Load Observer and Tuning Basics - Kendall Group - Home · PDF file ·...

Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. Rev 5058-CO900E

PUBLIC INFORMATION

Load Observer and Tuning Basics Feature Use & Benefits

Mark Zessin Motion Solution Architect Rockwell Automation

Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION

Questions Addressed

Why is Motion System Tuning Necessary?

What is the Autotune Test?

When is the Autotune Test Effective?

What is Load Observer?

What are the Key Advantages of Load Observer?

Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 3

Mass vs Weight


What is Mass? What is Inertia?


Compliance & backlash Changing inertia and torque

disturbances Machine to machine variation due to manufacturing tolerances Degradation over time

All machines are different and behave uniquely, with various levels of:

Why is Motion System Tuning Necessary?

Compliance Backlash

Load Variance

When Autotune provides unsatisfactory results…manual tuning is required Requires expertise and time Must connect to each axis to run the motor and tune Each axis must be tuned independently; so the configuration is axis-specific


Sources of Compliance


What is Backlash?


Simplified Control Loops

d^2/dt

d/dt

Σ

Acc FF Gain(Kaff)

Vel FF Gain(Kvff)

Pos P Gain(Kp)

Pos I Gain(Ki)

Σ Σ Vel P Gain(P)

Vel I Gain(I)

Σ

Motor

PositionCommand

d/dt Kd

d/dt Kd

Low PassFilter

Test for KiZone

UnlimitedCurrent

Command

MotorEncoder

Feedback

FeedbackPosition

FeedbackVelocity

ErrorAccumulator

CurrentLimiting

CurrentCommand

ErrorAccumulator


• Parallel Form - Proportional term (top) in parallel with the integral term (bottom) • The proportional term is the control loop bandwidth in [rad/sec] • The integral term has a squared relationship to the control loop bandwidth – rad/sec² • The 1000 factor is applied to counteract the squared relationship

Sercos Drive PI Controllers

9

Kpp = Position Proportional Gain [rad/s] Kpi = Position Integral Gain [rad/s/ms] Kvp = Velocity Proportional Gain [rad/s] Kvi = Velocity Integral Gain [rad/s/ms] Kvff = Velocity Feedforward [%] Kaff = Acceleration Feedforward [%]


Kpp = Position Loop Bandwidth [Hz] Kpi = Position Integral Bandwidth [Hz] Kvff = Velocity Feedforward [%] Kvp = Velocity Loop Bandwidth [Hz] Kvi = Velocity Integral Bandwidth [Hz] Kaff = Acceleration Feedforward [%]

• Series Form - Proportional term (top) in series

with the integral term (bottom) • All gains have a 2π factor applied this makes all

gains in [Hz] • All terms are proportional to each other and

represent physical Bandwidth

CIP Drive PI Controllers

10


Servo Loop Bandwidth

Bandwidth is the usable range of frequencies in [Hz] where the gain through the system is above -3dB • Bandwidth indicates servo drive performance and directly equates to transient response,

i.e. how fast the servo physically responds to the load • Higher bandwidth → higher performance • Factors affecting bandwidth are:

• Feedback resolution (higher is better) • Load inertia ratio (lower is better) • Drive update rate (faster is better) • Load compliance (rigid is better) • Drive Model Time Constant (lower is better)

11


Damping Factor

• Commonly referred to as zeta (z) • It affects the rise time for a given bandwidth

• Lower Damping → higher response • Higher Damping → lower response

• Sercos: default z = 0.8 • CIP: default z = 1.0 (Medium)

Under-damped

High: z < 1.0

Over-damped

Low: z > 1.0 Critically damped

Medium: z = 1.0

Highest possible rise time, but has overshoot

Fastest possible rise time without overshoot

Lowest possible rise time , similar to decreasing BW

12


What is the Autotune Test?

Calculates

combined motor & drive

characteristic values –

then sets control loop gains

Performs physical bump test to measure

the load – then sets

dynamic limits

Optimal for rigid mechanics & high dynamic systems with

rigid load ratios of 10:1 or less

Autotune Functionality


When is the Autotune Test Effective?

Autotune 10%

Additional Tuning Required

90%

Simple Loads • Rigid mechanics, non-changing loads • High dynamic systems with rigid load ratios ≤10:1

Remaining Applications • Compliant loads • Variable loads • Optimal performance • >10:1 load ratios

Autotune Effectiveness


What is an OBSERVER??


What is Load Observer?

Operates in real time as the

machine runs

Dynamically estimates the load torque &

provides a feedback signal

to cancel its effect

Causes the motor to behave as though it is

unloaded

Load Observer Functionality


What is Load Observer


[CATEGORY NAME]

[PERCENTAGE]

[CATEGORY NAME]

[PERCENTAGE]

Additional Tuning Required

5%

When is Load Observer Effective?

Systems Requiring Optimal Performance

Rigid/Compliant/ Changing Loads

• No 10:1 limitation on load ratio

Load Observer Effectiveness Systems

with Mechanical Resonance


[CATEGORY NAME]

[PERCENTAGE]

[CATEGORY NAME]

[PERCENTAGE]

When is Load Observer Effective?

Systems Requiring Optimal Performance

Rigid/Compliant/ Changing Loads

• No 10:1 limitation on load ratio

Load Observer Effectiveness


Load Observer Key Benefit #1 Enables Effective Tuningless Operation

Effective Tuningless Operation is Possible

• Configure axes without auto- or manual tuning • Commission axes without having to connect to machine • Utilize consistent configuration across axes and machines

Autotune Test and/or Manual Tuning was Required

• Autotune test was performed on all axes • Manual tuning required if autotune gave undesirable results • Tuning knowledge required to manually tune axes with success

In the past, without Load Observer…

Now, with Load Observer…


Load Observer Key Benefit #2 Effectively Manages Compliance in Mechanical Systems

Compliant Systems Operate with Superior Performance & Robustness

• Achieve greater performance in systems with compliance • Obtain greater robustness to system disturbances • Ease requirement to minimize compliance in machine designs

High Performance & Robustness was Impractical for Compliant Systems

• Systems with compliance were very challenging to tune • Performance sacrificed to avoid instability from disturbances • Mechanical systems were designed to minimize compliance




Load Observer Key Benefit #3 Provides Simple Control for Variable Loads

Simple, Dynamic Control of Variable Loads is Possible

• Torque signal is automatically adjusted as load varies • Gain values remain constant throughout operation • Control maintained even when load varies in unpredictable way

Complex Techniques were Utilized to Control Variable Loads

• Advanced techniques required to dynamically vary gains • Techniques required load to vary in a predictable manner • Gains set for lowest inertia to avoid instability during operation




Load Observer Summary Frequently Asked Questions

• Load Observer estimates the torque required to move the load and applies a feedback signal to cancel its effect.

• So, the motor behaves as though it is unloaded.

How does Load Observer work?

• Tuningless operation now possible for many applications. • Compliance is effectively managed. • Variable loads are easy to control and no longer require

additional code.

What are the key benefits of Load

Observer?

• Refer to Motion System Tuning Manual (MOTION-AT005). Where can I find more technical information?

http://literature.rockwellautomation.com/idc/groups/literature/documents/at/motion-at005_-en-p.pdf



Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved.

www.rockwellautomation.com

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