limit output controller signal to avoid saturation


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Hi everybody, this is my first post here so be patient please.

I have a system on whose output a sinusoidal disturbance is acting and I want to eliminate the disturbance using a unitary feedback of the measurements.

To do this I am using an unstable controller of the form:

R(s) = (a*s+b)/(s^2+w^2)

where s is the Laplace variable and w is the angular frequency. Now the problem is that the output of the controller (u) which is the signal that goes to the actuator diverges and assumes very very large values while the actuator have a finite range of excitation.

To avoid the saturation I was thinking to put a variable gain before the controller as a function of the error and the control variable u but I do not exactly which expression to use. Does anyone have any suggestion? Thanks in advance.