When to Use Closed-loop Control Instead of Open-loop Control

Tech Note

Control operations can be either closed loop or open loop. The key difference is feedback. An open-loop control system (also known as a non-feedback system) acts completely on the basis of input; the output has no effect on the control action. A closed-loop control system looks at the current output and alters it to the desired condition; also known as a feedback system, the control action in these systems is based on the output.

Each control type has its advantages and is best suited to particular types of control operations.

Here are some key signs that your control system should be running in closed loop, rather than open loop, fully automated, rather than with human intervention.

As control requires measurement, either direct or inferred, 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."
Before a closed-loop control system can be designed or tuned, all parts must be in good working order, so sticking valves, gear backlash, and erratic sensors, for example, must be fixed. When possible, dead times, such as transportation lag, should also be minimized, and shorter pipes are therefore a plus.

While closed-loop control systems may be expensive, they are usually less expensive than using human controllers. Consider, for example, the Manhattan Project, which used human controllers 24/7 to continuously adjust set points.

The advantages closed-loop control offers, however, are significant:
  • A process can be kept on set point within a given accuracy.
  • Corrections to process disturbances are automated.
  • Unstable processes can be stabilized.
On the other hand, open-loop control is the better choice when:
  1. Low cost is a priority, as open-loop control is inexpensive.
  2. An output changes rarely or not at all, for example, certain cooling pumps.
  3. No quantitative measurement is possible, as with an inaccessible process.
  4. A process is erratic, for example, a process with a sticking valve or erratic sensor.
  5. Process disturbances are extremely rare.
To ensure the processes in your operation receive the most effective control, it is essential to examine the pros and cons of closed-loop control vs. open-loop control for each particular situation.

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