What is On-Off Control?

On-Off Control Has a Role to Play in Industrial Automation…But Within Limits

On-Off Control is not a highly sophisticated control method. Even so, examples of its practical, everyday use can be found all around. Look no further than the appliances in your home. Kitchen appliances such as ovens and refrigerators utilize On-Off Control. Similarly the furnaces, water heaters, and air conditioners found in most basements rely on On-Off Control. Although it’s unsophisticated, On-Off Control plays a significant role in our lives.

Simplicity is the primary reason why On-Off Control is not used in commercial applications. Most homeowners would agree that a certain amount of variability in their oven’s temperature – as an example – is expected. Processes like an oven are simple. For one, they don’t interact with other processes. And second, if you’re a bad cook, then more precise control probably isn’t the answer. In contrast, industrial process manufacturing is complex. Processes are highly dynamic and interactive. Whether in terms of safety or economics, the benefits of more precise control are significant.

Following are a few aspects of On-Off Control that you should keep in mind when considering it for commercial application:

An Open or Shut Case

As its name implies, On-Off Control assigns the Controller Output (CO) to one of two positions such that the final control element (FCE) is either fully open or fully closed. Unlike intermediate value or PID control, there is no in between. Most industrial processes require greater latitude when it comes to adjusting the CO’s position.

Ups and Downs

On-Off Control can result in excessive variability as the controller has so few options for maintaining Set Point. A process equipped with On-Off Control will constantly overshoot its Set Point and cycle as a result. The work demanded of the FCE regularly accelerates the time to failure and increases maintenance costs.

Setting Boundaries

Deadband is a range of operation around the Set Point and within which the controller’s action will not change. On-Off Control with Deadband establishes upper and lower boundaries that are acceptable to the control loop’s operation. While Deadband’s “cushion” reduces wear on the FCE, variability remains within the process which can present challenges for other downstream processes. Most industrial processes require tighter control than is possible with On-Off Control. While it is frequently used in industrial HVAC and simple furnaces, On-Off Control is used sparingly in industrial process manufacturing. Given its ability to achieve tighter and more stable control, intermediate value PID-based control is well suited to industrial application requirements.   To learn more about these basic and advanced control strategies, consider taking a training course from a recognized authority on process control.

 

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