hi i am having a bit of a problem understanding the steady state error in proportional controllers.. The output from a P type Controller is proportional (u=kp(error))to the error so why does this error arise?? The output from the controller tries to achieve the S.P. Once the O/p = SP the error equals 0 the controller output should be 0 and when the there is no output from the controller again an error should occur..Our controller on sensing this error will again produce a proportional response till the error is zero. Once the error is zero again the cycle should continue.. Now if i am right here, what i don't get is why is the SS error a constant shouldn't our controller o/p keep fluctuating..achieving the set point and then again dropping..