A Contactless Transfer Devicce for Power and Data

Gwyn Roberts, IDB Ltd., University of Wales, Bangor, lJ.K.

-1-44 1248 382820 gwynr@sees. bangor.ac.uk

Nwyn R Owens, Phil M. Lane University of Wales, Bangor, U.K.

Martin E. Humphries, Rob K. Child MMS Space Systems Ltd., Filton, U.K.

FranGois Bauder Mecanex SA, Nyon, Switzerland

Jose M. Gavira Izquierdo ESA, Noordwijk, Netherlands

Abstract - This paper describes the initial development of a Contactless Transfer Device (CTD) for on-orbit applications. Target specifications called for the transfer of several hundred Watts of power, and several tens of digital data lines with a combined capacity of around 5 Mbps. The design accommodates a central axle or pole of up to 55 mm diameter, and may be used in an in-side-in or an in-side-out configuration.

A matrix evaluation technique based upon paired comparison between grouped evaluation criteria is described, and is used to select the preferred technologies for contactless data and power transfer. A modulated sub-carrier capacitive technique is selected for data- transfer, and a rotating transformer for power- transfer.

The electronic and mechanical aspects of the design and the build standard of the Breadboard Model are described.

TABLE OF CONTENTS

1. INTRODUCTION 2. APPLICATIONS AND

SPECIFICATIONS 3 . TECHNOL,OGIES SELECTION 4. DETAILED DESIGN 5. SUMRIIAR'YANDFUTURE

DEVELOPMENTS 6. REFERENCES 7. AUTHOR I3IOGRAPHY

1. INTRODUCTION

The transmission of electrical power antd data through rotating interfaces in spacecraft equipment is an essential requirement which needs to be maintained throughout the lifetime of a mission. Slipring and roll-ring type units are mainly suitable for low-speed applications. The utilisation of such units for high rotational rates presents imany drawbacks due to friction torque, electrical noise, high contact resistance,

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wear, debris and criticality concerning transmission system. Such a system would reliability and lifetime. allow higher rotational speeds, elimination of

wear and debris sources, allowing high Solar arrays, robotic joints, advanced remote reliability and a long lifetime for a mission. sensing instrumentation, general purpose This paper describes the initial development of scanning and pointing mechanisms and, in a such a Contactless Transfer Device (CTD) for generic way, any instruments which transmit on-orbit applications. power and signals through rotating or moving interfaces may benefit from a contactless

Table 1. CTD Target Specifications

Power-Transfer Total power transferred Input voltage Output voltage No. of channels

Data-Trans fer Spacecraft to rotating platform:

Rotating platform to spacecraft:

General Electrical Grounding impedance (across interface) Bonding impedance (not across interface)

Mechanical Outer diameter Inner diameter Axis length Mass Non-operating temperature range Operating temperature range No. of operations

' Rate of rotation

250 W 28 or 50 V DC 28 or 50 V DC 1 (+ 1 redundant)

2 dgital data 250 kbps 2 digital data 50 kbps 16 &gital data 5 kbps 4 digital data 1 kbps Total 24 lines, 750 kbps 8 digital data 250 kbps 8 dgital data 50 kbps 4 digital data 5 kbps 3 dlgtal data 1 kbps 1 compressed video 2 Mbps Total 24 lines, 5 Mbps

I l S 2 5 IO&

I 1 7 0 m m L 5 5 m m I320 mm 5 8 kg -45 to +90 "C -25 to +65 "C L5x106 I 3 l r p m

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2. APPLICATIONS AND SPECIFICATIONS

An extensive survey of potential users of a CTD and identification and harmonisation of their specific needs revealed specific application ranges, including scanning, pointing, rotating, tracking instruments, solar array drive mechanisms (SADM), and actuators for robotic joints and deployment mechanisms.

A single CTD capable of meeting all these applications would be over-engineered for any particular application. Available technology is already sufficient to adequately meet the majority of SADM requirements, (typically several kW at very high efficiency, one revolution per day), while deployment mechanisms tend to be very mission-specific. However, the requirements of scanning, pointing, rotating and tracking instruments are only moderately well met by current technology, and represent realistic data and power ranges for a CTD. This category of applications was therefore identified as the primary range at which to target the CTD.

