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Rigid Body Transformations

Rigid Body Transformations. Vijay Kumar. Remark about Notation. Vectors x, y, a, … A x u , v, p, q, … Matrices A, B, C, … A A B g , h, …. Potential for Confusion!. The 3 ´1 vector a and its 3 ´3 skew symmetric matrix counterpart a Ù. For any vector b a ´ b = A b. a.

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Rigid Body Transformations

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  1. Rigid Body Transformations Vijay Kumar

  2. Remark about Notation • Vectors • x, y, a, … • Ax • u, v, p, q, … • Matrices • A, B, C, … • AAB • g, h, … Potential for Confusion!

  3. The 3´1 vector a and its 3´3 skew symmetric matrix counterpart aÙ • For any vector b • a´ b = Ab a aÙ = A

  4. t Rigid Body Transformation • Rigid Body Displacement • Rigid Body Motion R3 O

  5. Coordinate Transformations and Displacements • Transformations of points • Transformation (g) of points induces an action (g*) on vectors • What are rigid body transformations? Displacements? • g preserves lengths • g* preserves cross products v p g (p) q g*(v) g (q)

  6. Translation of the origin of {B} with respect to origin of {A} Rotation of {B} with respect to {A} Rigid Body Transformations in R3 • Can show that the most general coordinate transformation from {B} to {A} has the following form • position vector of P in {B} is transformed to position vector of P in {A} • description of {B} as seen from an observer in {A}

  7. Rotational transformations in R3 • Properties of rotation matrices • Transpose is the inverse • Determinant is +1 • Rotations preserve cross products R u´R v = R (u´v) • Rotation of skew symmetric matrices For any rotation matrix R: RwÙRT = (R w)Ù

  8. Example: Rotation • Rotation about the x-axis through q z z q y y x Displacement x

  9. Rotation about the y-axis through q Rotation about the z-axis through q x x' z' q z y Example: Rotation x x' q z y y'

  10. Rigid Motion in R3 • The same equation can have two interpretations: • It transforms the position vector of any point in {B} to the position vector in {A} • It transforms the position vector of any point in the first position/orientation (described by {A}) to its new position vector in the second position orientation (described by{B}). Coordinate transformation from {B} to {A} Displacement of a body-fixed frame from {A} to {B}

  11. Mobile Robots y R q x W WAR

  12. The Lie group SE(3) http://www.seas.upenn.edu/~meam520/notes02/RigidBodyMotion3.pdf

  13. SE(3) is a Lie group • SE(3) satisfies the four axioms that must be satisfied by the elements of an algebraicgroup: • The set is closed under the binary operation. In other words, if A and B are any two matrices in SE(3), ABÎ SE(3). • The binary operation is associative. In other words, if A, B, and C are any three matrices Î SE(3), then (AB) C = A (BC). • For every element AÎ SE(3), there is an identity element given by the 4´4 identity matrix, IÎ SE(3), such that AI = A. • For every element AÎ SE(3), there is an identity inverse, A-1Î SE(3), such that A A-1 = I. • SE(3) is a continuous group. • the binary operation above is a continuous operation ¾ the product of any two elements in SE(3) is a continuous function of the two elements • the inverse of any element in SE(3) is a continuous function of that element. • In other words, SE(3) is a differentiable manifold. A group that is a differentiable manifold is called a Lie group[ Sophus Lie (1842-1899)].

  14. Composition of Displacements • Displacement from {A} to {B} • Displacement from {B} to {C} • Displacement from {A} to {C} • Note XAY describes the displacement of the body-fixed frame from {X} to {Y} in reference frame {X}

  15. Composition (continued) • Composition of displacements • Displacements are generally described in a body-fixed frame • Example: BAC is the displacement of a rigid body from B to Crelativeto theaxes of the “first frame” B. • Composition of transformations • Same basic idea • AAC = AAB BAC Note that our description of transformations (e.g., BAC) is relativeto the“first frame” (B, the frame with theleading superscript). • Note XAY describes the displacement of the body-fixed frame from {X} to {Y} in reference frame {X}

  16. Subgroups of SE(3)

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