A COLD FOCAL PLANE CHOPPER FOR HERSCHEL-PACS —CRITICAL COMPONENTS AND RELIABILITY

R. Hofferbert1, D. Lemke1, O. Krause1, U. Grozinger1, A. Bohm1 W. Bollinger2, J. Katzer21 Max-Planck-Institut fur Astronomie (MPIA), Konigstuhl 17, D–69117 Heidelberg, Germany

(correspondence: hofferbert@mpia.de, Tel 49-6221-528261, Fax 49-6221-528246)2 Carl Zeiss, Carl-Zeiss-Str. 22, D–73447 Oberkochen, Germany

ABSTRACT

A chopper acts as an optical switch within the en-trance optics of the PACS instrument onboard theESA Space Observatory HERSCHEL to discrimi-nate the light of faint celestial sources from a largebackground flux, which is basically due to the ther-mal radiation of the only moderately cooled 3.5m-telescope (≈ 80K). This chopper primarily consists ofa small electrically driven tilting mirror with a highposition accuracy for the full range of elongations upto ±9◦ to be operated at 4K with a power dissipationof less than 4mW. At a chopping frequency of up to10Hz in square wave modulation scheme a duty cycleof 80% is to be achieved. After a pre-developmentphase at MPIA, which resulted in an optimized andcold tested prototype, the industrial optimiza-tion and implementation of the PACS chopperstarted in 2000 in a cooperation with the companyZeiss (Oberkochen).We focus on the new design of the chopper and its

critical components, including the recently devel-oped double-stage flexural pivots as high reliablebearings of the chopping mirror axis. The mono-lithic CuBe2 layout has to resist vibration loads ofup to 45g during the launch of the ARIANE 5 car-rier and to guarantee for an accurate motion in acryogenic environment during a lifetime of at least 3years in space, which corresponds to 630 Mio cyclesof the chopper. Other components like the cryomo-tor and the rotor will be considered with respectto the selection of optimal materials, the reliabilityof mechanical linkages and the compensation of ther-mal stresses within the whole multi-material chopperassembly.

I. HERSCHEL SATELLITE

In 2007 ESA will launch the HERSCHEL satellite(formerly known as FIRST) as another cornerstonemission of its ”Horizon 2000 long term programme”[1]. This multi-user device will be the biggest andmost powerful infrared observatory of its time. Witha wavelength coverage between 60 and 670µm it willbe sensitive especially in the far-infrared and submil-limeter region, thus exploring radiation in a very un-known range of the electromagnetic spectrum. Sci-entific objectives, which will be addressed by HER-SCHEL, range from the exploration of the distant

universe and the question how first stars and galaxieswere born, across the formation of planetary systemsup to the investigation of our own solar system.

3.5mprimarymirror

sunshield +collector

cryostat +scientificinstruments

supplyunit

3.5mprimarymirror

sunshield +collector

cryostat +scientificinstruments

supplyunit

FIG. 1. The HERSCHEL Space Observatory.

HERSCHEL’s payload module is basically com-posed of a 3.5m Cassegrain telescope radiativelycooled to T ≈ 70-90K and three scientific instru-ments (HIFI [2], SPIRE [3] and PACS [4]) sittingin the corresponding focal plane inside a 1.7K cryo-stat filled with 2160l of superfluid helium, see Fig.1. At a total mass of roughly 5.300kg and a lengthof about 11m, HERSCHEL will be launched in apickapack configuration together with the PLANCKsatellite (cosmic microwave background survey, [5])by an ARIANE 5 carrier. After a transfer period of 3month both will be separated and operated individ-ually at the 2nd Lagrangian, 1.5 Mio km away fromearth in the anti-sunward direction. HERSCHEL isspecified as a multi-user, high accuracy pointed ob-servatory with a lifetime of at least 3 years.PACS, the Photodetector Array Camera and

Spectrometer [4], is one of the three scientific instru-

ments. It offers two principal observation modes inthe range between 60 and 210µm:

• Imaging photometry, using 2 bolometer detec-tor arrays with 32 × 16 resp. 64 × 32 pixels,allowing for simultaneous observation in twobands with a FOV of 3.5′ × 1.75′ and a sensi-tivity of 5mJy (5σ, 1 hour).

