THORLABS ITC4000 Series, Руководство по эксплуатации

Страница: 47 / 105

© 2018 Thorlabs
3 Operating the ITC4000 Series
45
3.13 PID Auto Tune
A very simple method to adapt the PID parameters to the characteristics of your thermal setup
is the use of the implemented PID Auto-Tune function. It is easy to use and gives good results
after a relatively short time (depending on the time response of your thermal setup).
The implemented PID Auto-Tune algorithm is using the Ziegler-Nichols method. That means
the I and D shares are set to zero and the algorithm is looking for the “critical gain” KC at which
the output of the control loop starts to oscillate. The algorithm is optimized for finding good PID
parameters as fast as possible.
The new parameters are displayed and you can set them active or discard them.
This gives a good transient oscillation to a new temperature set point, but it is more optimized
to avoid influences of disturbances from outside, such as changes in ambient temperature or
the air, changes in heat sink temperature, laser diode power (lost heat) and so on.
Strictly speaking, a set of PID parameters is valid only for the temperature set point active dur-
ing PID Auto-Tuning. However, a new temperature set point not too far away will also lead to an
adequate transient response. Therefore you will be able to control the temperature of your sys-
tem effectively within a certain temperature range of many degrees (depending on your thermal
setup).
If the system behavior for another temperature set point is not good enough, you can restart the
PID Auto-Tune function at the new temperature set point. In this case, a quick 5 step Auto-
Tune procedure is started, if the following points are true:
- You already finished a PID Auto-Tuning successfully before
- You did not change the PID parameters by hand in the meantime
- You did not unplug the cable between the ITC4000 and your thermal setup (e.g. changing
the mount).
To accelerate the process, the new quick PID Auto-Tune process only readjusts the PID para-
meters calculated before and is performed over a limited KP-range only.
Normally you will get better PID parameters for the new temperature set point after a short
time. If the KP tuning has to be greater than the allowed range the re-adjusting fails and the
complete PID Auto-tuning over the whole KP range is started automatically, resulting in a more
precisely tuned PID-Controller.
The PID Auto-Tune function may not finish successfully. In that case, the KC of your system
may be outside the possible tuning range (smaller than 0.1 or greater than 100). Please try to
manually tune the PID-Controller.
If the PID Auto-Tune function finishes very quickly but gives poor results, your system might be
too noisy, and the noise (around 50 or 60 Hz) might be interpreted as oscillations of your sys-
tem. Eliminate the noise and retry the PID Auto-Tune function.