# Basic structure and driving principle of thermal r

2022-10-20
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The basic structure and driving principle of the resistive touch screen

four wire resistive touch screen

the structure of the four wire resistive touch screen is shown in Figure 1. The glass or acrylic substrate is covered with two layers of turbine and uniformly conductive ITO layers, which are respectively used as X electrode and Y electrode, and they are separated and insulated by evenly arranged transparent lattice points. Among them, the lower ITO is attached to the glass substrate, and the upper ITO is attached to the PET film. The positive and negative ends of X electrode and Y electrode are led out from both ends by "conductive strip" (black strip part in the figure), and the positions of X electrode and Y electrode conductive strip are perpendicular to each other. There are four lines at the leading out end x-, x+, y-, y+, which is the origin of the name of the four wire resistive touch screen. When an object contacts the surface of the touch screen and applies a certain pressure, the upper ITO conductive layer deforms and contacts with the lower ITO. This structure can be equivalent to the corresponding circuit, as shown in figure 2:

Figure 1

Figure 2

Figure 3

calculating the X and Y coordinates of the contact is divided into the following two steps:

1 Calculate the Y coordinate, apply the driving voltage vdrive on the y+ electrode, ground the y- electrode, and measure the voltage at the contact point with x+ as the lead out end. Because the ITO layer is uniformly conductive, the ratio of contact voltage to vdrive voltage is equal to the ratio of contact y coordinate to screen height

2. Calculate the X coordinate, apply the driving voltage vdrive on the x+ electrode, ground the x- electrode, and measure the voltage at the contact point with y+ as the lead out end. Because the ITO layer is uniformly conductive, the ratio of contact voltage to vdrive voltage is equal to the ratio of contact X coordinate to screen width

the measured voltage is usually converted into digital signal by ADC, and then it can be used as coordinates to judge the actual position of the contact after simple processing

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 after the top layer is pressed, the upper and lower ITO contacts, and there is actually resistance on the contact, as shown in rtouch in the figure below. The greater the pressure, the fuller the contact, and the smaller the resistance. The pressure can be quantified by measuring the resistance

how to get the resistance value of RT when the equipment is installed and debugged? There are two ways

the first method: the following preparations should be made, as shown in the following figure:

1 X-grounding, x+ power supply, y+ ADC get the X coordinate of the contact

2 X-grounding, y+ what's wrong? Connect the power supply, x+ connect the ADC to get the position of Z1 point z1

3 X-grounding, y+ power supply, y-adc to get the position of Z2 point Z2

now that you know the X coordinate, that is, the output value of ADC, Z1, Z2, and also know the total resistance value of X-line and Y-line, you can calculate

the second method: do the following preparations

1 X-grounding, x+ power supply, y+ ADC get the X coordinate of the contact adcx

2 Y-grounding, y+ power supply, x+ ADC to get the Y coordinate of the contact adcy

2 X-ground, y+ connect to the power supply, x+ connect to the ADC to get the position of Z1 point Z1

also know the total resistance value of x-plate y-plate

the above calculation has a defect, that is, it does not consider the parasitic resistance of the electrode tap lead and the circuit driving the electrode, this part of the resistance is not included in the ITO resistance, and the resistance fluctuation is affected by the ambient temperature, which is likely to affect the correctness of the calculation, so the concept of eight wire resistance touch screen came into being

eight wire resistive touch screen

the structure of the eight wire resistive touch screen is similar to that of the four wires. The difference is that in addition to the four electrodes of X-drive, x+ drive, y- drive and y+ drive, a line is also led out at the end of each conductive bar: x- sense, x+ sense, y- sense and y+ sense, so there are eight lines in total. When the eight wire resistive touch screen works, first measure the voltage of the conductive strip:

· apply the driving voltage vdrive to the y+ electrode, and the Y-electrode is grounded. Measure the voltage of y+ sense and y-sense respectively, which are recorded as vymax and vymin,

· apply the driving voltage vdrive to the x+ electrode, and the X-electrode is grounded. Measure the voltage of x+ sense and x-sense respectively, which are recorded as vxmax and vxmin

and then calculate the X and Y coordinates of the contact, It is divided into the following two steps:

