ADXL202/ADXL210

–9–

REV. B

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 calcu-

lated 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 grav-

ity by using two accelerometers oriented perpendicular to one

another (see Figure 15). When one sensor is reading a maxi-

mum change in output per degree, the other is at its minimum.

Y

X

360 OF TILT

1

g

Figure 15. Using a Two-Axis Accelerometer to Measure

360

° of Tilt

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 ADXL202/ADXL210 AS A DUAL AXIS TILT

SENSOR

One of the most popular applications of the ADXL202/ADXL210

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 highest. 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 accelerom-

eter is perpendicular 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

1

g

X OUTPUT

Y OUTPUT (

g)

X AXIS

PER

PER

ORIENTATION

DEGREE OF

DEGREE OF

TO HORIZON ( )

X OUTPUT (

g)

TILT (m

g)

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

Figure 14. How the X and Y Axes Respond to Changes in

Tilt