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MicroElectroMechanical Systems (MEMS) - Transducers and Actuators, Study notes of Electrical and Electronics Engineering

Wright State University (WSU) - DaytonElectrical and Electronics Engineering

An overview of transducers and actuators in microelectromechanical systems (mems). Transducers are devices that transfer power between different forms, and they can be divided into sensors and actuators. Actuators act on the environment, and this document covers electrostatic actuators, their advantages and disadvantages, and various examples, such as parallel plate actuators and texas instruments digital micromirror device. The document also discusses the operation of electrostatic actuators, including displacement vs. Actuation voltage, spring constants, damping coefficient, and lumped element dynamic model.

Typology: Study notes

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EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)
Wright State University
Wright State University
EE480/680
Micro-Electro-Mechanical Systems
(MEMS)
Summer 2006
LaVern Starman, Ph.D.
Assistant Professor
Dept. of Electrical and Computer Engineering
Email: [email protected]
2
EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)
Transducers: Actuators
Transducers: Actuators
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EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^1

Wright State University Wright State University

EE480/

Micro-Electro-Mechanical Systems

(MEMS)

Summer 2006

LaVern Starman, Ph.D. Assistant Professor Dept. of Electrical and Computer Engineering Email: [email protected]

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^2

Transducers: Actuators Transducers: Actuators

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^3

OverviewOverview

  • Transducers
  • Basic Mechanics
  • Actuators
    • Electrostatic
    • Electro-Thermal
    • Bimorph Electro-Thermal
    • Residual Stress
    • Mechanical Components

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^4

Transducers Transducers

  • Transducer: a device that transfers power from one form to another
  • Transducers can be divided into two categories
    • Sensors – reacts to environment
    • Actuators – acts on environment
  • Can you think of common examples of sensors and actuators?

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^7

Transducers Transducers

  • Sensor Classification

Biological Sugars, proteins, hormones, antigens, and etc.

Humidity, pH level and ions, concentration of gases, vapors and odors, toxic and flammable materials, pollutants, and etc.

Chemical

Magnetic field, flux, magnetic moment, magnetization, magnetic permeability, and etc.

Magnetic

Position, displacement, velocity, acceleration, force, torque, pressure, mass, flow, acoustic wavelength and amplitude, and etc.

Mechanical

Gamma rays, X-rays, ultra-violet, visible, infra-red, micro-waves, radio waves, phase, and etc.

Radiation

Thermal Temperature, heat, heat flow, entropy, heat capacity, and etc.

Charge, current, voltage, resistance, conductance, capacitance, inductance, dielectric permittivity, phase, frequency, and etc.

Electrical

Signal Measurands Classification

After Gardner, Microsensors , 1994.

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^8

Transducers Transducers

  • Actuator Classification

Biological Provide mechanical actuation, computing, etc.

Change/Provide humidity, pH level and ions, concentration of gases, vapors and odors, muscle stimulation, and etc.

Chemical

Provide magnetic field, flux, magnetic moment, magnetization, magnetic permeability, etc.

Magnetic

Provide displacement, velocity, acceleration, force, torque, pressure, mass, flow, and etc.

Mechanical

Radiation emit light and other radiation

Thermal heat, cool, radiate, and etc.

Electrical Provide charge, current, voltage, and etc.

Signal Action Classification

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^9

Transducers Transducers

  • Ideal Sensor Characteristics
    • Linear Operation
    • Noise Free Response
    • Zero Baseline
    • Fast Response Time
    • Large Frequency Bandwidth
    • No Saturation
    • High Sensitivity
    • High Resolution
    • Reliable and Rugged
    • No Performance Drift
    • Intolerant to Interference
    • No Hysteresis, Repeatable
    • Low Power Consumption
    • Simple Construction
      • Ideal Actuator Characteristics
        • Aforementioned, plus ….
        • High Force Per Unit Volume
        • Large Deflections
        • Simplicity of Drive and Control
        • Simple Interface

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^10

OverviewOverview

  • Transducers
  • Basic Mechanics
  • Actuators
    • Electrostatic
    • Electro-Thermal
    • Bimorph Electro-Thermal
    • Residual Stress
    • Mechanical Components

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^13

F
K A
V
V
F
K A
V
V

Shear Stress & Strain Cont. Shear Stress & Strain Cont.

E = 2 G (1 + μ) = 3 K (1 − 2 μ)

For isotropic materials (those having identical properties in every direction, generally not the case for most single-crystal materials, Shear modulus, G, is related to the elastic modulus, E, by μ is Poisson’s ratio K is the bulk modulus

The bulk modulus is defined as the ratio of hydrostatic stress to

volume compression

The bulk modulus of a material represents its volume change under uniform pressure. In general, solids are less compressible than liquids due to their rigid atomic lattices Water – K = 2.0 x 10 9 N/m^2 Aluminum – K = 7 x 10^10 N/m^2 Steel – K = 14 x 10^10 N/m^2

For Ex.

hydrostatic stress volume compression in N/m

2

Micromachined Transducers Sourcebook G. Kovacs ©

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^14

Poisson’ Poisson’s Strains Strain

a o

t o

L

L

D

D

axial strain

transverse strain

t o a o

D

D

L

L

Poisson’s ratio ν or μ always defined as a positive value

t o a o

D

D

L

L

transverse strain longitudinal strain

Typical values are 0.2 to 0.5 for most materials For most metals, Poisson’s ratio is ~ 0. Rubber’s have a Poisson’s ratio closer to 0. Cork has a Poisson’s ratio close to 0 (^) Micromachined Transducers Sourcebook G. Kovacs ©

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^15

Actuators: Electrostatic Actuators: Electrostatic

  • Advantages
    • Simple Designs
    • Simple Fabrication
    • High Frequency Operation
    • Low Power
  • Disadvantages
    • Low Force Per Unit Volume
    • High Drive Voltages
    • Nonlinear Operation

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^16

Actuators: Electrostatic Actuators: Electrostatic

  • Parallel Plate
    • Two plate like structures facing each other, with a potential

difference between them, will be drawn together due to the

force of electrostatic attraction.

