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Magnetostrictive Dental Scalers
Paul Bartlett
Wolfson Centre for Magnetics
29 March 2007
Outline of Presentation
• Description of magnetostriction• Introduction to dental scalers• Description of the main dental scaler components• Wolfson Centre work on magnetostrictive
materials characterisation• Wolfson Centre work on the dynamic properties
of dental scalers• Wolfson Centre Finite Element Modelling• Wolfson Centre work on new dental scalers
What is Magnetostriction?
• When a magnetic field is applied to a ferromagnetic material (eg iron), the series of domains rotate to partially align themselves to the magnetic field direction.
• This results change in length (Δl) known as the strain (e).
Source:MTS Sensors
What is an Ultrasonic Dental Scaler
• Usually a magnetostrictive device that is used to remove ‘calculus’ from teeth
• Utilises a combination of ultrasonic vibrational abrasion and cavitation
• Comprises of the:– Power Supply
– Handle
– Insert
• Has interchangeable inserts’ with different geometry tips
Dentspy International
Analysis of typical dental scalers
• X-Ray EDS analysis showed the laminates were of high purity nickel
• Drive signal ramps up to 40 kHz when trying to identify resonance
• Drive system seeks for a resonance to lock on to
• This is done by identifying a phase change in a pick-up coil signal
Tip Grip
Handle
Magnetostrictive StackDrive Coil
Sensing coil (under
secondary drive coil)
DC Magnetostiction Characterisation
DC characterisation test rig that was used to obtain strain Vs. applied field data for various temperatures and pre-stresses for strips of magnetostrictive material.
Magnetostrictive sample
Pre-stress rig
DC drive coil
Sample holder
Opened Sample holder
Example DC Strain-H plots
AC Material Measurements• A characterisation rig has been developed for the AC
analysis of sample strips of candidate materials.
AC drive coil with strip of material inserted
Single-point laser
vibrometer head
Example dynamic analysis of strips via laser vibrometry
NiFe 298 A/m
0
1
2
3
4
5
6
7
10000 12500 15000 17500 20000 22500 25000 27500 30000
Frequency (Hz)
Dis
pla
cem
ent
(mic
rom
eter
s R
MS
)
1st Harmonic
2nd Harmonic
3rd Harmonic
Ni Fe 2nd Harmonic
0
1
2
3
4
5
6
7
10000 15000 20000 25000 30000 35000 40000
Frequency (Hz)
Dis
pla
cem
ent
(mic
rom
etre
s R
MS
)
298 A/m
222 A/m
148 A/m
Ist, 2nd and 3rd Harmonics for a single
drive field value.
2nd harmonic dominant
2nd Harmonic at differing drive
fields
y = -0.0029x + 98.362
R2 = 0.9942-60
-40
-20
0
20
40
60
80
100
0 10000 20000 30000 40000 50000
Frequency (Hz)
Ph
ase
(Deg
rees
)
Phase sensing
coil
Phase sign change in region of 30kHz
resonance
0
2
4
6
8
10
12
0 10000 20000 30000 40000 50000 60000
Frequency (Hz)
Dis
pla
cem
ent
Am
plit
ud
e (n
m)
Device
Wolfson CoilDynamic analysis of current inserts
Analysis of Current Dental Scalers
Resonance sensing system
Production of scalers and coilsStandard
Insert
Prototype Insert
Modified Drive Coil
Stacks were produced of the candidate materials and were insulated against eddy current effects with a spray enamel paint. This paint has an operating temperature of up to 425o C.
A Dentsply drive coil had to be modified to take the larger diameter stacks without damaging the coil systems.
Finite Element Modelling
• Simple 2d and 3d models produced
• Dynamic model shown
• Multi-element stack model also produced
Dynamic FE Analysis
Mechanical modelling showed;
•That there were flexural modes in the region of previously measured resonances
•That modelled tip vibrations closely matched actual results obtained by the Birmingham University team
•This suggests that modelling could be used for other tip-shape designs
Scanning laser vibrometer data of a tip
(University of Birmingham)
3d Finite Element mechanical model
Note the similarity of shape of the
modelled and real tip
vibrations
Scaler Failure Issue
Modelled flexural mode in the region of the insert resonance.(Vibration mode magnitude exaggerated)
So-called ‘broken’ insert.
Compare the form of the stack distortion to that of the model above
What was covered in the presentation
• Description of magnetostriction• Introduction to dental scalers• Description of the main dental scaler components• Wolfson Centre work on magnetostrictive
materials characterisation• Wolfson Centre work on the dynamic properties
of dental scalers• Wolfson Centre Finite Element Modelling• Wolfson Centre work on new dental scalers
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