Several aspects make this book different, and doing things not the usual way is its recurring motif. Therefore, in this chapter, we focus on establishing those unique aspects of the book that constitute its claimed titular “fresh look.” This expression has a double meaning here. On the one hand, it refers to the active disturbance rejection control (ADRC) as this unorthodox approach in the control landscape and on the other to the fresh look at how ADRC can be introduced to those interested in it. We will also answer here some basic important questions like what exactly is in the book, why is it constructed the way it is, to whom the book is for, and what is the best way to use it depending on the reader’s background.
What Is This Book About?
This book is on controls. But, as you will soon find out, this is not yet another typical control textbook, if there is even such a thing. Although the basics of control systems were developed in the first half of the twentieth century, time and technology continue to generate refinements. Here, we look at the area of controls from a particular perspective of active disturbance rejection control (ADRC). The idea of ADRC, manifested through its concepts, operating principles, and tools, allows its users to have an alternative view of how to look at, analyze, and solve control problems. Since its inception, ADRC gradually matured into a viable industrial control technology and—as evident through recent adoptions by major companies—found its way into numerous application domains. The unique, disturbance-centric perspective of ADRC helps develop high-performance controllers but with their overall complexity comparable to standard PID. The same cannot be said about the majority of so-called advanced control algorithms which were never successfully transitioned from academia to industry. The relative simplicity, high adaptability, robustness, and scalability are reasons ADRC is often mentioned as a potential contender to PID, which has dominated the field for over 100 years.
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How Is This Book Organized?
This book is divided into two main parts, following the book’s subtitle. The first one is what we consider the theoretical foundations of ADRC. This part intends to give the reader basic information on the subject in a tutorial style. It is meant to be a self-contained, one-stop shop, where the readers can get a solid footing on ADRC without needing to refer to external sources. The second part deals with all the aspects needed to put the ideas, methods, and tools introduced earlier into practice. In the book, one can find the Prelude, Interlude, and Postlude sections serving as framing devices, which, as the names suggest, discuss the opening, midpoint, and closing matters of the book, respectively.
The first, theoretical, foundation-laying part of the book is organized as follows.
Chapter 2 is the titular first contact with ADRC. It is an example-driven, step-by-step, elementary introduction aiming at developing an initial feel for ADRC. Through this chapter, we want to encourage the readers’ use of common sense and engineering intuition and thus liberate them from the limiting dichotomies of control theory versus practice and classical versus modern control algorithms. We want to show that alternatives exist, like ADRC, where synergistic solutions can be conceived by taking what is useful from different worlds.
Chapter 3 generalizes the concepts introduced earlier but does it in a more stripped-down and concise way using the standard control domain-related terminology and a bit more mathematical rigor. The result of the chapter is the commencement of a base form of ADRC, a kernel one can start with when building customized ADRC-based solutions.
Chapter 4 takes the previously established base ADRC and contextualizes it to deliver interpretations of it from the established field of linear control systems. This allows the positioning of ADRC among the established control methods.
Chapter 5 showcases ADRC in a predominantly visual manner. Here, we go deep into the capabilities and limitations of ADRC and foster the reader’s intuition and understanding of what is actually possible with this control framework.
Chapter 6 makes a case that the ADRC should be understood as a modular, flexible framework that could be tailored to specific applications. In this chapter, we show some potential extensions and modifications to the ADRC scheme that can help the designer develop customized solutions addressing specific control problems and/or characteristics of the controlled plant.
Chapter 7 is the concluding chapter of the first, theoretical part of the book. It is thus a good time to take a look back and around. Since ADRC was not conceived overnight, we give credit where credit is due and briefly describe the history of ADRC including explaining how the elements in the previous chapters came to life. This is also the place where we justify why we decided in this book to mainly focus on a particular form of linear ADRC. To widen the readers’ horizons in terms of ADRC, in the second half of the chapter, we give a list of proposed further readings addressing various aspects of ADRC not (at least directly) covered in this book.
The second, practical part of the book consists of the following chapters.
Chapter 8 opens the second part of book. Since putting ADRC in practice will almost always be in the form of a software-based implementation, we present several discrete-time variants of ADRC and discuss their specific features. We present different styles of implementation and show how it can be optimized for a low computational footprint.
