DEVELOPMENT OF A MOBILITY DRIVE UNIT FOR LOW GRAVITY PLANETARYBODY EXPLORATION

J. Reill, H.-J. Sedlmayr, S. Kuß, P. Neugebauer, M. Maier, A. Gibbesch, B. Schäfer, and A. Albu-Schäffer

DLR (German Aerospace Center), Robotics and Mechatronics Center, Muenchner Str. 20, 82234 Wessling, GermanyE-mail: Josef.Reill@dlr.de

ABSTRACT

The 10 kg asteroid lander package MASCOT (mobileasteroid surface scout), developed by DLR, is a contri-bution to the JAXA Hayabusa-II mission, intended tobe launched in 2014. MASCOT will provide in-situsurface science at several sites on the C-type asteroid1999 JU3. An innovative hopping mechanism, devel-oped at the Robotics and Mechatronics Center (RMC) inOberpfaffenhofen, allows the lander to upright to nom-inal position for measurements as well as to relocate byhopping. The mechanism concept considers the uncertainand harsh environment conditions on the asteroid surfaceby using the impulse of an eccentric mass with all rotat-ing parts completely inside the MASCOT structure. Thepaper gives an overview of the major development activi-ties which lead to a new promising powerful and scalabledrive system for low gravity planetary body exploration.

1. INTRODUCTION

The Robotics and Mechatronics Center (RMC) of DLR inOberpfaffenhofen gained a lot of experience in terrestrialrobotic systems as well as in in space applications. Dur-ing the ROKVISS (robotics component verification onISS) mission (see [1], [2] and [3]) there was a robotic armbuilt and launched towards the ISS in 2004. The roboticarm was telemanipulated with closed force feedback loopfrom ground while the robotic arm was mounted outsidethe ISS at the Zvezda service module. In 2011 the roboticarm was still working and returned to earth for further in-vestigations. Several other powerful mechatronic systemswith modular actuation units like the DEXHAND werebuilt (see [4], [5] and [6]).

This paper shows a new developed hardware for the MAS-COT landing unit, that was optimized to cope with thechallanging requirements of the long term space missionHayabusa-II. Especially the long cruise phase of approx-imately four years and additional eight months for tar-get characterization is a big challenge for the hardware.During this time almost no movement of the mechatronicsystem is expected which contributes to the risk of coldwelding within bearings and gear unit. Also thermal and

radiation issues have to be taken into account. The land-ing unit is about 10 kg and rectangular shaped with thesize of 300 mm x 300 mm x 200 mm. There are severalsubunits of MASCOT:

• MASCOT structure (German Aerospace CenterDLR Braunschweig FA-MFW, Germany)

• GNC, guidance, navigation & control system (Ger-man Aerospace Center DLR Bremen RY, Germany)

• PCDU, power and communication systems (CNESCentre National D’études Spatiales)

• OBC, on board computer (Telespazio VEGADeutschland GmbH, Germany)

• CAM, visible camera (German Aerospace CenterDLR Berlin PF, Germany)

• MARA, infrared radiometer (German AerospaceCenter DLR Berlin PF, Germany)

• MAG, magnetometer (Techincal University Braun-schweig, Germany)

• MicroOmega, near infrared hyperspectral micro-scope (IAS Paris, France)

• MOB, mobility unit (German Aerospace CenterDLR Oberpfaffenhofen RMC, Germany)

The integration of all subsystems and payloads is doneat the DLR Institute of Space Systems, Exploration Sys-tems in Bremen. In this paper the mobility mechanismhardware and electronics will be shown in detail. Theother components and systems are presented in differentother papers (see for example: [7]). Mission and decisiondetails to the mechanism are presented in [8].

Functional safety aspects during the mission forced therealization of a redundant electronic block. Furthermorethe 120 g mass of the eccentric arm must be acceleratedand decelerated in a fast way which needs a strong drive.Both requirements are contrary to the given space andmass restrictions. The realized electronics including con-trol, sensors and motor drive subsystems are realized ona single PCB, sized 95 mm x 105 mm in total for bothredundancy paths. The system is designed for -55 up to100◦C storage and -40 up to 80◦C operating temperature.

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2. MOBILITY SYSTEM OVERVIEW

The mobility mechanism is based on the reactive forcegenerated by an eccentric arm. This way MASCOT isable to hop on the asteroid surface. Solutions with drivewheels were rejected because of surface uncertanties andweight limitations.

