MOGlabs FZW600, Manual

Page: 27 / 52

4.7 PID control
21
Note that operating the FZW in environments with poor ambient
temperature control may experience much more rapid drift, and the
benefit of data averaging is greatly reduced.
4.7 PID control
The FZW includes an SMA output that provides an analog voltage
V
(
t
) that can be used for locking a laser at an arbitrary wavelength
typically by feeding back to the piezo through the laser controller.
The FZW implements a standard PID algorithm that includes pro-
portional, integral and differential control responses with adjustable
gains, as represented by the expression
V
(
t
) =
G K
p
e
(
t
) +
K
i
Z
t
0
e
(
τ
)
dτ
+
K
d
de
(
t
)
dt
+
V
0
,
where
e
(
t
)
≡ f
(
t
)
− f
0
is the “error function”, the difference between
the measured laser frequency and the setpoint frequency (in MHz).
The PID constants are in the range [
−
1
,
1] and
G
represents an
overall gain in MHz/V. These values can be set using the
PID,GAIN
command as shown in the example below, or using the software
interface (see
§
3.3.3).
The range of the analog output is
±
2
.
5 V, with an offset voltage
V
0
that may be useful for interacting with some laser controllers. It
is recommended to ensure that appropriate minimum and maximum
output values be set in accordance with the device being controlled.
# define the desired lock point (in THz)
PID,SET,384.22924169
# set the gain to 100 MHz/V
PID,GAIN,100
# define the PID gains
PID,KP,1
PID,KI,0.1
PID,KD,0
# set the PID to average over 100ms
PID,AVERAGE,100