Closed Loop Control

I have wondered for some time, why so many control engineers have problems with closed loop control systems. I think the answer is that these engineers are used to manual control systems. They do not realize that the loop propagation makes closed loop systems fundamentally different.

Let me explain you what I mean. In manual control systems, a human input (e.g. a joystick or lever) is converted into a signal that will drive an actuator (like a valve or a throttle) to run a machine. Although a manual control system may contain a lot of complex software, in essence it is a one way operation. That is why manual control systems are known as open loop control systems.

In automatic control systems, there is a feedback loop in which the response is measured and compared with a setpoint. Because of this feedback loop, automatic control systems are also known as closed loop control systems. It is this feedback loop that gives so many problems.

For example debugging. In open loop control systems, software errors are easy to find and solve. You simply track the control signal from the manual input to the actuator. At the point where the signal starts to deviate from the expected, the error is caused. An error in a closed loop control system is much harder to find. If somewhere in the system an error is caused, the resulting deviation will start to travel around the loop. Distinguishing between cause and result is very hard in closed loop systems. This loop propagation is why closed loop control systems are so different from open loop control systems.

If you want to know more, please read our product sheet on Closed Loop Control.