Generic specifications for this group are illustrated in Table 1, and have been taken as the baseline for the design of a Breadboard Model (BM) CTD. Note that each data channel is the equivalent of a unidirectional screened twisted pair. The maximum rate of rotation was taken as the maximum required by the Multi-frequency Imaging Microwave Radiometer (MIMR), though this is not critical.

Crucially, the CTD would not have axial access; the design would have to accommodate a central axle or pole of up to 55 m diameter, as required by applications such as MTMR. Size and mass of the unit were tightly controlled, while electromagnetic compatibility

(EMC) and susceptibility to lubricant contamination were critical areas for the design. Furhhermore, a modular design was required to emure configurational flexibility.

A survey of existing equipment for contactless transmission revealed none that were directly suited to the application. The lack of axial access was a problem for many systems including fibrt: optic rotary joints.

3. TECI3NOLOGIES SELECTION

A wide range of technologies were investigated in detail as potential candidates around which to base the design of the CTD.

Data- Transfer

It was apparent that several technologies could form the baisis of a practical contactless data-transfer system. For any technique to be considered as a candidate for the CTD, it must be capable of satisfying a number of fundamental requirements, which are considered to be the ability to:

provide specified number of channels 0 support specified data rates

In addition, the technology proposed must allow operatjon in the specified space environment. The following are considered ‘killer criteria ’; if any one cannot be satisfied, then that technology is immediately rejected:

operation in vacuum low out-gassing

e operation over specified temperature range operation at minimum and maximum rates of rotation and/or oscillation

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Data-transfer technologies which satisfied these preselection tests were: 0 axial guided wave 0 rotating directional coupler 0 microwave free space

magnetic 0 axial optical 0 annular optical

baseband capacitive 0 modulated sub-carrier capacitive

The next task was to identi@ the most suitable technology for this application by means of a trade-off evaluation, and to justirj.. that selection objectively.

Table 2. Evaluation Criteria for Data-Transfer Technology Selection

4. LijkLReliability 9. Inherent reliability of technology

B. Functional Performance a. Bit error rate performance b. EMC emission

C. Application Suitability - Crifical a. Volumetric efficiency b. Configurational flexibility c. Ease of implementing modularity

D. Application Suitability a. Low power requirement b. Low mass c. Low torque and cogging

E. Development RisWComplexity a. Use of space-approved technology b. Low development risWmanufacturabi1ity c. Complexity

F. Additional BenefidDetrimental Effects a. Capacity for future expansion b. Undesirable exported products

The evaluation criteria used are listed in Table

categories, labelled A-F. Having identified evaluation groups and individual criteria within those groups, it is now necessary to allocate weighting factors to each. This is a 2-step process:

0 Determine the weighting factor for each group of evaluation criteria. This is done by means of a paired comparison method, where the importance ofpairs of criteria are compared. (e.g. A is compared with B, then with C etc.). The most important criterion of each pair is identified; ‘equal importance’ is not allowed. In this way, the groups are ranked in order of priority. Weighting factors are assigned: 6 to the highest priority, 1 to the lowest.

Determine the weighting factor for each individual criterion within each group using the paired comparison method described above. This is then scaled by the group weighting factor.

Having allocated a weighting factor to each criterion, we can now score each candidate data-transfer technology against each criterion:

To each technology, allocate a score of between 1 and 5 to each criterion. A score of 5 indicates that it hlly satisfies that requirement while a score of 1 indicates poor compliance.

0 Calculate and add the weighted scores for each technology to determine the overall preferred solution.

High reliability is a major driver; unless the CTD exhibits a long lifetime then it offers little advantage over existing technology. It is therefore assigned a high weighting factor.

It is anticipated that optical techniques would 2. Note that the criteria are grouped into exhibit favourable EMC characteristics.

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Optical techniques have been used for data- transfer on earlier ESA contracts for microgravity applications [ 1,2]. However, their susceptibility to lubricant contamination, and uncertainty about the reliability of optical sources reduced their overall scores in the evaluation matrix.

