static optimization help

[Brody Hicks]

Hello,

I am trying to run static optimization for that rat hindlimb model. I have results for Inverse Kinematics as well as GRF in the form of a force data file. When I run the static optimization tool, the GUI shows the simulation going all the way through but the messages window keeps displaying warnings such as

"StaticOptimization.record: WARN- The optimizer could not find a solution at time = 2.21

The model appears too weak for static optimization.
Try increasing the strength and/or range of the following force(s):
BFa approaching upper bound of 1
VL approaching upper bound of 1
VI approaching upper bound of 1
VM approaching upper bound of 1
TA approaching upper bound of 1
EDL approaching upper bound of 1


time = 2.21 Performance =6.30914 Constraint violation = 4702.95
SimTK Exception thrown at InteriorPointOptimizer.cpp:261:
Optimizer failed: Ipopt: Maximum iterations exceeded (status -1)
OPTIMIZATION FAILED..."

and

"StaticOptimization.record: WARN- The optimizer could not find a solution at time = 2.24

The model appears unsuitable for static optimization.
Try appending the model with additional force(s) or locking joint(s) to reduce the following acceleration constraint violation(s):
sacrum_pitch: constraint violation = 239.68
sacrum_roll: constraint violation = 578.614
sacrum_yaw: constraint violation = -537.947
sacrum_x: constraint violation = 11.9721
sacrum_y: constraint violation = 5.37233
sacrum_z: constraint violation = -23.2284
hip_flx: constraint violation = -213.194
hip_add: constraint violation = -1114.42
hip_int: constraint violation = -16.6284
knee_flx: constraint violation = -762.289
ankle_flx: constraint violation = 1598.52
ankle_add: constraint violation = -2893.59
ankle_int: constraint violation = -331.296


time = 2.24 Performance =0.188418 Constraint violation = 3686.32
SimTK Exception thrown at InteriorPointOptimizer.cpp:261:
Optimizer failed: Ipopt: Infeasible problem detected (status 2)
OPTIMIZATION FAILED..."
___________________________________________________________________________________

I've tried to create an actuator file to append to the the model but I think it was unsuccessful as appending this file did not fix the problem. I have also looked at the "Working with Static Optimization" tutorial in the user guide. Does anyone have any suggestions or tips on how to get an accurate Static Optimization output?
Thanks,

William

[jimmy d]

The problem with this model is that the masses are so small that it may be very hard for the optimizer to find a solution (depending on the stiffness of the muscle). When I see this, I think of the problems described in this paper (it may or may not be).

Right now, it would be very difficult to figure out which (set of) muscle(s) is causing the issue. One thing you could try is to replace the muscles with PathActuators-- that have no force-length properties or passive properties-- and see if you can get a solution that even works. Then you can progressively replace the PathActuators with muscles, testing after each replacement.