Now we widely use the capacitive touch screen in industrial equipment as the mainstream technology of the current multi-touch interface. It is also called industrial touch screen in the industrial control industry. The anti-interference of the capacitive touch screen is one of the performance requirements of the touch screen. If the anti-interference is weak, it will affect the switchboard’s touch screen effect. Such as the touch is not sensitive, inaccurate and other issues. So the electromagnetic interference problem of industrial touch screen pcap is extremely challenging in the early stage of development in the early stage of design.
The projected capacitive touch screen can accurately locate where the finger touches the screen. It determines the position of the finger by measuring small changes in capacitance. In this type of touch screen application, we need to consider a key design issue .Because it is the impact of electromagnetic interference (EMI) on system performance. The performance degradation caused by interference may adversely affect the design of the touch screen. This article will discuss and analyze these interference sources.
We install a typical projected capacitive sensor under a glass or plastic cover. The transmitting (Tx) and receiving (Rx) electrodes are connected to transparent indium tin oxide (ITO) to form a cross matrix. Each Tx-Rx node has a characteristic capacitance. The Tx ITO is located under the Rx ITO, separated by a layer of polymer film or optical adhesive (OCA).
Let us not consider the interference factors for the time being, to analyze the work of the touch screen. The operator’s hand indicates that it is at ground potential. Rx is held at ground potential by the touch screen controller circuit. The Tx voltage is variable. The changing Tx voltage causes current to flow through the Tx-Rx capacitor. A carefully balanced Rx integrated circuit. Isolate and measure the charge entering Rx. Therefore,the measured charge represents the “mutual capacitance” connecting Tx and Rx.
We widely use projected capacitive touch screens in portable devices today, are susceptible to electromagnetic interference. Interference voltage from internal or external will be capacitively coupled to the touch screen device. However, these interference voltages will cause electric charge movement in the touch screen. This may confuse the measurement of charge movement when the finger touches the screen. Therefore, the effective design and optimization of the touch screen system depends on the understanding of the interference coupling path. And to reduce or compensate as much as possible. The interference coupling path involves parasitic effects. For example: transformer winding capacitance and finger-device capacitance. Properly model these effects. It can fully recognize the source and magnitude of interference.
For many portable devices, battery chargers constitute the main source of interference for touch screens. When the operator’s finger touches the touch screen. The resulting capacitance allows the charger to close the interference coupling circuit. So the quality of the internal shielding design of the charger and the proper grounding design of the charger are the key factors that affect the interference coupling of the charger.
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