·Table of Contents
·Methods and Instrumentation
AAPI Magnetic Field Sensor and its Application in NDTZhang Haixia
State key laboratory of tribology, Department of precision instrument & mechanology,
Tsinghua university,Beijing, 100084, China,
Beijing Research Insitute,
Shenzhen Huawei Techinology Co. Ltd., 100080, China
In this paper, a novel Amorphous Alloy Pulse-Induction (A2PI) magnetic field sensor was designed, fabricated and applied in NDT field. The A2PI magnetic field sensor employed Co-based amorphous alloys as its core materials and was based upon pulse-induction principle. Its twin-coil with one core magnetized in two directions, optimized by negative feedback technology. Due to its high sensitivity and resolution, by magnetic flux leakage (MFL) techniques, the A2PI magnetic field sensor was applied in picking up micro-cracks in pipeline successfully.
Keywords: magnetic field sensor, amorphous alloy, pulse-induction, NDT
Amorphous magnetic materials, due to their special forming procedure, have excellent soft magnetic characteristics: high saturation induction, high permeability, low coercivity, and low core loss. Since 1970's, amorphous alloys have been considered as ideal active materials for high-performance magnetic field sensors, researchers all over the world have obtained achievements in many fields [1,2]. In past 5 years, we have done work in designing A2PI magnetic field sensors , and done several useful optimization (varied-structures, feed back techniques, bias magnetic field techniques), all these efforts improved properties of the sensor greatly, such as: sensitivity, resolution and work range. So, This kind of sensor can be used to pick up micro magnetic field information in NDT, for example, to identify micro flaws and other bugs in steel plate or oil pipeline. In this paper, we set up an array pigs of the A2PI magnetic field sensors around the pipeline, their outputs were send to a computer and analyzed by our program, this system can pick up transverse surface flaws: 0.2-0.5mm in depth, 5-10mm in length.
The principle of this twin-coil A2PI magnetic field sensor is illustrated in Fig.1. Coil 1 and 2 are two identical coils, which are wrapped over amorphous core, connected in anti-series, their joint terminal is connected at ground.
|Fig.1: The scheme of twin-coil A2PI magnetic field sensor and its opimization|
Two identical pulse current source S1, S2 supply the coil 1,2 respectively. Therefore, the two coils are identical in structure and magnetize the amorphous core in opposite direction. According to the Faraday's law of induction, the emf in coil 1 and coil 2 are:
Here,which is a constant parameter independent on the external magnetic filed Hd, we call it the zero magnetic field voltage.
As seen in equations (1) and (2), and reflect the change of Hd. If the sensor's operating point is among the nonlinear range in M-H curve, especially when:
From equation (4), we can get the signal of the external magnetic field Hd.
In order to improve the sensor's properties, work was done to optimize both the parameters of the probe structure and the sensor system, such as optimizing parameters of coil, core and time, changing coils' combination, adding negative feedback coil and bias magnetic field, etc. Experiments showed that the above optimizing techniques could improve the sensor's performance effectively. Optimized by negative feedback technology, its sensitivity is higher than 1500V/T with 3.75% linearity in 10-7~1.5´ 10-3T dynamic range. Adjusted by bias magnetic field, it can test 10-7~10-2T magnetic field which is 10 times wider than other type A2PI magnetic field sensors. More, According to different requirements, this sensor can adjust its output properties by changing feedback factor and other parameters.
In oil industry, pipelines act as the important role, a micro-defect may cause tremendous dangerous, so, it is necessary to pick-up micro-flaws on pipelines or other instruments. Since the being-checked flaws are very small, the magnetic leakage flux induced by them are too weak to be check out by traditional magnetic sensors, such as fluxgate, Hall elements, it needs more accuracy and high sensitive sensor, for example: A2PI pulse-induction amorphous magnetic field sensors.
In this NDT field, we always use MFL techniques, because it is simple and high efficiency, the principle of MFL method is illustrated in Fig.2.
|Fig.2: The principle of MFL techniques in pipeline|
The magnet yoke magnetize the wall of pipe, and flaws in pipeline cause magnetic flux leakage, which can be sensed by magnetic field sensor. The sensitivity and accuracy of sensor are the key factors of this testing method. Pulse-induction Co-based amorphous magnetic sensor has such advantages.
|Fig.3: The scheme of 4-sensors in testing|
In our system, as shown in Fig.3. We magnetized the pipeline online, and set up an array pigs of the A2PI magnetic field sensors around the pipeline, their outputs were send to a computer and analyzed by our program. This structure can get more information from the whole circle of pipeline from four phases, and add up these four output data based on their angular. The collector will pick up these data every certain time, and then the program will analyze these data, put out the curve of flaws, help engineers to determine the incipient faults.
Using above scheme, we test micro defects on pipeline by MFL technique. Our experimental pipeline is 70-90mm diameter, 6mm in wall thickness, 250mm in length, with 10 transverse surface flaws: 0.2--0.5mm in depth, 5--10mm in length. Using permanent magnet yoke magnetize the wall of pipe, and flaws on pipeline cause magnetic flux leakage which can be sensed by this magnetic sensor. Fig.4 shows the testing data. According to this result, the micro flaws on pipeline can be detected easily.
|Fig.4: The experimental data of testing micro flaws on pipeline|
This system can pick up transverse surface flaws: 0.2-0.5mm in depth, 5-10mm in length. This technique was useful in testing the work condition of pipelines online, and can be used to predict the life of pipelines and other objects.
From above analysis, we can get conclusions as follow:
This project is sponsored by National post-doctoral scientific foundation of China.
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