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Introduction to Sensing And Actuation Methods
Sensing & Actuation Methods
Sensing
• Electrostatic• Thermal• Magnetic• Piezoelectric• Piezoresistive
Actuation
• Electrostatic• Thermal• Magnetic• Piezoelectric• Shape Memory Alloys
Tunneling ,Optical, FET, RF Resonance Sensing
Design considerations
Sensor
• Sensitivity• Linearity• Responsivity• SNR• Dynamic Range• Bandwidth• Drift• Reliability• Cross talk• Cost
Actuator
• Torque or force output capacity
• Range of motion• Dynamic response• Ease of fabrication• Power consumption &
energy efficiency• Linearity of displacement
as a function of driving bias
• Cross sensitivity & environmental stability
• Foot Print
Electrostatic Sensing and Actuation
Principle of operation
• A capacitor is broadly defined as two conductors that can hold opposite charges
• If the distance/relative position or dielectric medium between two conductors change as a result of applied stimulus,the capacitance value will change.This forms the basis of capacitive (Electrostatic) sensing.
• If a voltage or electric field is applied across two conductors,an electrostatic force would develop between these two objects resulting in actuation. This is defined as electrostatic actuation.
Principle of operation ……• Two Types of capacitive electrode geometries
* Parallel Plate Capacitors
* Interdigitated Finger (Comb Drive) Capacitors
• Two Parallel plates can move with respect to each other
* Normal Displacement
* Parallel sliding displacement
Equilibrium position of Electrostatic Actuator under Bias
Electromechanical model of a // plate capacitor
Electrostatic Actuation
Electrostatic energy stored by a capacitor
Maximum Energy stored is
Where Eb is the breakdown electric field
When a voltage V is applied, a force Felectric develops between the plates.The magnitude of force equals the gradient of the stored energy W
• The spatial gradient of Electric Force is defined as electrical spring constant, Ke
Ke changes with position (d) and the biasing voltage (V)
Effective spring constant of the structure: Km-Ke
Calculation of equilibrium displacement : x
Mechanical restoring force is
At equilibrium,
Equilibrium distance x can be calculated by solving this quadratic equation with respect to x.
Electrical and Mechanical Force as a Function of Spacing
Graphical solution
Amplitude of electrostatic & mechanical force
Balance of Electrical and Mechanical Force
Effect of different bias voltages on equilibrium distance, x
Balance of Forces at the Pull-in voltage
•At tangent,Magnitude of Fe equals Fm
• Pull-in Voltage
•The // plate electrostatic actuator becomes unstable for V greater than Vp
At Pull in Voltage, magnitudes of electrical and mechanical balance forces are same.By equating these two forces
Analytical Solution
We know
Only solution for x when Ke=Km is satisfied: Independent of Vp &
Spring constant
Putting V2 from above
We get
And consequently we get
Putting x = xo/3 in
at V = V pull in or Vp
For V>Vpull in, Snap in condition,•There is no equilibrium position and the two plates ‘snap in’ or come in contact
• Idealized case: Two sources of deviation - Fringe caps. & Restoring force considered linear
Two Types -Transverse - Longitudinal
• Many Parallel plates can increaseActuation force.
Perspective view of comb-drive sensors and actuators
Transverse Comb drive
Capacitance at Rest Csl=Csr=є0 l0 t/x0
Capacitance after movement x Csl= є0 l0 t/x0-x Csr= є0 l0 t/x0+x Total value of capacitance= Csl+Csr+Cf
The displacement sensitivity Sx=∂Ct ot/ ∂ xMagnitude of force(Actuator)Fx= | ∂ U/ ∂ x | = | ∂ / ∂ x (1/2CtotV2)I
Longitudianal Comb Drive
With lateral movement y,the capacitance of single finger Csl=Csr=є0 (l0 - y) t/x0
The displacement sensitivity: Sy= ∂Ct ot/ ∂ y Force (Actuator) Fy= ∂E/ ∂ y= ∂/∂y (1/2CtotV2)
Applications
• Electrostatic Motor• Inertia Sensor - Parallel plate- capacitive accelerometer - Torsional plate- capacitive accelerometer
• Pressure Sensor - Membrane parallel plate pressure sensor - Membrane capacitive condenser microphone
• Flow sensor• Tactile sensor
MEMS Electrostatic Actuators
MEMS Electrostatic Actuators
An out of plane accelerometer based on comb drive actuation
Typical Calculations
• The force constant associated with the mass is twice that of each individual fixed-guided cantilever. The overall force constant is
K= 24EI/L3
• The total capacitance at rest is contributed by eight fixed electrodes and therefore 16 vertical wall capacitors .The value of total capacitance is
C(t)= 16 (єo loto/d)
• The displacement in Z axis which is a function of the applied acceleration causes the effective thickness (t) to change. Upon displacement z,the capacitance becomes
C(t)= 16 {єo lo(to-z)/d} and
z= ma/K= maL3/24EI
The relative change of capacitance with respect to acceleration a is
∂C/ ∂a= 2 єo lomL3/3dEI
Fabrication Process of Torsional Acceleration Sensor
)2( fmfr ll
d
lnC
Change in capacitance under angular displacement
Where,lm :length of inertia masslf: length of sensing fingerd: gap distancen: number of sense fingers
Example: Force Balanced ADXL-50
ADXL-50
Accelerometer with Capacitive Sensing
Bulk micromachined capacitive accelerometer. Inertial mass in the middle wafer forms the moveable electrode of a variable differential capacitive Circuit.
Accelerometer with Capacitive Sensing
Fabrication Process Steps:
Parallel-Plate Capacitive Accelerometer
Surface Micromachined Parallel Plate Capacitor as an Accelerometer
RT Process
Ni Plate Size1x0.6mm2 in area5um Thick
Fabrication Process of Pressure Sensor with Sealed Cavity
Oxidation + Patterning
Anisotropic Etch ( 9um)
Oxide Etch
Oxidation
Patterning
B Diffusion 15um
Reoxidise+Pattern
Thin B-Doping
Dielectric + Patterning
Poly + Doping
CMP + Cr/Au deposition+Oxide Dep+Pattern
Flip chip Bonding
Silicon Etch
Surface Micromachined Pressure Sensor
Capacitance changes with deflecting membrane which can be measured using AC circuitry.
Comb-Drive Actuator for Optical Switching
Linearly graded comb teeth
MEMS Electrostatic Actuators
Electrostatic Optical Switch
Bulk Micromachined Parallel-Plate Capacitor as Differential Mode Tactile Sensor
dd
LC r
2
20
Capacitance change under normal force
Ld
LC r
5.0
20
Total Capacitance under shear force
Fabrication Process of Tactile Sensor
Buried n type layer (3um) +6um thick n epi layer
Scratch Drive Actuator
Square Pulse
SDA Supported by Elastic Beams
Fabrication Process: SDA
Oxidation+Poly Si+P Implant+Photolithography+ Nitride deposition
Sacrificial oxide+ Two step Lithography
Poly Si ( Buckling beam)+ P Implant +Photolith. + Dry Etch
Stress Anneal with thin oxide+Sacrificial layer removal
SDA Actuator and Linked Buckling Beam
Maximum Deflection Vs Horizontal Displacement
Generated Force By SDA and Applied Voltage
3-D Self-Assembled Polysilicon Structure
MEMS Electrostatic Actuators
MEMS Electrostatic Actuators
MEMS Electrostatic Actuators