The invention, a single element magnetic suspension actuator with bidirectional force capability along a single axis, includes an electromagnet and a nonmagnetic suspended element. A permanent magnet mounted on the suspended element interacts with a magnetic field established by the electromagnet to produce bidirectional forces in response to a variable force command voltage V (sub FC) applied to the electromagnet. A sensor measures the position of the suspended element on the single axis which is a function of force command voltage V (sub FC)

Groom, Nelson J.

English

NASA Technical Reports Server

United States Patent [19] 
Groom 
13 
It BZ - 
[ill Patent Number: 5,003,235 
[45] Date of Patent: Mar. 26, 1991 
[54] SINGLE ELEMENT MAGNETIC 
SUSPENSION ACWATOR 
[75] Inventor: Nelson J. Groom, White Marsh, Va. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 405,154 
[22] Filed: Sep. 11, 1989 
[51] Int. ( 3 . 5  ............................................. H02K 41/00 
[52] U.S. Cl. .................................... 318/135; 310/90.5 
[58] Field of Search ................. 310/90.5, 12; 361/143, 
361/146; 318/135, 632, 633; 335/234, 256, 266 
[561 References Cited 
U.S. PATENT DOCUMENTS 
4,629,262 12/1986 Hamilton ........................... 310/90.5 
4,642,501 2/1987 Kral et al. .......................... 310/90.5 
4,726,640 2/1988 Iwama et al. ...................... 310/90.5 
4,751,487 6/1988 Green, Jr. ........................... 335/234 
Primary Examiner-Steven L. Stephan 
Assistant Examiner-Judson H. Jones 
Attorney, Agent, or Firm-Harold W. Adams; John R. 
Manning 
[571 ABSTRACT 
The invention, a single element magnetic suspension 
actuator with bidirectional force capability along a 
single axis, includes an electromagnet 11 and a nonmag- 
netic suspended element 12. A permanent magnet 13 
mounted on the suspended element 12 interacts with a 
magnetic field established by the electromagnet 11 to 
produce bidirectional forces in response to a variable 
force command voltage VFC applied to the electromag- 
net 11. A sensor 14 measures the position of the sus- 
pended element 12 on the single axis which is a function 
of force command voltage VFC. 
10 Claims, 5 Drawing Sheets 
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SINGLE ELEMENT MAGNETIC SUSPENSION 
ACTUATOR 
ORIGIN OF T H E  INVENTION 
The invention described herein was made by employ- 
ees of the United States Government and may be manu- 
factured and used by or for the Government for govern- 
mental purposes without the payment of any royalties 
thereon or therefor. 
BACKGROUND OF T H E  INVENTION 
1. Field of the Invention 
The invention is a single element magnetic suspension 
actuator and more particularly to a single element mag- 
netic suspension actuator with bidirectional force capa- 
bility along a single axis. Applications for the invention 
may include magnetic actuators for shape and figure 
control of antennas and precision segmented reflectors, 
and magnetic suspension for payload pointing, isolation, 
and control systems. 
2. Description of the Prior Art 
Typically, the force produced by a single magnetic 
suspension actuator element is an attractive force that is 
directly proportional to the square of the flux in the 
magnetic gap. The flux, under ideal assumptions, is 
directly proportional to the current in the actuator ele- 
ment coil and inversely proportional to the magnetic 
gap. In order to produce a bidirectional force, prior art 
has utilized two or more actuator element pairs acting 
together along a single axis. Many approaches for con- 
trolling the force produced by this type of actuator are 
known. For instance see U.S. Pat. Nos. 4,629,262 and 
4,642,501 to Brian J. Hamilton and Kevin 0. Kral, re- 
spectively. 
A disadvantage of prior art devices is in the need for 
two or more elements controlled either differentially or 
one at a time in opposition to produce a bidirectional 
force along a given axis. This results in weight and 
volume disadvantages and configuration constraints in 
most applications. 
OBJECT AND SUMMARY O F  T H E  INVENTION 
An object of the invention is to provide a single ele- 
ment magnetic suspension actuator with bidirectional 
force capability along a single axis. In general, the in- 
vention includes an electromagnet 11 and a nonmag- 
netic suspended element 12 having a permanent magnet 
13 mounted thereon for providing a magnetic field that 
interacts with a magnetic field established by a variable 
force command voltage VFc applied to the electromag- 
net 11. The resulting bidirectional forces are sensed by 
measuring the change in position of the suspended ele- 
ment. A current feedback loop compensates for lag in 
movement of the suspended element effected by the 
inductance of the electromagnet. 
BRIEF DESCRIPTION O F  THE DRAWINGS 
