The history of control systems and servomechanism design is rooted in the essential human quest to automate and regulate processes. Long before the advent of modern control theory, engineers and inventors grappled with the fundamental challenge of achieving stability and precision in mechanical systems.
The origins of automatic control can be traced to ancient times. Early examples include the float valve used in ancient Greek and Roman aqueducts to regulate water flow. Centuries later, during the Islamic Golden Age, scholars like al-Jazari devised water clocks with feedback mechanisms. These rudimentary systems demonstrated basic principles of feedback control, though the concept itself had yet to be formally articulated.
The Industrial Revolution accelerated the development of mechanical control systems. James Watt's steam engine governor, invented in the late 18th century, is often cited as a pivotal innovation. The governor used centrifugal force to regulate engine speed, maintaining a balance between power and stability. However, as industries demanded more precise and responsive systems, engineers encountered increasingly complex stability challenges.
By the early 20th century, the emergence of electrical engineering introduced servomechanisms—automatic devices that use feedback to achieve desired motion or position—became central to military and industrial advancements. Systems for gun aiming, aircraft stabilization, and ship navigation required not only mechanical precision but also rapid, reliable control responses. Engineers began to adopt mathematical analysis to predict system behaviors, yet tools for visualizing and designing stable systems were still rudimentary.
In 1948, Walter R. Evans gave engineers a powerful new method to understand and design for system stability: the root locus method, Into Stability tells the story of how his insight changed the way control systems are designed. Blending biography and engineering history, the book follows Evans through a pivotal decade of invention and impact.
In 1948, Walter R. Evans gave engineers a powerful new method to understand and design for system stability: the root-locus method. First sketched on a classroom blackboard in 1948, root locus transformed a field that had relied on formulas and intuition into one where stability and performance could be visualized directly. It became both a practical tool and a teaching language, making its way into textbooks, classrooms, laboratories, and eventually into the software that every engineer uses today.
Blending biography and engineering history, the book follows Evans through a pivotal decade of invention and impact. But Into Stability is about more than a method. It is about the life and character of the man who created it. Walter Evans grew up in a family that prized education and problem-solving, studied under teachers who opened doors to new ways of thinking, and found in Arline Pillisch a partner whose steady support carried him through both triumphs and trials.
His career began in wartime laboratories at General Electric, moved into the booming aerospace industry at North American Aviation, and was shaped by friendships, mentors, and students. The invention of root locus did not happen in isolation—it grew out of these roots, was refined through feedback, and ultimately proved itself by bringing stability to both systems and lives.
The arc of the book follows these stages:
Part I: Roots
The foundations of Walter Evans’s thinking—family heritage, education, and the crucible of early career experiences— combined with the entrepreneurial environment of North American Aviation. These "roots" shaped both the man and the method.
Part II: Feedback
No innovation takes hold without conversation and correction. Between 1944 and 1954, Evans's ideas encounter- treed the influence of colleagues, students, and competitors. The questions of how to teach, refine, and spread root locus circulated in a feedback loop.
Part III: Stability
This section reflects on Evans as engineer and father, and on the steady rhythm of his life. It gathers reflections from colleagues, connecting the technical legacy to the personal qualities-humor, humility, integrity-that defined Walter Evans.
Epilogue
Tested by a stroke that left him disabled but never defeated, with Arline at his side, he demonstrated resilience and a joy in living, setting an example for young and healthy and infirm alike.
Appendices
Supporting material, technical notes, firsthand accounts and background documents for those who want to dig deeper into the details of the root-locus method and its developer, Walter Evans.
My Stories of Dad
Personal recollections from my perspective as his son. These stories bring forward the father who inspired by example.
Seventy-five years after its first publication in AIEE Transactions, root locus remains one of the most widely taught and used tools in control system design. Whether manually plotted with a Spirule or generated instantly by software, it carries forward the clarity and ingenuity of Walter Evans. This book is the record of how it came to be and the life that made it possible.
