Transcript Linear Motors

Sean DeHart
Smriti Chopra
Hannes Daepp
Overview
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DC Motors (Brushed and Brushless)
Brief Introduction to AC Motors
Stepper Motors
Linear Motors
Sean DeHart
2
Electric Motor Basic Principles
Interaction between magnetic field and current
carrying wire produces a force
 Opposite of a generator

Sean DeHart
3
Conventional (Brushed) DC Motors
Permanent magnets for
outer stator
 Rotating coils for inner
rotor
 Commutation
performed with metal
contact brushes and
contacts designed to
reverse the polarity of
the rotor as it reaches
horizontal
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Sean DeHart
4
2 pole brushed DC motor commutation
Sean DeHart
5
Conventional (Brushed) DC Motors
Common Applications:
 Small/cheap devices such as toys, electric tooth brushes,
small drills
 Lab 3
 Pros:
 Cheap, simple
 Easy to control - speed is governed by the voltage and
torque by the current through the armature
 Cons:
 Mechanical brushes - electrical noise, arcing, sparking,
friction, wear, inefficient, shorting
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Sean DeHart
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DC Motor considerations
Back EMF - every motor is also a generator
 More current = more torque; more voltage = more speed
 Load, torque, speed characteristics
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Shunt-wound, series-wound (aka universal motor),
compound DC motors
Sean DeHart
7
Brushless DC Motors
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Essential difference - commutation is performed
electronically with controller rather than
mechanically with brushes
Sean DeHart
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Brushless DC Motor Commutation
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Commutation is performed electronically using a
controller (e.g. HCS12 or logic circuit)
 Similarity with stepper motor, but with less #
poles
 Needs rotor positional closed loop feedback: hall
effect sensors, back EMF, photo transistors
Sean DeHart
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BLDC (3-Pole) Motor Connections
Has 3 leads instead of 2 like brushed DC
 Delta (greater speed) and Wye (greater torque)
stator windings
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Delta
Sean DeHart
Wye
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Brushless DC Motors
Applications
 CPU cooling fans
 CD/DVD Players
 Electric automobiles
 Pros (compared to brushed DC)
 Higher efficiency
 Longer lifespan, low maintenance
 Clean, fast, no sparking/issues with brushed contacts
 Cons
 Higher cost
 More complex circuitry and requires a controller

Sean DeHart
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AC Motors
 Two main types of AC motor, Synchronous and
Induction.
 Synchronous motors supply power to both the rotor
and the stator, where induction motors only supply
power to the stator coils, and rely on induction to
generate torque.
Sean DeHart
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AC Induction Motors (3 Phase)
Use poly-phase (usually 3) AC current to create a rotating
magnetic field on the stator
 This induces a magnetic field on the rotor, which tries to
follow stator - slipping required to produce torque
 Workhorses of the industry - high powered applications
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Sean DeHart
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AC induction Motors
 Induction motors only supply current to the stator,
and rely on a second induced current in the rotor
coils.
 This requires a relative speed between the rotating
magnetic field and the rotor. If the rotor somehow
matches or exceeds the magnetic field speed, there is
condition called slip.
 Slip is required to produce torque, if there is no slip,
there is no difference between the induced pole and
the powered pole, and therefore no torque on the
shaft.
Sean DeHart
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Synchronous AC Motors
 Current is applied to both the Rotor and the Stator.
 This allows for precise control (stepper motors), but
requires mechanical brushes or slip rings to supply
DC current to the rotor.
 There is no slip since the rotor does not rely on
induction to produce torque.
Sean DeHart
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Stepper Motor
A stepper motor is an electromechanical device which
converts electrical pulses into discrete mechanical
movements. The shaft or spindle of a stepper motor
rotates in discrete step increments when electrical
command pulses are applied to it in the proper sequence.
Smriti Chopra
Main features
The sequence of the applied pulses is directly related to
the direction of motor shafts rotation.
The speed of the motor shafts rotation is directly related
to the frequency of the input pulses.
