Magnetic Actuation

EE485A Lecture10/27/2008

Magnetic Terminology Magnetic Field Intensity, H [A/m]: driving magnetic influence

external to a material Magnetic Field Density, B [T, or Wb/m2 or Vs/m2]: induced total

magnetic field inside a piece of magnetic material. Also called magnetic induction or magnetic flux density. Sense of magnitude:

Refrigerator magnet: 10 to 100 mT Earth magnetic field (near equator): 0.1 mT Rare earth magnet used in MRI: 1-2 T

For small levels of H, H and B related by:

B = 0H+M = 0 (H+H)=r0H0 = magnetic permeability of space= 1.257 x 10-6 Wb/(Am)r = relative permeability of the material = r –1 = magnetic susceptibility

Magnetic Terminology, cont.

Paramagnetic: x is weak and positive Diamagnetic: x is weak and negative Ferromagnetic (nickel, iron, cobalt and some rare earths): x is very

large. True B vs. H curve shown below: 1. Saturation: Level at which

magnetization saturates.

2. Remnance: the fraction of the saturation magnetization that is retained upon removal of the field

3. Coercivity: the reverse field needed to drive the magnetization to zero.

4. The area enclosed by the hysteresis curve indicates the amount of magnetic energy stored in the material.

Remnance

Coercivity

Saturation

H

B

Magnetic Actuator Principles Lorentz Force Actuator

Force induced between current-carrying conductor and external magnetic field

F = qv X B, or in terms of magnitude: F = qvBsin Interaction between permanent magnet or “soft

magnet” and DC magnetic field Objects will experience torque to align with external field. If field is non-uniform, objects will also experience a force. In soft magnets, shape anisotropy plays an important role

in determining direction of magnetization

F M wt H M = Magnetization (H)

wt = cross-sectional area

H = difference in magnetic driving field seen by object

Magnetic Actuator Principles

Micromagnetic Fabrication Large thicknesses

generally required Electroplating is most

common method for depositing ferromagnetic materials Requires deposition of

conductive seed layer which is usually later removed from unwanted areas

Popular materials: Ni80Fe20 permalloy CoNiMnP permanent

magnet

Magnetic Actuation Case Studies See pp. 292-303

Sensing Method ComparisonMethod Advantages Disadvantages

Electrostatic sensing

Simplicity of materialsLow PowerRapid Response

Large footprintElectronics complexitySensitive to particles and humidity

Thermal sensingSimplicity of materialsElimination of moving parts

Large power consumptionSlower response (than electrostatic)

Piezoresistive sensing

High sensitivity achievableSimplicity of materials

Requires doping of silicon to achieve high performanceSensitive to environmental temperature changes

Piezoelectric sensing

Self generating – no power necessary

Complex material growth and process flowCannot sustain high temperature operation

Actuation Method Comparison

Method Advantages Disadvantages

Electrostatic actuation

Simplicity of materialsRapid Response

Trade-off between magnitude of force and displacementLimited by pull-in

Thermal actuationCapable of achieving large displacementModerately fast response

Relatively large power consumptionSensitive to environmental temperature changes

Piezoelectric actuation

Fast responseCapable of moderately large displacement

Requires complex material preparationDegraded performance at low frequencies

Magnetic actuationCapable of generating large angular displacements

Moderately complex processDifficult to form on-chip, high-efficiency solenoids