By: George Procter
V-P Motion Systems
Copley Controls Corp.
20 Dan Road, Canton, MA 02021
Tel: 781-828-8090
gprocter@copleycontrols.com
www.copeycontrols.com
Linear Motor Advantages In Machine Vision Systems
Advances in sensor speed and sensitivity have raised the
performance standard for machine vision scanning
mechanisms. No longer can traditional ballscrews, used for
linear axis motion control, meet today’s productivity challenges.
In contrast, direct electromagnetic drive enables linear motors
to deliver resolution and reliability unattainable with traditional
ballscrew scanners. Copley Controls’ fast response tubular
linear motors raise vision system throughput, reduce wear, and
shrink downtime.
Pushing Out The Envelope
Modern linear motors, benefitting from
advances in design, magnetic materials,
and electronic controls, raise machine
vision throughput, precision and reliability. With form factor matching a
ballscrew’s, Copley’s tubular linear
motors, Figure 1, provide a direct replacement and substantial scanning system
upgrade. The linear motors also integrate
easily into new machine vision designs.
Direct Drive
Optical sensing equipment conveniently
mounts directly onto the moving forcer.
The moving forcer has internal coils that
surround the permanent magnet thrust
rod. Interaction between the forcer’s
electrically energized coils and the thrust
rod’s permanent magnet field, produce
motor drive force. The forcer and its
camera load travel smoothly across the
workpiece.
Lead screw driven by
separate motor.
Flexible Coupling
lead screw
converts rotary
to linear motion
Drive
Motor
Load mounts
on platform
Load mounts
directly onto
rugged forcer
BallScrew
vs
Linear Motor
Large air gap
simplifies alignment
Forcer develops
electromagnetic
drive force
Linear encoder is
built into motor frame
Permanent magnets
sealed into thrust rod
Figure 1—Ballscrew scanning mechanisms, top, no longer
meet the productivity challenge of modern machine vision
applications. Linear motors, bottom, provide dramatically
superior specifications, plus similar form factor as
ballscrews, simplifying performance upgrading.
Upgrade machine vision ATE with linear motor scanning
Page: 2
Performance
With peak velocities of 23 m/s and accelerations of 20g, Copley linear motors surpass ballscrew
performance by an order of magnitude. Friction between forcer and thrust rod is totally absent
eliminating wear and maintenance, Figure 2.
Motor components are available in ready-toEnclosed drive
Thermally efficient design
use actuators equipped with optical position
coils minimize RFI
(coils on the outside)
sensors for positioning resolution to 0.1 µM.
eliminates forced air cooling
Non-Contact: Friction-Free
Friction free travel eliminates wear as a source
of linear motor positioning and scanning error.
Error reduction is particularly important for
machine vision applications that must resolve
microscopic visual characteristics. Freedom
from static and dynamic friction at the ultra low
scanning speed removes a potential source of
random variation in scanning velocity.
Large air gap eases
machine tolerances
Field replaceable
cable connector
Magnets enclosed
in thrust tube
Rugged load
bearing housing
Velocity Ripple
2—Large air gap between traveling forcer and
When scanning objects with feature dimensions Figure
stationery thrust rod eases mechanical integration.
measured in microns, minuscule deviations in
Freedom from friction and wear raises reliability and
scanning velocity can be interpreted as anoma- maximizes uptime for 24/7 scanning applications.
lies or flaws in the scanned object. To prevent
such misdiagnosis, sensor travel must be free from the cyclic velocity variations referred to as
velocity ripple. In ballscrew mechanisms, velocity ripple varies depending on preload and lubrication. Velocity performance degrades as the ballscrew wears. Copley’s linear motors are extremely low velocity ripple free, owing to their air-core, patented magnetic design.
Core Less Coils
To minimize force ripple, the linear motor’s magnetic drive coils are air cored, Figure 3. Use of
air cored drive coils eliminates variable magnetic permeability, and with it, nonlinear motor
drive force. The motor’s drive coils are energized by digitally generated sinusoidal excitation, which exerts a constant drive force.
Proof
A manufacturer of semiconductor wafer scanning equipment who upgraded to Copley linear
motors reports unprecedented velocity ripple
performance. The previous ballscrew scanning
system had been unable to meet the linearity
challenge.
