Electric Drive System for Automatic Guided Vehicles Using Contact-free Energy

TransmissionMarcel Jufer

EPFL, Lausanne, Switzerland, e-mail: marcel.jufer@epfl.ch

Abstract—Four technologies, combining mainlyinnovative solutions, offer the possibility of clean andflexible vehicles, all using only electricity. The main common components are contact-free energy transmission, storage on super-capacitors, holonomeaxles integrating wheel-motors and automatic guiding.Moreover, the complete energetic chain is managed by power electronics. Their applications are mainly in the field of public and industrial transportation.Several applications are described: electric busses,automatic guided vehicles for container handling,automatic people movers and automatic surveillancevehicles. For more information please visithttp://www.numexia.com/

Keywords—Wireless power transmission, automotive application, energy storage, energy system management,soft switching

I. INTRODUCTION

A new generation of vehicles, mainly based on 4different technologies, is presently in development for good or passenger transportation or special applications.

These technologies are :- Contact-free energy transmission- Wheel-motor with an external rotor - Energy storage on super-capacitances- The use of an energy chain based on power

electronics and control, from the ground to themotors.

The main applications are : - An automatic guided vehicle (AGV) for container

transportation and handling- An automatic people mover at 15 km/h- An automatic safety and control vehicle- An electric city bus

II. AIM

Starting from different technologies developed for thehigh speed Maglev Swissmetro (Fig 1 ) [1], the approachof technology transfer has been to apply one or severalcomponents to innovative vehicles, responding to specificconstraints and criteria:

- High flexibility of exploitation ; - Clean, quiet and sustainable ;

- High automation and control with or withoutdriver;

- Low investment in infrastructure;- Low maintenance costs.

In order to reach these goals, specifically in the range ofspeeds between 10 and 100 km/h, one more device had tobe implemented : the direct drive wheel-motor withexternal rotor, also developed at EPFL.

Fig. 1. From Swissmetro project to specific technologies

Fig. 2. Contact-free energy transmission - Principle

III. CONTACT-FREE ELECTRICAL ENERGYTRANSFER

A. Principle

This technique [2-7] offers the possibility to transferelectrical energy from a fixed coil on or in the ground to

Linear motors

Contact-free energytransmission

Aerodynamics Numericalplatform

Magneticlevitation

Moving coil linked to the vehicle

Fixed coil in theground

1

978-1-4244-1742-1/08/$25.00 c© 2008 IEEE

an other coil placed under a vehicle. The process isrealized without iron magnetic circuit, in the air. On Figure 2 the principle is illustrated.

In order to reach this ironless magnetic coupling, a high frequency is necessary (>>50 Hz). Effectively, fromthe back-EMF equation it is possible to write:

dtdui / = back-EMF

total flux airNBSNN = coil number of turnsB = magnetic flux density

airS magnetic air section

So the back-EMF equation becomes :

airi SBfNU ... (1)

By increasing the frequency and the magnetic section, it is possible to reduce the flux density B and the coppervolume by reducing the number of turns N. Thus, it is possible to suppress the iron magnetic circuit, using airinstead. An optimisation software, aiming to reduce thecopper volume, to reach a transmission efficiency > 98 % and to minimise the electromagnetic radiation has beendeveloped and applied. A system for a total power of 120kW (Fig 3) has been designed and built with thecorresponding power electronic to generate the high frequency and a controlled rectifier on the vehicle . A global maximum efficiency of 95 % has been measured at110 kW.

Fig.3. Contact-free energy transmission – 120 kW test facility

B. Magnetic field

A specific problem of inductive energy transfer is themagnetic field radiation, limited by normalization,according to the frequency.

The solution to reduce the intensity of radiation for passengers in the vehicle and nearby is to design thesystem using 3 safety elements:

- To switch on the primary coil only when thesecondary coil (i.e. the vehicle) is correctly positioned.

- To place a magnetic shield on the vehicle floor (athin lamination)

- To impose the same current volume for the primaryand the secondary coils

This last condition has been applied to an energytransfer system corresponding to the followingconditions:

- Power transferred : 108 kW - Coil sizes : 4*2 m- Distance d : 0.115 m- Primary voltage: 500 V - Floor level above primary coil : 0.3 m

With current volume opposition, the magnetic fieldlevels without shielding are represented on graphics, inrelative values, referred to the earth magnetic field. On Fig. 4, the relative flux density is calculated above themiddle of the coils, on a vertical axis (yy), from the vehicle floor level (0.3 m) up to 2 m. On the vehiclefloor, the relative amplitude is 0.3.

