Andy Ruina seemed to think that we should consider our inverse dynamics methods more carefully. He said Art Kuo is very critical of most people's inverse dynamics methods. Since we work up from the foot there is inevitably more error in the joint torque predictions that are higher up the chain, e.g. in our case the hip joint torque is more erroneous than the ankle joint torque. He was suggesting that since we rely heavily on the inverse dynamics calculations that using the standard methods may not be the best idea. He discussed the weighting choices you inevitably make and also mentioned that you can enforce periodicity in the inverse dynamics.

I mentioned that we did not make the assumption that the average gait cycle is the nominal cycle that the feedback controller is trying to push the state towards. We have always been assuming that the nominal (or limit cycle) state and torque trajectories were an unknown. Andy essentially convinced me that we should consider the average gait cycle trajectories as the nominal (commanded) trajectories. I don't think this would change the controller we find, but it would allow the torque contributions from the feedback and the nominal torques to be separated and quantified.

I can't really remember what Ton's reasoning was behind not assuming that the average gait cycle was the reference trajectory. I remember being convinced of it and believe(d) that there is no way to know the reference trajectory in any given gait cycle. I tried to argue to Andy that each gait cycle could have a slightly different reference trajectory but he didn't buy that. He said the definition of periodic walking implies a common reference trajectory for each gait cycle. So I'm not sure what I think now.