chapter 4 accelerometer testing - information and...
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Chapter 4
Accelerometer Testing
4.1 Introduction
The accelerometer designed and realized so far is intended for an
aerospace application. Detailed testing and analysis needs to be
conducted to qualify the product for the end use and establish the
suitability of the product for the intended use. These tests can take many
different forms including chip level probing for electrical characterization,
sensitivity estimation, bias stability, performance evaluation under
thermal and dynamic environments. Details of performance testing
procedure of commercially available accelerometers are presented by
number of researchers [47-49].
MEMS pendulous accelerometers have already demonstrated good
performance in automobile and other commercial applications. The
challenge of using of this technology in aerospace inertial navigation is
about, significantly improving the bias stability, cross-axis sensitivity,
temperature sensitivity and detailed measured performance
demonstration for the intended application.
The accelerometer was tested using procedure and data analysis
methods similar to the practices in defence departments which are
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broadly adopted from IEEE STD 337-1972 [50]. This describes the test
procedures for linear, single axis pendulous analog, torque balance
accelerometer.
The test procedure consists of observing the output of the test
device to input acceleration using Earth‟s gravitation field or an external
excitation source.
The objective of the test is to characterize the accelerometer
Bias stability
Linearity
Hysteresis
Cross- axis sensitivity
Temperature sensitivity
Bandwidth.
Shock response
It was presumed that physical sources of the errors described in the
reference [50] remain valid for the micromachined accelerometer also. In
both micromachined and conventional accelerometers, the temperature
sensitivity of performance is an important parameter. However, due to
the small scale of MEMS device, temperature is expected to play a larger
role.
The error model components selected for micromachined
accelerometers are bias, scale factor, non-linearity, cross-axis sensitivity
and temperature. The effect of misalignment of the input axis is not
investigated.
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The equation relating different error model components to the final
overall error expressed in % of FSO is as follows.
Average output = E1 + E2 + E3 + E4 x ∆T + E5.
E1 – Bias drift
E2 – Non linearity
E3 – Cross- axis sensitivity
E4 – Temperature sensitivity of bias drift
E5 – Misalignment of input axis.
∆T – change in device temperature during testing.
4.2 Chip level probing
The primary aim of the chip level probing is to validate the wafer
fabrication process and to certify the device functionality for further
packaging and integration operations.
Fig 4.1 Test set-up for electrical probing.
A standard Cascade make probe station is integrated with Agilent LCR
meter and is used for measuring the chip level electrical parameters. This
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set-up can measure capacitance from 10 atto Farads to 10 Farads with
an accuracy of 0.05%.
Ten number of accelerometers are selected for probing to verify the
consistency of the results. The average nominal capacitance values of
Co1 (capacitance between top electrode and proof-mass) and Co2
(capacitance between bottom electrode and proof-mass) are tabulated in
table - 4.1
Sl.No Average nominal
capacitance [Co1]pF
Average nominal capacitance
[Co2]pF
1 3.0102 3.1124
Table 4.1 Measured values of average nominal capacitance.
Observations and comments:-
1. The average nominal capacitance between electrodes and proof-
mass measured is around 3 pF which is very close to the simulated
value of 2.5 pF. The discrepancy in nominal capacitance may be
due to stray capacitance caused by electrical routing & pads and
also due to fine difference in air gap on both sides.
2. However the variation in nominal capacitances Co1 and Co2 is
small hence can be balanced using internal trim capacitors of the
ASIC during electronic integration.
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3. Finally the probing results proves that the device is successfully
realized as per the design without any stiction or shorts and the
gap between proof–mass and electrodes is same on both sides and
is cleared for further electronic integration and testing.
4.3 Packaging & electronics integration
The fabricated wafers are diced and the individual chips are die
bonded on a ceramic carrier in LCC package as shown in plate 4.1. Gold
layout is patterned on the ceramic substrate in such a way so as to
accommodate both the sensor chip and all other required electronic
components. The chip is wire bonded with 1 mil gold wire. The wire
bond is tested for its pull strength to ensure quality wire bond.