B/Be

0.3

0.2

0.1

Fig.4. Relative flux density distribution above the coil centre, on a vertical axis – 108 kW power – 4*2 m coils

0.2 0.5 1.0 1.5 yy[m] 2.0

On Fig.5, the same relative flux density is representedon the floor level (yy= 0.3m) on an horizontal axis, fromthe coil centre laterally up to 2m; xx = 1 m corresponds to the coil side level. The peak value is 0.75 above the coils.

B/Be

0.75

0.5

0.25

0.1

Fig.5. Relative flux density distribution on the vehicle floor level, fromthe centre to 2 m – 108 kW power – 4*2 m coils

On Fig.6, the same relative flux density is representedon the on a level of yy=1.2m, on an horizontal axis, fromthe coil centre laterally up to 2m; xx = 1 m corresponds to the coil side level. The peak value is 0.1 m above the coilcentre.

0.0 0.5 1.0 1.5 xx 2.0

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Fig.6. Relative flux density distribution on a level of 1.2 m above the primary coil, from the centre to 2 m – 108 kW power – 4*2 m coils

IV. ENERGY STORAGE ON SUPERCAPACITORS

Energy transfer can be done continuously from theground to a vehicle, using a continuous succession ofcoils (Fig.7) . This presents two drawbacks: theinvestment cost and the absence of track flexibility.

Fig.7.Continuous coil track – Test facility with 5 passengers, 15km/hvehicles

As a variant, contact-free energy transmission with intermediate fast storage on ultra-capacitors [8,9] is a very interesting solution. It allows an autonomy of track with a reduced investment. This solution requires an autonomy in the range of 2.5 to three times the distancebetween two loading stations.

On Fig.8, the global electrical scheme of such asolution is presented, including the energy transmissionsystem and ultra-capacitor loading control.

The rapid loading operation requires an importantpeak power. Thus, an interesting solution is to equip a station also with an ultra-capacitor intermediate storage,such as to have the possibility to smooth down the mainpower (see Fig. 18).

V. WHEEL-MOTOR

For the different categories of vehicles presentedhereafter, the electric drive solution has been focused ondirect wheel motor drive characterized by:

B/Be

0.1

0.09- No gear transmission- Brushless DC motors- External rotor0.08

The absence of transmission leads to a heavier motor.But the solution with external rotor [10] allows a betterintegration directly in the wheel and not laterally to thewheel (Fig 9) as sometimes realized.

Fig 10 shows a wheel motor according to the proposedsolution. An optimized design software leads to anacceptable mass with an important free volume at theinside.

0.07

xx1 0. 1.5 2.0 2.5 3.0

Ultracapa

Fig.8.Electric scheme of energy transfer system and storage on ultra-capacitors

Fig.9.Electric drive for wheel with classical gear transmission

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Fig 11 shows the stator (left) and the rotor (right) ofsuch a motor for a rating torque of 4000 Nm.

Based on such motors, vehicle axles with 2 wheelshave been developed, with integrated steering motorallowing a very important angle of rotation until ± 90o

(Fig 12). In this case, the damping devices have beenintegrated in the free space internal to the motors. Such an axle has been realized for the vehicle of Fig.14.

Fig.10.wheel motor with external rotor

Fig.11.Stator (left) and rotor of a wheel motor of 4000 Nm

Fig.13.Automatic Guided Vehicle for 40 T containers -120 kW powerat 25km/h.

rotor-rim

Fig.12.Vehicle axle with 2 wheel motors and a steering motor. Thedamping elements are integrated in the motor itself.

VI. VEHICLES INTEGRATING THE DIFFERENTTECHNOLOGIES

stator A. Automatic guided vehicles for container handling

An automatic guided vehicle for container handling has been developed integrating the following technologies:tire

- Contact-free energy transmission- Motor wheel axles - Energy storage on super-capacitors- Automatic guiding

The transportation capacity is designed for 40 Tcontainers. The maximum power is 120 kW with a maximum speed of 25km/h.

On Fig.13, a picture of the final vehicle is presented.On Fig 14, the first prototype vehicle is represented. It issupplied by the coil system of Fig.3.

Fig.14.Automatic Guided Vehicle for 40 T containers – First prototype

A first series of such vehicles will be built and tested in a harbor at the end of 2008.

B. Low floor electric bus

The same technology of energy transmission system and axles with wheel motors can be applied to busses with a

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very low floor, according to the integrated wheelrealization . The possibility to move the wheelsindependently (but in coordination) offers the advantageto reduce the stop area length. A test realization isforeseen for 2009.

