E AME ICA SOCIE Y O MEC A ICA E GI EE S 0 G 240 4 E. 4 St., Y r , .Y. 00
h S t h ll n t b r p n bl f r t t nt r p n n dv n d n p p r r .n n t t n f th S t r f t v n r S t n , r pr nt d n It p bl t n l n pr nt d nl It th p p r I p bl h d n n ASME n l. p r r v l bl I fr ASME f r f ft n nth lt r th t n . r 4 d n USA. Copyright © 1990 by ASME
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C AW O MEEKS nd IC O S E CE A CO — Adv n d C ntr l hn l , In . rthr d , C l f rn 24
A S AC (3) advances in miniaturized, integrated circuit, solid state electronics. A novel magnetic bearing design was created that uses permanent magnets to generate the primary magnetic field and attraction These developments have lifted the most significant barriers to electromagnets for stabilization and control. This approach uses a application of magnetic suspension to a multitude of modem machines. geometrically efficient arrangement with a combination of axially flowing permanent magnet field and a circumferentially flowing electromagnetic Magnetic bearings offer the advantages of very long life and high field. This design was compared analytically with other types of magnetic reliability by (1) the elimination of wear out and fatigue failure modes and, bearing designs. The design comparison showed the new design to be 50% (2) elimination of a lubrication supply and circulation system, and (3) by lighter weight and 50% lower in power consumption than all providing a way to avoid the single point failure limitation of conventional electromagnetic designs of equivalent performance. A demonstration bearing designs. Additionally, the very low rotational axis torques of model of this new approach built and tested for performance at low magnetic bearings make possible lower bearing power loss, higher shaft speeds. This test model successfully demonstrated the feasibility of accuracy pointing systems, high resolution instruments, and improved this new approach. rotor dynamics for pumps. Future pumps and turbine engines may be operated at higher efficiencies because the large clearances typically used in magnetic bearings, and elimination of operating temperature limitations of liquid lubricants, may allow the bearings to survive high and low n l t r temperatures associated with high efficiency thermodynamic cycles and cryogenic fluid pumping. A = p l f r ( 2 = fl x d n t (G The one serious disadvantage of magnetic bearings is they are b = tr v r bl = 2 significantly larger and heavier than their rolling element bearing = f r ( t n counterparts. The purpose of this work was to address methods for = r n p ( reducing the size and weight of magnetic bearings. The improvements = v l n th f n t nd t n v r t n suggested are compared with the known state-of-the-art in prior magnetic = p r b l t f fr p bearing design.
.0 ACKG OU 2.0 A COM A ISO S U Y O ESIG A OAC ES Magnetic levitation has been a topic of serious engineering interest for over 150 years. A literature survey by the author revealed that during the A brief comparison study was made to review the state-of-the-art in l t 2 r v r 0 d ff r nt n t p n n t h v b n magnetic bearings and to determine the optimum design approach for high developed and tested. However, the realization of the potential advantages stiffness, high load applications such as pumps, turbines, and pointing and of magnetic bearings escaped scientists and designers until three tracking gimbals. Both active and passive (semi-passive) design schemes important, relatively recent, developments: were studied and the relative merits of various design approaches were compared. (1) advances in high energy product permanent magnet materials (over 30 X 10 6 Gauss-Oersted The approach used was to establish a set of requirements typical of small energy product); pumps and to design four types of magnetic bearings to meet the requirements and to compare the resulting designs for size, weight and (2) the development of high-saturation flux ferromagnetic power consumption. Magnetic bearing design is, of course, a highly materials, and; heuristic process and there are always trade-offs between size, weight and
r nt d t th G rb n nd A r n n C n r nd Exp t n— n 4, 0— r l , l power consumption. However, by using the same materials, same coil A = wave length of magnetic induction variation power density, and same magnetic materials design limits (saturation flux, coercive strength, and magnetic induction), a reasonably objective The concept of Figure 2-1c uses the principle of a magnetic circuit comparison of relative merits of various design approaches can be made. always seeking a geometry that minimizes the reluctance of the ferromagnetic circuit to develop forces that align or i.e., maintain 2. r n pl f Op r t n n d n th th d f th r t n l r r t n l nt p l faces. In the configuration shown, radial stability is attained, but at the The state-of-the-art review