The overall winners of the selection process were the capacitive techniques. The relative merits of the baseband and modulated sub- carrier techniques are dictated by their operational environment. The baseband technique requires relatively simple drive requirements, and hence would be expected to exhibit high reliability. However, the data- transfer unit will be located in close proximity to the power-transfer system, which will have currents of several Amperes being switched at around 25 kHz. The use of a modulated sub- carrier of a sufficiently high frequency will ensure that no interference from the power- transfer system will pass the receiver bandpass filter. A further advantage of‘ using a modulated sub-carrier rather than baseband is the ability to select this frequency so as to minimise interference to other spacecraft systems.

Overall, the modulated sub-carrier is the preferred approach, but since the baseband capacitive technique requires similar hardware at the rotating interface, both techniques will be evaluated in the BM.

Finally, in order to assess the degree of objectivity of the matrix evaluation method, a sensitivity analysis was carried out. With identical weighting factors applied to each evaluation group, and to each criterion within each group, the capacitive methods still scored the highest.

Power- Tran.$er

For the power-transfer system also, several technologies were evaluated. In this case the trade-off was dominated by the need to achieve a high DC-to-DC conversion efficiency. Much work has been done on microwave power- transfer techniques [3, 4, 5, 61, However, as shown in Table 3 it was apparent that only the rotating transformer could operate at a useful efficiency level, hence this technology was selected for use within the CTD.

