Design criteria for spacecraft inertia-wheel suspensions are listed. The advantages of magnetic suspensions over other suspension types for spacecraft inertia-wheel applications are cited along with the functions performed by magnetic suspension. The common designs for magnetic suspensions are enumerated. Materials selection of permanent magnets and core materials is considered

Downer, J. R.

English

NASA Technical Reports Server

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1_85 13871 0_-_
MAGNETIC SUSPENSION OPTIONS
FOR
SPACECRAFT INERTIA-W_EEL APPLICATIONS
3. R. Downer
The Charles Stark Draper Laboratory,
CambridEe, Massachusetts
Inc°
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L_sted below are the criteria that should be used when evaluating a suspension
system for an inertia wheel in a spacecraft environment. The suspension must be com-
patible with a vacuum environment. It must exert minimal drag torques on the wheel
and must consume only small amounts of power. The suspension must be capable of ex-
tended life with little or no attention. Additional functions that could be performe_
by the suspension includ_ pointing the wheel's angular momentum vector to achieve
active attitude control and precisely measuring the torques exerted on the wheel so
that an attitude reference signal can be obtained.
282
DESIGN CRITERIA FOR
SPACECRAFT INERTIA-WHEEL SUSPENSIONS
-_
• VACUUM COMPATIBLE
• LOW LOSSES
-- DRAG
-- POWER CONSUMPTION
• LONG: LIFE
• Lr_ ;MAINTENANCE
;']';1"1'_" R EL IABI LITY
_ _ r_'-:_.'.,:'-rb A_iGOLAR MOMENTUM VECTOR POINTING
_ .¢_¢PA'BI LITY (OPTIONAL)
• CALIBRATION CAPABILITY (OPTIONAL)
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The prtma_, advantage of utilizing a magnetic suspemlton £or a spacecraft _erc_-
._el application _s the lack of physlcal contact beldam the r_or and the scator.
Ls leads to extended suspension l_e and reduced dra S. The tolerances that must be
Lcl in construction of the suspension can be recIueed from those required of ocher sus-
_slon system types such as preclslon ball and gas bear_s. Reduced vlbraclou and
.-uctural inCeractlon can also be obtalned, Slnce ma_t_ m_.ms_ons require no
>ricanc, they are quite compatible _r_th a vacuum envlro_ment, Properly designed
_necic suspensions also allow the £unctlons of aeeltude control aud attitude race
_slng to be performed.
I
ADVANTAGES OF MAGNETIC
SUSPENSIONS OVER OTHER SUSPENSION TYPES
FOR SPACECRAFT INERTI/SPWHEEL APPLICATION8
• NO ROTOR/STATOR CONTACT
-- EXTENDED LIFE
-- REDUCED DRAG
-- REDUCED TOLERANCES
-- REDUCED MECHANICAL VIBRATION
- REDUCED STRUCTURAL INTERACTION$
• NO PROBLEM OF PROVIDING LUBRICATION IN AVACUUM
• POTENTIAL TO ELIMINATE MECHANICAL GIMBAI_
• POTENTIAL FOR ESTIMATING SATELLITE RAlrF.B
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283
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A magnetic suspension system may perform one or more of the £ollovlng Junctions:
rotor support, torquing, and torque measurmeent. Regardless of gunctlon, the suspen-
stou design is typically dr£ven hy the lnertta vheel precession torques, the stiffness
o_ the surroLmd_ng structure, and by the angle _hroughvhtch _he rotor ::st be tipped.
• FUNCTIONS PERFORMED BY MAGNETIC SUSPENSION
- BEARING
- TORQUER (OPTIONAL)
- RATE SENSOR (OPTIONAL)
• FACTORS AFFECTING MAGNETIC BEARING DESIGN
- WHEEL ANGULAR MOMENTUM
- REQUIRED SLEW RATES OF SATELLITE
-- STRUCTURAL COMPLIANCES AND INTERACTIONS
-- MAXIMUM TILT ANGLE
284
• •
There are four common designs for magnetic suspensions. The oldest type, the
_sive dynamic f_eld b_sring (ref. 17, :is stabilize_ by tuning circuit parameters for
particular excitation frequency. This type typically consumes excessively hish power
r a spacecraft iner_la-_heel applica_iou. Earushaw's theorem states that a body in a
Itlc magnetic field is stable in at most two axes. The stiffness along passively
_ble axes is typically low. Hany designers uti_lize active control of passively un-
able axes to achieve a stable system. Several advantages exist for actively control-
aE all axes with servo control. These are addressed in detail on the next viewgraph.
MAGNETIC BEARING TYPES
TYPE COMM ENTS
PASSIVE DYNAMIC FIELD
PASSIVE STATIC FIELD
PASSIVE/ACTIVE
ACTIVE •
HIGH POWER CONSUMPTION
STABLE IN AT MOST TWO AXES
(EARNSHAW's THEOREM)
LOWSTIFFNESS
UNSTABLE AXIS OF PASSIVE
SYSTEM ELIMINATED WITH
SERVO CONTROL
SERVO CONTROL REQUIRED
POTENTIAL FOR ADVANCED
CONTROL STRATEGI ES
POTENTIAL FOR ELIMINATION
OF MECHANICAL GIMBALS
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285
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An actively controlle_ maEnetlc suspension "-'ill typically be much stiffer tha