illustrated that many different types of expense of axial instability that must be overcome with active control. magnetic bearing designs have been constructed and tested. Although there Several designs using the "reluctance centering" princi ple are reported in are a multitude of different magnetic bearing design approaches, they are the literature for spacecraft attitude control wheels (6,7,8,9,10,11) all simply combinations of three principles of operation. Figure 2-1 illustrates the three magnetic principles used in all practical magnetic The force per unit area, or "magnetic pressure", of reluctance Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1990/79085/V005T14A012/2400068/v005t14a012-90-gt-240.pdf by guest on 29 September 2021 bearings constructed to date. centering magnetic bearings in the passive plane is: (from reference 11) Q B2 COMME S CO CE A = [2 ] OS IUY S A O S SO S YA 0 AC I E Y CO O E EOUI ES 2 SE OS O EG EE O EE OM S A Y
O EQUI ES E EC ICA OWE O where: Q is a geometric constant that varies from a theoretical M A MAG E IC E S maximum of 0.33 to less than 0.1 O A GE CO S EQUI E O O UCE MAG E IC E S A 0 cons) n• A Y• U E The magnetic gaps of designs 2a and 2c can, of course, be energized by . d l .n,ln 0 passive permanent magnets or electromagnet coils. The use of permanent Attr t n l tr n t magnets to energize the air gaps, and electromagnets for control, yields some unique characteristics that are not apparent just from examination of [ 4YEM YAUYE MOS 0 A IA Y S A E A IA Y `. U S A E ( EQUI ES O YOY I SE O the characteristics of elements 2-la, and c. The use of permanent magnet
0 A IA COU E A U IS A IA Y energization of the bearing air gaps makes possible: S A E A IA Y U S A E
0 I C O O UCE UE O OMOGE EI ES MAG E * less power consumption for the same load capacity or same MA E IA S t ffn , O O UCS I Y O EMS WI MAG E S b p l n p r n nt n t * a meta-stable system that can be operated at almost zero control
COO . power when external disturbance loads are zero.
0 A IA Y S A E * a linear force vs control current system which can lead to 0 A IA Y U S A E considerable simplification in the servo control system design. 0 EQUI ES O Y I SE O CO O A new, fourth, concept, shown below in Figure 3-1, was developed by adding permanent magnets to energize the radial working air gap of the all electromagnetic, actively controlled bearing. This design has several l t n nt r n advantages over the all electromagnetic bearing approach as will be illustrated below. r 2 . M n t r n n r n pl . 2.2 lt f n C n pt C p r n St d Attraction electroma nets, as shown in Figure 2-la, have been used in a variety of designs ( ,2, ) with from one to five active servo controls Four candidate design were created from the concepts of Figure 2-1 (i.e., X, Y, Z, and cross coupling control for &x, ty and &z). for comparison with a new, permanent magnet bias concept. The designs were developed analytically using the common design criteria of Table The force per unit air gap area, or "magnetic pressure" (F/A), of 2 . electromagnetic attraction bearings is: (from reference 4) I PARAMETER VALUE 2 A — [2 ] Radial load ...... 356 N (80 pounds ) 2 Moment load ...... 18.0 N-M (160 in-lb) Shaft air gap diameter ...... 3.0 (1.187 inch) The use of permanent magnets in repulsion, as illustrated in Figure Radial stiffness ...... 4.38 x 104 N/cm (2.5 x 104 lb/in) 2-lb, was originally conceived of by Backers( ) and yields a bearing that is Ferromagnetic material magnetic saturation limit ...... 10,000 Gauss radially stable using passive magnets, however, it is axially unstable and Coil temperature rise ...... 70 0 C requires at least one degree of active control. Permanent magnet energy product ...... 30 x 106 Gauss-Oersteds
The force per unit air gap area, or "magnetic pressure", of bl 2 . n r t r nd t r l pr p rt d f r permanent magnet repulsion bearings is: (from reference 5) p r n t d
2 Figure 2-1 is a compilation of the results of this design comparison.
A b b [2 2] 4
where: b = 2ag/A ESIG CO CE
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E MA E MAG E A IA E U SIO E UC A CE CE E I G A E EC OMAG E IC IAS AC I E CO O AC I E A IA S A I I A IO A IA A IA COM I A IO A IA A AME E A IA AC UA O (S EA I G A A IA EA I G EA I G ( O A
OU SI E IAME E ( 6. 2 h .8 8. 8 ( .46 In. 6. ( . n. AI GA IAME E ( .0( . 8 In. .0 .0 .0 ( . 8 Irt .0 ( . 8 Irt E G 22 (4.8 In. 0. 22 8.8 . In. 22 (4.8 In. MA IMUM O CE OU U 6 80 b. A 6 6 (80 6 6 (80 lb. OWE A MA IMUM O CE W <60 60 00 20 S EA Y S A E OWE W < 0 0 0 0 WEIG O AC UA O K .2 26 b. 6 0.6 . 6 ( 6.28 lb. 244 . lb. t l W t . K
EMA KS S ll t, l ht t br t n f . M n t d W ht r fl t r d l nd El tr n ntr l t ht, l t d ff lt (0.0 t 0.04 In h th . x l b r n th l r r th n M d t p r n pt n r In p r bl In p f r l r b rr nt t h n tl In . M n t r l t n nt r n t r l n r t r bbl
Figure 2-1. Comparison of design approaches for magnetic bearings.