The length of rotation is directly related to the number of
input pulses applied.
Smriti Chopra
Stepper Motor Characteristics
Open loop
The motors response to digital input pulses provides open-loop
control, making the motor simpler and less costly to control.
Brushless
Very reliable since there are no contact brushes in the motor.
Therefore the life of the motor is simply dependant on the life of
the bearing.
Incremental steps/changes
The rotation angle of the motor is proportional to the input
pulse.
Speed increases -> torque decreases
Smriti Chopra
Torque vs. Speed
Torque varies inversely with
speed.
Current is proportional to
torque.
Torque → ∞ means Current →
∞, which leads to motor damage.
Torque thus needs to be limited
to rated value of motor.
Smriti Chopra
Disadvantages of stepper motors
There are two main disadvantages of stepper motors:
 Resonance can occur if not properly controlled.
This can be seen as a sudden loss or drop in torque at certain speeds which can
result in missed steps or loss of synchronism. It occurs when the input step pulse rate
coincides with the natural oscillation frequency of the rotor. Resonance can be
minimised by using half stepping or microstepping
.
 Not easy to operate at extremely high speeds.
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Working principle
Stepper motors consist of a permanent magnet rotating
shaft, called the rotor, and electromagnets on the
stationary portion that surrounds the motor, called the
stator.
When a phase winding of a stepper
motor is energized with current, a
magnetic flux is developed in the
stator. The direction of this flux is
determined by the “Right Hand
Rule”.
Smriti Chopra
At position 1, the rotor is
beginning at the upper
electromagnet, which is
currently active (has voltage
applied to it).
To move the rotor clockwise
(CW), the upper electromagnet
is deactivated and the right
electromagnet is activated,
causing the rotor to move 90
degrees CW, aligning itself
with the active magnet.
This process is repeated in the
same manner at the south and
west electromagnets until we
once again reach the starting
position.
Smriti Chopra
Understanding resolution
Resolution is the number of degrees rotated per step.
Step angle = 360/(NPh * Ph) = 360/N
NPh = Number of equivalent poles per phase = number of rotor
poles.
Ph = Number of phases.
N = Total number of poles for all phases together.
Example: for a three winding motor with a rotor having 4 teeth,
the resolution is 30 degrees.
Smriti Chopra
Two phase stepper motors
There are two basic winding arrangements for the
electromagnetic coils in a two phase stepper motor:
bipolar and unipolar.
unipolar
Smriti Chopra
bipolar
Main difference
A unipolar stepper motor has two windings per phase, one
for each direction of magnetic field. In this arrangement a
magnetic pole can be reversed without switching the
direction of current.
Bipolar motors have a single winding per phase. The
current in a winding needs to be reversed in order to
reverse a magnetic pole.
Bipolar motors have higher torque but need more complex
driver circuits.
Smriti Chopra
Stepping modes
Wave Drive (1 phase on)
A1 – B2 – A2 – B1
(25% of unipolar windings , 50% of bipolar)
Full Step Drive (2 phases on)
A1B2 – B2A2 – A2B1 – B1A1
(50% of unipolar windings , full bipolar
windings utilization)
Half Step Drive (1 & 2 phases on)
A1B2 – B2 – B2A2 – A2 ---(increases resolution)
Microstepping (Continuously
varying motor currents)
A microstep driver may split a full step into as many as 256 microsteps.
Smriti Chopra
Types of Stepper Motors
There are three main types of stepper motors:
 Variable Reluctance stepper motor
 Permanent Magnet stepper motor
 Hybrid Synchronous stepper motor
Smriti Chopra
Variable Reluctance motor
This type of motor consists of a soft iron multi-toothed
rotor and a wound stator.
When the stator windings are energized
with DC Current, the poles become magnetized.
Rotation occurs when the rotor teeth
are attracted to the energized stator
poles.
Smriti Chopra
Permanent Magnet motor
The rotor no longer has teeth as with
the VR motor.
Instead the rotor is
magnetized with alternating north
and south poles situated in a straight
line parallel to the rotor shaft.
