MOGlabs MWM, Manual, Page: 39 / 96

5. Signal out and PID
The
MWM
can output an analogue signal on the
SMA
connector (
V
out
=
−
5 to +5 V). That signal can be a fixed value, a signal proportional to
measured laser wavelength, or a
PID
control signal that attempts to drive
the wavelength of a laser towards a set wavelength.
The
MWM
implements servo feedback control via a standard
PID
(propor-
tional 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 proportional, integral
and differential terms respectively. Typical values are
k
p
= 0
.
03
−
0
.
8,
k
i
= 0
.
01
−
1
.
00 and
k
d
= 0. The offset allows the
MWM
to control
unipolar devices (e.g. 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 en-
tire signal processing chain, including the measurement time, which de-
pends on exposure time, and any delays in the laser response. A locking
bandwidth up to 100 Hz is feasible.
31