awesome.  i agree.  

On Apr 1, 2014, at 10:05 AM, jacob-izr <notifications@github.com> wrote:

> In systems/plants/RigidBodyAddedMass.m:
> 
> > +            
> > +            %Find the spatial inertia matrix
> > +            Imass = mcI(obj.mass,obj.com,obj.inertia);
> > +            
> > +            %Augment spatial inertia with added mass
> > +            manip = manip.setBody(body_ind,manip.body(body_ind).setOnlyInertial(Imass+obj.MaBody));
> > +        end
> > +        function force = computeSpatialForce(obj,manip,q,qd)
> > +            % Adds the gravity correction from added mass
> > +            % This force is ONLY necessary because the Featherstone solver
> > +            % imposes gravitational acceleration at the base of the
> > +            % manipulator rather than imposing a gravity for at each node
> > +            % Imposing at the base assumes that F_gravity = (M+Ma)*g,
> > +            % rather than F_gravity = M*g
> > +            % Essentially, gravitional and inertial mass are NOT the 
> > +            % same if added mass forces are assumed (isn't physics fun?)
> Yeah. It's a trade-off in amount of code I have to change. From what I've read (and please correct me), everyone accelerates the base instead of applying a gravity force at each node because it has a lower computational cost. That trick doesn't work in my case with the added-mass.
> 
> I have three options then from what I can tell: 
> 1) Modify the loop to apply gravity at each node (and slow down everyone else's code, since I doubt many people will be using added-mass). Then I also have to create a way to make gravity forces in the RigidBody class, or a new force element.
> 2) Add a bunch of conditionals in the featherstone loop, so that you can choose between using a gravity force or using an accelerating base. 
> 3) Keep as is, and use the anti-gravity correction. I think this has the same computational cost as 1&2, since I'm added a correcting force at every node. Seems best to me, as now I'm not mucking with as much of other people's code.
> Thoughts?
> 
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