Transcript Slide 1
PID Control
&
ACS550 and ACH550
© ABB Oy - 1 -
LV AC drives
expert days 2008
PID in every day life
Cruise function in a car:
User sets the speed (Input = SetPoint = Reference)
Car measures (Feedback = Actual)
Car automatically minimizes the error (Deviation) between these
two (Input and Feedback)
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”…as a result is a smooth ride.”
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PID: General
Widely used in automation and process industries.
Applications:
Motor Speed Control
Pressure & Temperature Control
Flow Control
(Water) Level Control
PID: General
Proportional-integral-derivative (PID) controller.
Control loop feedback method:
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VSD (controller) compares the reference signal (input) to the
actual signal (feedback) and controls the speed of the drive
according to the difference (error) of these signals.
PID: General
PID algorithm includes three different variables:
P(roportional): Proportional reaction to the error of the reference and measured
signals: improves the rise time!
I(ntegral): Determines the accumumulated proportional offset over time: eliminates
the steady state error!
D(erivative): Anticipated control: determines the slope of the error signal.
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Reduces the overshoot of the integral component.
PID: General
The output (v) of the PID controller:
de
v K P e K I edt K D
dt
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P
I
D
PID: General
Effects of controllers (P, I, D):
When increasing the value of the
variables…
CLOSE LOOP RESPONSE
Increasing variable’s value
RISE TIME
OVERSHOOT
SETTLING TIME
STEADY-STATE ERROR
Kp
Decreases
Increases
Small Change
Decreases
KI
Decreases
Increases
Increases
Eliminates
KD
Small Change
Decrease
Decreases
None
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Remember that the variables are dependent on each
other and thus by changing one value you might effect on the others!
The table above is only a good reference!
(Table @ http://www.engin.umich.edu/group/ctm/PID/PID.html)
PID: Tuning
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Traditional Ziegler-Nichols method:
1.
Pure P controller (disable D and I)
2.
Increase the gain untill the system oscillates
(critical gain KC)
3.
Read the oscillation period Tc
4.
Calculate the values according to the table
below:
Controller
P
I
P
0.5KC
PI
0.4KC
0.8TC
PID
0.6KC
0.5TC
Tc
D
0.125TC
PID: General
Control type by application:
Level Control: Usually P
Flow & Pressure & Temperature Control: PI
Motor Speed Control: PID
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(Figures @ http://www.engin.umich.edu/group/ctm/PID/PID.html)
PID: General
Using PID can be served:
Fast response/rise time
No overshoot
Low steady-state error
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SMOOTH AND FAST SYSTEM!!!
PID: General
Sometimes all three controllers are not needed to obtain
a good response!
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Depending on the system a good enough control can be
achieved by using only PI controller!
Always keep the system as simple as possible!
PID: ACS550
PID functions available both on ACS550 and ACH550
ACS550: PID control
Groups:
40 (Process PID SET 1)
41 (Process PID SET 2) and
42 (EXT / TRIM PID)
Pre-defined PID control macro:
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Set 9902
PID Assistant
PID: ACS550
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PID control macro: (9902)
PID: ACH550
ACH550: PID controller is in every HVAC macro.
Groups:
40 (Process PID SET 1)
41 (Process PID SET 2) and
42 (EXT / TRIM PID)
Macro selections:
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Set 9902
PID Assistant
PID: ACS550 and ACH550
ACS550 and ACH550 schematic of reference signal using parameter group 40:
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In order to use PID controller,
parameter 1106 must set to EXT2
(19).
PID: ACS550 and ACH550
Parameters 4001-4009:
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Used to adjust the PID:
Param. 4001: GAIN
Param. 4002: INTEGRATION TIME
Param. 4003: DERIVATION TIME
…
PID: ACS550 and ACH550
Rule of thumb in parameter setting:
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PID GAIN (parameter 4001)
increase until the system starts to oscillate
right value is half of this
PID INTEG TIME (parameter 4002)
decrease until the system starts to oscillate
right value is 2*Ti
PID: Summary
What is it for the customer?!
1.
Accurate:
2.
Easiness: (PID Macro)
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3.
Quality of the Process and Goods
Time Savings
Extent in Use:
Compatibility
Maintenance & Support