Piezoelectric Transducer Analysis

In the Polaroid design the membrane is a special foil consisting of a Kempton® film coated with gold. The foil is stretched over a V-grooved plate, Figure 2.5. The transducer is protected by stainless steel housing. A few models with different housings are available. The sizes of these transducers are the main drawback since they are from 30 up to 45 mm in diameter. A further analysis of this structure and its dynamic features is found in [Attila et. al., 1995].
The piezoelectric transducer is based on a piezoelectric crystal that either is forced into mechanic oscillation by an external voltage or generates a voltage when exposed to mechanic oscillation. Consequently both sending and receiving functions can, also here, be performed by the
Among them Matsushita- and Murata- elements are often found in low cost applications. Their most common transducer is a 40 kHz one, based on amplification and impedance matching with a mechanical horn-like design. It is capsuled in an open housing that allows air flow through one side. Water proof Transducers are also available (Murata) but are usually somewhat less sensitive. Elements for precision measurements are often based on thickness expansion mode of the piezoelectric material in combination with impedance matching layers. This design can easily be hermetically sealed but is primarily used for higher ultrasonic frequencies, larger than 100 KHz. In the work [Frere Bastes et al, 1994] the internal design of the transducers is shown more in detail together with a temperature test of a few different transducer
However, this is no major advantage since the piezoelectric crystal starts oscillating at this frequency due to many disturbances sources. This implies low noise immunity.
To design the interface circuits for the electrostatic element is a cumbersome task. The high voltage bias eliminates the direct use of standard components like OP-amps. To handle the combination of high voltage and high bandwidth can be a quite difficult task.

Finally the prices of the sensor elements are quite equal. Real low cost versions of the piezoelectric elements are available. These ones cost less than $10 in small quantities while electrostatic ones cost about $40. High quality piezoelectric elements or elements for higher frequencies are usually somewhat more expensive and cost up to $100.However, ultrasonic sensors are still comparatively inexpensive sensors. The different shapes of the ultrasonic echoes using these two types of transducers are illustrated