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Magnetic Field Sensor Based Corrosion Monitoring of Reinforced Concrete Beams
Indrani Mukherjee Department of Electrical Engineering
Indian Institute of Technology Mumbai, India
Jinit Patil Department of Electrical Engineering
Indian Institute of Technology Mumbai, India
Siddharth Tallur Department of Electrical Engineering
Indian Institute of Technology Mumbai, India
Abstract—Corrosion of iron and steel is detrimental to the
health of concrete beams and poses potential risks to safety. At present, eddy-current testing (ECT) is widely used to detect flaws in metals but is limited by its low detection range. This calls for a non-destructive testing method which can detect corrosion at depths inside concrete beams in the early stages and not after cracks appear on the structure indicating high levels of damage. This project aims to implement eddy current detection using anisotropic magnetoresistive (AMR) sensor having extremely high sensitivity to detect corrosion of different degrees and depths. Different levels of corrosion on the iron rod were tested with a lock-in amplifier and found to give different magnitude and phase shifts corresponding to the respective change in conductivity.
Keywords— Eddy current testing, anisotropic magnetoresistve sensor, corrosion, phase shift, lock-in, reinforced concrete beams
I. INTRODUCTION
Strength of a concrete beam depends highly on the quality of underlying iron rods[fig.1]. Monitoring the extent of corrosion of the rods is hence crucial for maintenance of structural health of the concrete beam. Since corrosion occurs in stages, it is important to make it detectable in the early stages to prevent irreparable damages. At present, eddy current based sensing is widely used to detect corrosion. However, it is restricted by its low detection range and is mostly applied to non-ferromagnetic materials. For ferromagnetic substances with high permeability and materials at depths, eddy current based sensing has poor performance due to small skin depth and large magnetic flux fluctuations. For a ferromagnetic substance these flux fluctuations are larger and hence output SNR is quite low. An anisotropic magnetoresistive(AMR) sensor has very high typical sensitivity of 3.2 mV/V/Gauss and hence can detect even small magnetic fields[1]. In this study, eddy current sensing with the help of an AMR sensor is proposed to detect changes in magnetic field strength due to corrosion[2]. Eddy current in a metal is directly proportional to its conductivity, given by the relation . BAσdϕ/dt I = Since corrosion would cause a change in conductivity of a metal, corresponding change in eddy current should be observed. Thus the output signal of AMR sensor taken at different corroded positions were analysed through a lock-in detector to separate amplitude and phase, in-phase and
quadrature phase components and observed individually. Tests were done with different levels of rusting on the same iron rod to minimise effects of any variations of the object dimensions. An excitation current of 10 mA rms, 1KHz sinusoid was applied and was lock-in detected with 1KHz, 1Vrms sinusoidal reference voltage. The strength of the magnetic field induced by eddy currents was found to be decreasing with corrosion. The phase measurements could not be resolved to provide any meaningful trend, but it is expected that phase response would improve when tested with higher excitation currents and at lower frequencies.
fig.1
II. EXPERIMENT
A. Sensor Assembly Design A measurement system was designed using SolidWorks
software to detect corrosion using an AMR sensor. The setup consists of a transmitter coil through which ac excitation current is applied which sets up a magnetic field, an AMR sensor (HMC1001) to detect the magnetic field induced in metals because of eddy current and a cancellation coil around the sensor. The cancellation coil was used to minimize the magnetic field at the sensor position under normal conditions by nullifying the field due to transmitter coil and achieve a good signal-to-noise ratio (SNR). A support structure was also designed to hold this test assembly and make it easier to move across different test points on the iron rod. The design was then 3D printed[fig.2][fig.3].
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fig.2
B. Experimental Setup The experimental setup consists of the previously
described sensor assembly and a lock-in amplifier (SR530)[3] which also has an in-built function generator. The transmitter coil is cylindrical in shape with diameter 2.5 cm, and length 2.5 cm. Copper wire of diameter 0.05 cm was wound 45 turns for the transmitter coil making the turn-to-length ratio (n) = 1800. The cancellation coil of diameter 1.5 cm ,length 1.4 cm and n = 1800 was wound 25 turns in the opposite sense to make the current direction opposite to that of the transmitter coil for magnetic flux cancellation at the sensor position. An excitation current of 10 mA was passed through both the coils and output of the AMR sensor was given as a differential input signal to the lock-in amplifier[fig.13].