1 calculate the Y coordinate, apply the driving voltage vdrive on the y+ electrode, ground the Y-electrode, and measure the voltage at the contact point with x+ as the lead out end

2 calculate the X coordinate, apply the driving voltage vdrive on the x+ electrode, ground the X-electrode, and measure the voltage at the contact point with y+ as the lead out end

the advantage of the four wire/eight wire resistive touch screen is that it can not only calculate the horizontal X and Y coordinates, but also measure the longitudinal Z coordinates through a series of methods, that is, the pressure of the finger. This is obtained by measuring the longitudinal contact resistance rtouch, because when contact occurs, the contact resistance is inversely proportional to the pressure. The greater the pressure, the smaller the contact resistance. The measured value of this resistance can be used to quantify the contact pressure

the disadvantage of the four wire/eight wire resistive touch screen is that it is not durable enough. Long time touch and pressure will damage the device. Because pet and ITO on the upper layer will be deformed every time you touch it, while ITO material is brittle and easy to be damaged when deformation often occurs. Once the ITO layer breaks, the uniformity of conductivity will be destroyed, and the proportional equivalence when deriving coordinates above will no longer exist. This kind of fracture is very easy to occur in the area that is often touched, such as the position of the "confirm" key. Another disadvantage is that the ITO attached to the pet movable substrate will not be fully oxidized. Once the plastic is used on household appliances and exposed to a humid or heated environment, oxidation will lead to the rise of resistance, which will also destroy the conductivity uniformity and make the coordinate calculation error, that is, the phenomenon of "drift". This gave birth to the concept of five wire resistance screen

five wire resistive touch screen

in view of the shortcomings of the four wire resistive touch screen, the five wire resistive touch screen adopts the structure that X and Y electrodes are made on the ITO layer attached to the glass substrate, while the upper ITO is only used as a movable electrode. The X and Y electrodes of the bottom ITO lead out UL, ur, ll, LR from four corners, plus the upper active electrode, so there are five lines in total. The advantage of five wire resistive touch screen is that the glass substrate is relatively firm and not easy to deform, and it can fully oxidize the ITO attached to it. The glass material will not absorb water, and its expansion coefficient is very close to that of ITO, and the resulting deformation will not cause ITO damage. The ITO on the upper layer is only used as the lead out electrode, and there is no current flowing, so it is not necessary to require uniform conductivity. Even if it is damaged due to deformation, it will not cause "drift" of the resistance screen

the electrodes of a five wire resistive touch screen cannot be led out from four sides by conductive strips like a four wire resistive screen, which will cause a short circuit. The electrodes are dispersed into many resistance patterns distributed around the touch screen, and then led out from the four corners. The function of these patterns is to make the voltage gradient in X and Y directions of the touch screen linear, which is convenient for coordinate measurement

when the five wire resistive touch screen works, UL applies the driving voltage vdrive and LR is grounded. Measuring the X and Y coordinates of the contact is divided into the following two steps:

1 calculate the Y coordinates, apply the driving voltage vdrive on the ur electrode, and the LL electrode is grounded. The movable electrode is used as the lead out end to measure the voltage of the contact point

2 calculate the X coordinate, apply the driving voltage vdrive on the LL electrode, ground the ur electrode, and measure the voltage at the contact point with the movable electrode as the lead out end

six wire resistive touch screen

based on the five wire resistive touch screen, the six wire resistive touch screen adds a grounded conductive layer on the back of the glass substrate to isolate the signal crosstalk from the back of the glass substrate

seven wire resistive touch screen

like the four wire resistive touch screen, the five wire resistive touch screen also does not consider the parasitic resistance of the electrode tap lead and the circuit driving the electrode. This part of resistance is not included in the ITO resistance, which is likely to affect the accuracy of the calculation. Therefore, on the basis of the five wire resistive touch screen, the seven wire resistive touch screen leads a line from both ends of UL and LR to sense the actual terminal voltage of the touch screen, They are respectively recorded as Vmax and Vmin. The working principle is the same as that of the five wire resistive touch screen

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