Resistive touch screen

Resistive touch screen

The resistive touch screen uses pressure sensing for control. The main part is a resistive film screen that fits well with the surface of the display. This is a multi-layer composite film. It uses a glass or hard plastic plate as the base layer and is coated with a transparent metal oxide (ITO) conductive layer. Layer, covered with a layer of hardened outer surface, smooth and scratch-resistant plastic layer (the inner surface is also coated with a layer of ITO coating), there are many small (about 1/1000 inch) transparent spacing points between them Separate and insulate the two ITO conductive layers.

When a finger touches the screen, the two conductive layers that are usually insulated from each other make a contact at the touch point. Because one conductive layer is connected to a 5V uniform voltage field in the Y-axis direction, the voltage of the detection layer changes from zero to zero. Non-zero, after the controller detects this connection, it performs A/D conversion and compares the voltage value obtained with 5V to get the Y-axis coordinate of the touch point, and the X-axis coordinate is obtained by the same reason. The most basic principle common to all resistive technology touch screens.

The key to resistive touch screens lies in material technology. Commonly used transparent conductive coating materials are:

① ITO, indium oxide, weak electrical conductor, the characteristic is that when the thickness drops below 1800 angstroms (angstroms = 10-10 meters), it suddenly becomes transparent, the light transmittance is 80%, and the light transmittance decreases when it is thinner When the thickness reaches 300 angstroms, it rises to 80%. ITO is the main material used in all resistance technology touch screens and capacitive technology touch screens. In fact, the working surface of resistance and capacitance technology touch screens is ITO coating.

② Nickel-gold coating, the outer conductive layer of the five-wire resistive touch screen uses a nickel-gold coating material with good ductility. Due to frequent touches, the purpose of using a nickel-gold material with good ductility is to extend the service life. However, the process cost is relatively high. Although the nickel-gold conductive layer has good ductility, it can only be used as a transparent conductor. It is not suitable for the working surface of the resistive touch screen because of its high conductivity and the metal is not easy to be very uniform in thickness. It is not suitable for the voltage distribution layer and can only be used as a probe. Floor.

1. Five-wire resistive touch screen:

The base layer of the five-wire resistive technology touch screen adds the voltage field in two directions to the conductive working surface of the glass through a precision resistance network. We can simply understand it as the time-sharing of the voltage field in the two directions.

The work is added to the same working surface, and the outer nickel-gold conductive layer is only used as a pure conductor. After touching, the X and Y axis voltage values of the inner ITO contact points are time-divisionally detected to measure the position of the touch point. The five-wire resistive touch screen requires four leads on the inner layer of ITO, and only one conductor on the outer layer. There are 5 lead wires for the touch screen. Another proprietary technology of the five-wire resistive touch screen is to correct the linearity problem of the inner layer of ITO through a precise resistor network: the uneven thickness of the conductive coating may cause uneven voltage distribution.

Resistive screen performance characteristics:

① They are a working environment completely isolated from the outside world, not afraid of dust, water vapor and oil

②It can be touched with any object, can be used for writing and drawing, which is their big advantage

③The accuracy of the resistive touch screen only depends on the accuracy of the A/D conversion, so it can easily reach 4096*4096• In comparison, the five-wire resistance is better than the four-wire resistance in ensuring the resolution accuracy, but the cost is high. So the price is very high.

Improvement of the five-wire resistive touch screen:

First of all, the A side of the five-wire resistive touch screen is conductive glass instead of a conductive coating. The process of conductive glass greatly improves the life of the A side and can improve the light transmittance. Secondly, the five-wire resistive touch screen delegates the tasks of the working surface to the long-life A side, while the B side is only used as a conductor, and uses a nickel-gold transparent conductive layer with good ductility and low resistivity. Therefore, the B side Life expectancy is also greatly improved. Another proprietary technology of the five-wire resistive touch screen is to correct the linearity problem of the A side through a precision resistance network: due to the inevitable thickness unevenness of the process engineering, the voltage field is unevenly distributed, and the precision resistance network flows during operation Pass most of the current, so it can compensate the possible linear distortion of the working surface. The five-wire resistive touch screen is currently the best resistive technology touch screen, which is most suitable for military, medical, and industrial control fields.

2. Four-wire resistive touch screen

The touch screen is attached to the surface of the display and is used in conjunction with the display. If the coordinate position of the touch point on the screen can be measured, the toucher's intention can be obtained according to the display content or icons of the corresponding coordinate point on the display screen. Among them, resistive touch screens are used more in embedded systems. The resistive touch screen is a 4-layer transparent composite film screen. The bottom layer is a base layer made of glass or organic glass. The top layer is a plastic layer whose outer surface is hardened to make it smooth and scratch-resistant. In the middle are two metal conductive layers. On the base layer and the inner surface of the plastic layer, there are many small transparent isolation points between the two conductive layers to separate them. When a finger touches the screen, the two conductive layers touch at the touch point. The two metal conductive layers of the touch screen are the two working surfaces of the touch screen. A silver glue is coated on both ends of each working surface, which is called a pair of electrodes on the working surface. Voltage, a uniform and continuous parallel voltage distribution will be formed on the working surface. When a certain voltage is applied to the electrode pair in the X direction and no voltage is applied to the electrode pair in the Y direction, in the X parallel voltage field, the voltage value at the contact can be reflected on the Y+ (or Y-) electrode , By measuring the voltage of the Y+ electrode to the ground, the X coordinate value of the contact can be known. Similarly, when a voltage is applied to the Y electrode pair and no voltage is applied to the X electrode pair, the Y coordinate of the contact can be obtained by measuring the voltage of the X+ electrode.

Defects of four-wire resistive touch screen:

The B side of the resistive touch screen has to be touched frequently. The B side of the four-wire resistive touch screen uses ITO. We know that ITO is a very thin oxide metal. During use, small cracks will quickly occur. Once the cracks occur, The current flowing through the place is forced to go around the crack, and the evenly distributed voltage is destroyed accordingly, and the touch screen is damaged, which shows that the cracks are not accurate.