Normalizing a nonlinear plant in system identification in control
For best results, I suggest you need; 


to convert to linear control e.g. 1/x, log x or at least piecewise linear control, then 
have  tuning weights for MIMO signals all of order unity and not higher order terms.  e.g. position feedback for position control, current or acceleration feedback for likewise control.
use per unit variables for grid control