Control loop: Difference between revisions

Revision as of 17:43, 16 May 2021

Revision as of 05:21, 14 October 2022

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<ref>{{cite web|last1=Cooper|first1=Douglas|title=Control Guru|url=http://controlguru.com/the-components-of-a-control-loop/|website=Control Guru terminology|accessdate=16 September 2017}}</ref>

In this example, the value of the PV is always the same as the MV, as they are in series in the pipeline. However, if the feed from the valve was to a tank, and the controller function was to control the level using the fill valve, the PV would be the tank level, and the MV would be the flow to the tank.

The controller function can be a discrete controller, or a function block in a computerised control system such as a distributed control system or a [[programmable logic controller]]. In all cases, a control loop diagram is a very convenient and useful way of representing the control function and its interaction with plant. In practice at a process control level, control loops are normally abbreviated using standard symbols in a [[Piping and instrumentation diagram]], which shows all elements of the process measurement and control based on a [[process flow diagram]].<ref>{{cite web|title=Piping and instrumentation diagram P&ID|url=http://processflowsystems.com/piping-instrumentation-diagram-pid/|website=Process Flow Systems|accessdate=16 September 2017}}</ref>

At a detailed level the control loop connection diagram is created to show the electrical and pneumatic connections. This greatly aids diagnostics and repair, as all the connections for a single control function are on one diagram.

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Based on the standards [[ANSI]]/[[Instrument Society of America|ISA]] S5.1 and [[ISO 14617]]-6, the identifications consist of up to 5 letters.

The first identification letter is for the measured value, the second is a modifier, 3rd indicates the passive/readout function, 4th - active/output function, and the 5th is the function modifier. This is followed by loop number, which is unique to that loop.

For instance, FIC045 means it is the Flow Indicating Controller in control loop 045. This is also known as the "tag" identifier of the field device, which is normally given to the ___location and function of the instrument. The same loop may have FT045 - which is the flow transmitter in the same loop.

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