MOGlabs MWM, Manual, Page: 41 / 96

5.3 Integrator windup
33
Gain (V/unit)
Defines the gain,
G
. The gain is in volts per unit, where the
units are the currently active measurement units if operating using the
MWM
standalone
PID
controller, or via the drop-down unit selector in the
mogwave
PID
menu. The gain and output voltage do not change if the
current
measurement
units are changed. A typical value is 10 V
/
unit if
units are nm. A negative gain can be used to invert the sign of the output
voltage.
kp, ki, kd
Proportional, integral and differential coefficients, each limited to a
range of 0 to 1. Set
k
i
=
k
d
= 0 to use the analogue output as a measure
of laser wavelength, where
k
p
and the gain
G
define the relationship
between the output voltage and the measured wavelength relative to the
setpoint wavelength
λ
0
.
V
max
,
V
min
These set the upper and lower output voltage limits, and also the
integrator windup limits. See section below regarding windup.
V
zero
The offset voltage; that is, the output when the laser is at the correct
wavelength,
λ
=
λ
0
. For
MOGL
abs
DLC
controllers, set
v
zero
= 1
.
25.
5.3 Integrator windup
Integrator windup is caused by saturation of an output actuator. Once the
MWM
output voltage reaches the maximum (or minimum) value defined by
V
max
or V
min
, the integrator will continue to accumulate error (“wind up”),
but the actuator is not able to respond to an increased control voltage. For
example, on
MOGL
abs
DLC
controllers, the piezo control voltage is limited
to 0 to 2.5 V range and output voltages exceeding that range will not
have any additional effect. If the actuator (laser) finally returns to within
range, the “wound up” integrator has to reset from a value far from where
it should be for the existing actuator value. That recovery can introduce
a long time lag in the response, or prevent the servo controller from ever
reaching stable equilibrium. The solution is to stop integrating when the
integrator reaches the saturation limit. For
MOGL abs
DLC
controllers, set
V
max
= 2
.
5,
V
min
= 0.