Download Sensor Characterization Report

 P.W.L.S. Innovations Chris Landry, Project Manager Kosta Papasideris, Systems Engineer Brad Sutter, Hardware Engineer Archie Wilson, Software Engineer Sponsor: Afshin Shaybani, Avicen Corporation Advisor: Dr. Ben Zoghi Sensor Characterization March 10, 2008 Introduction
For use with the ExacTrak system, this characterization report was developed. Sensors
used in the development of positioning information such as accelerometers, gyroscopes
and magnetometers are the integral modules of the ExacTrak system and must first be
characterized and calibrated for use. This report outlines test routines and the data
collected for the STMicrolelectronics LIS3LV02DQ accelerometer, InvenSense IDG-300
gyroscope and Honeywell HMC6343 Magnetometer, and its purpose is to develop a
sensor characterization knowledgebase.
Definitions
In order to outline the process undertaken to develop an understanding of how the
ExacTrak’s sensors should operate in-system, more than just reading datasheets is
necessary. Sensor characterization and calibration must be defined.
Characterization
Sensor characterization is the process of understanding the functionality of the sensor.
This includes maximum and minimum output data values, how this data is transmitted,
in what form, and what it means to be maximum or minimum. Also contributing to
characterization are data sampling rate and data drift between trials.
Calibration
Sensor calibration is the final step involved in sensor knowledgebase development.
When sensor output data is confirmed and consistent, the datasheet’s expected values
can then be compared to trial results for each sensor. This data can be interpreted as
an error and adjusted accordingly if necessary.
PWLS Innovations | pwlsinnovations.com 2 System Data Requirements
The ExacTrak System will be used to determine and display the location and orientation
of a person through knowledge of:
1. Distance traveled, and
2. Angle of rotation when direction is changed
In order to find changes in these criteria we used an accelerometer to measure
acceleration which will be integrated twice in software to determine distance traveled,
and a gyroscope and magnetometer to measure angular change.
Before system data can be collected and interpreted in software, however, each sensor
must be characterized so we can understand what the raw sensor data equates to with
regards to system requirements.
Accelerometer
The STMicrolelectronics LIS3LV02DQ accelerometer is a 3-Axis, ±2g/±6g Digital
Output, Microelectromechanical System (MEMS) sensor. Being tri-axis, this
accelerometer provides orientation information for distances in all three directions of
interest (forward/backward, left/right and up/down).
Theory of Operation
Accelerometers measure the linear acceleration of a system in the system's own
reference frame (but without direction), providing data in the form of changes in
acceleration per time period based solely on the movement of the system.
Figure 1a depicts the accelerometer under no forces. This accelerometer is composed
of two fixed capacitive sensing cells and a central mass which is free to move in the
direction of the sensed acceleration. When acceleration is applied to the
accelerometer, the central mass displaces from its rest position, changing capacitance
between its plates and registering the imbalance according to the relationship:
where capacitance, C, relates inversely to a change in distance d between the plates. A
is the area of each capacitive plate, and ε refers to the dielectric constant of the material
between the plates (vacuum = 1). Figure 1b represents the accelerometer under
acceleration’s pull and depicts a change in δx between fixed electrodes and movable
electrodes, acting as the capacitive plates.
PWLS Innovations | pwlsinnovations.com 3 Anchor
Point
Fixed
odes
electro
Beam
B
Mass
ble
Moveab
electrod
des
x0+δx
x0-δx
(1a)
Figure 1a,b:
Accelerometer at rest
r
and unde
er acceleration
(1b)
Charactterization an
nd Data Interpretation
The LIS3LV02DQ accelerome
a
eter commu
unicates dig
gitally follow
wing I2C pro
otocol and
ch can be in
nterpreted as
a changess in accelerration over a given (or
transmitts data whic
user spe
ecified) sam
mpling time.. The datassheet for th
his accelero
ometer state
es that whe
en
accelera
ating in one
e axis, norm
mal to gravitty and pointting in gravvity’s pull (re
efer to Figu
ure
2), a deccimal value
e representing -1g or -1024 is tran
nsmitted. Likewise,
L
fo
or accelerattion
normal to
t gravity and pointing away from
m gravity’s pull,
p
a decim
mal value re
epresenting
g 1g
or 1024 is transmittted.
