This invention relates to brushless DC motors having position-sensing
Electrical servo systems are widely utilized for many different applications
in which position control is necessary; typically, they are used to drive robots
and X-Y tables, to position mirrors for laser applications and gun turrets, in
integrated circuit production equipment, and the like. Primary criteria for such
systems are of course high precision and responsiveness to input commands, as well
as ecohomy and facility of manufacture, reliability and durability.
Stepper motors have been used for such motion control applications,
and are advantageous from the standpoint of providing high stiffness and positioning
capability, coupled with comparatively low cost and relative simplicity. However,
they do not generally offer optimal dynamic characteristics and, because of the
absence of feedback capability, elements of uncertainty are inherent.
Closed loop DC servo motor systems provide considerable dynamic performance
benefits, including speed range, acceleration, torque-to-inertia ratios, and frequency
response; however, they tend to be deficient in static performance characteristics.
Moreover, the feedback devices employed in such servo motor systems for the extraction
and utilization of information for commutation, and for speed and position control
(e.g., shaft angle position detectors, tachometers encoders and resolvers), are
expensive, tend to be fragile in some instances, and may give rise to unreliability.
In general, systems of this kind are relatively complex and troublesome to install
United States Patent No. 4,687,961, which issued on August 18, 1987
to Ralph W. Horber, provides a motion control system, and a brushless DC motor
for use therein, which are relatively incomplex and inexpensive to manufacture,
which afford a highly desirable balance of accuracy of rotor-position information,
speed and torque characteristics, and which are highly effective, efficient and
reliable in and for their intended purposes. The motor operates in a closed-loop
mode without need for any added feedback device.
It is of course desirable to increase accuracy and improve other
operating characteristics in any such apparatus. Accordingly, it is the broad object
of the present invention to provide an improved brushless DC motor of the kind
described and claimed in the above-identified Horber patent, in which the accuracy
of the feedback signal is increased substantially.
Additional objects of the invention are to provide a motor having
the foregoing features and advantages, which is highly responsive to commands,
which enables extraction of exact rotor position information at standstill and
has a very large number of angular resolution points to provide extremely precise
position information, and which generates increased torque and has very smooth
running, and full power starting, torque characteristics.
The prior art shows a wide variety of systems and motors, some of
which may be employed for motion control applications and may have certain of the
features hereinabove discussed, as indicated by the following United States patents:
Polakowski No. 3,453,512 provides a brushless DC motor which employs
silicon controlled rectifiers in the armature switching circuits, the turn-on signal
being generated by an angular position detector and the turn-off signal being
generated by a capacitor. The position detector may consist of a series of stationary
coils which are sequentially inductively coupled, by a member mounted to rotate
with the field structure, with a common coil.
In Veillette No. 3,501,664, a system is disclosed for regulating
a DC motor having an internal stator field that is rotated in space 90&sup0; ahead
of the rotor field. The stator has teeth on its inner periphery arranged in non-diametrically
oriented pairs, which carry secondary windings through which current, applied
through a primary winding on the main body of the stator, is transferred sequentially
as the rotor poles align with the teeth during rotation.
The DC motor taught by Lahde No. 3,541,407 utilizes two-terminal
field coils as both a pickup, to sense the position of the rotor, and also as a
power coil to provide driving torque.
Kobayashi et al No. 3,590,353 shows an electronically commutated
motor having an outside and an inside rotor positioned on a common shaft, the inside
rotor serving a position detecting function. Primary and secondary windings on
an internal detect- ing stator are variable electromagnetically coupled, depending
upon the position of the rotor.
A phonograph turntable, driven directly by an electrically controlled,
variable speed brushless DC motor, is shown in Kobayaski et al No. 3,683,248. Winding
pairs within the stator are selectively coupled, depending upon the position of
an internal position detector rotor, to control current flow through particular
outer stator windings to drive an external rotor.
In Coupin et al No. 3,794,895, an electronically commutated DC motor
is described in which the stator is wound with pairs of power and detector coils,
the latter providing a speed-dependent signal which is dephased by 90&sup0; for
control of the power amplifier.
