Pid Autotune (Model 2510-At Only); Autotune Operation; Response Options; Short Lag And Tau Time Example - Keithley 2510 User Manual

4-6 PID Control Concepts
4.
5.
PID autotune
The Model 2510-AT autotune algorithm provides the user with a tool to help in tuning the
Model 2510-AT Temperature PID loop. It is intended to give a set of PID tuning parame-
ters that will give close to the optimum system performance, but it may not result in ideal
tuning parameters. These PID parameters can be improved upon by iterative adjustments,
which are covered later in this section.

Autotune operation

The Model 2510-AT PID autotune algorithm obtains its information from the system by
forcing a step function in voltage across the TEC and then observes the system response to
that step function. The Lag and Tau times of this response waveform are then extracted
and applied to a modified version of the Ziegler-Nichols tuning equations.

Response options

There are two different options for the autotune function: minimum settling time and min-
imum overshoot. The discussion below compares these two options based on short and
long Lag and Tau (τ) times.

Short Lag and Tau time example

Typical response characteristics for the two response options are shown in Figure 4-4. In
these examples, the Laser Diode Module has a Lag time of 0.77 seconds and a Tau of 7.70
seconds and is subjected to a +3°C step in temperature setpoint. For the minimum settling
time plot, the temperature overshoot occurred at 3.274 seconds (with a peak value of
26.09°C). It settles to within ± 0.1% of the final temperature (0.003°C) in 11.14 seconds.
In the minimum overshoot case, the temperature overshoot occurs at 4.96 seconds (with a
peak value of 25.67°C). It settles to within 0.1% of the final temperature (25.503°C) at
27.32 seconds. (See
Table 4-1
Response time comparison example 1 (laser diode Lag Time 0.77sec, Tau Time 7.70sec)
Condition
Max Overshoot: 26.09°C @ 3.274 Sec after Step
Settle to 0.1%: 25.497°C @ 11.14 Sec after Step
Slowly increase the Integral (I) value until oscillation just begins, then reduce this
level by a factor of two or three. This reduction should be enough to stop the oscil-
lation. Use the lowest I value that gives adequate performance.
Check that the overall temperature control characteristics of the Model 2510 using
these settings is satisfactory under the conditions it will be used.
(Model 2510-AT only)
Table 4-1.)
Minimum Settling Time
Models 2510 and 2510-AT User's Manual
Minimum Overshoot
25.67°C @ 4.96 Sec after Step
25.503°C @ 27.32 Sec after Step