These magnetized rotor poles provide an increased
magnetic flux intensity and because of this
the PM motor exhibits improved torque characteristics
when compared with the VR type.
Smriti Chopra
Hybrid Synchronous motor
The rotor is multi-toothed like the VR motor and
contains an axially magnetized concentric
magnet around its shaft.
The teeth on the rotor provide an even
better path which helps guide the
magnetic flux to preferred locations in
the air gap.
Smriti Chopra
Applications
Stepper motors can be a good choice whenever controlled
movement is required.
They can be used to advantage in applications where you
need to control rotation angle, speed, position and
synchronism.
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These include
printers
plotters
medical equipment
fax machines
automotive and scientific equipment etc.
Smriti Chopra
Linear Motors
Hannes Daepp
Basics of Linear Motors [1],[4]
 Analogous to Unrolled DC Motor
• Force (F) is generated
when the current (I)
(along vector L) and the
flux density (B) interact
• F = LI x B
I
Hannes Daepp
Linear Motors in Action
 http://www.parkermotion.com/video/Braas_Trilogy_T3E_Video.MPG
Hannes Daepp
Analysis of Linear Motors [1],[5]
 Analysis is similar to that of rotary machines
 Linear dimension and displacements replace
angular ones
 Forces replace torques
 Commutation cycle is distance between two
consecutive pole pairs instead of 360 degrees
Hannes Daepp
Benefits of Linear Motors [2]
 High Maximum Speed
 Limited primarily by bus voltage, control electronics
 High Precision
 Accuracy, resolution, repeatability limited by feedback device, budget
 Zero backlash: No mechanical transmission components.
 Fast Response
 Response rate can be over 100 times that of a mechanical
transmission  faster accelerations, settling time (more throughput)
 Stiffness
 No mechanical linkage, stiffness depends mostly on gain & current
 Durable
 Modern linear motors have few/no contacting parts  no wear
Hannes Daepp
Downsides of Linear Motors [2]
 Cost
 Low production volume (relative to demand)
 High price of magnets
 Linear encoders (feedback) are much more expensive than rotary
encoders, cost increases with length
 Higher Bandwidth Drives and Controls
 Lower force per package size
 Heating issues
 Forcer is usually attached to load  I2R losses are directly coupled to
load
 No (minimal) Friction
 No automatic brake
Hannes Daepp
Components of Linear Motors
[2],[3]
 Forcer (Motor Coil)
 Windings (coils) provide current (I)
 Windings are encapsulated within core
material
 Mounting Plate on top
 Usually contains sensors (hall effect
and thermal)
 Magnet Rail
 Iron Plate / Base Plate
 Rare Earth Magnets of alternating
polarity provide flux (B)
 Single or double rail
Hannes Daepp
F=
lI x B
Types of Linear Motors [1],[2],[3]
 Iron Core
 Coils wound around
teeth of laminations
on forcer
 Ironless Core
 Dual back iron
separated by spacer
 Coils held together
with epoxy
 Slotless
 Coil and back iron
held together with
epoxy
Hannes Daepp
Linear Motor Types: Iron Core [1],[2]
Distinguishing Feature
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Copper windings around forcer laminations over a single magnet rail
Advantages:
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Highest force available per unit volume
Efficient Cooling
Lower cost
Disadvantages:
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High attractive force between forcer & magnet track
Cogging: iron forcer affects thrust
Laminated forcer
force as it passes over each
assembly and mounting
magnet (aka velocity ripple)
plate
Coil wound Around
Forcer lamination
Hannes Daepp
Rare earth magnets
Iron Plate
Hall effect
and thermal
sensors
Top View
Linear Motor Types: Ironless
[1],[2]
Distinguishing Feature
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Forcer constructed of wound coils held
together with epoxy and running
between two rails (North and South)
 Also known as “Aircore” or “U-channel”
motors
Advantages:
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No attractive forces in forcer
No Cogging
Low weight forcer - No iron means
higher accel/decel rates
Disadvantages:
Front View
Forcer
Mounting
Plate
Winding, held
Rare
by epoxy
Earth
Magnets
Hall Effect and
Horseshoe
Thermal
Shaped
Sensors in coil
backiron
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Low force per package size
Lower Stiffness; limited max load without improved structure
Poor heat dissipation
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Higher cost (2x Magnets!)
