Transcript Slide 1

PID Control
&
ACS550 and ACH550
© ABB Oy - 1 -
LV AC drives
expert days 2008
PID in every day life
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Cruise function in a car:
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User sets the speed (Input = SetPoint = Reference)
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Car measures (Feedback = Actual)
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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
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Widely used in automation and process industries.
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Applications:
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Motor Speed Control
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Pressure & Temperature Control
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Flow Control
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(Water) Level Control
PID: General
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Proportional-integral-derivative (PID) controller.
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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
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PID algorithm includes three different variables:
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P(roportional): Proportional reaction to the error of the reference and measured
signals: improves the rise time!
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I(ntegral): Determines the accumumulated proportional offset over time: eliminates
the steady state error!
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D(erivative): Anticipated control: determines the slope of the error signal.
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Reduces the overshoot of the integral component.
PID: General
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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
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Effects of controllers (P, I, D):
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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:
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Level Control: Usually P
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Flow & Pressure & Temperature Control: PI
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Motor Speed Control: PID
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(Figures @ http://www.engin.umich.edu/group/ctm/PID/PID.html)
PID: General
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Using PID can be served:
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Fast response/rise time
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No overshoot
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Low steady-state error
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SMOOTH AND FAST SYSTEM!!!
PID: General
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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
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PID functions available both on ACS550 and ACH550
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ACS550: PID control
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Groups:
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40 (Process PID SET 1)
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41 (Process PID SET 2) and
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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
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ACH550: PID controller is in every HVAC macro.
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Groups:
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40 (Process PID SET 1)
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41 (Process PID SET 2) and
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42 (EXT / TRIM PID)
Macro selections:
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Set 9902
PID Assistant
PID: ACS550 and ACH550
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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
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Parameters 4001-4009:
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Used to adjust the PID:
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Param. 4001: GAIN
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Param. 4002: INTEGRATION TIME
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Param. 4003: DERIVATION TIME
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…
PID: ACS550 and ACH550
Rule of thumb in parameter setting:

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PID GAIN (parameter 4001)
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increase until the system starts to oscillate

right value is half of this
PID INTEG TIME (parameter 4002)
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decrease until the system starts to oscillate

right value is 2*Ti
PID: Summary
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What is it for the customer?!
1.
Accurate:
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2.
Easiness: (PID Macro)
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3.
Quality of the Process and Goods
Time Savings
Extent in Use:
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Compatibility
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Maintenance & Support