• Spectroscopy, using 2 photoconductor detectorarrays with 25×16 pixels each (Ge:Ga, stressedand low-stressed), covering a FOV of 50′′×50′′at a resolution of 175km/s with a sensitivity ofdown to 2.5 · 10−18W/m2 (5σ, 1 hour).

In comparison to its very successful predecessor ISO[6], ESA’s ”Infrared Space Observatory”, whichwas operated between 1995 and 1998, HERSCHEL-PACS features a roughly one order of magnitudehigher sensitivity per pixel for compact celestialsources at a drastically increased number of pixelsand a higher angular resolution.

II. PACS CHOPPER

Since the primary mirror of the HERSCHEL tele-scope is only moderately cooled to a temperaturearound 80K, a small tilting mirror at 4K, the chop-per, located right within the entrance optics ofPACS, will act as an optical switch to discriminatethe light of faint celestial sources (down to 10−16W)from the orders of magnitudes larger backgroundflux. In addition the PACS chopper is specified toallow for in-orbit calibration of the whole camera byswitching flux from internal black bodies onto thedetector arrays. Both demands can be fulfilled, ifthe tilting mirror is driven at different elongationsusing appropriate modulation schemes. The full listof requirements for the chopper as adapted for anoptimized operation of PACS itself [7] can be seen inFig. 2.Note, the critical items are given by a duty cycle of

80% in square wave modulation scheme at 10Hz driv-ing frequency with an accuracy of 1 arcmin acrossthe whole deflection range of 4.1◦, to be achievedat 4K with only 4mW of total power consumption.Those stringent requirements together with its lo-cation in the entrance optics of PACS, making it asingle point failure source of the whole experiment,demand a high degree of reliability and accuracy forthe chopper and its critical components.In a two year pre-development phase at MPIA the

technical feasibility of such a cryogenic mechanismhas been demonstrated, yielding an optimized andcold-tested prototype [8–10], which at least fulfilledthe given functional requirements as given in Fig. 2.In Fig. 3 the principal design of the advanced pro-totype can be seen:

Dimensions envelope 125 mm X 80 mm X 40 mm

Mass � 300 g

Mirror optical performanceflatnessroughnessreflectivity

< 150 nm< 50 nm

� 98%

Mirror Size elliptical 26 mm x 32 mm

moving axis � 0.1 mmsharp edges 10° wedge shaped

Chopper inclinationa) operationb) calibration

� 0° – 4.1° accuracy � 1 arcmin

� 9° accuracy � 2 arcmin

Modulation functiona) operationb) calibration

arbitrary waveformsquare wave

Frequency ranges 0 Hz ... 10 Hz, adjustable

final position time (duty cycle)a) operationb) calibration

� 80%

� 70%

power consume. 10 Hz square wave10% calibration 9°; 90% observation 4.1° � 4 mW at 4 K

Peak current ( � � 8,9°) 100 mA

resistance (complete) for drive current appr. 20 �

fail save position zero position � 1 arcminenvironmental condition for operation vacuum, 4 K (operation at RT possible)

Cool down-/Warm up-cycles � 15

vibration loadsquasistaticsinusrandom

45 g tbd30 g tbd15 g RMS tbd

Lifetimea) operationb) calibration

� 6 years, operation 33 % of time570 Mio. cycles

63 Mio. cycles

FIG. 2. Specifications for the PACS chopper.

FIG. 3. Design of the advanced MPIA chopper proto-type.

The mirror is an integral part of the rotor (oneblank, Al 6061 T6), which also carries two drivingmagnets (Vacodym 344 HR), a position sensor mag-net (Vacodym 344 HR) and mounting bushings fortwo flexural pivots. The latter in general provide forboth, a frictionless bearing as well as a linear restor-ing force. This embedded, free swinging rotor is ac-tuated by three alternating driving coils leaving onlya small air gap for the moving permanent magnets onthe rotor. To account for a very accurate deflectionmeasurement, which is also important for controllingthe externally pre-shaped modulation function of therotor, a newly developed position sensor (closed Ul-traperm/Cryoperm core with magnetic flux sensitivesemiconductor fieldplates) picks up the signal of themoving position sensor magnet. Further informationabout design and functionality of this position sensorcan be found in [11].