A self-exciting DC motor, having means for preventing rotation in
one direction, is disclosed in Kanamori No. 3,903,463. The stator poles are wound
with both field and armature coils, and the position detecting elements include
cores that are magnetically saturated by a permanent magnet and high frequency
Machida No. 3,997,823 teaches a circuit for a brushless DC motor,
in which the stator employs star-connected fixed windings. Means is provided for
detecting position signals induced in the fixed windings by rotation of the rotor,
which signals are employed to control switching means for supplying driving current
to at least one of the windings.
In Gosling et al No. 4,096,420, an oscillator with an LC resonance
circuit is employed in the control circuit for a brushless DC motor, oscillation
of the oscillator being modulated in response to induction caused in a sensing
coil by the rotor field.
Wright No. 4,162,435 discloses a circuit for a brushless DC motor,
wherein the voltage induced across one unenergized winding is sampled, integrated
and compared to a predetermined, position-indicating voltage to derive a control
signal while at least one other winding is energized, for selective commutation.
A commutatorless DC motor drive system is provided by Gelenius No.
4,262,237, in which a permanent magnet rotor induces AC potential waveforms in
phase displaced stator phase windings. Means is provided for initiating rotor rotation
from standstill, to initially induce the potential waveforms in the stator phase
windings, means is provided for producing a switch point reference signal, and
means responsive to the induced waveforms is provided for sustaining rotor rotation
by sequentially completing and interrupting individual stator phase winding energizing
circuits, in controlled relation to the reference signal.
Dittman et al No. 4,297,622 discloses a two phase gyrosystem which
employs two series-connected, motion-sensing reference coils, located 180&sup0;
apart, for motor drive and control.
A brushless DC motor is disclosed in Muller No. 4,481,440 which utilizes
a permanent magnet rotor in which the poles, viewed in the direction of rotation,
have approximately rectangular or trapezoidal magnetisation curves. The harmonic
fields included in such poles induce voltages in a sensor winding of the stator
which corresponds to the harmonic wave for which the winding is dimensioned.
A control device for a brushless DC motor is taught in Tokizaki et
al US-A-4 495 450. It has a rotor position detecting circuit in which voltages
induced in stator coils by rotation of the rotor are compared to neutral voltage
at a virtual neutral point to detect polarity changing points. Based thereupon,
an invertor controls the conducting modes of the stator coils to control rotation
of the motor.
The DC motor of Rhee, US-A-4 551 658, includes brushes and a commutator.
After starting, the brushes are centrifugally displaced from the commutator to
break the starting circuit.
In June of 1985, an article entitled "Multiple-Pole Stepping Motor"
was published. It described a hybrid stepping motor wound for two-phase operation,
wherein every two adjacent poles of each group are connected in series, but for
According to the present invention there is provided in an electric
motor, such as a polyphase direct current electric motor or the like, the combination
comprising: a stator of magnetic material comprising a body portion and a multiplicity
of pole elements extending outwardly from said body portion at spaced locations,
said stator being constructed to cooperate with an armature for said motor; a
sensor coil wound about each of said pole elements; a phase coil wound about each
of said pole elements; sensor means operatively connected to all of said sensor
coils for receiving an electrical signal therefrom indicative of the position
of an associated armature; and power supply means operatively connected to all
of said phase coils for generating forces for driving such an associated armature.
The position of the armature is preferably determined relative to
the pole elements.
Preferably the number of pole elements is no smaller than eight,
and usually divisible by either four or six.
