Prismatic

Related class: VxDynamics::Prismatic

prismatic.png

The prismatic joint is imagined as a bar with a block that slides along it without rotating. The name prismatic refers to the imagined shape of the bar whose non-circular cross-section restricts rotation by keying into the matching hole in the block.

A prismatic joint leaves a single, linear, degree of freedom (DoF) which can be controlled by a displacement coordinate. It constrains the two remaining linear DoFs and all three angular DoFs, as it acts to maintain the relative orientation of the parts.

The prismatic joint is not symmetric as its two attachments have different geometric roles:

  • If one attachment is absolute it should usually be the first one as this fixes a line in the world.
    The bar is a line defined by an axis fixed in the first attachment.
  • The block is a point position corresponding to the origin of the second attachment.
    Making the second attachment absolute fixes a point in the world. Combined with relaxation of the angular constraint this point acts as a springy pivot.