To demonstrate the achieved performance and ac-curacy of the advanced MPIA prototype, Fig. 4presents the nominal square wave modulation func-tion at 10Hz with a maximum deflection of 4.1◦ anda duty cycle of even more than 80% as measured inthe cryovacuum at 4K. The attached table comparescritical operational parameters, indicating that theprototype exceeds specification.

Specifications Prototype

Chop throw [°] 9 12

Chop frequency [Hz] 10 15

Position accuracy [arc sec] 60 20

Power dissipation [mW] 4 0.4

Duty cycle [%] 80 83

Specifications Prototype

Chop throw [°] 9 12

Chop frequency [Hz] 10 15

Position accuracy [arc sec] 60 20

Power dissipation [mW] 4 0.4

Duty cycle [%] 80 83

FIG. 4. Cold-measured modulation function of the ad-vanced MPIA prototype and a comparison of specifiedand achieved physical properties. While curve a) is mea-sured under closed-loop control, curve b) was recorded ina feed-forward loop without control.

Based on this prototype phase at MPIA, the de-velopment of a space-qualified chopper Flight Model(FM) for HERSCHEL-PACS has been finally com-missioned to the company ZEISS in 2000. Theirproject philosophy is defined by

• the adoption of the MPIA prototype concern-ing functionality and performance,

• optimization with respect to lifetime andlaunch conditions,

• the fabrication of four models, including twotest-specimen (lifetime test, qualification test),

• the space qualification of all components andthe whole unit,

• the preparation of a complete documentationas demanded by ESA and

• supervision of the whole project by industrialsystem engineering and quality assurance.

A technical drawing of the PACS chopper as elabo-rated during the ZEISS design phase together with aphotography of the corresponding hardware can beseen in Fig. 5.

housing

mirror

drive unitwith 3 coils

positionsensorunit

flexiblepivot

temperaturesensor

pigtail withI/F connector

housing

mirror

drive unitwith 3 coils

positionsensorunit

flexiblepivot

temperaturesensor

I/F point

pigtail withI/F connector

FIG. 5. ZEISS design of the PACS chopper togetherwith a photography of parts for the Lifetime Model (LM).

III. CRITICAL COMPONENTS OF THEPACS CHOPPER

We will now focus on the design, manufacture andtesting of critical components of the PACS chopperlike the flexural pivots, the driving coils and the ro-tor. Special emphasis will be put on the selectionof optimal basic materials, the reliability of linkagesand the compensation of thermal stresses within thewhole multi-material chopper assembly. All sub-levelverifications serve as preparation for the assembly ofthe Lifetime Model (LM), which will be hard-testedsoon.

A. Flexural Pivots

Flexural pivots present an optimal solution to con-struct maintenance-free bearings with one axis of ro-tation for the cryovacuum. The possibility to run

them friction-less without any lubricant makes themsuperior to any other type of suspension. The me-chanical principle basically rests on two or morecylindrical stages in a suspension unit, which are onlycoupled and positioned to each other by a systemof embedded crossed blade springs providing both,bearing as well as restoring force. Figure 6 shows atypical realization, which has been used in the ad-vanced MPIA prototype.

suspension unit

blades

FIG. 6. Typical design of a flexural pivot (model: Lu-cas 5005-800), which has been used in the advancedMPIA prototype.

Three different kinds of loads have to be consid-ered:

• Fatigue during the whole lifetime of the satel-lite mission. Verification of this feature is themain goal of the lifetime test. Mainly thechoice of material determines this item.

• Mechanical stress during the launch. This fea-ture has been addressed in a detailed FEMmodel structural analysis of the chopper andthe flexural pivot as its most critical mechani-cal subcomponent. Here also the limiting loadsand the eigenfrequencies of the flexural piv-ots have been determined. To avoid any dam-age caused by shock loads during the launch,the chopper will be additionally equipped withan electro-mechanical launch lock mediated byelectrically shortened motor coils. Tests ofthese items are also part of the LM verifica-tion phase.