These decimal
d
valu
ues are indicative of th
he accelero
ometer’s 12-bit data register,
th
represen
nted as 2 bytes
b
(16 bitts). The 11 bit repressents the sign of the acceleration
a
n
and will be configurred in softw
ware to reprresent forward or backkwards acce
eleration. The
T
12
en is confine
ed to a rang
ge of 0 to 2 (4096) decimal valu
ues. Underrstanding th
hese
data the
signed values
v
is im
mportant in interpreting
g the data correctly. As this accelerometer has
h
a chosen maximum
m g-force ra
ating of ±2g, we can exxpect signe
ed values be
etween -20
048
47.
and 204
2048
3072
4096, 0
1024
2047
Figure 2:
Accellerometer refe
erence (±2g unsigned
u
data
a)
bsolute minimum and maximum sensor
s
data
a readings are
a known, the
Once ab
accelero
ometer can be rotated from -1g to
o +1g, an angle chang
ge of 180°, representin
r
ng
PWLS Innovations | pw
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r
of values under observation. This rota
ation mimiccs, in a controlled
environm
ment, the fo
orward and backward movement of a person
n with a rela
atively consstant
accelera
ation, and under
u
gravitty’s constan
nt pull. Refferencing Figure 3, the
e data reflected
by Figurre 2 is interpolated to a rotational motion insstead of a la
ateral one. A test for 0g
0 is
necessa
ary to reflec
ct the accele
erometer’s stillness be
etween axe
es. When placed
p
flat on
o a
table, the accelerom
meter’s xy-plane is un
naffected byy the pull off gravity on it, thus
transmittting 0g, 0 decimal
d
for its accelera
ation.
1024
(1024)
0 (0)
0 (0
0)
3072
(-1024)
Figure 3:
Accelerom
meter referen
nce 180° Testt (±1g signed data)
Calibratiion
elerometer reflected tyypical datassheet-expeccted resultss,
Since initial testing of the acce
sensor calibration
c
consisted
c
o incorpora
of
ating and exxtracting the
e sign bit, making
m
sensor
output data
d
reflect true ±2g accceleration. This step is importan
nt because utilizing va
alues
which arre all positiv
ve gives uss no immediate knowle
edge of movvement, so
o signed datta is
critical.
e the accele
erometer, a software filter
f
was incorporated as a step
In order to calibrate
ng data tran
nsmission to the user. This filter looks at the
e 11th bit (ssign bit) and
d if it
precedin
is high, the
t sign is negative. This
T
sign is saved and appended to the num
mber once it is
subtractted from 2048, making
g it fit within the range we require (-2048 to 2047).
2
Befo
ore
this filterr is utilized,, values are
e representa
ative of acccurate data, yet subtra
acting each
value fro
om 2048 turns high va
alues (>204
48) negative
e and low values (<204
48) positive
e.
Please refer
r
again to Figure 2.
2
PWLS Innovations | pw
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The accelerometer under test reflected datasheet-expected results with relation to
simple acceleration tests. The accelerometer was taken from 1g to -1g, with minimum
and maximum values of 1024 to 3072, respectively. These results yielded the expected
range of 2048, from -2g to 2g. When applying the software filter, and repeating the 180°
rotation test, values indicative of negative and positive acceleration (-1024 to 1024)
were recorded. Average values of -1023.52 to 1073.76 reflect the test ranges, and
Figure 4 displays this test data, stopping at +x, 0, -x and 0 again.
1g, 1024
0g
-1g, -1024
Figure 4:
Accelerometer Test Routine (-1024 to 1024)
PWLS Innovations | pwlsinnovations.com 6 Gyrosco
ope
enSense ID
DG-300 gyro
oscope is an
a integrate
ed dual-axiss, trimmed full
f scale
The Inve
range: ±500°/sec
±
MEMS
M
senssor. Being dual-axis,
d
th
his gyrosco
ope providess orientatio
on
data in the
t form of voltage pottential drop
ps within a 0 to 3.3V ra
ange.
Theory of
o Operatio
on
Gyrosco
opes use vibrating mecchanical ele
ements to sense
s
rotation. This vibratory sen
nsor
is based
d on the transfer of ene
ergy between two vibrration mode
es of a struccture cause
ed
by Corio
olis accelera
ation. Corio
olis accelerration is an acceleratio
on arising in
n a rotating
g
referencce frame an
nd is proporrtional to the rate of ro
otation.
Vibrating
g gyroscopes use Corriolis accele
eration effeccts to sense
e when they rotate,
establish
hing an osc
cillatory motion orthogo
onal to the input axis in
i a sensing
g element
within th
he gyro. An
n internal circuit precissely sets an
n oscillation for each mass.
m
When
n
the senssor is rotate
ed about the
e X or Y-axxis, the Coriiolis Effect causes a vibration (with
velocity, v) that can
n be detecte
ed by a cap
pacitive nettwork. Figu
ure 5 displays the Coriolis
Effect an
nd its accelleration com
mponent.
