To meet the great needs for MFL (magnetic flux leakage) inspection of drill pipes at wellheads, a lift-off-tolerant MFL testing method is proposed and investigated in this paper. Firstly, a Helmholtz coil magnetization method and the whole MFL testing scheme are proposed. Then, based on the magnetic field focusing effect of ferrite cores, a lift-off-tolerant MFL sensor is developed and tested. It shows high sensitivity at a lift-off distance of 5.0 mm. Further, the follow-up high repeatability MFL probing system is designed and manufactured, which was embedded with the developed sensors. It can track the swing movement of drill pipes and allow the pipe ends to pass smoothly. Finally, the developed system is employed in a drilling field for drill pipe inspection. Test results show that the proposed method can fulfill the requirements for drill pipe inspection at wellheads, which is of great importance in drill pipe safety.

This paper presents an anti-windup self-tuning euzzy proportional-integral-derivative (PID) controller for the speed control of brushless direct-current (BLDC) motor. The proposed controller consists of a self-tuning fuzzy PID controller and an snti-windup controller, which combines the merits of these two controllers. The anti-windup controller is designed as a fuzzy proportion controller and is switched by a logic function based on the output of proposed controller and the speed error. A detailed simulation study in MATLAB/Simulink is performed to investigate the performance of proposed controller, and the simulation experiments are conducted on different conditions. The control system parameters of the proposed controller such as overshoot, settling time and rising time are compared with that of the conventional PID controller. The simulation results show that the proposed controller can accelerate the response speed of BLDC motor, strengthen the ability of anti-windup and make motor runs smoothly, it is proven that anti-windup self-tuning fuzzy PID controller has better performance than conventional PID controller.

In order to improve the sensitivity of online magnetic flux leakage (MFL) testing for steel pipe, a sensing method based on the magnetic guiding effect is proposed and investigated in this paper. Compared to the conventional contact sensing method using a non-ferromagnetic support, the proposed method creatively utilizes a ferromagnetic one to guide more magnetic flux to leak out. Based on Hopkinson’s law, the principle of the magnetic guiding effect of the ferromagnetic support is theoretically illustrated. Then, numerical simulations are conducted to investigate the MFL changes influenced by the ferromagnetic support. Finally, the probe based on the proposed method is designed and developed, and online MFL experiments are performed to validate the feasibility of the proposed method. Online tests show that the proposed sensing method can greatly improve the MFL sensitivity.

To improve the accuracy of the magnetic flux leakage(MFL) testing in practical application, we analyzed the signal characteristics of rectangular induction coil affected by the sensor arrangement and scanning direction. On the basis of magnetic dipole theory, the descriptions for the MFL field distributions of standard discontinuity were established. Further, based on Faraday's law of induction, the signal characteristics of rectangular induction coils were calculated and it was found that the signal waveform of the horizontal coil is similar with the x component of MFL; in contrast, the signal waveform of the vertical one has the same changing trend with the z component of MFL. Finally, the influence of scanning direction was analyzed and found that different scanning directions will generate different signal characteristics; and that with the angle β between the coil and discontinuity orientation changing from 0 to π/2, the signal amplitudes of both induction coils decrease, which was verified by relevant experiments.

Eddy current effect essentially occurs when relative movement happens between steel pipe and axial magnetization coil in Hi-speed MFL (magnetic flux leakage) testing. Based on the forming mechanism of eddy-current, the eddy-current magnetic field distributions in different parts were obtained, and it was found that the eddy-current magnetic field and the original magnetizing field in pipe tail have the same direction, resulting in the increase of the magnetization intensity; however, the direction of the eddy-current magnetic field is opposite to the one of the original magnetizing field in pipe head, which brings about the reduction of magnetization intensity; and that the eddy-current magnetic field in pipe body nearly has no influence on the magnetization. Due to the different magnetization intensity in different parts of the steel pipe, sensitivity difference is formed, lowering the accuracy of MFL testing. Further, the elimination method for sensitivity difference by building extending magnetic pole was proposed. Lastly, the relevant experiments for steel pipe were conducted to verify the influence of the eddy-current magnetic field and the effectiveness of elimination method.