Preceding blogs have described the workings and principles of ultrasonic transducers. Today’s blog will summarize the information on piezoelectric ultrasonic transducers and give readers a view of what the real hardware looks like. An upcoming blog will concentrate on magnetostrictive hardware.
Piezoelectric Transducers –
Piezoelectric transducers, which may also be called electrostrictive transducers, nearly all have the same general construction. One or more piezoelectric elements are “sandwiched” between two pieces of metal. These pieces of metal are often called “masses.” This arrangement, known as a Langevin transducer, was first developed for sonar applications and is named after its inventor Paul Langevin. The following illustration shows the general arrangement of a Langevin ultrasonic transducer.
The piezoelectric driver(s) are compressed between the two masses M1 and M2 by a bolt through the center of the assembly. Some ultrasonic transducers use a series of bolts around the perimeter of the masses to apply the compression. The result is the same – compression of the transducer components. The entire assembly is resonant at the desired operating frequency with the piezoelectric element being only a small component of the overall assembly. A typical transducer uses one or or two piezoelectric elements. If there are two elements, they are arranged in such a way that their motion is additive. In the above illustration, the two piezoelectric elements are placed so that their positive faces contact a center electrode which is insulated from the rest of the assembly. The remaining parts of the assembly including the two masses M1 and M2 are at negative or ground potential and complete the circuit for the negative poles of the piezoelectric elements. A picture of a cross section of a real ultrasonic transducer is shown below along with a picture of one of the piezoelectric elements.
The faces of the piezoelectric ceramic elements are coated with a conductive silver material to provide the electrical connection. The small blue dot on the edge of the piezoelectric element is placed nearest to the positive pole to indicate which way the element should be arranged when the transducer is assembled. The hole through the center of the ceramic is to accommodate the bolt which clamps the assembly together. At assembly, the bolt is tightened to provide a precise amount of compression which is important to the proper operation of the transducer. In general, ultrasonic transducers are only assembled under controlled conditions in the ultrasonic manufacturer’s facility.
Once assembled, the piezoelectric ultrasonic transducer is resonant over its length. This resonance, as described in previous blogs, is essential to its proper operation. By changing the dimensions and contours of the transducer masses, the operating frequency along with electrical and acoustic characteristics can be customized for specific applications. An upcoming blog will describe these effects and their usefulness in more detail.
– FJF –