In one embodiment of the invention the stator comprises a multiplicity
of at least eight identical pole elements, said sensor coils being arranged as
equal-number sets of at least four series-connected coils, with said coils of
a first set being wound on a first set of associated pole elements and said coils
of a second set being wound on a second set of associated pole elements, said
pole elements of said first and second sets alternating in positions along said
body portion of said stator, each of said sets of coils and associated pole elements
being sub-divided into equal-number subsets of at least two stator coils and pole
elements, said pole elements of one of said subsets of each set alternating, in
positions along said body portion of said stator, with said pole elements of the
other subset of the same set; the sensor means having at least two electrical circuit
legs, one of said circuit legs being connected to said first set of coils, at
a first junction between said subsets thereof, and the other of said two circuit
legs being connected to said second set of coils, at a second junction between
said subsets thereof, said sensor means-connected coils adapting said associated
pole elements to function as sensor pole elements, providing at least two sensor
channels for the generation of signals indicative of the position of an armature
assembled with said stator, said signals constituting averaged values of electrical
effects produced simultaneously upon a plurality of pole elements; said power
supply means being connected to said phase coils for providing at least two phases
of current, said supply means having one circuit portion connecting a first group
of said phase coils wound about a first group of pole elements in series for energisation
by one phase of current, and having another circuit portion connecting said second
group of said phase coils wound about a second group of said pole elements in
series for energisation by a second phase of current, said one and another circuit
portions of said supply means, so connected, adapting said pole elements to function
as first and second phase force-generating pole elements, respectively, the total
number of pole elements on said body portion divided by the number of phases provided
by said power supply means being an even number.
The armature may comprise a rotor having a generally cylindrical
body with an array of pole elements disposed circumferentially thereon, said stator
being constructed to so cooperate with said rotor.
In another embodiment of the invention the stator comprises a generally
cylindrical body portion and a multiplicity of at least eight identical pole elements
extending radially from said body portion at locations spaced thereabout, said
sensor coils being arranged as equal-number sets of at least four series-connected
coils, with said coils of a first set being wound on a first set of associated
pole elements and said coils of a second set being wound on a second set of associated
pole elements, said pole elements of said first and second sets alternating in
positions about said body portion of said stator, each of said sets of stator coils
and associated pole elements being sub-divided into equal-number subsets of at
least two coils and pole elements; the sensor means having at least two electrical
circuit legs, one of said circuit legs being connected to said first set of coils,
at a first junction between said subsets thereof, and the other of said two circuit
legs being connected to said second set of coils, at a second junction between
said subsets thereof, said sensor means-connected coils adapting said associated
pole elements to function as sensor pole elements, providing at least two sensor
channels for the generation of signals indicative of the angular orientation of
a rotor assembled with said stator, said signals constituting averaged values of
electrical effects produced simultaneously upon a plurality of pole elements; said
power supply means being connected to said phase coils for providing at least
two phases of current, said supply means having one circuit portion connecting
a first group of said phase coils wound about a first group of said pole elements
in series for energisation by one phase of current, and having another circuit
portion connecting said second group of said phase coils wound about a second group
of said pole elements in series for energisation by a second phase of current,
said one and another circuit portions of said supply means, so connected, adapting
said pole elements to function as first and second phase torque pole elements,
respectively, the total number of said pole elements on said body portion divided
by the number of phases provided by said power supply means being an even number.
Preferably, all of said sensor coils of said first set are wound
in the same direction, and all of said sensor coils of said second set are wound
in the opposite direction. Each of said sensor coils may be connected directly
to a sensor coil other than the one that is mechanically most proximate to it,
and said pole elements of one of said subsets of each set may alternate, in positions
about said body portion of said stator, with said pole elements of the other subset
of the same set.
The phase coils may be arranged as pairs, with the members of each
pair wound on pole elements that are disposed directly adjacent one another, said
members of each pair of phase coils being wound in opposite directions to magnetically
couple said directly adjacent pole elements when said phase coils thereon are energised.
One of said directly adjacent pole elements may constitute a first-set pole element,
and the other of said directly adjacent pole elements may constitute a second-set
pole element. The coupled pole elements of the first group may alternate, about
said body portion of said stator, with coupled pole elements of said second group.
The pole elements may be equidistantly spaced from one another on
said stator body portion, for example when there are 24 such pole elements they
will be spaced by 15° mechanical.
The pole elements may extend inwardly from said stator body portion
to define a space for a rotor therewithin. The motor may include a rotor comprising
a cylindrical body with an array of pole elements circumferentially disposed thereon.