Slow Scanning: Fast Return
Slow speed scanning applications still benefit
from the linear motor’s fast travel capability. In
unidirectional scanning, a high speed return
Figure 3—Forcer’s cylindrical magnetic coils are
wound without an iron core. Air-cored coil design enables sinusoidal excitation currents to develop smooth
drive force without force variation. Constant drive
force, free from force variation (“force ripple”), is key
to machine vision inspection of parts with feature dimensions measured in microns.
Upgrade machine vision ATE with linear motor scanning
enhances productivity. The optical sensor must
also be slewed from its rest position to the
edge of the target object at the start of each
operation.
High resolution characterizes one extreme of
the machine vision field: the opposite extreme
involves 24/7 scanning of high volume products. Example: cell phones and other assemblies. In such applications, productivity is
critical and downtime is costly. High volume
applications can fully capitalize on the linear
motor’s unprecedented reliability.
No Lubrication Necessary
The linear motor’s “forcer,” which carries the
machine vision sensor, does not make contact
with the thrust rod, Figure 4. Freedom from
contact eliminates friction-induced wear and
subsequent performance degradation.
Page: 3
Thrust Rod Linear Actuator
Forcer develops
Load mounts
drive thrust
directly onto forcer
Large air gap
Thrust rod contains
permanent magnets
Retractable power
cable system
Linear encoder built into
actuator frame
Forcer supported
by recirculating bearings
Figure 4—Linear motor actuator surpasses the speed,
acceleration, reliability and accuracy of traditional
ballscrew mechanisms. Electromagnetic drive force
is developed directly by air cored forcer coils for
smooth, ripple-free performance. There’s no gear train
and ballscrew to increase inertia and limit acceleration. The large air gap between forcer and thrust rod
eliminates friction and wear as a source of long term
positioning error, easy integration of custom designs.
Scanning velocity is unconstrained by ballscrew whip
and lubrication spin-off. The module can be equipped
with air bearings for sensitivity unmatchable with
ballscrew mechanisms. (See Table, page 4, for motor/
ballscrew comparisons).
No friction means no need for lubrication,
simplifying maintenance. The linear motor’s
freedom from thrust rod lubrication provides a
further important advantage. Increasing scanning velocity in a ballscrew implementation can cause lubricant to spin off and contaminate both
the workpiece and the work environment. Lubricant cleanup involves costly downtime.
Wear Free
Ballscrew wear requires costly replacement, incurring extended shutdown for ballscrew replacement. Shutdown for any cause is costly. If the service technician has to travel across country to
replace the ballscrew, both down time and maintenance costs escalate.
Further Linear Motor Benefits
Smaller Overall Footprint
Some scanner upgrades have been driven by the compact size of a linear motor versus ballscrew
and motor. Attaching a motor to the end of a ballscrew significantly increases the actuator footprint.
Dynamic Stiffness
Linear motors provide direct drive with excellent dynamic stiffness. There are no intervening
belts, gears, ballscrew with associated whip at high speed, or flexible couplings to introduce
mechanical resonances. Servo loop bandwidth, no longer constrained by mechanical resonance,
can be increased to deliver optimal precision.
Upgrade machine vision ATE with linear motor scanning
Bearing Versatility—Air Bearings
Linear motors can be equipped with a variety
of modern bearings for lowest friction and
maximum long-term reliability. Use of air
bearings, together with the linear motor’s noncontact forcer travel, afford a significant
sensitivity benefit unattainable by ballscrew
mechanisms. (Ballscrew friction would defeat
the advantage conferred by air bearings).
Miniature TT Micro Motor
For small scale scanning applications, Copley’s
miniature TT Micro
Specifications
linear motors provide
travel distance from a
Max Speed
centimeter to half a
meter. The TT Micro
Backlash, Cogging
motors embody the
same tubular design as
Resolution
their larger thrust motor
counterparts, and use air Acceleration
cored magnetic coils for Max Force
ripple free operation.
Dynamic stiffness
These motors provide a
potent alternative to
Settling time
voice coil positioning
systems owing to their
Controllability
much long travel distance.