Chapter 9 examines some of the issues one can encounter when implementing a controller in the real world. Here we focus on those aspects that are most commonly seen in practice. At the same time, we offer specific solutions to those problems that retain the simplicity of ADRC, which is one of its trademarks.
Chapter 10 shows that going from principles to practice is immediately possible based on the results from previous chapters. Here we demonstrate the implementation of discrete-time variants in software form using both a model-based environment and low-level C language code. We take full advantage of the “cooking recipes,” which are handy, ready-to-use procedures, introduced throughout the book, that streamline getting the ADRC tuning parameters for all variants in the book.
Chapter 11 puts ADRC to work after establishing all the necessary components for its practical implementation. Here we utilize laboratory testbeds to mimic real control problems from process and power control areas and apply knowledge from this book to solve them. To make the experiments reproducible by the readers, we limit ourselves only to affordable and off-the-shelf hardware platforms. With this choice, we hope the readers will be able to first go through our hardware examples and then be ready to take ADRC and deploy it in their applications.
Chapter 12 deals with the future, both nearest, and that a bit further. Regarding the former, we show what is still coming in the book, and we guide the reader to the Appendices. Regarding the latter, we show what the reader can do after reading this book, discuss possible avenues of further development, and speculate which direction ADRC can evolve in.
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The substantive part of the book does not end with the numbered chapters. Appendix A is a reference guide for returning readers that repeats and summarizes all relevant equations required to implement and tune all variants of ADRC that have been introduced in this book. Appendix B provides an overview of some of the existing ADRC toolboxes for MATLAB/Simulink, including the “Linear ADRC Blockset” in Sect. B.3, which is our original toolbox facilitating the implementation of ADRC, in particular those variants and formulas covered in this book.
With the above chapter list and descriptions, one can start to see a pattern here. This book is written in a way that the further you go into the book, the more practical and concise it gets. As the book progresses, the previously minutely explained and analyzed theoretical concepts are gradually transformed and boiled down into succinct practical tools and solutions. All this reaches its culmination point in the Appendix, where different variants of ADRC, covered throughout the book, are blatantly distilled into plug-and-play formulae, which one can simply take and directly deploy in embedded systems, on programmable logic controllers (PLCs), etc.
To Whom Is This Book Addressed and How to Use It?
These two questions need to be answered jointly because how to use this book depends on who you are on the control spectrum. First of all, the book is for those who design and tune control loops regularly and those still learning that. It can play different roles while addressing a wide range of users with different levels of knowledge and skills, wishing to discover and learn ADRC. It also aspires to bridge the theory-practice gap and put ADRC into the hands of every control engineer.
For students, it can be a textbook being part of an introductory course in control systems at both senior undergraduate and first-year graduate levels in several engineering departments. Its two-part structure organically creates a two-semester course. The first semester (Chaps. 2–7) could be focused on building the foundations of ADRC from scratch, while the second semester (Chaps. 8–12) could take those fundamental ideas and tools and show how to apply them to practical scenarios. If the book is used as didactic material, it guides the reader through the subject bit by bit, covers a wide range of topics within ADRC, and provides in-depth information and explanations. The same hardware platforms from Chap. 11 could be used in class to aid students in their ADRC learning experience as those are easily accessible and reasonably priced laboratory testbeds.
For practitioners, it can be viewed as a compact guide on how to use ADRC, in other words, an introduction if you already have a control background in methods like PID. For more experienced users, it can be perceived as a handbook of straightforwardly deployable control solutions that offer improved performance over conventional control applications. We wrote the book having a strong belief that theory must be accessible to engineers and that there is little point in offering a theory that, because of its complexity, is unlikely to find its way into engineering practice. While avoiding overly complicated scientific jargon, the book also explains some ready-to-use modifications and extensions that allow practicing engineers to customize their ADRC algorithms and implement solutions that are tailored to the actual real-world control scenarios.
For scientists, the book offers a systematic and carefully distilled body of work, being a solid foundation of ADRC one can build on. Since the theory-practice gap is heavily tackled in the book and even manifested through its two-part structure, scientists can get a good grasp on the current state of things and be able to further explore basic and applied research questions in the area of ADRC.
