Three Problems in Robotics - helix

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λ iaibiFigure 1: A Rigid Body Suspended by a System of SpringsThe infinitesimal screws or motors were elements of the Lie algebra of this group.Many other elements of Lie theory were also present in Ball’s screw theory. But perhapstheir significance was not fully appreciated. For example, the Lie product or Liebracket is simply the cross product of screws. For Ball this was just a geometricaloperation, the analogue of the vector product of 3-dimensional vectors.Some authors refer to Screw theory and Lie group methods as if they were differentapproaches. This author’s view is that there is no distinction between them, screwtheory is simply the specialisation of Lie theory to the group of rigid body transformations.The name screw theory is useful in three ways, it is a useful shorthand, it isdescriptive and lastly it reminds us that it was Ball who worked out almost all of thetheory before Lie groups were invented!In this work the fact that a Lie group is a differential manifold is used. To minimisea smooth function on such a space we do not need the machinery of Lagrange multipliers.We can work on the manifold directly, we do not need to think of the group asembedded in Cartesian space, as would be implied by the use of Lagrange multipliers.To find equations for a function’s stationary points we differentiate along tangentvector fields. The most convenient vector fields to use are the left-invariant fields onthe group. These are just elements of the Lie algebra of the group, the screws. Hencethis technique could be thought of as, “differentiating along a screw”.2 SpringsConsider a rigid body supported by a system of springs, see figure 1. Further assumethat the springs have natural length 0, obey Hook’s law and can both push and pull. Thespring constants λ i , of the springs can be different. Let ã T i =(a T i , 1) be the points wherethe springs are attached to the ground or frame, and ˜b T i =(b T i , 1) the correspondingattachment points on the rigid body when the body is in some standard ‘home’ config-1
uration. If the body undergoes a rigid motion the attachment points will move to,( ) ( )( )b′i R t bi=1 0 1 1which we will abbreviate to ˜b ′ i = M ˜b i .Thefirst question we can ask about this situationis: Is there an equilibrium configuration for the rigid body?The problem is to minimise the potential energy of the spring system, this is givenby the following function,Φ = 1 2 ∑ iλ i(ãi − M ˜b i) T (ãi − M ˜b i)Notice that this function is defined on the group SE(3), that is Φ : SE(3) −→ IR,asMvaries over the group we have different values of the potential.If this was just a function on IR n we would find the partial derivatives and set themto zero to find the stationary points. The standard method of tackling this problemwould be to minimise the matrix elements using Lagrange multipliers to take accountof the constraint that the matrix must be a group element.However, we can imitate the simpler method for unconstrained functions usingsome manifold theory. To find the stationary points of a function defined on a manifoldwe simply differentiate the function along a vector fields on the manifold. As usual weset the results to zero and then solve the resulting equations to find the stationary point.For this to work we do need a set of vector field which span the space of all vector fieldon the manifold.As the manifold under consideration is the underlying manifold of a Lie group,such a complete set of vector fields is always available. We can use the elements of theLie algebra thought of a left invariant vector fields.To differentiate along a vector field we move a little way along a path tangent to thevector field and compare the function values at the two points, then we take the limit ofthe difference between values of the function at these neighbouring points as the pathget shorter and shorter.Let:S =( )Ω v0 0be a Lie algebra element or screw given in the 4 × 4 representation. Ω is a 3 × 3 antisymmetricmatrix corresponding to a vector ω, Ωx = ω × x.If M is a group element written in the 4 × 4 representation, then the action of S onM, is given by the left translation,M(t)=e tS MThis takes M along a path tangent to the vector field defined by S. Taking the derivativealong the path and then setting t = 0gives,∂ S M = SMHence the derivative of the potential is given by,) T∂ S Φ = −∑λ i(ãi − M ˜b i SM ˜b ii2
Page 1: Three Problems in RoboticsJ.M. Seli
Page 5 and 6: into an orthogonal matrix and a non
Page 7 and 8: onal and D diagonal. Now we can wri
Page 9 and 10: So we see that the rows of this Jac
Page 11 and 12: an eigenvector of Q. So let us assu
Page 13: [15] J.M. Selig and P.R McAree. Con

Three Problems in Robotics - helix

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