Current Probes

At sufficiently high frequency, the electromagnetic skin depth is several times smaller than a typical defect and induced currents flow in a thin skin at the conductor surface and the crack faces. It is profitable to develop a theoretical model dedicated to this regime. Making certain assumptions, a boundary value problem can be defined and solved relatively simply leading to rapid numerical calculation of eddy-current probe impedance changes due to a variety of surface cracks. 

Fig. 1 Shematic of eddy current probe for corrosion detection.

SQID Eddy Current Technique Applying Conformable Eddy Current Probes. 

Lew frequency Eddy current probing For frequencies below some 100 Hz the SQUID is coupled with a completely superconducting flux antenna. This antenna has to be within the cryogenic vessel. The Eddy current excitation is done in a conventional way. But care must be taken, that interference between the excitation field and the flux anteima and SQUID is... 

We realized an Eddy current SQUID system of the high frequency type a room temperature Eddy current probe is connected to a SQUID sensor at hquid nitrogen temperature. Fig.3 gives an overview over the components of the system, fig, 5 shows a schematic diagram of the electronics. 

Several types of Eddy current probes were used with the SQUID system and the commercial system as well. High inductance wire wound probes with a ferritie eore and low induetance planar thick frhn coils were applied. The wire wound probe is the commonly used probe for high resolution conventional testing. The low inductance planar cod is more suited to be apphed in combination with the SQUID system. It is well adapted for surfaee defects and shallow defects. 

As stated above the SQUID amplifier demands a low inductance Eddy current probe in order to be able to amplify signals up to 1 MHz. Low inductance Eddy current probes can be obtained by reducing the number of turns and by loosing the magnetic coupling between the tums. So magnetic cores should be avoided as well as tight wounded tums. For this purpose planar coils are the best... 

This type of coil was prepared from copper cladded printed circuit board material by applying photolithographic techniques. The p.c. board material is available with difierent copper thicknesses and with either a stiff or a flexible carrier. The flexible material offers the opportunity to adapt the planar coil to a curved three dimensional test object. In our turbine blade application this is a major advantage. The thickness of the copper layer was chosen to be 17 pm The period of the coil was 100 pm The coils were patterned by wet etching, A major advantage of this approach is the parallel processing with narrow tolerances, resulting in many identical Eddy current probes. An example of such a probe is shown in fig. 10. 

New Aspects for Remote Field Eddy Current Probe Development. 

The development of Remote Field Eddy Current probes requires experience and expensive experiments. The numerical simulation of electromagnetic fields can be used not only for a better understanding of the Remote Field effect but also for the probe lay out. Geometrical parameters of the prohe can be derived from calculation results as well as inspection parameters. An important requirement for a realistic prediction of the probe performance is the consideration of material properties of the tube for which the probe is designed. The experimental determination of magnetization curves is necessary and can be satisfactory done with a simple experimental setup. 

P.Y. lotibert, D. Miller, D. Placko, and E. Savin. New eddy current probe and associated preprocessings for 3d image reconstruction in non ferromagnetic tubes testing. In To be published in the proceedings of the Seventh ECNDT, 1998. 

New Eddy Current Probe and Associated Preprocessing for 3D Image Reconstruction in Non Ferromagnetic Tube Testing. 

Progress in mean of modelisation and inverse problem solving [1] let us hope to dispose soon of these tools for flaws 3D imaging in Non Destructive Control with eddy current sensors. This will achieve a real improvement of the actual methods, mainly based upon signature analysis. But the actual eddy current probes used for steam generators tubes inspection in nuclear industry do not produce the adequate measurements and/or are not modelisable. 

Therefore we present here a new eddy current probe designed with imaging objectives. The elements of this probe have been choosen after a relevant analysis of electromagnetic interactions. 

The second method applies eddy-current probes in a new measurement method. The principle circuit of this method is presented in fig. 2. 

Fig. 1 The view of the polar display with monitors arcs and control panel for SFT6000N board parameters. Recorded signal is from the eddy current probe moved along in a brass tube of inner diameter 20 mm with 2 mm holes as artificial flaws. SFT6000N card operates with 40 kHz injection voltage firequency.

Current probe - to measure the third harmonic component of/p It is then converted to actual I, from the ZnO characteristic data provided by the arrester manufacturer, /r versus /jr, corrected to the site operating temperature and voltage. The value of is then used to assess the condition of the arrester. 

An ac and/or dc current probe for the oscilloscope. Especially needed for switching power supply design. Some appropriate models are Tektronics P6021 or P6022 and A6302 or A6303, or better. 

Displacement, or eddy-current, probes are designed to measure the actual movement, or displacement, of a machine s shaft relative to the probe. Data are normally recorded as peak-to-peak in mils, or thousandths of an inch. This value represents the maximum deflection or displacement from the true centerline of a machine s shaft. Such a device must be rigidly mounted to a stationary stmcture to obtain accurate, repeatable data. See Figure 43.21 for an illustration of a displacement probe and signal conditioning system. 

Displacement or eddy-current probes are designed to measure the actual movement, i.e. displacement, of a machine s shaft relative to the probe. Therefore, the displacement probe must be rigidly mounted to a stationary stmcture to gain accurate, repeatable data. 

The bottom line is—to measure Fet current, insert the current probe into its Drain, never in its Source. Put in a small sense resistor in the Source if you must, but nothing inductive please. 

To check if our bulk decoupling is good enough, we can place a current probe on the leads coming from the bench supply and check if the wobble seems too much. 

Note that while prototyping, it is a bad idea to insert a current probe (through a loop of wire), anywhere in a critical trace section. The current loop becomes an additional inductance that can increase the amplitude of the noise spikes dramatically. Therefore practically speaking, it can often become virtually impossible to measure the switch current or the diode current individually (especially in the case of switcher ICs). In such cases, only the inductor current waveform can really be measured properly. Sometimes we can place a small sense resistor instead of a current loop, because a good resistor will not create inductive kicks at least. 

Figure 6-17 Leave Options on the Board for Easy Connection to Current Probe for Measuring Inductor Current...

Recommendation 10 (Figure 6-17) Similar to Recommendation 9,1 leave a provision for inserting a current probe to monitor the inductor current. 

Finally, there seemed to be no pattern to the failures So we had to hook up a scope and current probe to check the 5 V diode waveforms to rule out excess current and voltage stresses. Note that a freewheeling diode failure will always precede a Fet failure, very rarely the other way around. In other words, if the diode failed, we would expect the Fet to fail soon thereafter, but if the Fet was what started it all, the diode would usually be found intact. So at least we were reasonably sure we were heading in the right direction by looking at the diode, not the Fet We looked at all the diode waveforms, and we were sure... 

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