From telephones, office equipment, speakers, digital photo frames, TV control keys, remote controls, GPS systems, car keyless control, to medical monitoring equipment, touch devices everywhere! Every industry, every product type, all kinds Dimensions, every application, and even at every price point, are inseparable from touch technology. It can be said that touch technology is everywhere.

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In fact, if there is an LCD or keyboard on the product, the designer may need to consider how to design a product that uses touch technology. Unfortunately for designers, when designing touch screens, there are many different solutions, a wide range of performances, and of course different design considerations. So now is the time to understand the technology and evaluate your product line. Only in this way can we become the market leader, and good design is the starting point of design.

In-depth analysis of touch technology

Knowing the design needs is the most important first step in touch product design. Many providers on the touchscreen supply chain often offer many different components that are confusing, and more often some providers come together to provide a value chain for end customers. The composition of the touch screen ecosystem is shown in Figure 1. Interestingly, the ecosystem is the same whether it's on the latest laptops or the latest touchscreen phones.

　　

 

Figure 1: Touch screen controller resolution

#1front panel or frame

The front panel or frame is the top layer of the end product. In some products, the frame encloses a transparent cover to protect it from the harsh weather or moisture of the exterior and to prevent the underlying sensor product from being scratched and damaged (see #3). Sometimes, the outermost frame simply covers the top of the touch sensor, in this case just a decoration.

#2Touch Controller

Typically, the touch controller is a small microcontroller chip that sits between the touch sensor and the PC/ or embedded system controller. The chip can be mounted to the controller board inside the system or to a flexible printed circuit (FPC) attached to a glass touch sensor. The touch controller will extract information from the touch sensor and convert it into information that the PC or embedded system controller can understand.

#3 Touch Sensing

The touch screen "sensor" is a transparent glass plate with a touch-responsive surface. The sensor is placed over the LCD so that the touch area of ​​the panel covers the viewable area of ​​the display. There are many different touch sensing technologies on the market today, each using a different method to detect touch input. Basically, these techniques are such that when touched, current is passed through the panel to produce a voltage or signal change. This voltage change will be sensed by the touch controller to determine the touch location on the screen.

#4液晶显示器(LCD)

Most touch screen systems are used on conventional LCDs. The LCD selection method for touch products is basically the same as in conventional systems, including resolution, resolution, refresh rate, cost, and the like. But another major consideration in touch screens is the level of radiation. Since the technology in the touch sensor is based on a small electrical change caused by the touch of the panel, an LCD capable of radiating a lot of electrical noise is a difficulty in design. Before selecting an LCD for use in a touch system, it should be negotiated with the touch sensor provider.

#5系统软件

The touch screen driver software can come from the original manufacturer (such as the embedded OS in the mobile phone), or it can be installed later (like adding a touch screen to a traditional PC). The software should enable the touch screen to work with the system controller. It tells the product's operating system how to resolve touch event information from the touch controller. In PC-type applications, most touchscreen drivers work like a mouse. This makes the touch screen very similar to the continuous mouse click on the same position on the screen. In an embedded system, the embedded control driver must compare the information that appears on the screen with the location where the touch was received.
Three touch technologies

Resistive touch technology: Resistive touch technology is the most commonly used touch screen technology. It is used for high-traffic flow applications and is immune to water droplets and other residues on the screen. Resistive touch screens are often the lowest cost solution. Since it is a reaction to stress, you can touch it with your fingers, a gloved hand, a touch pen, or other object like a credit card.

Surface capacitive touch technology: Surface capacitive touch technology provides display clarity that is much clearer than the plastic film typically used in resistive touch. In the surface capacitive display, sensors located at the four corners of the display detect changes in capacitance due to touch. Such touch screens can be touch activated with a finger or other capacitive object.

Protective capacitive touch: Protective capacitive touch is a technology that has only recently entered the market. This technology also provides excellent light transmission, but it also has some advantages over surface capacitive touches. Projected capacitive touch does not require position calibration and provides much higher positional accuracy. Another exciting thing about projected capacitive touch is that it can support multi-touch at the same time.

