FOREWORD

  Two things came together in the late 1940s: The remarkable young people in the auto-navigator division of North American Aviation's Aerophysics Laboratory and one of its new leaders, Walter Evans. This team was solving very difficult engineering problems one after another, to produce, for the first time anywhere, precise navigation systems for very long-range, unmanned aircraft and for submarines that went exactly to the North Pole, among many firsts. These were very hard systems to achieve. 1

Control systems had to remain stable under conditions and deliver precise performance at speeds and ranges never before attempted. Classical analysis tools—while mathematically sound—provided little intuitive guidance. And perhaps the biggest gap: While it was clear that poles and zeros controlled a system’s behavior, there wasn’t a reliable way to move them around on the complex plane. That made it hard to design for specific dynamics, especially when stability had to be rock-solid. (Note to reader: Refer to Appendix 1 for Professor Cannon’s mathematical explanation of Evans’s root-locus method.)

 Walter Evans (1920–1999), better known to his colleagues as Walt, was a person of remarkable insight and a major leader in the understanding of automatic control and how to design excellent systems very well and very quickly. One of his field-leading contributions was the invention of the root-locus method for seeing instantly the natural dynamic behavior a linear system will have, seeing it directly in terms of the control parameters at the designer's disposal. The method presents—in seconds—a plot of the system's stability, speed of response, and the damping quality of all of its natural motions.  

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