Calculating Velocity with Accelerometer & Pitch for Robot Positioning

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SUMMARY

The discussion focuses on using an Ocean server 4000 compass/accelerometer to calculate the velocity of a robot based on accelerometer readings (Ax, Ay, Az) and pitch. The user, Eric, experiences discrepancies in velocity calculations, particularly when the robot is brought to a stop, indicating that the resting Ax values differ. The solution involves converting local accelerometer readings to world coordinates, taking into account the changing pitch angles, rather than relying on simple arithmetic adjustments to Ax values.

PREREQUISITES
  • Understanding of accelerometer data (Ax, Ay, Az)
  • Familiarity with pitch and its effects on motion
  • Knowledge of coordinate transformation techniques
  • Basic grasp of Euler angles and rotation matrices
NEXT STEPS
  • Research methods for converting local accelerometer readings to world coordinates
  • Study Euler angles and their application in 3D motion analysis
  • Learn about rotation matrices and their role in transforming coordinate systems
  • Explore advanced filtering techniques for accelerometer data, such as Kalman filters
USEFUL FOR

Robotics engineers, motion control specialists, and anyone involved in sensor fusion and positioning algorithms for mobile robots.

erickulcyk
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Hello, I have an Ocean server 4000 compass/accelerometer, and I am trying to use it to gauge the position of a robot on which it is placed. I get Ax, Ay, Az, pitch, roll, and heading as the output currently. I also have the accelerometer set to do a moving average of 16 readings. I did a simple experiment where I moved the robot forward in roughly the direction and stopped it. When I calculate the velocity straight from the Ax readings, the velocity doesn't end at 0 like it should (I brought the robot to stop for several seconds at the end). Furthermore the resting Ax at the start is different then at the end. The pitch has also changed though. In fact, the Ax values followed the pitch very well. I think there must be a relation to get the true x value given the two. I have tried Ax-pitch, Ax+pitch, Ax-sin(pitch), Ax+sin(pitch), √(Ax^2-sin(pitch)^2), however non of these gives me a velocity of close to 0 at the end. Can you help me out?

Thanks,
Eric
 
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Yes, if angle readings change throughout, you have to take it into account. There is no simple formula, though. You need to convert Ax, Ay, Az in local coordinates to Ax, Ay, Az in "world" coordinates. The later can be arbitrary, but they need to be fixed. These two articles might help you get started.

Euler Angles
Rotation Matrix
 

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