All ultrasonic cleaning systems utilize 1 of 2 available types of ultrasonic transducers; piezoelectric transducers and magnetostrictive transducers.  There are many differences between these transducer designs, including the way in which they are bonded to the "radiating surface" or ultrasonic diaphragm, the frequencies which can be generated, and the electrical efficiency of the system.  We will discuss each difference in detail.

How Each Transducer Design Functions

Both transducer designs produce ultrasonic activity by rapidly oscillating the ultrasonic diaphragm to which they are mounted.  However, each design performs this action differently.  Magnetostrictive transducers are essentially electromagnets made of a heavy nickel or alloy core which is wound with wire.  As electrical current is pulsed through the wires, the core vibrates at a frequency which matches the output frequency of the ultrasonic generator, thereby producing the ultrasonic cleaning effect in the tank.

Piezoelectric transducers are manufactured of lead zirconate titanate, a common piezoelectric material which expands and contracts when provided with the appropriate electrical frequency and voltage.  As the transducer expands and contracts rapidly, the ultrasonic diaphragm vibrates to introduce ultrasonic activity into the cleaning tank. 

Bonding Method

Piezoelectric transducers are bonded to the ultrasonic diaphragm with a combination of high-temperature epoxy, and a metallurgic bond usually composed of a welded and threaded stud.  Magnetostrictive transducers are bonded using vacuum brazing of the transducer base to the ultrasonic diaphragm. 

Manufacturers of magnetostrictive ultrasonic designs are quick to point out that a vacuum-brazed transducer bond is superior to an epoxy-bonded transducer, which is true to some extent.  Epoxy-bonded transducers cannot withstand highly abusive environments, such as environments in which objects may be dropped onto the diaphragm which may cause damage to the epoxy bond.  However, these environments do not exist in 99% of ultrasonic cleaning operations.  Additionally, transducers can easily be protected against such damage, and this protection is included when required on Zenith ultrasonic cleaning systems.

Electrical Efficiency

Ultrasonic cleaning systems operate by converting electrical energy into mechanical vibration to produce ultrasonic cavitation in the cleaning fluid.  When an ultrasonic system is highly-efficient, most of the incoming electrical power is converted to mechanical vibration.  For example, piezoelectric ultrasonic cleaning systems manufactured at Zenith are 95-98% electrically efficient.  Most of the in-coming power is being converted into mechanical vibration.  This efficiency is common to most piezoelectric ultrasonic systems, and is one of the primary reasons why over 95% of all ultrasonic equipment manufacturers utilize piezoelectric transducers.

Magnetostrictive transducers are highly in-efficient in design.  In some cases, systems are only 50-60% efficient.  More electrical power will be required to generate the same amount of ultrasonic cleaning action as a comparable piezoelectric system.  Not only do these systems require more electrical current, but the generators are also very large, and may require air-conditioning or other special cooling methods to keep components within acceptable operating temperatures.

Ultrasonic Frequency Choices

Ultrasonic operating frequency is perhaps the single most important consideration when choosing an ultrasonic cleaning system.  Each frequency has its own unique characteristics.  Low frequencies are used for large, un-detailed parts with heavy contamination and produce un-even cleaning action in the fluid, while higher frequencies produce more evenly-distributed cleaning action, and have the ability to penetrate small blind holes, threaded areas, and other detail.  More information is available under the "Ultrasonic College" drop-down menu above, under "Frequency Selection."

Choosing an ultrasonic frequency for a magnetostrictive system is easy, since there really is no choice at all.  Magnetostrictive system designs typically operate at frequencies below 30kHz, making these systems unsuitable for most ultrasonic cleaning applications.  Most parts being cleaned ultrasonically require the removal of lightly-bonded contaminants on the surface of precision parts, applications which are addressed with 40kHz, 80kHz or CROSSFIRE Multiple Frequency Ultrasonics operating in this range.  Low frequencies would produce in-consistent cleaning results on such parts, and parts may be damaged by cavitational erosion.  The photo on the left depicts a 1/4" thick glass plate that has been damaged by 40kHz ultrasonic cleaning action. 

Although Zenith manufactures 25kHz ultrasonic systems, they are rarely recommended for any ultrasonic cleaning applications, unless they are combined with 40kHz in a CROSSFIRE system.  This is the direct result of the thousands of sample parts which have been test-cleaned ultrasonically at Zenith.  Our Ultrasonic Testing Service is used to develop complete processes for our potential customers.  Parts are submitted and tested in various ultrasonic frequencies and cleaning agents to determine the best process to use for a given application.  Low frequency ultrasonic cleaning systems in the 25kHz-30kHz range never produce a better cleaning result than higher frequency systems do.  In fact, it is exactly the opposite.  Parts are cleaned more effectively in systems operating at 40kHz and above.  Since these systems are less damaging to components, quieter in operation, and better at cleaning in fine detailed areas, these systems are usually recommended for any cleaning application.

