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+--- Thread: Help Obi Rope mass ratio (/thread-3692.html)
Obi Rope mass ratio - natko1412 - 13-12-2022
Hi,
I have a 2700kg mass rigidbody with another rigidbody of mass 200 attached via hinge joint to the first one.
The second body is mass 2300 and I have rope attached to the 2300 one with the 200 one on the other side.
I start with very short length rope and I want the rope to extend to 200m or even more. What is the reasonable mass for rope particles for this configuration?
For now, the mass is 30kg for each particle. This seems to be a problem as I add more particles, the second object (2700 +200 mass one) cannot move upwards because of the weight of the rope...
Can you please give me some advice?
Thanks in advance!
RE: Obi Rope mass ratio - josemendez - 13-12-2022
(13-12-2022, 10:02 AM)natko1412 Wrote: Hi,
I have a 2700kg mass rigidbody with another rigidbody of mass 200 attached via hinge joint to the first one.
The second body is mass 2300 and I have rope attached to the 2300 one with the 200 one on the other side.
I start with very short length rope and I want the rope to extend to 200m or even more. What is the reasonable mass for rope particles for this configuration?
Hi!
Same as it is in most iterative engines: mass ratios should be kept below 1:10. So if your rigidbody is 2700kg, rope particles should be around 270 kg. Note you can always use more substeps to improve convergence at the cost of performance, and go for larger mass ratios.
(13-12-2022, 10:02 AM)natko1412 Wrote: For now, the mass is 30kg for each particle. This seems to be a problem as I add more particles, the second object (2700 +200 mass one) cannot move upwards because of the weight of the rope...
That's probably the other way around... the second object cannot move upwards because the rope does not have enough mass to withstand the object's weight. The combined weight of both objects is dragging the rope down.
kind regards,
RE: Obi Rope mass ratio - natko1412 - 13-12-2022
(13-12-2022, 10:09 AM)josemendez Wrote: Hi!Thank you for swift reply!
Same as it is in most iterative engines: mass ratios should be kept below 1:10. So if your rigidbody is 2700kg, rope particles should be around 270 kg. Note you can always use more substeps to improve convergence at the cost of performance, and go for larger mass ratios.
That's probably the other way around... the second object cannot move upwards because the rope does not have enough mass to withstand the object's weight. The combined weight of both objects is dragging the rope down.
kind regards,
I will try it out and get back if I have other questions.
best regards
RE: Obi Rope mass ratio - natko1412 - 14-12-2022
(13-12-2022, 10:09 AM)josemendez Wrote: Hi!Hi,
Same as it is in most iterative engines: mass ratios should be kept below 1:10. So if your rigidbody is 2700kg, rope particles should be around 270 kg. Note you can always use more substeps to improve convergence at the cost of performance, and go for larger mass ratios.
That's probably the other way around... the second object cannot move upwards because the rope does not have enough mass to withstand the object's weight. The combined weight of both objects is dragging the rope down.
kind regards,
I increased the mass and it seems to work better.
However,
i have another problem. Let me first paint the picture:
So, the 2700kg mass object is underwater ROV and it is connected to the 2300 object (tether management system).
Both objects are moveable and subject to forces like water current and waves. The problem is when start moving and rotating the ROV, the rope starts to swing and move more and more until it just launches the objects into space. How would I proceed about solving this?
Thanks in advace and kind regards
RE: Obi Rope mass ratio - josemendez - 14-12-2022
(14-12-2022, 09:22 AM)natko1412 Wrote: Both objects are moveable and subject to forces like water current and waves. The problem is when start moving and rotating the ROV, the rope starts to swing and move more and more until it just launches the objects into space. How would I proceed about solving this?
Thanks in advace and kind regards
Hi,
No idea what could cause this. Position-based dynamics (the simulation paradigm used by Obi) is unconditionally stable, in fact it dissipates energy quite fast so there should be no way for energy to accumulate. Could you share a more detailed description of what you're doing? Maybe you can share a scene that reproduces this issue, you can send it to support(at)virtualmethodstudio.com so I can take a closer look.
RE: Obi Rope mass ratio - natko1412 - 14-12-2022
(14-12-2022, 10:03 AM)josemendez Wrote: Hi,
No idea what could cause this. Position-based dynamics (the simulation paradigm used by Obi) is unconditionally stable, in fact it dissipates energy quite fast so there should be no way for energy to accumulate. Could you share a more detailed description of what you're doing? Maybe you can share a scene that reproduces this issue, you can send it to support(at)virtualmethodstudio.com so I can take a closer look.
It is impossible for me to send you the scene since it uses some external hardware and software to reproduce all this.
I will try to figure it out myself
Can you just explain max bending and max compression parameters to me?
Thanks
RE: Obi Rope mass ratio - josemendez - 14-12-2022
(14-12-2022, 01:46 PM)natko1412 Wrote: Can you just explain max bending and max compression parameters to me?
Thanks
Sure,
maxBending determines the maximum amount of bending that the rope can undergo before it starts resisting bending. The higher the value, the more the rope can bend before it actively resists bending. Now, how strongly it will resist bending once it's past the max bending threshold is determined by the bending compliance parameter.
So if you set both the max bending and bending compliance at zero, the rope will resist any amount of bending as strongly as the solver settings allow for it.
maxCompression is similar, but for distance constraints. It determines the maximum amount of compression (shortening) distance constraints can undergo before they start working against it. It's expressed as a percentage of their rest length, so if you set maxCompression to zero, the rope will resist any compression, if you set it to 1, it will not resist compression at all.
These are explained in the bending constraints manual page and the distance constraints manual page respectively, pasting links here for your convenience:
http://obi.virtualmethodstudio.com/manual/6.3/bendingconstraints.html
http://obi.virtualmethodstudio.com/manual/6.3/distanceconstraints.html
let me know if I can be of further help,
kind regards
RE: Obi Rope mass ratio - josemendez - 14-12-2022
(14-12-2022, 01:46 PM)natko1412 Wrote: It is impossible for me to send you the scene since it uses some external hardware and software to reproduce all this.
I will try to figure it out myself
Just to make sure, check whether you've attached the rope inside a collider that's set to collide against the rope . This results in an impossible to solve configuration in which the collider tries to push the rope outside, but it cannot because the attachment tries to keep the rope inside. So the rope and the collider will keep trying to get out of each other unsuccessfully and twitching/jerking around as a result.
This problem along with its solution are described in the manual, see "attachments inside colliders" in the attachments page:
http://obi.virtualmethodstudio.com/manual/6.3/attachments.html