Feedback control systems

BOOK REVIEWS FEEDBACK CONTROL SYSTEMS,by Otto J. M. Smith. 694 pages, diagrams, 6 X 9 in. New York, McGraw-Hill Book Co., Inc., 1958. Price, $13.50. The sociological phenomenon of the "vicious circle" has long been recognized as positive feedback. The stabilizing effect of negative feedback has been advocated even longer as illustrated by the quotation " I f anyone strikes you on the right cheek, turn the other to him, and if anyone sues for your shirt, give him your coat too." To intelligently apply controls to such feedback systems, there is a need for developing general methods of closed loop analysis and synthesis. This book is written, in the author's words, as "a unified philosophy for the analysis and design of all types of feedback systems." He includes social, economic, government, and biological systems with the more clearly defined physical ones. The book is designed as a reference and textbook for graduate study, assuming prior courses in basic feedback control and statistical analysis. The four main divisions of the text are: linear analysis, linear synthesis, non-linear analysis and non-linear synthesis and each is treated extensively. The first quarter on linear analysis begins with the conventional t r e a t m e n t of a single feedback loop. It expands into the analysis of multiple loops and a particularly lucid description of power spectra. The second section, dealing with linear synthesis, opens with the definition of the author's choice of an optimum system, namely, one with minimum error power. The importance of a statistical description of the expected input is stressed. A new function is introduced, called the "realizability operator" which removes, somewhat magically, all unrealizable poles in the optimum design. Coming back down to earth for an interval, the author presents an excellent section on design techniques, indicating an intimate knowledge of the mechanics of synthesis by graphical methods. These techniques are extended to cover the simpler non-linear sys66

tems having dead time and distributed parameters. The third and fourth sections present what is probably the most comprehensive approach to non-llnear systems to date, even considering the excellent work of John Truxal. Relay systems with and without multilevel signals are treated at length by the method of two loci in the L-plane. Chapters on unilateral and bilateral non-linearities and discontinuities follow. Examples of these are saturation hysteresis and dead zone as the unilateral type, and backlash and coulomb friction as the bilateral type. Both may lead to discontinuities such as jump phenomena and lock-up oscillation. Non-linear synthesis is handled primarily by the phase-plane method, graphically and with an eye toward real components. Predictor control for compensating non-linearities is described and a general t r e a t m e n t of carrier systems completes the section. The appendix contains a set of compensating networks with their mathematical and graphical description. The book is an outstanding contribution to the field, being valuable as a text for advanced graduate study as well as a working reference for the designer of complex systems. It effectively fills out the McGraw-Hill Series in Control Systems Engineering. CHARLES A. BELSTERLING The Franklin Institute Laboratories DISLOCATIONS AND MECHANICAL PROPERTIES OF CRYSTALS, edited b y J. C. Fisher, W. G. Johnson, R. Thomson and T. Vreeland. 634 pages, illustrations, 6 X9 in. New York, John Wiley & Sons, hlc., 1957. Price, $15.00. This book is the result of a small international conference held Sept. 6-8, 1956 at Lake Placid, New York initiated by N. F. M o t t and J. H. Hollomon who recognized the need to review the present state of research both theoretical and experimental on dislocations and their relationship to the mechanical behavior of crystals. It represents the final results of considerable recent work on dislo-