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ADXL05 Datasheet(PDF) 16 Page - Analog Devices

Part No. ADXL05
Description  Single Chip Accelerometer with Signal Conditioning
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADXL05 Datasheet(HTML) 16 Page - Analog Devices

 
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ADXL05
REV. B
–16–
control of system and mounting resonances are critical to proper
measurement. Refer to the application note AN-379, available
from Analog Devices.
CALIBRATING THE ADXL05
If a calibrated shaker is not available, both the 0 g level and scale
factor of the ADXL05 may be easily set to fair accuracy by using
a self-calibration technique based on the 1 g (average) accelera-
tion of the earth’s gravity. Figure 30 shows how gravity and
package orientation affect the ADXL05’s output. Note that the
output polarity is that which appears at VPR; the output at VOUT
will have the opposite sign. With its axis of sensitivity in the
vertical plane, the ADXL05 should register a 1 g acceleration,
either positive or negative, depending on orientation. With the
axis of sensitivity in the horizontal plane, no acceleration (the
0 g bias level) should be indicated.
0
g
(a)
0
g
(b)
–1
g
(c)
+1
g
(d)
INDICATED POLARITY IS THAT
OCCURRING AT V
PR .
Figure 30. Using the Earth’s Gravity to Self-Calibrate the
ADXL05
To self-calibrate the ADXL05, place the accelerometer on its
side with its axis of sensitivity oriented as shown in “a.” The 0 g
offset potentiometer, Rt, is then roughly adjusted for midscale:
+2.5 V at the buffer output (see Figure 25).
Next, the package axis should be oriented as in “c” (pointing
down) and the output reading noted. The package axis should
then be rotated 180
° to position “d” and the scale factor poten-
tiometer, R1a, adjusted so that the output voltage indicates a
change of 2 gs in acceleration. For example, if the circuit scale
factor at the buffer output is 400 mV per g, then the scale factor
trim should be adjusted so that an output change of 800 mV is
indicated.
Adjusting the circuit’s scale factor will have some effect on its
0 g level so this should be readjusted, as before, but this time
checked in both positions “a” and “b.” If there is a difference in
the 0 g reading, a compromise setting should be selected so that
the reading in each direction is equidistant from +2.5 V. Scale
factor and 0 g offset adjustments should be repeated until both
are correct.
REDUCING POWER CONSUMPTION
The use of a simple power cycling circuit provides a dramatic
reduction in the ADXL05’s average current consumption. In
low bandwidth applications such as shipping recorders, this
simple, low cost circuit can provide substantial power reduction.
If a microprocessor is available, only the circuit of Figure 31 is
needed. The microprocessor supplies a TTL clock pulse to gate
buffer transistor Q1 which inverts the output pulse from the
µP
to a 5
° tilt error over the entire temperature range. Straight-
forward calibration schemes discussed in this data sheet may be
used to reduce or compensate for temperature drift to improve
the absolute accuracy of the measurement.
Using the ADXL05 in Inertial Measurement Applications
Inertial measurement refers to the practice of measuring accel-
eration for the purpose of determining the velocity of an object
and its change in position, or distance traveled. This technique
has previously required expensive inertial guidance systems of
the type used in commercial aircraft and military systems. The
availability of a low cost precision dc accelerometer such as the
ADXL05 enables the use of inertial measurement for more cost
sensitive industrial and commercial applications.
Inertial measurement makes use of the fact that the integral of
acceleration is velocity and the integral of velocity is distance.
By making careful measurements of acceleration and math-
ematically integrating the signals, one can determine both veloc-
ity and the distance traveled. The technique is useful for
applications where a traditional speed and distance measure-
ment is impractical, or where a non-contact, relative position
measurement must be made.
A practical inertial measurement system uses multiple acceler-
ometers to measure acceleration in three axes, and gyroscopes to
measure rotation in three axes, the requirement for a 6 degree of
freedom system. For simpler systems where one or more of the
axes can be constrained, it is possible to build a system with
fewer accelerometers and gyros.
The measurement system must take the acceleration sensor and
calibrate out all static errors including any initial inaccuracy or
temperature drift. A mathematical model is used to describe the
performance of the sensor in order to calibrate it. If these errors
are not removed, then the process of double integration will
quickly cause any small error to dominate the result. Most prac-
tical systems use microprocessors for error correction and a tem-
perature sensor for temperature drift compensation. Another
approach is to maintain all of the sensors at a controlled tem-
perature. The microprocessors have the additional advantage of
providing a low cost method of performing the single and
double integration of the acceleration signal.
The stability and repeatability of the accelerometer is the most
important specification in an inertial system. The ADXL05 is
“well behaved” that is, its response and temperature characteris-
tics are easy to model and correct, and once modeled they are
very repeatable. For example, temperature performance can be
adequately modeled using first order, (straight line) approxima-
tions for most applications, and other errors such as on-axis and
pendulous rectification are minimal. This greatly simplifies the
math required to correct the sensor.
Vibration and Shock Measurement Applications
The ADXL05 can measure shocks and vibrations from dc to
4 kHz. Typical signal processing for vibration signals includes
fast Fourier transforms, and single and double integration for
velocity and displacement. It is possible to build a single inte-
grator stage using the ADXL05’s output buffer amplifier in
order to provide a velocity output.
The sensitivity of the accelerometer will typically vary only
±0.5% over the full industrial temperature range, making it one
of the most stable vibration measurement devices available. In
vibration measurement applications, mechanical mounting and
OBSOLETE


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