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5. PID locking
The
FZW
can output an analogue signal on the
SMA
connector (
V
out
=
−
2
.
5 to +2
.
5 V). That signal can be a fixed value or a
PID
control
signal that attempts to drive the wavelength of a laser towards a set
wavelength.
The
FZW
implements servo feedback control via a standard
PID
(pro-
portional integral differential) function:
V
(
t
) =
Gk
p
e
(
t
) +
Gk
i
Z
t
0
e
(
τ
)
dτ
+
Gk
d
de
dt
+
V
offset
where
V
(
t
) is the feedback response, limited to [
−
2
.
5
. . .
+ 2
.
5] V.
The error is calculated as
e
(
t
) = (
λ − λ
0
) where
λ, λ
0
are the most
recent wavelength and setpoint wavelength.
G
is an overall gain,
and
V
offset
is a constant voltage offset. The
PID
coefficients
k
p
, k
i
, k
d
are floating-point values in the range [0
,
1] which correspond to pro-
portional, integral and differential terms respectively. Typical values
are
G
= 10 V
/
THz,
k
p
= 0,
k
i
= 1 and
k
d
= 0. The offset allows the
FZW
to control unipolar devices such as the
MOGL
abs
DLC
controllers
which have 0 to 2.5 volt control input range.
When optimising a
PID
control loop, it should be kept in mind that
the achievable loop bandwidth is limited by the propagation delay
of
the
entire
signal
processing
chain,
including
the
measurement
time, which depends on exposure time, and any delays in the laser
response.
A locking bandwidth of 10 Hz is feasible.
21
Summary of Contents for FSW4
Page 1: ...Fizeau Wavemeter and Fibre Switcher FZW600 FSM2 FSW4 FSW8 Firmware v0 9 5 mogfzw v1 4 5 ...
Page 36: ...30 Chapter 5 PID locking ...
Page 50: ...44 Appendix A Specifications ...
Page 60: ...54 Appendix C Command language ...
Page 62: ...56 Appendix D Errors and troubleshooting ...
Page 67: ...F Dimensions and PCB 83 120 146 32 9 30V PID TRIG FSW Figure F 1 FZW chassis dimensions 61 ...
Page 68: ...62 Appendix F Dimensions and PCB 146 33 37 120 18 Figure F 2 FSW fibre switch dimensions ...
Page 71: ......