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6 Appendix
49
6.3 PID Controller Theory
The temperature controller in the TED4015 series is using a digital proportional-integral-derivat-
ive controller (PID controller) to correct the difference between a measured temperature and a
desired temperature set point. The temperature can be adjusted accordingly by calculating and
then outputting a corrective current.
The PID controller calculation (algorithm) involves three separate parameters; the Proportional,
the Integral and Derivative values. The Proportional value determines the reaction to the cur-
rent temperature error, the Integral value determines the reaction based on the sum of recent
temperature errors, and the Derivative value determines the reaction based on the rate at
which the temperature error has been changing. The weighted sum of these three terms is
used to adjust the temperature via the current supply of a cooling/heating element (Thermo
Electric Cooler (TEC) – Peltier Element).
The PID control scheme is named after its three correcting terms, whose sum constitutes the
manipulated variable (MV). Hence:
where P
out
, I
out
, and D
out
are the contributions to the output from the PID controller from each of
the three terms.
By "tuning" the three constants in the PID controller algorithm, the controller can provide control
action designed for specific process requirements. The response of the controller can be de-
scribed in terms of the responsiveness of the controller to an error, the degree to which the con-
troller overshoots the set point and the degree of system oscillation.
You can set every constant (P, I, D) to zero to disable it. That means you can use the PID con-
troller only as PI controller by setting the Derivative value to zero. This may be useful in an
noisy environment since derivative action is very sensitive to measurement noise. On the other
hand, the absence of an integral value may prevent the system from reaching its target temper-
ature and is not recommended.
