Translate

Friday, 9 September 2016

PIEZOELECTRIC TRANSDUCER IN ULTRASONIC TESTING.

The transducer is a very important part of the ultrasonic instrumentation system. The transducer incorporates a piezoelectric element which converts electrical signals into mechanical vibrational (transmit mode) and mechanical vibrations into electrical signals (receive mode) many factors, including material, mechanical and electrical construction, and the external, mechanical and electrical load conditions,influence the behaviour of a transducer. Mechanical construction includes parameter such as the radiation surface area mechanical damping, housing connector type and other variables of physical construction.As of this writing, transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.
A cut a way of typical contact transducer is shown above. It was previously learn that the piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible and impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that it's thickness is 1/4 of the desired wave length. This keeps waves that were reflected with in the matching layer in phase when they exit the layer (as illustrated in the image to the right) for contact transducer, the matching layer is made from a material that has an acoustical impedance between the active element and steel. Immersion transducers have a matching layer with an acoustical impedance between the active element and water.Contact transducer also incorporate a wear plate to protect the matching layer and active element from scratching.
The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with an impedance similar to that of  the active element will produce the most effective damping such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases material penetration increases but transducer sensitivity is reduced.