The interface circuit for converting the variation in capacitance to
voltage is implemented using a standard capacitance to voltage
conversion ASIC, MS3110 from Irvine Sensors. MS3110 is a general
purpose, ultra noise CMOS IC that requires only a single +5V DC supply
and some decoupling components. It has gain & DC offset trim functions
and on chip EEPROM for storage of program coefficients. The circuit
gives a DC bias of 2.5V at 0 „g‟, which is also equal to the reference
voltage of the ASIC.
The fabricated accelerometer chip shows a deviation in nominal
capacitance values from the designed values and hence the capacitance
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bridge is not balanced at zero „g‟. As a result there is an initial offset in
the output voltage when at zero „g‟. The offset is nullified by using the
internal capacitances in the ASIC. In case where the offset is much more
than the limit of the internal capacitances, provision is made to add an
external capacitor of suitable value in parallel with the lower capacitance
in the bridge. Provision is made in the interface circuit board for the
tuning of the ASIC coefficients after the final assembly of the components
to cater for packaging effects also.
Plate 4.1 Packaged sensor with electronics.
4.4 Scale factor test
Scale factor or sensitivity of an accelerometer is the ratio of the
sensor electrical output to mechanical input typically rated in mV /g.
This is the fundamental parameter to specify a sensor and forms the
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basis for further detailed performance testing. The test method block
diagram is shown in fig 4.2
Fig 4.2 Test set-up for scale factor measurement.
The accelerometer sensor is mounted on an automatic
accelerometer calibrator and connected to power supply and the output
is monitored using precision digital oscilloscope. The calibrator generates
a physical excitation signal of magnitude ±1 „g‟ at a frequency of 159Hz.
The screen shot of the oscilloscope captured during testing is presented
in fig 4.3
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Fig 4.3 Sensitivity of accelerometer
It can be seen from the oscilloscope output that the sensitivity of the
accelerometer measured is 62 mV/g in both directions.
Observations and remarks:-
The sensitivity of the accelerometer is 62 mV/g at room
temperature. Change in sensitivity can be obtained by programming the
gain of the ASIC. The sensor is programmed to have an offset voltage of
2.5 V at zero „g‟. The maximum output at +30 „g‟ is 4.5V and at -30 „g‟ it
is 0.5V. Hence the maximum sensitivity that can be obtained is
66mV/g. With measured sensitivity of 62mV/g the sensor yields 3.1 mV
for 50 milli „g‟ resolution, which can be detected easily.
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4.5 Hysteresis test
A sensor should be capable of following the changes of the input
parameter regardless of which direction the change is made, hysteresis is
the measure of this property. An example of hysteresis within an
accelerometer is the presence of residual deflection/strain within the
sensor's spring after acceleration has been applied and then removed. In
the presence of hysteresis, an accelerometer will not be able to
successfully repeat its null position; this will lead to unstable bias.
Hysteresis is expressed as % of FSO.
Fig 4.4 Block diagram of Hysteresis testing
As shown in the fig 4.4, the sensor is mounted on a centrifuge in such
a way that the sensing axis is in radial direction. The sensor is suitably
rotated at different speeds to obtain upto + 30 „g‟ acceleration
in steps. The output measured at different accelerations
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applied from 0 „g‟ to 30 „g‟ and while returning from 30 „g‟ to 0 „g‟ are
plotted in fig 4.5 and the output values are given in table 4.2.
Fig. 4.5 Hysteresis test result
Table - 4.2 Accelerometer output at different ‘g’
0.00
500.00
1000.00
1500.00
2000.00
2500.00
3000.00
3500.00
4000.00
4500.00
-30 -20 -10 0 10 20 30
Ou
tpu
t V (
mV
)
Acceleration g
Hysterisis
Up
Down
Acceleration (‘g’) UP (mV) Down (mV)
-30 320.00 322.00
-18 1063.00 1064.00
-6 1800.00 1802.00
0 2181.00 2180.00
6 2548.00 2550.00
18 3294.00 3297.00
30 4041.00 4041.00
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Observation and remarks:-
From the readings presented it can be seen that the maximum deviation
in output is at 18 „g‟. The hysteresis is given as
(∆y)/2 X 100 = 0.08 % of FSO
FSO The measured value of hysteresis is 0.08%, which is much less than the
specified value of 0.15 % of FSO.
