A new coating thickness measuring instrument based on eddy More Info at Open Access Database current sensor

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1 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic A new coating thickness measuring instrument based on eddy More Info at Open Access Database current sensor FU YanLing, XIAO HaiQing, MENG Hui, WANG Xing, BAI Hong, YU HongMei Institute of Mechanical and Electrical Product Safety, Chinese Academy of Inspection and Quarantine (CAIQ); Beijing, China Phone , Fax ; fuyanling82@163.com Abstract In this paper, a kind of coating thickness measuring instrument based on eddy current sensor was developed. The functions of data acquisition, data processing, data communication and data display are integrated in the system. In the design of hardware, the system is developed with eddy current sensor technology and ARM Cortex-M3 microprocessor control technology to achieve nondestructive measurement on the metal plating and plate thickness. In software, the deviation of the real time data can be checked and corrected by data processing algorithm to further improve data precision and accuracy. The system has been successfully applied to the non-contact measurement of coating thickness. Experiment results indicate that it is a sensitive, wide frequency range and highly adaptable method. Keywords: coating thickness measuring instrument, eddy current sensor, ARM Cortex-M3, metal plating and plate thickness, nondestructive measurement 1.Introduction The covering material which is used to surface protection or decoration, such as coating, adhesive layer, chemical conversion film, is called coating in the relevant national and international standards. Coating thickness measurement has become an important part of the processing industry and surface engineering quality detection, and is a necessary way to achieve excellence in product quality standard. Methods for measuring thickness of coating are mainly: wedge resection method, light section method, electrolytic method, the difference of the thickness measurement, weighing method, X-ray fluorescence spectrometry, β-ray backscatter method, capacitance method, magnetic measurements and eddy current measurement, etc. The first five measurements in these methods are damage detection, and they are mostly used in sampling inspection because of complicated measuring and slow speed. With development of modern technology, especially in recent years the introduction of computer technology, thickness gauge is step to miniature, intelligent, multi-functional, high precision and practical direction. Therefore, this paper presents a thickness meter design based on the portable layer eddy current sensor. The

2 thickness measuring instrument using eddy current nondestructive method will not destroy the coatings and the substrate, can make lots of inspection work carried out economically by fast detection speed. 2.Specific instructions 2.1 The measurement principle of eddy current sensor The metal conductor is placed in the changing magnetic field, and the conductor will produce induced current. This current is like water vortex in conductor circle, so it is called electrical eddy current or eddy current. This phenomenon is called eddy current effect. Eddy current sensor is based on the eddy current effect. The formation of vortex must have the following two conditions: the presence of an alternating magnetic field; electric conductors in the alternating magnetic field. Therefore, eddy current sensor is mainly composed of the current coils, producing alternating magnetic field, and the metal conductor in the alternating magnetic field. The metal conductor can also be measured object itself. By using this eddy current effect, eddy current sensor converts some non-electrical parameters into the changes of load impedance (or the changes of the inductance) in order to measure the non-electrical parameters. As shown in Figure 1, a sensor coil with the alternating current of I1 produces a magnetic field H1 around the coil by the changing current. If the measured conductor is in the magnetic field range, the measured conductor will produce the eddy current of I2. This eddy current will generate a new H2, which has the opposite direction with H1 in magnetic field, so the offset part of the original magnetic field will lead to the changes of inductance, impedance and quality factor. Figure 1. Schematic diagram of eddy current sensor

