PID%20Control%20Loops

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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
Mathematical Calculation
Input Control Variable
Output Device 0-100
Setpoint
Input Outside Air Temperature Setpoint
50F Output Chiller on/off (0-100) Action
Direct (input output )
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Open Loop Control / Reverse Acting
Mathematical Calculation
Input Control Variable
Output Device 0-100
Setpoint
Input Outside Air Temperature Setpoint
60F Output Boiler on/off (0-100) Action
Reverse (input output )
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Closed Loop Control Reverse Acting
Mathematical Calculation
Input Control Variable
Output Device 0-100
Setpoint
Input Room Temperature Setpoint 70F Output
Radiator Valve (0-100)
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PID Control by Mathematical calculation

• Proportional
• Output varies in PROPORTION to input error
• Integral
• Output varies over TIME based on input error
• Derivative
• Output varies based RATE of CHANGE of input error
• P control
• P I control
• P 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

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Definition OFFSET (error)

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

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Definition Throttling Range (Proportional Band)
Amount of Change in INPUT that equals a 0-100
change in OUTPUT
100
ControllerOutput
50
0
70
75
65
Setpoint
Throttling Range (Proportional Band) (10F)
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Proportional Control (with Offset Error)
Control Point
Setpoint
Offset
0100
Time
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Proportional Integral Control (eliminates
offset)
Control Point
Setpoint
Offset
0100
Time
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Derivative Calculation based on RATE of CHANGE
Control Point
Setpoint
Offset
D
P I D Not used in our industry!
Time
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Integral Windup Condition
Setpoint
100
Control Point
Time
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
50
0
100
100
50
50
0
0
T1
T2
T3
T4
T5
T6
T1
T2
T3
T4
T5
T6
Output 50 at Setpoint
Output 0 at Setpoint
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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)

UNOCC
Setpoint
OCC
OCC
Time
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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)
100
ControllerOutput
50
0
70
Setpoint
69
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Proportional BandThrottling Range (2F)
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

• Integral Time to Zero
• Throttling Range ½ Input Sensor Range
• Adjust Throttling Range to point where about
  first begins to hunt.
• Increase Throttling Range 1.5 X
• Add Integral Time based on system response time
• (typically 60 2500 seconds)
• 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?