Docs examples breaking?

[cmhyett]

Perusing the docs, and found

Which referenced the plot:

These are obviously not the same, but ought to be?

Two code snippets:

With joint

```@example pendulum
@named body_0 = Body(; m = 1, isroot = false, r_cm = [0, 0, 0])
@named damper = Translational.Damper(d=1)
@named spring = Translational.Spring(c=10)
@named joint = Prismatic(n = [0, 1, 0], axisflange = true)

connections = [connect(world.frame_b, joint.frame_a)
               connect(damper.flange_b, spring.flange_b, joint.axis)
               connect(joint.support, damper.flange_a, spring.flange_a)
               connect(body_0.frame_a, joint.frame_b)]

@named model = System(connections, t, systems = [world, joint, body_0, damper, spring])
model = complete(model)
ssys = structural_simplify(multibody(model))

prob = ODEProblem(ssys, [], (0, 10))

sol = solve(prob, Rodas4())
Plots.plot(sol, idxs = joint.s, title="Mass-spring-damper system")

Without joint

```@example pendulum
@named root_body = Body(; m = 1, isroot = true, r_cm = [0, 1, 0], phi0 = [0, 1, 0])
@named multibody_spring = Multibody.Spring(c=10)

connections = [connect(world.frame_b, multibody_spring.frame_a)
                connect(root_body.frame_a, multibody_spring.frame_b)]

@named model = System(connections, t, systems = [world, multibody_spring, root_body])
model = complete(model)
ssys = structural_simplify(multibody(model))

defs = Dict(collect(root_body.r_0) .=> [0, 1e-3, 0]) # The spring has a singularity at zero length, so we start some distance away

prob = ODEProblem(ssys, defs, (0, 10))

sol = solve(prob, Rodas4())
plot(sol, idxs = multibody_spring.r_rel_0[2], title="Mass-spring system without joint")

Here, we used a Multibody.Spring instead of connecting a Translational.Spring to a joint. The Translational.Spring, alongside other components from ModelingToolkitStandardLibrary.Mechanical, is a 1-dimensional object, whereas multibody components are 3-dimensional objects.

Internally, the Multibody.Spring contains a Translational.Spring, attached between two flanges, so we could actually add a damper to the system as well:

push!(connections, connect(multibody_spring.spring2d.flange_a, damper.flange_a))
push!(connections, connect(multibody_spring.spring2d.flange_b, damper.flange_b))

@named model = System(connections, t, systems = [world, multibody_spring, root_body, damper])
model = complete(model)
ssys = structural_simplify(multibody(model))
prob = ODEProblem(ssys, defs, (0, 10))

sol = solve(prob, Rodas4(), u0 = prob.u0 .+ 1e-5 .* randn.())
plot(sol, idxs = multibody_spring.r_rel_0[2], title="Mass-spring-damper without joint")

[baggepinnen]

This package hasn't had a working compiler for years now, so I'm not surprised if some things are broken 😅 Give it a few more months and I hope it'll be fully up and running again

[cmhyett]

Ah, my bad. No reason to track if package isn't working.