Capacitive touch screen

The working principle of the capacitive screen: The capacitive screen control chip periodically generates a driving signal, which is then received by the receiving electrode to measure the charge. When the capacitive screen is pressed, it is equivalent to introducing a new capacitance to the electrode, thereby changing the magnitude of the charge measured by the receiving electrode. Single-layer ITO has the advantages of low cost, high light transmittance, but poor anti-interference ability. Single-sided double-layer ITO has good performance, high yield, but high cost. Double-sided single-layer ITO has good performance and strong antistatic ability, but Poor anti-interference ability.

Surface Capacitive Touch, SCT for short

The SCT panel is a uniformly spread ITO layer, and each of the four corners of the panel has an electrode (UR, UL, LR, LL) connected to the SCT controller. The SCT controller must first establish a uniform electric field on the SCT panel, which is achieved by charging the panel by the driver circuit inside the IC. When a finger touches the screen, the current from the four-side electrodes will flow to the contact; the strength of the current is proportional to the distance between the finger and the electrode. At this time, the sensing circuit in the IC will analyze the current on the four electrodes separately. The surface capacitive type has the disadvantage of uneven light transmittance and color distortion, which is easy to cause blurring of image characters. Evenly deposited ITO can also cause pincushion distortion .When a large area of the palm or a hand-held conductive object is close to the capacitive screen instead of touching it can cause the capacitive screen to malfunction. This situation is particularly serious in humid weather. There is no response when touched with gloved hands or holding non-conductive objects. This is because a more insulating medium is added. When the environmental temperature and humidity change, and the environmental electric field changes, it will cause the capacitive screen to drift and cause inaccuracy. The outermost layer of extremely thin glass has very good scratch resistance under normal conditions, but it is fragile.

Projected capacitive screen, referred to as PCT

One or more etched ITO layers are required. The ITO layer is etched to form multiple horizontal and vertical electrodes, which are driven by a capacitive sensor chip. The chip can not only transmit data to the main processor, but also process the XY position of the contact by itself. Normally, both horizontal and vertical electrodes are driven by a single-ended sensing method, that is, one row and one column drive circuit is the same, which is called "single-ended" sensing (self-capacitance). In addition, one axis is driven by a set of AC signals, and the response across the touch screen is sensed by electrodes on other axes. This method is called "transverse" induction, because the electric field is transmitted from one electrode group (such as rows) to another electrode group (such as columns) (mutual capacitance) through the dielectric layer of the upper panel in a transverse manner. Axial-coordinate sensing unit matrix of projected capacitance: The separate rows and columns of the axis-coordinated sensing unit are realized by two crossed sliders. X-axis slider and Y-axis slider are used to detect the capacitance change of each grid sensing unit.

Self-capacitive

On the glass surface, ITO is used to make horizontal and vertical electrode arrays, and respectively form a capacitor with the ground. This capacitor is the so-called self-capacitance, that is, the capacitance of the electrode to the ground. When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the capacitance of the screen body, which increases the capacitance of the screen body. During touch detection, the self-capacitance screen detects the horizontal and vertical electrode arrays in turn, according to the changes in capacitance before and after the touch. , Determine the horizontal and vertical coordinates respectively, and then combine them into the touch coordinates of the plane. The self-capacitance scanning method is equivalent to projecting the touch points on the touch screen to the X-axis and Y-axis directions respectively, and then respectively in the X-axis and Y-axis directions The coordinates are calculated and finally combined into the coordinates of the touch point.

Mutual capacitance

ITO is used to make horizontal electrodes and vertical electrodes. The difference between it and self-capacitance is that capacitors will be formed where the two sets of electrodes cross, that is, these two sets of electrodes respectively constitute the two poles of the capacitor. When the finger touches the capacitive screen, it affects The coupling between the two electrodes near the touch point changes the capacitance between the two electrodes. When detecting the mutual capacitance, the horizontal electrodes send out excitation signals in turn, and all the vertical electrodes receive signals at the same time. In this way, the capacitance value of the intersection of all the horizontal and vertical electrodes can be obtained, that is, the capacitance of the two-dimensional plane of the entire touch screen. When the human finger approaches, it will cause the local capacitance to decrease. According to the two-dimensional capacitance change data of the touch screen, the coordinates of each touch point can be calculated. Therefore, even if there are multiple touch points on the screen, the true coordinates of each touch point can be calculated.