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LV AC drives expert days 2008

PID Control& ACS550 and ACH550

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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)

”…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

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PID: General

Proportional-integral-derivative (PID) controller.

Control loop feedback method:

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.

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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.

Reduces the overshoot of the integral component.

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PID: General

The output (v) of the PID controller:

dt

deKedtKeKv DIP

P I D

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PID: General

Effects of controllers (P, I, D): When increasing the value of the

variables…

CLOSE LOOP RESPONSEIncreasing 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

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)

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PID: Tuning

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:

Tc

Controller P I D

P 0.5KC

PI 0.4KC 0.8TC

PID 0.6KC 0.5TC 0.125TC

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PID: General

Control type by application:

Level Control: Usually P

Flow & Pressure & Temperature Control: PI

Motor Speed Control: PID

(Figures @ http://www.engin.umich.edu/group/ctm/PID/PID.html)

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PID: General

Using PID can be served: Fast response/rise time

No overshoot

Low steady-state error

SMOOTH AND FAST SYSTEM!!!

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PID: General

Sometimes all three controllers are not needed to obtain a good response! Depending on the system a good enough control can be

achieved by using only PI controller!

Always keep the system as simple as possible!

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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: Set 9902

PID Assistant

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PID: ACS550

PID control macro: (9902)

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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: Set 9902

PID Assistant

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PID: ACS550 and ACH550

ACS550 and ACH550 schematic of reference signal using parameter group 40:

In order to use PID controller, parameter 1106 must set to EXT2 (19).

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PID: ACS550 and ACH550

Parameters 4001-4009: Used to adjust the PID:

Param. 4001: GAIN

Param. 4002: INTEGRATION TIME

Param. 4003: DERIVATION TIME

…

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PID: ACS550 and ACH550

Rule of thumb in parameter setting:

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

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PID: Summary

What is it for the customer?!

1. Accurate: Quality of the Process and Goods

2. Easiness: (PID Macro) Time Savings

3. Extent in Use: Compatibility

Maintenance & Support