Table 3. DC-DC Conversion Efficiency of Contactless Power-Transfer Methods

~~~

Method DC-DC Efficiency Rffi

Transformer 90-95% WI

Microwave 54% max. demonstrated [7]

optical Solar cell I 18% P I Thermal Thermopile 11-13% typ.

Overall 17-8%

4. DETAILED DESIGN

In this section we present a design for the Breadboard Model CTD in line with the envisaged sce:nario of operation. Figure 1 provides an overview.

Throughout the design process, the underlying philosophy has been to minimise the number of components mounted at the rotating interface by mounting the electronic circuitry associated with both data and power-transfer sub-systems remotely. This allows:

the design of the rotating interface to be electrically totally symmetric between fixed and moving elements, thus allowing the CTD to be used as an ‘in-side-in’ or in-side-

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out’ configuration without modification of the interface, high reliability,

the rotating interface. These components the use of non-flight-representative are passive, robust and do not require electronics for the BM, maintenance or adjustment,

0 ease of thermal management, resulting in

only a few components to be mounted at 0 improved access to the electronics,

‘ V remote data electronics

data hamesses , ~

/

xpacitance plate (4 fixed, 4 rotating)

resonating torroidal transformer

-3

t

data harnesses

\ redundant data-transfer module b U

_ _ _ _ _ - _ _ - - - - _ _ _ _ _ _ _ _ - - - - - mwer harness

‘ transformer wound on ferrite Ucores

‘ redundant power-transfer module

DATA

POWER

Figure 1 Overview of breadboard model Contactless Transfer Device

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multiplexing the data remotely enables the composite data stream to be connected to the rotating interface by means of a single thin coaxial cable, rather than by individual screened twisted pairs, as would be the case with sliprings. This leads to a significant reduction in the mass of the harness.

Build Standard of Breadboard Model

The general design philosophy adopted is that the BM CTD should ‘demonstrate those aspects of its design that are not already proven’. The design may be broken down into 2 elements:

rotating interface remotely-mounted electronics (fixed and rotating sides)

The aim has been to produce a rotating interface design that is space-compatible in order to minimise subsequent effort to develop a Flight Model (FM). However the bearings, which are flight-representative with respect to tolerances, are not traceable. r-blso, mass optimisation has not been carried out on the BM design.

Ease of prototype assembly, test and development are the prime drivers for the remotely-mounted electronics of the BM. Commercially-rated passive and active components are therefore used; however future upgradeability is a critical factor which has been addressed at all stages throughout the development process. e.g. FPGAs will be used in the BM which may be replaced by discrete logic for the FM.

Data- Transfer

The data-transfer system will be described in two sections, firstly the components at the rotating interface and secondly the remotely- mounted elecitronics.

Rotating Inteirface4s illustrated in Figure I, a data-transfer module consists of 4 fixed and 4 rotating capacitance tracks at the rotating interface. Each track comprises a thin copper strip on an insulating kapton backing. The air (vacuum) gap between each fixed-rotating pair is nominally 1 mm, giving a nominal capacitance value of around 10 pF. 4 pairs of tracks provide bi-directional differential data- transfer capability. Earthed guard rings between the tracks (not illustrated) will contain the field and minimise interference and crosstalk. A cylindrical arrangement has been selected for ease of assembly and disassembly.

Since data-transfer is by means of a modulated sub-carrier, rtrsonating the interface to the carrier frequency offers advantages of not only reduced drive requirements but also improved interference-reiection. Using a simple passive circuit to accomplish this offers considerable advantages; no electrical power need be provided at the interface, it is inherently reliable, and is symmetrical in nature and thus equally well suited to in-side-in and in-side-out configurations.

Remote Data ElectronicsAn describing the remote electronics, we will follow the flow of data from the iixed to the rotating side of the interface; the E5M CTD actually demonstrates 24 channels in each direction.

Data lines arrive at the fixed side remote electronics as screened twisted pairs. Each is connected to a differential line receiver and is then input to tlhe multiplexer. The output of

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the multiplexer is a bit stream where each bit is data from one of the input lines. A fiame of data will contain data from all the input lines - one bit from the slowest line and many bits from the highest rate line. A framing pattern is then inserted into the data to identifjr the start and the end of each frame.

This composite data stream is then used to modulate a carrier. This carrier is at a nominal frequency of 100 MHz, which offers a good compromise between ease of circuit fabrication and high data-carrying capacity. The precise frequency will be selected so as to minimise interference to and from other spacecraft systems. Since Amplitude Shift Keying (ASK) is used, high stability is not required and a crystal oscillator is adequate to generate the carrier. Finally, this ASK signal is amplified and connected via the miniature coaxial data harness to the rotating interface.

After passing across the capacitive interface to the rotating side, the signal is amplified and demodulated using a level detector and shaping filter. The recovered data signal is then passed to the demultiplexer.

Clock recovery is simplified by the use of return-to-zero (RZ) data, which contains a spectral component at the clock frequency. Frame detection logic identifies the start of an individual frame of data. Then a 5-bit select word, generated to match that used at the transmitter, is used to drive a demultiplexer to recover the individual data lines. Latches are required to maintain the state of the data lines between samples. Finally, the latch outputs connect to line driver amplifiers that provide the differential outputs appropriate to the twisted pair cable that is to be driven.

