The coordinate systems in mobile robotics

  1. world coordinate system
  2. ego-vehicle coordinate system

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Rigid Body Assumption

Rigid Body is a solid body in which deformation is zero or so small it can be neglected.

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rigid body assumption implied that movement of a sensor mounted on the robot should be the same as other co-located sensors and the whole platform.

Motion in general

Motion that can be described by a rotation and translation. There are 6 degree of freedom (DoF) in 3D space for rigid body.

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Coordinate transformation
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Rotation

rotation matrix
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Direct Cosine Matrix

Direct Cosine Matrix (DCM), unit vectors, 3 assumptions
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Eular Angles

Euler Angles, single axis rotation, in an intrinsic sequence (X, Y, Z).
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Gimbal lock, loss of one DoF, cause singularity.
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Quaternions

Quaternions and 3d rotation, explained interactively
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Small summary

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Order of rotation, rotation in 3D are non-commutative.
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Reference Frame

Earth Centered Earth Fixed Frame

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North East Down Frame

North East Down (NED) frame

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Sensors’ frame

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GNSS

Global Positioning System
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Position in ECEF coordinate via accurate ephemeris information. Time of signal transmission via onboard atomic clock.
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Reducing uncertainty every time we add a new circle.
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In this case we have sphere.
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Ephemeris & clock errors, Multipath effect affecting precision
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ionosphere delay
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Future Reading

  1. Understanding Direct Cosine Matrices, Euler Angles and Quaternions.
  2. Representing Robot Pose: The good, the bad, and the ugly.
  3. Rethinking GPS: Engineering Next-Gen Location at Uber

Origin: Dr. Chris Lu (Homepage)
Translate + Edit: YangSier (Homepage)

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