The book can be used for self-study and as a one-volume reference for both control novices and the already initiated. Having in mind its introductory nature, the prerequisites for studying the considered subject are intentionally not very strict. Regardless of the intended audience, to get a meaningful grasp of the described underlying concepts, methods, and tools in ADRC, we recommend having at least basic knowledge of systems and controls, as well as an understanding of core mathematical concepts of calculus, linear algebra, and differential equations. Basic programming skills, even though not mandatory, could turn out to be helpful, especially in the more practically oriented second part of the book.
What Are This Book’s Guiding Principles?
With this book, our goal is to offer a “fresh look” at how ADRC can be introduced. So if at any point in reading the book, you get a sense that it is peculiar in terms of our choices of language, structure, and content, then most likely it is because we tried to align all elements of the book to the below main motives. To us, this is the book’s strength and one of its selling points.
Self-containment: You may have noticed already that we are almost at the end of Chap. 1 and we have not yet provided a single literature reference, which is not typical for the first chapter of a book. In fact, you will not encounter any references until the end of Part I. Such a different use of literature is not a coincidence but rather this book’s leitmotiv. It is done to not distract you from the story we are telling and to give you an uninterrupted introduction to ADRC. It is on purpose to make the book as self-contained as possible. We deliberately delay giving credit, context, and historical perspective to Chap. 7, which serves as a thematic midpoint where all the literature references are concentrated and briefly discussed. But even then, there is no need to read any of the papers or books mentioned there to get from start to finish as the book aims to stand on its own. The discussed references should be thus treated as useful pointers rather than prerequisites. Our goal for writing the book was to provide a relatively complete look around the ADRC area without external dependencies besides fundamentals of control systems (like state space and transfer functions).
Multiplicity of entry points: We have structured and written the book to facilitate not only who you are in the control field but also what is your prior experience with ADRC. This means that how to find your way through the book depends on your initial knowledge of ADRC. We offer several entry points where a person can get acquainted with the core concepts of the book. We try to pick people up where they are in terms of experience and knowledge and through a customized guide give here some hints and personal recommendations of what may be of particular interest. Given the fact that the book is intended for multiple audiences, it is important for us to help the readers find their way through the book and quickly filter the information they truly seek.
Practicality and reproducibility: Everything that went into this book had to positively answer the question: Is it practically useful? Such a selection process of what to include was applied to the presented ADRC variants, tuning methodology, methods of implementation, etc. This was our litmus test for deciding whether something contributes to bridging the theory-practice gap in control, which we aim for. That is why we deliberately keep scientific jargon in the book to a minimum and focus on implementability as we want the reader to try ADRC out for oneself. That is also why we provide ready-to-use “cooking recipes” for different ADRC variants in the book. One can easily take those and try them out in application. Furthermore, in the experimental section of the book, we use cheap and commercially available target platforms, so anyone can reproduce all of the results seen in this book, including the hardware ones.
Lasting value: This book is not intended to be read only once. We expect it to be revisited, whenever deemed necessary. When reading it for the first time, one probably moves intuitively from front to back. But when one returns to the book, it can also be read starting at the end as a way to recap the key practical takeaways. For those who have already gone through the book, you may want to go to the Appendix upon subsequent reads, where we have condensed and summarized the material. You can then use the book as a reference manual, designed to be consulted for information on a specific topic and organized in a way to help locate specific information quickly. That is why we go the extra mile in the book by offering toward the end some specially arranged information like the ADRC software implementation or the MATLAB/Simulink blockset that has the ADRC variants covered in the book ready to be deployed.
In Fig. 1.1, we provide some subjective suggestions to readers on what to read next. For the newcomers to the topic of ADRC, we recommend starting with Chap. 2, where we begin explaining ADRC without any assumptions (besides basic control fundamentals) and then proceeding with the book in chronological order. For those who already have some knowledge of ADRC and know the terminology usually associated with it, we suggest starting to find out what this book is about in Chap. 3.
Fig. 1.1
Reading guidance with proposed entry points for the book’s first-time readers (left-hand side) and for those who return to the book seeking a specific topic (right-hand side)
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For returning readers, we provide several potential paths, the choice of which depends on what particular aspect of ADRC the reader wants to revisit.
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