5 Replies Latest reply on Aug 4, 2006 12:33 PM by

# Cosmos Motion and ropes?

Does anyone know of any way to simulate ropes in Cosmos Motion? We're currently using VisualNastran Motion for this, as it allows us to build expressions based on length (less than rope length- do nothing, greater than rope length- act like a spring). However Cosmos Motion help describes that expressions can only be built with time as a variable, adding lengths will cause an error. Plus, expressions are not available when defining springs.

We'd like to stop using VN Motion, as it's completely unsupported and no longer sold, but we need to simulate these ropes in our kinematic system. Thanks!
• ###### Cosmos Motion and ropes?
Thanks for the thoughts. Hari from COSMOS had also sent an email with an excellent response that I've posted below. I've also attached the example file in case it can help someone else- I found it quite educational. I've noticed that you have to be careful with the damping values and "activation length" as it can get unstable quickly:

Response
--------------------------------------------------
We currently do not have the option to model ropes directly. You can do it using the action reaction force (internally used by spring) with an expression using the result dependent functions.

Here is a quick sample one of people put together to model a cable using an action reaction force. It utilizes the underlying algorithm for the impact function, but I converted it to be based on a max separation distance instead of a minimum distance.

IF((DM(38,37)-200):0,0,-10000*((DM(38,37)-200)**1.5+STEP(DM(38,37),200,5
0,200.1,0)*VR(38,37)))

Where:

200 is the length of the cable
DM(38,37) is for the attachment points for the AR force to represent the cable. Hence DM is the distance between the cable ends
VR(38,38) is the relative velocity of the cable ends
10000 is the stiffness of the cable
50 is the max damping value for the cable.
1.5 is the exponent for the spring stiffness variation (f=k.x^n)
200.1 is the max penetration distance at which damping occurs (goes from Cmax at cable length to zero at a length of cable +0.1)

Watch the motion and see if this meets your needs.

Hope this helps.

Regards,
Hari
• ###### Cosmos Motion and ropes?
I think something can be made using MARKERS and expressions using marker values.
They are not so simple to implement (and not so parametric in the classic SWX meaning).
I think they are an old ADAMS feature, but they work.
I used them once to simulate a ribbon accumulation in a system of moving pulleys.
Try to find a Motion expert that knows how to use them.

• ###### Cosmos Motion and ropes?
This is a HUUUGE tool!
• ###### Cosmos Motion and ropes?
Try waybackmachine.org

http://web.archive.org/web/*/ http://www.eif.ch

Peter Gillespie
SolidWorks 2004 SP1 / COSMOS 2004
• ###### Cosmos Motion and ropes?
Thank you Kevin for sharing!

For those interested, I have checked, and expressions such as "DM(38,37)" are functions referencing markers.

I have too poor knowledge to advance solutions on my own on this subject.
I only want to outline the markers enable the user to "read" (real time) at each frame any kinematic/dynamic property of any component, and to calculate selective reactions or something similar, conditioned by the reaching of a given speed, or distance.

I remember MSC had on their site a good training course on the old DDMotion, dealing with similar subjects, with examples.
It had been done by someone at http://www.eif.ch.
I checked today, but it vanished.

Anyone can find a copy of this?