Joints

A joint connects body1 to body2 (default: the ground) at a joint frame – a point (location) and axes (orientation), given globally at t = 0 and fixed on both bodies. It fixes some of the six relative directions of that frame: translations dx dy dz along and rotations ax ay az about its axes.

joints:
  hinge:
    type: revolute
    body1: rod
    body2: ground
    location: [0, 0, 0]
    orientation: [0.7071068, -0.7071068, 0, 0]   # joint z axis = global y: a hinge about y

type	fixes	leaves free
fixed	dx dy dz ax ay az	nothing
spherical	dx dy dz	all rotations
revolute	dx dy dz ax ay	rotation about z
cylindrical	dx dy ax ay	translation along and rotation about z
prismatic	dx dy ax ay az	translation along z
generic	the directions in fix	the others

  planar_slide: {type: generic, body1: block, location: [0, 0, 0], fix: [dz, ax, ay]}

key	value
id	id
type (required)	fixed | spherical | revolute | cylindrical | prismatic | generic	Presets fix directions of the joint frame (joint axis z): revolute about z, prismatic and cylindrical along z. generic: the fix list.
body1 (required)	string	A body.
body2	string	A body or 'ground'. Default ground.
location	[x, y, z] or a point name	Joint point, global, at t = 0.
orientation	list of 4 or a frame name	Joint frame as Euler parameters, global, at t = 0. Default [1,0,0,0].
fix	list	generic only: fixed translations (d) and rotations (a) along/about the joint axes.

The joint's free axis is always z. Turn the joint frame with orientation to put z where the motion is; the orientation table has the common cases.

The constraint force of each fixed direction is a result channel: hinge.dx.lambda, hinge.ay.lambda, … (Result channels).

Redundant constraints

Closed loops built from joints often fix the same direction twice (a four-bar linkage of four revolute joints in a plane fixes the out-of-plane motion more than once). The checks report it as more constraints than degrees of freedom; replace a joint by one that fixes fewer directions (a spherical or a generic joint).