Fundamentals
How does EMAT – ultrasound generation work
EMAT is short for Electromagnetic Acoustic Transducer. EMAT systems can be used for non-destructive ultrasound testing. This system overcomes many problems associated with conventional piezo ultrasonic transducers.
An EMAT transducer generally comprises an induction coil and a permanent magnet. The induction coil is fed with an alternating current which generates an electromagnetic alternating field. The alternating field induces eddy currents on the surface of the test object. The eddy current interacts with the permanent magnetic field and generates elastic deformations directly in the test object. With adapted excitation, the deformations propagate as ultrasonic waves in the test object. Part of the ultrasonic signal is reflected at fault areas, and the reflected ultrasound can be detected in the receiver.
Unlike conventional (piezo) ultrasonic transducers, an EMAT testing system does not require a coupling medium or a mechanical contact as the ultrasound is generated directly in the test object. This increases the test reliability, as the physical properties of the transmission path do not change. In addition, the required tolerance for the position and propulsion of the test object within the EMAT system is very low.
How does a phased array ultrasound system work
A phased array (phase or time controlled) system (antenna array) refers to a configuration whereby several transmitters transmit at different times in order to achieve a directivity. In the rod test, a row of coils forms the transmitters of the ultrasonic waves. An electrical pulse through the first transmitter coil generates an ultrasonic wave (see EMAT) in the rod to be tested, and the wave propagates longitudinally in the rod at the speed of sound.
As it passes through additional coils, the pulse is amplified in one direction by further excitations. The wave from each transmitter to the rear is eliminated. The focusing and amplification of the ultrasound is thus fully electronic. The transmission direction can be changed very quickly and purely electronically.
The receipt of ultrasonic signals functions in the same way. Here, the wave reflected back at fault areas induces a small electrical pulse in a receiver coil. As it runs through additional receiver coils, more impulses are generated. The signals from all the receiver coils are added electronically with a time offset. This amplifies the ultrasonic wave signals, while signals from noise and other such
The receipt of ultrasonic signals functions in the same way. Here, the wave reflected back at fault areas induces a small electrical pulse in a receiver coil. As it runs through additional receiver coils, more impulses are generated. The signals from all the receiver coils are added electronically with a time offset. This amplifies the ultrasonic wave signals, while signals from noise and other such interfering signals are muted. The receipt direction can also be changed very quickly and purely electronically at the receiver.
This directivity achieves an extremely high signal-to-noise ratio and thus an extremely good sensitivity to faults.
How does the conventional surface test with eddy currents work
An eddy current is generated on the surface of the test object with an exciter coil. Material faults lead to changes in the eddy current field which can be detected in a receiver coil.
For rod material, there are continuous coils and mechanically intricate rotating probes. In both designs, the probe must be laboriously refitted when changing the material, since the distances between material and probe must be maintained very accurately. Depending on the process, the penetration depth into the surface is only approx. 30-300 µm.
How does the conventional internal fault test with a piezo ultrasound work
During an ultrasonic test using piezo technology, the ultrasonic signals are transmitted from the piezo probe to the test object through a coupling medium. Due to the use of media and the required close tolerances for the test object, the test equipment is very expensive to purchase and to operate. The test itself takes place in the radial direction, leaving an area of about 2 mm below the surface untested.
