FOREWORD

Walter Evans invented the root locus method in 1948 and published it in 1950. At North American Aviation's Aerophysics Laboratory where Evans worked, it had already become the primary method for designing automatic pilots for high performance aircraft. It soon became widely known and used and took its place as the central context for the study and design of feedback control systems.

There are several reasons for that. The roots of a dynamic system's characteristic equation reveal directly and quantitatively the natural behavior it will have at what frequencies it will vibrate and how quickly the vibrations will damp out. By plotting the locus of these characteristic roots, as Evans' method does, versus the design parameter being chosen one can see precisely which values will give good behavior and which will not values will make the system unstable, for example. By plotting the locus of these roots versus the design parameter being chosen (control strength, for example) one sees precisely which values give good behavior and which will not. 

Working with Walt was enormously   and full of surprises. No period in my life did I learn real engineering more rapidly, or more deeply, or with more enjoyment. Walt shared his small office with Bill Mullins and me. That was one great office for a young, green fresh out of school guy to be in.It was in those days that Evans led the development and construction of the stable inertial platforms for the guidance systems. For each system Evans saw the many technical problems coming, and, in problem after problem, his agile mind made giant leaps over the details to the key to a good answer. After he'd left the office, Bill Mullins and I would sit there and scratch our heads. After about an hour we'd figure out what it was he'd just told us in five minutes. 

Above all, Walt Evans was devoted to his children and his wife, Arline, who was his super-supportive and brilliant companion throughout his life.