THORLABS MTD415T, Техническое описание

Страница: 20 / 30

© 2016 Thorlabs GmbH
20
MTD415T
MTD415T Data Sheet Rev. 1.0
7 PID Tutorial
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.
The MTD415T microcontroller incorporates a digital PID controller. The P, I and D shares can
be programmed manually or calculated automatically by the firmware by entering the results of
a loop oscillation test. Below an example procedure is explained in detail.
Example of a PID adjustment
Pre-conditions:
·
TEC current limit is set correctly
·
all connections are made properly
In order to observe the temperature change, connect an appropriate instrument that allows to
display the temperature change vs. time, to the
TEMP input of the MTD415T.
1. Configure the PID loop:
Set temperature = 25°C: " T25000 "
P share = 1000 mA/K: " P1000 "
I share = 0: " I0 "
D share = 0: " D0 "
Cycle time = 30 ms: " C30 "
2. Enable the TEC. The actual temperature
Te
approximates the set value.
3. Now, find the critical P share (critical gain) value at which the system starts to oscillate for a
minimum of 20 cycles without amplitude drop as a reaction to a changed set temperature.
An example procedure is described below:
·
Set P to 10.000 mA/K: " P10000 ".
In order to trigger loop oscillation, increase the set temperature for 0.1 K: " T25100 "
·
Lower P to 5.000 mA/K: " P5000 ", decrease the set temperature for 0.1 K: " T25000 "
and observe the loop behavior.
·
If the loop still oscillates, lower the P share again, change the set set temperature for
0.1K and observe the loop behavior.