Examination of Figure 2-1 will illustrate that the radially active, permanent .0 EW, COM AC , E MA E MAG E IAS, magnet bias magnetic bearing system is: AC I E Y CO O E ESIG
* smaller in size The permanent magnet bias actively controlled bearing, as shown in * lighter weight Figure 3-1, offers significant advantages over the all electromagnetic * and, lower in power consumption than the other three design bearing. In this approach, permanent magnets are used to energize the approaches. working air gaps, and electromagnet coils are used for stabilization and control only. In this design, a hollow, circular ring of axially polarized, The all electromagnetic design (which is the most common design rare earth alloy permanent magnet material generates high density found in the literature) is somewhat larger and approximately twice the magnetic fields in circular gaps at each end of the modular bearing. weight of the radially active, permanent magnet bias design. The power consumption of the permanent magnet bias design was also lowest of the designs compared. This weight and power efficiency advantage results OUSI G because in the all electromagnetic bearing, the entire levitation magnetic field is provided by electrical power. The result is that the coils must be OSI IO SE SO r 0p large and designed for continuous duty, and therefore, the actuator must be larger and heavier. In addition, the power required to generate the high intensity levitation magnetic field requires a larger servo electronics power amplifier and larger electronics control system. This study did not compare the added electronics weight which would increase the advantage of the permanent magnet bias, actively controlled approach.
The radial repulsion design was significantly larger and much higher in weight. There are also producibility problems in manufacturing S A O (2 ACES the radially polarized, permanent magnet disks in the required thin sections E MA E of 7.0 to 10 mm thick. MAG E
The reluctance centering design is much larger and heavier than the O E IECE th r ppr h d t th l r x l l tr n t nd l rl n t S EE E suitable for high load, high stiffness applications. However, because of its O O A MA U E simplicity, requiring only one servo loop, it does have applicability to systems where moderate radial stiffness is acceptable.
Because of the analytically demonstrated superiority of the radially Figure 3-1. Novel, permanent magnet bias actively controlled active, permanent magnet bias design, a model was constructed to evaluate d n n pt. ( t nt p nd n the feasibility of this new approach. The permanent magnets produce an axially flowing magnetic flux which flows through the cylindrical pole pieces and into the laminated field magnet assemblies and across the two radial air gaps. These high density fields produce a radial force system that is metastable when the shaft (or rotor) is centered within the field magnet bores.
A multiplicity of electromagnetic coils in the fixed laminated field magnet assemblies are servo controlled to cancel the metastable "negative spring" effect of the permanent magnet field and to stabilize the bearing in the optimum, centered position. The electromagnet coils generate magnetic flux that flows circumferentially around the laminated field magnet stators, but not through the permanent magnets. Variations in external forces are neutralized by the actively controlled electromagnets producing a "positive
spring", or high stiffness restoring force system that keeps the shaft Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1990/79085/V005T14A012/2400068/v005t14a012-90-gt-240.pdf by guest on 29 September 2021 centered in the field magnet assemblies. This approach permits much r 4 . r n nt n t b n t b r n t t smaller electromagnet coils because they only provide control currents, not d l. the primary magnetic field currents and can operate at a low "duty cycle". All electromagnetic bearings generally operate the field magnet coils with a large, constant current to linearize the control law. This requires significantly larger coils and stator field magnet frame to enclose the coils than required for a permanent magnet bias bias design.
Position sensor pairs at each end of the bearing assembly measure the shaft position and the error from the desired optimum rotor position is fed into the four channel servo-control system. The servo control system stabilizes the shaft and maintains it in the optimum centered position.
One of the unique features of this permanent magnet bias approach is the axial flow magnetic field used in combination with the circumferential flow electromagnetic fields. This arrangement results in a very compact, light-weight, power efficient system. The magnetic efficiency is optimized because the electromagnetic coils do not have to provide electromotive force to drive flux through the permanent magnets which have a very low permeability (approximately 1.0); i.e., they are like high magnetic resistance "air gaps". Consequently, the efficiency of the electromagnets is maximized. r 4 2. C p n nt p rt f p r n nt n t b f b l t t t d l. The permanent magnet bias, actively controlled bearing was shown in this study to be smaller than the other types of designs. It is smaller than all electromagnetic bearings because the electromagnet coil window is smaller, and the active magnetic circuit laminated structure is smaller. In addition, the permanent magnet linearizes the control law of force versus input control current.
