CONSTRUCTIN OF TRANSDUCER:
When an alternating electric potential is applied to crystals then they
exhibit the property of expanding and contracting". This property is
known as PIZEO-ELECTRICITY and the crystals are called as PIEZO-ELECTRIC
CRYSTAL. Due to an alternation of supplied potential, crystals alternatively
close together and move apart as shown in fig. (a). This is called as
process of polarization. The polarizing voltage is in the order of 200 volts
for every millimeter of thickness and the temperature must be raised above
the Curie point.
Mainly ceramics (lead zirconate) crystals are used in ultrasonic cleaners.
It is composed of barium, lead, titanium and zirconium. As crystals become
thinner its capacitance rises and its impedance falls. The below fig. (b)
shows the construction of transducer, the assembly of the transducer is as
follows.
Fig. (b)
The two crystals are sandwich between aluminum block and mild steel block
along with brass shims at the maximum torque of 150 PSI. The brass shims are
use to apply the output signal from the generator across the crystal.
The arrangement is such that aluminum block and mild steel are fixed with a
bolt made up of mild steel. The result of this is that both the crystals
will get opposite polarity of excitation at a particular instant of time.
Because of this one crystal will expand while the other will contract and
for the other half cycle the first will contract and the second will expand.
In this fashion expansion of one crystal is compensated by the contraction
of the other crystal. The crystal is lead zincronate titanate (PZT- 4). It
is a ceramic crystal that exhibits the piezoelectric property.
Features of the crystals:
1. Comparatively cheaper.
2. Its curie temperature is well above 300deg./cm.
3. It has higher power output with lower input voltage.
4. Its impedance is 150-ohm/sq. cm.
5. It can be easily machined to desired shape.
Ultrasonic Cleaning Equipments
