pid controllers

33
Shell Global Solutions Tuning Workshop PID Controllers

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Easiest and coolest way of explanation.working with diagrams and tables,must Read those who are from instrumentation background....

TRANSCRIPT

Page 1: PID controllers

Shell Global Solutions

Tuning Workshop

PID Controllers

Page 2: PID controllers

Shell Global Solutions 2

CP0420

FurnaceTT

TC

What is Controller?

Outlet temperature

Fuel flow

Feed flow

Set Value

Page 3: PID controllers

Shell Global Solutions 3

PID Controller Key Concepts

• The PID control algorithm does not "know" the correct output to bring the process to the set point.

• It merely continues to move the output in the direction which should move the process toward the set point.

• The algorithm must have feedback (process measurement) to perform

Page 4: PID controllers

Shell Global Solutions 4

Open Loop / Controller in Manual

e.g. Furnace, TC in MANUAL

TC

Process

Page 5: PID controllers

Shell Global Solutions 5

Closed Loop/ Auto Mode

TC

Process

e.g. Furnace, TC in AUTO

Page 6: PID controllers

Shell Global Solutions 6

PID Controller Tuning Parameters

• Proportional Band (Controller Gain)

• Integral Action ( Reset)

• Derivative Action

Page 7: PID controllers

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Proportional action

CP0520

Controller output varies proportionally to input :

G = Controller Gainin = deviation or (PV-SV)

c

out = G * inc

= Proportional Band (PB)100%cG

Also:

Proportional action therefore provides a signal proportional to the size of the deviation

Page 8: PID controllers

Shell Global Solutions 8

Controller output

Controller input

0 50 100%20 80

100%

50

0

Output span: 100%

Input span: 60%

CP0585

Proportional-only control

( PV )

( MV )

Page 9: PID controllers

Shell Global Solutions 9

Proportional-only control

CP0560

Offset

Offset

Process variable

Deviation

Set value

Set value

Deviation

time

time

Low Gain

High gain

- smaller offset - less damping of measured value

Page 10: PID controllers

Shell Global Solutions 10

Proportional-only control - Offset

CP0550

tem

pera

ture

Disturbance in feed flow

Offset

Set value change by operator

Set value

Offset

Desired value

time

Process variable (actual measured value)

Process variable trend (if no feedback control)

Page 11: PID controllers

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Proportional action• Proportional action gives an output signal proportional to the size

of the error.

• Proportional action can be tuned via the Controller Gain (Gc)

• Proportional action will leave an offset between SP and PV.

OP = K *

With:

= (PVP – SPP)

PVP = PV / “Range” * 100 % = PV / (PVEUHI – PVEULO) * 100 %

SPP = SP / “Range” * 100 % = SP / (PVEUHI – PVEULO) * 100 %

Page 12: PID controllers

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Integral Action Definition

CP0570

Integral action therefore provides a signal which depends the length of time a deviation has existed.

Integral action time i is defined as the time taken for the

controller output to change by the same amount from integral action as that from proportional action.

out = (t) dt1

i

0

t

Page 13: PID controllers

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Integral actionIntegral action

Deviation

TimeController Output

Integral Action Only

Time

Proportional + Integral ActionController Output

Proportional Action Only

Change due to the Proportional Action

i

Time

Page 14: PID controllers

Shell Global Solutions 14

Integral action at different ValuesIntegral action at different Values

Time

Proportional + Integral ActionController Output

Proportional Action Only

Change due to the Proportional Action

i

Time

Proportional + Integral ActionController Output

Proportional Action Only

Change due to the Proportional Action

i

Page 15: PID controllers

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Proportional & Integral actionInitial kick in output from proportional action.

After that integral action increases output

Process value reaches set point – no off set due to integration

Proportional & Integral controller

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Time (minutes)

Set

po

int

/ P

roce

ss V

alu

e (E

U)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Ou

tpu

t (%

)

SP

PV

OPInitial kick from proportional action

Integral action eliminates the off set

Page 16: PID controllers

Shell Global Solutions 16

Proportional & Integral action

• Proportional action gives an output signal proportional to the size of the error

• Integral action gives a signal which magnitude depends on the time the error has been there. The longer the error is present the higher the contribution of the Integral action to the total output.

Page 17: PID controllers

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Proportional + Integral control

CP0590

Process Variable

Deviation

Set value

time

Long Integral action time

Short Integral action time Set value

Deviation

time

Process Variable

1

2

Integral Action removes offset

Page 18: PID controllers

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Derivative Action

CP0610

time

Controller output

Derivative action only

Deviation

time

dtime

Proportional + Derivative actionController output

Proportional action only

change due to the Proportional action

Page 19: PID controllers

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CP0600

Derivative action therefore provides a signal which depends on the rate-of-change of deviation

Derivative action therefore provides anticipatory action

Derivative action time d is the time taken for the change in controller output due to proportional action to equal the same change as that from derivative action

Derivative Action Definition

out = d ddt

Page 20: PID controllers

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• Disturbance at t = 10 (min). Initial kick in output from proportional and derivative action; integral action increases OP further; when PV starts to move derivative action brings OP quickly back to steady state value.

