A Permanent Magnet Repulsive Type Magnetic Bearing Balance System

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A Permanent Magnet Repulsive Type Magnetic
Bearing Balance System

• Tomotada Okada, Alaa Hussien, Takahisa Ohji,
• Sotoshi Yamada, Masayoshi Iwahara

Institute of Nature and Environmental Technology,
Kanazawa University Department of Electrical
Engineering, Toyama University
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Background
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Magnetic Bearing

• It does not produce noise, and has less vibration
• It has non-contact nature
• It does not need lubrication

A Permanent Magnet Repulsive Type Magnetic
Bearing

• Passive ( Permanent magnet )
• Active ( Electromagnet with control )

• Simplification
• Miniaturization
• Cost reduction

of the equipment are possible
Application High sensitive balance
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Background
Required performances ofa micro-mass
measurementbalance system
Features of Magnetic bearing

• Measuring very small masses
• High measuring sensitivity

Using A Permanent Magnet Repulsive Type Magnetic
Bearing is adequate
A Permanent Magnet Repulsive Type Magnetic
Bearing Balance System
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System Configuration
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Structure of the axial shaft part
Axial shaft part

• RTMB is used Both passive bearing (radial
  direction) and
  active bearing (axial direction) are employed
• Only one direction (Axial direction) needs to be
  controlled

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Structure of the balance beam part
Balance beam part
For Balance beam control

• Electromagnet
• which has iron core
• Voice Coil Motor (VCM)

are used individually
The mass measurement in both cases is compared
Axial shaft part
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Structure of the gap sensor
Axial shaft part Gap sensor
Balance beam part Gap sensor
The luminescence side ? a LED The light reception
side ? a bisection photodiode
The quantity of the incident light on the
bisection photodiode is changed with the change
of the position of the shutter
The shutter, which is installed atthe axial
shaft part or the balance beam part, obstructs
the light
The LED emits light
The output voltage of the bisection photodiode
will be changed
The displacement of the axial shaft or the
balance beam is detected
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Characteristics of the Gap sensor
Out put voltage vs. displacement characteristics
of a gap sensor
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Control system design of A Permanent Magnet
Repulsive Type Magnetic Bearing Balance System
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Control system design-1
Repulsive Type Magnetic Bearing Balance System
Axial shaft part (RTMB)
Balance beam part
The unstable axial direction is controlled
The levitation position is controlled
Mass measurement

• The axial shaft part and the balance beam part
  are controlled independently

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Control system design-2
Axial shaft part
An optimal regulator --- The controlled
object is kept at an equilibrium state
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Control system design-3
Balance beam part
An integration type optimal servo --- The
controlled object in our case, the
position of the balance beam is kept at a fixed
position
The levitation position should not be changed
when the masses on the holder are changed
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Control system design-4

• Control system
• The axial shaft part and the balance beam part
  are controlled digitally through a DSP

Control-flow diagram
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Characteristics of A Permanent Magnet Repulsive
Type Magnetic Bearing Balance System
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Balance beam control (Using an electromagnet
with iron core)

• The vertical movement of the balance beam is
  controlled by the electromagnet
• The additional weight is transferred to the
  current of the controlled electromagnet so,
  the change in control current expresses the
  weight value

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System characteristics (Using an electromagnet
with iron core)
Control current of electromagnet and
Displacement vs. weight characteristics in case
of increasing mass gradually
Control current of electromagnet vs. weight
characteristics in case of increasing and
decreasing mass
Removing the errorwhich may result fromthe
non-linearity of theforce-current relation of EM

• Non linear
• The hysteresis error while increasing and
  decreasing the weight
• The maximum hysteresis error 730 µg
• ?less than 1 of 100 mg that is full weight

The use of VCM
F k ( i / x )2
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Balance beam control (Using a VCM)

• The vertical movement of the balance beam is
  controlled by the VCM
• The additional weight is transferred to the
  control current of the VCM so, the change in
  control current expresses the weight value

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System characteristics (Using a VCM)
Structure of VCM
F magnetic force I current B flux density L
length of coil
F IBL
Control current of VCM vs. weight
characteristics in case of increasing mass by 20
mg

• The relation between Control current and Weight
  is linear

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System characteristics (Using a VCM)
Control current of VCM vs. weight
characteristics in case of increasing and
decreasing mass ( 0 100 mg )
Close-up view around 20 mg

• The hysteresis error while increasing and
  decreasing the weight
• The maximum hysteresis error 48 mg
• ?less than 0.05 of 100 mg that is full weight

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Conclusion
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Conclusion

• A Permanent Magnet Repulsive Type Magnetic
  Bearing is applied for designing a balance
  system to measure small masses with high
  sensitivity

• The comparison of the mass measuring sensitivity
• in case of using an electromagnet and a VCM for
  the balance beam control

• The resolution is improved by using a VCM

Future Improve the system sensitivity and reach
the error to be less than 1 mg
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End of slide