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Closed-Loop Control Vs. Open-Loop Control

September 26, 2017

Writers at Dataforth shared a useful technical note about when it is best to use closed-loop control instead of open-loop. Below is a quick summary of the article followed by the official link to the article/Dataforth's website.

The one differential characteristic between closed-loop or open-loop control is feedback. A non-feedback system or a open-loop control system acts on the basis of input; the output has no effect. A closed-loop is of course the opposite. A closed-loop control system looks at the current output and changes it to the desired condition; also known as a feedback system, the control action in these systems is based on the output.

According to the authors at Dataforth, there are several key signs that your control system should be running in closed-loop rather than open, fully automated, rather than with human intervention.

Closed-loop control is the operation of choice when:

  1. Measurement is feasible.
  2. The process has a degree of predictability, i.e., a known approximate response to an input control or controls.
  3. An output varies from a desired outcome and is not "Set and Forget".

Closed-loop control systems are expensive however, they are usually less expensive than using human controllers.

The advantages of closed-loop control are as follows:

  • A process can be kept on set point within a given accuracy.
  • Corrections to process disturbances are automated.
  • Unstable processes can be stabilized.

What about when open-loop control is the better choice?

  1. Low cost is a priority, as open-loop control is inexpensive.
  2. An output changes rarely or not at all.
  3. No quantitative measurement is possible, as with an inaccessible process.
  4. A process is erratic.
  5. Process disturbances are extremely rare.

To receive the most effective control, be sure to weigh the pros and cons for each situation!

View "When to Use Closed-loop Control Instead of Open-loop Control" by Dataforth, 2017

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