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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 4
What is Mass? What is Inertia?
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 6
Sources of Compliance
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 7
What is Backlash?
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 8
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
• 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 [%]
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 15
What is an OBSERVER??
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION 17
What is Load Observer
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
[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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
[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
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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…
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
In the past, without Load Observer…
Now, with Load Observer…
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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
In the past, without Load Observer…
Now, with Load Observer…
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. PUBLIC INFORMATION
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?
Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved.
www.rockwellautomation.com
Follow ROKAutomation on Facebook & Twitter. Connect with us on LinkedIn.
Rev 5058-CO900F
PUBLIC INFORMATION
Questions?