Electro Magnetic Transducers

Electro Magnetic Transducers-By Group No. 9-Mechanical Dept.-4th Semester-Mechanical Measurements and Metrology

IntroductionIn inductive type transducer, the magnetic characteristic of electric circuit change due to the motion of the object.Electro-magnetic transducer is the self generating type inductive type transducer in which voltage signal is generated because of relative motion of conductor and magnetic field.

ConstructionElectromagnetic transducer contains of a coil is wound direct on a permanent core as shown in the figure.When a plate of iron or ferromagnetic material is moved with respect to the magnet, the flux field expands or collapse and a voltage is induced in the coil.

Working of EMTPractical application of this transducer lies in the angular speed measurement.When pickup is placed near the teeth of rotating gear, speed measurements can be made with great accuracy.This transducer is linear only for small motion as the flux intensity changes due to change in air gap.

AdvantagesCompared to Piezoelectric Transducers, EMT probes have the following advantages:No couplant is needed. Based on the transduction mechanism of EMT, couplant is not required. This makes EMT ideal for inspections at temperatures below the freezing point and above the evaporation point of liquid couplants. It also makes it convenient for situations where couplant handling would be impractical.EMAT is a non-contact method. Although proximity is preferred, a physical contact between the transducer and the specimen under test is not required.

AdvantagesDry Inspection. Since no couplant is needed, the EMT inspection can be performed in a dry environment.Less sensitive to surface condition. With contact-based piezoelectric transducers, the test surface has to be machined smoothly to ensure coupling. Using EMT, the requirements to surface smoothness are less stringent; the only requirement is to remove loose scale and the like.

AdvantagesEasier for sensor deployment. Using piezoelectric transducer, the wave propagation angle in the test part is affected by Snells law. As a result, a small variation in sensor deployment may cause a significant change in the refracted angle.Easier to generate SH-type waves. Using piezoelectric transducers, SH wave is difficult to couple to the test part. EMT provide a convenient means of generating SH bulk wave and SH guided waves.

DisadvantagesThe disadvantages of EMT compared to piezoelectric UT can be summarized as follows:Low transduction efficiency. EMAT transducers typically produce raw signal of lower power than piezoelectric transducers. As a result, more sophisticated signal processing techniques are needed to isolate signal from noise.Limited to metallic or magnetic products. NDT of plastic and ceramic material is not suitable or at least not convenient using EMT.

DisadvantagesSize constraints. Although there are EMT transducers as small as a penny, commonly used transducers are large in size. Low-profile EMT problems are still under research and development. Due to the size constraints, EMT phased array is also difficult to be made from very small elements.Caution must be taken when handling magnets around steel products.

Comparison of Piezoelectric and Electromagnetic Transducers

ApplicationEMT has been used in a broad range of applications and has potential to be used in many other applications. A brief and incomplete list is as follows:Thickness measurement for various applications.Flaw detection in steel productsPlate lamination defect inspectionBonded structure lamination detection

ApplicationLaserweld inspectionfor automotive componentsVarious weld inspection for coil join, tubes and pipes.Pipeline in-service inspection.Railroad and wheel inspectionAustenitic weld inspection for power industryMaterial characterization

EMT Plate Testing Procedure