THORLABS ITC8 Series, Руководство по эксплуатации

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Optimization of temperature control 25
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© 2010 Thorlabs
To 2. If the internal power dissipation in the laser changes (e.g. the laser current is
changed) also the temperature gradient between laser and sensor will change.
This results in a measurement error depending on the mechanical set-up
laser/sensor. Slow changes of the ambient temperature, however, will be
compensated well by the control loop since the retroaction of the air on the
laser diode can be neglected.
Possible solution: optimized thermal design
To 3. The transient response by setting a new temperature is limited since the heat
transport in the copper bloc is relatively slow. Furthermore, the temperature
slope in the copper must stabilize anew. The sensor must settle to the laser
temperature - it also has a non negligible heat capacity.
Possible optimization: careful adjustment of PID parameters
1.7.2 PID adjustment
Temperature control loops are comparatively slow with control oscillations in the
Hertz range.
The PID adjustment will optimize the dynamic behavior. With the TED8xxx the three
parameters P, I and D can be set independently from 0.1% to 100%.
Example of a PID adjustment
(Pre-conditions: All limit values have been set correctly, all polarities are correct, all
relevant calibration values are entered)
•
Switch off the I-share.
•
Set the P-, I- and D-share to 1%.
(refer to section 2.2.11,”Setting the P-, I- and D-share of the control loop” on page
36)
•
Switch on the output and observe the temperature.
P-share
•
Change repeatedly between set temperatures of 18 °C and 22 °C while observing
the settling behavior of the actual temperature.
Increase the P-share gradually. Higher values will increase the settling speed, too
high values make the system oscillate.
The P-share has been set correctly when the actual temperature remains stable
near the set temperature after only 2-3 overshoots.