• Thermal stress due to cryogenic shrinkage ofdifferent materials. Not only the embrittlementof certain raw materials at low temperaturesbut also the mechanical interplay with othermaterials had to be considered. Furthermore,this also determines the kind of linkage betweencomponents (screwing, clueing, etc.).

All those considerations led to a trade-off phase atthe ZEISS laboratories and the present layout of the

PACS chopper flexural pivots, see Fig. 7.

Item PACSType 1

PACSType 2A

PACSType 2B

ISOPHOT

Type Monolithicsingle

Monolithicdouble

Monolithicdouble

Monolithicsingle

Material CuBe2 CuBe2 TiAl6V4 CuBe2

Dimensionof blades

T=L=W=

30µm2.3mm3mm

40 µm2.3mm2x4.5mm

45µm2.3mm2x4.5mm

50µm1,7mm3mm

Spring rate ofone bearing

0.22Ncm/rad 0.28Ncm/rad 0.28Ncm/rad 1.6Ncm/rad

Quasistatic loadwithout safety

60g 56g(=45g x SF)

72g 36g required

Bendingstress

Calibration

Operation

�4.1°

�9.0°

128MPa282MPa

�4.1°

�9.0°

72MPa157MPa

�4.1°

�9.0°

64MPa141MPa

�1.7°

�4.3°

134MPa344MPa

Lifetimesafety

Calibration

Operation

570Mio63Mio

4

2

570Mio63Mio

7

3.5

570Mio63Mio

8

4

35Mio0.2Mio

3.4

4.2

Eigenfrequencies591Hz (R)857Hz (R)1874Hz (R)1915Hz (R)

389Hz (FP)564Hz (R)759Hz (R)1706Hz (R)1744Hz (R)

455Hz (FP)564Hz (R)759Hz (R)1706Hz (R)1744Hz (R)

Thermalstresses

R-bushingFP-housingFP-blade

� Type 2A 84MPa172MPa55MPa

187MPa338MPa120MPa

W coupling rod

pivot blades

L

T

Y

Z X

cut

view

FIG. 7. Results of the ZEISS trade-off for the PACSchopper flexural pivots. The whole investigation baseson a detailed FEM model structural analysis using typi-cal grids as shown in the lower part (PACS Type 2A/2Bin cut view). For comparison also the properties of theISOPHOT chopper [12], which flew on the ISO satel-lite, are given. Eigenfrequencies are labelled by (FP) forflexural pivot excitations and (R) for rotor excitations,respectively.

While the advanced MPIA prototype used com-mercial, modularly composed, stainless-steel flexu-ral pivots Lucas type 5005-800 (Fig. 6), the lifetimeresp. launch requirements demand an optimized geo-metrical design [13] with respect to spring rate, bend-ing stresses and load capacity. Therefore, on the onehand side a monolithic layout has been foreseen fa-voring CuBe2 resp. TiAl6V4 as raw material, bothof which are suitable for individual geometrical struc-turing by wire eroding (EDM). The basic advantagesof monolithic vs. modular-type manufacturing is theprevention of parasitic strength enhancement at tautpoints of the spring blades as well as a hysteresis-free momentum - angle of deflection characteristics.On the other hand, a design with crossing springblades unseparated on the axis of rotation has been

preferred. This guarantees internal prevention of amoving axis during deflection and therefore crucialreduction of additional stress by perturbing torque.Anyhow, since the stiffness or spring rate d (in

Nm/rad) of an unseparated flexural pivot is in linearapproximation 4 times larger than for a ”separatedtwin” of the same dimensions [14],

dunsep =23E

WT 3

L= 4 · dsep , (1)

the maximum deflection ϕ is in the same way reducedby a factor of 4,

ϕunsep =12

σ

E

L

T=

14· ϕsep . (2)

Here, the properties L (length), T (thickness) and W(width) define the geometry of the blades, see Fig.7, while E (modulus of elasticity) and σ (fatiguestrength) characterize the used raw material. Thisin general enforces to choose long and thin bladesfor applications with large deflection angles as in thecase of the PACS chopper. Now, an additional de-sign feature had to be implemented to account forthe disadvantageous factor of 4 and to again relaxthe bending stress loads of the bearings: A double-stage design divides the full range of elongations of atleast ±9◦ into two substages with half the deflectioneach, both of which are connected by an intermedi-ate bar or coupling rod. Figure 8 gives an impressionof this double-stage design and the way it could benowadays manufactured by wire EDM.

connecting bar

stages

blades

connecting bar

stages

blades

FIG. 8. The monolithic, unseparated, double-stageflexural pivot presents the final result of the sophisticateddesign phase at the ZEISS laboratories. The photogra-phy shows a first prototype, which has been manufac-tured from one blank of CuBe2 raw material in EDM(Electrical Discharge Machining, i.e. wire eroding) tech-nique.