Fig.15.Bus with independent wheel axles and very low floor

C. Automatic people mover

Automatic people mover (APM) are small vehicles with a capacity of 8 to 10 people (1200 kg) with a speedof 15 km/h (Fig 16). The rating power is 14 kW with a maximum slope of 16%. The peak power for the energytransfer is 50 kW. This allows a loading time ofmaximum 10 seconds for an autonomy of 1 km (Fig 17).In order to avoid such peak energy consumption on themain, an intermediate energy storage at each loadingstation is introduced (Fig 18). So the power request on the main is smoothed.Such transportation system can be applied to pedestrianstreets, airports, large parking areas, exhibition centers,university campus, large factories, etc. Two test vehicles are in construction and a first pilot track is foreseen for2009, in Lausanne.

Fig.16.Automatic people mover vehicle

Fig.17.Automatic people mover system with loading stations every 400 m

1

2

3

4

7

65

Fig.18.Automatic people mover with energetic chain and loading station 1-Vehicle structure 2-Loading station with intermediate storage

3-Power electronics 4-Super-capacitors 5-Obstacle detection6-Primary coil 7-Secondary coil

D. Automatic surveillance vehicle

Many different installations or situations require safetysurveillance and reconnaissance. They are generallyknown under the name of Mobile Detection AssessmentResponse System (MDARS).Automatic systems based on the same energetic chain as the APM system have theadvantage of reliability, absence of noise, insensitivity toany type of pollution and a smaller size. Such a vehiclehas been designed and will be equipped with devices suchas radar, NBC sensors, IR camera, etc.

Among the characteristic applications: airports,nuclear power plants, gas and fuel production, high orlow temperature environment, catastrophe evaluation andrescue, military investigation, etc. Such a vehicle, basedon similar technologies as AGV and APM, have theadvantage of no noise, no pollution and high flexibility. A first prototype will be built soon. On Fig 19, the vehicledesign is represented.

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ACKNOWLEDGMENT

The author thanks particularly the Swiss FederalCommission for Technology and Innovation (CTIhttp://www.bbt.admin.ch/kti/index.html?lang=en ) whichsupported financially these projects, the FoundationNumexia and the Company Numexia S.A. (http://www.numexia.com/index.php?page=accueil&hl=en_GB ) which designed the different vehicles and realizedthe prototypes.

REFERENCES

[1] M. Jufer, V. Bourquin, M. Sawley, “Global Modelisation of the Swissmetro Maglev using a numericalPlatform”, Proceedings MAGLEV 2006, Dresden 13-15 sept 2006

[2] N. Macabrey, “Alimentation et guidage sans contact” PHD thesis, EPFL, No 1840, 1998.

[3] M. Jufer, N. Macabrey, M. Perrottet, “Modeling and Test of Contactless Inductive Energy Transmission”, 5th InternationalConference Electrimacs, 1996, Saint-Nazaire (France), Vol 3/3, pp1199-1204.

Fig.17.Automatic people mover system with loading stations every 400 m

[4] M. Jufer, L.Cardoletti, B. Arnet, N. Macabrey, M.Perrottet, “Inductive powered vehicles for semi-personal transport- The Serpentine project”, EVS 15, Brussels, October 1998 (CD-ROM).

VII. ENERGETIC CHAIN

All these different vehicles use basically the sameenergetic chain, with powers from 10 kW to 250 kW. Forthese developments, power electronics is a key factor,mainly for the following functions:

[5] M. Jufer, N. Macabrey, P.Germano, M. Perrottet,“Contactless Energy Transmission for MovingDrive,”Proccedings 27th Annual Symposium on IncrementalMotion Control Systems and Drives (IMCSD), San Jose (Ca), July 1998, pp 47-53.- High frequency supply generator for energy

transmission, with a high efficiency [6] M. Jufer, M. Perrottet, N. Macabrey, "ContactlessEnergy Transmission for Electric Drives," EPE ChapterSymposium on Electric Drive Design and Applications, Nancy,June 1996, pp 7-12

- Super-capacitors load and control- Propulsion and steering motor control and

drive[7] M. Jufer, R. Perey, “Contact-less Energy Transmission

for Maglev”, Proceedings MAGLEV 2006, Dresden 13-15 sept2006

Fig 20 describes the complete energetic chain.

[8] B. Destraz, P. Barrade, A. Rufer, and M. Klohr, “Studyand Simulation of the Energy Balance of an Urban TransportationNetwork”, EPE Conference Aalborg, 2-5 September 2007

DSP

DSP

DSP

DSP

DSP

DSP

DSP

DSP

DSP

DSP

DSP

DSP

C

L

LC

[9] A. Rufer, “Benefits of short and long term energy storage in the context of renewable energies and sustainableenergy consumption”, Grimaldi Forum, Monaco, March 29 – April 1 2007.

[10] M. Jufer, “Limit Performances of direct Electric Drives”,International Symposium on Advanced Electromechanical MotionSystems, Electromotion’99, Patras, July 1999, pp 1-6.

Fig.20.Energetic chain including energy contactless transmission,energy storage and motor drive and control

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