It was not until 1922, however, that a formal control law for what we now call PID or three-term control was first developed using theoretical analysis, by
Russian American engineer
Nicolas Minorsky.
[9] Minorsky was researching and designing automatic ship steering for the US Navy and based his analysis on observations of a
helmsman. He noted the helmsman steered the ship based not only on the current course error but also on past error, as well as the current rate of change;
[10] this was then given a mathematical treatment by Minorsky.
[4] His goal was stability, not general control, which simplified the problem significantly. While proportional control provided stability against small disturbances, it was insufficient for dealing with a steady disturbance, notably a stiff gale (due to
steady-state error), which required adding the integral term. Finally, the derivative term was added to improve stability and control.
Trials were carried out on the
USS New Mexico, with the controllers controlling the
angular velocity (not the angle) of the rudder. PI control yielded sustained yaw (angular error) of ±2°. Adding the D element yielded a yaw error of ±1/6°, better than most helmsmen could achieve.
[11]
The Navy ultimately did not adopt the system due to resistance by personnel. Similar work was carried out and published by several others in the 1930s.
