about resistive and capacitive screens below.
Resistive: Great force!
The resistive touch panel is mainly composed of two thin film substrates coated with ITO (indium tin oxide) on one side. A transparent elastic insulating spacer dot needs to be filled between the upper and lower plates to separate them. As shown in the figure, the lower plate must be a rigid thick glass to prevent deformation, while the upper plate needs to induce external force to produce deformation, so glass or plastic needs to be broken.
During normal operation, the upper and lower plates are connected to voltage and in a disconnected state. When external force is applied, the upper plate deforms and makes contact with the lower plate, resulting in a voltage change. This voltage change can be used to accurately measure the coordinates of the touch point (because when the upper and lower plates are touched and make contact, the resistance of the upper and lower plates changes from the original overall resistance to a divided resistance, and the divided resistance value is proportional to its distance from the edge to calculate the X and Y coordinates). So the accuracy of resistive touch screens mainly depends on the conversion accuracy of the coordinate voltage, which is very dependent on the accuracy of the A/D converter (voltage sensitivity of the force level).
Because the resistive screen is controlled by pressure, it does not necessarily need to be controlled by hand. Pen, credit card, etc. can be operated, even with a cover, and it is isolated from the outside world, so it has the advantage of dust and dirt prevention; However, if touched too lightly, the resistive screen will not respond and needs to be lightly poked. Resistive screens are low-cost and have a low technical threshold. Moreover, when operating a resistive touch screen, it requires a light tap, which makes it easy to break, and the sensitivity is not very good. Drawing and writing are not smooth.
Capacitive: Enjoy a smooth and comfortable ride!
However, what truly brought the trend of smartphones was the capacitive touch screen, which is composed of a glass substrate coated with conductive film on both sides and covered with a thin SiO2 dielectric layer on the upper plate. As shown in the figure, the upper electrode is used to form a flat capacitor with the human body (ground) to sense changes in capacitance, while the lower electrode is used to shield external signal interference.
When working, the upper transparent electrode needs to be connected to voltage and four electrodes need to be drawn out at the four corners. Therefore, when a finger touches the SiO2 layer on top, due to the conductivity of the human body, sufficient coupling capacitance is generated between the human body and the upper transparent electrode. The touch position coordinates are calculated based on the changes in capacitance values measured at the four corners (or surroundings) (the closer to the touch position, the larger the capacitance).
However, this type of surface capacitive touch still cannot meet the current trend of multi touch. To achieve multi touch, a new technology called Projected Capacitive Touch must be used. Its main change is to lay the sensing electrodes on the surface in one or two layers and pattern them (mainly diamond shaped), with one layer responsible for the X direction and the other for the Y direction.
Then locate by the changes in electrode capacitance in the X and Y directions.
Due to the current mainstream being multi touch, I will briefly explain its evolution process. There are two main types of projected capacitance for multi touch: self capacitive and mutual capacitive.
Self capacitance directly scans the capacitance of each X and Y electrode, so when two touch points are reached, two additional virtual points (Ghost Points) will be generated, as shown in the figure.
The left side is a schematic diagram of two electrode patterns (mostly diamond shaped), which only requires one layer of ITO to form X and Y electrodes through photolithography. On the right is the schematic diagram.
From the schematic diagram, when touching (X2, Y0) and (X1, Y3) simultaneously, two additional points (X1, Y0) and (X2, Y3) will be added due to measuring the capacitance of the four electrodes. This is Ghost Points and can only be solved by software.
Although self capacitors have Ghost Points issues, their precise positioning and high sensitivity make them ideal for single layer ITO films.
However, as the size increases (>15 inches), the increase in points leads to an increase in pins, resulting in high costs. Moreover, with too many points, the middle circuit cannot be formed, and the ITO must be made thinner, resulting in increased resistance and longer scanning time.
Although this may seem like no advantage, it seems that Apple phones are now using self capacitive touch technology, which should have all broken through.
And mutual capacitive or trans capacitive requires two layers of ITO film, which separate the X and Y electrodes at each node through a special structure, so that it scans the intersection capacitance instead of the electrode capacitance. However, the contact between these two layers of ITO at the intersection must be separated, and MEMS technology is needed to connect them like an overpass.
Both self capacitance and mutual capacitance rely on conducting capacitance from the human body to the electrode, so both technologies are called charge transfer capacitive touch.
The advantage of capacitive touch is its fast speed, which allows for sliding without the need for poking. However, it can only be controlled with conductive objects, and it also has a disadvantage that if the touch area is relatively large (palm), you may have actions before you touch it, because the large area and coupling capacitance trigger the screen, so it is sensitive to changes in the electric field caused by external electric fields or temperature and humidity. But it is a layer of glass plate structure, so its transmittance can be over 90% higher than that of resistive type.
However, both resistive and capacitive touch screens are difficult to achieve a uniform electric field, so they can only be used for panel sizes below 20 inches. If you want to use a large screen touch, you must use wave type touch technology (mainly surface acoustic waves or infrared waves). It mainly installs infrared or acoustic wave transmitters/receivers at the corners or edges. When the touch blocks the sound waves or infrared waves, the corresponding receiver cannot receive the signal, and the coordinates can be determined. This type of touch screen is afraid of dirt, dust, and oil, too delicate, and is easily affected by environmental fluctuations.
Each type of touch screen has its own advantages and disadvantages. To understand which type of touch screen is suitable for which occasion, the key is to understand the working principle and characteristics of each type of touch screen technology.
Circuit part of touch panel:
Most of the space was spent introducing the principle and structure of the sensing module of the touch panel, but like Sensor (which is also a type of Sensor), if there is a sensing part, there must be a circuit part. The main responsibilities of the touch circuit part are signal detection, coordinate positioning, and gesture recognition (sliding/zooming/clicking)
For MCU circuits, the most important circuit units are ADC (which is essential for all sensors), followed by Scan Control and DSP (signal processing). Scanning circuits require clock signals, so Timer is needed. Gesture recognition is achieved through a unit called Finger Tracking, and ultimately, the user configure code needs to be saved using EEPROM or Flash.
There are two main difficulties in design: 1) the problem of high resistance increasing capacitance, and 2) noise coupling. The former is mainly due to the increasing size of the screen, which causes the ITO electrodes to become longer and the resistance to increase. In addition, the longer electrodes lead to an increasing capacitance area, resulting in an increase in capacitance. The final problem is the extension of RC delay. The solution is either to increase the voltage to accelerate the scan or to replace the metal wiring (Ag). The latter (Noise coupling) is mainly due to the increasing size of the panel, which is easily affected by environmental noise interference. Shielding is mainly used to avoid interference, while other interference comes from the switching power supply, which can only be achieved by adding Noise Cancellation before the ADC.