Contact-less Manipulation of Millimeter-scale Objects via ...
Contact Less Energy Transfer 1
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Transcript of Contact Less Energy Transfer 1
ABSTRACT
In this paper a new topology for contactless energy transfer is proposed and tested
that can transfer energy to a moving actuator using inductive coupling. The proposed
topology provides long-stroke contactless energy transfer capability in a plane and a
short-stroke movement of a few millimeters perpendicular to the plane. In addition, it is
to lerant to small rotations. The experimental setup consists of a platform with one
secondary coil, which is attached to a linear actuator and a 3-phase brushless
electromotor. Underneath the platform is an array of primary coils that are each
connected to a half-bridge square wave power supply. The energy transfer to the
electromotor is measured while the platform is moved over the array of primary coils by
the linear actuator. The secondary coil moves with a stroke of 18cm at speeds over 1m/s,
while up to 33W power is transferred with 90% efficiency.
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INTRODUCTION
Most high-precision
machines are positioning stages
with Multiple degrees of
freedom(DOF), which often
consist of cascaded long-and short-
stroke linear actuators that are
supported by mechanical or air
bearings. Usually, the long stroke
actuator has micrometer accuracy,
while the Submicron accuracy is
achieved by the short-stroke
actuator. To build a high-precision
machine, as much disturbances as
possible should be eliminated.
Common sources of disturbances
are vibrations, Coulomb and
viscous friction in bearings,
crosstalk of multiple cascaded
actuators and cable slabs.
A possibility to increase throughput,
while maintaining accuracy is to use
parallel processing, i.e. movement and
positioning in parallel within section,
calibration, assembling, scanning, etc.
To meet the design requirements of high
accuracy while improving performance,
a new design approach is necessary,
especially if vacuum operation is
considered, which will be required for
the next generation no lithography
machines. A lot of disturbance sources
can be eliminated by integrating the
cascaded long-and short-stroke actuator
into one actuator system. Since most
long-stroke movements are in a plane,
this can be done by a contactless planar
actuator.
The topology proposed and
tested in this paper provides long-
stroke contact less energy transfer
(CET) in a plane with only small
changes in power transfer
capability.
DESCRIPTION
ACTUATOR
Actuator is a mechanical device
used for moving or controlling a
mechanism or system. It converts
electrical signals into motion.
Here we are using a linear actuator;
it converts electrical signals into
linear motion i.e. the movement is
linear in manner along a plane.
CET TOPOLOGY
The design of the primary and
secondary coil is optimized to get a
coupling that is as constant as
possible for a sufficiently large
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area. This area should be large
enough to allow the secondary coil
to move from one primary coil to
the next one without a large
reduction in coupling. If this can be
achieved, the power can be
transferred by one primary coil that
is closest to the secondary coil.
When the secondary coil moves
out of range the first primary coil is
turned off and the next one will be
energized. To ensure a smooth
energy transfer to the moving load,
the position dependence of the
coupling should be minimized,
while keeping the coupling high
enough to get a high-efficiency
energy transfer.
The drawing in Fig.3 shows
one secondary coil above nine
primary coils. The black square
shows the area in which the center
of the secondary coil can move
while maintaining good coupling
with the middle primary coil. The
secondary coil is situated in the
bottom-left corner of the area of
interaction with the middle primary
coil. The coupling between the
primary coil and the secondary coil
within that area is calculated with
Maxwell 3D 10Optimetrics and
measured
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STEADY-STATE ELECTRIC
CIRCUIT ANALYSIS
Since the system will
be used in a maglev application
based on repulsive forces between
coils and permanent magnets, the
use of iron or ferrites is prohibited.
In addition, the use of cores will
limit the stroke of the system.
Therefore, a coreless or air core
inductive coupling is used to
transfer the energy. To keep the
efficiency of an air core inductive
coupling high a resonant capacitor
is used for both the primary and the
secondary coil. Moreover, due to
the position dependent coupling, a
series resonant capacitor is used for
both coils to ensure that the
resonant frequency of the circuit
does not depend on the coupling.
