In eddy current testing, the condition of the workpiece is reflected by the change of the eddy current probe. As long as the components that are sensitive to changes in the magnetic field, such as Hall elements, magneto-sensitive diodes, etc., can be used as eddy current probes, the detection coils are currently more commonly used. According to the principle of eddy current detection, the probe first needs an excitation coil so that the alternating current can pass through and excite the electromagnetic field around it and in the inspected workpiece; at the same time, in order to detect the signals that reflect the various characteristics of the workpiece under the action of the electromagnetic field, it also needs a detection coil. Because the excitation coil and detection coil of the eddy current probe can be two coils with different functions, or the same coil has both excitation and detection functions. Therefore, when there is no need to distinguish the functions of the coils, the excitation coil and the detection coil are usually collectively referred to as the detection coil, or eddy current probe.
There are two types of eddy current probes: parameter type and transformer type. The parametric coil is generally a coil that generates an excitation magnetic field and a coil that picks up the eddy current signal of the workpiece, so it is also called a self-inductive coil. For example: absolute probe and differential probe. Transformer-type coils are generally composed of two sets of coils, one is dedicated to generating an alternating magnetic field (or primary coil), and the other is used to pick up eddy current signals (or secondary coil), also called mutual inductance coil.
There are three types: outer-through probe, inner-through probe and placement probe.
(1) Outer-through probe: This kind of eddy current probe is to pass the workpiece through the inside of the detection coil for detection. Because the probe and the workpiece do not need to contact, it is easy to realize batch and high-speed inspection of eddy current flaw detection, and it is easy to realize automatic detection. Therefore, it is widely used in the surface quality inspection of small diameter pipes, bars, and wire specimens.
(2) Internal penetrating probe: In the inspection of pipe fittings, it is sometimes necessary to put the probe into the inside of the pipe. This kind of probe inserted into the pipe fitting for inspection is called the penetrating probe, which is suitable for condenser pipes ( such as titanium tube, copper tube) in-service inspection.
(3) Placed probe: also known as point probe. During flaw detection, place the probe on the surface of the tested workpiece for inspection. This kind of probe is small in size, and the coil is generally equipped with a magnetic core, so it has the property of magnetic field focusing and high sensitivity. It is suitable for surface inspection of various plates, strips, large-diameter pipes and bars, and it can also perform local inspections on a certain area of complex-shaped workpieces.
It can be divided into two types: self-comparative and other-comparative.
(1) Self-comparative probe: also called differential probe, which uses different parts of the same test piece as a comparison. This kind of probe can effectively suppress the change of the coil impedance caused by the slow change of the ambient temperature and the shape of the workpiece. It has high detection sensitivity for various small defects.
(2) He-comparative probe: This kind of eddy current probe is used to detect a certain position of the object to be inspected by comparing the difference in electromagnetic characteristics with another object. Usually, this reference object is a comparative sample. This detection method has the advantage of being able to detect slow changes in the dimensions and chemical composition. Corresponding to the above-mentioned differential probe, we are accustomed to call this other comparative probe an absolute probe.