Principle of four-wire resistive touch screen

Principle of four-wire resistive touch screen

1, structure

The surface touched by the finger is a hard coating to protect the underlying PET layer. The PET layer is a thin, flexible PET film that bends downward when the surface is touched, and enables the two layers of ITO coating below to contact each other and connect the circuit at this point. Between the two ITO layers are some isolation fulcrums about one thousandth of an inch thick to separate the two layers. At the bottom is a transparent hard bottom to support the structure above, usually glass or plastic.

2. Calculation of touch coordinates

The ITO ceramic layer is divided into upper and lower layers, separated by an isolation fulcrum in the middle, the two layers are the X layer and the Y layer. The resistances from X- to X+ and Y- to Y+ on the X layer are evenly distributed. There are two steps when calculating the touch points:

(1) Calculate the Y coordinate, apply the driving voltage V to the Y+ electrode, Y- is grounded, and the chip measures the voltage of the contact point through X+. Since the ITO layer is uniformly conductive, the ratio of the contact voltage to the V voltage is equal to the ratio of the contact Y coordinate to the screen height.

(2). Calculate the X coordinate, apply the driving voltage V on the X+ electrode, ground the X- electrode, and measure the voltage of the contact point as the lead terminal. Since the ITO layer is evenly conductive, the ratio of the contact voltage to the Vdrive voltage is equal to the contact X coordinate Ratio to screen width. The measured voltage is usually converted into a digital signal by ADC, and then simple processing can be used as a coordinate to judge the actual position of the contact.

3, measure the pressure of the contact

The four-wire resistive touch screen can not only get the X/Y coordinates of the contact, but also measure the pressure of the contact. This is because after pressing the touch screen, the upper and lower ITO layers are in contact, and there is actually resistance on the contact. The larger the contact, the smaller the resistance. The pressure can be quantified by measuring the resistance.

The advantage of the four-wire/eight-wire resistive touch screen is that it can not only calculate the horizontal X, Y coordinates, but also measure the vertical Z coordinate through a series of methods, that is, the pressure of the finger, which is obtained by measuring the vertical contact resistance Rtouch. Because when contact occurs, the contact resistance is inversely proportional to the pressure. The greater the pressure, the lower the contact resistance. The measured value of this resistance can be used to quantify the contact pressure.

The shortcoming of the four-wire/eight-wire resistive touch screen is that it is not durable enough, and the device will be damaged by pressing and pressing for a long time. Because every time you touch, the upper layer of PET and ITO will be deformed, and the ITO material is more brittle and easily damaged when the deformation occurs frequently. Once the ITO layer is broken, the uniformity of conductivity is destroyed, and the proportional equivalence when deriving the coordinates above no longer exists. This kind of breakage can easily occur in areas that are frequently touched, such as the position of the "confirm" button. Another disadvantage is that the ITO attached to the PET movable substrate will not be fully oxidized. Once exposed to a humid or heated environment, the oxidation will cause the resistance to rise, which will also damage the conductivity uniformity, and cause errors in the coordinate calculation, that is, "drift" phenomenon. This gave birth to the concept of five-wire resistive screen.