L

d

Top Electrode

A

b

a

Flexure

Bottom Electrode

Anchor V

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^19

Actuators: Electrostatic Actuators: Electrostatic

  • Notes:
    • Displacement vs. Actuation Voltage
    • Spring Constants
    • Damping Coefficient
    • Lumped Element Dynamic Model

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^20

Actuators: Electrostatic Actuators: Electrostatic

  • Rotary

Cronos^ Cross section of motor

Pin

Stator Rotor

(^3 3) Cronos Torque Motor

Wobble Motor

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^21

Actuators: Electrostatic Actuators: Electrostatic

  • Comb Drive

Anchor

Folded Spring Suspension Truss

Moveable Comb

Stationary Comb

n = 30

Drive Line

Sense Line Ground

Stationary Comb

Bumper/Limiter

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^22

Actuators: Electrostatic Actuators: Electrostatic

  • Comb Drive
    • Sandia Example

Anchor

Folded Spring Suspension Truss

Moveable Comb

Stationary Comb

Ground

Stationary Comb

Bumper

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^25

Actuators: Electrostatic Actuators: Electrostatic

  • Scratch Drive

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^26

Actuators: Electrostatic Actuators: Electrostatic

  • Scratch Drive
75 V0-P
150 V0-P
30 V0-P
30 V0-P
V0-P
1/ fDrive
Function Generator & Amplifier
Probe

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^27

Actuators: Electrostatic Actuators: Electrostatic

  • When driving with a zero-bias input signal, the frequency of operation is twice the input signal frequency!

Drive Voltage

1/ fDrive +

Actuator Displacement V

1/2 fDrive

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^28

Actuators: Electrostatic Actuators: Electrostatic

  • Cantilever
    • Simpler Structure
    • Modeling Voltage vs. Deflection more
complicated.

K. E. Petersen, “Dynamic Micromechanics on Silicon: Techniques and Devices,” IEEE Transactions on Electron Devices , vol. ED-25, no. 10, 1978.

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^31

Actuators: ElectroActuators: Electro--ThermalThermal

• Advantages

  • Simple Designs
  • Simple Fabrication
  • High Force Per Unit Volume
  • Low Voltage

• Disadvantages

  • Temperature Dependent
  • High Electric Power Consumption
  • Low Frequency Operation

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^32

Actuators: ElectroActuators: Electro--ThermalThermal

  • Material expands due to Ohmic or Joule Heating causing motion of
actuator structure.
L

t

t

A
T(0) = T 1 T(L) = T^2

0 x

I

+ V -

2

q
x
T
k &
T T x T
L
Lx x
k
q
T x = − + − +

Heat Transfer

( ) =∫ [ + ( ( )− )]

x

Lnew x T T d

0

Thermal Expansion

q &

L
V A
R
V
A
IL
q I R

2 2 2

=Power =^2 = -or- =

2

2 2

2

  • or-
Volume
Power
L
V
A
I
q & = =

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^33

Actuators: ElectroActuators: Electro--ThermalThermal

  • Laterally/Horizontally Deflecting
    • Motion that is parallel to the plane of the substrate

d

h Lc Lf Lh Electrically Insulated Substrate

Wc

Wh g

anchors

cold arm hot arm

direction of actuation

polysilicon

Wf

200 μm

Example properties needed for modeling an electro- thermal actuator: ρ = electrical resistivity = 2.3 × 10 -5^ Ωm α = coefficient of thermal expansion = 29 × 10 -7^ K- αr = temperature coefficient of resistance = 1.25 × 10 -3^ K- k = thermal conductivity = 32 W/mK E = Young’s modulus = 169 GPa ν = Poisson’s ratio = 0. Comtois et al., 1995 (^) Optimum Dimensions:

  1. g = as small as possible
  2. h = as tall as possible
  3. Wc/Wh = 7
  4. Wh = as small as possible
  5. L (^) f ≈ L (^) h/
  6. Increasing the temp. difference between the cold and hot arm increases deflection.

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^34

Actuators: ElectroActuators: Electro--ThermalThermal

  • Laterally (Horizontally) Deflecting 35 μm 200 μm

μ

m

V. Bright et al., AFIT, 1996

Comtois et al., 1995

Lh = 200 μm Wh = Wf = g = 2 μm Lf = 35 μm Wc = 14 μm R = 1558 Ω Force ≈ 20 μN

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^37

Actuators: ElectroActuators: Electro--ThermalThermal

Yoke Dimple Tab Actuator Wiring

R. Reid, AFIT, 1996

R. Reid, AFIT

AFIT
AFIT

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^38

Actuators: ElectroActuators: Electro--ThermalThermal

R. Reid, AFIT, 1996

V. Bright, AFIT

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^39

D. Burns et al., AFIT

252 μm

Actuators: ElectroActuators: Electro--ThermalThermal

  • Double Hot Arm

dimple

current path cold arm

flexure

outer hot arm

anchor

substrate contact

inner hot arm anchor

Design measurements for thermal actuators

Comparison of single hot-arm (1-H) and double hot-arm (2-H) actuator operating properties

direction of movement

EE 480/680, Summer 2006, WSU, L. Starman MicroElectroMechanical Systems (MEMS)^40

Actuators: ElectroActuators: Electro--ThermalThermal

  • Double Hot Arm

D. Burns et al., AFIT