a passive suspension. The advantages of a high dc stiffness include more repeatable
performance and reduced core losses due to chansin S flux in the rotor as it spins.
Stiff suspensions are required if conventi,,nal electrical machines (such as inductio
motors), which produce significant s_deloadlng forces, are to be utilized.
Actively controlled magnetic suspensions may be used to provide set point con
_rol of the inertla wheel's angular momentum vector to provide attitude control with
mechanical gimbals. By actively varying the dampin E of the suspension, performance
near rotor critical speeds may be improved. Advanced control and estimation technlq
aimed at suppressing whirl instabilities can also be applied.
ACTIVELY CONTROLLED MAGNETIC SUSPENSIONS
• HIGHER dc STIFFNESS THAN PASSIVE MAGNETIC
SUSPENSIONS
- MORE REPEATABLE PERFORMANCE
- LOWER DRAG LOSSES
- UTILIZATION OF CONVENTIONAL ELECTRICAL
MACHINES POSSIBLE
• POTENTIAL FOR UTILIZING ADVANCED CONTROL
STRATEGIES
- SET POINT CONTROL OF ANGULAR MOMENTUM
VECTOR
-- ACTIVE VARIATION OF DAMPING
- WHIRL MODE SUPPRESSION
286
'1] -."- ;all
_,'_gnetic suspensions exer_ forces either through _he attraction of a ferromag-
netic body or by the Lorentz force.
F@rromagnetlc attraction type magnetic suspensions can be fur=her classified by
the manner through which the magnetic field is produced, Purely electro-magnetic sus-
pensions have very low gain near zero current and are highly non-linear. Permanent
magnet biased electromagnets, however, possess relatively high Eain near zero current
and are more nearly linear.
Magnetic suspensions that produce forces by the Lorencz force across a fixed
length air gap have an advantage in terms of core loss since the core material sees a
constant flux. Lorentz force suspensions are very nearly linear. By utilizing a per-
manent magnet rather than a wound core to produce the magnetic field, copper losses
are reduced.
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ACTIVE MAGNETIC BEARING DESIGNS
• FERROMAGNETIC ATTRACTION
-- PURE ELECTROMAGNET
B LOW GAIN
• NON-LINEAR
-- PERMANENT MAGNET (PM) BIASED
ELECTROMAGNETS
• HIGH GAIN
a LINEAR
• LORENTZ FORCE
-- FIXED GAP LENGTH
• ELIMINATES CORE LOSSES
a LINEAR
-- PERMANENT MAGNET FIELD
• REDUCES COPPER LOSSES
287
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The Charles Stark Draper Laboratory (CSDL) has been involved in magnetic suspen-
siov technology for more than 30 years. The definitive reference on float suspensions
was wrlCten by three CSDL engineers in 1974 (ref. I). A 5-axis actively controlled
flywheel suspension supportin E a 12 ib rotor was constructed in 1978 (refs. 2, 3, 4).
A magnetic suspension system for removing all but a nominal preload from the ball bear-
ing support of a 200 Ib flywheel was constructed in 1981.
There are currently three advanced magnetic suspension programs at CSDL. A
single axis actively controlled magnetic vibratlon-lsolator and a magnetically sus-
pended inertial reference unit _re being constructed and tested. A study of a Combined
Attitude, Eeference, and Energy Storage (CARES) system (ref. 5) is also in progress.
The goal of this study is to demonstrate the feasibility of performing the energy
storage, attitude control, and attitude reference functions of a satellite with hlgh
energy density, magnetically suspended inertia wheels. A novel Lorentz force type
magnetic suspension is be_ng constructed and tested as part of this program.
DRAPER LAB MAGNETIC SUSPENSION EXPERIENCE
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288
• F LOAT SUSPEI_SIONS
• 5 AXIS ACTIVELY CONTROLLED FLYWHEEL SUSPENSION
(12 Ib WHEEL)
• HYBRID FLYWHEEL SUSPENSION (200 Ib WHEEL)
• PRECISION ACTIVE MAGNETIC VIBRATION ISOLATION
• MAGNETICALLY SUSPENDED INERTIAL REFERENCE UNIT
• COMBINED ATTITUDE, REFERENCE, AND ENERGY
STORAGE (CAR ES) SYSTEM
. ,._. _:- -_ -.,---
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As an example of a typical _old.heel masnetic suspensioz_, the 5-axis actively
controZ1ed flywheeE shown here wiE1 be discussed.
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289
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The figure below shows the internal arrangement of components. This system
uses three actively controlled bearing elements, two radial and one axial, to control
five degrees of freedom. Three inductive position sensors and an optical speed sensor
are also used.
The f)ywheel module was designed to have a vertical spin axis so that the sus-
pension that supported the weight of the wheel would not see changing fluxes due to
rotor eccentricity and thereby reduce losses.
.
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II
290
ORIGINAL P_" "
OF F_OR QUALITY
