A Brief History of Feedback Controls Bin Yao Intelligent and Precision Control Laboratory School of Mechanical Engineering Purdue University West Lafayette, IN47907, USA It is my pleasure to have this opportunity to visit my friends here at HKUST and present you some of the research that I have been working on, namely, nonlinear adaptive robust control and its applications. This is a control approach that I first conceived when I went to UC Berkeley for my Ph.D. study in 1992. Over the years, rigorous theoretical analysis and various extensions have been developed. Applications to various electro-mechanical/hydraulic systems have demonstrated the high performance nature of the approach. The results have led me to believe that this method is the right approach to the intelligent and precision control of electro-mechanical systems. So I am very excited to present the approach to you and hope that it will benefit you in some ways.

History of Feedback Controls Feedback Controls have been implicitly used throughout human civilization; Automatic Control Systems were first developed over two thousand years ago (Ktesibios’ water clock) The first feedback control device on record is thought to be the ancient water clock of Ktesibios in Alexandria Egypt around the around 270 B.C. It kept time by regulating the water level in a vessel using a float (just like the flush toilet used today) and, therefore, the water flow from that vessel. Cornelius Drebbel (1572–1633), developed the first automatic temperature regulator, i.e. the thermostat, of a furnace.

History of Feedback Controls Automatic Control Systems have been an essential factor in the spread of industrialization (Watt’s steam engine) James Watt (1736–1819) did not invent the steam engine as many so often claim. Steam engines had been around for decades before Watt saw one for the first time. Watt did, however, improve the steam engine in many different ways, the most revolutionary being Watt’s “flying balls”, i.e. the motor’s rotation speed regulator (fly-ball governor), which Watt made public in 1769. Before Watt’s invention, the rotation speed of steam engines varied significantly and controlling the motors was extremely difficult. http://www.history.com/media.do?action=clip&id=mm_gi_steam_broadband

History of Feedback Controls The need for a formal Mathematical Control Theory appears with the invention and industrial use of sophisticated Automatic Control Systems In his 1868 paper "On Governors", J. C. Maxwell (who discovered the Maxwell electromagnetic field equations) was able to explain instabilities exhibited by the flyball governor using differential equations to describe the control system. This demonstrated the importance and usefulness of mathematical models and methods in understanding complex phenomena, and signaled the beginning of mathematical control and systems theory. Elements of control theory had appeared earlier but not as dramatically and convincingly as in Maxwell's analysis. Formal Stability Analysis Techniques: Routh Stability Criterion (1877) (Hurwitz 1895) for LTI systems Lyapunov’s Stability Theorems (1893) for general nonlinear systems Nyquist Stability Criterion (1932) in frequency domain originally developed for the design of feedback amplifiers by Black at Bell Lab, a critical element to telecommunications ( )

History of Feedback Controls Classic Control Theory was developed in conjunction with the ability to implement simple linear controllers using analog operational amplifiers (analog computer) around the World War II The realization that feedback described common phenomena in a variety of settings did not crystallize until World War II, when new institutions brought engineers from diverse backgrounds together to construct military control systems. Only then were the techniques developed at BTL to deal with feedback, frequencies, and noise applied to mechanical and hydraulic systems, and to the human operators themselves. Only then did feedback become prominent as a general principle in engineering, and only afterward, with the work of Wiener, Shannon, and numerous others, did Black’s, Bode’s, and Nyquist’s ideas move beyond amplifiers and into a broad range of disciplines Various Classical Control Techniques for LTI Systems: PID control: Callender et al (1936) (time-domain) Root Locus: Evans (1948) (time-domain) Frequency Domain: Bode (1945) (gain and phase margin)

History of Feedback Controls The fundamental role of Feedback Controls to modern technology was well recognized after World War II Benefits of Feedback Controls: Stabilize Unstable Systems Improve System Performance (e.g., speeding up system response) to meet stringent performance requirements Reduce (Attenuate) the effect of modeling uncertainty and various disturbances for consistent performance

History of Feedback Controls The fundamental role of Feedback Controls to modern technology was well recognized after World War II Various Professional Societies Formed: The ASME Dynamic Systems and Control Division (DSCD) was founded in l943 (http://divisions.asme.org/dscd/) The IEEE Control Systems Society (CSS) was founded in l954 (http://www.ieeecss.org/) The American Automatic Control Council (AACC), an association of the control systems divisions of eight member societies: AIAA, AIChE, AISE, ASCE, ASME, IEEE, ISA, and SCS, was formed in 1956 (http://www.a2c2.org/) 中国自动化学会（Chinese Association of Automation) 于1961年成立 (http://caa.gongkong.com/) The International Federation of Automatic Control was founded in 1957 (http://www.ifac-control.org/)

History of Feedback Controls Modern Control Theory was developed based on state-space representations of dynamic systems. More sophisticated mathematical and computational design tools were developed with the advent of digital computers Control theory made significant strides in the next 100 years. New mathematical techniques made it possible to control, more accurately, significantly more complex dynamical systems than the original flyball governor and feedback amplifiers. Samples of Modern Control Theory Design Techniques: Observer Design and Filtering (50s) Optimal Control (Bellman’57, Pontryagin’58, Kalman’60) Stochastic, Robust, and Adaptive Control of LTI systems (70s and 80s) Digital Controls (70s and 80s)

Current Status of Control Theory Rapid advances in microelectronics and microprocessor technologies during the past decades have made the computer based control implementation platform rather affordable and a standard choice for any modern machines. Such a hardware configuration enables control algorithms to be constructed in the same way as what a human brain normally does – a decision making process making full use of all information available including feedback signals Diverse Advanced Control Design Techniques: Computation based Multivariable Linear Controls (since 80s) Nonlinear Control Theory (since 80s) Nonlinear Robust and Adaptive Controls (since 90s) Control of Discrete Event and Hybrid Systems (since 80s) …

General Structure of Controller In general, a controller is nothing but a strategy to determine a control action based on all available information; information not only comes from the measured output but also from the measured internal state variables, measured disturbance, reference trajectory, and plant model structure. It can have any form and is illustrated below: So there will be no end to new control schemes !

Control Applications Feedback is fundamental to our technology world. Applications of control methodology have helped make possible space travel and communication satellites, safer and more efficient aircraft, cleaner auto engines, cleaner and more efficient chemical processes, to mention but a few.

Examples of Feedback Control Robotics

Examples of Feedback Control Manufacturing

Examples of Feedback Control Automotive Applications Active Suspension Systems Brake-by-Wire

Examples of Feedback Control Aerospace & Astronautics

Examples of Feedback Control External Vibrations Eccentricity Pivot Friction Non-Circular Track Profile Flow-induced Aero-elastic Forces Hard-Disk Drive Control Issues

Examples of Feedback Control Medical and Daily Life Applications CyberKnife full-body radiosurgery using Image-guided robotics

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