|The Technology and Operation of Piezo Sounder|
|A piezoelectric ceramic element is a sintered body of many crystals (Poly-crystals). Distortion of this crystal occurs when a stress is applied to the element, either thermally, mechanically or electrically. These distortions create many possible uses including alarm and sensor applications.
In using piezoelectric elements in audible output an application, a metal plate is attached to the ceramic element because the resonant frequency of the ceramic is too high to produce an audible tone by itself. This metal plate vibrates as shown in Fig. 1 due to the contraction and expansion of the piezo ceramic, and an audible signal is produced.
|The equivalent circuit for piezoelectric elements is shown in Fig. 3.The mechanical Resonance of the element is show by R, L, C where L and C determine the Resonant frequency (Fig. 3). Because the shunt capacitor is larger than the series combination the total impedance is capacitive.|
|MODES OF VIBRATION AND SUPPORTING METHODS FOR THE SOUND ELEMENT|
|Three principal modes of vibration can be created in the element depending on the style of mounting. This is illustrated in Fig. 2|
|(1) Node Support The sound element shown in Fig. 2(a) is node mounted, allowing it to vibrate in a free state. The node, a circumference where no vibration takes place, is created as shown by the broken line in Fig. 1. Mounting at the node causes the least mechanical suppression of vibration, thus allowing the greatest amplitude. Hence this mounting method, as illustrated in Figure 5(a),gives the highest sound pressure output and the most stable oscillation frequency of the three choices. As a result, this is the most appropriate design for high output, self-drive applications.(2) Edge Support Fig. 2(b) shows the mode of vibration when the sound element is supported at the edges. In this mounting configuration, the whole sound plate vibrates up and down as is illustrated by the broken line in the diagram. Hence, the edge method as illustrated in Fig. 5(b), suppresses the fundamental resonant frequency by moving the node. This offers the possibility of a wide frequency response, and is most advantageously used with external drive.(3) Center Support Fig. 2(c) shows the mode of vibration when the sound element is supported at the center. As the main vibration area is forcefully supported, large sound pressure levels are not possible when this method is used. This too is appropriate for external drive but due to design difficulties center support is not useful as an alarm.|
|CIRCUIT DESIGN CONSIERATIONS1. Driving Wave
The piezo elements may be driven by either sinusoidal, pulsed, or square wave, depending upon the particular application. If a sine wave is used, the device will operate at a frequency lower than the resonant frequency (Fo) with a lower sound pressure level. The reason for this is the loss of energy, through the time lag between peak deflections as shown in Fig. 7. It is important that a clean sinusoidal signal be provided, as any clipping of the waveform can result in frequency instability. If square waves or pulsed waves are used to drive the elements, a higher acoustic output will be realized, along with an increase in harmonic levels. A parallel capacitor can reduce these harmonics.2. Driving Frequency
For maximum output, a frequency of between 500Hz and 4KHZ should be used, as recommended by the specific part chosen.
3. DC Precaution
4. High Voltage Precautions
5. Booster Coil Applications:
7. Mounting Glue:
8. Design of Resonating Case:
9. Electrostatic Capacitance
10. Soldering Recommendations