Generation of Audible Noise

The lower the operating frequency of the ultrasonic system, the more audible noise generated since the frequency of the ultrasonic system is closer to the human hearing range.  25kHz and 30kHz ultrasonic systems are so loud that they commonly require expensive acoustic insulation to reduce decibel levels.  In some cases, it may be impossible to reduce the decibel level to within OSHA guidelines for un-protected hearing in such systems, another reason why higher ultrasonic frequencies are typically recommended.  Since magnetostrictive ultrasonic systems operate at these low frequencies, the decibel level which is produced is very high, which must be considered when purchasing an ultrasonic system where operators will be located.

Reliability

Magnetostrictive ultrasonic generators operate at such high temperatures that it is common for these systems to require extensive additional cooling, such as air conditioning equipment dedicated to cooling the generators.  This excessive heat may cause premature failure of certain components in the system.  To overcome these issues, most manufacturers of magnetostrictive systems utilize more expensive, large electronic components which are able to withstand these high operating temperatures, and generator enclosures are significantly larger as a result.

Piezoelectric cleaning systems are highly-reliable electronic devices when properly engineered.  Generators are manufactured using surface mounted devices, transistors, and common MOSFET components, and run cool when compared to a magnetostrictive design.  Generators are relatively small, lightweight, and do not require air conditioning or excessive cooling. 

With regards to transducer reliability, one cannot argue that magnetostrictive transducers are more reliable.  The only way a magnetostrictive transducer can fail is if the wire winding on the transducer is broken.  Piezoelectric transducers are bonded with epoxy and mechanical means, and is manufactured using a more sensitive material, making them more sensitive to shock damage caused by heavy objects dropped on the transducer diaphragm.  However, as mentioned earlier, protection to prevent this from occurring is included in systems manufactured by Zenith for abusive environments.

Long-term operational reliability and power of piezoelectric transducers has been questioned by manufacturers of magnetostrictive ultrasonic systems, which claim that transducers lose power over time, and continuously deteriorate internally.  Zenith has not found this to be the case, and has over 73 years of experience proving otherwise.  Zenith performs repairs on piezoelectric systems which have been in operation for 20-30 years.  These systems have perfect transducer bonds that have not deteriorated, and the cleaning power is just as good or better than when originally installed, leading one to conclude that piezoelectric transducer systems, when properly engineered, can last for decades depending upon the application.

Erosion of the Diaphragm

Since magnetostrictive ultrasonic systems operate at very low ultrasonic frequencies, the cavitational erosion created by such systems is much greater than that of lower ultrasonic frequencies.  Cavitational erosion is the deterioration of the transducer diaphragm, and is normal for all ultrasonic cleaning systems.  Over time, the ultrasonic diaphragm will slowly erode, and will eventually deteriorate to the point where replacement is required.  The lower the operational frequency of the system, the faster erosion will occur, and since magnetostrictive transducers operate at low frequencies, one can expect significant erosion of the ultrasonic diaphragm.

To overcome this limitation, magnetostrictive system manufacturers construct their radiating diaphragms from very thick materials which extends the life of these systems.  However, by increasing the thickness of the diaphragm, it also becomes less flexible as well, and reduces the effective cleaning power in the ultrasonic tank.  The radiating diaphragm must be flexible enough to produce compression/expansion cycles in the cleaning fluid, a task which requires enormous power with diaphragms which are 3/8" thick.

Piezoelectric ultrasonic systems utilize thinner radiating diaphragms to maximize the ultrasonic power and energy distribution in the cleaning tank.  To overcome excessive cavitational erosion, Zenith applies hard-chrome plating to the radiating surface to decrease the rate of erosion.  Cavitational erosion is a direct indicator of ultrasonic power being generated.  If your diaphragm is not eroding, there is not enough power on the diaphragm to damage the surface.

Energy Distribution

Since magnetostrictive systems operate at lower frequencies, the distribution of the cleaning action in the fluid is poor.  Every ultrasonic cleaning system produces a cleaning action which is distributed in the fluid as a series of equidistant bands of cleaning action, with very little cleaning action produced between these bands.  At 30kHz, the active cleaning bands, or standing waves, are approximately 1" apart from one-another.  If the parts being cleaned have small blind holes, and these holes happen to rest between standing waves, they may not be cleaned. 

Since piezoelectric systems typically operate at higher ultrasonic frequencies, the cleaning action produced in the tank is more evenly-distributed when a higher frequency is chosen.  For example, at 80kHz, the standing waves produced are less than 1/4" apart.  Even the smallest blind holes or detailed part areas are effectively and evenly cleaned in such a system.  For more information about ultrasonic energy distribution, and its relationship to frequency, select the Ultrasonic College drop down menu and select "Frequency Selection."

Conclusion

After reading the above, one can see why most ultrasonic equipment manufacturers choose piezoelectric transducers.  While magnetostrictive designs are not damaged by shock, this is easily overcome by the use of protection devices mounted in the tank.  Piezoelectric systems are superior in every other regard, including frequency selection, energy distribution in the tank, decibel levels generated, reliability, equipment size, and efficiency.

For additional information, click here.

Zenith Mfg. & Chemical Corp.
85 Oak St.
Norwood, NJ  07648-0412
800-432-SONIC (7664)
FAX: 201-768-6999
sales@zenith-ultrasonics.com