PID Control Loops
Guy Zebrick
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
Setpoint
Output Device
0-100%
Input: Outside Air Temperature
Setpoint: 50’F
Output: Chiller on/off (0-100%)
Action: Direct (input output )
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Open Loop Control / Reverse Acting
Mathematical
Calculation
Input
Control
Variable
Setpoint
Output Device
0-100%
Input: Outside Air Temperature
Setpoint: 60’F
Output: Boiler on/off (0-100%)
Action: Reverse (input output
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Closed Loop Control Reverse Acting
Input
Control
Variable
Mathematical
Calculation
Output Device
0-100%
Setpoint
Input: Room Temperature
Setpoint: 70’F
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
7
0% 30% 40%
60%
0%
40%
70%
25%
40%
P
PI
PID
PID
PI
PID
PI
PID
PI
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Definition: (SETPOINT)
• The desired value of the input variable
Input Value
Setpoint
T1
T2
T3
T4
T5
T6
Time
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Definition: OFFSET (error)
• The DIFFERENCE between the input variable and
the desired setpoint.
Input Value
Setpoint
T1
T2
T3
T4
Offset
T5
T6
Time
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Definition: Throttling Range (Proportional Band)
Amount of Change in INPUT that equals a 0-100% change in OUTPUT
100%
Controller
Output
50%
0%
65°
70°
Setpoint
75°
Throttling Range
(Proportional Band)
(10’F)
T1
10
T2
T3
T4
T5
T6
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Proportional Control (with Offset Error)
Control Point
Setpoint
Offset
0=100%
T1
T2
T3
T4
T5
T6
Time
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Proportional + Integral Control (eliminates offset)
Control Point
Setpoint
Offset
0=100%
T1
T2
T3
T4
T5
T6
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!
T1
13
T2
T3
T4
T5
T6
Time
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Integral Windup Condition
Setpoint
100%
Control Point
T1
T2
T3
T4
T5
T6
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
T1
T6
Output = 50% at Setpoint
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T2
T3
T4
T5
T6
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%
Controller
Output
50%
0%
70°
Setpoint
69°
71°
Proportional Band
Throttling Range
(2’F)
T1
20
T2
T3
T4
Time
T5
T6
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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 Zero
2. Throttling Range = ½ Input Sensor
Range
3. Adjust Throttling Range to point
where about first begins to hunt.
4. Increase Throttling Range 1.5 X
5. 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?
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