Simulation of shaped comb drive

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Simulation of shaped comb drive as a stepped
actuator for microtweezers application
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Introduction

• Capacitance-based sensors and actuators have been
  extensively used in micro electromechanical
  systems (MEMS) devices.
• The basic design of a comb drive relies on the
  theory of parallel-plate capacitors, which in
  turn is a function of the plates area and shape.

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The following model of an electrostatically
actuated comb drive opens and closes a pair of
microtweezer.
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The following definitions applied to the system

• The material used in the fabrication
  (polysilicon) was assumed homogeneous and
  isotropic.
• The thickness dimension was small compared to the
  length.
• The stress in the normal Z-direction was ideally
  zero.

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Electromechanical principles of the comb drive

• In the air surrounding the comb drive, the
  electrostatic problem is described by Laplaces
  equation (in rectangular coordinates)

where
Vpotential energy, which is defined as
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• The electric energy We is computed at all
  elements according to

where
electric field vector, which is defined as
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• For capacitance C and electrostatic force Fes

where
V0actuation voltage.
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Modeling in COMSOL Multiphysics

• Because electrostatic forces attract the combs to
  each other, and geometric change has an impact on
  the electric field between them. To account for
  this effect, the model uses an arbitrary
  Lagrangian-Eulerian(ALE) method implemented in
  COMSOL Multiphysics Moving Mesh application
  mode.
• In this model the displacements are relatively
  large and support for large deformations in the
  Plane Stress application mode is used.

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Modeling
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Create rectangles
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Array
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Union
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Create another rectangle
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Intersection
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Create rectangles
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Create composite object
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Create another rectangle
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Scale
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Subdomain Setting for Electrostatics
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Subdomain 1
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Subdomain 2 , 3
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Force tab.
COMSOL Multiphysics then automatically generates
the variables Fes_nTx_emes and Fes_nTy_emes for
the electrostatic force components. Later on we
will use these variables to define the boundary
load in the Plane Stress application mode.
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Boundary Conditions
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Zero charge/symmetry
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Continuity
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Vin
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Ground
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Subdomain Setting for Moving Mesh
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Subdomain 1(Air)
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Subdomain 2 , 3 (Comb drives)
These are the displacement calculated in Plane
Stress application mode.
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Boundary Conditions
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Symmetry
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Fixed
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Comb drive
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Subdomain Setting for Plane Stress
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Subdomain 1
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Subdomain 2 , 3
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Boundary Conditions
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Symmetry
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Fixed
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Free
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Es force
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Initialize Mesh
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Solver Parameters
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Solver
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Result
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Result
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Result
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Reference

• Isabelle P. F. Harouche and C. Shafai,
  Simulation of shaped comb drive as a stepped
  actuator for microtweezers application, Sensors
  and Actuators A Physical, 2005.
• Michel A. Rosa, Sima Dimitrijev and H. Barry
  Harrison, Improved Operation of Micromechanical
  Comb-Drive Actuators through the Use of a New
  Angled Comb Finger Design, Journal of
  Intelligent Material Systems and Structures,
  1998, 9, 283