4.6 Bias stability or drift
Bias stability is specified as a percentage of FSO at constant temperature
over a specified time period.
Fig 4.6 Test set-up for bias stability measurement.
The accelerometer is positioned in such a way that its sensitive axis (Z) is
perpendicular to the earth gravitation vector. In this way the sensor is
not subjected to any acceleration. The sensor output is connected to a
data logger. The offset variation is measured using a digital multi meter
(DMM) and logged over a period of two hours and plotted as in Fig 4.7.
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Fig 4.7 Bias stability with time.
Observations and comments
Long term bias stability measurements have demonstrated an
overall measured value of ±0.5mV over a period of 120minutes. This
works out to be 0.025% of FSO, which is well within the requirement of
0.15% of FSO.
4.7 Linearity test
The transfer function of the sensor (input/output relationship) is
not perfectly linear. Non-linearity is expressed as the ratio of maximum
deviation of output voltage from a best fit straight line to full scale output
of the device. This is expressed as a percentage of FSO and the equation
is given below.
2.1700
2.1750
2.1800
2.1850
2.1900
0 20 40 60 80 100 120
Ou
tpu
t(V
)
time(Min)
Drift Testing of SE03 for 2 Hours
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Non linearity = Maximum Deviation(Volts) X 100 %
Full Scale Output(Volts)
Non-linearity is one of the major sources of error in aerospace class of
accelerometers and shall be limited to less than 1% of the FSO.
Fig 4.8 Block diagram of linearity test
To conduct the test, Modalshop make automatic accelerometer
calibration workstation is used. The system uses back to back
comparison calibration method as per ISO 16063-21 [51] and generates
test reports automatically. It can apply a peak acceleration of ± 20 „g‟ at
a reference frequency of 100Hz. Hence the test range is limited to ± 20 „g‟
instead of full range of ± 30 „g‟.
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The Accelerometer is mounted on the shaker with its sensitive axis
(Z- axis) along the shaker excitation axis. The sensor is subjected to „g‟
sweep from ±1 „g‟ to ±20 „g‟ progressively at a reference frequency of
100 Hz. The „g‟ output from the sensor at different „g‟ values is compared
with that of a standard acceleration sensor and report generated
automatically as shown in Fig 4.9.
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Fig 4.9 Linearity test result (calibrator output)
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Observations and remarks
From fig 4.9 it can be seen that the linearity error of the sensor is
0.32% of FSO, which is well within the specified value of 1%.
4.8 Cross-axis sensitivity test
Aerospace systems experience acceleration forces along all three axes
i.e. pitch, roll and yaw. Accelerometer with its sense axis mounted along
a particular direction shall sense acceleration in that direction only and
shall be immune to the accelerations applied on other axes.
Cross-axis sensitivity is the output that is obtained on the sensing
axis for an acceleration applied on a perpendicular axis. This is
expressed as a percentage of the full scale output sensitivity. The sensor
has two cross-axis sensitivities 𝑆𝑍𝑌 and 𝑆𝑍𝑋 . The first subscript is the
sense axis and the second subscript is the off-axis direction. Cross-axis
sensitivity is given by
𝑆𝑍 𝐶𝑟𝑜𝑠𝑠 =𝑆𝑧𝑥
𝑆𝑧× 100
𝑆𝑍 𝐶𝑟𝑜𝑠𝑠 =𝑆𝑧𝑦
𝑆𝑧× 100
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Fig 4.10 Cross-axis testing block diagram
The sensor is mounted on a precision centrifuge in such a way that
its sensitive axis (Z) is along the Earth‟s gravitational vector. This method
of mounting eliminates both radial and tangential components of
acceleration acting on the sense direction. The sensor output for 1‟g‟
acting due to gravity is nullified and set to zero in the DMM. Now by
suitably rotating the centrifuge at appropriate speed the required cross
axis acceleration is applied on the accelerometer and output of the
sensor is recorded through a data logger.
