Load Observer and Tuning Basics Feature Use & Benefits Mark Zessin Motion Solution Architect Rockwell Automation PUBLIC INFORMATION Rev 5058-CO900E
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?
Mass vs Weight PUBLIC INFORMATION 3
What is Mass? What is Inertia? PUBLIC INFORMATION 4
Why is Motion System Tuning Necessary? All machines are different and behave uniquely, with various levels of: Compliance & backlash Changing inertia and torque disturbances Machine to machine variation due to manufacturing tolerances Degradation over time Load Variance Compliance Backlash 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 PUBLIC INFORMATION 6
What is Backlash? PUBLIC INFORMATION 7
8 Simplified Control Loops d^2/dt Acc FF Gain (Kaff) d/dt Vel FF Gain (Kvff) Position Command Σ Pos P Gain (Kp) Σ Σ Vel P Gain (P) Σ Test for Ki Zone Low Pass Filter Error Accumulator Pos I Gain (Ki) Error Accumulator Vel I Gain (I) Unlimited Current Command Feedback Position d/dt Kd Feedback Velocity d/dt Kd Current Limiting Current Command Motor Encoder Feedback Motor
9 Sercos Drive PI Controllers 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 [%] 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
10 CIP Drive PI Controllers Kpp = Position Loop Bandwidth Kpi = Position Integral Bandwidth [Hz] [Hz] Kvff = Velocity Feedforward [%] Kvp = Velocity Loop Bandwidth Kvi = Velocity Integral Bandwidth [Hz] [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
11 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)
12 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 Fastest possible rise time without overshoot Lowest possible rise time, similar to decreasing BW Highest possible rise time, but has overshoot Under-damped High: z < 1.0 Critically damped Medium: z = 1.0 Over-damped Low: z > 1.0 Sercos: default z = 0.8 CIP: default z = 1.0 (Medium)
What is the Autotune Test? Autotune Functionality Calculates combined motor & drive characteristic values then sets control loop gains Optimal for rigid mechanics & high dynamic systems with rigid load ratios of 10:1 or less Performs physical bump test to measure the load then sets dynamic limits
When is the Autotune Test Effective? Autotune Effectiveness Simple Loads Rigid mechanics, non-changing loads High dynamic systems with rigid load ratios 10:1 Autotune 10% Remaining Applications Compliant loads Variable loads Optimal performance >10:1 load ratios Additional Tuning Required 90%
What is an OBSERVER?? PUBLIC INFORMATION 15
What is Load Observer? Load Observer Functionality 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
What is Load Observer PUBLIC INFORMATION 17
When is Load Observer Effective? Systems with Mechanical Resonance Systems Requiring Optimal Performance [CATEGORY NAME] [PERCENTAGE] Additional Tuning Required 5% Load Observer Effectiveness Rigid/Compliant/ Changing Loads No 10:1 limitation on load ratio [CATEGORY NAME] [PERCENTAGE]
When is Load Observer Effective? Systems Requiring Optimal Performance Load Observer Effectiveness [CATEGORY NAME] [PERCENTAGE] Rigid/Compliant/ Changing Loads No 10:1 limitation on load ratio [CATEGORY NAME] [PERCENTAGE]
Load Observer Key Benefit #1 Enables Effective Tuningless Operation In the past, without Load Observer 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 Now, with Load Observer 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
Load Observer Key Benefit #2 Effectively Manages Compliance in Mechanical Systems In the past, without Load Observer 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 Now, with Load Observer 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
Load Observer Key Benefit #3 Provides Simple Control for Variable Loads In the past, without Load Observer 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 Now, with Load Observer 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
Load Observer Summary Frequently Asked Questions How does Load Observer work? 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. What are the key benefits of Load Observer? Where can I find more technical information? Tuningless operation now possible for many applications. Compliance is effectively managed. Variable loads are easy to control and no longer require additional code. Refer to Motion System Tuning Manual (MOTION-AT005).
Questions? PUBLIC INFORMATION Follow ROKAutomation on Facebook & Twitter. Connect with us on LinkedIn. www.rockwellautomation.com Rev 5058-CO900F