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Co-innovating Tomorrow PowerPoint Template 04-2016

PID Control BasicsPID TuningRob SinkTechnical Support SpecialistJune 14th, 2016

Copyright Yokogawa Corporation of America #

1

What will be covered:Common Process Control TechniquesProcess DynamicsWhat is PIDPID Control ComponentsHow to Tune a PID Loop

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2

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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3

Common Process Control Techniques quesManual Control

ON / OFF Control

Closed Loop Control

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4

Manual ControlOperator observes the process error and adjusts the control output

PID CONTROL

Set Point

Measurement(Process Variable)

Process

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In manual control, an operator monitors the difference between the process variable and the setpoint.

The operator then makes changes to the control output to reduce or eliminate the error.5

ON / OFF ControlSimplest form of feed back control

Can be used for processes not requiring extremely tight control

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ON/Off control is the simplest form of feedback control. In this type of control, the output is driven from fully closed to fully open depending on the relationship of the process variable to the setpoint and hysteresis.

Every one listening today has experience with the thermostat in their home which is a common example of simple on off control.6

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

Final Control Element

PVSet Point

Measurement(Process Variable)

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PID control is a form of closed loop control. In closed loop control, the process variable (PV) is measured and compared to the desired value called setpoint (SP). The controller changes its output, or manipulated variable (MV), until the measured variable equals the setpoint.

Because process dynamics vary greatly and the controllers are made to be universal, controllers muse be TUNED to match the process.

7

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

OutputProcess Variable

Lag Time

DeadTime

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For example, lets say we have a large tank of liquid for which we are controlling the temperature. When the output comes on, if there is no change in temperature for 10 minutes, then the dead time of the process is 10 minutes.8

Process Dynamics: Lag TimeLag is defined as the time required for the process variable to adjust to a steady state after an output change is performedLag time affects the control action

OutputProcess Variable

Lag Time

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9

Process Dynamics: Output vs. Process Change

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In a direct acting process, as the PV increase towards the SP, the output also increases. A common direct acting process is chiller being used for air conditioning.

In a reverse acting process, as the PV increases towards the SP, the output decreases. Flow control, furnaces, and pressure are all examples of direct acting processes.10

What is PID?

PID control refers to process control using the coefficients Proportional, Integral and DerivativeIt is not P&ID which refers to Piping & Instrumentation Diagram

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11

PID Control DefinedPID 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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The concept of PID control and the terms associated with it will be the same whether we are talking about simple single loop controllers, more advanced programmable controllers, PLCs and distributed control systems.12

PID Control TermsProportional 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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Put simply:

The proportional band adjusts the output amplitude.

The integral reduces or eliminates any error between the setpoint and process variable.

And the Derivative monitors the rate of change of the error in an attempt to anticipate process upsets.13

Proportional BandThe 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 setpointSome manufacturers use Gain instead of Proportional Band

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Proportional Band is expressed as a % of the full operating span of the controller and is centered around the SP assuming that the manual reset is set to 50%.

For example, an operating range of 0-1,000 with a P of 5%, would equal a P of 50 straddled 25 above and below the setpoint.

Lets say that this is a reverse acting process like a furnace with a SP of 500. If the PV drops to 475 or below, the output will go to 100%. Similarly, if the PV rises to 525 or above, the output will go to 0%.

The output will stay at these extreme values until the PV re-enters the proportional band between 475 and 525. When the temperature is within the proportional band, the output is PORTIONAL to the error.

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Proportional with Manual ResetWith 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.

1000

Manual Reset Adjusted Here500 Set PointProportionalBandTime

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Using proportional only control, once the optimum proportional band is set, the process variable will be offset from the setpoint.

Manual reset can them be used to shift the control output to compensate for the offset.

Manual reset is available on all Yokogawa controllers, with a default to 50%.

Using manual reset doesnt provide very stable control. As parameters in the environment, or process system changes through the day, the process variable can drift.

To prevent having to constantly change the setpoint, we need an automatic reset.

15

IntegralIntegral action is used with proportional to eliminate the inherent offsetThe integrating term observes how long the error has existed, summing the error over timeThe sum becomes a value added to the outputOutput

Time200 sec/repeat

Integral ActionProportional ActionIntegral Time Constant

Error

-10%+10%

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16

Integral cont.Engineering units: Repeats/minuteMinutes/repeatSeconds/repeatThe integral action ceases at a no error condition

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The smaller the integral number the more often the proportional action we be repeated.

If integral is too small, the process variable will oscillate through the set point and create erratic control action.

If the number is too large, the action will be sluggish and unable to compensate for process upsets.

The integral number should be approximately 5 times the dead/lag time of the process variable.17

Integral at Work

IIIIIntegral started.

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

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The integral term continues to increase or decrease the output until a zero error condition is obtained.18

A Note About Integral WindupIntegral 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 saturationTypical 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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Sometimes referred to as reset windup, integral windup occurs when the integral action continues to add to the control output past the operational range of the valve, variable speed drive, heater, etc.

(READ LAST POINT ON SLIDE)

19

DerivativeEngineering units: minutes or secondsAnticipates the error rate and applies the brakesDerivative has no effect if the error is constant

Output

Time50 seconds

Derivative ActionIntegral ActionDerivative Time Constant

Error-10%+10%

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Derivative can simply be thought of as a prediction of the error in the future based on the time set.20

P, I and D Working Together

P only

P and IP I D

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21

How to Tune a PID LoopManually tuning the loop

Using the contro