In Fig. 1 the Simpack model structure of the Mascot mo-bility system is shown as implemented in the MBS for-malism. By introducing the polygonal contact model ap-proach (PCM) arbitrary surfaces can be modelled on themobility and on the asteroid surface side. With the elasticlayer contact and the multi-body system approach highlyreliable simulation results are possible for hopping anduprighting manoevers. The asteroid gravitational force isimplemented by means of the gravitational force law ofEq. 1

F = G · masteroid ·mmascot

r2(1)

with G = 6.6738410−11, gravitational constant.

MBSSIMPACK

statescontact forces

MBS

asteroid gravitation

contact model PCM

desired trajectory

external inputs

CAD

PCM parameter

motor control parameter

MASCOT bodymotion

Figure 1. Mascot Simpack Architecture

A general overview of the mechanical system is givenin Fig. 2. The MobCon on the left hand side representsthe mobility controller that is described in more detail inFig. 3. The drive unit consists of a brushless dc motorwith very high power density and a high reliability be-cause of only very few mechanical parts and the absenceof brushes. The motor design was done by DLR while themanufacturing of the leightweight torque servo motor isactually done by RoboDrive [9] which is a DLR spin-offcompany. The motor is mounted to a harmonic drive gear-ing with a 1:30 reduction. For commutation reasons andalso to calculate the position of the eccentric arm, digitalhall sensors are used.

The electronics consists of two fully separated cold re-dundant signal paths. This concept enables the OBC toswitch to the other redundancy path in case of some fail-ure. Due to weight and volume restrictions the motorcould not be realized redundant. Also the hall sensorsin the motor housing are not redundant due to the verysmall motor outer diameter of 25 mm. To ensure func-tionality, radiation hard hall sensors are used. With themotor and the sensors being not redundant, the redundantelectronics needs to be connected to the non redundand

motor and hall sensors by a special decoupling network(patent pending).

HallSensors

Ref.-Pos.-Sensor

MobUnit

3-Phase BLDC-Motor

ILM 25Harmonic

DriveBearing

EccentricArm

EccentricMass

Mechanics

Mo

bC

on

PC

B

Figure 2. Block diagram of the mobility system.

HallSensors

FPGA(Actel)

UART

SPI

PWM

GPIO

Industrial BLDC Motor Controller

3-Phase MOSFET Bridge

Redundancy Coupling

BLDCMotor ILM25

Line Driver RS422

ADC

Analog Circuits:

(Current, Voltage And Temperature Measurement)MobCon PCB

HallSensor

MobUnit

Ref.-Pos.-Sensor

OBC Power (CNES) Connectionto MASCOT

Figure 3. Block diagram of the electronics (MobCon).

The main control unit of the mobility is a flash based Mi-crosemi (formerly Actel) FPGA. The main tasks are:

• communication by RS422 with OBC

• collecting measurement data of ADCs by SPI bus

• provide the control algorithm for the position controlloop of the motor

• generate PWM signal for motor control

• calculate the eccentric arm absolute position by useof motor hall sensors and an additional reference po-sition sensor

• realize safety monitoring of the unit

Since there is the harmonic drive gearing between motorand eccentric arm, the motor hall sensors do not show theabsolute eccentric arm position. This is why there is anadditional reference position sensor that detects referencemagnetes mounted on the eccentric arm. After power on,an initalize sequence needs to be run so that the controlalgorithm can calculate the correct absolute position. Inthe following sections more details are shown on the hard-ware as well as on the electronics.

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3. DETAILS ON THE HARDWARE

Fig. 4 shows a photo of the QM version of the mechan-ics. The design is the result of FEM analyses, simula-tion and the experience of the ROKVISS and DEXHANDMission. Several parts of the MobUnit, were alreadytested and used in the DEXHAND. The suitable lengthof the eccentric arm and the weight of the eccentric masswere identified by mock-up tests and MBS simulationsrespectively. The mobility-mechanism is mounted on theMASCOT common electronics box because of mountingspace, temperature and sealing requirements. The tem-perature range inside the E-box is from -20 up to approx-imately 40◦C. The main part of the MobUnit is locatedinside the E-Box. The eccentric arm, the reference posi-tion sensor and a small part of the housing are situated inthe actuation space of the arm, outside the E-Box.

