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Page 1: PID Control Basics

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PID Control Basics

PID Tuning

Rob SinkTechnical Support Specialist

June 14th, 2016

Page 2: PID Control Basics

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What will be covered:1. Common Process Control Techniques2. Process Dynamics3. What is PID4. PID Control Components5. How to Tune a PID Loop

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Why do I Need to Understand PID

Every process is different

Makes manual tuning easier

Helps companies save money

Helps facilities remain safe

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Common Process Control Techniques ques

Manual Control

ON / OFF Control

Closed Loop Control

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Manual Control

Operator observes the process error and adjusts the control output

PID CONTROL

Set Point

Measurement(Process Variable)

Process

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ON / OFF Control

Simplest form of feed back control

Can be used for processes not requiring extremely tight control

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Closed Loop Control

The PID controller measures the process variable, compares it to the setpoint and then manipulates the output accordingly.

Final Control Element

PV

Set Point

Measurement(Process Variable)

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Process Dynamics: Dead Time

Dead time is defined as the time before the process variable BEGINS to react to a change in the control output

Output

Process Variable

Lag TimeDeadTime

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Process Dynamics: Lag Time

Lag is defined as the time required for the process variable to adjust to a steady state after an output change is performed

Lag time affects the control action

Output

Process Variable

Lag Time

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Process Dynamics: Output vs. Process Change

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What is PID?

PID control refers to process control using the coefficients Proportional, Integral and Derivative

It is not P&ID which refers to Piping & Instrumentation Diagram

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PID Control Defined

PID control can be described as a set of rules with which a precise regulation of a closed-loop control system is obtained.

Temp (PV)

TempSetpoint (SP)

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PID Control Terms

Proportional Band adjusts output amplitude (reciprocal of Gain)

Integral eliminates offset error (automatic Reset or simply Reset)

Derivative looks at the rate of change of the error (Rate)

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Proportional Band

The Proportional Band (P) is defined as the range over which the control output is adjusted from 0-100%

Proportional does the heavy lifting getting the temperature close to the setpoint

Some manufacturers use Gain instead of Proportional Band

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Proportional with Manual Reset

With proportional only control, an offset will be present between set point and process variable. Manual Reset allows a user to bias or shift the output to compensate for the steady state offset.

1000º

Manual Reset Adjusted Here

500º Set Point

ProportionalBand

Time

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Integral

Integral action is used with proportional to eliminate the inherent offset

The integrating term observes how long the error has existed, summing the error over time

The sum becomes a value added to the output

Output

Time 200 sec/repeat

Integral Action

Proportional Action

Integral Time Constant

Error-10%

+10%

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Integral – cont.

Engineering units: Repeats/minuteMinutes/repeat

Seconds/repeat The integral action ceases at a no error condition

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Integral at Work

I I II

Integral started.

Setpoint

Each time period where the error is not zero, the output is increased (or decreased) by the Integral term.

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A Note About Integral Windup

Integral windup refers to the situation in a PID controller where the integral, or reset action continues to integrate (ramp) indefinitely

This usually occurs when the controller's output can no longer affect the controlled variable, which in turn can be caused by controller saturation

Typical causes of Integral Windup are: The input has been removed from the process, output device has failed, a furnace door has been opened keeping the process from reaching temperature

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Derivative

Engineering units: minutes or seconds Anticipates the error rate and applies the “brakes” Derivative has no effect if the error is constant

Output

Time 50 seconds

Derivative Action

Integral Action

Derivative Time Constant

Error-10%

+10%

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P, I and D Working Together

P only P and I P I D

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How to Tune a PID Loop

Manually tuning the loop

Using the controllers Auto/Self Tune

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Manually Tuning a Loop

These values are good starting points Change only (1) term at a time Make small changes observing the result

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Fine-tuning the Proportional Band

Work from larger to smaller numbers (wider to narrower)

If cycling appears, the proportional band is too narrow

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Fine Tuning the Integral Time

The main goal is to reduce the offset

Adjust from longer to shorter time

If an oscillation exists at a longer period then the integral time is too short

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Fine Tuning the Derivative Time Adjust from shorter to longer time

If short-period oscillations develop, the time is to long.

The larger the Derivative, the stronger the corrective action and the more likely the output will become oscillatory

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Tuning Loops with Dead Time

Output

Process Variable

Lag TimeDeadTime

Set P to 5% and the I & D to 0%

Start the process with a setpoint that will allow the process variable to stabilize

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Tuning Fast Reacting Loops

Set P to 100% and the I & D to 0%

Start the process with a setpoint that will allow the process variable to stabilize

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Using Auto Tune to Determine PID Values

The output is varied between 0% and 100% three times (these values may be limited). The process variable must ascend and descend through set point for the output to change state. The auto tune algorithm observes the PV response to these output changes and installs the appropriate PID terms.

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Ways to Prevent Overshoot

Limit the working output or enable an output ramp rate (if available)

Limit the output range which will have an effect on the time it takes to get to setpoint

Ramp the setpoint at a slow rate

Use fuzzy logic (if available)

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Fuzzy Logic

Fuzzy logic is used to help reduce setpoint overshoot

Used in addition to PID control

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Yokogawa Products that Use PID Control

Single loop controller

Programmable controller

PLC

DCSPLC/RTU

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UTAdvanced Line of controllers

1-2 loops of control

Built in ladder sequence control

Software used in Webinar

Nuclear qualified

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YS1000 Family of Controllers

1-2 loops of control

Nuclear qualified

Hard manual backup

Function block programming

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FA-M3 PLC

Modular PLC design

4 control loops per PID module

PID control is not done in ladder logic

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Questions

Questions?Feel free to email us with further questionsat [email protected] put “PID Webinar” in the subject line.

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Thank you for attending!

Feel free to email us with further questionsat [email protected] put “PID Webinar” in the subject line.


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