685 Series
ComboPak
User’s Manua
l · Page 33
The controller can calculate PID values using the AutoTune function, discussed
in detail in the next section.
To manually adjust the PID, start by changing the I and D values to zero, and
adjust the P value so that it reaches the set point as quickly as possible without
overshooting the set point an unacceptable amount. Gradually increase the I
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
ComboPak
is capable of automatically determining PID parameters for
most applications. Using a form of the Ziegler-Nichols method, the
ComboPak
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 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 ambient temperature (23
°
C). While this starting
point 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, turning the output on, if necessary. 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 never started the AutoTune process.