The fiber dynamics is not a function of excitation (thinking about the jacobian); that is, the element of the jacobian relating fiber velocity to excitation is zero. Are you saying OpenSim's API should have a way to tell you such information?bradh wrote: Recently I have tried to understand how/if excitation plays a roll in computeStateVariableDerivatives.
Brad
Does OpenSim report state equations?
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Christopher Dembia
- Posts: 506
- Joined: Fri Oct 12, 2012 4:09 pm
Re: Does OpenSim report state equations?
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Ton van den Bogert
- Posts: 164
- Joined: Thu Apr 27, 2006 11:37 am
Re: Does OpenSim report state equations?
These pieces of information from Ross are very valuable. This is a good place to share these details which usually don't make it into the publications.
I generally use a 1 N/m linear (bidirectional) spring in addition to the ground contact model, and I do the same in the elastic elements of muscle. It seemed like the right thing to do.
I do something similar in the contractile element force-velocity relationship, by adding a small linear damping in parallel. Currently I am using a damping of 1e-5 (that is in units of Fmax per m/s). That removes the horizontal asymptote. In explicit dynamics, this addition makes it harder to solve for the CE shortening velocity. In implicit dynamics, this is a very easy change to make.
For neither of these I know how critical they currently are for successful convergence of a direct collocation problem with IPOPT. These tricks tend to accumulate and never get removed or critically examined again, even when other improvements may make them unnecessary.
I do know that the CE damping is essential for certain implicit ODE solvers. Our fixed-step real time solver would not work without it.
Ton
I generally use a 1 N/m linear (bidirectional) spring in addition to the ground contact model, and I do the same in the elastic elements of muscle. It seemed like the right thing to do.
I do something similar in the contractile element force-velocity relationship, by adding a small linear damping in parallel. Currently I am using a damping of 1e-5 (that is in units of Fmax per m/s). That removes the horizontal asymptote. In explicit dynamics, this addition makes it harder to solve for the CE shortening velocity. In implicit dynamics, this is a very easy change to make.
For neither of these I know how critical they currently are for successful convergence of a direct collocation problem with IPOPT. These tricks tend to accumulate and never get removed or critically examined again, even when other improvements may make them unnecessary.
I do know that the CE damping is essential for certain implicit ODE solvers. Our fixed-step real time solver would not work without it.
Ton
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Brad Humphreys
- Posts: 79
- Joined: Thu Feb 03, 2011 11:32 am
Re: Does OpenSim report state equations?
Chris,
I wanted to put in one more plug for implicit dynamics formulations. Wouldn't having implicit dynamics also provide a means to better diagnose problems during Static Optimization? It would make it possible to diagnose why static optimization fails to solve due to under-actuated joints.
For example using computerStateVariableDerivatives and equilibriateMuscles, I do a "poor man's static optimization" to get a set of balanced/static initial conditions for models. (I use fmincon to solve muscle lengths, all velocities=0, constrained to achieve 0 joint accelerations, while minimizing an activation cost function; you could of course do this with an OpenSim work flow too). But then I am also able to look at which joint accelerations are having trouble getting to 0 across the optimization. One of the limitations to that troubleshooting is that all muscles lengths can effect all joint accelerations; they are coupled. But as Ton points out in his 2011 IUTAM paper, the implicit formulation produces a Jacobian with non-zero terms only in locations where muscles cross joints - it decouples the joints from each other. So you would be able to diagnose under actuated joints.
Maybe this has been said before, but in case it has not, another +1 for implicit dynamics.
Brad
I wanted to put in one more plug for implicit dynamics formulations. Wouldn't having implicit dynamics also provide a means to better diagnose problems during Static Optimization? It would make it possible to diagnose why static optimization fails to solve due to under-actuated joints.
For example using computerStateVariableDerivatives and equilibriateMuscles, I do a "poor man's static optimization" to get a set of balanced/static initial conditions for models. (I use fmincon to solve muscle lengths, all velocities=0, constrained to achieve 0 joint accelerations, while minimizing an activation cost function; you could of course do this with an OpenSim work flow too). But then I am also able to look at which joint accelerations are having trouble getting to 0 across the optimization. One of the limitations to that troubleshooting is that all muscles lengths can effect all joint accelerations; they are coupled. But as Ton points out in his 2011 IUTAM paper, the implicit formulation produces a Jacobian with non-zero terms only in locations where muscles cross joints - it decouples the joints from each other. So you would be able to diagnose under actuated joints.
Maybe this has been said before, but in case it has not, another +1 for implicit dynamics.
Brad
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Christopher Dembia
- Posts: 506
- Joined: Fri Oct 12, 2012 4:09 pm
Re: Does OpenSim report state equations?
Brad:
That's a good point; I hadn't realized this added benefit. We have talked about creating a tool to diagnose issues with CMC, in particular, to give information about if a model is too weak to generate the necessary forces to track a motion. It seems the implicit dynamics might help here. Here's a related issue on GitHub: https://github.com/opensim-org/opensim-core/issues/337.
-Chris
That's a good point; I hadn't realized this added benefit. We have talked about creating a tool to diagnose issues with CMC, in particular, to give information about if a model is too weak to generate the necessary forces to track a motion. It seems the implicit dynamics might help here. Here's a related issue on GitHub: https://github.com/opensim-org/opensim-core/issues/337.
-Chris