Lift-off Value Research Articles

Study on the lift-off effect of EMAT

Non-contact inspection is the main advantage of the electromagnetic acoustic transducer (EMAT). However, it lowers the transduction efficiency between ultrasonic energy and electromagnetic energy. The lift-off of the transducer has a close relationship with transmitting and receiving ultrasonic signals, because it reduces the magnitude of the alternating magnetic field in the specimen. The distribution and intensity of the alternating magnetic field at different lift-off values are simulated in ANSOFT. The relationship between the lift-off and received signals is analyzed through experiments. Results show that the suitable lift-off value for this specific EMAT system should be around 2 mm, considering both sensitivity and stability of the transducer.

Forces and charges on an undeformable droplet in the DC field of a plate condenser

A single conducting drop resting on the lower plane electrode of a horizontal condenser and surrounded by a dielectric fluid is considered. When a DC field is applied to the electrodes, the drop acquires electric charges and is subjected to an electrostatic force normal to the electrode. This driving force may eventually detach the drop if the applied field strength exceeds a threshold value. For small drops, the gravitational field and the electrical force effect can be neglected with respect to the surface tension. In this case, it may be assumed that drops are undeformable and keep a spherical cap shape. Based on this model, charges and forces are calculated analytically in any wetting conditions. To this end, previous studies concerning solid spheres are conveniently extended. The usefulness of the above model is then considered to determine the lift-off threshold value. For non-wetting conditions, new experimental results are presented: they fit precisely the derived theoretical lift-off conditions. For wetting conditions, the preceding calculations are shown to be unable to provide any testable criterion. The undeformability assumption has to be relaxed.

A Pathway Separate from the Central Channel through the Nuclear Pore Complex for Inorganic Ions and Small Macromolecules

Nuclear pore complexes (NPCs) are supramolecular nanomachines that mediate the exchange of macromolecules and inorganic ions between the nucleus and the cytosol. Although there is no doubt that large cargo is transported through the centrally located channel, the route of ions and small molecules remains debatable. We thus tested the hypothesis that there are two separate pathways by imaging NPCs using atomic force microscopy, NPC electrical conductivity measurements, and macromolecule permeability assays. Our data indicate a spatial separation between the active transport of macromolecules through the central channel and the passive transport of ions and small macromolecules through the pore periphery.

An adaptive method for channel equalization in MFL inspection

Influenced by the lift-off value between the pipeline and coil sensors, the various properties of electronic component and the different location of coil sensor, the output signal's amplitude and phase of each channel are different despite the same flaw in magnetic flux leakage (MFL) inspection. The channel-to-channel mismatch may severely degrade the performance of testing equipment and disturb the evaluation of the level of flaw unless some form of compensation is employed. In this paper an adaptive method for channel equalization in MFL inspection is presented by using the finite impulse response filter. Both theoretical analysis and experimental results have shown the effectiveness of the proposed method.

Transient and harmonic eddy currents: Lift-off point of intersection

It has been previously determined experimentally that the transient responses of pulsed eddy current excitation of a conductive semi-infinite plane pass through a common point, regardless of the lift-off value. The origin of this intersection point is investigated and related to the harmonic components of the Fourier series representation of the pulse. Discrete sinusoidal signals, when individually sent through the same inspection system, also exhibit this common point feature when only the lift-off is varied. Moreover, the transient responses can be reconstructed from the harmonic outputs by using truncated Fourier series.

THE RELATION BETWEEN MEASURED VALUE OF MAGNETIC FLUX LEAKAGE AND SENSOR SIZE

The size of magnetic sensor influences the detectability of defect and the evaluation of magnetic flux leakage quantity. However the relation between the sensor size and the measured value of magnetic flux leakage has not been clarified quantitatively so far. In the present report, five Hall elements of different sizes are used as the magnetic sensor and the value of the normal component of magnetic flux leakage is experimentally obtained on a structural steel with an artificial defect or a fatigue crack. The effect of the sensor size on the results are discussed. It is clearly shown that the measured value of magnetic flux leakageis large in a smaller sensor, and the influence of sensor size on the measured value of magnetic flux leakage is large in a smaller lift-off value. An experimental equation is proposed for the relationship among the magnetic flux leakage,sensor size, lift-off distance and defect size. It is alsoconfirmed that the influence of sensor size on the measured value of magnetic flux le...