Electromagnetic braking arrangement for controlling rotor rotation in electric moto

Perkins, G. S.

English

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AUTICS 
FIG. I 
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FIG. 3 
FIG. 2 
BY 
The windings are analogous to field windings except for 
being short circuited. When the motor is energized and 
its rotor rotates, the permanent magnet turns in unison 
therewith, inducing maximum current in the windings 
which generate a counter electromagnetic force (EMF). 
This force in essence produces a torque on the rotor which 
is counter to  that developed by the motor torque. How- 
ever, it is very small compared to the motor torque, so 
that its effect on the rotation of the rotor can be ne- 
glected. However, once the power to the motor is turned 
ABSTRACT OF THE DISCLOSURE off the counter torque produced by the counter EMF be- 
net defining opposite poles supported on a shaft, coexten- finally stopping it when the magnetic poles of the perma- 
sive with a motor's shaft. A pair of pole pieces are radially nent magnet are aligned with the two stator-like poles, 
positioned with respect to the permanent magnet and form 15 which together with the Yoke Provide a return Path for 
a magnetic yoke therefor. When the motor is energized, *e flux lines Of the Permanent magnet. 
a counter EMF is induced by currents in short circuited The novel features that are considered characteristic 
windings supported on the pole pieces. This force tends of this invention are set forth with particularity in the 
to  slow the shafts' rotation once the motor is &-energized. appended claims. The invention itself, both as to its 
The shafts firlally come to rest when the poles of the 20 organization and method of operation, as well as addi- 
permanent magnet are aligned with the pole pieces. tional objects and advantages thereof, will best be under- 
stood from the following description when read in con- 
nection with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIGURE 1 is a diagram of the electromagnetic arrange- 
ment of the present invention with a part & the housing 
removed; 
FIGURE 2 is a cross sectional view along line 2-2 of 
FIGURE 3 is a cross sectional view of another embodi- 
3,487,281 
IDETENTING SERVOMOTOR 
James E, Webb, Administrator of the Aero- 
nautics and space Admi&&ation with respect to an 
invention by Gerald S. Perkins, Altadena, Calif. 
Filed May 17, 1967, Ser. No. 640,786 
Int. C1. H02p 3/02, 3/06 
W.S. C1. 318-382 4 Claims 
A braking arrangement comprising a permanent mag- comes significant, tending tQ Slow down the rotor and 
ORIGIN OF THE INVENTION 
formance of work under a NASA contract and is subject 
to the provisions of Section 305 of the National Aero- 
nautics and Space Act of 1958, Public Law 85-568 (72 
Stat. 435; 42 USC 4257). 
BACKGROUND OF THE INVENTION 
Field of the invention 
The invention described herein was made in the per- 25 
3o FIGURE 1; and 
ment of the invention. 
DESCRIPTION OF THE PREFERRED 
This invention relates to electromagnetic circuitry and, EMBODIMENTS 
more particularly, to an electromagnetic braking arrange- 35 
merit for controlling the rotation of a rotor in an electric Attention is now directed to the figures, in which like 
motor. elements are designated by like numerals. In FIGURE 1, the electromagnetic braking device of the present inven- 
Description of the prior art tion is designated by numeral 10 and shown comprising 
In the prior art, whenever the rotor or an electric motor 40 Of a housing 1% Which supports a shaft 14, having One 
is to be prevented from coasting after the power to the end 146 extending through the housing. This end is 
motor is turned off, braking or electromechanical clutch adapted to be Connected to a shaft Or rotor of any motor 
arrangements are generally employed. This is particularly whose shaft's rotation to be controlled to minimize 
of the rotor of a servo motor is very important. The use 45 coup1ed to a shaft motor 20, bY means of 
of prior art electromechanical braking or clutch arrange- a coupling member 22, SO that shaft 14 rotates in unison 
ments, though often a necessity, is quite undesirable, both with shaft 16. For explanatory Purposes, motor 21) is 
from increased cost and performance points of view. Such assumed to include a Pair of leads 23 and 24 connectable 
arrangements produce undesirable surface friction, and to a Power Source (not shown) which supplies energizing 
break away torque. Also, they generally include lubricants 50 power to the motor. 
which are temperature sensitive, a characteristic which Within housing 12, a cylindrical permanent magnet 25 
is most undesirable in spacecraft applications. is supported on the shaft 14, so that as shaft 14 rotates 
the magnet 25 rotates therewith. Aligned with the magnet 
OBJECTS AND SUMMARY O F  THE INVENTION 25 are magnetic pole-pieces 27 and 28 which are mag- 
55 netically connected by a cylindrical yoke 29, supported 
It is, therefore, a primary object of the present invention within the housing. 
