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How do I simulate a cable or wire in Python?

Use scene.add_rod in the Alloy physics engine, installed with pip install alloy-engine. A rod is a chain of segments that bends, twists and stretches like the material you name, and you read its centre line back as numpy points. This page clamps one end of a cable and lets the rest sag under its own weight.

1. Clamp a cable and let it sag

import alloy

scene = alloy.Scene(dt=0.01)
scene.add_rod("cable", length=0.4, radius=0.003, pos=(0, 0, 0.5), direction=(1, 0, 0), segments=16,
              clamp="start", route="chain")                    # held horizontal at z = 0.5, then released to sag
sim = alloy.build(scene)
print(sim.routes()["cable"]["route"])

chain

clamp="start" welds the first segment where it is built. route="chain" keeps a clamped rod on the fast chain of rigid segments instead of the slower elastic-rod solver.

2. Run it and read the centre line

sim.run(2.0)
points = sim.state(["cable"], world=0)["cable.points"]
print(points.shape, points[-1][[0, 2]].round(2))               # the free end's x and z
print(sim.check()["ok"])

(17, 3) [0.35 0.31]
True

"cable.points" has one row per segment end, 17 for 16 segments. The free end hangs at x = 0.35 and z = 0.31 m, below the clamp at 0.5. A rod that yields, such as a copper_c110 wire, keeps a bend; it steps on the slower solver (Rods guide).

One measured number: on an AMD EPYC 7B13 CPU, one core, a 0.4 m cable on the ground costs 0.9 s of wall time per simulated second on the chain route and 2.5 s on the elastic-rod solver.

Keep in mind

  • A clamped cable steps on the slower solver unless you pass route="chain". Beside a robot on the CPU that solver costs 83-95 s per simulated second on an AMD EPYC 7B13 (details).
  • A chain held by a robot on the CPU slowly loses its twist, 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 uses the slower solver.
  • Full list: Keep in mind.

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