How touch screen works

We'll take a closer look at the two most common touchscreen technologies. The most widely used technology is resistive touch. Most people may have used such resistive touch technology in ATMs on banks, in credit card checkers in many shopping malls, and even in order to order a meal order in a restaurant. The projection type capacitive touch screen has not been used in a wide range, but has a rapid development momentum. Many mobile phones and portable music players with projection-type capacitive interfaces are on the market. Both resistive and capacitive technologies have a robust electrical component that utilizes ITO (indium tin oxide, transparent conductor), both of which are used for long periods of time.

The resistive touch screen includes a flexible top layer followed by a layer of ITO, an air gap, and then another layer of ITO. The panel has four wires attached to the ITO layer: one on each of the left and right sides of the "X" layer, and one at the top and bottom of the "Y" layer.

A touch is detected when the flexible top layer is pressed into contact with the underlying layer. The position of the touch is measured in two steps: first, "X Right" is driven to a known voltage, and "X Left" is driven to ground to read the voltage from the Y sensor. This provides the X coordinate. Repeat this process for another axis to determine the exact finger position.

The resistive touch screen is available in 5-wire and 8-wire versions. The 5-wire type replaces the uppermost ITO layer with a more durable low resistance "conductor layer". The 8-line panel achieves higher resolution by better calibrating the panel characteristics.

There are several disadvantages to resistive technology. The flexible top layer has only 75%-80% transmittance, and there are more sources of error in the resistive touch screen measurement process. If the ITO layers are inconsistent, the resistance will not change linearly in the sensing range. A measurement voltage accuracy of 10-12 bits is required, which is difficult in many environments. In order to align touch points with underlying LCD images, many existing resistive touch screens require periodic calibration.

Conversely, the projected capacitive touch screen has no moving parts. There is only ITO and a glass plate with a transmittance of almost 100% between the LCD and the user. Projection-type capacitive sensing hardware consists of a glass top layer (see Figure 2), an X sensor array, a layer of insulating glass, and a Y sensor array on the glass substrate. The panel is connected to each of the X and Y sensors, so the 5 x 6 panel has a total of 11 wires (as shown in Figure 3 below), while the 10 x 14 panel has 24 sensor wires.

Figure 2: Stacked layers for “resistive screen” (left) and “capacitive screen” (right)

When a finger or other object is brought close to the screen, a capacitance is created between the sensor and the finger. Although this capacitance is relatively small relative to other capacitors in the system (approximately 0.5pF in 20pF), it can be measured using centralized techniques. One such technology is the use of Cypress Semiconductor's PSoC device, known as CSD. It involves charging the capacitor quickly and then measuring the discharge time to a discharge resistor.

The purpose of designing a projected capacitive sensor array is to enable a finger to interact with more than one X sensor and more than one Y sensor at the same time (see Figure 3). This is software that can interpolate to determine the exact location of a finger very accurately. For example, if the signal strengths sensed by sensors 1, 2, and 3 are 3, 10, and 7, respectively, the center of the finger should be at (1*3+2*10+7*3) / (3+10+7) = 2.2.

Figure 3: The signal strength of the row and column sensors determines the location of the touch

Because the projected capacitive panel has many sensors, it can be combined with other technologies to detect multiple fingers simultaneously. In fact, the projected capacitor can detect up to 10 fingers simultaneously. It is possible to achieve exciting new applications based on multiple finger presses. Imagine, can you play the piano on your mobile phone? How about playing games with multiple fingers on the PDA?

There is no doubt that the touch screen has an excellent appearance. They began to define a new type of user interface and industry design standards that are widely accepted worldwide. From a variety of devices, from heart rate monitors to the latest all-in-one printers, touch screens are rapidly becoming a technical design standard. But beyond its beautiful appearance, the touch screen offers unmatched security, weather resistance, wear resistance, and the ability to take advantage of new touch technologies like multi-touch to open up a whole new market. Many types of products can be implemented using touch technology, so designers must have an in-depth understanding of the ecosystem of the technology and the availability of the technology currently in use.

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