Understanding The Pid Auto-Tune; Expanded Loop Table - Siemens SIMATIC S7-200 System Manual

S7-200 Programmable Controller System Manual

Understanding the PID Auto-Tune

Introduction
The auto-tuning algorithm used in the S7-200 is based upon a technique called relay feedback
suggested by K. J. Åström and T. Hägglund in 1984. Over the past twenty years relay feedback
has been used across a wide variety of industries.
The concept of relay feedback is to produce a small, but sustained oscillation in an otherwise
stable process. Based upon the period of the oscillations and the amplitude changes observed in
the process variable, the ultimate frequency and the ultimate gain of the process are determined.
Then, using the ultimate gain and ultimate frequency values, the PID Auto-tuner suggests a value
for the gain, reset and rate tuning values.
The values suggested depend upon your selection for speed of response of the loop for your
process. You can select fast, medium, slow or very slow response. Depending upon your process
a fast response may have overshoot and would correspond to an underdamped tuning condition.
A medium speed response may be on the verge of having overshoot and would correspond to a
critically damped tuning condition. A slow response may not have any overshoot and would
correspond to an overdamped tuning condition. A very slow response may not have overshoot
and would correspond to a heavily overdamped tuning condition.
In addition to suggesting tuning values the PID Auto-tuner can automatically determine the values
for hysteresis and peak PV deviation. These parameters are used to reduce the effect of the
process noise while limiting the amplitude of the sustained oscillations set up by the PID
Auto-Tuner.
The PID Auto-Tuner is capable of determining suggested tuning values for both direct-acting and
reverse-acting P, PI, PD, and PID loops.
The purpose of the PID Auto-Tuner is to determine a set of tuning parameters that provide a
reasonable approximation to the optimum values for your loop. Starting with the suggested tuning
values will allow you to make fine tuning adjustments and truly optimize your process.

Expanded Loop Table

The PID instruction for the S7-200 references a loop table that contains the loop parameters. This
table was originally 36 bytes long. With the addition of PID Auto-Tuning the loop table has been
expanded and is now 80 bytes long. The expanded loop table is shown in Table 15-1 and
Table 15-2.
If you use the PID Tuning Control Panel, all interaction with the PID loop table is handled for you
by the control panel. If you need to provide auto-tuning capability from an operator panel, your
program must provide the interaction between the operator and the PID loop table to initiate, and
monitor the auto-tuning process, and then apply the suggested tuning values.
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