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Editorial: Special issue on connecting theory and practice with ADRC
From Qiang Xuesen [1], to Guan Zhaozhi [2], and finally to Han Jingqing [3], active disturbance rejection control (ADRC) arose from their unwavering conviction that theory must be connected to practice, and that control theory of any practical significance must not simply be a branch of mathematics, predicated on accurate mathematical model of physical processes. It is their vision and wisdom that have guided us through difficult times. In particular, Han’s provoking question of “Is this a theory of control or a theory of model?” [3] awakened generations of scholars and compelled them to reexamine the very premise on which modern control theory has been built: what is the object of our study? Is it the control of a physical process, or is it the control of a mathematical model? True to his conviction, Han went on to demonstrate, through ADRC, how to make control systems inherently immune to unmodeled dynamics and disturbances [4–6]. Such developmentwould surely impact engineering practice, but not before it cleared one final hurdle: attaining backward compatibility with PID and with Bode and Nyquist’s language of frequency response. Ignorance of such hurdle arguably led to the stagnations of many well-established schools of so-called advanced control [7] and to the questioning of their relevance to engineering practice. For ADRC, the turning point is the parameterization of all controller gains as functions of bandwidth [8], making it user friendly and a viable alternative to PID on the account of simplicity, robustness, performance, and ease of tuning. Furthermore,multiple researchers have shownindependently that, with some simplification and for low order plants, ADRC is indeed backward compatible with PID. This helps to explain the staying power of PID and to pave theway for the bandwidth parameterization of the PID gains, with profound implications. Most notably, the seminal work of Ziegler and Nichols on PID tuning [9] is now made understandable by practitioners “in their own language of bandwidth and phase characteristics” [10]. And this was just a beginning. Seeing the significant performance improvements, engineerswould often ask: doesADRCachieve this at the expense of phase margin? Sheng Zhong and Yi Huang answered this question here resoundingly in their paper titled “Quantitative analysis on the phase margin of ADRC”. Against a typical second-order plant, phase margins of ADRC with four different ESOs are given analytically and verified in both simulation and experimentation. The results would put users at ease and give them design options. It also begs the question of “how to manage systematically various competing design objectives?” and it is addressed by the paper titled “On tuning of ADRC with competing design indices: a quantitative study”. Here, the relationship is finally established analytically between theADRCtuning parameters and the following measures of any control system: stability margin, tracking, disturbance rejection, and noise sensitivity. Engineers would also ask that, as ADRC is shown to be backward compatible to PID, does it really matter one or the other? In other words, why fix something that is not broken? In “On disturbance rejection proportional–integral–differential control with model-free adaptation”, readers will find an ingenious integration, in the standard PID form, of active disturbance rejection and parameter adaptation, leading to a new kind of model-free adaptive control, distinctly different from those well-known work in the literature. This helps to greatly expand the range of operation and disturbance rejection, while maintaining the simplicity and intuitiveness of PID. If this still does not make this special issue a good read, there is more to come....
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