4.0 EASI I I Y MO E ES ESU S
A model of the permanent magnet bias radial bearing concept was built to evaluate the feasibility and performance capability of this new, novel approach. The test model detail design parameters are summarized in Table 4-1. PARAMETER VALUE Air gap diameter 31.1 mm (1.187 inch) Lamination length (2 places) 19.0 mm (0.750 inch) Air gap magnetic path area 36.0 cm2 (5.59 inches2) Magnetic bearing outer diameter 6.35 cm (2.50 inch) r 4 . b l t t t d l f n t b r n . Permanent magnet outer diameter 6.35 cm (2.50 inch) Permanent magnet inner diameter 4.57 cm (1.80 inch) The radial magnetic bearing consists of two field magnet stator Permanent magnet thickness 4.68 mm (0.187 inch) assemblies made of .040 thick low carbon steel laminations shown in Rotor weight 2.84 Kg mass (6.25 pounds) Figure 4-4. Hollow, circular cylindrical pole pieces of low carbon steel Coil resistance 2.4 ohm magnetically connect the field magnets to a circular, axially polarized Coil inductance 4.5 milli Henries neodymium-boron-iron permanent magnet. The shaft is made of low Force constant 44.4 N/amp (10.0 pounds/amp) carbon steel and has a small flywheel on one end to make the center of mass centered between the two radial bearing field magnets. This bl 4 . M n t b r n t t d l d n p r t r arrangement minimizes dynamic disturbances that can induce nutation and gyroscopic instabilities. Figure 4-1 illustrates the design of the feasibility demonstration model. The the component parts, prior to assembly, are shown in Figure 4-2. The A permanent magnet bias, double-acting, "solenoid-like" axial assembled bearing is shown in Figure 4-3. bearing was used to stabilize the system in the axial direction.. Eddy current inductive position sensors were used to detect shaft position in the X and Y axes and also the axial, or Z, axis. A hall effect commutated, brushless D.C. motor was provided to spin the rotor. The bearing and servo system was designed to operate at speeds up to 10,000 RPM. Duplex linear with control current. The linear saturation range of vanadium-iron- pair ball bearings were utilized at each end of the shaft to capture the rotor cobalt alloys is over 16,000 Gauss. The unit load capacity of a magnetic when the system is non-operational or in the event of inadvertent power bearing varies as the square of the air gap flux density, (equation 2-1) and loss. The magnetic bearing system is enclosed in an aluminum alloy therefore, a similar unit constructed of vanadium-iron-cobalt laminations housing structure. could have produced 102 N/cm2 (148po undlinch2) airap unit loading. (i.e. (16 KG/10 KG)2 x 40 = 102 N/cmz (148 pound/mchZ). 4. n t n l t lt The advantage of lower weight (1.21 Kg) for the permanent magnet The feasibility test model was tested for static performance bias actuator, versus 3.17 Kg, 3.76 Kg, and 2.44 Kg for the other designs characteristics and the results are summarized in Table 4-3. High speed studied is a significant factor in components such as spacecraft hardware, tests, up to 10,000 RPM are planned, but results were not available at the aircraft equipment and even ship-board equipment. In addition, the time of publication of this report. significant reduction in power consumption to less than 60 watts versus Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1990/79085/V005T14A012/2400068/v005t14a012-90-gt-240.pdf by guest on 29 September 2021 120 watts for the all electromagnetic design has a significant effect on the PARAMETER VALUE servo-control electronics. The lower power requirement allows the use of Load capacity 355.8 N (80 pounds) smaller power amplifiers and output transistors which are the primary Negative stiffness -4307 N/cm (2460 pound/inch) determinants of electronics size and weight. Consequently, the permanent (due to permanent magnet) magnet bias electronics system will also be significantly smaller and Radial stiffness 175,118 N/cm (25,000 pound/inch lighter weight than the competing all electromagnetic approach. (static stiffness zero RPM) Power consumption 40 Watts total This overview study has identified three basic approaches to (including servo electronics) magnetic bearings. A comparison of designs using these approaches has Speed designed for speeds up to shown that the use of permanent magnets for energizing the air gaps 10,000 RPM combined with actively controlled electromagnets yields the ll t. (tested only to 100 RPM as of l ht r ht. l t p n pt n design of the concepts publication date) evaluated.
