Configuration space

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In classical mechanics, the parameters that define the configuration of a system are called generalized coordinates, and the vector space defined by these coordinates is called the configuration space of the physical system. It is often the case that these parameters satisfy mathematical constraints, which means that the set of actual configurations of the system is a manifold in the space of generalized coordinates. This manifold is called the configuration manifold of the system.

Configuration spaces in physics[edit]

Particle[edit]

The position of a single particle moving in ordinary Euclidean 3-space is defined by the vector r=(x, y, z), and therefore its configuration space is R3. If the particle is constrained to lie on a sphere, then its configuration space is the subset of coordinates in R3 that define points on the sphere S2.

For n particles the configuration space is R3n, or possibly the subspace where no two positions are equal.

An important problem in physics considers the set of all trajectories of a particle between two points, which is a configuration space that is also known as a function space M. In quantum mechanics one formulation uses histories, or trajectories, as configurations.

Rigid body[edit]

The set of coordinates that define the position of a reference point and the orientation of a coordinate frame attached to a rigid body in three-dimensional space form its configuration space, often denoted \mathbb{R}^{3}\times\mathrm{SO}(3) where \mathbb{R}^{3} represents the coordinates of the origin of the frame attached to the body, and \mathrm{SO}(3) represents the rotation matrices that define the orientation of this frame relative to a ground frame. A configuration of the rigid body is defined by six parameters, three from \mathbb{R}^{3} and three from \mathrm{SO}(3), and is said to have six degrees of freedom.

In Robotics, configuration space refers to the set of positions reachable by a robot's end-effector considered to be a rigid body in three-dimensional space.[1] Thus, the positions of the end-effector of a robot can be identified with the group of spatial rigid transformations, often denoted SE(3).

The joint parameters of the robot are used as generalized coordinates to define its configurations. The set of joint parameter values is called the joint space. The robot's forward and inverse kinematics equations define mappings between its configurations and its end-effector positions, or between joint space and configuration space. Robot motion planning uses these mappings to find a path in joint space that provides a desired path in the configuration space of the end-effector.

Phase space[edit]

In Mechanics, the configuration of a system is considered to consist of its position and its velocity, or more generally its momentum. The set of velocities available to a system defines a plane tangent to its configuration manifold. Momentum vectors are linear functionals on the tangent plane, known as cotangent vectors. Thus, the set of position and momenta of a mechanical system forms the cotangent bundle of the configuration manifold.

This larger manifold is called the phase space of the system. The usual configuration space can be viewed as "half" of the phase space of a mechanical system.

Configuration spaces in mathematics[edit]

The configuration space of 2 not necessarily distinct points on the circle is the orbifold T^2/S_2, which is the Möbius strip.

In mathematics a configuration space refers to a broad family of constructions closely related to the state space notion in physics. The most common notion of configuration space in mathematics C_n X is the set of n-element subsets of a topological space X. This set is given a topology by considering it as the quotient C_n X = F_n X / \Sigma_n where F_n X = \{(x_1,\cdots,x_n) \in X^n : x_i \neq x_j \forall \ i \neq j \} and \Sigma_n is the symmetric group acting by permuting the coordinates of F_n X. Typically, C_n X is called the configuration space of n unordered points in X and F_n X is called the configuration space of n ordered or coloured points in X; the space of n ordered not necessarily distinct points is simply X^n.

If the original space is a manifold, the configuration space of distinct, unordered points is also a manifold, while the configuration space of not necessarily distinct unordered points is instead an orbifold.

Configuration spaces are related to braid theory, where the braid group is considered as the fundamental group of the space C_n \Bbb R^2.

A configuration space is a type of classifying space or (fine) moduli space. In particular, there is a universal bundle  \pi\colon E_n\to C_n which is a subbundle of the trivial bundle  C_n\times X^n\to C_n, and which has the property that the fiber over each point  p\in C_n is the n element subset of  X_n classified by p.

The homotopy type of configuration spaces is not homotopy invariant – for example, that the spaces F_n \Bbb R^m are not homotopic for any two distinct values of m. For instance, F_n\Bbb R is not connected, F_n\Bbb R^2 is a K(\pi,1), and F_n \Bbb R^m is simply connected for  m \geq 3.

It used to be an open question whether there were examples of compact manifolds which were homotopic but had non-homotopic configuration spaces: such an example was found only in 2005 by Longoni and Salvatore. Their example are two three-dimensional lens spaces, and the configuration spaces of at least two points in them. That these configuration spaces are not homotopic was detected by Massey products in their respective universal covers.[2]

See also[edit]

References[edit]

  1. ^ John J. Craig, Introduction to Robotics: Mechanics and Control, 3rd Ed. Prentice-Hall, 2004
  2. ^ Salvatore, Paolo; Longoni, Riccardo (2005), "Configuration spaces are not homotopy invariant", Topology 44 (2): 375–380, doi:10.1016/j.top.2004.11.002 

External links[edit]