rdeits
commented
6 years ago You are correct that RigidBodyDynamics.jl is the key here, as all of the logic for kinematics and dynamics is entirely handled in that package.
set_configuration!(vis, args...) does exactly two things:
- It forwards your arguments to
set_configuration!(state, args...)wherestateis the RigidBodyDynamics.MechanismState holding the state of the mechanism in the visualizer. - It uses that MechanismState to update the rendered positions of all the bodies
To see the possible arguments to set_configuration!(state, ...), you can see the RigidBodyDynamics docs here: http://www.juliarobotics.org/RigidBodyDynamics.jl/stable/mechanismstate.html#RigidBodyDynamics.set_configuration!-Tuple{RigidBodyDynamics.MechanismState,AbstractArray{T,1}%20where%20T}
The simplest way to set the configuration is simply set_configuration!(vis, q) where q is a vector of the generalized joint positions for whatever your robot happens to be. For the Acrobot model, q is just a 2-element vector of joint angles.
Note that the key here is that set_configuration!() is used to set the generalized coordinates of the mechanism itself. That is, only the <joint> tags in the URDF. If you want to just move the visualizer around in a way that isn't supported by the actual robot's joints, then you need the lower-level settransform!() from MeshCat.jl.
For example, to move the whole visualizer:
using MeshCat: Visualizer, settransform!
using CoordinateTransformations: Translation
using MeshCatMechanisms: MechanismVisualizer, URDFVisuals
vis = Visualizer()
# move the root of the visualizer around
settransform!(vis, Translation(0., 0, 1))
# Add the mechanism visualizer below the root, at the path `/mechanism`.
mvis = MechanismVisualizer(robot, URDFVisuals(urdf), vis[:mechanism])does that help?
han-so1omon
It is unclear from the demo and code comments how to manipulate an arbitrary robot model. I have a single body, single joint (base to world) model loaded. My intent is to run a dynamics model in a networked server and use MeshCatMechanisms as a display. My goal is simply to load the model and then apply arbitrary transforms onto it as the display refreshes. The demos work fine, so that is not the issue.
When it comes to setting up the model, it is unclear how to do things like: set origin at center of display, set model transformations. It is also unclear what variables are involved in
set_configuration!()function. It seems like the mechanism visualizer, the joints, and some transform parameter are the inputs. However, it is not clear what the transform parameter is. The third parameter is a list of Float64 numbers corresponding to joints. I can't think of a transform that can be fully described by a single number, though.After digging, it seems like the key may be in the
RigidBodyDynamicspackage, but it has taken a long time to figure this out.Apologies if this response is unorganized, but I am a bit rushed to write this and am still quite confused in how a robot can be arbitrarily assigned a transformation. Perhaps adding documentation for just this will be of use to others as well. My current attempt is shown below:
` homog(t::Translation) = homog(AffineMap(eye(3),t.v))
vis = Visualizer() urdf = joinpath(pwd(), "resources", "quadrotor.urdf")
urdf = joinpath(pwd(), "resources", "Acrobot.urdf")
robot = parse_urdf(Float64, urdf) delete!(vis) mvis = MechanismVisualizer(robot, URDFVisuals(urdf), vis) body = bodies(robot)[end] body_frame = default_frame(body) setelement!(mvis,body_frame) at = homog(Translation(0,0,-1)) set_configuration!(mvis, findjoint(mvis.state.mechanism, "base_link_to_world"), [1.0])
open(vis) ` Edit: Above example fails with: ERROR: LoadError: BoundsError: attempt to access 0-element SubArray{Float64,1,Array{Float64,1},Tuple{UnitRange{Int64}},true} at index [Base.OneTo(1)]