Title: Silicon Resonant Accelerometer for Inertial Navigation Systems
1Silicon Resonant Accelerometer for Inertial
Navigation Systems
- Yong Ping XU
- Dept of Electrical and Computer Engineering
- National University of Singapore
2Outline
- Introduction
- The proposed silicon resonant accelerometers
- Sense resonator
- Circuit chip design
- Measurement results
- Conclusion
3Introduction
- Shortcomings of global positioning system (GPS)
- It is subject to signal jamming
- It cannot be used indoor
- GPS has low update rate and is therefore not
suitable for high-speed tracking
4Introduction (cont.)
- Inertial navigation system (INS)
- INS employs inertial sensors (accelerometer and
gyroscope) to track the position and orientation
of an object. - An INS is a self-contained system. Once the
initial position is known, it can track the
object without the need of any reference. - It can be used when GPS is not available
- It can also used as a complement to the GPS system
5Inertial navigation system
King, A.D., Inertial navigation Forty Years
Evolution, GEC Review, Vol.13, No.3, 1998,
pp.140-149
6Inertial navigation system (cont.)
King, A.D., Inertial navigation Forty Years
Evolution, GEC Review, Vol.13, No.3, 1998,
pp.140-149
7Inertial navigation system (cont.)
Global Accel
Acceleration
Position
Velocity
Bias errors
Position error (due to a constant bias error, eb)
eb 0.01g
Due to the nature of integration, INS requires
the accelerometer to have low bias error
8Sources of bias errors
- DC bias
- Output offset of the accelerometer when the
acceleration is zero. - Random/white noise
- Originated from thermal noise, both mechanical
and electrical - Flicker noise
- Device flicker noise and offset in readout
circuit - Temperature
9Bias stability
- Bias stability
- Bias change over a specified period of time,
typically around 100 seconds, at zero
acceleration. - Bias stability is usually measured by Root Allan
Variance floor - Bias stability is usually specified as 1s value
with a unit of mg/hr (milligravityper hour) - Typical requirement for inertial navigation is lt
100 mg
10Displacement sensing silicon accelerometers
- Displacement sensing
- The acceleration is measured by the displacement
of the proof mass - The displacement can be detected by optical,
capacitive, piezoresistive tunneling principles
a Acceleration m Mass k Spring constant
Amini, B.V., et al., A 4.5-mW closed loop DS
micor-gravity CMOS SOI accelerometer, IEEE JSSC,
pp.2983-2991,Dec 2006
11Resonant silicon accelerometer
Force sensing
Df
Where
P Axial force applied a - Acceleration fo
resonant frequency at zero acceleration f
frequency of oscillation under acceleration SF
Scaling factor (Hz/g)
Hopkins, R.E., et al., The Silicon Oscillating
Accelerometer, Draper Laboratory, MA, USA
- Advantage
- Radiation resistant
- Axially loaded, allowing large dynamic range
- Quasi digital output
- Potential to achieve good bias stability
12The proposed silicon resonant accelerometer
13Block diagram of one channel
Oscillator core
Sense resonator
Amplitude control
- Differentiator differentiates the position signal
DCs, to make the feedback force in phase with the
velocity of the resonator beam
14Challenges in readout circuits
- Low phase noise in the close-in (carrier) region
- Flicker and thermal noise
- MEMS resonator nonlinearity
- Low noise interface circuit
- Low polarization voltage requires sensitive
interface circuit - Extremely small capacitance change (0.520fPF) to
be sensed - Nonlinearity of MEMS resonator
- Low noise amplitude control
- Parasitic feed-through in MEMS resonator
15SOI sense resonator
Cross section
Acceleration axial
16Differential resonator
Double-ended tuning fork (single-ended operation)
Modified for differential operation
17Measred frequency response
Vp 25V Q 30,000_at_0.1mbar
Vp 3.3V No resonant peak can be seen due to
parasitic feed-through
18Readout circuit chip design
- Low noise capacitive sensing interface
- Offset free differentiator
- Amplitude control circuits
- CHS peak detector and error amplifier
- VGA and buffer
- Driving scheme with separate sense and driving
phase to avoid feedthrough
19Low noise capacitive sensing interface
Cp1700fF Cp2400fF f0 135kHz fs 5MHz
Transfer function
20Operations in four phases
Autozero phase
Clear phase
Sense phase
Drive phase
21Main features of the sensing interface
- Two step CDS (error stored in CA and CB)
- Fast-settling OTA
- Compensation resistor Rc to improve the settling
time - Capacitive isolation, Cc, during drive phase
22Amplitude control scheme
- Amplitude control is to set the oscillator
amplitude to a desired value (VR0) to maximize
the SNR, while keep the oscillator from the
nonlinear region, since the nonlinearity causes
large close-in phase noise, hence poor bias
stability.
23CHS peak detector and error amplifier
Vicm Vdm
I0
Vx
VT transistor threshold voltage Vov Overdrive
voltage (VGS VT)
24Complete chip
25Measurement results
SOI sense resonators and proof mass
Circuit chip
- SOI MEMS process from MEMSCAP
- 0.35 CMOS process from AMS
Tested _at_1.25mbar
26Frequency readings
Output waveform from VGA
Frequency reading after a PLL, multiplied by 420.
27Scale factor measurement
Scale factor 145 (Hz/g)
28Measured Allan variance
Root AVAR (0.4 mHz)
Bias stability Root AVAR/Scale factor 0.4
mHz/145 Hz/g 2.9 mg
29Summary
30Comparison
Comparison with previous resonant accelerometer
1
3mg
1 T. A. Roessig et al., "Surface-micromachined
resonant accelerometer," in Transducers97, June
1997, pp. 859-862.
31Comparison (cont.)
Comparison with previous capacitive accelerometer
2
3
4
2 M. Lemkin and B.E. Boser, "A three-axis
micromachined accelerometer with a CMOS
position-sense interface and digital
offset-trim electronics," in IEEE J. Solid-State
Circuits, vol. 34, pp. 456-468, Apr. 1999. 3 H.
Luo, et al. A post-CMOS micromachined lateral
accelerometer, in J. of MEMS, Vol. 11, No. 3,
pp. 188-195, June, 2002. 4 J. Chae, H. Kulah,
and K. Najafi, A monolithic three-axis micro-g
micromachined silicon capacitive accelerometer,
in J. of MEMS, Vol.14, No. 2, pp. 235-242,
Apr. 2005
32Conclusion
- A high performance silicon resonant accelerometer
with CMOS readout circuit has been demonstrated - The accelerometer, operating under 3.3V, achieves
3mg bias stability and 20mg/Hz1/2 resolution in
1Hz offset - The good performance is made possible by
- Differential MEMS resonator
- Low noise capacitive sensing interface
- Effective amplitude control scheme and low noise
implementation - Chopper stabilized rectifier and error amplifier
- Separate sensing and driving phase
- High and CMOS compatible Polarization voltage
through charge pump - The accelerometer is suitable for high-precision
INS
33Acknowledgements
- Dr Lin He,
- Shanghai Institute of Microsystem and Information
Technology, China - Dr Moorthi Palaniapan
- Dept of Electrical and Computer Engineering,
National University of Singapore
34Thank you!
35Mechanical leverage
36Allan Variance
Allan Variance
- y average value of the measurement in bin i,
- averaging time
- n total number of bins
t
t
t
t
t
t
n x t
- Allan variance is a function of averaging time
- Originally proposed and used for characterize the
clock systems