PRT 140: Lesson 8 Introduction to Control Loops



  • Describe Process Control
  • Explain the function of a control loop
  • Compare “Closed Loops’ and “Open Loops’
  • Identify the components of a control loop
  • Describe signal transmission types


Terms to Know

  • Setpoint
  • Open Loop, Closed Loop, Feedback
  • Control, Measure, Manipulate
  • Sensor, Transmitter, Controller, Transducer, Final Control Element
  • Live Zero
  • Loop Error

What is Process Control?

The act of regulating one or more process variables so that a product of a desired quality can be produced’

How to control a process variable?

  1. sense/measure it
  2. compare to the desired value, ‘setpoint’
  3. calculate necessary change — the error
  4. make the change — correction


Controlled — sense this value to initiate signal

Measured — determine actual condition of variable

Manipulated — adjust a quantity or condition

Not always the same process variable — not always the same process stream.


  • Instrument provides data
  • No connection to the change in the process — someone has to open/close the valve
    • No ‘feedback’
  • “Manual’ mode
A diagram of an open control loop
An open control loop
[image 140-8-01]


  • Instrument provides data, and also determines the necessary corrections to make
  • Instruments control the valve position
  • ‘Feedback’ — as level changes, control loop will register the change, and valve position will change as needed
  • “Automatic’ mode
A diagram of a closed control loop
A closed control loop
[image 140-8-03]
A diagram of a control loop block flow
Control Loop Block Flow
[image 140-8-5]

Control Loop – Components

Sensor Sensing
Transmitter Converting, Transmitting
Controller Compare, Calculate, Correct
Transducer Converting (signal type)
Final Control Element Manipulating
Indicator Displaying (values)
Computer Calculating, Converting

Sensor     (FE, TE, PE, LE, etc)

Flow Sensor in a Control Loop
[Image 140-8-06]

Transmitter (FT, TT, PT, LT, etc)

Flow Transmitter in a control loop
[image 140-8-7]

Controller (FC, TIC, PC, LIC, etc)

Flow Indicating Controller in a control loop
[Image 140-8-8]

Transducer (FY, TY, PY, LY, etc)

I/P Transducer in a control loop
[Image 140-8-9]

Final Control Element (FCV, etc…)

Final Control Element (pneumatic control valve) in a control loop
[Image 140-8-10]

Signal Types

  • Pneumatic — gas   std. 3-15 psig
  • Electronic — analog signal   std. 4-20 mA, 1-5 VDC
    • Often uses the same wires that provide power to instrument
  • Digital — binary — computerized — no std. range
  • Mechanical — physical linkage — no std. range

Signal Types on PID’s – Recap

Identify the analog electrical, digital, and pneumatic signals in this loop:

A diagram depicting various signal types in a loop
Various signal types in a loop
[image 140-8-11]

LIVE ZERO — Why isn’t 0 just 0?

  • 3-15 psig, 4-20 mA, 1-5 VDC — why not 0-12, 0-16, 0-4?
  • If 0 is 0, how do you tell the difference between a reading of 0 and a dead transmitter?
  • If 0 is 0, how do you handle any values <0?
  • How do you calibrate <0?
  • Remember that the range of an instrument is not necessarily 0-something — usually has a LRV and URV, so 0 doesn’t enter into it.

Control Loop Error

  • Each component in the loop has an error factor.
  • Cumulative error = Loop Error
  • Eloop = √[(E1)2 + (E2)2 + (E3)2 …(En)2]
  • Where E1, E2, …En = errors of all components in the loop.

Sample Problem, Loop Error

A control loop is composed of a transmitter (accuracy 0.5%); controller (accuracy 0.25%); I/P Transducer (accuracy 0.5%); and control valve (accuracy 1.5%).

  • Error = √[0.52 + 0.252 + 0.52 + 1.52]
  • Error = √[0.25 + .0625 + 0.25 + 2.25]
  • Error = √[2.8125]
  • Error = 1.68%

Accuracy calculation

[(measured value — true value)/(true value)] x 100%

“Accuracy’ is usually expressed as “accurate +/- x%’.

It doesn’t matter if the value from the calculation is positive or negative…

Sample Problem, accuracy:

Pressure gauge true value is 100 psig, and it is reading 98 psig

  • [(98-100)/100] x 100%
  • [-.02] x 100%
  • -2%
  • Gauge is accurate +/- 2%

Loop Analysis procedure…

  1. List all instruments, full tag numbers
  2. Start at the sensing element, move through the loop to the final control element
  3. ‘Variable being controlled’ = variable being controlled OR manipulated
    • This variable changes as you move through a loop
    • Control Valves almost always manipulate FLOW

Flow Control Loop

Discuss components with class


Flow Control Loop
[image 140-8-12]
ComponentElement TypePV being controlledComponent Function (table 8-1)
FE-100Flow elementFlowSensing
FT-100Flow transmitterFlowConvert/Transmit
FC-100Flow controllerFlowCompare/Calc/Correct
FY-100Flow transducerFlowConvert signal
FCV-100Flow control valveFlowManipulating

Level Control Loop

  • Discuss components with class
Diagram for homework 11b
Level Control Loop
ComponentElement TypePV being controlledComponent Function (table 8-1)
LE-100Level elementLevelSense
LT-100Level transmitterLevelTransmit/convert
LC-100Level ControllerLevel/FlowCompare/calc/correct
LY-100Level transducerFlowConvert signal
LCV-100Level control valveFlowManipulate

Temperature Control Loop –

  • Discuss components with class
Temperature Control Loop
[Image 140-8-16]
ComponentElement TypePC being controlledComponent Function (table 8-1)
TTTemperature TransmitterTemperatureSensing/Convert/transmit
TICTemperature Indicating ControllerTemperature/flowCompare/calc/correct/display
TYTemperature transducerFlowConvert signal
TCVTemperature control valveFlowManipulate