According to the number of sensing lines on the touch screen, resistive touch screen panels can be further divided into three categories: 4-wire, 5-wire and 8-wire.
① The strip electrodes of the 4-wire touch screen are installed on two different resistance layers (X+ and X- are on the same layer, Y+ and Y- are on the other resistance layer).
② The 5-wire touch screen only has circular electrodes (X+, X-, Y+ and Y-) on the bottom layer. The top layer is used to measure voltage during touch, and the voltage gradient is only applied to the bottom layer.
③ The working principle of 8-wire touch screen is similar to that of 4-wire touch screen, just a reference voltage line to each line is added, so the final number of buses reaches 8. The 4 newly added lines are used to provide reference voltages to the original 4 lines. The 8-wire touch screen adopts the measuring principle of proportional measurement analog-to-digital converter.
Because of the low cost and simple touch sensing algorithm, 4-wire touch screens are widely used in low-end consumer electronics products. 5-wire and 8-wire touch screen panels are mainly used in expensive high-end medical equipment and important industrial controllers.
1. The resistive touch screen panel is a transparent glass plate covered with a touch-responsive film.
2. The resistive touch screen panel consists of two resistive layers (indium tin oxide), with a thin separation layer in the middle.
3. The two film layers of the resistive touch screen panel form a resistive network, which acts as a voltage divider circuit for the touch position detection function.
4. The touch screen will cause a voltage change on the voltage divider composed of the resistor network, and this voltage is used to determine the contact position of the touch screen.
5. The touch screen controller (TSC) converts the captured analog voltage signal into a digital touch coordinate signal. The built-in analog to digital conversion channel acts as a voltmeter to measure the analog voltage.
6. After touching the screen, the touch controller acting as a voltmeter first applies a voltage gradient VDD at the X+ point and a ground voltage GND at the X- point. Then, detect the analog voltage on the Y-axis resistance, and convert the analog voltage into a value, and use the analog-to-digital converter to calculate the X coordinate (Figure 2). In this case, the Y-axis becomes the induction line. Similarly, by applying voltage gradients at the Y+ and Y- points, the Y coordinate can be measured.
7. Some touch controllers also support touch pressure measurement, that is, Z-axis measurement. When measuring the Z-axis coordinate, the voltage gradient is applied on the Y+ axis and X- axis.
There are two main forms of resistive touch sensing: software touch sensing solutions and dedicated touch screen controller chips.
(1) In the software tactile solution, the microcontroller must be responsible for all touch detection and coordinate calculation tasks. The software algorithm based on the microcontroller uses the internal microcontroller to measure the touch position voltage, and perform touch detection functions and coordinate processing functions.
(2) In the dedicated touch screen controller, the controller initiates an interrupt request to detect the touch event to the system host (microcontroller), and outputs digital data representing the touch coordinates. Then the Microprogrammed Control Unit (MCU) reads the digital data and executes the operation commands expected by the customer.
The design method based on MCU calculation parameters requires the main processor to be very fast. Only in this way can frequent touch operations be managed. For fast touch detection applications, this is not a very reliable design because there is no data averaging and touch detection delay functionality. Such kind of design has relatively low detection accuracy. The special touch screen controller chip with data sampling, measurement value averaging, touch detection delay configuration and digital touch coordinate calculation function is the real touch screen controller. Such chips are easy to integrate into product designs and have higher performance.
In the past, due to factors such as cost and technology, the amount of resistive control panel adopted far exceeded that of capacitive touch technology. However, in the recent stage, with the advancement of technology and mass production, the price of capacitive touch screens is continuously falling, and the price gap with resistive touch screen panels is getting smaller and smaller, which makes it increasingly able to compete with resistive touchscreen panels on price.
With the prosperous of Apple products such as iPhone and iPod Touch, not only major manufacturers have begun the research and development of capacitive touch screen products, but also many touch screen manufacturers such as 3M and Synaptics have followed suit, investing in the development and production of capacitive touch screens. It is believed that with the continuous upgrading of competition, capacitive touch screen will have great development in the short term, and capacitive touch screens will not only be used in existing mobile phones, portable video players and other products, but may also be extended to cameras, GPS navigators, game consoles and other products.
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