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Ultrasonic Transducers: Part I—How They Work
cleaningtimes Ultrasonic Transducers: Part I—How They Work
T
his column is about technology common to many metal cleaning shops: ultrasonic cleaning systems. These equipment components are used in both aqueous and solvent cleaning applications. Chiefly employed for removing solid particulate matter, they are agents of agitation that can dislodge soil components that can’t be removed solely by chemical action. In common use for decades, they are becoming (or have become) commodity equipment products despite the best efforts of suppliers to provide differentiation. A recent spate of requests for information about how ultrasonic transducers work (Part I), how to select the right frequency for use (Part II, slated for the March/April edition of Metal Finishing), and how to select the right power level for them (Part III) prompted this set of three columns.
GOOD VIBRATIONS Ultrasonic transducers produce waves of fluid pressure that bombard part surfaces (and all surfaces under immersion). The waves are produced by diaphragms that vibrate under immersion in fluids. The device producing the vibration is called a “transducer.” Frequency of vibration is high— from tens of thousands to hundreds of thousands of oscillations (cycles) per second (cps or Hertz). Consequently, the effect of each cycle of vibration is negligible—but their cumulative and continuous effect can be either positively or neg34 I metalfinishing I January/February 2013
atively dominant. There are two methods by which transducer diaphragms are caused to vibrate.
PIEZOELECTRIC TRANSDUCERS A piezoelectric material has two unusual and interrelated characteristics. They are basically the reverse of one another: • When a force is applied to a piezoelectric material, a tiny electric current is produced. • When an electric current is passed through piezoelectric materials they deform, i.e., change in size (volume) by a few percent.
It is the latter characteristic that produces a vibrating diaphragm. A rigid connector (arm) causes the diaphragm to move slightly when the piezoelectric material changes shape
upon application of an electric current (see Figure 1). Repeated application of the electric current, followed by its relaxation, enables a diaphragm to move forward and backward in one direction. Most piezoelectric materials are ceramics, many of which contain silicon, lead, aluminum, or titanium oxides.
MAGNETOSTRICTIVE TRANSDUCERS There is a magnetic analog to the piezoelectric effect. A ferromagnetic material (magnetic Iron) will respond mechanically to magnetic fields. This effect is called “magnetostriction.” Magnetostrictive materials transduce or convert magnetic energy to mechanical energy. As with the piezoelectric effect, the reverse is also true. When a magnetostrictive material is magnetized, it elongates—that is, it changes dimension in one direction. As shown in Figure 1, that dimensional change can be used to cause a
Figure 1
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cleaningtimes Piezoelectric
Magnetostrictive
Material
Ceramic
Metal
Normal attachment of diaphragm to housing wall
Epoxy bonding, or brazing with copper under vacuum
Brazing with silver
Location relative to liquid tank
Within (as an immersible unit) or externally attached to sides or bottom
Relative mass of transducer element
Low
High
Normal thickness of housing wall
Thin (typically 14 gauge [1.7mm] )
Thick (1/16 [5 mm] or greater)
Normally used frequency range
20, 30, 40, kHz and up
20, 25 and no higher than 30 kHz
This is the frequency of alternating current supplied to the transducer element by a matched and provided power generating unit. The power generating unit is supplied locally with 60 Hz electricity Potential for part damage via excessive sonic action
Much less, because frequencies are higher
Much more, because frequencies are lower
Applied energy level – at the same frequency
Lower
Higher
Applications by mass of parts
Used with smaller part masses, where energy absorption is not significant. If a larger part mass is added, it will absorb sonic energy and that available for surface cleaning will be reduced. Hence, it is important to match the transducer to the mass being cleaned.
Used with larger part masses, which absorb significant quantities of supplied sonic energy.
Application by type of parts
Small assemblies, machined fasteners, rolled sheet and strip, compact disks, etc.
Foundry castings, extruded pipe, injection molds, or parts which are highly resistant to damage
Application by type of soil
Single particles, spores or other biological contamination
Mill scale, some coatings
Concern about useful life
General maintenance life is quite Some manufacturers warranty the acceptable, but failures do occur due to: transducer and housing for whatever • Stress corrosion cracking, period the purchaser desires to use it • Cavitation damage on radiating surface, • Fatigue of the epoxy bond, • Cracking of the ceramic Users should expect a useful life of at least two years, but perhaps not three years.
