Fluid PID Autotuning Using Relay and Step Identification
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Solution Overview
Problem
Existing PID autotuning methods struggle to accurately tune PID controllers for fluid systems without prior knowledge, especially in industrial processes with load disturbances, and often result in unstable oscillations due to inaccurate identification of critical points.
Innovation Solution
A method for estimating a process transfer function using a first-order approximation with a feedback sensor, involving periodic relay processes and step injections to identify critical points, allowing for precise calculation of PID parameters without prior knowledge, and providing fallback calculations for non-convergence scenarios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If relay based autotuning is used to identify critical points, then PID tuning can be performed online, but the system may exhibit unstable oscillations due to inaccurate identification
Solution Approach 1:
The patent introduces an intermediary signal processing layer that uses autocorrelation analysis to accurately identify the critical oscillation period and frequency. This intermediary step between the relay signal and PID parameters ensures precise identification of critical points, preventing unstable oscillations while maintaining online tuning capability
Solution Approach 2:
The patent replaces traditional mechanical relay switching methods with a software-based autocorrelation algorithm that analyzes the relay response signal. This substitution eliminates the instability issues of pure relay methods by using mathematical analysis to precisely determine critical points without relying on noisy switching signals
2Extent of automation
If step based autotuning is used for process identification, then PID parameters can be calculated online, but prior knowledge of the process is required
Solution Approach 1:
The patent enables the system to perform self-identification of process parameters through autocorrelation analysis of the relay response. The system automatically determines the critical period, frequency, and amplitude without requiring external process knowledge or manual intervention, making the online tuning truly autonomous
Solution Approach 2:
The patent performs preliminary autocorrelation analysis on the relay response signal to extract critical process characteristics before calculating PID parameters. This preliminary identification step prepares accurate process data in advance, eliminating the need for prior process knowledge while enabling subsequent PID tuning
3Reliability
If hysteresis is introduced in the relay to prevent noise switching, then the relay becomes more stable, but the obtained point no longer exactly corresponds to the critical point
Solution Approach 1:
The patent uses autocorrelation feedback to continuously analyze the relay response signal and accurately identify the critical period even when hysteresis is present. The autocorrelation method compensates for the phase shift introduced by hysteresis, maintaining both relay stability and critical point identification accuracy simultaneously
Data Source
AI summary
A method for calculating PID parameters of a fluid system including a motor driving a pump, a compressor or a fan, including further a feedback sensor providing a feedback signal on the system and a PID regulation controlling the motor speed. The method includes:after an initial approximation of the fluid system by a first order transfer function with delay where three parameters of the transfer function of the process to be identified are: K Static gain, θ Delay, τ Time constant,one or more sequences of:a—bypassing the PID regulation and implementing the following processes:a periodic relay process providing a first point (ω−180; G−180) at −180° of phase named critical point,a periodic relay with integration process providing a second point (ω−90; G−90) at −90° of phase and,a step injection providing a third point G0 at ω=0° of phase and at null frequency,b—resolving the relevant equations in equation systems according to the points obtained through the processes to calculate the transfer function parameters available amongK=G0,τ=1ω-180(KG-180)2-1,θ=(tan -1(ω-180·τ)+π)·1ω-180c—applying the transfer function parameters obtained to calculate PID parameters for the system regulation{Kp=τK·(λ+θ)Ti=τTd=θ