Unable to make mechanism floating with RigidBodyDynamics

General Usage
1

I’m using RigidBodyDynamics and am trying to get my mechanism to be floating rather than fixed-base. However, when I set floating=true in parse_urdf, I get a “type QuatRotation has no field w” error after running MechanismState.

I’ve seen an example of this work, so it’s odd: example

I also tried using an earlier version of RigidBodyDynamics (v2.0.0), which didn’t throw any errors, but in this case the model is inexplicably missing from the visualization (And yes, I checked the first timestep).

curdir = pwd()
urdfpath = joinpath(curdir, "../res/flyhopper_robot/urdf/flyhopper_robot.urdf")
doublependulum = parse_urdf(urdfpath, floating=true)
state = MechanismState(doublependulum)

type QuatRotation has no field w

Stacktrace:
  [1] getproperty(x::Rotations.QuatRotation{Float64}, f::Symbol)
    @ Base ./Base.jl:33
  [2] set_rotation!
    @ ~/.julia/packages/RigidBodyDynamics/8B04X/src/joint_types/quaternion_floating.jl:37 [inlined]
  [3] zero_configuration!
    @ ~/.julia/packages/RigidBodyDynamics/8B04X/src/joint_types/quaternion_floating.jl:169 [inlined]
  [4] zero_configuration!
    @ ~/.julia/packages/RigidBodyDynamics/8B04X/src/joint.jl:353 [inlined]
  [5] (::RigidBodyDynamics.var"#57#58")(joint::Joint{Float64, QuaternionFloating{Float64}}, qjoint::SubArray{Float64, 1, Vector{Float64}, Tuple{UnitRange{Int64}}, true})
    @ RigidBodyDynamics ~/.julia/packages/RigidBodyDynamics/8B04X/src/mechanism_state.jl:294
  [6] macro expansion
    @ ~/.julia/packages/TypeSortedCollections/Z4ytl/src/TypeSortedCollections.jl:196 [inlined]
  [7] foreach
    @ ~/.julia/packages/TypeSortedCollections/Z4ytl/src/TypeSortedCollections.jl:187 [inlined]
  [8] zero_configuration!(state::MechanismState{Float64, Float64, Float64, TypeSortedCollections.TypeSortedCollection{Tuple{Vector{Joint{Float64, QuaternionFloating{Float64}}}, Vector{Joint{Float64, Revolute{Float64}}}}, 2}})
    @ RigidBodyDynamics ~/.julia/packages/RigidBodyDynamics/8B04X/src/mechanism_state.jl:293
  [9] zero!(state::MechanismState{Float64, Float64, Float64, TypeSortedCollections.TypeSortedCollection{Tuple{Vector{Joint{Float64, QuaternionFloating{Float64}}}, Vector{Joint{Float64, Revolute{Float64}}}}, 2}})
    @ RigidBodyDynamics ~/.julia/packages/RigidBodyDynamics/8B04X/src/mechanism_state.jl:318
 [10] (MechanismState{Float64, M, C, JointCollection} where {M, C, JointCollection})(mechanism::Mechanism{Float64})
    @ RigidBodyDynamics ~/.julia/packages/RigidBodyDynamics/8B04X/src/mechanism_state.jl:189
 [11] MechanismState(mechanism::Mechanism{Float64})
    @ RigidBodyDynamics ~/.julia/packages/RigidBodyDynamics/8B04X/src/mechanism_state.jl:198
 [12] top-level scope
    @ In[80]:6
 [13] eval
    @ ./boot.jl:360 [inlined]
 [14] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
    @ Base ./loading.jl:1116

Edit: It seems with v2.0.0, the issue is caused when running set_configuration!(mvis, zeros(11)). Before this, the visualization appears as normal. I’m not sure how to set the starting position of the body at the origin if using zeros isn’t the answer.

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2

That error is probably due to a change in how quaternions are handled in Rotations.jl. I don’t know the history of it, but there’s an open issue and a pending PR. It appears that things got switched from UnitQuaternion to QuatRotation, and field access to w, x, y, z was not retained. The PR should restore this with appropriate getproperty, but RigidBodyDynamics could also avoid field access altogether, since it is brittle.

You can try out the PR and see if it fixes everything for you. I don’t think this has anything to do floating vs fixed base.

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