Fig. 5 shows the CAD assembly of the MobUnit. Themechanism mainly consists of motor and gear (pink hous-ing), bearing (orange housing), hall sensors, eccentricarm (purple), eccentric mass (brown) and the MLI stand-off (red). The hall sensor (grey) to detect the referenceposition of the eccentric arm is installed right in behindthe arm.

The first model of the MobUnit was built up with stan-dard components. This means that standard lubricatedball bearings were used as well as an off the shelf Robo-Drive motor, and a standard steel Harmonic-Drive gear-box. The EQM of the MobUnit was built with a mix ofstandard and space proofed components. This means thatDicronite coated ball bearings were used as well as anoff the shelf RoboDrive motor, and a Dicronite coatedsteel Harmonic-Drive gear-box. The material of the man-ufactured parts consists of aluminium 3.4364 (7075). Allscrews and other standard parts were made by stainlesssteel A2. The eccentric mass consists of a 99.97% tung-ster alloy because of the very high density. The Robo-Drive motor will be obtained in a special space versionfor the FM where the manufacturing process and testingis more strict and selected materials are used.

Figure 4. Photo of the qualification model of the motorand eccentric arm (mobility unit MobUnit).

Figure 5. CAD image of the motor and eccentric arm(mobility unit MobUnit).

3.1. Lubrication

A very important influence for the energy consumptionand safe operation is the lubrication of the ball-bearingsand the gearing. The lubrication in deep space missionhas two tasks. The first is the minimization of the fric-tion and the second one is the prevention of cold welding.Therefore a wrong chosen lubrication leads to a signifi-cant decrease of mission safety. Three different conceptswere investigated.

Castrol BraycoteThe first concept provides that all bearings and Harmonic-Drive parts where lubricated with Braycote. The lubrica-tion, Castrol Braycote 602EF was chosen because of thegood experiences made during the previous ROKVISSmission. Braycote 602EF is unique among the Braycotefamily of products because it contains a fine particle dis-persion of MoS2. The disadvantage of Braycote is theconsiderable increase of the friction for high dynamicslike in the proposed mechanism. In a test series this lu-bricant showed quite bad behaviour especially in cold en-vironment. In summary it was decided to use better solu-tions if possible.

Cold-welding resistant materials combinationThe second concept prevents cold-welding by well se-lected combinations of materials. The best solution is acombination of non-metallic and metallic materials. Thatleads to a cold-welding resistant solution. There is thepossibility to take hybrid ball-bearings. These bearingsconsist of stainless-steel rings, Peek bearing cages andceramic balls. Unfortunately it is not possible to selectthe material in this way for the harmonic-drive gear unit.

Cold-welding resistant coatingThe third concept is to coat the surfaces of the mechani-cal parts with dry lubrication. For the MASCOT missionWS2 (Dicronite) and MoS2 were selected as the best so-lutions. WS2 has a wide temperature range, extremelylow outgassing behavior and a well-defined layer thick-ness. Dicronite was used in different space missions likethe Mars-Exploration-Rovers. WS2 forms no extensivecoating but selective tungsten carbides. Therefore notenough cold welding prevention is given. MoS2 has also

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a wide temperature range, extremely low outgassing be-havior and the lowest friction coefficient. MoS2 was usedin many space missions over the last 50 years. The exten-sive surface of MoS2 guarantees prevention of cold weld-ing. This is the main reason why MoS2 was chosen asthe best lubrication under the extreme conditions at theMASCOT mission.

3.2. Shaker test

The mechanical design of the hopping mechanism is sim-ple, but a single mass oscillator. For this reason it is nec-essary to have a more detailed look at the performanceand eigenfrequency of the MobUnit. To gain more experi-ence with this behavior and in order to validate the FEMsimulation, the MobUnit was tested on subsystem level.The test setup is shown in Fig. 6. After the first teststhe y-direction turned out to be the most critical direc-tion from which to stimulate the MobUnit. The MobUnitreached only 78 Hz eigenfrequency at the first subsystemshaker test at the shaker facility DLR Bremen (see Fig. 7).The mission requirements were an eigenfrequency above100 Hz. The following two points show the reasons forthe low eigenfreuency:

• Grooved ball bearings are much softer than ex-pected.

• Connection between driving shaft and eccentric armis too flexible.

