Eddy-current testing

Eddy current testing (English Eddy current method ) is an electrical method for nondestructive testing of materials. It is used for testing electrically conductive materials.

Principle

When considering an alternating magnetic field generated by a coil which induces eddy currents in the material to be examined. In the measurement of the eddy current density is measured by a sensor, which usually also contains the excitation coil, is detected by the magnetic field generated by the eddy current. The measured parameters are the amplitude and the phase shift of the excitation signal. Is usually used to measure them, a second coil in the sensor. One then speaks of a fluxgate magnetometer, which is colloquially referred to in the German language as a Ranger probe. Occasionally, other magnetic field sensors, such as GMR sensors or SQUIDS are used.

Wherein eddy-current testing of the effect is utilized, that most of the impurities and damages in an electrically conductive material have a different electrical conductivity, or a different permeability than the material itself.

Since the measurement signal from the three parameters conductivity, permeability, and distance between the detector and the material surface is determined, the eddy current testing has three different applications:

By altering the frequency of the excitation voltage, the depth of penetration of the eddy currents (skin effect), whereby adaptation to the test conditions can be changed.

Practical implementation of a sorting test structure

Here will be explained how parts can be tested and sorted on the microstructure and material confusion.

PRIOR ART It is today, with a plurality of frequencies to check parts. The test setup often looks like that two pairs of coils ( from transmitter and receiver coil consisting respectively ) are connected in the same direction and the receiver coils in opposite directions at the transmitter windings. In the one pair of coils ( the compensation coil ) a major chunk is firmly positioned as compensation to make the type of the output signals so that in the event of a Gutteiles the level in the range of 0 V remains; the easier it is then to capture the variations in bad parts, whether as a rash on a monitor ( oscilloscope) or for digital processing (A / D converter and digital signal processor), as current devices common. The other pair of coils is automatically loaded by hand or with the parts to be tested.

Before a sorting test can begin, a number of known good parts are first save as a reference in the instrument. This will already be selected in experiments and by the user's experience certain frequencies at which the best separation conditions are achieved for the application. This is performed automatically for modern system, in which case the widest possible frequency band of eight test frequencies and a ratio of 1:1000 is used, a typical frequency range would be e.g. 25 Hz to 25 kHz. For each frequency, the " response " of the parts is stored, from which the test instrument then determines the tolerance ranges, in which the parts are to be tested. If not achieved for a frequency of this range, then the part is sorting out a bad part.

This type of testing on the microstructure of a material is very sensitive to even slight deviations, unexpected errors are well recognized by using multiple frequencies, and yet the throughput is very high. Depending on the type of inspection and the size and geometry of the parts and the required accuracy of the order of up to about ten parts per second testable and sortable.

The audit of hand is mainly for sampling, testing, and small quantities of parts into account. For a 100% check an automatically operating system is integrated into the production line, PLC - controlled and fully adapted to the production process in general. Ideally, the current throughput by expanding with such a system is not reduced, which means a huge increase in quality without sacrificing productivity.

Especially in the case of automated testing is to distinguish between static and dynamic test. The dynamic test allows higher throughput, which is bought by lower accuracy. The individual components (either isolated or adjacent to each other ) transported in a continuous flow through the coil pair, while the position of the parts is monitored by appropriate sensors to start the test at the right time and exit. For a number of part types and combinations of materials to be tested, this method is generally impossible, because the separation is too imprecise. Dynamic testing in the verification of balls is used frequently ( for ball bearing ) to detect the correct heat treatment or surface hardening. It does not run coil pair is used, but one of the specimens one side facing Probe, without additional compensation coil, but with a compensating winding in the same enclosure.

In the static testing of the device under test is stopped in the coil and tested at rest. Although the actual process of testing takes no more, so the mechanical parts handling reduces the throughput significantly in certain circumstances; the part must be stopped, you must wait a certain time, so that part is also really come to rest, and after that it has to be transported and ultimately sorted again.

Not be confused with

• Magnetic induction method

• EddyCation - an eddy current system for training

• Material testing
• Electrical Measurement
• Sensors