to provide a new arrangement for preventing the coasting Windings 31 and 32 are shown wound about the pole- 
of a motor rotor after power turn off. pieces 27 and 28, respectively. In a preferred embodi- 
Another object is to provide a novel electromagnetic ment, the windings are closed loops or short circuited. As 
braking arrangement to minimize the coasting of a rotor eo will be explained hereafter, the function of the windings 
of a servo motor and thereby control the final rest position is to generate a counter electromagnetic force which is 
thereof. produced as a result of the currents induced to flow 
A further object of the invention is to  provide an elec- therein, as the permanent magnet 25 rotates, thereby 
tromagnetic braking arrangement for use with a simple causing the windings to  cut the flux lines of the magnet's 
servo motor for accurate positioning of the rotor thereof e5 magnetic field. 
without a complex closed loop correction system. In practice, when power is applied to motor 20 and 
These and other objects of the invention are achieved shaft 16 thereof rotates at a selected speed of rotation, 
by providing an electromagnetic braking arrangement shaft 14 coupled thereto and the magnet 25 rotate at the 
which includes a permanent magnet, positioned on a shaft same speed. Thus, a rotating magnetic field is produced 
which is coextensive with the motor's rotor. At least two in the housing 12. Since short circuit windings 31 and 32 
stator-like poles are positioned near the outer surface of 'O are located in this rotating field, currents are induced 
the magnet. The poles are interconnected by a yoke. A therein, which in turn generate a counter or back electro- 
short-circuited winding is wound about each of the poles. magnetic force (EMF) in a manner well known in the 
true in Sen0 systems in which the accurate positioning coasting. In FIGURE 1, the end 14e Of shaft 14 is shown 
Of a 
3,487,281 
art of electrical motors. Sin the windings 31 and 32 are 
perferably short circuited maximum currents flow therein, 
and a maximum counter EMF is generated. This force 
(counter EMF) produces a torque on the shaft 14 which 
is counter to that proudced by the motor 20. However, 
the counter torque produced by the counter EMF and is 
only a small fraction of the total motor torque, so that 
it has substantially no effect on the shaft’s speed of rota- 
tion, as long as power is applied to the motor. 
Once, however, the power is cut off, and the shafts con- 
tinue to rotate or coast, the magnitude of the counter 
torque is sufficient to reduce the free coasting of shaft 14 
and therefore the coasting of shaft 16. 
As a result, after several revolutions the shafts come 
to a practical standstill. As the coasting is reduced and 
the shafts are nearly at rest, the magnetic field produced 
by the permanent magnet tends t o  rotate the shaft until 
the magnetic poles of the magnet 25, designated in FIG- 
URE 2 by N for North and S for South, are aligned 
with the two pole-pieces 27 and 2S, so that the pole pieces 
together with the yoke 28 provide a magnetic path of 
least resistance for the magnetic flux lines between the 
N and S poles of the magnet. Once so aligned, the shafts 
6 remain in a substantially locked position until 
power is again applied to motor 20. 
In FIGURE 2, the N and S poles of magnet 25 are 
shown aligned with pole-pieces 27 and 28, however it 
should be appreciated that with only two pole-pieces, the 
shaft 14 may be locked in a position when the N and S 
poles are aligned with poles 28 and 27 respectively. Thus, 
in the arrangement diagrammed in FIGURE 2, the shaft 
14 may be locked in either of two positions. Also in the 
foregoing description, the pole-pieces 27 and 28 are as- 
sumed to be connected by yoke 28 to form a return path 
For the flux lines of magnet 25. However, it should be 
appreciated that the three parts (27, 28, 29) may be 
formed of a single member such as a multipole stator, 
well known in the art of electrical motors. 
It should further be pointed out that although in the 
foregoing description the poles (N and S )  of magnet 25 
are on diametrically opposite ends thereof, and therefore 
pole-pieces 27 and 28 are aligned along a diameter of 
housing 42 on opposite sides of the axis of rotation of 
shaft 14, the invention is not limited thereto. Rather, 
when using a permanent magnet 25 with a pair of mag- 
netic poles, whatever the shape of the magnet, the pole- 
pieces 27 and 28 have to be disposed so that when the 
shaft 14 comes to rest, one pole of magnet 25 is aligned 
with one pole-piece and the other with the other pole- 
piece. Also, more than one pair of pole-pieces may be 
included, so that the magnet 25 may align itself with any 
one of the pairs of pole-pieces, thereby increasing the 
number of positions in which the shaft may be magneti- 
cally locked. 
FIGURE 3 to  which reference is made herein is a cross- 
sectional view of an arrangement similar to that shown in 
FIGURE 2, except for two additional pole-pieces 27x 
and 28x having windings 31x and 32x wound thereon. 
From the foregoing, it should be appreciated that the 
4 may be magnetically locked when pole N is 
aligned with any one of the four pole-pieces and the pole 
S is aligned with the opposite pole-piece. 
In reducing the invention to practice, it has been found 
that a shaft of a servo, which in the absence of braking 
arrangement tends to coast about 500 revolutions before 
stopping after power cutoff, can be stopped with the novel 
arrangement disclosed herein within 3 turns, thus greatly 
increasing the accuracy with which a device (not shown) 
coupled to shaft 16 of motor 20 may be positioned. For 
example, assume that a load is coupled to the shaft 16 
by a gear reduction assembly having a 10,000/1 ratio. 
