IMU

Acceleration sensor

An acceleration sensor measures earth gravity force (m*s^2) in all 3 robot axes (x/y/z) allowing you to calculate the orientation towards earth center of the robot (Roll, Pitch). However, it cannot detect movement around Yaw - this requires a gyro.

Gyro

A gyro measure rotation speed (degree/second) in all 3 robot axes (Rol, Pitch, Yaw). This allows us to correct the robot if it is driving 'off-course' due to wet conditions or slope.

Compass

If a robot drives on a wet lawn, you'll notice quickly that it will not drive straight forward, but instead tends to drive into the direction of the slope. A compass can solve this problem and the robot will drive again straight forward.

However, a compass has a problem: if tilting the robot, the compass measurements (x,y,z) relate to the tilted robot. So, you need to correct them using knowing the tilt angles. This requires the use of the accerlation sensor (x,y).

Sensor fusion

To eliminate the issues of all sensor, they are merged to compute the course (degree) and pitch/roll angles (IMU - Intertial Measurement Unit). The IMU is key component of an intertial navigation system.

We use a complementary filter to fusion all sensor values.

IMU module GY-80

• Acceleration sensor: ADXL345B
• Compass sensor: HMC5883L
• Gyro: L3G4200D
• Pressure sensor: BMP085 (not used here)

Assembly

This photo shows how the IMU module is placed in driving position (the black arrow shows the driving position). The shown tube was a prototype. Actually, you can now place the IMU module directly in the robot.