Although the titanium type spring is superior incapacity against vibration loads during launch andpresents higher safety factors during lifetime cycling,it is exposed to drastically enhanced thermal stressduring cooldown as linking element between chopperrotor and housing, both made out of Al 6061 T6,see Fig. 7. Since the present design does only allow

for limited mechanical compensation, CuBe2 was theonly suitable solution and hence the baseline for thePACS chopper (Type 2A).In a series of experimental investigations the phys-

ical properties of those finally realized flexural pivotshave been analyzed. Although the structural analy-sis clearly points out capability against quasi-staticloads of 56g, which correlates to roughly 45g takinginto account an ESA specific vibration load safetyfactor of 1.3 for components inside PACS, see Fig.7, the blade thickness of first prototype flexural piv-ots were increased from 40 to 50µm, also to some-what relax the manufacturing process. Although thisenlarges the power dissipation and reduces the life-time safety factor of the chopper by about 10-15%,the load capacity is drastically enhanced due to theT 3-dependency in Eq.(1). Optical measurements in-dicate thickness variations of only 2µm across thewhole lengths and widths of several blades. Using abalance with implemented flexural pivot as bearingelement, the stiffness of the spring could be mea-sured with different test masses at different leverarms. This yields (0.50 ± 0.02)Ncm/rad, which isin good agreement with the FEM-analysis for the40µm-blades scaled by the T 3-law of Eq.(1), whichgives (0.54±0.06)Ncm/rad for the (50±2)µm-blades.In a final step a set of measurements has been

performed to check for the load capacity of thesprings using a Zwick material testing machine (type1484 WN:074797). According to the increased bladethickness, also the specified quasi-static load emerg-ing from the FEM-analysis, see Fig. 7, has to berescaled using the T 3-dependency. This results ina new value for the 50µm-blades of ≈110g for eachflexural pivot resp. ≈220g for the total setup withtwo bearings (both values again to be reduced by theabove mentioned vibration load safety factor of 1.3).To simulate this specified quasi-static load on thechopper, two prototype flexural pivots were mountedin a jig and laterally exposed to a tuneable force. Intwo sequences the springs were either symmetricallyor asymmetrically loaded (with up to 85% portionon one of the two sides). The recorded force vs. dis-placement diagrams indicated up to forces of 12N,which corresponds to a quasi-static load of 120g at arotor mass of 10 grams, a very linear behavior and afull restoration after force decrease without any dam-age and only a slight hysteresis, see Fig. 9. In theasymmetrical case the linear region ended approx-imately at 12N, first damage via inelastic bucklingof the spring blades could be observed beyond 22N.This is also in very good agreement with the specified220g as mentioned above.Additional experimental investigations also tested

the rupture limit of the blades themselves. For thispurpose the coupling rod inside the flexural pivot hasbeen exposed to tensional forces of up to 500N withrespect to the housing of the flexural pivot, see Fig.10.

1

flexural pivot

link

jig

force F force F

2

displacement a

Test case 1(symmetric)

2(asymmetric)

Force F 6N(60g at rotormass = 10gr)

12N 12N 18N �22N

Displacement a (13�1)µmfull restoration

(33�1)µmfull restoration

�41µmend oflinearrange

�94µm non-linearbuckling

FIG. 9. Setup and results of the load capacitytests using the ZEISS prototype flexural pivots with50µm-blades.

jig

flexural pivot

F

FIG. 10. Setup used for the rupture tests. On the leftthe flexural pivot can be seen in the jig, which is mountedin the material testing machine on the right, exposing upto 500N of tensional forces to the blades.

