pid control loops
DESCRIPTION
PID Control Loops. Guy Zebrick. Contents. Open & Closed Loops Direct and Reverse Acting Proportional Control Integral Control Derivative Control Definitions Loop Tuning. Open Loop Control / Direct Acting. Mathematical Calculation. Input Control Variable. Setpoint. Output Device - PowerPoint PPT PresentationTRANSCRIPT
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?