Pid Loops And Tuning - Siemens SIMATIC S7 System Manual

PID loops and tuning

PID auto-tune capability is incorporated into the CPU, and STEP 7-Micro/WIN SMART adds
a PID Tune control panel. Together, these two features greatly enhance the utility and ease-
of-use of the PID function provided.
You can initiate auto-tune in the user program, from an operator panel, or by the PID Tune
control panel. The PID Auto-Tuner computes suggested (near optimum) values for the gain,
integral time (reset), and derivative time (rate) tuning values. You can select tuning for fast,
medium, slow, or very slow response of your loop.
With the PID Tune control panel, you can initiate the auto-tuning process, abort the auto-
tuning process, and monitor the results in a graphical form. The control panel displays any
error conditions or warnings that might be generated. You can apply the gain, reset, and rate
values computed by auto-tune.
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.
The auto-tuning algorithm used in the CPU 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 relay feedback concept produces 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 can overshoot and corresponds to an underdamped tuning
condition. A medium speed response can be on the verge of overshoot and corresponds to a
critically-damped tuning condition. A slow response cannot have any overshoot and
corresponds to an overdamped tuning condition. A very slow response cannot have
overshoot and corresponds 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. You use these parameters 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 can determine suggested tuning values for both direct-acting and
reverse-acting P, PI, PD, and PID loops.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
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