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The Secrets of TuningAdvanced Control Strategies
for Servo Systems
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George Ellis
Chief Engineer, Servo Systems
Kollmorgen Corp.
Alex ZaatariGraduate Student, Modeling of Dynamic Systems and Controls
University of Texas, Austin
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What well talk
about today Classical Control Theory
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Auto Tuning
Four Tuning Secrets
Deploying Your Control System to Hardware
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Some Things Controls Can Help With
Selecting controls equipment
Configuring controllers
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Designing transmissions and end effectors
Diagnosing problems in the field
Predicting performance before a machine is built
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The Simple LoopThe Basis of Classical Controls
R(s) C(s)G(s)
H s
+
-
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R(s) C(s)H(s)G(s)1
G(s)+
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The Simple LoopThe Basis of Classical Controls
R(s) C(s)G(s)
H s
+
-
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R(s) C(s)H(s)G(s)1
G(s)+
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The Basis of Classical Controls
R(s) C(s)G(s)
H (s)
+
-
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R(s) C(s)H(s)G(s)1
G(s)
+
Keep G(s).H(s) far away from 1!
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An Example Control Loop:
A Servo System Velocity LoopVelocity
PICurrent Motor
Filters
Position
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ommanoa
Position
sensorFilters Differentiation
Velocity
Feedback
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G(s)
A Motor Controller
Velocity
CommandPI
Current
Loop
Motor
& LoadFilters
Position
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H(s)
Position
sensorFilters Differentiation
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Calculation:The academic way to get G(s) x H(s)
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H(s)
G(s)
Velocity
Command
P I
C u r r e n t
L o o p
M o t o r
& L o a d
P o s i t i o n
s e n s o r
F i l t e r s
F i l t e r s
D i f f e r e n t i a t i o n
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If calculation isnt practical,
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what about measurement?
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Open Loop Measurement:
(The old way to measure G(s) x H(s))
R(s) C(s)+
-
G(s)Error(s)
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H (s)Feedback(s)
G(s) x H(s) = Feedback(s)Error(s)
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Error(s)Excitation
How We Do It Now!1. Inject an excitation signal into the closed loop
2. Measure Error(s) and Feedback(s)
3. Calculate the open loop
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R(s) C(s)G(s)
H (s)
-
Feedback(s)
G(s) x H(s) = Feedback(s)
Error(s)
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A Demonstration:
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Velocity Command
Well-Behaved: Rapid, Stable Response
2 mSec settling
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VL.KP = 0.1
VL.KI -= 10
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GH for a Well Behaved ServoFocus in
this
range
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Keep G(s).H(s)Keep G(s).H(s)
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ar away rom0dB-180!ar away rom 1!=
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Gain = 0dB
GH for a Well-Behaved Servo
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Phase = -180
Wide separation
(Ratio ~ 3)
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Another Demonstration:Marginal Stability Induced
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with the Phase Lagof a Low-Pass Filter
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G(s)
Velocity
CommandPI
Current
Loop
Motor
& LoadFilters
Add a Low-Pass Filter to the System
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H(s)
Position
sensorFilters Differentiation
350 Hz, 2-pole
Low-pass
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Velocity Command
Velocity Feedback
Adding a Low-Pass Filter
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Gain = 0dB
For reference:
GH for a Well Behaved Servo
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Phase = -180
Wide separation
(Ratio ~ 3)
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Gain = 0dB
GH and Marginal Stability
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Keep G(s).H(s) far away from 0dB-180!
Phase = -180
Almost no separation!
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The 1st Rule of Controls
Minimize phase lag
Specify products with fast sampling
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When tuning, use filters only when you need them
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Rigid vs. Compliant
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Motor Load
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Compliant TransmissionsBelts and pulleysLead screws
Shaft couplings
Gear boxes
Shafts
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A Simple Example:
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7x Load
Rigid coupling
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GH for a (pretty) Well-Behaved Servo
3 mSec settling Velocity FeedbackVelocity Command
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(For ref: retuned & used a 600 Hz Low-Pass Filter)
VL.KP = 0.3
VL.KI -= 10
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Gain = 0dB
Still have a
wide separation
But not in high
Frequencies1400 Hz!
GH for a (pretty) Well-Behaved Servo
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Phase = -180
in low requencies
(For ref: used a 600 Hz Low-Pass Filter)
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(For ref: used a 600 Hz Low-Pass Filter)
0.75ms ~ 1400 Hz!
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Another Simple Example:
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7x Load
Compliant coupling
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GH for a Not-So-Well-Behaved Servo
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This system has the exact same tuning as the previous example!
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Gain = 0dB
GH for a Not-So-Well-Behaved Servo
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Phase = -180
This system has the
exact same tuning asthe previous example!
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Autotuning
The process of
1 Measurin the motor/load
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characteristics2) Applying gains and filter
settings to match the motor &
load
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G(s)
Velocity
CommandPI
Current
Loop
Motor
& LoadFilters
What Autotuning Adjusts
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H(s)
Positionsensor
Filters Differentiation
+ Position Loop Gains
+ Acceleration, Velocity Feed-forward
+
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Q. How Does Autotuning Work?A. It Depends.
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1. Previous Generation Autotune
2. New Generation Autotune
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Previous Generation Autotune
Use the motor to shake the load at low frequency andcalculate the load inertia
Make assumptions about compliance effects at higher
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frequenciesSet the tuning gains based on the measured load
inertia and the assumptions
Results were only as reliable as theassumptions!
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New Generation Autotune
Excite the load at the entire frequency range
Measure the motor/load characteristics across the
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entire range of frequenciesSet the tuning gains based on the full set of
measurements
Results are much more reliable.
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17 mSec settling
A Well-Tuned
(if not so-well-designed) System
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Spring effect
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Autotuning Can Adjust a Lot of Gains
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Gain = 0dB
Results for Auto-Tuned System
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Phase = -180
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Taking Controls to the
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Hardware
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Free standing driveWindows
Host- PC
Drive and
Motor
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Torque
Control
Loop
Position
Control
Loop
Trajectory
Generator
Velocity
Control
Loop
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Deployment Options Motion on WindowsWindows
Host- PC
cRIO Ethernet
ExpansionBackplane
Drive Interface
Hardware
Drive and
Motor
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Position
Control
Loop
Trajectory
GeneratorTorque
Control
Loop
Velocity
Control
Loop
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Four Secrets of Tuning
1. Think in terms of frequency. You can measure thefrequency response with modern drives!
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Four Secrets of Tuning
1. Think in terms of frequency. You can measure thefrequency response with modern drives!
2. Start with the best mechanical design you can afford.
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Four Secrets of Tuning
1. Think in terms of frequency. You can measure thefrequency response with modern drives!
2. Start with the best mechanical design you can afford.
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3. You need autotuning that works over the entirefrequency range.
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Four Secrets of Tuning
1. Think in terms of frequency. You can measure thefrequency response with modern drives!
2. Start with the best mechanical design you can afford.
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3. You need autotuning that works over the entirefrequency range.
4. You can deploy your control system to standard
hardware
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For more on the topic
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