The rotor body may comprise a core having said pole elements of said rotor disposed
on the outer surface thereof. The power supply means may provide two phases of
current, and said rotor pole elements may be permanent magnets the polarities
of which alternate in said array, the ratio of the number of said stator pole elements
to said rotor pole elements being either 4:3 or 4:5. In an optimal, two-phase
motor configuration, the stator will have 24 of said pole elements and said rotor
will have 18 of said magnet elements thereon. However, when the power supply means
provides three phases of current, and said rotor pole elements are either permanent
magnets or steel poles, the ratio of the number of said stator pole elements to
said rotor pole elements will desirably be 3:2.
The sensor means may have two circuit legs, with the power supply
means providing two phases of current.
For maximum signal quality, the sensor coils may be disposed adjacent
the innermost ends of said pole elements, preferably as close as possible to the
rotor magnets. The phase coils may be disposed on said pole elements so as to
be outwardly of said sensor coils relative to said body portion; they may be wound
to overlap, or indeed to fully overlie, the associated sensor coils.
For a better understanding of the present invention and to show more
clearly how it may be carried into effect reference will now be made, by way of
example, to the accompanying drawings in which:
- Figure 1 is a perspective view of a motor embodying the present invention;
- Figures 2, 3 and 4 are front, rear and side-elevational views of the motor;
- Figure 5 is an exploded perspective view thereof;
- Figure 6 is an end view of the stator of the motor drawn to an enlarged scale,
with the wiring schematically illustrated;
- Figure 7 is an end view of the rotor of the motor drawn to substantially the
scale of Figure 6; and
- Figure 8 is a diagrammatic representation of a novel motion control system
utilizing the motor of the present invention.
Turning now in detail to Figures 1 to 7 of the appended drawings,
therein illustrated is a brushless DC motor, generally designated by the letter
"M", embodying the present invention. It consists of a permanent magnet rotor,
generally designated by the numeral 10, a stator, generally designated by the numeral
12, and front and rear end caps, generally designated respectively by the numerals
14 and 16. The rotor consists of a cylindrical core 18 with axial shaft portions
20, 22, and an array of high energy permanent magnets 24 secured (such as by adhesive
bonding) on the surface thereof and extending longitudinally (axially) therealong;
the magnets are disposed with their polarities alternating in the array, and will
advantageously be made of a samarium/cobalt alloy. Suitable bearings 25 are mounted
within the end caps 14, 16, and serve to receive the opposite end portions 20,
22 of the shaft for rotatably supporting the rotor 10, in a conventional fashion.
The stator 12 is comprised of numerous laminae of punched or stamped
steel fabrication, the configuration of which is indicated in Figure 6 (although
characterized as an end view of the stator, the Figure may also be considered to
show the structure of an individual laminae). As can be seen, the laminae consist
of an annular body element 26, with 24 pole elements 34 extending radially inwardly
therefrom at equidistantly spaced locations about its inner circumference; each
laminae is of course of one-piece, integrally formed construction. The pole elements
34 are of rectangular configuration and, as a result, define slots 36 therebetween
which are relatively wide at the base and taper in a radially inward direc- tion.
As will be appreciated, the stacked laminae 24 coopera- tively provide the poles
34 of the stator (for convenience, the parts of the stator and the elements of
the laminae from which they are formed are given the same numbers); the poles are
of substantially rectangular configuration in both their axial and also their
transverse planes, and thus they have a single thickness dimension and a single
axial length dimension, at all points taken along their radial length. Although
shown only in Figures 1 and 5, the lamina may cooperatively define longitudinally
extending ribs 28, 30, 32, to serve as integral radiators for dissipation of the
heat that is built up during operation of the motor; the ribs are dimensioned and
configured to afford sufficient surface area for efficient heat transfer, while
lying substantially entirely within the corner areas defined between the outer
circumference of the stator body portion 26 and an imaginary square figure disposed
thereabout and having a side dimension equal to the diameter thereof. Four long
bolts 37 extend through apertures in the front end cap 14 and are secured within
tapped openings in the rear cap 16, to hold the parts in assembly.
Each pole 34 of the stator 12 has two coils 38, 38' disposed adjacently
thereupon and insulated therefrom with paper or a synthetic resinous material,
in a conventional manner. The coils 38, 38' are wound and interconnected as schematically
shown in Figure 6.