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Fig 4.11 Cross-axis sensitivity plot
Cross axis
acceleration Output (V)
0 2.1812
5 2.1823
10 2.1834
15 2.1845
20 2.1855
25 2.1866
30 2..1874
Table 4.3 Cross-axis sensitivity output
2.1600
2.1650
2.1700
2.1750
2.1800
2.1850
2.1900
0 5 10 15 20 25 30 35
sen
sor
ou
tpu
t (
Vo
lts)
cross-axis acceleration applied (g)
Cross-axis sensivitty
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Observations and remarks
Since the sensor is symmetrical along X-axis and Y-Axis the cross-
axis test is done along one direction only.
The measured value of cross-axis sensitivity is 0.313% of FSO,
which is well within the specified value of 1% of FSO. However it is
more than the simulated value of 0.01% of FSO. This may be due
to the initial deflection present in the accelerometer because of the
Earth‟s gravity, fabrication error in positioning the beams at the
centre and due to sensor mounting misalignment in the package.
4.9 Temperature sensitivity test
Aerospace systems, during their operational period are exposed to
harsh environmental conditions, which includes vibration and wide
operational range of temperatures. The two most important performance
parameters that need to be studied for their temperature effects are bias
stability and offset variation. The temperature sensitivity of the
accelerometer is the sensitivity of a given performance characteristic to
operating temperature. It is expressed as the change of the characteristic
per degree of temperature change, typically in ppm/°C for scale factor
and mg/°C for bias. This figure is useful for the estimation of maximum
sensor error with temperature as a variable while modelling.
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4.9.1 Temperature sensitivity of offset value:
Temperature sensitivity of zero „g‟ offset is, the variation in the zero
„g‟ offset value over the operating temperature range. The offset variation
is measured by placing the accelerometer in a thermal chamber fig 4.12
and subjecting it to different operating temperatures. The accelerometer
is mounted in such a way that its sense axis is perpendicular to the
Earth‟s gravitational axis. The output is noted down using a precision
DMM which in turn is connected to a data logger.
Fig 4.12 Temperature sensitivity of offset test block diagram
Fig 4.13 Temperature sensitivity of offset
2.1760
2.1780
2.1800
2.1820
2.1840
2.1860
2.1880
-20 0 20 40 60 80
off
set
var
iati
on
(V
olt
s)
Temperature deg C
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Temp (ºC) Nominal output
(V)
-20 2.1773
0 2.1786
20 2.1812
40 2.1837
60 2.1860
80 2.1871
Table 4.4 Temperature sensitivity of offset
The zero „g‟ offset voltage is measured at -20, 0, 20, 40, 60, 80°C.
The zero „g‟ offset voltage at -20°C is subtracted from the value obtained
at 80°C. The resulting value obtained is divided by the accelerometer's
FSO to express the change in output in terms of % of FSO or alternately
it can be expressed as ppm also.
Observations and remarks:
From Table 4.4, it can be seen that over the operating temperature
range - 20°C to + 80°C the maximum change in zero „g‟ output is
9.8mV. Hence the temperature sensitivity of zero „g‟ error works
out to be 0.52% of FSO (or) alternately this can be expressed as
1.58mg/°C (at 62mV/g sensitivity).
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The offset variation is fairly linear with temperature, hence by
implementing suitable temperature compensation techniques, the
effect can be reduced considerably.