The electric circuit of the CET
system is shown in Fig.5, where V
1 is the RMS voltage of the power
supply, I 1 is the RMS current
supplied by the power supply, I 2
the RMS current induced in the
secondary circuit. C 1and C 2 are
the series resonant capacitors in the
primary and secondary circuit, R 1
is the resistance of the primary
coil, R2 is the resistance of the
secondary coil. L 1 and L 2 are the
self inductance of the primary and
secondary coil, respectively. k is
the inductive coupling factor
between the primary and secondary
coil, and R L is the resistance of
the load. The load R L represents
the rectifier and additional power
electronics.
Simplified versions of the
circuit are shown in Fig.6a and b,
where Z R is the reflected load of
the secondary circuit on the
primary circuit and Z 1is the load
seen by the power supply.
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EXPERIMENTAL SETUP
An experimental setup was built to
test the CET design, which consists
of an array of three stationary
primary coils that are fixed in a
row on top of a ceramic structure.
The ceramic structure is used to
allow heat from the coils to be
conducted to the iron base frame
and at the same time to prevent
eddy current losses in the iron base
frame. The primary coils are made
of litz wire. Each bundle of litz
wire consists of 60 strands of 71
5
µm and the strands are wrapped
together with a layer of cotton. The
strand size has been chosen after
examining the AC losses. The
turns of the coil are fixed by glue
that has been applied during the
winding process. Approximately
120 turn fitted in the cross-section,
resulting in a 0.3 filling factor.
Each primary coil is connected in
series with a resonance capacitor.
Each resonant circuit is driven by a
separate half-bridge power supply
that applies a square wave voltage
of 191 kHz over the resonant
circuit. The schematic of the half-
bridge power supply is shown in
Fig. 7. An overview of the primary
coils and the corresponding series
capacitors is shown in Table II.
The secondary coil is fixed onto a
ceramic plate that is bolted to the
mover of a linear actuator. Again
ceramic material is used for heat
conduction and the minimization
of eddy current losses. The linear
actuator can move the secondary
coil over the three primary coils.
The position of the secondary coil
with respect to the array of primary
coils is measured by the encoder of
the linear actuator.
The secondary coil is
connected in series with a resonant
capacitor. The circuit is then
connected to a full-bridge diode
rectifier to generate a DC output.
The DC output of the rectifier is
connected to the load, which is an
electromotor of
a CD drive running at 12 VDC.
All subsystems are connected to a
ds1103 dSpace system running the
control program at 8 kHz. This
way the DC bus voltage of the
primary coil power supplies is
controlled as well as which of the
primary coil power supplies is
enabled. The position of the linear
actuator is controlled using a PID
controller running on the dSpace
system. Depending on the position
of the linear actuator the dSpace
system enables the primary coil
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that is completely overlapped by
the secondary coil.
The primary coil
activation is controlled by a multi-
port switch. The multi-port switch
has four active coil states; state1
enables the power supply of the
first primary coil, state 2 and 3
enable the power supply of the
second and third primary coil,
respectively. State 4 disables all
power supplies and this state is
used for switching from one power
supply to the next. When the
secondary coil moves out of range
of primary coil 1 (active coil state
1), the active supply is switched off
(active coil state 4) and one sample
time later the second supply is
switched on (active coil state 2).
For one sample time none of the
power supplies is active (active
coil state 4), which is necessary to
allow the triac in the power supply
that is switched off to block the
circuit after the current in the
resonant circuit is damped. There
is no other control mechanism in
the power electronics, and the
system operates without any
measurement on the secondary site,
except for the position of the
secondary coil
SCHEMATIC DIAGRAM
OF POWER SUPPLY
RESULTS
An electromotor of a
CD drive that runs on 12 VDC is
connected to the rectifier. The
voltage and current from the DC
bus supply as well as the voltage
and current to the CD drive are
measured and shown in Fig. 10 and
11. The secondary coil is moving
over all three primary coils
following a sinusoidal position
reference, which represents a total
displacement of 18 cm (i.e. the
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amplitude of the sine wave is 9
cm). The frequency of the
sinusoidal position reference is 2
Hz, so in one second the secondary
coil makes two cycles (one cycle
implies moving from primary coil
1 over primary coil 2 to primary
coil 3 and back). The cycle is
clearly visible from the Active Coil
plot in Fig. 10 and 11, which
represents the state of the active
coil multi-port switch. The
secondary coil reaches a maximum
speed of 1.1 m/s over the second
primary coil. Due to this speed the
secondary coil is in range of the
second primary coil for only 60
ms.
