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Cables

Add a cable with scene.add_rod: it bends, twists and stretches like the material you name, and rests on objects.

1. Lay a cable over a table edge

import alloy

scene = alloy.Scene(dt=0.01)
scene.add_ground()
scene.add_box("table", size=(0.3, 0.3, 0.2), pos=(0, 0, 0.1), static=True)       # top at z = 0.2
scene.add_rod("cable", length=0.4, radius=0.0015, pos=(-0.15, 0, 0.21), direction=(1, 0, 0), segments=16)
sim = alloy.build(scene)
print(sim.routes()["cable"]["route"])

chain

The cable starts flat, 1 cm above the table, and 10 cm of it sticks out past the table edge at x = 0.15. Without a clamp it is a fast chain of rigid segments joined by springs, which is what sim.routes() reports.

2. Run it and read the centre line

sim.run(1.0)
points = sim.state(["cable"], world=0)["cable.points"]
print(points.shape, points[-1][[0, 2]].round(2))       # the free end's x and z

(17, 3) [0.24 0.15]

"cable.points" is the centre line from start to end, one row per segment end. The cable settled onto the table and its overhang bent down over the edge: the free end is at x = 0.24, z = 0.15 m, below the table top at 0.2.

3. Save a picture

sim.render(width=640, height=480, path="cables.png")
print(sim.check()["ok"])

True

Keep in mind

  • A clamped cable (clamp="start") steps on a slower solver. Beside a robot on the CPU it costs 83-95 s per simulated second on an AMD EPYC 7B13, and route="chain" keeps it on the fast chain, about 60x faster (cost).
  • A chain held by a robot on the CPU slowly loses its twist while held, about 0.14 % per second (details). Clamp a cable that must hold a twist for long.
  • A cable that needs more than 40 segments (long or soft) also steps on the slower solver, wherever it lies.

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