Calculating tilt angles

AN3182

12/18

Doc ID 17289 Rev 1

3.2.2

Dual-axis tilt sensing

When a dual-axis tilt sensing approach is used, the user should be aware of two different

situations in which this approach could limit overall accuracy or even inhibit tilt calculation.

●

Figure 7

, Example A: Rotate the accelerometer counter-clockwise around the dotted

arrow with

β angle. When β is less than 45°, the X axis has higher sensitivity, while the

Y axis has lower sensitivity. And when

β is greater than 45°, the X axis has lower

sensitivity while the Y axis has higher sensitivity. Therefore, when the two-axis

approach is used, it is always recommended to calculate the angle based on the

orthogonal axis to a ±1 g condition.

●

Figure 7

, Example B: At this position, both the X and Y axes have high sensitivity.

However, without the help of a third axis (for example the Z axis), it is impossible to

distinguish a tilt angle of 30° from one of 150° because the X axis has the same outputs

at these two tilt angles.

Figure 7.

Tilt sensitivity of a dual-axis accelerometer

3.2.3

Tri-axis tilt sensing

With a 3-axis accelerometer, the user can use the Z axis to combine with the X and Y axes

for tilt sensing, to improve tilt sensitivity and accuracy (see Figure 8).

There are two ways to calculate 3 tilt angles in Figure 8. The first is use basic trigonometric

Equation 5

Equation 5

60

0.866

8.594

75

0.966

4.37

90

1.000

0.152

Table 3.

Tilt sensitivity of single axis accelerometer (continued)

Tilt [°]

Acceleration [g]

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

α

g

A

arcsin

1

x