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Laser Solutions Short Courses

Laser Solutions Short Courses

Short Course #1:
State-of-the-Art Precision Motion
Systems & Their Applications in
Advanced Laser Materials
Processing





Jim Johnston
Course Instructor


Monday, October 29 1:30PM

Room: Narcissus / Orange

1
State of the Art Precision
Motion Systems and Their
Applications in Advanced Laser
Materials Processing
Jim Johnston
Aerotech, Inc.
October 29, 2007
Outline
Typical Motion System Components
Stages
Mechanical basics / stage design
Controller
Control theory basics
Drive electronics
Amplifier overview and design
Analysis of a system
Contribution of stack-up errors
Calibration metods
Application examples
Sizing a motor
Application review
Mechanical Basics
Bearing technologies
Mechanical
Air bearing
Motor technologies
Rotary motors
Linear motors
Feedback devices
Stage Dissection
Linear
Bearing Rails
Linear Motor
Ball Screw
Brushless
Rotary Motor
Cable Management
Chain
Moving
Carriage
Magnet Track
Linear Encoder
Bearing Styles: Ball Cage
Bearing Styles: Linear
Ball bearings:
Self cleaning (point of contact does not allow debris to interfere
Lower cost (components are common and easily manufactured
Cross roller bearings:
Higher line contact offer 8-10x load capacity over ball slides
Higher stiffness, smoother operation
Limited travel and potential for higher pitch errors with high
cantilever loads 2
Bearing Styles: Air Bearing
Bearing established by transferring pressure force through
orifices, generating a gap across the bearing area.
Can support a force approximated by F = P
avg
*Area
Actual pressure varies based on bearing design and other
parameters, but generally considered to be 30% efficient, or F =
0.3 * P
avg
*Area
Calculation fairly straightforward for flat
pads and rectangular bearings. Spherical
bearings require many other considerations
Bearing Styles: Air Bearings
Typically three phase
Stator contains the windings, rotor contains
permanent magnets
Similar to stepper motors, but with less poles
Preferred over brush motors
Increased reliability
Lower maintenance costs
Higher power
Higher torque/inertia ratio
Brushless Rotary Motors
Brushless Rotary Motors
Stators formed by stacking
iron laminations and
wrapping the phase coils
around the slots of the
laminations
Iron laminations increase the
torque of the motor by
directing the magnetic flux
radially into the airgap
Also add cogging torque,
which can be reduced by
skewing the laminations
No cogging in slotless motors
Laminations
Windings
Skewing
Advantages: High Acceleration High Torque No Maintenance Highest Max Temperature Rating
Disadvantages: Complicated Amplifier Design Cogging Torque Ripple
High Strength
Neo-dymium magnets
Skewed Laminations
Copper wire
Motor Overview
Linear Motors 3
Linear Motors
Essentially a rotary motor unrolled flat
The forcer (rotor) is made up of coils of wires
encapsulated in epoxy. The track is constructed by
placing magnets on steel.
Like a brushless rotary motor, the forcer and track
have no mechanical connection (no brushes
)
The control of linear motors is identical as with rotary
motors
History
Eric Laithwaite (1921-1997) is generally
credited with inventing linear motors
Early proponent of Maglev for
transportation
Spent later years investigating
gyroscopes and possible uses for them
in propulsion
Last project was electromagnetic
launch system for NASA (Maglifter),
uncomplete at time of death
Coil Construction Types
T-Shape
Easier manufacturing
Non-overlapped windings
Lower force/unit volume
I-Shape
More difficult to manufacture
Overlapped windings
Highest force/unit volume
I-Shape Coil
Potting with thermal
conductive epoxy under
vacuum maximizes heat
removal
Embedded Hall Effect
Sensors aligns motor
phases with magnets
Ultra high-flex cables for
motor power, hall effect
and thermister feedback
3 phases of coils tightly
wound for highest force
output per area
Linear Motors
Moving Forcer:
Forcer weight small compared to load
Requires cable management system with high flex cable
Smaller heatsink
Moving Track:
Must move load plus weight of the track
No cable management system required
Can use mounting base as a larger heatsink
Magnet Tracks
U-Channel
Usually ironless
Good for applications requiring
smooth motion (scanning or scribing
application)
Typical magnet assembly U-shaped
with magnets facing each other
across the gap.
Flat