Thus, the ambiguity in the stopping position of the load 
is 1/10,000 that of the stopped position of the motor 
shaft. Since the motor has a 3 turn ambiguity, the load 
would have a .0003 turn ambiguity or 0.1 degree posi- 
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tioning accuracy, making the motor well suited for many 
remote position control functions, without complex seryo 
closed loops or prior art complex clutches or braking 
devices. 
From the foregoing, it should be appreciated that in 
accordance with the teaching of the invention, an electro- 
magnetic braking arrangement is provided to  minimize 
the free coasting of a motor shaft and magnetically lock 
the position thereof after power cut off. The reduction in 
coasting is provided by producing a counter EMF which 
does not affect the motor’s torque when power is applied 
to the motor, but which provides a counter torque to  the 
motor’s shaft after power cut off, sufficient to practically 
stop the shaft after a minimum number of coasting turns. 
A permanent magnet is used to bring the shaft to a com- 
plete stop and magnetically lock the shaft in any one of a 
plurality of defined positions. 
It is appreciated that those familiar with the art may 
make modifications in the arrangements as shown without 
departing from the true spirit of the invention. Therefore, 
all such (modifications and/or equivalents are deemed to 
fall within the scope of the invention as claimed in the 
appended claims. 
What is claimed is: 
1. An electromagnetic braking device for reducing the 
coasting of a rotor of a motor after the energizing power 
applied to  said motor is cut off, the device comprising: 
a shaft adapted to be coupled to the rotor of a motor 
so as to rotate in unison therewith; 
cylindrical means defining first and second opposite 
magnetic poles supported on said shaft to  rotate 
therewith; 
at least two stator-like magnetic pole-pieces, positioned 
adjacent said cylindrical means with only air gaps 
defined therebetween; 
a magnetizable yoke coupling said magnetic pole-pieces; 
and 
a short-circuited winding wound about each magnetic 
pole-piece, whereby currents induced in said windings 
when said cylindrical means rotate on said shaft gen- 
erate electromagnetic forces tending to inhibit the ro- 
tation of said shaft, with said shaft stopping when 
said two magnetic pole-pieces are aligned with said 
first and second magnetic poles to provide together 
with said magnetizable yoke a path for magnetic flux 
between said magnetic poles. 
2. The electromagnetic braking device as recited in 
claim 1 wherein said cylindrical means comprise a perma- 
nent magnet defining first and second magnetic pores 
which are at diametrically opposite ends of said perma- 
nent magtlet and said two stator-like magnetic pole- 
pieces comprising first and second pole-pieces are on op- 
posite sides of the axis of rotation of said shaft, whereby 
said shaft stops either when said first magnetic pole is 
aligned with said first pole-piece and said second mag- 
netic pole is aligned with said second pole-piece, or when 
said first and second magnetic poles are aligned with said 
second and first pole-pieces, respectively. 
3. An electromagnetic braking device for limiting the 
coasting of a rotor of a motor after the removal of the 
application of power thereto comprising: 
a shaft adapted to be coupled to the rotor of said motor 
to rotate in unison therewith; 
a permanent magnet supported on said shaft, said mag- 
net defining first and second opposite magnetic poles; 
at least first and second magnetic pole-pieces positioned 
on opposite sides with respect to the axis of rota- 
tion of said shaft said pieces being coupled by a mag- 
netic yoke; and 
a short circuited winding wound about each of said 
magnetic pole-pieces, each having a current induced 
therein as a function of the rotation of said perma- 
nent magnet, whereby said currents generate a 
counter electromagnetic force to inhibit the rotation 
of said permanent magnet and the shaft on which it 
is supported. 
5,487,281 
4. The electromagnetic braking device as recited in 2,415,022 1/1947 Morrison _y_l___i___ 318-184 
claim 3 further including third and fourth magnetic pole- 2,704,343 3/1955 Streubfr _ _ _ _ _ _ _ _ _ _ _ _  3 1 8 4 5  
pieces, said pole-pieces equidistantly positioned about said 2,805,373 9,1957 Bonnell _ _ _ _ _ _ _ _ _ _ _ _  310-191 
permanent magnet, whereby said shaft is magnetically 
locked in any one of four positions when the first magnetic 3~0463463 711962 JohnSon ------------ 318-184 
pole is aligned with one of said pole-pieces and the sec- 
ond magnetic pole is alined with an opposite pole-piece. 
References Cited 
ORIS mER, primary Examiner 
IC. L. CROSSON, Assistant Examiner 
UNITED STATES' PATENTS 10 U.S. C1. X.R. 
2,796,572 6/1957 Tallant _________c-.. 318-382 310-1563 318-184 
2,971,105 2/1961 Jaeschke ______T__c___ 31&93 


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