pid

i should feel ashamed in writing about this but i guess one has to confess his or her sins at one time or another.

the transfer function of a pid controller is NOT strictly proper

U(s) kd*s^2 + kp*s + ki
----- = -------------------- (1)
E(s) s

doesn't it create any problems ? all undergraduate control texts say that transfer functions are(should be?) strictly proper. there is something amiss here.

my guess is that since its physically impossible to construct a pure differentiator
( kd*s term ) its therefore impossible to physically construct a PID controller