By calculating the RMS
values of the voltages and currents
the power from the DC bus supply
Pin as well as the power to the CD
drive load Pout and the efficiency
η according to Eq. 14 can be
calculated. This calculation
includes losses in the power
electronics. The values are listed in
Table III.
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In Fig. 12, the
transient behavior is shown when
the secondary coil is moving from
primary coil 1 to primary coil 2. It
is clearly visible that all power
supplies are switched off when the
active coil state has value 4. There
is also some delay between the
active coil state switch and the
response from the power
electronics, which is caused by a
slow rising edge of the enable
signal and by delay in the power
electronics. In Fig. 10 and 11 the
switching is also visible in the
current waveforms, since no
current is drawn from the DC bus
supply and no current is available
for the electromotor of the CD
drive.
The ripples visible in
the voltage and current waveforms
from the DC bus power supply and
to the CD drive are related to the
changing coupling. However, since
the CD drive does not represent a
purely resistive load, the ripple is
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somewhat smoothed by the
inductance of the load. This effect
is more visible when a purely
resistive load will be connected to
the system. In addition, the CD
drive does not need much power to
operate and a resistive load can be
operated at higher power levels.
Therefore, a 50 Ω resistive load is
used at a higher power level. The
same trajectory is used for the
secondary coil. The measured
voltage and current waveforms of
the DC bus supply and the load are
shown in Fig. 13 and 14
respectively. The RMS values of
voltage, current and power as well
as the efficiency are shown in
Table IV.
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The variation in coupling is now
clearly visible in
the current and voltage waveforms
of the load. This suggests that the
power transfer can be further
smoothed by measuring the
coupling and changing the voltage
of the DC bus supply accordingly.
The results are very similar to the
results of the CD drive.
Higher power levels have not been
tested using the linear actuator,
since the capacitors in the resonant
circuit cannot operate above 800
V. Operating at higher power
requires new capacitors which
have not been realized yet. It is
expected that power transfer up to
300 W is feasible
BLOCK DIAGRAM
Primary
Coil 1
Primary Coil 2
Primary Coil 3
Half bridge power supply
Half bridge power supply
Half bridge power supplywer
Secondary coil
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IMPORTANCE OF CET
A Better Solution for a Mobile
World
Talk to any plant engineer or
production system designer and you’ll
find that electrical wiring is the bane of
their existence. From installing the
wires, to rewiring as production lines
need to be changed, to repairing
damage caused by careless workers,
electrical wires represent an ongoing
cost and risk for downtime in
manufacturing plants. Until recently,
the miles of electrical wiring that
snake around any manufacturing
facility, hanging down from ceilings
and extending across corridors between
equipment, have been viewed as a
necessary aspect of industrial
automation. But today industry is
moving toward a wireless world. Like
consumers with their cell phones,
laptops and PDA’s, industrial
companies want wireless technologies
that improve versatility, reduce costs
and maintain connectivity. One of the
latest developments to draw interest
among engineering personnel is
contactless energy transfer for
powering and controlling motors.
While wireless communication is now
common in factories, wirelessly
transferring 16kW of electricity
through the air to power equipment is a
relatively new phenomenon in the
United States.
