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.

1

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.

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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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