Translate steady state requirements

Hi Guys,

I hope that this section is the right one. I think that there are also some thread relevant to control system design.
In details, I have a feedback contro system as follow:


r---->O---->Gc----->Gp----->O---->y
| |
<----Gf-----0<----Gs<---
|
ds

(where ds is a sinusoidal disturbance ds=a*sin(w*t) with a<0.1 and w> 40 rad/s
with known Gp

and I have to translate the following stead-state requirements in order to design the controller Gc:
|e_ds_inf|<0.0002

Then, from the above requirement I have already computed the constraint on sensitivity function, Mt_hf=e_ds*Gp/a
What I have still to impose is the constraint relevant to the crossover frequency. Do you have ideas on this subject?

Thanks!