PID Control Loops

PID Control Loops

Guy Zebrick

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Contents

•Open & Closed Loops•Direct and Reverse Acting•Proportional Control•Integral Control•Derivative Control•Definitions•Loop Tuning

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Open Loop Control / Direct Acting

InputControlVariable

Output Device0-100%

MathematicalCalculation

Setpoint

Input: Outside Air TemperatureSetpoint: 50’FOutput: Chiller on/off (0-100%)Action: Direct (input output )

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Open Loop Control / Reverse Acting

InputControlVariable

Output Device0-100%

MathematicalCalculation

Setpoint

Input: Outside Air TemperatureSetpoint: 60’FOutput: Boiler on/off (0-100%)Action: Reverse (input output )

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Closed Loop Control Reverse Acting

InputControlVariable

Output Device0-100%

MathematicalCalculation

Setpoint

Input: Room TemperatureSetpoint: 70’FOutput: Radiator Valve (0-100%)

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PID Control by Mathematical calculation

ProportionalOutput varies in PROPORTION to input error

IntegralOutput varies over TIME based on input error

DerivativeOutput varies based RATE of CHANGE of input error

P controlP + I controlP + I + D control (not used in our industry!)

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PID closed loop control – driving at speed

0%

P

30%

PID

40%

PI

60%

PI

0%

PID

40%

PID

70%

PI

25%

PID

40%

PI

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Definition: (SETPOINT)

• The desired value of the input variable

T1 T2 T3 T4 T5 T6

Input Value

Setpoint

Time

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Definition: OFFSET (error)

• The DIFFERENCE between the input variable and the desired setpoint.

T1 T2 T3 T4 T5 T6

Input Value

Setpoint

Time

Offset

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Definition: Throttling Range (Proportional Band)

Throttling Range (Proportional Band)

(10’F)

100%

50%

0%

65° 70° 75°Setpoint

ControllerOutput

T1 T2 T3 T4 T5 T6

Amount of Change in INPUT that equals a 0-100% change in OUTPUT

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Proportional Control (with Offset Error)

T1 T2 T3 T4 T5 T6

Control Point

Setpoint

Time

Offset

0=100%

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Proportional + Integral Control (eliminates offset)

T1 T2 T3 T4 T5 T6

Control Point

Setpoint

Time

Offset

0=100%

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Derivative Calculation based on RATE of CHANGE

T1 T2 T3 T4 T5 T6

Control Point

Setpoint

Time

Offset

DDP + I + D

Not used in our industry!

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Integral Windup Condition

T1 T2 T3 T4 T5 T6

Control Point

Setpoint

Time

100%

Prevented by Disabling Loop when system is OFF

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PID Enhancements

• Setpoint Reset• Setpoint Recovery Ramp• Setpoint Demand Limit “BUMP”• Setpoint Select(s)

• PID Output BIAS• PID Output Start Point• PID Output Start Ramp• PID Output Adjust Delay• PID Output Adjust Threshold• PID AUXILLARY Output• PID Output Sequencer

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Definition: PID Output Bias

T1 T2 T3 T4 T5 T6

100%

50%

0%T1 T2 T3 T4 T5 T6

100%

50%

0%

Output = 50% at Setpoint

Output = 0% at Setpoint

50%

0%

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PID Setpoint RESET

• Requires RESET sensor (typically outside air)• Automatically adjusts setpoint

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PID Setpoint Recovery Ramp

• Degrees/Hr• Requires multiple schedule information

- Current State (occupied/unoccupied)- Next State (occupied/unoccupied)- Time until next state (minutes)

Setpoint

UNOCC

OCC

TimeOCC

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PID options (not available on ALL PID loops)

•Output Start Value (%)-Provides starting value for initial control

•Output Start Ramp (seconds)-limits action during initial startup to prevent overshoot

-Output Dead Band (%)

•Output Adjust Delay & Threshold-Prevents repeated minor adjustment to output

-Prevents needless wear & tear on mechanical devices

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Definition: Hunting (over-reacting)

Proportional BandThrottling Range

(2’F)

100%

50%

0%

69°

70°

71°

Setpoint

ControllerOutput

T1 T2 T3 T4 T5 T6

Time

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Tuning Hints:

• PI control for closed-loop applications only.• The narrower (smaller) the throttling range, the more precise the control operation. The wider (larger) the throttling range, the more stable the control action.

• Generally the throttling range required for PI control is greater than what is used for proportional control only.

• The integral time value is set in seconds. A slow process such as space temperature control requires a long integral time (600 seconds or more), while a fast process such as static pressure control requires a short integral time.

• An integral time of 0 eliminates the integral function for the control loop.

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Typical Values

• From Engineering Manual

• Default Space Temperature Control

(CV AHU controller)

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Field Tuning

1. Integral Time to Zero2. Throttling Range = ½ Input Sensor

Range3. Adjust Throttling Range to point

where about first begins to hunt.4. Increase Throttling Range 1.5 X5. Add Integral Time based on system

response time • (typically 60 – 2500 seconds)

6. Check for Stable Operation

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PID POP Quiz

• Define open loop• Define closed loop• A cooling loop is (direct/reverse) acting• An AHU static pressure control loop is (direct/reverse) acting

• Define P, I, D• Define throttling range• Define integral time• When do you use Differential control?• What adjustment(s) reduce ‘hunting’?

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Questions?