## Measuring Tension in a rope

Vytas
Posts: 3
Joined: Wed Nov 16, 2011 4:09 am

### Measuring Tension in a rope

Hey All!
I'm working on simulating a robotic tensegrity structure in bullet, which means i'm controlling the lengths and forces in the ropes to change the shape of the structure. See image below.

I can now drive the ropes to various lengths/positions and am now trying to measure the forces being experienced by the ropes (to implement force controlled motion). I don't really want contact forces, rather I want the tension in the rope itself.

I tried looking at the force accumulators in the nodes of the rope (node.m_f) but they are always 0, so that was not helpful.
I thought to look at the impulse matrix in the anchors, but to be honest, I have no idea what to do with that data -- not really sure what it is representing.

The best idea I have so far is to look at the difference between the restlength of the rope and the actual length of the rope (i.e. calculated from node distances), use that as a measure of stretch and somehow combine that with the elasticity to calculate the tension in the rope. But, this approach seems odd, since it feels like I'm recreating information that must already exist, since the force in the rope must be calculated somewhere to generate that stretch in the first place.

does anyone have any good ideas about how to get the current tension in a rope?

thanks
vytas

hmm.. the attached image does not appear to show up. you can find it here:
http://www.sunspiral.org/vytas/random/t ... works.tiff
Tensegrity structure
tensegrity_simulation_works.tiff (86.14 KiB) Viewed 3396 times

hsvdg
Posts: 1
Joined: Wed Jan 29, 2014 4:04 pm

### Re: Measuring Tension in a rope

How do you determine the length of the rope? The obvious answer, SetRestLength, does not seem to do anything ....

If you have an idea, I would be grateful.

Thanks.

Basroil
Posts: 463
Joined: Fri Nov 30, 2012 4:50 am

### Re: Measuring Tension in a rope

The question her is why bother with bullet for the ropes? You'll just end up estimating hooke's law with lower accuracy. Only reasons for the rope not having the same tension at the contact points as within the rope at some point are :

1) High inertia/gravity related force compared to tension forces
2) Contact with something else
3) Rope magic

The first two are usually ignored for tensegrity robots, so your internal tension should be equal to the contact point forces (and those forces should be equal). Are either of the first two not being met? If it's only the first one, it might just be easier to calculate the dynamics yourself (perhaps some derivative of the wave equation or FEM method) even if it might not be as self-contained.