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ADXL202AE Datasheet(Arkusz danych) 11 Page - Analog Devices

Numer części ADXL202AE
Szczegółowy opis  Low-Cost -2 g Dual-Axis Accelerometer with Duty Cycle Output
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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REV. A
ADXL202E
–11–
A DUAL AXIS TILT SENSOR: CONVERTING
ACCELERATION TO TILT
When the accelerometer is oriented so both its X and Y axes are
parallel to the earth’s surface it can be used as a two axis tilt sensor
with a roll and a pitch axis. Once the output signal from the
accelerometer has been converted to an acceleration that varies
between –1 g and +1 g, the output tilt in degrees is calculated as
follows:
Pitch = ASIN (Ax/1 g)
Roll = ASIN (Ay/1 g)
Be sure to account for overranges. It is possible for the acceler-
ometers to output a signal greater than
± 1 g due to vibration,
shock or other accelerations.
MEASURING 360 OF TILT
It is possible to measure a full 360
° of orientation through gravity
by using two accelerometers oriented perpendicular to one another
(see Figure 5). When one sensor is reading a maximum change
in output per degree, the other is at its minimum.
360 OF TILT
1g
Y
X
Figure 5. Using a Two-Axis Accelerometer to Measure
360
° of Tilt
USING THE ANALOG OUTPUT
The ADXL202E was specifically designed for use with its digital
outputs, but has provisions to provide analog outputs as well.
Duty Cycle Filtering
An analog output can be reconstructed by filtering the duty cycle
output. This technique requires only passive components. The
duty cycle period (T2) should be set to <1 ms. An RC filter with a
3 dB point at least a factor of >10 less than the duty cycle fre-
quency is connected to the duty cycle output. The filter resistor
should be no less than 100 k
Ω to prevent loading of the output
stage. The analog output signal will be ratiometric to the supply
voltage. The advantage of this method is an output scale factor of
approximately double the analog output. Its disadvantage is that
the frequency response will be lower than when using the XFILT,
YFILT output.
XFILT, YFILT Output
The second method is to use the analog output present at the
XFILT and YFILT pin. Unfortunately, these pins have a 32 k
output impedance and are not designed to drive a load directly.
An op amp follower may be required to buffer this pin. The
advantage of this method is that the full 5 kHz bandwidth of the
accelerometer is available to the user. A capacitor still must be
added at this point for filtering. The duty cycle converter should
be kept running by using RSET <10 M
Ω. Note that the acceler-
ometer offset and sensitivity are ratiometric to the supply voltage.
The offset and sensitivity are nominally:
0 g Offset = VDD/2
ADXL202E Sensitivity = (60 mV
× VS)/g
Table V. Trade-Offs Between Microcontroller Counter Rate,
T2 Period, and Resolution of Duty Cycle Modulator
Counter-
ADXL202E Clock
Counts
RSET Sample
Rate
per T2
Counts Resolution
T2 (ms) (k ) Rate
(MHz)
Cycle
per g
(mg)
1.0
124
1000
2.0
2000
250
4.0
1.0
124
1000
1.0
1000
125
8.0
1.0
124
1000
0.5
500
62.5
16.0
5.0
625
200
2.0
10000
1250
0.8
5.0
625
200
1.0
5000
625
1.6
5.0
625
200
0.5
2500
312.5
3.2
10.0
1250 100
2.0
20000
2500
0.4
10.0
1250 100
1.0
10000
1250
0.8
10.0
1250 100
0.5
5000
625
1.6
STRATEGIES FOR USING THE DUTY CYCLE OUTPUT
WITH MICROCONTROLLERS
Application notes outlining various strategies for using the duty
cycle output with low cost microcontrollers are available from
the factory.
USING THE ADXL202E AS A DUAL-AXIS TILT SENSOR
One of the most popular applications of the ADXL202E is tilt
measurement. An accelerometer uses the force of gravity as an
input vector to determine orientation of an object in space.
An accelerometer is most sensitive to tilt when its sensitive axis
is perpendicular to the force of gravity, i.e., parallel to the earth’s
surface. At this orientation its sensitivity to changes in tilt is high-
est. When the accelerometer is oriented on axis to gravity, i.e.,
near its +1 g or –1 g reading, the change in output acceleration
per degree of tilt is negligible. When the accelerometer is perpen-
dicular to gravity, its output will change nearly 17.5 mg per degree
of tilt, but at 45
° degrees it is changing only at 12.2 mg per
degree and resolution declines. The following table illustrates
the changes in the X and Y axes as the device is tilted
± 90°
through gravity.
Y
X
+90
0
–90
1g
X Output
Y Output (g)
X Axis
per
per
Orientation
Degree of
Degree of
to Horizon ( )
X Output (g)
Tilt (mg)
Y Output (g)
Tilt (mg)
–90
–1.000
–0.2
0.000
17.5
–75
–0.966
4.4
0.259
16.9
–60
–0.866
8.6
0.500
15.2
–45
–0.707
12.2
0.707
12.4
–30
–0.500
15.0
0.866
8.9
–15
–0.259
16.8
0.966
4.7
0
0.000
17.5
1.000
0.2
15
0.259
16.9
0.966
–4.4
30
0.500
15.2
0.866
–8.6
45
0.707
12.4
0.707
–12.2
60
0.866
8.9
0.500
–15.0
75
0.966
4.7
0.259
–16.8
90
1.000
0.2
0.000
–17.5
BOTTOM VIEW
Figure 4. How the X and Y Axes Respond to Changes
in Tilt




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