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The Enhancement of a Multi-Terrain Mechatron for Autonomous Outdoor Applications

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6.2.3 Tilt Measurements

The accelerometer and compass can both provide information on pitch and roll and

therefore enable sensor redundancy. If one unit fails or cannot provide accurate data,

the other is used. Sensor fusion algorithms can take advantage of this property by

giving the data from the preferred sensor a higher weighting.

An important application of compass and accelerometer sensor redundancy for tilt

applications is near magnetic disturbances such as power lines. The stray magnetic

fields interfere with the electronic compass’s Magnetoresistive sensors, producing

erroneous results. The accelerometer however, works by measuring acceleration due

to Earth’s gravity and is therefore unaffected by magnetic fields and can be used as a

replacement for the compass in this case.

A comparison between the pitch and roll outputs from the compass module are

compared to those calculated (using Equation 5.5) from the accelerometer in Figures

6.6 and 6.7. Both modules are connected together and rotated through an angle range

of ± 90° for both the pitch and roll axes.

The data show a close correlation between the tilt angles of both units throughout the

different degrees of pitch and roll. A slight offset is consistently observed between the

data from each sensor as indicated by region A in Figure 6.6. This shows an example

of accelerometer data having a higher magnitude for pitch angles however, the

compass data exhibits the higher magnitude for roll angles (Figure 6.7). Although the

difference is negligible during normal operation, the slight variations (approx 1.9°)

can be attributed to the non-exact mounting of the devices explained in section 6.2.1

and therefore may be eliminated by including the offset in the tilt equation.

The figures also show the tilt limitation of the compass compared to the

accelerometer. Technical data for the HMR3300 suggest a maximum tilt range of

±60° in both pitch and roll and this is verified by the experimental data. In contrast the

accelerometer is capable of the full ±90° range of tilt angles as shown in region B of

Figure 6.7.

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