The above and numerous other objects and advan- 
tages of the invention will become apparent from the 
following description when read in view of the ap- 
vended drawings wherein: 
5 
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5 5  
2 
FIG. 3 is an electrical schematic of the preferred 
FIG. 4 is a schematic of an alternative embodiment 
FIG. 5 is an electrical schematic of the invention 
embodiment that includes current feedback; 
utilizing both current and flux feedback; and 
illustrated in FIG. 4. 
BRIEF DESCRIPTION O F  THE INVENTION 
Referring now to the drawings FIGS. 1-3 illustrate a 
preferred embodiment of the invention as generally 
comprising a circular electromagnet 11 having a longi- 
tudinal axis that represents a single axis Bz, a nonmag- 
netic suspended element 12, a permanent magnet ele- 
ment 13 mounted on the suspended element 12 and 
positioned on the single axis Bz  to provide a magnetic 
field for the electromagnet to interact with to produce 
bidirectional forces along the Bz  axis in response to a 
variable force command voltage VFC connected to a 
coil 15 of electromagnet 11 by a first circuit means that 
includes a first conventional operational amplifier 16. A 
sensor 14 measures the change in position of the element 
12 being controlled, the position being directly related 
to the force command voltage VFC. 
In order to describe the operation of the single ele- 
ment actuator, the simplified schematic of FIG. 2 show- 
ing a single turn coil C with radius r and a permanent 
magnet element PM located a distance z from the center 
of the coil C on the centerline through the coil C is 
illustrated. The flux density along the centerline Bz, 
generated by the coil C in FIG. 2 can be written as: 
B ~ =  (pd/2)(+~(+ + r 2 ) 3 / 2 )  (1)  
B s t  the center of the coil C, defined as Bzo, becomes 
Bzo=poi/Zr (2) 
Equation (1) can be written in terms of the ratio z/r by 
factoring r out of the denominator 
Bz= (pd/2)(;/41 + ( ~ / r ) ~ ] ~ / ~ )  (3) 
From (2), (3) can be written as 
B z = B z d ( i +  [ ~ / r ] ~ ) ~ / ~  
The force on the element, along the z - axis, can be 
written as 
Fz= (Vol)MzaBz/az ( 5 )  
where M z  is the magnetization of the permanent mag- 
net element PM along the z - axis and Vol is the volume 
of the element. Taking aBz/az results in 
aBt/az= (- 3Bzo/r)([z/r]/li+ ( ~ / r ) ~ ] ~ / * )  (6) 
and the force becomes 
60 
FAi,  z)= (Vol)MA- 3pd/29)([z/r]fii+ ( ~ / r ) ~ ] ~ / ~ )  17) 
Linearizing F d i ,  z) about a nominal operating point io, 
zo  results in 
FIG. 1 is a schematic of a preferred embodiment of 65 
the invention utilizing current feedback; 
FIG. 2 illustrates a single turn electrical coil C repre- 
sentative of the principle of operation of the invention; 
&Fz= K$i+ K 8 . z  
where 
5,003,235 
3 4 
mentation of the single element actuator, as disclosed, 
uses current feedback to control the flux produced by 
the coil. 
K/  = aFAI, z)/ar 1 , Kz = aFAr. z)/azl (9) 
1,220 IGlO 
An alternative embodiment using flux feedback in 
5 combination with current feedback is shown in FIGS. 4 Taking the partial derivatives in (9) results in 
and 5. As shown in FIG. 4 a flux feedback loop means 
z)/a i= (VOI)MA - 3pd2rZ)( [z / r ] / [  I + (z/r)2]5/2) (10) 30 includes a conventional Hall Effect sensor 31 
mounted on electromagnet 11 and arranged to sense the 
flux produced by the electro- magnet. As shown in 
lo FIG. 5 the output 32 of the sensor 31 which may be an 
integrated circuit combining am amplifier and Hall 
Effect device is summed with the force command volt- 
age VFCin a second operational amplifier 33 the output 
34 of which is summed with the feedback voltage VFB 
The alternative embodiment may be used to compen- 
sate for nonlinearities in a single element actuator using 
a coil with a magnetic core 40 rather than an air core 
2o which is linear and requires no flux feedback compensa- 
tlon. 
in the electromagnet 11 
However, when a magnetic core 40 is used Equation 1 
no longer holds as hysterisis and other nonlinearities are 
introduced by the magnetic core. Using flux feedback 
dFz(i, 
and 
aFz(i, 
z)/az= (VOOMA - 3poi/2&[i- 4 (~ /r )~ i -  
Ai+ (z/r)2I7/2) (11) 
Under the assumption that b=o, the linearized force 
at the nominal operating point zo becomes l 5  from the sense resistor Rs  in operational amplifier 16. 
SFz= ([ - 3(vol)Mzpd2r2][(zo/r)/(i+ [~o /r ]~ )~ /* ] -  
( I 2 )  
An implementation of the actuator using a current 
feedback loop means 17 is shown in FIG. 3. This circuit 
current feedback configuration. The first operational 
amplifier 16 input sums the input force command volt- 
age VFC with a voltage feedback VFB generated by 25 
current flowing through a resistor Rs in series 
)Si 
The use of a magnetic components and is set in a increases the maximum flux possible for a given current, 