The key technologies that have made possible practical magnetic bl 4 . lt f p rf r n t t . bearings are (1) high saturation flux density ferromagnetic materials, (2) miniaturized servo electronics components and systems, and (3) high The test results of the feasibility test model demonstrated the energy product permanent magnets. Of these emerging technologies, capability for producing air gap unit loading of over 40 N/cm2 (58 permanent magnet technology is showing the greatest improvements in pounds/inch2) using silicon steel laminations in the electromagnetic recent years. The energy product of commercially available rare earth circuit. i.e. alloy magnets has increased from 16 x 106 Gauss-Oersteds in 1970 to over 34 x 10 in 1989. If this trend continues, then the present advantages of the effective radial bearing area is from data of Table 2-1 permanent magnet bias magnetic bearings will be further increased.
A = l// x (air gap dia) x (length of lamination stack)
A = (3.0) x (1.90) = 4.04 cm2
and the magnetic pressure (F/A) is:
F/A = 162 N/4.04 cm2 = 40.2 N/cm2
The test model clearly demonstrated the feasibility of this new REFERENCES magnetic bearing concept and the performance data corroborates the advantages over other design approaches for high load, high stiffness applications. 1. Sindlinger, Rainer S., "Magnetic Bearing Momentum Wheels with Magnetic Gimballing Capability for 3-Axis Attitude control and .0 CO C USIO S A E A UA IO O ESU S Energy Storage," Paper presented at The 11th Aerospace Mech. Symp. NASA Goddard. The permanent magnet bias, actively controlled bearing was shown in this study to be smaller than the other designs approaches compared. It 2. Haberman, H., and G. Laird, "An Active Magnetic Bearing is smaller than the all electromagnetic bearing because the electromagnet System," Tribology, International, April, 1980. coil window is smaller, and consequently, the active magnetic circuit laminated structure can be made smaller. In addition, the permanent 3. Daniels, A., M. Gasser, A. Sherman, "Magnetically Suspended magnet linearizes the control law of force versus input control current. Stirling Cryogenic Space Refrigerator-- Status Report," North This simplifies the servo control because. it is linear system. American Philips Labs., Briarcliff Manor, NY, NASA Goddard Space Flight Center, Greenbelt, MD. The design comparison in section 2.2 above was based on using low carbon steel laminations with a magnetic saturation flux density of 10,000 4. Roters, "Electromagnetic Devices", John Wiley & Sons, New York, Gauss. A similar machine made of a high saturation flux alloy such as 1941. vanadium-iron-cobalt would be even smaller and lighter. The extrapolation of the test results to a unit constructed of this high efficiency alloy is 5. Backers, F.T., "A Magnetic Journal Bearing," Philips Technical relatively straight forward using the ratio of the square of the flux Review, Vol. 22, NO.7, 1961. densities. 6. Studer, P.A., "Magnetic Bearings for Instruments in the Space The magnetic saturation flux of the low carbon steel used in the test Environment," NASA Technical Memorandum 78048, NASA model is about 10,000 Gauss before the magnetizing force becomes non- Goddard, 1978. . St d r, .A., "A r t l M n t r n ," IEEE r n t n n M n t , l. MAG , . , S pt. . 8. b , .C., "S t ll t M nt nd t n Wh l W th M n t r n ," r d n f AOCS C nf r n h ld n rd j , O t. 6, (ESA S 28, v. . . b , .C., " v l p nt f S t ll t l h l Ut l z n M n t r n W th v d l C nt r n C n pt nd lt ," AIAA 8 , C n t n S t ll t S t C nf. th, S n , C l f., Apr l 24 2 , 8. Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1990/79085/V005T14A012/2400068/v005t14a012-90-gt-240.pdf by guest on 29 September 2021 0. M r , C. t. l., "A p f M n t G b ll d M nt Wh l nd It Appl t n t Att t d C ntr l n Sp ," p r pr nt d t rd C n r f Intl. A tr n t l d r t n, r , r n , S pt. 2 O t. 2, 82. . Kn rr h n, ., nd h. Lange, "M d l r n nd n t n A t v r n M nt Wh l ," I AC A t t C ntr l n Sp , rd j rh t, h th rl nd , 82. 2. Albr ht, ., nd W l t, .A., "A h r t l nd Exp r nt l Inv t t n f th M n t ld nd r Ar n n M n t S p n n S t ", M h n l hn l , In . p rt 2 , n, .