There exist two types of control techniques: open loop control and closed loop (feedback) control. Both strategies attempt to control the process by adjusting the adjustable(s) of the process.


In open loop control, the control action (the input) is predefined and is independent of the process outcome (the output). As such, open loop control systems do not have any sensor to measure the process output. Open loop controllers are relatively straightforward and inexpensive but limited to situations where events are quite predictable.  Timer based cycles on household appliances such as dishwashers and washing machines are examples of open loop control.  As long as the cycle is selected correctly, the outcome (clean dishes or clean laundry) will be acceptable, i.e. the open loop control provides adequate performance.

Closed-Loop Controllers

In closed-loop control (or feedback control), the control action depends on the output. Closed loop control involves measuring the output and continuously adjusting the input to keep the measured output at or within a pre-defined range.  Closed-loop controllers have many advantages over open-loop controllers, including but not limited to a) disturbance rejection, b) stabilizing unstable processes, c) enhanced reference tracking, and d) process dynamics modification. A commonly used closed-loop controller scheme is the Proportional+Integral+Derivative (PID) control.

To further improve reference tracking performance, closed-loop and open-loop control can be used, simultaneously. In such systems, the open-loop control is called feedforward control. This combination is commonly used in motion control applications.

In some applications, a control system can be put together with just having an appreciation of the behavior of the process to be controlled and the right hardware. But there are applications which need advanced control systems knowledge for effective solutions. DEICON has the expertise and experience in advanced control systems design and implementation for such systems.

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