How to compute the parameters of the PID regulator
from the available description of the plant
Note
This introduction to autotuning has been extracted from the IFAC Professional Brief
Hands-on PID Autotuning: a Guide to Better Utilisation (A. Leva, C. Cox, A. Ruano), 2002.
What is an Autotuner
An autotuner is something capable of computing the parameters of a regulator connected to a plant
(i.e. tuning that regulator) automatically and, possibly, without any user interaction apart
from initiating the operation.
The best way to understand autotuners is to analyze how a PID regulator may be manually tuned by a
human. The basic steps of a tuning process may be summarized as follows
observing the process behavior, eventually stimulating it somehow and turning this
knowledge into a description of the process behavior
establishing the desired closed loop behavior on the basis of the obtained process
description;
computing the regulator parameters in order to achieve the desired closed loop behavior.
Formalizing these steps establishes a tuning method. An autotuner is then an implementation
of a tuning method capable of running automatically.
The "description" of the process may be obtained in two ways:
the first approach identifies a model of the process from the data, where model means
something that can be simulated and that captures the process dynamics precisely enough to allow
forecasting once the regulator is tuned (model based autotuner);
the other approach is to employ the process data immediately for the subsequent tuning (non model based
autotuner). The relevant fact is that these representations cannot be simulated. Non model based autotuners are also identified as
characteristics based autotuners.
Tuning rules can be constructed in several ways:
choosing the regulator parameters so that the expected closed loop behavior (forecasted using the
process model) is as similar as possible to that of a reference model (model based, model
following autotuner);
computing the regulator parameters so that some time or frequency domain properties are satisfied
by the closed loop system (model based, characteristics following autotuner).
