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