Co-Creator of Control Loop Tuning Equation Dies

Inspired innovators live on in their enduring discoveries. Such is especially true for John G. Ziegler, 88, the father of modern process control loop tuning, who died Dec. 9, 1997, at his retirement home in Scottsdale, Ariz.A literal cornerstone of the control field, Mr.

01/01/1998


Inspired innovators live on in their enduring discoveries. Such is especially true for John G. Ziegler, 88, the father of modern process control loop tuning, who died Dec. 9, 1997, at his retirement home in Scottsdale, Ariz.

A literal cornerstone of the control field, Mr. Ziegler is revered by engineers worldwide for essentially setting the stage for much of the process control field as it exists today. While working at Taylor Instrument Co. (Rochester, N.Y.), Mr. Ziegler and Nathaniel B. Nichols experimented with many processes and eventually devised the Ziegler-Nichols Closed Loop method and equation.

Mr. Ziegler developed the empirical method and hardware, while Mr. Nichols developed the mathematics to support the equation. Their work was first described in Ziegler and Nichols' now-famous 1942 paper, 'Optimum Settings for Automatic Controllers,' which was published in ASME [American Society of Mechanical Engineers] Transactions , Vol. 64. The hardware that Mr. Ziegler developed to make the Z-N method successful included the first pneumatic controller with reset (integral) added to the proportional (gain) control, as well as the first controller with rate (derivative) function on the same device.

Mr. Ziegler later formed his own company, Ziegler & Associates, in the mid-1960s. The company supplied insertion sensors for in-line measurement, and it was eventually sold to Seco Control Systems (Concord, Calif.). Mr. Ziegler also helped develop the Durbi Dimeter for measuring low levels of suspended solids in fluids, a suspended solids monitor, a syrup concentration monitor, a sugar over-saturation monitor, and sugar consistency monitor/viscometer.

An equally towering figure in the control field, Mr. Nichols went on to work at MIT's Radiation Lab during World War II and later in the aerospace industry for many years. Mr. Nichols, 82, died April 17, 1997, at his home in El Segundo, Calif.

The Ziegler-Nichols method and equation was considered a breakthrough because it was the first systematic, mathematical procedure for tuning control loops. Though it became one of the most common tuning methods, the Z-N equation has itself been fine tuned over the decades. However, the equation has endured as one of the foundations of modern process control because it was based on sound mathematics. The Z-N method and equation are still taught in most academic and corporate control engineering departments, such as those at DuPont and Monsanto.

'There were about 15 other mathematical routines that could be used instead of the Ziegler-Nichols method, and each was tried and championed—only to succumb to the simplicity and ease of use of Z-N tuning,' says George Blickley, Control Engineering 's consulting editor. 'The control industry practically snubbed the Z-N method when it was introduced to the ASME, but it soon gained wide acceptance over intuitive and flawed methods used at the time. If there is ever a museum built to honor PID, statues of John Ziegler and Nat Nichols should be at the entrance.'

The open-loop method is useful for most process control loops. To use the method the loop is tested with the controller in automatic. The Z-N Closed Loop method determines the gain at which a loop with proportional only control will oscillate and then derives the controller gain, reset, and derivative values from the gain at which the oscillations are sustained and period of oscillation at that gain.

'Control tuning was very chaotic before Z-N tuning because it was all based on guess-and-test. The equation allowed engineers to conduct methodical, mathematically based tuning,' says David St. Clair, an engineer and Z-N equation fan and historian. 'There was even an ASME panel on how to tune loops at that time, and the Z-N equation won hands down over all other methods. It was no contest.'

Mr. St. Clair is currently seeking input from engineers on what the Z-N paper, method, and equation have meant to them. He plans to post comments received on a web site and send them to the families of both Mr. Ziegler and Mr. Nichols. Send contributions to dwstclair@aol.com .

'Though many people have honed the Z-N method since it was invented, it is still the same essential tool,' says Mr. St. Clair. 'One of Mr. Ziegler's friends, Larry Braun, has said the Z-N equation is the 'Rosetta Stone' of process control, and that is truly what it is.'

To view the original 'Reference Guide to PID Tuning' Click here .





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