How to use the 'cable' composite object for modelling crane and payload system #1951
Replies: 5 comments
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Hi, I use this to attach the body to the cable: <body name="payload" pos="0 0.015 0.02">
<geom type="sphere" size=".001 .001" density="100" rgba=".5 .2 .8 1" mass="0.01"/>
<freejoint/>
</body>
<composite type="cable" count="50 1 1" curve="s 0 0" offset="0 0 0" initial="ball" size="0.39 1 1">
<joint kind="main" damping="0.01" armature="0.001"/>
<geom type="capsule" size=".0005 .001 0.3" rgba=".8 .2 .1 1"/>
</composite>
<equality>
<weld body1="payload" body2="B_last" active="true" anchor="0 0 0" torquescale="0"/>
</equality>Your cable is defined with many small bodies, and if you want to refer to a specific one, you can use B_0 for the first segment, B_N for the Nth segment, or B_last for the last segment. I hope this helps |
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If you look the code I attached, I do use the equality constraint with B_last. The issue is that there is a gap between the position of the payload and the end of the cable, and if I lengthen the cable, it does not seem to reduce this gap (essentially the payload is suspended in free space below the end of the cable" |
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If I understand it correctly, it's because constraints are soft in mujoco. You can use https://mujoco.readthedocs.io/en/latest/XMLreference.html#equality |
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I think this is because the connect constraint assumes it is satisfied in the starting position (and keeps any starting offset constant). Move the payload to the end of the cable in your xml, and it will get rid of the offset! (e.g. pos="0.2 0 0.2") Beyond that, there will be some small stretching as well from soft constraints like Alessio described as well, which you can stiffen up if needed. |
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I'm experiencing the same issue, and despite trying to tune the solref parameters, I couldn't fix it. It seems that as the payload increases, the cable stretches, but this deformation is not visually observable. |
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Hi Mujoco community!
I'm very new to Mujoco and am working on a project design control systems for a crane system. We want to use the cable composite to model our crane's cable down to its payload, since the cable has nonneglible mass and we will be operating the crane in a fairly dynamic environment (dynamics of the cable itself will come into effect). I'm trying to get a simple model of the crane, with a payload and cable attached up and running but am struggling with the cable composite plugin.
I don't fully understand how to enforce attachment points between the end of the cable and the crane, and the end of the cable and the payload, so would appreciate some assistance.
My very simple xml is below and the run script attached.
cable_demo.zip
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