How Does Eddy Current Flaw Detection Realize Automatic Full Detection?

Non-contact measurement systems include: non-destructive testing using eddy currents (eddy current testing) and non-contact measuring machines (infrared/CCD photoelectric technology). Applying these basic technologies, on-line inspection can sort out defective products without compromising product performance, and realize high-level quality management under high productivity.

The technology of eddy current flaw detector: Whether there are defects such as cracks on the surface of mechanical parts is very important, because it will cause serious quality problems. In order to ensure the safety and reliability of the processed parts, visual inspection alone cannot meet the requirements, so the requirements for the detection of the workpiece processing surface have increased. Therefore, in order to achieve the required quality in the production line, a total inspection system that uses non-contact measurement methods to inspect the parts produced in a short time is indispensable.

It is a kind of electromagnetic induction inspection that uses the eddy current of the conductor to conduct various inspections, and its typical application is eddy current flaw detection. Here is a brief introduction to the inspection of defects such as cracks on the surface of conductive workpieces.

1. Characteristics of eddy current in eddy current flaw detector

Non-contact detection of workpieces made of magnetic and conductive non-magnetic materials (aluminum alloy, stainless steel, copper, etc.). In addition to cracks, the eddy current can be used to detect the following defects: casting voids and holes on the surface, grinding burns, scratches, black skin residues, local material changes or foreign objects (material classification inspection) and local hardness changes (Inspection of material composition and structure), etc.

2. The principle of eddy current flaw detector

An alternating current flows through the coil, creating a magnetic field around it (represented by lines of force) and acting on nearby conductors. Since the magnetic field of the coil is changing, the magnetic field passing through the conductor changes with time both in direction and magnitude. At this time, the conductor hinders the change of the magnetic field to generate electricity, which is called electromagnetic induction. The electric field generated in the conductor generates a current (alternating current) under the action of the changing conductor magnetic force line, which is called an eddy current according to the shape of the current. The size and distribution of the eddy current generated in the conductor vary with the frequency, the conductivity and magnetic permeability of the conductor, the size and shape of the test body, the shape, size, current and distance from the conductor of the coil, and the shape of casing.

If there is a crack on the surface of the plate-shaped workpiece, the eddy current will detour and flow away from the cracked defect area (reducing the load on the coil), and as a result, the magnetic field will change. In this way, whether there is a change in the properties of the conductor caused by damage such as cracks and the distribution of the eddy current will change accordingly, and the magnetic field lines generated by the eddy current will also change accordingly. Therefore, as long as the change of the magnetic field line is detected, it is possible to know whether there is damage and the properties of other conductors.

There is no magnetic field in the air in the center of the two coils, and the output voltage is 0; when the conductive workpiece approaches the probe, the loop impedance changes, and the output voltage is no longer 0. Due to the change of impedance, the alternating current (that is, voltage) of the coils at both ends changes, and then the analog signal sent from the probe becomes a digital signal after being amplified.

By monitoring the change in impedance, it is possible to determine whether the workpiece is acceptable or not. According to the presence or absence of defect signals, defective products can be removed from the production line.