Cost Of Ownership
Linear motor drive
systems tend to be
regarded as costly
solutions to intractable
and highly specialized
problems. Invariably,
someone else’s problem! In reality, the
linear motor is often the
most economic solution,
even where alternative
drives meet technical
specifications.
Several factors contribute to the motor’s cost
Page: 4
Figure 5— Copley provides the TT Micro family of miniature linear motors and motor modules with travel
distances from one centimeter to half a meter. A major advantage of these small linear motors is ability to
provide far longer travel than the voice coil positioning mechanisms. Like their larger ThrustTube counterparts, the TT Micro motors’ large air gap between
forcer and thrust rod relaxes the mechanical specifications for system integration.
Linear Motor
BallScrew
100 inches/second
20 inches/second
None
Caused by lead screw
wear; gearbox if used
Sub micron
Microns
10g
2g
200 pounds
1000 pounds
16 - 21 kgf/mm
9 - 18 kgf/mm
10 - 20 ms
100 ms
Superior: direct
digital servo drive
Moderate. Flexible
coupling can cause
resonance
Drive Method
Direct
Gear train can
introduce backlash
Bearings
Can use wide range, Lead screw friction
defeats air
including air
bearingbenefit
bearings
Maintenance
requirements
Very low
Moderate to high
Environmental
Contamination
Minimal
Oil, grease, swarf
contamination
Noise level
Lowest
Moderate
Life cycle cost
Very low, single
moving part
Moderate, many
wearable parts
Features
Table 1—Linear motor specifications demonstrate compatibility with advances
in machine vision scanning. The only attribute in which ballscrew performance
outstrips the linear motor’s lies in maximum force... which is not particularly
relevant to scanning applications
Upgrade machine vision ATE with linear motor scanning
Page: 5
effectiveness. Linear motor costs have declined significantly in recent years, thanks to the benefits of scale and to steady reduction in the cost of rare earth magnets. In addition, the high
performance servo amplifiers needed for powering and controlling them have made the transition
to all-digital technology.
Finally, the linear motor’s enduring uptime
ensures profitable operation, bringing down
life cycle costs— and cost-of-ownership —
well below that of the typical ballscrew installation.
Mechanical Integration
From a mechanical compatibility point of
view, the load mounts directly onto the
thruster’s surface, exactly as it mounts on a
ballscrew’s load platform. Consequently,
Thrust Rod™ linear motors can directly
replace ballscrew mechanisms with minimal
mechanical alteration. The linear motor’s
mechanical form factor works well with the
gantry construction of many X-Y vision systems
CANopen
Network
Cool Operation
Conventional U-channel linear motors—whose
coils are enclosed within the magnetic track—
suffer from thermal problems. The Thrust
Rod™ motor design is inherently self cooling.
The magnet coils are on the outside, surrounding the thrust rod, for effective heat dissipation. Cooling fins on the thruster surface are
completely unobstructed, enabling them to
radiate heat energy.
Figure 6—Technological strides have enabled linear
motors to make the transformation from the other
engineer’s last resort to today’s obvious answer.
Among the advances are new high strength permanent magnets costing a fraction of traditional prices.
Advanced field oriented control enables the motor to
run faster and cooler. Sinusoidal modulation of motor
drive power develops ripple free thruster power; carrier cancellation modulation of the servo amplifier’s
semiconductor power circuits ensures true zero
crossing linearity, superimposes minimal noise current on motor excitation currents. Finally, Copley’s
recently introduced CANopen compatible Accelnet™
servo amplifiers put multi-axis drive motors under
networked control.
Modern Networked Control
Technological strides have enabled linear motors to make the transformation from the other guy’s
exotic solution to today’s obvious answer. Among the advances are new high strength permanent
magnets costing a fraction of traditional prices. Innovations in brushless motor design, allied
with servo amplifiers that provide advanced field oriented power control, raise the maximum
speed bar. Sinusoidal modulation of motor drive currents produces a ripple free drive force;
carrier cancellation modulation of the servo amplifier’s MOSFET or IGBT power circuits ensures true zero crossing linearity. Carrier cancellation modulation also minimizes noise current
on motor drive power. Finally, Copley’s recently introduced CANopen compatible Accelnet™
servo amplifiers put multi-axis drive motors under networked control.
END