Know when to use open- or closed-loop control
Control operations can be either closed loop or open loop. The key difference between the two 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. Closed-loop control is better for automated settings while open-loop controls require some human intervention.
As control requires measurement, either direct or inferred, closed-loop control is the better choice when:
- Measurement is feasible.
- The process has a degree of predictability, i.e., a known approximate response to an input control or controls.
- 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. That means sticking valves, gear backlash, and erratic sensors 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 often less expensive than using human controllers because they have to continuously adjust set points.
The advantages closed-loop control offers include:
- A process can be kept on set point within a given accuracy.
- Corrections to process disturbances are automated.
- Unstable processes can be stabilized.
Open-loop control is the better choice when:
- Low cost is a priority; open-loop control is inexpensive.
- An output changes rarely or not at all, for example, certain cooling pumps.
- No quantitative measurement is possible, as with an inaccessible process.
- An erratic process such as a sticking valve or erratic sensor.
- Process disturbances are extremely rare.
To ensure the processes in an operation receive the most effective control, it is essential to examine the pros and cons of closed- vs. open-loop control for each particular situation.
Lee Payne is founder and president, Dataforth Corp. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, firstname.lastname@example.org.
- Feedback is the key difference between open- and closed-loop controls.
- Closed-loop control advantages include automatic corrections to process disturbances and they can be kept on a set point.
- Open-loop controls are useful for situations where output rarely changes and the process is erratic.
What other situations and settings are beneficial for open- or closed-loop control and why?