PI controller - derivative action removed(Kp = 1; Tp = 6; Td = 2; Kc = 2.3; Ti = 6; Tder = 0)

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Time (minutes)

Set

po

int

/ P

ro

cess

Valu

e (

EU

)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Ou

tpu

t (%

)

SP

PV

OP

Disturbance of - 5

PI controller - with derivative action(Kp = 1; Tp = 6; Td = 2; Kc = 2.3; Ti = 6; Tder = 0.6)

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Time (minutes)

Set

po

int

/ P

ro

cess

Valu

e (

EU

)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Ou

tpu

t (%

)

SP

PV

OP

Disturbance of - 5

Proportional & Integral & Derivative action

Page 21: PID controllers

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Proportional & Integral & Derivative action

• Proportional action gives an output signal proportional to the size of the error

• Integral action gives a signal which magnitude depends on the time the error has been there

• Derivative action gives a signal proportional to the change in the PV. It gives sort of “anticipatory” control. Sensitive to measurement noise !

Page 22: PID controllers

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Proportional + Integral + Derivative control

CP0620

Process Variable

Deviation

Set value

time

P + I + D action

Set value

Deviation

time

P + I action (without Derivative action)

Process Variable

Page 23: PID controllers

Shell Global Solutions 23

PID controller actions - summary

• Proportional action

• Integral (reset) action

• Derivative action

• P + I + D controller

CP0630

out = (t).dt1

i0

t

out = d ddt

out = Gc * in = Gc *

Out = Gc * + (t).dt + d ddt

1

i0

t

for a P-only controller Out = Gc * + 50%

Page 24: PID controllers

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Proportional controlProportional control

Proportional + Integral controlProportional + Integral control

without Derivative action with Derivative action

Long Integral action time vs. Short Integral action time

Low Gain vs. High Gain

OffsetOffset

CP800

Controller Responses

Page 25: PID controllers

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Controller Selection

• P-Only Controllers:

•Used when offset is uncritical (some level applications).

• PI controllers are used in 95% of the applications:

•When offset is undesirable

• On applications having long dead time and time constant use PID except if the measurement is noisy.

•Examples

•Large Volume Pressure

•Temperature Control

Page 26: PID controllers

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Controller Algorithm Selection - Type Td < 0.2*Tp 0.2*Tp < Td < Tp Tp < Td < 5*Tp Td >

5*Tp

Dominant first orderDominant dead time

- PI x x x x

- PID x x

- PIT x x x

- I-only x

- Td = process dead time (minutes)

- Tp = process time constant (minutes)

- Use PI as standard

- Use Derivative action (together with P& I action) only as exception; most effective when 0.5*Tp < Td < 5*Tp

- Proportional Integral Time-delay (PIT) controller when dead time exceeds time constant. Special algorithm and relatively easy to implement in DCS.

Page 27: PID controllers

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Desired Response Desired Response

Response to a Step Change

Set Value

Time

Process Variable

One Overshoot

One Undershoot

• Desired behavior or response must be decided upon before loop can be tuned.

• Response below could be in general considered as “good” (fast and well damped).

Page 28: PID controllers

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Valve Failure ActionValve Failure Action Spring to Close (Air Failure Close)

(Air to Open)

Spring to Open (Air Failure Open)(Air to Close)

Decision Fail Action is taken in Process/Safeguarding design

Page 29: PID controllers

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Reverse and Direct Controller Polarity

If the measurement

increases

The controller output has to be reduced to

counter act

Reverse

PV↑ OP↓ Reverse

If the measurement

increases

The controller output has to be increased to counteract

Direct

PV ↑ OP ↑ Direct

Page 30: PID controllers

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Controller Action

LC

C-1

Direct-Acting Controller

Output

Pump

(Air Failure Close)

increase of PV leads to increase of OP of controller with constant SP

Page 31: PID controllers

Shell Global Solutions 31

Reverse-Acting Controller

LC

C-2

Output

Pump

(Air Failure Close)

increase of PV leads to decrease of OP of controller with constant SP

Controller action

Page 32: PID controllers

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Valve Linearity 

Valves are usually non-linear.That is, the flow through the valve is not the same as the valve position. Several types of valves exist:

LinearSame gain regardless of valve position

Equal Percentage (most commonly used)Low gain when valve is nearly closedHigh gain when valve is nearly open

Quick OpeningHigh gain when valve is nearly closedLow gain when valve is nearly open

Page 33: PID controllers

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End of The PID Controllers