C. Methodology An iron rod was taken having different levels of rusting at three different positions - non-corroded, lightly corroded and heavily corroded as Position 1, Position 2 and Position 3 respectively[fig.4]. The iron rod was fixed in a wooden platform[fig.5]. To prepare a heavily corroded spot, the iron rod was dipped in saline water and left aside[fig.6]. Before beginning to experiment with the rod, the value of offset magnetic field measured by the sensor was tested. Upon giving a bridge voltage of 5 V to the AMR sensor, under normal conditions, a minimum voltage output corresponding to 0 Gauss and maximum output of 0.42 Gauss was obtained which matches with Earth’s magnetic field (0.25 to 0.65 Gauss). This offset field is very small in terms of voltage output of the AMR sensor: 6.72 µV and would not affect our output range. The circuit diagram shows the two coils carrying current in opposite directions and the AMR output being given to lock-in amplifier. The magnitude (R) and phase (𝛳) were noted[fig.5]. Next, to validate the proof of concept, tests were done on aluminium and bronze plates and changes in both magnitude and phase of AMR output voltage was observed. Fig.7 shows AMR sensor slid into cancellation coil.
fig.4
fig.3
Two potentiometers of were used with each coil. At the transmitter coil, it was used to control magnitude of excitation current while at the cancellation coil, it was used to nullify the AMR output at normal conditions. Total fifteen sets of readings were taken at each of the three test points on the rod.
It was noticed that for very small changes in phase and magnitude, it was difficult to nullify and observe the effect on a digital oscilloscope. Offset nullification through a lock-in amplifier by making the reference phase zero at normal condition gave more accurate cancellation.
fig.5
fig.6 fig.7
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III. OBSERVATIONS AND RESULTS
The magnitude and phase of of the AMR output can be seen changing by different amounts for aluminium and bronze[fig.9],[fig.10] respectively. Fig.8 shows AMR output at normal conditions after nullification and phase adjustment.
fig.8
fig.9
fig.10
As can be observed from above figures, the magnitude of AMR output is more in case of aluminium whereas the phase shift is more is case of bronze.
Similarly, magnitude and phase changes in AMR output were noted for the three differently corroded positions of the iron rod. Fifteen such sets were plotted and analysed using Origin software. Both magnitude and phase outputs were plotted separately[fig.11],[fig.12]. It can be observed that
strength of the magnetic field induced by the eddy current in the iron rod is decreasing with increasing corrosion levels. For the phase response a clear trend is not observable. However, the phase shift at the uncorroded location can be seen to have lesser spread about the mean phase shift.
fig.11
fig.12
It can be concluded that corrosion can be detected using the proposed method of using an AMR sensor based eddy current sensing. Since corrosion occurs non-uniformly over a surface it is logical to categorise levels of corrosion into broad ranges and improve the overall system responsivity.
It can also be concluded that to detect small changes in corrosion a larger eddy current generation is required. Therefore, power amplifiers could be used to drive larger coil currents in place of general purpose op-amps (LM741) used in this project. The AMR output of a corroded surface is in microvolts for excitation currents in tens of milliamperes. The measurement system needs to made on a more noise proof and sturdy structure as a printed circuit board (PCB) as opposed to noise prone bread boards since the experiment included detecting small changes in magnetic flux. Electromagnetic inferences also needs to be shielded properly.
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● fig.13 (Schematic of the measurement system)
IV. DISCUSSION
In this project, the principle of metal discrimination using dependence of eddy current on conductivity of material has been exploited for detecting corrosion. Since corrosion is a slow and gradual process, there are slight changes in conductivity over long periods of time. Since this work involved detecting changes of small magnitudes, a system with high sensitivity was desired. After due literature survey, AMR sensor was selected as the magnetic field detecting device in place of another induction coil used in earlier metal discriminator projects[4]. Next challenge was to construct a system that could provide enough support to keep the small sensor chip oriented in the desired direction for best performance. The 3D design for such a structure was performed by Jinit Patil and the literature survey to choose the sensor was done by Indrani Mukherjee. The remaining project work was equally contributed by both.
Due to cylindrical structure of the rod, lesser magnetic flux is cut by the AMR, hence creating the need of a high resolution phase sensitive detection system. Future challenge of this project include developing a mobile alternative to the ready-made lock-in amplifier SR530 which can be integrated on a PCB to be carried out for field testings.
ACKNOWLEDGMENT
We would like to thank TATA Centre for assisting us with 3D printing . We would also like to thank Prof. K. L. Narasimhan for providing guidance on usage and handling of SR530. Also we express our gratitude to WEL lab and AIMS lab for constant help with component procurements, lab availability and conducting experiments.
REFERENCES [1] https://aerospace.honeywell.com/~/media/aerospace/files/datasheet/1a
nd2axismagneticsensorshmc1001-1002-1021-1022_ds.pdf [2] https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7410073 [3] https://www.thinksrs.com/downloads/pdfs/manuals/SR530m.pdf [4] http://www.ee.iitb.ac.in/~stallur/wp-content/uploads/2017/02/Project_
report.pdf
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