3 In general, the changes of inductance, impedance and quality factor are related to the sensor coil geometry, the conductivity and the permeability. And they are also related to the geometric parameters, the coil current frequency and the distance between the coil and the conductors. If we can control to change one parameter, while the others remain unchanged, a variety of sensors can be designed in temperature measurement, displacement measurement hardness, etc. 2.2 The composition and working principle of thickness gauge system An electromagnetic field is formed in high frequency alternating current by eddy current detector in the system. The eddy current on the metal substrates generates a feedback effect on the coil side of the head, when the probe contacts with the cover layer. The covering layer thickness can be deduced by the size of the feedback effect. As shown in figure 2, the hardware of the system controlled by ARM Cortex-M3 embedded processor, composed of precision eddy current probe acquisition circuit, keyboard input circuit, LCD display circuit and print output circuit. When the system works, initialized first and the power indicator light; Before measurement, the system is on standby, and the operator can set the measurement parameters via buttons Then the system collect and process the data of the tested products according to the parameters Finally,the system equipped with a standard USB interface, can transfer the collected data to the PC,or accept callback data from PC for further processing. Figure 2. Hardware design of system structure In order to reduce development costs and improve the stability and be ease to hardware upgrades, eddy current type coating thickness meter adopts self-developed micro-systems. This system uses ARM Cortex-M3 processor as the main control chip to meet the demand of the processing speed of the system and controls interface quantity etc., The ARM Cortex-M3 processor is a 32 bit industry-leading processor. It is suitable for real-time applications with higher certainty, and development of high performance and low cost platform by specially developing.

4 Eddy current detector using N1 type eddy current probe transmits the signal into electric signal. The N1 probe is a standard probe in eddy current measurement. Parameters are as follows: the measurement range is 0 ~ 1250um; the resolving power is 0.1um, the diameter of the measured area is more than 5mm; the critical thickness of substrate is 0.3mm. This kind of probe covers most coating thickness range. The physical signal acquisition by Eddy current detector as the initial signal, need to be amplified through the differential amplification circuit. As shown in Figure 3, the magnification is 10 ~ 20 times. The amplified data is convenient for transmitting to the AD module in the micro controller. Figure 3. Principle diagram of amplifying circuit USB communication function: provide reliable and high-speed data transmission channel when exchange the data with the other equipment or transfer data to the PC. The USB chip in this system is EZ-USB FX2 CY7C68013A from Cypress Ltd. Its versatility, simple development and the highest 96MB/s instantaneous transmission rate can meet the demand of communication. 3.The Software Design of thickness gauge system For software design, structured and modular way make the procedures simple and ease to maintenance because of big data information and high real-time and complexity. Meanwhile, this kind of modularized design improves system reliability, flexibility and maintainability. The main program, interrupt service routine (ISR) and subroutines are designed by use of modularized develop ways. The main program is the command center of the system software. Combined with the other modules, the main program completes the system initialization, real-time condition monitoring, real-time control output and data display etc.. Its flow chart is given in Figure 4.

5 According to the functional requirements of the system and the hardware circuit structure, system software includes the main program, data acquisition program, data processing program, serial communication program, print output program, LCD display program and keyboard program, etc.. The main program function is to complete the initialization of each module, make each module get into the normal working state and coordinate the workflow of the various modules to achieve detection function. Start Initial setup N Whether setthe function parameters? Y Set parameters through the human-computer interaction Whether start data acquisition? N Y Set the acquisition symbol Data acquisition Data processing Display and Clear signs Figure 4. Flow chart of main program 4.Conclusions In this paper, a kind of coating thickness measuring instrument based on eddy current sensor was developed. Compared to other methods of measurement, eddy current measuring instrument for coating thickness can realize the non-contact measurement, and have high sensitivity, wide frequency response range and strong adaptability, which can be widely used in the measurement of thickness of coating. References 1. ZHANG HongRen, The sensor technology, Beijing University of Aeronautics and Astronautics Press, pp , HE WenHui, Analysis of a novel eddy current sensor in theory, Journal of Shanghai Jiao Tong University, Vol 40, No 3, March 2006.

6 3. LIN Jun, eddy current probes for testing materials properties of metals, Nondestructive Testing, Vol 18, No 10, October SUN XiangYang, Eddy Current Testing Application in Railway Catenary Defect, Southwest Jiaotong University, 2005.