2.2 Operating the TEC controller
2 – 9
(see page 4 – 39 ; and section 3.3.3.10, page 3 – 12 ). The
reversed polarity setting will be remembered by the LDC500
through power cycling and can be saved and recalled as part
of user parameters (see
SPAR
and
GPAR
, page 4 – 48 ). The
polarity setting will return to normal, however, in response to
a hardware reset or the
*RST
command.
2.2.4
Auto-tuning
If you already have determined values for the
P
,
I
, and
D
loop
gains (either from separate measurement or earlier operation with
the LDC500), you can enter them directly from the TEC [Config]
menu tree. In general, however, autotuning is recommended.
•
Press the [Config] button to select the TEC configuration menu
tree. Use the [ ] and [ ] buttons to navigate to the “Autotune”
item:
%%
&'#
.
•
Pressing [Enter] now will initiate an autotuning cycle. The
TEC controller will temporarily turn on, and a thermal step
response is measured to estimate the optimum loop gains and
time constants.
•
The default current step used is 10 % of the maximum TEC
module current (set previously). If a di
ff
erent current step is
desired for autotuning, press [ ] to “Autotune tuneStep:”, and
change the setting to the desired step size (in amperes). Press
[ ] to navigate back to the Autotune “start” item.
•
If you need to cancel autotuning, you can press the [Clear
/
Cancel]
button; otherwise wait until the display indicates that autotun-
ing has completed.
For best initial results, the temperature should be stable at room
temperature, and the TEC module should have been o
ff
for a few
minutes (to allow the heat-sink side to also reach equilibrium). You
can check the results by pressing [ ] to display the “PID vals” for
P
,
I
, and
D
. Note that all loop gain settings are given in physical
units (A
/
◦
C, 1
/
s
, or
s
). See section 4.5.12 for more details on the
temperature control algorithm.
The time needed for autotuning is dependent on the thermal mass
of the stage—the larger the thermal mass, the longer the autotuning
process. If the thermal time constant is longer than around 60 sec-
onds, default autotuning may fail because of a weak or noise signal.
In this situation, a possible solution is to increase the tuning step size
to a larger value (see above). The allowed range is from 0 to 25 %
LDC500 Series Laser Diode Controllers