with the coil 15 of electromagnet 11. The input resistor 
18 and feedback resistors 19 and 21 are selected to pro- 
loop 30 insures that the current through the Of 
to l1 is -adjusted to the proper 
vide a current gain of 1 Amp/volt in the current feed- produce a commanded flux density. Current feedback 
back loop 15. A conventional buffer amplifier 22 pro- 30 loop 17 provides accurate control of the coil current 
vides the necessary drive current for output stage 23 within the flux feedback loop 30. 
which is a complementary configuration consisting of While preferred embodiments have been described in 
power transistors 24 and 26 and power transistors 27 detail, numerous changes in modification can be made 
and 28. within the principle of the invention which is to be 
more accurate control of current in the electromagnet What is claimed is: 
11 which is important since the force produced by the 1. A single element magnetic suspension actuator for 
single element actuator at a nominal operating point is producing bidirectional forces along a single axis com- 
directly proportional to current (see equation 12). The prising: 
current feedback Provided by the loop 15 also reduces 40 an electromagnet having a longitudinal axis that rep- 
the effect of lag in movement of the suspended element resents said single axis; 
12 introduced by the coil inductance. a detached single non-magnetic suspended element 
A typical application for a single element actuator in spaced from said electromagnet; 
accordance with the invention may be a magnetic sus- a magnet mounted on said single non-mag- 
pension and pointing platform with six degree of free- 45 netic suspended element and positioned on said 
single axis; dom (DOF) control such as the Annular Suspension 
mand voltage VFC to said electromagnet to pro- figuration uses six magnetic suspension actuators to 
duce opposing directional forces on said single achieve six D O F  control. Currently these six actuators are conventional actuators which requires two electro- 50 non-magnetic suspended element; and magnet elements per actuator to produce bidirectional means for sensing the resulting change in position of force. said single non-magnetic suspended element as a Substituting a single element actuator in accordance 
function of said force command voltage VFC. with the invention for the prior art design eliminates the 
need for a total of six electromagnet elements, thus 5 5  2. The invention as defined in claim 1 wherein said 
resulting in a significant reduction in power and weight 
requirements, A similar single actuator arrangement 3. The invention as defined in claim 2 wherein said 
may be used to control a precision reflector segment first circuit means includes a first amplifier means and a 
which is part of a large segmented reflector. sense resistor means series connected to said electrical 
coil in combination with a Source of magnetic flux on 4. The invention as defined in claim 2 wherein said 
the suspended element, such as permanent magnet ma- electromagnet further includes a magnetic core sur- 
terial, to provide a bidirectional force along a single rmnded by said electrical coil. 
axis. Alternate embodiments of the invention could 5. The invention as defined in claim 2 including a 
include multiple elements interacting with the same flux 65 current feedback loop connected around said amplifier 
source attached to the suspended element, for example 
a single permanent magnet core of a given shape, to 6. The invention as defined in claim 5 wherein said 
produce multiaxis forces and torques. Also, the imple- 
The use of the current feedback loop 17 provides 35 limited only by the scope of the impending claims. 
and Pointing System (Asps)‘ The current ASPS ‘On- first circuit means for applying a variable force corn- 
an ‘Oil. 
Thus, the invention, uses a single actuator element or 60 Coil of said electromagnet. 
means. 
first amplifier means is a first operational amplifier. 
5,003,235 
5 6 
7. The invention as defined in claim 4 including flux 
feedback loop means for compensating for non-lineari- 
ties in flux density introduced by said magnetic core. 
8. The invention as defined in claim 7 including a Hall 
for sensing magnetic flux produced 
by said electromagnet and providing an output repre- 
sentative thereof. 
9. The invention as defined in claim 8 including a 
second operational amplifier for summing said output of 
said Hall Effect sensor means and said force command 
voltage vFc. 
10. The invention as defined in claim 9 wherein said 
second operational amplifier is series connected with 
said first operational amplifier. 
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Single element magnetic suspension actuator

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