In a typical automated
manufacturing environment, where
carts full of parts must be moved
between the different stages of a
production process, a contactless
system transfers electrical energy
inductively from an insulated
conductor in a fixed installation to one
or more mobile loads. Electromagnetic
coupling is realized via an air gap, so it
is not subject to wear and costly
maintenance. Contactless energy
transfer reduces costs in several ways:
It eliminates festooning or
overhanging utilities. The underground
wiring is compact and poses no trip
hazards. There is no carriage to run out
on the shop floor. There are also no
pits to be dug to drop in trailing
utilities.
In addition to lower
costs, a mobile system using
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contactless energy transfer provides
greater versatility: The contactless
system enables more flexible track
layout with curves and switches,
simple segmentation of tracks, which
makes it easy to extend a track or
change travel directions, and higher
speeds.
APPLICATIONS
Contactless energy transfer is ideal for
applications where:
• The mobile equipment has to cover
long distances
• A variable, extendable track layout is
required
• High speeds have to be achieved
• The energy transfer has to be
maintenance free
•Additional environmental contaminants
are not permitted in sensitive areas
• The operation takes place in wet and
humid areas
Maintenance and ambient conditions
are important factors in constructing
systems for material handling and
transportation applications, such as
automotive assembly, storage and
retrieval logistics and sorting. Typical
applications that could benefit from
Contactless energy transfer includes:
• Overhead trolleys
• Conveyor trolleys
• Guided floor conveyors
• Push-skid conveyors
• Storage and retrieval units
• Pallet transportation systems
• Baggage handling
• Panel gantries
• Elevator equipment
• Amusement park rides
• Battery charging stations
By replacing a drag-chain
system in a conveyor trolley that
transports and sorts pallets, for
example, contactless energy transfer
enables pallets to transverse over
longer distances. Complicated holders
for drag chains are eliminated, as is
downtime for repairing cable breaks
and battery charging. Repairs for wear
from bending or torsion are also
eliminated. The wear-free power
supply in a contactless system has
many advantages in designing and
maintaining push-skid conveyors used
in automotive assembly, for example,
or in storage and retrieval units in a
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high-bay warehouse. Because there is
no conductor rail, there is no danger of
introducing contaminants from system
leakages and no components that are
difficult to reach for maintenance.
Problems with fitting the platforms
into conveyor belts are also eliminated,
since there’s no need for high
mechanical tolerances between the line
cable and pick-up.
Perhaps the biggest advantage
of a system based on contactless
energy transfer is higher system
availability because the system is
essentially maintenance free. In a
manufacturing environment where
change is a constant and speed is an
imperative, the versatility, flexibility
and reliability of contactless energy
transfer systems can reduce the wear-
and-tear on plant engineers as well as
equipment.
ADVANTAGES
1. It is not subject to wear and
costly maintenance.
2. Contactless energy transfer
reduces costs in several
ways.
3. It eliminates festooning or
overhanging utilities.
4. The underground wiring is
compact and poses no trip
hazards.
5. There is no carriage to run
out on the shop floor.
6. There are also no pits to be
dug to drop in trailing
utilities.
7. In addition to lower costs, a
mobile system using
contactless energy transfer
provides greater versatility.
8. The contactless system
enables more flexible track
layout with curves and
switches
9. simple segmentation of
tracks, which makes it easy
to extend a track or change
travel directions, and higher
speeds.
14
CONCLUSION
A new topology for
contact less energy transfer (CET)
to a moving load has been
proposed, built and tested. The
CET topology allows for a long-
stroke movement in a plane and a
short-stroke movement of a few
millimeters perpendicular to the
plane. In addition, it is tolerant to
small rotations. The power
electronics consist of a half-bridge
square wave power supply for each
primary coil and series resonant
capacitor and a full-bridge diode
rectifier at the load.
Power transfer up to
33 W with resistive load of 50 Ω
has been demonstrated The CET
system was used to power a 3-
phase brushless electromotor of a
CD drive and showed stable power
transfer of 3.44 W. The power was
transferred at approximately 90 %
efficiency, while the secondary coil
was moving with speeds up to 1.1
m/s over the primary coils
REFERENCES
Jeroen de Boeij, Student
Member, IEEE
www.wikepedia.com
www.siemens.com
www.ieee.org
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