How to find error (e(t)) in PI tuning for transfer function????

I am doing a DC voltage controller for FACTS controller. I have a second order system (PI controller connected to the plant transfer function with unity feedback). To use genetic algorithm to optimise the PI gain values i require to find the error to manipulate ITAE or ISE.I dont know how to extract the error part separately from the model using matlab codes.

[zain yousaf]

did u get an answer, plz share with me, cuz im doing same kind of work and confused as bad.