MEMS Tuning-Fork Gyroscope
Group 8:
Amanda Bristow
Travis Barton
Stephen Nary
Outline
1.
Introduction: What is a MEMS Gyroscope?
2.
Purpose
3.
Mechanical Structure: Design and Optimization
4.
Electrical Structure: Theoretical Calculations
5.
Design Results
6.
Manufacturing Process
7.
Testing Equipment: PCB
8.
Conclusion
What is a MEMS Gyroscope?
A small, silicon device
that senses rotation
●
Benefits:
●Small size
●Low power
●Cheap to mass produce
●
Common Applications:
•iPhone 4
•Wii Motion Plus
MEMS Gyroscope viewed under a microscope
Purpose
To assist Dr. Hao’s research by refining an existing MEMS
Gyroscope design to improve accuracy
●
To examine the method of fabrication for MEMS devices
●
To gain practical experience with the testing of MEMS devices
using the existing design
●
Structure of a MEMS Gyroscope
Symmetric mechanical
structure
●
Gyroscope structure
free to move, except at
anchors
Sense
electrodes
●
Proof mass
Proof mass
Comb Drive
Transducers used to
drive proof masses
●
Sense electrodes used
to detect rotation
●
Gyroscope Structure
Drive Mode
Structure made to vibrate
at natural frequency
●
Vibration of proof mass
provides necessary
velocity for Coriolis
Acceleration
●
Drive Mode Vibration
Sense Mode
When rotation is
applied, Coriolis force
causes proof mass
direction to change
●
Coriolis Acceleration
ac  2 V
Sense Mode Vibration
Optimization of Mechanical
Structure
Ideal frequency range: 15
kHZ – 30 kHz
●
L2
Ideal difference between
drive mode and sense mode:
50 Hz – 100 Hz
●
L1
Key Dimensions for
adjusting frequency:
●L2 → Sense Mode
●L1 → Drive Mode
●
Key Dimensions
Optimization of Mechanical
Structure
L1, μm
5
520
13
494
Trial
Drive
L2, μm
14924.587
620
15872.906
620
Sense
Difference
15904.967
980.381
15951.401
78.495
Comb Drive Transducers
Form capacitors with proof
masses
●
Use “fingers” to increase
surface area
●
Proof mass
AC voltage applied to one
transducer to excite proof
masses to vibrate
●
Other transducer used to
monitor drive mode
vibration
Proof mass
●
Comb Drive
Transducers
Designing the Comb Drive
Transducers
Proof Mass Vibration Amplitude
VDC
2nε 0 hVDC v AC
q0 =
Q
gk
~
vAC
Voltage Sources
Magnified View: Proof Mass and
Comb Drive Fingers
Designing the Sense Electrodes
Sense Electrodes form parallel plate
capacitor with proof mass

Sensitivity of Gyroscope (ratio of current to
input signal):

i 2ε0ωs h ωs ωd m q0Qs
=
Ω
d 0 k s  kelec 
Design Results
Drive Mode Frequency
Sense Mode Frequency
15872.9 Hz
15951.4 Hz
Vibration Amplitude
AC Voltage Amplitude
Drive Mode Current
Sensitivity
3.636x10-6 m
0.4810 V
6.4214x10-8 A
1.1554x10-6
CAD Model for Mask
-Calculations determine structure
dimensions
-Total of 25 fingers on proof
mass
-2 micron interference for comb
transducers
-3 micron sense gap
Manufacturing of MEMS Gyroscope
1. Design is laser etched onto a chrome plated mask.
2. Align mask above silicon wafer.
3. Expose wafer and mask to UV light.
Mask
Photo resist
Silicon
Substrate
Manufacturing of MEMS Gyroscope
5. Developing
Manufacturing of MEMS Gyroscope
Mechanical structure needs to move freely. Therefore special wafer is
used.
6. Etch SiO2
Note: HF does not etch away all
SiO2
Gyroscope Testing - PCB
Operational
Amplifiers
•Weak sense current from
gyroscope requires amplification
to aid detection
•Operational Amplifiers and large
resistors used to increase current
Resistors
PCB with major components highlighted
Conclusion
Successes:
Desirable Further Work:
•Gyroscope frequencies within
design parameters
•Fabrication
•Theoretical calculations for
electrical components within
acceptable ranges
•Testing
•Comparison with existing
design
Questions?