control loop

What is a control loop? How does a control valve in a control loop affect its transmitter in a control loop when both are in the normal operating condition?

Rgds man_4422

jee snipped-for-privacy@pacific.net.sg (man_4422) wrote in news: snipped-for-privacy@posting.google.com:

For answers to your specific questions, more detail is needed. What type of valve?...Hydraulic...or pneumatic, what type of transmitter?

A closed circuit control loop could be simply explained as a circuit, where you have an input, a control unit that creates a change from the input, a measuring device to tell the control unit how much change actually took place, which feeds back to the control unit that corrects for inaccuracies in the commanded change vs. the actual change.

An open circuit control loop is one where a input is sent, the control makes the change, but there is no feedback, and hence, no correction.

Dear man_4422:

A control loop is comprised of two major components:

- a measurement device,

- a process device that ultimately ends up affect the measurement. Beyond this, there are a plethora of implementations. Some type of feedback is required to control the process device, and achieve some sort of value at the measurement device.

As to your second question, the other responder is correct, more is required to be known.

As an example, I implemented the following control scheme: Ozone generator, with a requirement to make x ppd at y wt%. One control loop measured pressure in the generation space, and altered control valve opening to bleed gas out to maintain the set pressure (ozone output ot the process). Positive feedback: more pressure requires more valve opening. A second control loop determined the mass inlet flow rate from the numeric targets above, and allowed gas to flow in through a control valve achive the desired measured mass flow rate. Negative feedback: more flow requires less valve opening. A third control loop used the wt% number, and controlled applied power to the power supply, and measured output concentration. Negative feedback: more concentration requires less power applied.

Note that feedback can be interpreted as either positive or negative. An error in measured value will be used to either increase or decrease control valve position.

David A. Smith