More particularly, it can be seen that there are two sensor circuit
legs, designated CT1 and CT2, which are electrically parallel to one another and
join at the junctions designated S1 and S2. All coils 38 of the set comprising
the circuit leg CT1 are wound in one direction upon the odd-numbered stator poles
34, and all of the coils 38 comprising the circuit leg CT2 are wound in the opposite
direction upon the even-numbered poles. Thus, the sensor coils of the two sets
produce oppositely directed magnetic fluxes, as indicated by the positions of the
small circles adjacent either the outer or the inner end of the coil winding.
The junctions at which the terminals 48 and 50, for CT1 and CT2 sensor
circuit legs, respectively, are connected effectively subdivide each set of sensor
coils 38 into two, equal-number subsets. In addition to having the poles of each
sensor coil set alternatingly interposed with one another, it can be seen that
the poles of each coil subset also alternate with one another about the stator
With regard to the phase, or torque coils 38', those wound on the
poles numbered 1, 2, 5, 6, 9, 10, 13, 14, 17, 18, 21 and 22 are connected in series
as a first phase, designated "A", and those wound on the remaining poles are interconnected
as a second phase, designated "B". Thus, the poles of each phase group are arranged
as adjacent pairs, and the phase coils 38' on the paired poles are wound in opposite
directions so as to cause the magnetic flux to follow a path through the pairs
of poles when the phase is energized, thus magnetically coupling them.
It will be appreciated from the foregoing that the wiring of the
stator described adapts each pole to function as a torque pole and a sensor pole.
The particular arrangement illustrated affords optimal magnetic balance in both
the power and the sensing functions of the motor, when the stator is connected
to operate on two phases of equal voltage and to provide two-channel feedback
signals, in addition to affording extremely high levels of resolution of rotor
position and hence exceptionally accurate position information.
Figure 8 constitutes a functional block diagram of a system employing
the instant motor. Power for the motor M is provided by an amplifier, which operates
on DC voltage applied to the power terminal 60 and ground connections (not shown),
and which is comprised of an H-bridge for each phase, designed HA and
HB, connected respectively through lines 62, 64, and having current-regulating
minor loops, as illustrated; the H-bridges will advantageously employ power field
effect transistors, and will typically be operated at 50 KHz chopping frequency.
The terminals 52, 54 for the sensor channels of the motor are connected
through lines 66, 68 to a DC powered sensor 70, which may also take the form of
an H-bridge and will typically operate at a predetermined constant frequency of
100 KHz to supply an alternating current to the pairs of sensor coils of the two
channels, CT1 and CT2, respectively. The sensor 70 is synchronized with the synchronous
detector 72, which is connected through lines 74, 76 to terminals 48 and 50 of
the motor M and performs inversion functions upon the signals from the sensor
channels, to provide single-level voltages of varying amplitude to a position decoder
78, the latter normally being a function of a microprocessor controller integrated
into the system. The microprocessor also perform gain adjustment functions for
standard PID controllers 80, 82, 84, and provides an indexing signal which is summed
at the junction 86 with the vector signal from the position decoder 78, for input
to the controllers. The signal from the output summing junction 88 controls the
H-bridges HA and HB, through lines 90 and 92, for coil energization
and pulse-width modulation appropriate to produce desired operation of the motor,
depending upon rotor position and the command signals input into the system from
the control terminal 94, through the interface port 96 and the microprocessor
monitoring, sequencing, indexing, gain adjustment and memory functions 98, 100.
Line 102 establishes a commutation loop from the position decoder 78 to the H-bridges,
and terminals 104 are provided to accommodate external indexing, if so desired.
