introduction

The history of control systems and servo-mechanism 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. This struggle defined the early trajectory of servo-mechanism development, paving the way for innovations like the root locus method that would later revolutionize the field.

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. Oscillations and hunting behaviors in mechanical systems underscored the need for a deeper understanding of feedback dynamics.

By the early 20th century, the emergence of electrical engineering introduced new layers of complexity to control systems. Servo-mechanisms—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 and manage system behaviors, yet tools for visualizing and designing stable systems were still rudimentary.

It was within this context of challenge and necessity that the root locus method was born. Offering a graphical means to trace the paths of system poles as parameters varied the method transformed control system design. It provided a bridge between mathematical rigor and intuitive visualization, empowering engineers to design stable systems with greater confidence and precision.

This book chronicles the invention of the root locus method, its early adoption, and its profound impact on the field of control systems. But before delving into that story, it is crucial to appreciate the historical landscape that shaped its development—a landscape marked by ingenuity, trial, and the relentless pursuit of control in an increasingly complex technological world. Realizing Root Locus is structured into three parts:

Part 1 Roots – The invention of the root locus method emerged from a confluence of factors: a lineage of analytical thinking passed down through family, the influence of inspirational teachers, a life partner who provided unwavering support, a habit of testing concepts through extreme cases, and the entrepreneurial environment at North American Aviation. Each of these was a "root" of root locus, shaping the way my father approached engineering. This section explores the experiences that shaped Evans’ thinking and laid the groundwork for the method.

Part 2 Feedback - No innovation stands alone. While Evans developed the method, others played crucial roles in refining and popularizing it. How should it be taught to engineers unfamiliar with the concept? Dad had strong opinions on these questions, but as with any evolving idea, external influences—feedback—shaped the outcome in unexpected ways. Understanding this process within the pivotal decade of 1944–1954 became central to this book.

Part 3 Stability – By 1954, the root locus method was widely adopted. Walter Evans’ personal life reached a steady, fulfilling rhythm. This section offers reflections on Evans the engineer and Evans the father through the eyes of his colleagues and the author. After his stroke, we see Walter and Arline as a team, demonstrating what it means to be true to ones values when facing challenges.

The story told in Realizing Root Locus is one of perseverance, ingenuity, and a relentless pursuit of understanding—qualities just as vital to engineering today as they were in Walter Evans’ time. Seventy-five years after its introduction, the root locus method remains relevant. Whether sketched by hand with a Spirule or rendered instantly by software, it continues to serve as one of the most powerful tools in control system design.

This is how it all came to be realized.