Damage only occurred in the case of asymmetrictension across the rod, thus defining the rupture limitof the used material.After this static tests verifying the critical item

”quasi-static load” of the flexural pivots, a wholebundle of experimental investigations will be devotedto launch vibrations between 5 and 100Hz (sine ex-citations) and between 20 and 2000Hz (random ex-citations). Those values could also be derived fromthe FEM structural analysis of the chopper, yieldinglimiting loads of 30g (sine) and about 18g rms (ran-dom), which are based on the mentioned 56g quasi-static load for the 40µm-blades. Verification of thoseitems will be mainly part of the qualification tests onthe PACS chopper QM, which are also planned forthe second half of this year.

B. Driving Coils

In contrast to the flexural pivots the driving coilsof the cryomotors were already optimized with re-spect to the chopper specifications during the pro-totype phase at MPIA [8–10]. Not only the design

but also the choice of different raw materials weresubject of detailed investigations, including e.g. afull electromagnetic FEM simulation of the wholeactuator unit using the MAFIA package. Resultsof these simulations were in perfect agreement withcryogenic measurements defining a driving mecha-nism, which is understood in every physical detaillike specific torque, power dissipation and stray fieldcharacteristics. This allowed for instance to operatethe whole chopper in a feed-forward loop (curve b inFig. 4) without any control unit, i.e. position sensor,since the desired modulation function can be directlyconverted into an exciting voltage via the electricalproperties of the cryomotor and the mechanical, alsowell-known properties of the rotor.

ZEISS had to address important modificationsto guarantee a reliable and space-qualifiable design.The principle task comprised (a) to properly issu-ing a procedure of how to manufacture and assembleevery single part of the motor block (”the recipe”)and (b) to further optimize the whole unit with re-spect to the choice of raw and bonding materials(”the ingredients”). Main drivers of this optimiza-tion process are on the one hand a further reduc-tion of the power consumption basically mediatedthrough Ohmic losses in the coils and eddy currentsin the cores. On the other hand a sufficient thermalcoupling of all components towards the housing, i.e.heat sink of the chopper, has to be guaranteed. Incontrast to the MPIA prototype, where the motorchassis was realized as an integral part of the chop-per housing, ZEISS preferred an exchangeable motorblock made from polished Al 6061 T6, which could bemodularly mounted and dismounted from the rest ofthe unit. Here, wide mounting feet equipped withshrinkage compensated screws (via Invar washers)account for an optimal thermal coupling. A detaileddrawing of this final state can be found in Fig. 11.

driving coils

screws +

washers

adjust pin hole

Vespel cap

motor chassis

FIG. 11. Final outcome of the cryomotor design ac-cording to ZEISS.

The recipe for the coils as mediator of the drivingmagnetic flux looks as follows: Cryoperm 10 (Vaku-

umschmelze), which defines an optimal raw mate-rial for magnetic applications with high permeabil-ities (µr

>∼ 105) and high saturation fields (Bs>∼

0.9T), both at very low temperatures around 4K,has been used to produce laminated cores with typ-ically 112 sheets of 100µm thickness, clued and en-capsulated with Stycast 1266 layers of approximately10µm. The corresponding manufacture process hasbeen optimized by a dedicated procedure, which isknown from circuit board etching: Pre-cutting ofthe Cryoperm sheet material, removing of the insula-tion and chemical abrading of the surface (nitric acid,50%), rinsing and drying, applying of photoresist, ex-posing to UV light, processing with NaOH, removingthe photoresist with KOH, etching with FeCl3 and fi-nal rinsing and drying. The manufacture process iscompleted by a homogeneous clueing of all sheets in-side a pressuring tools and an outgassing sequencein vacuum to exclude embedded air. In a subsequentfine milling the core edges are finally prepared forcoiling the very thin (80-100µm) high purity (5-6N)wire (970-1020 windings, aluminum resp. copper).Furthermore, a slight lateral oversizing of the

cores, see Fig. 11, guarantees an appropriate guid-ing of parasitic stray field in the outer regions of themotor block. This not only prevents the actuatorfrom a non-linear torque vs. current behavior, butalso reduces eddy losses in the aluminum chassis ofthe motor block. Finally, a Vespel cap also equippedwith shrinkage compensated screws covers the chas-sis and cares for a sufficient mechanical stress to fixthe coils during cooldown.

