Research On The Capacitive Sensor And Online System For Thickness Measurement Of Plastic Film

In the industry of plastic film manufacturing, the thickness is the most important physical indicators. The uniformity of film thickness is related to the performance of the product, grading rate, prices and benefits.Uneven thickness influence the product quality directly.For example, the packaging film(POF), such as packaging films for tableware, are more prone to rupture in the thin places when heat shrinkage;the coating film, such as films for the Greenhouses (PVC polyvinyl chloride), which is surfacecoated with a layer of fog flow drops, will have a short use life,because the uneven film thickness will result in uneven coating thickness.Research and development the on line film thinkness measurement system is a very necessary means, With it we can timely adjust and correct the parameters of the production line to reduce the defect rate, saving raw materials. Development and application of coplanar type capacitive thickness measurement system is one of the important method for online testing.It will play a decisive role to improve the quality of plastic film products and reduce the consumption of raw materials.Domestic plastic film manufacturing industry have a low percentage of using plastic film thickness measurement system, and mainly buy foreign products.Because the imported thickness measurement is expensive,only a small number of manufacturers with strength economy and large-scale can afford. Not equipped manufacturers can only based on experience or manual measurement, which can not guarantee the quality of the product.Measuring non-electrical parameters such as the thickness of the plastic film, the measured non-electrical volume should be converted into electricity and then enter. Capacitance sensor changes the thickness of the plastic film into capacitance change.The precision sine wave excitation source to generate standard sine wave, which amplitude modulate the measurement signal.The precision amplifier proportional integral signal amplification, and then the signal using full-wave rectifier circuit and the active filter circuit for demodulation to extract useful information.Finally, the measurement signal is corrected by the calibration circuit, so that the conversion of the plastic film thickness to a voltage signal is completed. Voltage signal through the data acquisition card for AD converter, then through RS-485 bus upload voltage signal to the PC.The PC software completed the quantification of signal, the synchronization temperature compensation and thickness fluctuations in the polar coordinate curves and trend curve shows with which the measured thickness corresponds exactly to the position of the die bolts, so we can regulate and limit alarm of the thickness.Firstly, this article give an overview of the development status of the plastic film thickness measurement at home and abroad, and then expand the capacitive plastic film thickness testing system design. Capacitor plastic film thickness sensor probe design is the key, and describes the hardware detection system, in which highlight the detection mechanism of circuit design, including the sine wave oscillator circuit,expression measurement circuit,full-wave rectifier circuit,active filter circuits,precision low noise regulated power supply. The design of the system software is divided into the underlying program design and the upper interface design.The underlying program included the serial communication module, data processing module, the temperature compensation module and the data recording and query module. The upper interface included the polar coordinates contour,the transverse real-time trend graph,the parameter setting interface and record the query interface. Finally,the article describes the sensor static calibration and systematic error analysis.