Buck Converter PID Autotuning Using Modified Relay Feedback
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Solution Overview
Problem
Existing autotuning methods for DC-DC buck switching power converters are complex and require multiple stages, often performed during regular operation, and do not guarantee stability or specified gain or phase margins, making them inefficient and prone to errors.
Innovation Solution
The modified relay feedback test (MRFT) autotuning method, which involves a single-step test and tuning stage using a PID controller with specific equations to update parameters based on measured oscillations, ensuring a specified gain or phase margin and near-optimal transient performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional RFT-based autotuning is used with an integrator in series, then oscillations occur at resonant frequency for tuning, but the test must be performed during regular operation at voltage below nominal and the tuning process is complex and long
Solution Approach 1:
The patent segments the multi-stage conventional RFT tuning process into a single unified MRFT stage. Instead of performing separate steps for identifying resonant frequency, tuning zeros, and adjusting gain iteratively, the modified relay feedback test combines all these functions into one coordinated process that simultaneously determines all PID parameters in a single test sequence.
Solution Approach 2:
The patent applies preliminary action by pre-calculating the relationship between relay parameters and the desired phase margin before performing the actual test. The coordination between test parameters and tuning objectives is established in advance, allowing the system to directly compute optimal PID parameters from the measured oscillation characteristics without requiring iterative adjustments during the test.
2Reliability
If multiple stages of iterative tuning are performed, then desired phase margin can be achieved, but the tuning process becomes long and complex
Solution Approach 1:
The patent implements feedback by using the measured oscillation characteristics (amplitude and frequency) directly to compute the PID parameters through a coordinated relationship. The test itself provides the feedback information needed to determine the optimal controller settings, eliminating the need for separate iterative tuning stages while guaranteeing the desired phase margin through the pre-established coordination between test parameters and controller parameters.
Solution Approach 2:
The patent changes the parameters of the relay feedback test (specifically the relay hysteresis width and amplitude) in a coordinated manner with the desired controller performance specifications. By pre-defining the relationship between test parameters and controller parameters, the system can achieve the desired phase margin in a single test run without requiring iterative parameter adjustments.
3Adaptability or versatility
If autotuning is performed online during regular operation, then the converter can be retuned based on actual system parameters, but the test must be performed at voltage below nominal which may affect system performance
Solution Approach 1:
The patent makes the relay feedback test universal by coordinating it with the normal operational modes of the converter. The MRFT can be performed across different operating conditions and voltage levels, and the same test procedure adapts to provide optimal tuning for each condition. This multi-functionality allows the system to perform both normal operation and autotuning using the same control infrastructure without compromising stability.
4Productivity
If Ziegler and Nichols PID tuning rules are used with RFT, then satisfactory performance is usually achieved, but stability is not guaranteed and gain or phase margin cannot be specified
Solution Approach 1:
The patent introduces an intermediary layer of coordination between the relay feedback test and the PID controller design. Rather than directly applying empirical rules like Ziegler-Nichols, the MRFT uses a mediating set of coordinated relationships that translate the measured oscillation characteristics into PID parameters while explicitly ensuring the desired phase margin. This intermediary coordination layer guarantees stability while maintaining tuning efficiency.
Data Source
AI summary
A method of automatically tuning a proportional-integral-derivative (PID) controller (102) controlling a DC-DC converter (100) includes substituting the PID controller (102) with a modified relay feedback test (MRFT) (104) controller having a threshold parameter (β) and a magnitude parameter (h), iteratively determining a current error signal (e[k]) by measuring an output voltage (Vo[k]) of the DC-DC converter (100) and comparing it to a desired reference output voltage (Vo-ref), input the error signal (e[k]) to the MRFT (104), determining an output value (u[k]) of the MRFT, update the duty cycle (D) based on the output value (u[k]), operating the DC-DC converter (100) using the updated duty cycle (D), exciting oscillations in the loop containing the MRFT (104) and the DC-DC converter (100), measuring a frequency (Ω0) and amplitude (α0) of the output voltage (Vo[k]), updating PID controller parameters based on the frequency (Ω0) and amplitude (α0), and substituting the MRFT (104) with the PID controller (102).