Factory engineers once tuned automatic industrial valves through stressful trial-and-error that frequently blew out boilers and ruined chemical batches; John Ziegler and Nathaniel Nichols created simple mathematical formulas to tune PID controllers for optimal stability. Published in 1942, the Ziegler-Nichols tuning rules established the operational foundation for modern industrial automation, controlling over ninety percent of all automated factory loops, chemical plants, and thermostats in the world.
In the massive factory build-up of World War II, automated pneumatic controllers were introduced to regulate temperatures in oil refineries and steam pressures in chemical reactors. However, setting the knobs on these automatic controllers was pure guesswork—if an engineer turned a knob too far, the boiler went into violent oscillations and exploded.
Engineers John Ziegler and Nathaniel Nichols discovered two practical tuning shortcuts. By pushing a controller just to the brink of continuous oscillation and measuring that single critical frequency, their simple table of mathematical formulas instantly tells the engineer the exact knob settings needed to stabilize the machine with a graceful quarter-decay response.
Ziegler-Nichols tuning became the most famous rule of thumb in industrial automation. By regulating furnace temperatures in steel mills, by maintaining precise flight speeds in aircraft cruise control, and by controlling life-support systems in hospitals, PID tuning keeps modern industry running smoothly.
Optimum Settings for Automatic Controllers
Abstract In this paper, the three principal control effects found in present controllers are examined and practical names and units of measurement are proposed for each effect. Corresponding units are proposed for a classification of industrial processes in terms of the two principal characteristics affecting their controllability. Formulas are given which enable the controller settings to be determined from the experimental or calculated values of the lag and unit reaction rate of the process to be controlled. These units form the basis of a quick method for adjusting a controller on the job. The effect of varying each controller setting is shown in a series of chart records. It is believed that the conceptions of control presented in this paper will be of assistance in the adjustment of existing controller applications and in the design of new installations.
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