pid control basics
TRANSCRIPT
Copyright © Yokogawa Corporation of America
1
PID Control Basics
PID Tuning
Rob SinkTechnical Support Specialist
June 14th, 2016
Copyright © Yokogawa Corporation of America
2
What will be covered:1. Common Process Control Techniques2. Process Dynamics3. What is PID4. PID Control Components5. How to Tune a PID Loop
Copyright © Yokogawa Corporation of America
3
Why do I Need to Understand PID
Every process is different
Makes manual tuning easier
Helps companies save money
Helps facilities remain safe
Copyright © Yokogawa Corporation of America
4
Common Process Control Techniques ques
Manual Control
ON / OFF Control
Closed Loop Control
Copyright © Yokogawa Corporation of America
5
Manual Control
Operator observes the process error and adjusts the control output
PID CONTROL
Set Point
Measurement(Process Variable)
Process
Copyright © Yokogawa Corporation of America
6
ON / OFF Control
Simplest form of feed back control
Can be used for processes not requiring extremely tight control
Copyright © Yokogawa Corporation of America
7
Closed Loop Control
The PID controller measures the process variable, compares it to the setpoint and then manipulates the output accordingly.
Final Control Element
PV
Set Point
Measurement(Process Variable)
Copyright © Yokogawa Corporation of America
8
Process Dynamics: Dead Time
Dead time is defined as the time before the process variable BEGINS to react to a change in the control output
Output
Process Variable
Lag TimeDeadTime
Copyright © Yokogawa Corporation of America
9
Process Dynamics: Lag Time
Lag is defined as the time required for the process variable to adjust to a steady state after an output change is performed
Lag time affects the control action
Output
Process Variable
Lag Time
Copyright © Yokogawa Corporation of America
10
Process Dynamics: Output vs. Process Change
Copyright © Yokogawa Corporation of America
11
What is PID?
PID control refers to process control using the coefficients Proportional, Integral and Derivative
It is not P&ID which refers to Piping & Instrumentation Diagram
Copyright © Yokogawa Corporation of America
12
PID Control Defined
PID control can be described as a set of rules with which a precise regulation of a closed-loop control system is obtained.
Temp (PV)
TempSetpoint (SP)
Copyright © Yokogawa Corporation of America
13
PID Control Terms
Proportional Band adjusts output amplitude (reciprocal of Gain)
Integral eliminates offset error (automatic Reset or simply Reset)
Derivative looks at the rate of change of the error (Rate)
Copyright © Yokogawa Corporation of America
14
Proportional Band
The Proportional Band (P) is defined as the range over which the control output is adjusted from 0-100%
Proportional does the heavy lifting getting the temperature close to the setpoint
Some manufacturers use Gain instead of Proportional Band
Copyright © Yokogawa Corporation of America
15
Proportional with Manual Reset
With proportional only control, an offset will be present between set point and process variable. Manual Reset allows a user to bias or shift the output to compensate for the steady state offset.
1000º
Manual Reset Adjusted Here
500º Set Point
ProportionalBand
Time
Copyright © Yokogawa Corporation of America
16
Integral
Integral action is used with proportional to eliminate the inherent offset
The integrating term observes how long the error has existed, summing the error over time
The sum becomes a value added to the output
Output
Time 200 sec/repeat
Integral Action
Proportional Action
Integral Time Constant
Error-10%
+10%
Copyright © Yokogawa Corporation of America
17
Integral – cont.
Engineering units: Repeats/minuteMinutes/repeat
Seconds/repeat The integral action ceases at a no error condition
Copyright © Yokogawa Corporation of America
18
Integral at Work
I I II
Integral started.
Setpoint
Each time period where the error is not zero, the output is increased (or decreased) by the Integral term.
Copyright © Yokogawa Corporation of America
19
A Note About Integral Windup
Integral windup refers to the situation in a PID controller where the integral, or reset action continues to integrate (ramp) indefinitely
This usually occurs when the controller's output can no longer affect the controlled variable, which in turn can be caused by controller saturation
Typical causes of Integral Windup are: The input has been removed from the process, output device has failed, a furnace door has been opened keeping the process from reaching temperature
Copyright © Yokogawa Corporation of America
20
Derivative
Engineering units: minutes or seconds Anticipates the error rate and applies the “brakes” Derivative has no effect if the error is constant
Output
Time 50 seconds
Derivative Action
Integral Action
Derivative Time Constant
Error-10%
+10%
Copyright © Yokogawa Corporation of America
21
P, I and D Working Together
P only P and I P I D
Copyright © Yokogawa Corporation of America
22
How to Tune a PID Loop
Manually tuning the loop
Using the controllers Auto/Self Tune
Copyright © Yokogawa Corporation of America
23
Manually Tuning a Loop
These values are good starting points Change only (1) term at a time Make small changes observing the result
Copyright © Yokogawa Corporation of America
24
Fine-tuning the Proportional Band
Work from larger to smaller numbers (wider to narrower)
If cycling appears, the proportional band is too narrow
Copyright © Yokogawa Corporation of America
25
Fine Tuning the Integral Time
The main goal is to reduce the offset
Adjust from longer to shorter time
If an oscillation exists at a longer period then the integral time is too short
Copyright © Yokogawa Corporation of America
26
Fine Tuning the Derivative Time Adjust from shorter to longer time
If short-period oscillations develop, the time is to long.
The larger the Derivative, the stronger the corrective action and the more likely the output will become oscillatory
Copyright © Yokogawa Corporation of America
27
Tuning Loops with Dead Time
Output
Process Variable
Lag TimeDeadTime
Set P to 5% and the I & D to 0%
Start the process with a setpoint that will allow the process variable to stabilize
Copyright © Yokogawa Corporation of America
28
Tuning Fast Reacting Loops
Set P to 100% and the I & D to 0%
Start the process with a setpoint that will allow the process variable to stabilize
Copyright © Yokogawa Corporation of America
29
Using Auto Tune to Determine PID Values
The output is varied between 0% and 100% three times (these values may be limited). The process variable must ascend and descend through set point for the output to change state. The auto tune algorithm observes the PV response to these output changes and installs the appropriate PID terms.
Copyright © Yokogawa Corporation of America
30
Ways to Prevent Overshoot
Limit the working output or enable an output ramp rate (if available)
Limit the output range which will have an effect on the time it takes to get to setpoint
Ramp the setpoint at a slow rate
Use fuzzy logic (if available)
Copyright © Yokogawa Corporation of America
31
Fuzzy Logic
Fuzzy logic is used to help reduce setpoint overshoot
Used in addition to PID control
Copyright © Yokogawa Corporation of America
32
Yokogawa Products that Use PID Control
Single loop controller
Programmable controller
PLC
DCSPLC/RTU
Copyright © Yokogawa Corporation of America
33
UTAdvanced Line of controllers
1-2 loops of control
Built in ladder sequence control
Software used in Webinar
Nuclear qualified
Copyright © Yokogawa Corporation of America
34
YS1000 Family of Controllers
1-2 loops of control
Nuclear qualified
Hard manual backup
Function block programming
Copyright © Yokogawa Corporation of America
35
FA-M3 PLC
Modular PLC design
4 control loops per PID module
PID control is not done in ladder logic
Copyright © Yokogawa Corporation of America
36
Questions
Questions?Feel free to email us with further questionsat [email protected] put “PID Webinar” in the subject line.
Copyright © Yokogawa Corporation of America
37
Thank you for attending!
Feel free to email us with further questionsat [email protected] put “PID Webinar” in the subject line.