"A marriage of theory and practice"

Everything under control

Leonhard's greatest achievement is his work on control engineering. His book "Automatic control systems in electrical engineering", published in 1940 by the Springer publishing house, was reprinted in the USA in 1944, even before the end of the war, and was also translated into Turkish and Japanese. In Japan the book became a standard textbook and was reprinted several times.

The book brought together all aspects of control engineering, which until then had only been familiar within special application fields, and created a theoretical basis that was independent of any specific application. In his book, Leonhard was the first to describe a control loop as link between a controlled system and a control device, and introduced the system of illustrating control loops with flow diagrams that is still in use today. Control engineering systems based on these fundamental principles are found in practically all modern technical appliances, from cars to large-scale production facilities.

In a control loop a critical variable is monitored and regulated using one or more actuators. A simple practical example is a modern industrial water boiler in sanitary installations, in which the temperature of the water it contains is measured by a thermometer and kept constant by means of a heating element. The thermometer and heating element are part of the controlled system, and the control device has the task of comparing the measured value with the target value and activating the heating element if necessary.