Automatic Closed Loop Control Systems

The previous page introduction to control systems ended with a description of a manual closed loop control system.

The next step is to remove the human from the equation. In the temperature control example, this is achieved with a thermostat:

This process is now fully automatic. The only thing a human has to do is set the thermostat to his desired temperature (the temperature setpoint).

Probably the earliest automatic closed loop control system was the water level regulator invented by the ancient Greeks. This uses a float valve to open a pipe if the level of water drops and close it when it is at setpoint:

As you can see here this principle is still in use for the incredibly important control challenge of keeping Fido well watered. When the greedy mutt drinks enough water the level drops, the float drops and the valve opens to fill the bowl until the level is high enough to raise the float and close the valve.

The next major automatic feedback system to be invented was the Watts Flyball Governor by James Watt. This controls the speed of steam engines and is shown below.

As the shaft (engine) speed increases the balls move out due to centrifugal force. A linkage connected to the balls converts this into a linear movement that closes the steam supply valve in proportionto the ball's movement. This reduces the steam to the engine and thus the speed. Think of it as cruise control for steam engines.

These examples are both fully mechanical. Nowadays control systems are mostly software driven which means they come in many different flavours and each vendor can and usually does implement them differently. This creates a challenge for those of us that have to modify, design or tune the control systems.

The next tutorial will show the stages of designing a modern digital control system, focussing on the most common form: The PID Control Loop.