Hannes Daepp
Linear Motor Types: Slotless
[1],[2]
Side View
Distinguishing Feature
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Mix of ironless and iron core: coils with
back iron contained within aluminum
housing over a single magnet rail
Front View
Advantages over ironless:
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Lower cost (1x magnets)
 Better heat dissipation
 Structurally stronger forcer
 More force per package size
Advantages over iron core:
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Lighter weight and lower inertia forcer
 Lower attractive forces
 Less cogging
Hannes Daepp
Thermal
sensor
Coil
Back
assembly iron
Rare
Earth
Magnets
Mounting
plate
Iron
plate
Linear Motor Types: Slotless
[2],[3]
Side View
Disadvantages

Some attractive force and cogging
 Less efficient than iron core and
ironless - more heat to do the same job
Front View
Thermal
sensor
Coil
Back
assembly iron
Rare
Earth
Magnets
Hannes Daepp
Mounting
plate
Iron
plate
Linear Motor Type Comparison [2]
Linear Brushless DC Motor Type
Feature
Iron Core
Ironless
Slotless
Attraction Force
Most
None
Moderate
Cost
Medium
High
Lowest
Force Cogging
Highest
None
Medium
Power Density
Highest
Medium
Medium
Forcer Weight
Heaviest
Lightest
Moderate
Hannes Daepp
Components of a “Complete” Linear
Motor System [3]
Motor components
2. Base/Bearings
3. Servo controller/feedback
elements
1.
• Typical sensors include Hall
Effect (for position) and thermal
sensors
4. Cable management
Hannes Daepp
Sample Pricing
 $3529
 Trilogy T1S Ironless linear
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motor
110V, 1 pole motor
Single bearing rail
~12’’ travel
magnetic encoder
Peak Velocity = 7 m/s
Resolution = 5μm
Hannes Daepp
Applications [3],[5],[6]
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Small Linear Motors
 Packaging and Material Handling
 Automated Assembly
 Reciprocating compressors and
alternators
Large Linear Induction Machines
(3 phase)
 Transportation
 Materials handling
 Extrusion presses
Hannes Daepp
References
[1] S. Cetinkunt, Mechatronics, John Wiley & Sons, Inc., Hoboken 2007.
[2] J. Barrett, T. Harned, J. Monnich, Linear Motor Basics, Parker
Hannifin Corporation,
http://www.parkermotion.com/whitepages/linearmotorarticle.pdf
[3] Trilogy Linear Motor & Linear Motor Positioners, Parker Hannifin
Corporation, 2008,
http://www.parkermotion.com/pdfs/Trilogy_Catalog.pdf
[4] Rockwell Automation,
http://www.rockwellautomation.com/anorad/
products/linearmotors/questions.html
[5] J. Marsh, Motor Parameters Application Note, Parker-Trilogy Linear
Motors, 2003. http://www.parkermotion.com/whitepages/
Linear_Motor_Parameter_Application_Note.pdf
[6] Greg Paula, Linear motors take center stage, The American Society
of Mechanical Engineers, 1998.
References (continued)
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http://www.physclips.unsw.edu.au/jw/electricmotors.ht
ml
http://www.speedace.info/solar_car_motor_and_drivet
rain.htm
http://www.allaboutcircuits.com/vol_2/chpt_13/1.html
http://www.tpub.com/neets/book5/18d.htm single
phase induction motor
http://www.stefanv.com/rcstuff/qf200212.html
Brushless DC motors
https://www.geckodrive.com/upload/Step_motor_basic
s.pdf
http://www.solarbotics.net/library/pdflib/pdf/motorbas
.pdf
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