Working principle and application of resistive touch panel

General Structure of a Resistive Touch Panel

The resistive touch panel is a resistive film screen tightly attached to the surface of the display. It is composed of multiple layers of composite films. A piece of glass or organic glass serves as the lower substrate, with its upper surface coated with an ITO transparent conductive layer. The lower substrate is covered by a plastic sheet or glass sheet, whose lower surface is also coated with an ITO transparent conductive layer, while its upper surface (the outer surface) is hardened, smooth, and scratch-resistant. Between the two conductive layers, numerous small transparent insulating isolation points keep them separated. When a finger touches the screen, the two conductive layers come into contact at the touch point. In a resistive touch panel, the two-layer ITO film working surface must remain intact. A strip of silver conductive glue is applied to both sides of each working surface, with a 5V voltage applied to one end and 0V to the other, creating a uniform and continuous voltage distribution across the working surface.

The four-wire resistive touch panel is an early resistive touch screen. It consists of an upper diaphragm (0.2mm thick glass or plastic film) and a lower substrate (0.7~1.1mm thick glass). The inner surface between the upper and lower sheets is coated with ITO film. The ITO film is used as a uniform resistance layer, and the sheet resistance is about 200~300/port. The lower substrate uses a photolithography process to form dense and evenly arranged small bumps on the ITO film layer. The purpose is to separate the ITO film layer into several small intervals to improve the resolution of the touch screen.

Use screen printing to print two silver electrodes on both lateral sides of the lower substrate ITO film layer and on the vertical sides of the upper diaphragm ITO film layer. The sheet resistance should be more than 3000 times smaller than that of ITO. There is usually a gap of 0.1~0.3mm between the upper and lower pieces. A spherical gasket is installed between the gaps. The height of the gasket is 5~10um. The distribution diameter of the spherical gasket should be less than 50um. The four electrodes are led out through flexible cables, and the upper and lower pieces are pressed together with sealant or double-sided tape to form a complete touch screen. By attaching the touch screen to the LCD surface and connecting the corresponding control circuit and the interface circuit between the touch screen and LCD, it can be used in various products.

The following picture shows the  Hot Display four-wire resistive touch screen: Figures 1 to 2 show the touch screen and the effect after assembly on the display screen respectively.

Working principle of LCD Display with resistive touch panel 

Resistive touch screens mainly operate and control screen content through the principle of pressure sensing. This type of resistive touch panel uses two layers of highly transparent conductive layers to form a touch screen. When a finger touches the screen, the two conductive layers that are usually insulated from each other come into contact at the touch point position. Because one of the conductive layers is connected to a 5V uniform voltage field in the Y-axis direction, the voltage of the detection layer changes from zero to non-zero. After the controller detects this connection, it performs A/D conversion, and by comparing the obtained voltage value with 5V, the Y-axis coordinate of the touch point can be obtained. Similarly, the X-axis coordinate can be obtained. This is the most basic principle common to all resistive technology touch screens.

When a person's finger presses the touch screen surface, the elastic PET film will bend downward, allowing the upper and lower ITO coatings to contact each other to form a touch point. An ADC is used to detect the voltage at this point to calculate the X and Y axis coordinate values.

The working principle of the four-wire resistive touch screen is as follows:

1. Add a constant voltage Vref to the X+ and X- electrodes, and connect Y+ to a high-impedance ADC.

2. The electric field between the two electrodes is uniformly distributed in the direction from X+ to X-.

3. When the hand touches, the two conductive layers come into contact at the touch point, and the potential of the X layer at the touch point is directed to the ADC connected to the Y layer.

Get the voltage Vx.

4. Through Lx/L=Vx/Vref, the coordinates of the x point can be obtained.

5. In the same way, connect Y+ and Y- to the voltage Vref, the coordinates of the Y-axis can be obtained, and then connect the X+ electrode to the high-impedance ADC to obtain. At the same time, the four-wire resistive touch screen can not only obtain the X/Y coordinates of the contact, but also measure the pressure of the contact.

This is because the greater the pressure, the more complete the contact, and the smaller the resistance. By measuring the resistance, the pressure can be quantified. The voltage value is proportional to the coordinate value, so it needs to be calibrated by calculating whether there is a deviation in the voltage value of the (0, 0) coordinate point.

How to do the Detection for the resistive touch panel？

The four-wire resistive touch panel (also commonly referred to as a four-wire resistive touch screen) is pressed with a finger or a hard object, causing its resistance to change so that the display starts to work. Then, it is very important to detect whether the touch layer is effective before the product is completed. This article introduces the detection method of touch occurrence on a four-wire resistive display.

All resistive touch panel types can detect when a touch occurs by using a weak pull-up resistor to pull one layer up and a strong pull-down resistor to pull the other layer down. If the measured voltage of the pull-up layer is greater than a certain logic threshold, it indicates that there is no touch, and vice versa. The problem with this approach is that the touch screen is a huge capacitor, and you may also need to increase the capacitance of the touch screen leads to filter out the noise introduced by the LCD. A weak pull-up resistor connected to a large capacitor will increase the rise time and may cause spurious touches to be detected.

The four-wire resistive touch panel includes two resistive layers, the ITO layer we talked about earlier. One layer has a vertical bus on the left and right edges of the screen, and the other layer has a horizontal bus on the bottom and top of the screen. As shown below.

The resistances in the X layer and Y layer are evenly distributed, forming a uniform electric field, which is equivalent to the following figure

Therefore, to determine whether a touch occurred and where it occurred, we need to calculate the XY coordinate position. The specific calculation method is divided into two steps:

1. Calculate the X-axis coordinate position

Step 2, calculate the Y-axis coordinate position

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