Therefore a redesign of the MobUnit was necessary. Theone modification is to change the grooved ball bear-ing against angular contact bearings with a wavespring.Thereby the prestressed bearings are very stiff and not sus-ceptible regarding temperature changes. The other modi-fication is a more inflexible design with one M5 screw toconnect the shaft with the eccentric arm With these twomodifications the mobility was tested again. At the testbench the new setup showed an increase of the eigenfre-quency up to 119 Hz (see Fig. 8).

Figure 6. Test setup of the MobUnit mounted to theshaker.

Figure 7. Shaker results with the first MobUnit prototype(eigenfrequency at 78 Hz).

Figure 8. Shaker results with the redesigned MobUnitprototype (eigenfrequency at 119 Hz).

4. DETAILS ON THE ELECTRONICS

Not only size and weight limitations were the driver fordeveloping an own electronics for the MASCOT projectbut also radiation and thermal issues. During the longcruise phase of MASCOT a very robust electronics hard-ware is mandatory. Whenever useful Components off theShelf (COTS) were used to save space on the PCB. Fur-thermore ITAR free parts were used wherever possible.

4.1. Radiation Hard Components

Flash based FPGAFor the MASCOT electronics the flash based FPGA (RT3series) from Actel was the best solution for our needs.Following topics made the decision towards the ITARpart Actel FPGA:

• small package (Ceramic Column Grid Array;CCGA)

• moderate power needs

• high system gate count

• no external peripheral units needed

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(a) Top side of the mobility electronics PCB (b) Bottom side of the mobility electronics PCB

Figure 9. Photo of the qualification model PCB (top side and bottom side).

Other solutions for the control unit e.g. the usage ofprocessors were not possible due to the limited availablespace. The following Fig. 9(a) shows the top side of themobility electronics, with the FPGA on it.

MOSFETsInfineon developed new radiation tolerant power MOS-FETs (BUY25CSJ) based on DLR/ESA contracts(50PS0301, 50PS0409, 50PS0601, 50PS0903) within theEuropean Component Initiative (ECI). These brand newpower MOSFETs in the SMD0.5 package were selecteddue to their performance data and to overcome the ITARrestrictions. Furthermore the small package was helpfulto realize a small and powerful motor drive unit. The fol-lowing Fig. 9(b) shows the bottom side of the mobilityelectronics, where the MOSFETs of both redundancy lev-els are located.

4.2. Components Off The Shelf

A central part of the MASCOT electronics is the motordrive electronics. For a Brushless DC (BLDC) motor6 power MOSFETs must be driven and controlled in aproper way to optimize the performance of the motor andto minimize losses. The existing radiation hard compo-nents for BLDC motor control could not be used due totheir huge form factor and the need for higher voltage lev-els than the MASCOT battery can supply. Therefore theSpin-In of a BLDC motor controller and MOSFET driverwas treated as best way to meet the space and system re-quirements.

Military parts having a small form factor and high scaleintegration were not found, therefore a part for automo-tive application was used. For these parts an operatingtemperature range of -40◦C up to +150◦C is mandatory.This made it easy to meet the mission temperature require-ments. Of course, radiation tests must be succeeded be-

fore this parts were treated as possible solution for a spacemission.

TID resultsFor the MASCOT mission a total ionizing dose (TID) of4.2 kRad (incl. margin) is expected for the electronicsbox. To provide additional margin for unforeseen eventsa minimum of 10 kRad was treated as criterion for thesuccess / fail decision of the TID test. This test was per-formed at the Helmholtz Zentrum Berlin Wannsee, Ger-many [10]. The HZB offers a Cobalt 60 source with anactivity of 5.29 · 1013Bq (June 2009). Multiple biased in-situ tests with a fully operable controller were performedat room temperature each. During these tests multiple pa-rameters were measured.

At the end of the tests, all parts were fully operable at14.5 kRad (water equivalent dose) which correlates to ap-prox. 13 kRad(Si). Beyond this limit some parts stoppedtheir operation due to the degradation of internal refer-ences which prevents the controller from erroneous oper-ation.

The following Fig. 10 shows one result of the TID radia-tion tests. One measurement of the supply currents wasdone while the motor was rotating in clock wise direc-tion (left graph). The next measurement shows the supplycurrent while the motor was rotating counter clock wise(right graph). Although the supply current after the irradi-ation doubles almost the value of the unradiated parts, wesaw no functional errors or discovered any thermal prob-lems. Please note that the dose is noted as water equivi-lant dose value.