4.9.2 Temperature sensitivity of the scale factor:
Temperature sensitivity of the scale factor is the change in the
sensitivity of the accelerometer from the room temperature sensitivity as
the temperature changes. The variation is measured using special test
set-up in which accelerometer is placed on one end of an arm which is
inside the thermal chamber and the other end is outside and is
connected to a precision rotary table. Now the accelerometer is subjected
to the temperatures -20, 0, 20, 40, 60, 80°C. By rotating the rotary table
the accelerometer is subjected to ± 1 „g‟ acceleration and the scale factor
value is noted down at different temperatures as shown in fig 4.14. After
the testing is complete, the data is analyzed. The sensitivity at 25°C is
subtracted from each of the measurements. The resulting maximum
change in sensitivity is divided by the accelerometer's sensitivity at 25°C
to express the change in output in terms of ppm change in scale factor.
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Fig 4.14 Temperature sensitivity of the scale factor
Temp (ºC) Sensitivity (mV/ ‘g’)
-20 61.906
0 61.923
20 61.948
40 61.965
60 61.989
80 62.006
Table 4.5 Temperature sensitivity of scale factor
61.9
61.92
61.94
61.96
61.98
62
62.02
62.04
-20 0 20 40 60 80
sen
siti
vit
y
mv/g
'
temperature C
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Observations and remarks:
From Table 4.5 it can be seen that the scale factor variation over
the temperature range is a maximum of 0.1mV. Hence the scale
factor stability is 1612 ppm. This stability figure is adequate for
control class aerospace applications.
By adopting closed loop control techniques, scale factor variation
can be reduced considerably and more precise accelerometers can
be realized.
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4.10. Bandwidth test
The bandwidth is defined as the useful frequency range, in which
the output of the sensor is within ±3dB of the nominal value. The test
set-up block diagram is shown in fig 4.15.
Fig 4.15 Test set-up for bandwidth measurement.
The accelerometer is mounted on Modalshop make automatic
dynamic shaker. In this system, the output of the accelerometer under
test is compared with an inbuilt reference accelerometer output and the
performance is compared. A sine sweep signal of 1 „g‟ magnitude is
applied from a frequency of 10 Hz to 10000 Hz taking the output at 100
Hz as the reference value. The deviation in amplitude response as a
function of frequency is shown in fig 4.16.
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Fig 4.16 Frequency sweep output of accelerometer
Observations and remarks
From fig 4.16 amplitude response vs. frequency plot it can be seen that,
±3dB deviation in output is occurring at 800Hz. Hence the sensor meets
the operational bandwidth of 100Hz.
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4.11 Shock test
Accelerometers used in aerospace applications are subjected to high
shocks during the operation. The sensor shall withstand the shock and
exhibit normal performance after the shock is withdrawn.
Fig 4.17 Block diagram of shock testing
As shown in the fig 4.17 the sensor is mounted on a shock tester in
such a way that the sensing axis is along the shock input axis. A half
sine shock signal of 50 „g‟ magnitude is applied for a duration of
11msec. Fig. 4.18 gives the input shock spectrum and fig. 4.19 is the
response shock spectrum of the accelerometer, the graph plots the
response of three different accelerometers with different sensitivities. Also
the shock response spectrum provides a measure of response time of the
sensor.
Power
Supply Accelerometer Data
Logger
Shock
Tester
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Fig 4.18 Input shock spectrum
Fig 4.19 Response shock spectrum
- Sensor 1
- Sensor 2
- Sensor 3
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Observations and remarks:-
From the test results, it can be seen that, all the three
accelerometers have responded in a similar way to the applied
shock and the response is along the lines of input shock.
It can be seen that the accelerometers have very fast response of
less than 1msec.
4.12 Results & discussion
The sensitivity of the sensor measured is 62 mV/g.
The sensor demonstrated linearity error and cross-axis sensitivity
less than 1% of FSO as designed.
The hysteresis and bias stability values measured are less than
0.15% of FSO and meet the sensor specifications.
The temperature sensitivity of zero „g‟ error or offset error is 0.52%
of FSO (or) 1.58mg/°C (at 62mV/g sensitivity) and is fairly linear
over the temperature range.
The scale factor stability over the operational temperature range is
1612 ppm.
The operational bandwidth of the sensor is >100Hz as designed.
Sensor response time measured from shock test is less than
one msec.