Purchase price for the same frequency and power level
~~ $3 / watt, and up
~~ $5 to 6 / watt, and up
Author’s characterization of noise level
An electric razor
A machine gun (hearing protection should always be used by workers)
Table 1 Comparison of Piezoelectric and Magnetostrictive Transducers
www.metalfinishing.com
January/February 2013 I metalfinishing I 35
cleaningtimes diaphragm to move though driven by a different factor. Most magnetostrictive materials are metal alloys of nickel or contain significant quantities of nickel compounds. Magnetostrictive transducers are not used at frequencies above around 30 kHz. The main reason is that the difficulty and cost of controlling the motion of the relatively large mass of material (dense nickel) associated with magnetostrictive transducer elements becomes too severe at frequencies above that level. The two methods of generating pressure waves for cavitation are compared in Table 1.
MAKING A CHOICE Some may inform managers that the choice is between the higher purchase price and longer maintenance life of magnetostrictive trans-
36 I metalfinishing I January/February 2013
ducers vs. the opposite for piezoelectric transducers, or to achieve a lower level of operating noise. That's a false choice. The choice should be totally based on the character of the parts. • No one, for example, would consider using magnetostrictive transducers for cleaning of disk drive components where piezoelectric transducers are commonly used. The components would “dance" in the water bath and be destroyed. • Nor would anyone consider using piezoelectric transducers for removal of scale prior to painting of small engine blocks for lawn mowers. Nothing would be removed.
WHAT ABOUT FREQUENCY? In next month’s column, we’ll review the science a manager should use to choose one frequency of
ultrasonic waves vs. others. And in the follow-up column thereafter (May/June issue), we’ll review how to size the power input.
BIO John Durkee is the author of the book, “Management of Industrial Cleaning Technology and Processes”, published by Elsevier (ISBN 0-0804-48887). In 2013, Elsevier will publish in print his two landmark books, “Science and Technology of Cleaning with Solvents” [ISBN 9781455731312]. and “Handbook of Cleaning Solvents,” (ISBN-13:978145573144), as well as a four-part e-Book Design of Solvent Cleaning Equipment. Durkee is an independent consultant specializing in metal and critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or [email protected].
www.metalfinishing.com
T
his column is about technology common to many metal cleaning shops: ultrasonic cleaning systems. These equipment components are used in both aqueous and solvent cleaning applications. Chiefly employed for removing solid particulate matter, they are agents of agitation that can dislodge soil components that can’t be removed solely by chemical action. In common use for decades, they are becoming (or have become) commodity equipment products despite the best efforts of suppliers to provide differentiation. A recent spate of requests for information about how ultrasonic transducers work (Part I), how to select the right frequency for use (Part II, slated for the March/April edition of Metal Finishing), and how to select the right power level for them (Part III) prompted this set of three columns.
GOOD VIBRATIONS Ultrasonic transducers produce waves of fluid pressure that bombard part surfaces (and all surfaces under immersion). The waves are produced by diaphragms that vibrate under immersion in fluids. The device producing the vibration is called a “transducer.” Frequency of vibration is high— from tens of thousands to hundreds of thousands of oscillations (cycles) per second (cps or Hertz). Consequently, the effect of each cycle of vibration is negligible—but their cumulative and continuous effect can be either positively or neg34 I metalfinishing I January/February 2013
atively dominant. There are two methods by which transducer diaphragms are caused to vibrate.
PIEZOELECTRIC TRANSDUCERS A piezoelectric material has two unusual and interrelated characteristics. They are basically the reverse of one another: • When a force is applied to a piezoelectric material, a tiny electric current is produced. • When an electric current is passed through piezoelectric materials they deform, i.e., change in size (volume) by a few percent.
It is the latter characteristic that produces a vibrating diaphragm. A rigid connector (arm) causes the diaphragm to move slightly when the piezoelectric material changes shape
upon application of an electric current (see Figure 1). Repeated application of the electric current, followed by its relaxation, enables a diaphragm to move forward and backward in one direction. Most piezoelectric materials are ceramics, many of which contain silicon, lead, aluminum, or titanium oxides.