FIG. 12. Cryomotor hardware for the PACS chopper.

First hardware can be seen in Fig. 12, first testswill be performed in Summer 2001. Here also thetype of wire will be ultimately fixed. While 5N Al incontrast to 5N Cu has been proven to be better bya factor of ≈ 10 in the remaining Ohmic resistanceat 4K (MPIA prototype: residual resistance ratioRRR = R300K/R4K ≈ 800 for Al and 70 for Cu, re-spectively), a reliable workmanship is more difficult,maybe even impossible for the foreseen delicate 6N

Al wire specified for ≈ 1000 windings.Note, that due to the excellent properties of the

MPIA prototype, also the SPIRE experiment on-board the HERSCHEL Space Observatory plans touse the present coils as integrated part of a BeamSteering Mechanism.

C. Rotor

Concerning installation and adjustment, the chop-per rotor can be regarded as the most important sub-component of the whole assembly. Due to its highnumber of interfaces to (i) the incident optical beam,to (ii) the housing via two flexural pivots, to (iii) theactuator via two driving magnets and to (iv) the po-sition sensor via another fieldplate magnet, only amonolithic realization (Al 6061 T6) based on an op-timized geometrical design could guarantee the nec-essary stiffness and compactness for the desired dy-namical behavior. A monolithic construction is alsonecessary to allow for a precise alignment of the flex-ural pivots as rotor suspensions towards the chopperhousing.Although the MPIA prototype already presented

a well-advanced development, certain modificationswere necessary to comply with the present chopperspecifications:

• While the prototype rotor was constructed onthe baseline of a circular mirror with 25mm ofdiameter, the present PACS design demandsan elliptical mirror with 26mm×32mm. Thisslightly increases mass and momentum of inter-tia (MoI), thus the geometry had to be adaptedconstituting a somewhat different lightweightconcept on the mirror back (cross members in-stead of honeycombs).

• The driving and sensor magnets (Vacodym 655TP) were slightly changed with respect to sizeand position above the rotor axis to coincidewith the modified driving resp. position sensorunit.

• The bushings at the rotor ends have been well-adapted to the new double-stage flexural piv-ots. Here another shrinkage compensation fea-ture has been foreseen: While on the hous-ing side the flexural pivot will be fixed by ascrewed clamp again with compensating wash-ers, see. Fig. 5, the counterpart stage willbe clued into the rotor bushing (using Sty-cast 1266). Within this bushing a small glu-ing gap of 2mm length allows to reduce lat-eral stress between the strongly shrinking alu-minum (α = ∆l/l = −420 ·10−5) and the moreweakly shrinking CuBe2 (α = −320 · 10−5).

• Apart from those changes, which basically op-timize rotor performance during the nominal

operation with typical angular accelerations ofup to 20000rad/s2 during the mirror transi-tions, the ZEISS design in particular had toaddress optimization with respect to the launchconditions. In this context the FEM modelstructural analysis focused on two importantrequirements: (a) Eigenfrequencies of the ro-tor have to be far beyond the principle reso-nance frequency of the PACS instrument (f0 ≈125Hz) to avoid resonant amplification duringthe launch and (b) to reduce vibration excita-tions of the chopper and to improve the launchlock mechanism the rotor needs to be equippedwith a tuneable balancing system, which mightbe adjusted and fixed even after the assemblyof the rotor-bearing system.

integratedlightweightmirror

sensormagnet

drivemagnets

integratedlightweightmirror

sensormagnet

drivemagnets

tuneable

balancing

masses

MoI: 3.23x 10 kg m-7 2

Mass: 12 gramsMaterial: Al 6061 T6

FIG. 13. ZEISS design of the chopper rotor.

