WALTER M. WONHAM (1934-2023)

BY CRISTINA AMON AND DEEPA KUNDUR

WALTER MURRAY WONHAM, a control theorist whose pioneering research spanned multivariable geometric control, stochastic systems, and discrete-event control via formal languages, passed away on May 14, 2023, at age 88.

A University Professor Emeritus in the Edward S. Rogers Sr. Department of Electrical & Computer Engineering at the University of Toronto, Walter was widely recognized for the Wonham filter, the internal model principle, and the co-founding of supervisory control theory.

Born in Montreal in 1934, he attended high school in the city, where he developed a preference for independent pursuits over team sports — interests that later evolved into lifelong passions for sailing and tennis. He studied engineering physics at McGill University, earning his undergraduate degree in 1956, and completed his Ph.D. in control engineering at the University of Cambridge in 1961. Over the course of his early career, he held positions at Purdue University’s Control and Information Systems Laboratory, Martin Marietta’s Research Institute for Advanced Studies, Brown University, and the National Aeronautics and Space Administration’s (NASA) Office of Control Theory and Application.

In the 1960s, Walter emerged as a pioneer in stochastic control theory. His foundational work on the stochastic filter — now known as the Wonham filter — and the separation theorem remains widely cited, including in the emerging field of quantum systems. His insights into the Matrix Riccati equation and stochastic systems set a new standard for theoretical rigor in control engineering.

He is best known for his groundbreaking contributions to linear multivariable control. In 1967, he became the first to prove the pole assignment theorem, introducing the terms “stabilizability” and “detectability,” now fundamental to control theory curricula around the world. As Manfredi Maggiore observed, “He invented the state feedback stabilization problem.”

In 1970, after 15 years abroad, Walter returned to Canada to join the University of Toronto as an associate professor. He became a full professor in 1972 and went on to shape generations of researchers within the Systems Control Group. During his tenure, he introduced the geometric approach to linear time-invariant systems and co-formulated the internal model principle with his student Bruce Francis. This milestone offered an explicit formulation of the Conant and Ashby good regulator theorem, and the framework remains central to robust control design.

In the 1980s, recognizing the dawn of the digital age, he turned his focus to discrete-event systems. Alongside doctoral student Peter Ramadge — now a professor at Princeton University — he laid the foundations for supervisory control theory, a method for synthesizing control systems using automata and formal languages. This research culminated in several influential books, including his final volume, Supervisory Control of Discrete-Event Systems (Springer Cham, 2019), co-authored with Kai Cai.

Walter’s textbook Linear Multivariable Control: A Geometric Approach, first published in 1979 (Springer-Verlag), is widely considered one of the most elegant and influential texts in the field. Issued in three editions and translated into Russian and Chinese, it remains a cornerstone for control engineering education worldwide.

In 1996, he was appointed a University Professor—the University of Toronto’s highest academic honor—and became University Professor Emeritus upon his retirement in 2000. He held visiting lectureships at institutions around the world, including the Massachusetts Institute of Technology, Washington University, the Indian Institute of Technology in Kanpur, Zhejiang University, and many others. His lectures sparked curiosity and creativity in students across continents, and stories from his academic travels were warmly remembered upon his return.

His accolades included election as a foreign member of the National Academy of Engineering, fellow of the Royal Society of Canada, life fellow of the Institute of Electrical and Electronics Engineers (IEEE), and honorary professor at Beihang University. He received the IEEE Control Systems Science and Engineering Award in 1987, the Brouwer Medal in 1990, and the Giorgio Quazza Medal from the International Federation of Automatic Control in 2020.

Yet, beyond his intellectual brilliance, Walter was deeply admired for his character. His office door bore a simple sign — “Knock and enter” — an invitation that reflected his open-hearted generosity toward students and colleagues alike. He was polylingual, a bibliophile, a sailor, a skier, a tennis player, and a lover of classical Chinese poetry.

He is survived by his beloved wife, Anne, and their daughters, Cynthia and Marjorie. He will be remembered not only for his towering achievements but for the joy he found in discovery, the elegance of his ideas, and the profound influence he had on all who had the privilege of learning from him.