1. PID Control Loops
Guy Zebrick

2. Direct and Reverse Acting Proportional Control Integral Control
Contents
Open & Closed Loops
Direct and Reverse Acting
Proportional Control
Integral Control
Derivative Control
Definitions
Loop Tuning

3. 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 )

4. 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 )

5. Closed Loop Control Reverse Acting
Mathematical
Calculation
Input
Control
Variable
Output Device
0-100%
Setpoint
Input: Room Temperature
Setpoint: 70’F
Output: Radiator Valve (0-100%)

6. 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!)

7. PID closed loop control – driving at speed0% P
30%
PID
40% PI
60% PI 0%
PID
40%
PID
70% PI
25%
PID
40% PI

8. Definition: (SETPOINT)
The desired value of the input variable T1 T2 T3 T4 T5 T6
Input Value
Setpoint
Time

9. Definition: OFFSET (error)
The DIFFERENCE between the input variable and the desired setpoint. T1 T2 T3 T4 T5 T6
Input Value
Setpoint
Time
Offset

10. Definition: Throttling Range (Proportional Band)
Amount of Change in INPUT that equals a 0-100% change in OUTPUT
100%
Controller Output
50% 0%
70°
65°
75°
Setpoint T1 T2 T3 T4 T5 T6
Throttling Range (Proportional Band)
(10’F)

11. Proportional Control (with Offset Error)
Control Point
Setpoint T1 T2 T3 T4 T5 T6
Offset
0=100%
Time

12. Proportional + Integral Control (eliminates offset)
Control Point
Setpoint T1 T2 T3 T4 T5 T6
Offset
0=100%
Time

13. Derivative Calculation based on RATE of CHANGE
Control Point
Setpoint T1 T2 T3 T4 T5 T6
Offset D
P + I + D
Not used in our industry!
Time

14. Integral Windup Condition
Setpoint T1 T2 T3 T4 T5 T6
100%
Control Point
Time
Prevented by Disabling Loop when system is OFF

15. 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

16. 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

17. PID Setpoint RESET Requires RESET sensor (typically outside air)
Automatically adjusts setpoint

18. 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

19. 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

20. Definition: Hunting (over-reacting)
100%
Controller Output
50% 0%
70°
Setpoint T1 T2 T3 T4 T5 T6
69°
71°
Proportional Band Throttling Range
(2’F)
Time

21. 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.

22. Typical Values From Engineering Manual
Default Space Temperature Control
(CV AHU controller)

23. Throttling Range = ½ Input Sensor Range
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

24. 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’?

25. Questions?