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TECPak
User’s Manual
value until the set point is achieved without oscillation. In many systems, the D
term is not needed and may be left at zero.
For additional information on PID loop tuning, consult online resources such as
Wikipedia (http://en.wikipedia.org/wiki/PID_controller) or search for terms such
as “Ziegler-Nichols method”, or “PID Loop”.
Using the AutoTune Function
The
TECPak
is capable of automatically determining PID parameters for most
applications. Using a form of the Ziegler-Nichols method, the
TECPak
will step
through a process to determine the thermal response of the mount, which can
then be used in a mathematical model to calculate the PID parameters.
The PID parameters generated by AutoTune are not necessarily the
ideal
PID
parameters, and small improvements may be possible by further refining the
results manually.
Before starting the AutoTune function, it is best to begin from ambient
conditions, either with the TEC off and the mount stabilized at ambient, or the
TEC on and the set point around 25
°
C. While this is not required, it can produce
better results. AutoTune only functions in temperature mode. PID parameters
must be manually determined for R mode.
Make sure the current and temperature limits are set prior to starting AutoTune.
AutoTune will intentionally cause your mount to oscillate, so the temperature
limit should be at least 5 to 10 degrees away from the test point to avoid tripping
a limit during the process.
The TEC:AUTOTUNE command starts the process, and requires one parameter,
which is the AutoTune temperature test point. The command will immediately
put the instrument into AutoTune mode. The TEC:AUTOTUNE? query can be
used to monitor the process of AutoTune. A response of 1 indicates AutoTune is
in progress. Once complete, the instrument will respond with a 2 if AutoTune
failed or a 3 if AutoTune succeeded. A response of 0 is returned if the instrument
has never started an AutoTune process.
The AutoTune process can take several minutes, and even longer on larger
mounts with higher thermal mass.
Once the AutoTune process is complete, the output will remain on. If the
AutoTune process fails, an E-436 AutoTune Failed error will be generated and
the output turned off. Any of the following can cause the AutoTune to fail:
Noisy temperature measurements, which make it difficult to accurately
measure oscillations