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Description
This test demonstrates a problem in our description of orientations
Two opposing forces (green arrows) are both acting on a body. Another identical body is connected to the first body.
- If the second body is removed, the tests pass with
quat = trueorneg_w = true - If the second body is present, it passes with
quat = false, neg_w = false - It passes with
quat = true, if the transpose onΩis removed innonunit_quaternion_equations. This further indicates that that this transpose is equivalent to the neg_w - When the test fails, the result corresponds exactly to one of the forces being set to zero
Questions
- What's the relation between the negative sign in the angular velocity and the force?
- Why is negative sign needed sometimes but not other times? I.e., why does the second body
b0make a difference here?
@mtkmodel TestWorldForce begin
@components begin
world = W()
forcea = WorldForce(resolve_frame=:frame_b)
forceb = WorldForce(resolve_frame=:frame_b)
b0 = Body(m=1, state_priority=0)
body = Body(m=1, state=true, isroot=true, quat=true, neg_w=false)
end
@parameters begin
f[1:3]
end
begin
f = collect(f)
end
@equations begin
connect(forcea.frame_b, body.frame_a, b0.frame_a)
connect(forceb.frame_b, body.frame_a)
forcea.force.u ~ f
forceb.force.u ~ -f
end
end
@named testwf = TestWorldForce()
testwf = complete(testwf)
ssys = structural_simplify(IRSystem(testwf))
prob = ODEProblem(ssys, [testwf.world.g => 0; collect(testwf.f) .=> [1,0,0]], (0, 1))
sol = solve(prob, Tsit5())
# plot(sol)
@test sol(1, idxs=testwf.body.r_0) ≈ [0, 0.0, 0.0] atol=1e-3Metadata
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