Usually have iron-core and have
higher force output than the ironless
designs. Attractive force to iron core
may consume large percentage of
bearing load, requiring larger stage
Leads to cogging not suitable for
applications needing smooth motion
Good for high speed point-to-point
motion control 4
Linear Motors vs. Ballscrew
Advantages over ballscrew:
Frictionless, no contact or lubrication -> no screw wear
No backlash
Applicable for high speeds / accelerations
No temperature effects (reduces accuracy on screws)
Smooth velocity profile
No windup/compliance
Characteristics do not change over time
Accuracy limited only by feedback sensor
Disadvantages:
Higher cost per travel
No holding force when disabled
Linear Motor vs. Ball Screw Velocity
Encoders: Absolute
Provide an absolute position on
power-up
One design uses separate tracks
for each bit of information
N tracks yield 2
N
counts per
revolution
Usually coded in Gray scale (only
one bit to change between
successive numbers)
Other types of absolute encoders
use two tracks corresponding to
v
1
=sin() and v
2
=cos(), with he
angle calculated from
=atan(v
1
/v
2
).
Encoders may combine absolute
tracks for coarse position with
incremental tracks for finer
resolution
Source: Heidenhein
Encoders: Incremental
Fundamentally count a series of
equally-spaced pulses as they pass
by a detector
An index pulse is needed to provide
an absolute position reference
Available in long travel (limited only by
the scale length)
Absolute position may not be
available at startup
Speed limitation at high resolutions
Quantization errors occur at small
motions and low speeds
Quadrature decoding increases the
resolution by four
Source: Heidenhein
Quadrature Decoding
Two channels (A and B) spaced 90° apart increase
the fundamental resolution 4x.
Missed states can indicate an error condition.
CHANNEL B
CHANNEL A
Linear Stage Thermal Expansion
Typical coefficients of thermal expansion
(PPM per C)
Aluminum: 24
Float glass: 8
(default material used for most Aerotech encoders)
Pyrex glass: 4
Invar: < 1.4,
Zerodur: << 1, essentially zero 5
Effects of Thermal Expansion
Linear Accuracy Error Induced by Thermal Growth of the Encoder Scale
Example: Temperature increase of 2 degrees C
0
10
20
30
40
50
60
0
100
200
300
400
500
600
700
800
900
1000
Nominal Travel (mm)
L
i
n
ear
Po
sit
i
o
n
i
n
g
Er
r
o
r
(
m
icr
o
n
s
)
This example assumes uniform heating
of the encoder scale. For mechanical
bearing linear stages, however,
localized heating of the encoder scale
can be caused by the motor forcer
coil.
Scale on Aluminum
Scale on Float glass
Scale on Pyrex glass
Scale on Invar
Control Theory Basics
Open loop and closed loop control
Motor model
PID overview
Loop analysis
Closed Loop System
Open Loop / Forward Path
Closed Loop
Controller
Plant
+
-
Input
Output
PID Loops
The PID loop is responsible for keeping the error
between the position command and feedback to a
minimum
Kpos
Ki/S
Kp
Kv S
Pos Cmd
+
+
+
+
-
+
+
-
Pos Feedback
Vel Feedback
Vff S
Aff S
2
+
+
Icmd to amps
Vel Cmd
Feedforward Terms
Friction ff
Simple Motor Model
Kp = Proportional gain
Ki = Integral gain
Kd = Derivative gain
The variable (e) represents the tracking error, the difference between the
desired input value (R) and the actual output (Y)
This error signal (e) will be sent to the PID controller, and the controller
computes both the derivative and the integral of this error signal
The signal (u) just past the controller is now equal to the proportional gain
(Kp) times the magnitude of the error plus the integral gain (Ki) times the
integral of the error plus the derivative gain (Kd) times the derivative of the
error
PID Controller +
+
=
dt
de
D
I
p
K
edt
K
e
K
u 6
Effect of Kp, Ki, & Kd
CL RESPONSE RISE TIME OVERSHOOT
SETTLING
TIME
S-S
ERROR
Kp
Decrease
Increase
Small
Change
Decrease
Ki
Decrease
Increase
Increase
Eliminate
Kd
Small
Change
Decrease
Decrease
Small
Change
A proportional controller (Kp) will have the effect of reducing the rise
time and will reduce, but never eliminate, the steady-state error.
An integral control (Ki) will have the effect of eliminating the steady-
state error, but it may make the transient response worse.
A derivative control (Kd) will have the effect of increasing the
stability of the system, reducing the overshoot, and improving the
transient response.
Simple Mass Example
Simple Mass Example
Simple Mass Example
Simple Mass Example
Simple Mass Ex