Tutorial; Operating Principle Of A Temperature Controller - THORLABS MTDEVAL User Manual

9.3 Tutorial

9.3.1 Operating Principle of a Temperature Controller

In general, a temperature controller (within the blue frame) is a closed loop system. A
temperature sensor measures the temperature of the controlled object (e.g., a laser diode).
This actual temperature signal is amplified and compared with the temperature set value.
The differential signal out of the comparator controls then the current of the thermoelectric
cooler in order to maintain the temperature of the object constant. Ideally, the temperature
settling is carried out in the shortest times, with minimum settling error and without temperature
overshoots.
A thermoelectric coolers is a Peltier element that produces a temperature gradient depending
on the current direction trough the TEC. For this reason, the TEC current must be bidirectional.
In order to adapt the control loop to different thermal loads, and to optimize the temperature
controller's response characteristics, a PID amplifier is used.
The general requirements to a temperature control loop are:
· fastest settling time after power on or changing the set temperature
· minimum residual temperature error
· settling without temperature overshoots
· fastest response to changes of the thermal load
PID amplifiers can fulfill these requirements. Temperature control loops are comparatively slow;
control oscillations appear with a frequency in the range of several Hz or parts of Hz. The PID
adjustment allows to optimize the dynamic behavior.
The P share is the proportional share, or the gain of the amplifier, that defines the settling time.
The higher the P share, the faster the settling and the less residual temperature error. The
downside is that high P shares lead to oscillations.
The I share is the integrating share of the amplification, or the gain at low frequencies. It allows
to minimize the residual temperature error.
Optimal settings of the P and I shares result in a fast approach to the set temperature, without
oscillations and with a minimum residual temperature error. However, such a loop is not able to
quickly react to sudden changes of the thermal load, for example, if a thermally stabilized laser
diode is set to a higher or lower output power that changes the laser's heat dissipation. The D
share (differential share, or the gain at high frequencies) allows the system to quickly react to
temperature changes, without generating oscillation of the temperature around the set point.
© 2016 Thorlabs GmbH
9 Appendix
33