The first plot suggests that the controller essentially behaves like a PI controller acting on
REF-Y (the difference between the target and actual values of the controlled variables).
The second plot suggests that the transfer from measured disturbance to manipulated
variables could be replaced by a gain in series with a lag network. Altogether this
suggests the following simplified control structure consisting of a MIMO PI controller
with a first-order disturbance feedforward.13 Control System Tuning Examples