The result of this considerations as part of the de-velopment phase at the ZEISS laboratories can befound in Fig. 13, the eigenfrequencies are given inFig. 7. Beside design drawings also a photographyof the corresponding hardware and a list of the fun-damental properties are shown. The hardware man-ufacture turned out to be an enormous challenge forthe MPIA mechanical workshop as the whole chop-per did. The strict compliance of precise annealingprocesses before (6h at 215◦C, slow cooling to ambi-ent temperature) and after (8h at 180◦C, very slowcooling to ambient temperature) the machining ofthe coarse structure and a very skillful handling dur-ing the fine structuring yielded a nearly perfect ro-tational symmetry between the mirror axis and thetwo flexural pivot bushings with a mechanical errorof only about 1µm.In a final step the mirror surface has been dia-

mond milled and gold coated at the ZEISS work-shops. Optical measurements on two rotors yieldedvery promising surface conditions with worst casevalues of 88nm/81nm rms of flatness (before/afterthe coating) and approximately 10nm rms of rough-ness at a typical coating layer thickness of 200nm,compare Fig. 2. Figure 14 presents the completedrotor ready for the implementation of flexural pivotsand magnets and the final adjustment in the PACSchopper housing.

FIG. 14. The diamond milled and gold coated rotor.

IV. CONCLUSION

Three most critical components, namely the flex-ural pivots, the cryomotor and the chopping rotorhave been investigated with respect to design andchoice of optimal raw materials. Their behavior andinterplay during the launch and under cryogenic op-eration has been analyzed and verified in numericalsimulations and first experimental component tests.This resulted in a completely new monolithic, un-separated, double-stage design for the flexural piv-

ots, which could be technically realized in a wireEDM process on CuBe2. Static tests on first proto-type springs indicated the expected high load capac-ity and a proper functionality under nominal con-ditions. The cryomotor coils were optimized usinga new and sophisticated manufacture procedure forthe cores. Finally, also the rotor was subject to someimportant changes with respect to the MPIA proto-type. Beside optimizing its balancing features withrespect to a larger mirror and the specified launchconditions, also the optical processing of the mirrorsurface has been completed within specification.

The next milestone in this development will be thefunctional test phase of the complete chopper unitfollowed by a 2.5 month lifetime cycling at approx-imately 100Hz to simulate the specified 3 year life-time of HERSCHEL-PACS. Before this cycling thePACS chopper LM will be exposed to vibration loadssimulating the launch of the ARIANE 5 carrier. Ex-perimental setups for both test phases can be seen inFig. 15. With completion of the verification phase infall 2001, the reliability, accuracy and performance ofthe PACS chopper will be demonstrated.

77K cryostat 4K cryostat

FIG. 15. Test setup for the vibration (left) and lifetimetest (right).

V. ACKNOWLEDGMENTS

This work is funded by DLR (Deutsches Zentrumfur Luft und Raumfahrt), Bonn, and MPIA. Theauthors are responsible for the content of this pa-per. We are grateful to our colleagues at MPE(Max-Planck-Institut fur Extraterrestrische Physik),Garching, for smooth cooperation within the PACSproject.

[1] http://astro.estec.esa.nl/SA-general/Projects/First/first.html

[2] http://saturn.sron.nl/hifi/[3] http://www.ssd.rl.ac.uk/SPIRE/[4] http://pacs.ster.kuleuven.ac.be/[5] http://astro.estec.esa.nl/SA-general/Projects/Planck/

[6] http://isowww.estec.esa.nl/[7] R. Katterloher, N. Geis, ”Chopper Specifications”,PACS internal document PACS-ME-RS-001, Issue 1,2000.

[8] D. Lemke, U. Grozinger, O. Krause et al., Proc.SPIE 3759, p. 205, 1999.

[9] O. Krause, D. Lemke, U. Grozinger et al., Proc.SPIE 4131-39, p. 120, 2000.

[10] O. Krause, Diploma thesis, Universitat Heidelberg,2000.

[11] O. Krause, ”Magnetoresistive position sensors forcryogenic space applications”, these proceedingsresp. 9th ESMATS poster session.

[12] Lemke, D., Klaas, U., Abolins, J. et al., ISOPHOT– capabilities and performance, 1996, A&A 3125,L64.

[13] R. Haberland, Feinwerktechnik & Messtechnik 92(1984) 8, p. 417.

[14] M. Steinbach, ”Uber die Berechnung der Kreuzfeder-gelenke (KFG)”, Industrial investigation, Bochum(1998).