Overall, it can be seen that the system is near transparent as far as the transfer of data is concerned. The only difference that may be

anticipated between input and output signals is a small degree of jitter caused by asynchronous sampling. This will be quantified during the testing activities.

Power- Transfer

The power-transfer system will also be described in two sections, firstly the components at the rotating interface and secondly the remotely-mounted electronics. The power-transfer section is essentially a resonant-mode switched-mode power supply. Note that the power-transfer system uses the ‘ring-main, principle, where a single bus is transferred across the interface, with local regulation being employed on the secondary side as necessary.

Rotating Interface-A module consists of one fixed and one rotating ring of ferrite cores carrying stranded, heavy-gauge copper wire, to make up the primary and secondary sections respectively of the rotating transformer. As for the data-transfer system, a cylindrical approach is used. This allows the inner and outer to be completely separated, and is relatively insensitive to axial misalignment. Earlier work [l] has shown that by resonating the leakage inductance to the switching frequency, very efficient power transfer may be achieved even though the leakage inductance is very large with this configuration. Since a completely symmetric arrangement is used, primary and secondary fbnctions may be reversed. For flexibility and ease of fabrication, discrete ferrite cores are used. Stranded copper wire is used to minimise resistive losses at the frequency of operation (nominally 25 kHz). The wire size, number of strands and number of turns have been optimised in order to provide maximum transfer efficiency. Apart from the transformer assembly itself, there are no other components at the rotating interface.

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Remote Power Electronics-The driver circuit on the fixed (primary) side consists of an array of MOSFET switches arranged in a H configuration, together with the drive- waveform generating circuits. The system is designed such that the drive circuits always switch at instants when the current level is low, thus minimising power losses in the drive circuits and also reducing the amount of electromagnetic radiation. Due to the zero- switching feature, a large heatsink is not required.

In order to minimise radiation, ,the power- transfer harness comprises a multi-twisted-pair cable where each power line is twisted with its return. Identical harnesses are used on the fixed (primary) and rotating (secondary) sides.

The remote electronics on the rotating (secondary) side consist of an array of power diodes and a large reservoir capacitor. Characteristics required of the power diodes are a sharp on-characteristic with low slope resistance and good high frequency switching properties because they also form part of the resonant circuit.

The contactless power-transfer technique provides a unique advantage due to its inherent regulation. For a given input voltage, it is possible to change the output voltage by modification to the electronics only. No modification to the equipment located at the rotating interface is necessary.

Mechanical

The mechanical construction of the BM CTD is illustrated in Figures 2 and 3.

In the side-view of Figure 2, the main and redundant capacitive data-transfer modules may be seen on the left hand side. Cables from

the inner (rotating) side are routed through a slot machineid in the housing, while the cables from the outer are routed directly outwards.

The main and redundant power-transfer modules are seen to the right of the data- transfer modules. The ferrite cores are supported by a moulded resin housing. A shield of magnetic material is fitted between the power and the data-transfer sections in order to minimise interference.

The design used for the bearings consists of a preloaded pair of matched ball-bearings and one single bearing which is axially fiee and transfers only radial loads. This arrangement requires a relatively low preload which is 1/3 of the axial launch load for the bearings pair (100 N for ithe BM). A low preload is desirable for the BM in order to enable measurement of transfer-system-induced torque. Preload for the single bearing is provided by a spring washer. The solution does suffer from a thermal limitation: at high temperatures the preload is reduced due to differential expansion between the bearings (steel) and the housing (aluminium).

It is necessary to provide a ground path from the fixed to the rotating side of the interface for bonding purposes. Since a non-contact ground connection at all frequencies down to DC is not possible, the bearings are used to provide this electrical path.

As lubricant, Braycote 60 1 space-qualified grease is used. Grease is preferred to oil as the lubrication is easier, and tightness or labyrinth are not necessary for confinement.

The estimated mass of the BM CTD rotating interface is 5.4 kg. Its nominal dimensions are: 0 outer diameter 166 mm

inner diameter 55 mm 0 length 168 mm

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Figure 2. Breadboard Model CTD Side View (Remote electronics not illustrated)

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Figure 3. Breadboard Model CTD Cross-section (Remote electronics not illustrated)

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A point worthy of note is that the CTD is capable of operation while stationary and while oscillating or rotating in either direction up to the maximum rate. The maximum rate, limited only by the bearings, is 9500rpm, which is well in excess of the target figure of 3 1 rpm.

5 . SUMMARY&FUTUlRE DEVELOPMENTS

After application of a matrix evaluation technique, a modulated sub-carrier capacitive technique was selected for data-transfer, and a rotating transformer for power-transfer.

The design of the Breadboard Model Contactless Transfer Device has been described in detail. Manufacturing of the BM CTD is underway during Q 1 : 1996.

