Open and Closed Loop Control: What They Are and How They Work

Open and Closed Loop Control

A control loop is a process management system that maintains a variable at a desired set point.

There are two types of control loops: Open Loop Control and Closed Loop Control.

How Open Loop Control Workscontrol loop, open loop control

In Open Loop Control, there is no direct measurement of the process variable available for use in making compensating adjustments to the input of the system.

Inputs come in and travel through a timer system that makes an operation run. It goes through the process and then we have an output.

One household example of this is a clothes dryer. In older style dryers, you would put wet clothes into the dryer, turn the timer dial, and the dryer would run until the timer ran out. Then you would touch the clothes to see if they were dry. If they were not, you would have to re-start the dryer via the timer dial.

This means any adjustments you need to make will be manual adjustments based on inspection.

Benefits and Challenges of Using Open Loop Control

  • Simplest form of control
  • Low cost
  • Reliable
  • Inherent loop stability
  • As long as the load flow does not change, there is no variation in the process variable
  • Can be operator dependent
  • Long term drift can be a problem​

In oil and gas, systems that employ open loop include mechanical valves and ball valves.

How Closed Loop Control Works

control loop, closed loop controlIn Closed Loop Control, there is direct measurement of the process variable allowing for feedback to make compensating adjustments to the input of the system maintaining control of the process to a desired set point.​

Inputs come in and travel through the process. Then as they come out there will be a feedback loop, which will be provided by some kind of sensor. This will make adjustments to what’s happening in the process.

So in our dryer example, newer dryers come equipped with moisture sensors. You start your dryer and it has a moisture set point it is trying to get the clothes do.

As the clothes go through the process, the moisture sensor measures the moisture of the clothes and will not stop until the clothes are dried to that set point.

Benefits and Challenges of Using Closed Loop Control

  • Can handle frequent load fluctuations
  • Automatic control of the process variable
  • No operator needed
  • More accurate & consistent control
  • Introduces loop instability

Most Kimray control products are part of a closed loop system, where they use pneumatic, electric, or mechanical signals to adjust to production conditions and achieve a set point.

​What is Gain?

One term we use often in control loops is “Gain.”

Gain refers to the ratio of a change in output magnitude to the change in input magnitude of a process. Gain is a measurement of sensitivity of an element to changes in the input.

Equipment or processes with high gain are very sensitive.

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