How do we treat of three stage frequency compensation?

In primary control system, we were told that the ideal compersation
curve for a open loop system is a three stage frequency compensation
illustrated as follow:
| ;
| ; (40db/dc
| ;
| \
| \ (20db/dc
|---------------\-------------
| \
| \
| ;
| ;(40db/dc
;

at low frequency section,slope is 40db, middle frequency is 20db and
cross x-axis, at high frequency will be 40db else.
as we all know, that at the low frequency section,slope is 40db/dc
and phrase magin is zero and its amplication is greater than 1
is it oscillation at the section?

 

[Winfield Hill]

wcn@public.zz.ha.cn wrote...

In primary control system, we were told that the ideal compersation
curve for a open loop system is a three stage frequency compensation
illustrated as follow:
| ;
| ; (40db/dc
| ;
| \
| \ (20db/dc
|---------------\-------------
| \
| \
| ;
| ;(40db/dc
;

at low frequency section,slope is 40db, middle frequency is 20db
and cross x-axis, at high frequency will be 40db else.
as we all know, that at the low frequency section,slope is 40db/dc
and phrase magin is zero and its amplication is greater than 1
is it oscillation at the section?

No.


--
Thanks,
- Win

 

[Jon]

The slope at the 0dB gain point is 20 db/decade, so there will be no
oscillation, even though the slope is -40 db/decade at lower
frequencies. This is the reason that a 2 or 3 slope compensation is
used; to ensure a positive phase margin at the 0 db gain frequency.
Regards,
Jon