Test scheduled to be carried out on the BM CTD during 42: 1996 include:

data-transfer functional testing, power-transfer hnctional testing, EMC characterisation (including power-to- data interference), thermal cycling, torque measurement, vibration testing.

While the CTD design presented here is aimed at a particular group of applications, the design is readily scaleable both in terms of power and data-handling capacity, and in order to accommodate the constraints of different mechanical envelopes. It is anticipated that the Contactless Transfer Device will find many diverse applications in the aerospace and associated fields.

6. REFERENCES

[ 13 Owen, R.G., Jones, D.I., Owens, A.R, R0berts, G., Hadfield, P, “The microgravity Isolation Mount - a Columbus facility for improving the microgravity quality of payloads”, Proc. International Worlkshop on Vibration Isolalion Technology for Microgravity Science Applications, NASA Lewis Research Centre, Cleveland, Ohio, April 199 1.

[2] Owens, A.R, Jones, D.I., Owen, RG., Roberts, G., Robinson, A.A., “Progress in the design of a microgravity facility for in-orbit experiments”, Proc. of the 4th European Symposium on Space Mechanisms and Tribology, Ch”es, Sept 1989, pp229-234.

[3] Glaser, P.E., “Power from the sun,; its future”, Science, vol. 162, pp857-861, Nov. 22, 1968.

[4] Brown, W.C., Eves, E.E., “Beamed microwave power transmission and its application to space”, IEEE Trans. on Microwave Theory and Techniques, vol. 40, no. 6, pp1239-1250, June 1992.

[5] Mallavarpu, R., Puri, M.P., “High CW power with multi-octave bandwidth from power-combined mini- TWTs”, IEE MTT-S Int. Microwave Digest, pp1333- 1336, 1990.

[6] Koert, P., Clxi, J.T., ‘Millimeter wave technology for space power lbeaming”, IEEE Trans. on Microwave Theory and Techniques, vol. 40, no. 6, pp1251-1258, June 1992.

[7] Dickinson, R.M., Brown, W.C., “Radiation microwave power transmission system efficiency measurements”, ‘Tech. Memo 3 3 -727, Jet Propulsion Lab., Cal. Inst. Tech., Mar. 15, 1975.

[SI Brandhorst, H.W., “Space power - what’s old is new again”, IEEE AES Systems Magazine, pp21-24, Nov. 1994.

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Gwyn Roberts was awarded a BSc. with lst class honours in electronic engineering fiom the University of Wales, Bangor in 1982. He also received the IEE Award. Subsequent studies into advanced techniques for automating the processing of seismic data led to the award of a Ph. D. in 1988. From 1984-1 98 7, he worked as a postdoctoral research fellow on a European Space Agency contract at the University of Wales, Bangor, to develop a microgravity isolation mount (MGIW. Since 1987 he has been with the University's design consultancy company, IDB Ltd., where he has been responsible for a wide range ofprojects including the development of novel radio navigation and positioning solutions, programmable logic design, control instrumentation and computer modelling of transmitter systems. He is currently Project Manager on the CTD project described in this paper. Dr. Roberts is a Chartered Engineer and a Member of the IEE,

Phil M Lane was born in Wales in 1964. He receivedpom the University of Wales - Bangor a BSc degree, with the highest honours in the University, in Electronic Engineering in 1989 and a PhD in 1992. His PhD addressed realisation strategies for very high bit-rate optical receivers. He spent a period working for Industrial Development (Bangor) Ltd where he designed navigation and control systzms for>ee$oating ocean buoys used for environmental monitoring, audio distribution systems and fast pulse amplijiers for nucleonic instrumentation. From 1992 to mid-1995 he was employed

at the University of Wales as a Research Fellow and later as a lecturer. He is currently employed as a lecturer in telecommunications at University College London. His research interests include optical/mm-wave interactions, high bit- rate optical communications systems, signal design andprocessing for communications systems, stochastic processes and telecommunication networks. He is an associate member of the IEE and a member of the IEEE.

Rob Child is a Spacecraft Systems Engineer in the Directorate of Science and Radar at the Matra Marconi Space site in Bristol, UK. His primary area of activio* is spacecrafr concept and feasibility studies for new European space missions. This workgives him an overview of total space system requirements. His other area of activity is developing spacecraft system design and mission analysis tools. He has a Bachelor of Science in Physicspom Birmingham University, and a Master of Science in Astronautics and Space Engineering @om Cranfield University (CIlJ.

Martin Humphries is Head of Mechanism Design in the Directorate of Science and Radar at Matra Marconi Space. He heads a small team of engineers who are responsible for the design and development of specialised

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mechanisms for spacecraft and scientiJic instruments. Mechanisms under his responsibility have included the mechanisms on the Giotto space probe, the Hubble solar array and a variety of scanning and deployment systems. He is currently acting as a mechanism consultant on the European EWISAT Space Platform. He is the holder of a number ofpatents and has a Bachelor of Science Degree in Mechanical Engineering.

advanced slip-ring systems for power and signal ,transmission. He has a B.Sc in Mechanical Engineering from the Polytechnic University of Madrid.

Francois Bauder is an engineer in Microtechnoloa9om Swiss Federal -. ~

Institute of Technology (EPFL) . He has been involved at MECANEX in many space project developments such as mechanisms and sliprings in the frame of European Space Agency programs. He is now technically responsible for and project manager of various space projects.

Jose' Manuel Gavira Izquierdo is an engineer of the European Space Agency. He is working as an expert in space mechanisms development for science,

-

telecommunications and Earth Observation Instruments. Among other space-related activities he has lead European technology developments in the fields of tether mechanisms for capsule re-entry mission, scanning and pointing mechanisms for earth observation instruments and

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