PID Controller Tuning via Modified Relay Feedback
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Solution Overview
Problem
Existing PID controller tuning methods, such as relay feedback autotuning, can lead to inaccurate gain and phase margins due to the selection of test points not accounting for phase shifts introduced by the controller, resulting in reduced or excessively large stability margins.
Innovation Solution
A modified relay feedback test is developed that selects a different test point for exciting oscillations and designs tuning rules to account for the phase shift caused by the controller, ensuring specified gain or phase margins are met by measuring frequency and amplitude under these conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If relay feedback autotuning is used with test point selection at phase cross-over frequency, then the tuning method is simple and reliable, but the gain and phase margins become inaccurate due to unaccounted controller phase shifts
Solution Approach 1:
The patent applies preliminary action by pre-calculating and compensating for the controller's phase shift before conducting the relay feedback test. The test frequency is selected based on the process phase characteristic minus the controller phase shift, ensuring that the controller's effect is accounted for in advance. This allows the simple relay feedback method to maintain both ease of operation and measurement precision.
2Device complexity
If the test point is selected at phase cross-over frequency without accounting for controller phase shift, then the testing procedure is straightforward, but the resulting stability margins are reduced or excessively large
Solution Approach 1:
The patent changes the test frequency parameter from the conventional phase cross-over frequency to a modified frequency that accounts for controller phase shift. Specifically, the test frequency ω is selected such that the process phase at this frequency, minus the controller phase shift, equals -180 degrees. This parameter change maintains straightforward testing while ensuring accurate stability margins.
3Productivity
If conventional relay feedback test is used, then oscillations are excited at phase cross-over frequency, but the controller introduction causes unaccounted phase shift leading to tuning inaccuracy
Solution Approach 1:
The patent incorporates feedback by using the known controller phase shift characteristic to adjust the test frequency selection. The method feeds back the controller's phase effect into the test design, ensuring that oscillations are excited at a frequency that accounts for the controller's influence. This feedback mechanism maintains fast tuning while improving the accuracy of extracted dynamic information.
Data Source
AI summary
In tuning a PID controller for a process in a feedback control system, a method is provided for bringing the system into symmetric self-excited oscillations for measuring the frequency and the amplitude of the oscillations, and tuning the controller in dependence on the measurements obtained. A control algorithm referred to as the modified relay feedback test is introduced into a system in series with a process to generate self-excited oscillations. Tuning includes the steps of selecting a desired gain margin or phase margin, generating oscillations with the algorithm parameter corresponding to a selected gain or phase margin, measurements of the amplitude and the frequency of these oscillations, and computing PID controller tuning parameters. Data and formulas are given for the computation of the specific parameter of the modified relay feedback test and for tuning the parameters of the PID controller. An apparatus for performing the method is disclosed.