3.3 Fast servo loop
19
G
ain (dB)
20
0
40
60
–20
10
2
10
1
10
3
10
4
10
5
10
6
10
7
Fourier frequency [Hz]
Inte
grato
r
Double in
teg
rat
or
Proportional
High fr
eq
. cut
off
Diff
eren
tia
tor
Inte
grato
r
10
8
FAST LF GAIN (limit)
FAST
GAIN
DIFF GAIN (limit)
FAST INT
FAST DIFF
SL
OW INT
Filt
er
Figure 3.6:
Conceptual Bode plot showing action of the fast (red) and
slow (blue) controllers. The slow controller is either a single or double
integrator with adjustable corner frequency. The fast controller is a
PID
compensator with adjustable corner frequencies and gain limits at the low
and high frequencies. Optionally the differentiator can be disabled and
replaced with a low-pass filter.
High frequencies (1 MHz) typically require the differentiator loop
to dominate for improved locking. The differentiator provides phase-
lead compensation for the finite response time of the system and has
gain that increases at 20 dB per decade. The corner frequency of
the differential loop can be adjusted via the
FAST DIFF/FILTER
knob
to control the frequency at which differential feedback dominates.
If the
FAST DIFF/FILTER
is set to
OFF
, then the differential loop is
disabled and the feedback remains proportional at higher frequen-
cies. To prevent oscillation and limit the influence of high-frequency
noise when the differential feedback loop is engaged, there is an
adjustable gain limit,
DIFF GAIN
, that restricts the differentiator at
high frequencies.
A differentiator is often not required, and the compensator may in-
stead benefit from low-pass filtering of the fast servo response to
further reduce the influence of noise. Rotate the
FAST DIFF/FILTER