Ω
v
Figure 5:
Dual-axis Gyroscope
G
Exxperiencing Coriolis
C
Accele
eration
Coriolis acceleratio
on is given by
b
2Ω
where ac is the Corriolis accele
eration, Ω iss the angula
ar rate or ro
otation, and
d v is the
velocity of vibration
n. See also
o Figure 5.
Charactterization an
nd Data Interpretation
enSense ID
DG-300 gyro
oscope com
mmunicatess an analog
g signal and
d transmits
The Inve
data which can be interpreted as a chang
ge in angular orientation over a given
g
(or usser
specified
d) sampling
g time. Perr the datash
heet the datta is transm
mitted at 2m
mV/°/sec. Using
U
this relattionship, if one variablle changes then so do
oes anotherr. For exam
mple, if we
sample every ½ se
econd, then we must decrease
d
on
ne of the other variable
es either byy a
PWLS Innovations | pw
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that is steady at 1.5V when not sensing any rotation in the orthogonal axis. The voltage
then decreases or increases based on the direction of rotation: decreasing voltage
counter clock-wise and increasing voltage clock-wise. Based on the difference between
the initial voltage and the final voltage we can then use the given relationship to
calculate the change of degree per time interval.
Data/Results
The Gyroscope was taken through a simple rotation test, similar to the accelerometer.
Because the gyroscope recognizes a rotational change in the orthogonal axis to the
measured axis (in this case, the X-axis), when rotated from a stationary, 0° position
clockwise to 90° and finally to 360°, values representative of rotation about a complete
circle were recorded. Figure 6 displays the data collected during this test.
360°
270°
180°
90°
0°
Figure 6:
Gyroscope Test Routine (0°-360°)
PWLS Innovations | pwlsinnovations.com 8 Magnetometer
The Honeywell HMC6343 Magnetometer is a 3-axis magneto-resistive device
incorporating three 3-axis MEMS accelerometers. The accelerometers provide the
ability for magnetic true-north detection despite system orientation. Magnetometers
measure the strength and direction of magnetic fields in their vicinity. This information
can show heading (cardinal directions) and is represented digitally.
Theory of Operation
The HMC6343 Magnetometer is Anisotropic Magnetoresistive (AMR), indicating that its
ability to “sense” the earth’s magnetic field is proportional to a change in resistance.
The basic AMR device consists of strips of a thin film of nickel-iron alloy deposited on a
silicon wafer. Typically four of these resistors are connected in a Wheatstone bridge
configuration (see Figure 7) so that both magnitude and direction of a field along a
single axis can be measured.
Figure 7:
AMR Sensor Circuit
The magnetoresistive characteristic of the nickel-iron alloy causes a resistance change
in the bridge induced by the presence of an applied magnetic field. This causes a
corresponding change in voltage output from the bridge.
PWLS Innovations | pwlsinnovations.com 9 Charactterization an
nd Data Interpretation
The HMC6343 mag
gnetometerr communiccates data digitally
d
following I2C protocol.
p
This
data is a real-time measure off degrees around
a
a cirrcle, with tru
ue-north is represente
ed as
0° and 360°.
3
Figure
e 8 graphically represe
ents the ma
agnetomete
er’s range.
Figure 8:
Magneto
ometer Compass Values (F
Flat on Table--top)
haracterizattion is a tesst of magne
etometer ca
alibration. Values matcching
The nexxt step to ch
those in Figure 8 should be ga
athered. Th
he magneto
ometer whe
en placed fllat on a tab
ble
and rota
ated on a prre-designed
d compass designating
g cardinal direction
d
an
nd degree
values frrom 0°-100
0° resulted in
i data interpreted as both accura
ate and app
propriate.
Further, it was not apparent whether
w
the HMC6343 magnetom
meter could provide reliable
data when taken ou
ut of its norrmal, flat an
nd absolute orientation
n. It seemss that the three
3-axis accelerometters allow th
his magneto
ometer the freedom to
o determine
e cardinal
es) despite orientation.
o
w
the magnetome
m
eter module
e can
heading (in degree
In other words,
be flippe
ed, rotated and taken out
o of norm
mality with th
he ground and
a still tran
nsmit data in
the direcction of orie
entation with respect to
o the earth’’s magneticc north.
Data/Re
esults
gnetometerr under testt reflected datasheet-e
d
expected re
esults with relation
r
to
The mag
simple rotational te
ests. The magnetomet
m
ter was takken from 0°,, clockwise and back
again, with
w North = 0°, East = 90°, South
h = 180° and West = 270°. These
e results
yielded the
t expecte
ed range off 360°.
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