Proton test resultsSimilar to the TID test, the proton tests were performedat the HZB, too. The HZB offers protons up to 68 MeV[11]. For this test campaign a fluence of

ΦTest = 1.7 · 1010 Protonscm2

6

Figure 10. Motor controller TID test summary.

within five different proton energies equally spaced from30 MeV up to 68 MeV were applied to the controller. Dur-ing the biased in-situ tests several parameter were mea-sured, but despite of the tacho signal no significant distur-bances or Single Event Functional Interrupt (SEFI) weremonitored. Within the MASCOT project the speed infor-mation is derived from the hall sensors which are usedfor motor commutation by the control unit. Thereforethis effect won’t affect the performance of the MASCOTelectronics.

The following Fig. 11 shows the hall sensor supply volt-age which is provided by the motor controller in more de-tail. For this plot, all proton subtest measurements wereconcatenated starting with the 68 MeV test results downto 30 MeV results. Therefore the x-Axis is scaled in"Samples" which could be transfered to seconds of testtime by factor of two. In general, some minor spikescould be detected, but the major degradation is based onthe deposited total dose which was injected during the ir-radiation with protons.

Figure 11. Motor controller proton test summary.

Ion test resultsAfter delidding of the parts the ion tests were performedat the radiation effects facility in Jyväskylä, Finland [12].Up to an effective LET(Si) of 27.4 MeV

mg cm2 (at the braggpeak) no disturbances, SEFIs or Single Event Latchups(SEL) were monitored. Only a few voltage spikes on ana-logue input or output voltages could be monitored. As anexample for the seen effects the hall sensor supply volt-age is shown in the next Fig. 12. The left graph showsthe result of sample 1 while the right graph shows theresult of sample 2.

Figure 12. Motor controller ion test summary.

5. DETAILS ON THE CONTROL ALGORITHM

The controller must ensure the movement of the eccentricarm within given constraints to generate a defined jerk forthe MASCOT system. Therefore no overshoot for the po-sition must occur. The trajectory can be separated intothe three phases acceleration, constant speed and deceler-ation (see Fig. 13). The time periods are dependent on theparameters acceleration, deceleration, maximum velocity,start and end position. Assumed that starting position and

Figure 13. Movement phases of the eccentric arm.

velocity are zero at t = 0, meaning that the system is atstandstill in the beginning, Eqs. (2) show the basic calcu-lation of the trajectory.

v =∫ t

0amax. dτ = amax. · t

x =∫ t

0v dτ =

1

2· amax. · t2

(2)

Both equations can be solved for the time to get the de-sired velocity (3). This way the parameters acceleration,deceleration, speed limit for the constant speed phase,starting position, end position are sufficient to charac-terise the complete trajectory of the eccentric arm and toensure precise movement.

∆x = xdes. − xact.vdes. =

√2 · amax. · |∆x| · sign (∆x)

(3)

The difference of desired and actual position leads di-rectly to the desired velocity. This value is computed

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online by the position controller. Because of massive non-linear friction an additional velocity PI-controller is usedto prevent steady state position difference. Fig. 14 showsa block diagram of the cascaded controllers. The param-

v =sqrt(.)des.xdes.

xact.

vdes.

PI

vact.

PWM

Figure 14. Position controller and PI velocity controllercascaded to compute the necessary motor actuating sig-nals.

eters for the trajectory are taken from simulation results.Due to the very low gravity on the asteroid it is very diffi-cult to extract that data from a test bench in the laboratory.Only scaled measurements were taken in the laboratory tovalidate the simulation results.

6. CONCLUSION

The presented hardware is developed and tested for therough environments in space applications. Furthermore itshows very high power density and two complete redun-dant signal paths. The use of brushless motors leads to amuch higher torque output at the given weight limitation.With the better efficiency of the drive train, comparedto brushed motors, less power consumption is required.This is of great advantage when the system is powered bybattery only.

The module may also be used for different other applica-tions like for example a pan-tilt drive [6] or even a smalltraction system. With slight modification the system isalso scalable to an encreased power output. The mainlimitation is of course given by the environmental condi-tions as it is necessary to dissipate the heat loss. For lowgravity missions in future it would also be possible to ex-tend the hopping concept by additional eccentric arms ar-ranged orthogonally. This way a more accurate hoppingperformance could be achieved which leads to more pre-cise science possible on asteroids. With more degrees offreedom the system gets more robust and the dependancyon the mass distribution of the system is decreased.

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