How to route a cable (over a pulley) #1997
Replies: 13 comments 1 reply
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Hello, could you answer a couple questions? Does the fact that the cable can bend and twist at any point important, or would it be fine if it was piece-wise straight and only bent at the pulley? Is the payload supposed to be able to be pulled on top of the beam, or only lifted up in the air? Depending on the answers it may be more appropriate to simulate this system with tendons instead of cables. |
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Well, it would act like a fishing pole and string, so it would only bend as it goes over the edge of the rod, but it will then swing and spin around with the payload. The payload will never be on top of the beam, just to the edge. Is it true that tendons can stretch? In this case, like with a regular line/rope, it should maintain its length and not stretch or shrink. Thanks! |
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Tendons can be stretchable, but you can just as well define rigid, or rope-like tendons by defining a max length they can be. You can define geoms that a tendon must wrap around if its path would cross it as well. |
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One other question, do tendons work with MuJoCo XLA (mjx)? That's the package we need to use. Thanks for helping! And I'm guessing cables are not designed (yet?) to be routeable? |
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Tendons are coming soon to MJX. If I understand correctly, plugins such as the elasticity are further down the line for support to be added for MJX (could you confirm @yuvaltassa)? |
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Do you have a rough ETA for the tendons? Weeks, months, ...? Thanks! |
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I'm not involved with MJX development, however based on the pace of releases and the priority I heard it referenced to, my guesstimate is around a month. (Maybe ask for a more accurate estimate in the discord for a quicker reply?) As features are released when close to 1:1 behaviour is expected, I can recommend starting with a CPU MuJoCo now, and then transitioning to MJX once it is updated. |
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Hi, I wanted to follow up and ask how could one go about doing this via a cable? I dont want tendons as they can change their lenghts and would really prefer a cable instead. However it's unclear to me how to route the cable over a pulley (as the question says)? is there an easy way to do this? |
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Tendons are coming to MJX! I would recommend using tendons with length constraints, unless performance doesn't matter to you at all. Cables are expensive! |
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Thanks for the reply. I'm trying to simulate questions like these from physics textbooks (also i dont need mjx), where it's requirement that the length of the string doesn't change, however if i understand correctly it's not possible to entirely fix the length of a tendon, as when i keep the range limits to be the same value, it doesn't work. I don't have any speed issues right now, so i was thinking of going about with cable. However not sure if there is an easy way to route cable over a pulley. Any thoughts? |
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You can make the tendon length limit as stiff as you like... The model mentioned in this video is an example |
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It's mainly like this right?
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Hi, I'm currently using cables in a similar manner in a crane model, but it is very slow and unstable. I wanted to see if tendons are a good alternative. I have some questions about tendons. Is there a way to enable collisions with a tendon? Currently the tendon is passing through objects that are in its way instead of colliding with them. If there is a way to enable collisions, is it also possible to make a tendon wrap around other object that it collides with, similar to a cable? |
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Could you share how you achieved this with a cable? I'm trying to do a similar thing to @danachee , but with a pulley on the beam rather than a slider joint |
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Hi,
I'm a engineer that is very new to this space and I'm trying to use MuJoCo for a simulation project.
I'm looking for some help with connecting both ends of a cable. One to the top of a beam (with a slider joint) and one to its payload.
Here is a model which explains my question:
Anyway, google hasn't turned up anyone who has connected a cable this way. Any hints??
Thanks!
Here is a screenshot illustrating my question:
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