MAGNETOSTRICTIVE TRANSDUCERS There is a magnetic analog to the piezoelectric effect. A ferromagnetic material (magnetic Iron) will respond mechanically to magnetic fields. This effect is called “magnetostriction.” Magnetostrictive materials transduce or convert magnetic energy to mechanical energy. As with the piezoelectric effect, the reverse is also true. When a magnetostrictive material is magnetized, it elongates—that is, it changes dimension in one direction. As shown in Figure 1, that dimensional change can be used to cause a
Figure 1
www.metalfinishing.com
cleaningtimes Piezoelectric
Magnetostrictive
Material
Ceramic
Metal
Normal attachment of diaphragm to housing wall
Epoxy bonding, or brazing with copper under vacuum
Brazing with silver
Location relative to liquid tank
Within (as an immersible unit) or externally attached to sides or bottom
Relative mass of transducer element
Low
High
Normal thickness of housing wall
Thin (typically 14 gauge [1.7mm] )
Thick (1/16 [5 mm] or greater)
Normally used frequency range
20, 30, 40, kHz and up
20, 25 and no higher than 30 kHz
This is the frequency of alternating current supplied to the transducer element by a matched and provided power generating unit. The power generating unit is supplied locally with 60 Hz electricity Potential for part damage via excessive sonic action
Much less, because frequencies are higher
Much more, because frequencies are lower
Applied energy level – at the same frequency
Lower
Higher
Applications by mass of parts
Used with smaller part masses, where energy absorption is not significant. If a larger part mass is added, it will absorb sonic energy and that available for surface cleaning will be reduced. Hence, it is important to match the transducer to the mass being cleaned.
Used with larger part masses, which absorb significant quantities of supplied sonic energy.
Application by type of parts
Small assemblies, machined fasteners, rolled sheet and strip, compact disks, etc.
Foundry castings, extruded pipe, injection molds, or parts which are highly resistant to damage
Application by type of soil
Single particles, spores or other biological contamination
Mill scale, some coatings
Concern about useful life
General maintenance life is quite Some manufacturers warranty the acceptable, but failures do occur due to: transducer and housing for whatever • Stress corrosion cracking, period the purchaser desires to use it • Cavitation damage on radiating surface, • Fatigue of the epoxy bond, • Cracking of the ceramic Users should expect a useful life of at least two years, but perhaps not three years.
Purchase price for the same frequency and power level
~~ $3 / watt, and up
~~ $5 to 6 / watt, and up
Author’s characterization of noise level
An electric razor
A machine gun (hearing protection should always be used by workers)
Table 1 Comparison of Piezoelectric and Magnetostrictive Transducers
www.metalfinishing.com
January/February 2013 I metalfinishing I 35
cleaningtimes diaphragm to move though driven by a different factor. Most magnetostrictive materials are metal alloys of nickel or contain significant quantities of nickel compounds. Magnetostrictive transducers are not used at frequencies above around 30 kHz. The main reason is that the difficulty and cost of controlling the motion of the relatively large mass of material (dense nickel) associated with magnetostrictive transducer elements becomes too severe at frequencies above that level. The two methods of generating pressure waves for cavitation are compared in Table 1.
MAKING A CHOICE Some may inform managers that the choice is between the higher purchase price and longer maintenance life of magnetostrictive trans-
36 I metalfinishing I January/February 2013
ducers vs. the opposite for piezoelectric transducers, or to achieve a lower level of operating noise. That's a false choice. The choice should be totally based on the character of the parts. • No one, for example, would consider using magnetostrictive transducers for cleaning of disk drive components where piezoelectric transducers are commonly used. The components would “dance" in the water bath and be destroyed. • Nor would anyone consider using piezoelectric transducers for removal of scale prior to painting of small engine blocks for lawn mowers. Nothing would be removed.
WHAT ABOUT FREQUENCY? In next month’s column, we’ll review the science a manager should use to choose one frequency of
ultrasonic waves vs. others. And in the follow-up column thereafter (May/June issue), we’ll review how to size the power input.
BIO John Durkee is the author of the book, “Management of Industrial Cleaning Technology and Processes”, published by Elsevier (ISBN 0-0804-48887). In 2013, Elsevier will publish in print his two landmark books, “Science and Technology of Cleaning with Solvents” [ISBN 9781455731312]. and “Handbook of Cleaning Solvents,” (ISBN-13:978145573144), as well as a four-part e-Book Design of Solvent Cleaning Equipment. Durkee is an independent consultant specializing in metal and critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or [email protected].
www.metalfinishing.com