simtk.org

Hi,

OpenSim looks like a great tool though I am a little confused about the difference between using the SimBody engine and SDFast. Would it be possible for someone to summarise the benefits of purchasing SDFast?

I'm really interested in extracting the Dynamic Equations of motion symbolically from a BioMechanical model, I realise you need SDFast for this (though it appears to generate an interative solver in C as opposed to the 'pure' symbolic equation). I can't seem to find any documentation on OpenSim that details how easy it is to do this equation extraction (I've already been through the SDFast user guide). Has anyone else done this?

Many Thanks,

Hi Adam:

SDFast is a great and reliable multibody dynamics solver and its speed is quite difficult to match. But, it is a commercial product and not open source and therefore not extensible. Simbody is open source and freely available. The biggest advantage of Simbody is the ability to build up a model programmatically, which enables OpenSim to create models on the fly without generating separate code and requiring a compile step.

The upcoming release of Simbody should do everything SD/Fast can and uses an O(n) recursive formulation (which was an extra cost option for Sd/Fast). Simbody does not generate symbolic equations like Autolev, but there are methods for getting all the elements of the EOMs, like the mass matrix, generalized forces, Coriolis and centripetal contributions, etc ...

There is more documentation on Simbody on SimTK.org. As for requirements for symbolic equations - there are a few applications where it could be advantageous, but we have not come up with an example where they are necessary.
It would be interesting to know what application you have in mind.

Cheers,

Thank you Ajay. I have investigated SD-Fast a bit more and discovered that the equations etc are built into a series of C-functions generated in a file when the model is compiled.

I'd be interested to see what these functions look like for Simbody, do you know if a similar file (of function) is generated when simbody is used instead of sdfast? I'm interested in implementing control algorithms around the dynamics of the human musculoskeletal system which is why I'm curious about viewing the generated equations of motion.