Support variable mass properties

[chrisdembia]

@nickbianco and I are hoping to support optimizing a body's mass in Moco. Right now, changing a body's mass require's invoking OpenSim::Model::initSystem() which is too expensive to do during direct collocation.

I had hoped that Simbody would allow editing the mass properties in the state (at stage Instance). I found the following in the source code:

simbody/Simbody/src/MobilizedBodyImpl.h

Lines 257 to 261 in c6f465b

	const MassProperties& getBodyMassProperties(const State& s) const {
	// TODO: these should come from the state if the body has variable mass props
	const SBInstanceVars& iv = getMyMatterSubsystemRep().getInstanceVars(s);
	return getMyRigidBodyNode().getMassProperties_OB_B();
	}

This indicates that Simbody does not yet support updating mass properties after realizeTopology().

@sherm1, do you have a sense of how difficult it would be implement a MobilizedBody::setMassProperties(SimTK::State&)?

[chrisdembia]

@nickbianco

[sherm1]

Not difficult, I think -- mostly bookkeeping to take care of. We have examples of this already. The first one that came to my mind is Thermostat which has the standard patttern: for example, setDefaultBathTemperature(temp) and setBathTemperature(State&, temp).

Then the code in its realizeTopology() method allocates Model and Instance stage variables that hold the runtime values for those parameters. And those get initialized from the default values.

For mass properties of MobilizedBodies I think I already assumed we would want them to vary so there may be a generic instance stage variable belonging to SimbodyMatterSubsystem that holds them all and could be used to implement the variable ones (been a while since I looked at that).

[sherm1]

Yeah, this state variable is what I was thinking of. That already holds runtime values for all the mass properties and other parameters. Those get initialized to the default values -- I think all you would need to do is add APIs for modifying these (and make sure they don't accidentally get overridden afterwards from the defaults).

[chrisdembia]

Thanks @sherm1 that's very helpful. Maybe we'll give it a shot.