The photograph below shows the magnetlc actuators that were used _ the 5-axls
actlvely controlled £1ywheel. Each is a permanent magnet biased electromagnet which
uses a permanent magnet to produce a bJ_slng £1eld and control coils to ob_aln active
control.
OR._ .._. 7
OF POO_ C_-.-..:=;_='
AXIAL SUSPENSION
%.
291
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As an example of the operation of a permanent magnet biased magnetic suspension:
consider the axial bearing of the 5-axis actively controlled flywheel that is shown
below. The primary flux for suspension is provided by an axially oriented permanent
magnet. Control of the air gap flux is obtained by usin 8 a control coil to shunt flux
through an alternate flux path _hat has been provided in the magnet housing.
-- DELRIN SLEEVE
_Y FLUX PATH
L FLUX PATH
CONTROL
I
SHIM
FERRITE SLEEVE
STRUCTURAL SHAFT
-WEDGE RING
ASSEMBLY
292
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I The CARES system magnetic bea_:ing is shown below. Forces sre exerted on the
flywheel due to the interaction of rotating permanent magnets and stationary control
coils. The figure also lhovs the wlndl_E confIEuretloo required to produce these
forces. The wlndinEs of the control coll structure are shown assembled and in exploded
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CARES MAGNETIC SUSPENSION
CONCEPT
CONTROL COILS ASSEI_3LE!)
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CONTROL COILS DETAILED
293
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Proper _ater_l Jelectlon in terms of either permanent _agnets or core _aterials
is cr±tical to a successful maEnetic suspension design.
When selecting a permanent magnet, maximum energy product is of pr_y ".'_por-
tance since Chls dlrectly affects the magnet size. The proper mix of re_anence and
coerclvlty must also be considered to make a s_all cross sectional area or long air Eap
design work. The re/a_lwe ease w£th which a permanent magnet can be machined should
also be considered.
Core materials are typically selected on the basis of their core loss character-
istlcs (as measured by their hysteresis loop and volume resistivity) but ease of mag-
net£zatlon mast also be e_Idered. Ease of magnetization is measured by a macer/al's
permeab_llty (one of three types, depending on appllcatlon). Operating a core ma_er_al
in its saturation region is typlcally wasteful of power.
MATER IAL CONSIDERATIONS
294
• PERMANENT MAGNETS
- MAXIMUM ENERGY PRODUCT
-- PERMANENT FLUX DENSITY
-- COERCIVE FORCE
-- MACHINABILITY
• (MRRE MATERIALS
-- HYSTERESIS LOOP
-- RESISTIVITY
-- PERMEABILITY
"" • INITIAL
. • MAXIMUM
.. • INCREMENTAL
-- SATURATION FLUX DENSITY
• ,_. . _. _- 'ibm" ,"" _. =.
J_ _in
Xt is safe to conclude that t_e basic technology requlzed: to utilize an actlvely
Itrolled _agnetic suspension syst_ iu a spacecraft ine_tla wheel has been satis-
"-torily d_monstrated, and that the design and control _peri_ce necessary to produce
Ls hardware is available. -_. ""
In addition, there is curre_ly zn" progress a great deal of. effort aimed at _m"-
Yv_-ng magnetlc mater£als. These-advances are taking place, in the areas of high er.ergy
>duct rare earth/cobalt magnets and low loss £erroceramic core materials. Tbese _d-
,zces are certaln eo _acilltate magnetic suspension design.
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CONCLUSIONS
• THE BASIC TECHNOLOGY REQUIRED FOR=MA'G_]ETIC
SUSPENSION OF A SPACECRAFT INERTIA WHEEL
EXISTS:
-- SUSPENSION DESIGN EXPERIENCE
- CONTROL TECHNOLOGY
• RECENT ADVANCES IN MAGNETIC MATEPr_.._I_.J_ACIL-
ITATE MAGNETIC SUSPENSION DESIGN " "_*"
295
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5.
REFERENCES
Frazier, R.; Gilllnson, P.; and Oberbeck, G.: Magnetic and Electric Suspensions,
Cambridge, MA, The MIT Press, 1974.
Eisenhaure, D.; Oberbeck, G.; and Downer, J.: "Development of a Low-Loss Flywheel
Magnetic Suspension," Proceedings of the 14th Inter-Society Ener_ Conversion
.Engineering Conference, American Chemical Society, Vol. I, 1979, pp. 357-362.
Downer, J.: Analysis of a Single Axis Magnetic Suspension System, MS Thesis, MIT,
1980.
Eisenhaure, D.; Downer, J.; and Rockney, R.: "Factors Affecting the Control of a
Magnetically Suspended Flywheel," Proceed£n_s of _e 19,90 Flywheel Technology
Symposium, The American Society of Mechanical Engineers, Vol. I, 1980,
pp. 380-391.
Eisenhaure, D.; Downer, J.; 31iamptis, T.; and Hendrie, S.: "A Combined Attitude
Reference, and Energy Storage (CARES) System for Satellite Applications," AIAA-
84-0565_ The American Institute of Aeronautics and Astronautics, 1984.
296


Magnetic suspension options for spacecraft inertia-wheel applications

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