As will be appreciated, the amplitude of the voltages detected at
terminals 48 and 50 will depend upon the inductance of the sensor coils 38, in
turn providing an indication of magnetic flux in the poles, as affected by the
rotor magnets. Because of the geometry of the stator and rotor poles, and the
timing of H-bridge switching, the signals from the sensor channels will vary as
the rotor turns, and will be 90° electrical out-of-phase to represent sine and
cosine functions of the rotor angle; this will enable generation of a vector signal
by the position decoder that is indicative of the actual angular position of the
rotor. Moreover, because each signal will effectively represent an averaging of
the effects of the rotor magnets upon six stator poles (rather than upon only one,
as in the motor previously described in the identified Horber patent) the construction
and magnetic configuration of the rotor and stator permit extremely high angular
resolution and hence accuracy of positioning. A position error value less than
±8 arc-minutes has been achieved with an exemplary motor constructed as hereinafter
described and employed in combination with a driver of suitable design.
As will also be appreciated, the microprocessor of the system will
determine, from the information obtained through the integrated feedback system
of the motor, precise distances of travel from a home position. By controlling
power supplied to the phases through the H-bridge circuits, control of motor speed,
as necessary to carry out operational commands, is also afforded.
Although a two-phase system has been illustrated, it will be understood
that the concepts of the invention are equally applicable to other polyphase systems;
specific modifications will of course have to be made, as will be apparent to those
skilled in the art. For example, rather than configuring the rotor and stator
to generate signals in the sensor channels that are 90° out-of-phase, for a three-phase
system the signals would be 60° electrical out of phase and three channels (comprised
of at least four sensor poles per channel) would be provided as a matter of preference,
albeit that the provision of only two channels is feasible and would be less expensive.
Also, while the number of stator poles for a two phase motor must total at least
eight and be divisible by four, for three-phase operation the number must total
at least 12 and will preferably be divisible by 6; in both cases however the quotient
of the number of poles divided by the number of phases must be an even number.
In addition to the foregoing, it has been found that the numerical
relationships that exist between the stator and rotor poles of motors embodying
the invention must conform to certain criteria. More particularly, the stator pole:rotor
pole ratio in a two-phase motor must be either 4:3 or 4:5; whereas in a three-phase
motor it must be 3:2; Table One below sets forth illustrative combinations of numbers
of stator and rotor poles, reflecting these relationships:
6 or 10
12 or 20
18 or 30
24 or 40
30 or 50
Despite the foregoing, in most instances the minimum total number
of stator poles will generally be 24, since that will normally provide optimal
magnetic balance and performance in a two-phase motor system.
It is of course also desirable that the motor be of a physically
symmetrical nature, again to provide optimal magnetic balance. As can be seen,
the illustrated stator is highly symmetric, physically as well as in all magnetic
functions. Although the circuit arrangement of phase and sensor coils shown in
the drawings is preferred, other arrangements are possible. For example, the sensor
coils of the sets and subsets described might be grouped together rather than being
interpositioned and wound as shown; however, the magnitude and accuracy of the
feedback signal would be compromised as a result, and undesirable phase shifts
Especially important advantages of the present motor stem from the
fact that it employs all of the pole elements as torque poles, thus maximizing
power output, and because it employs coupled torque poles that are located directly
adjacent to one another. Flux paths are very short as a result and there is no
crossing of the fluxes of the different phases, which factors in turn minimize
magnetic losses and attenuation due to phase interaction, and thereby maximize
the efficiency of operation. The use of high energy magnets for the rotor poles,
and their close proximity to the stator poles, also aid the development of high
torque values and low inductances. As to pole geometry, the confronting faces of
the rotor poles will generally be wider than the faces of the stator poles, but
narrower than the distance across two adjacent poles including the gap therebetween.
Motors embodying the invention are characterized by torque/speed
curves that are virtually flat throughout the major portion of their speed range,
and this is true under both continuous and also intermittent duty conditions. They
are also constructed to provide cycle counts of 8-20 electrical cycles per revolution
of the rotor, to afford an optimal balance of accuracy, speed and torque. Cycle
counts of six or less provide levels of sensor accuracy that would be inadequate
for most applications for which the motors and systems of the invention are intended,
and motors with counts of 25 or more will generally be too slow.
An exemplary motor embodying the invention has been produced, and
was constructed, wired and controlled as hereinabove described and illustrated.
It had a 24-pole steel laminate stator approximately 2-1/4 inches (5.7 centimeters)
square in transverse exterior cross-section, and employed an 18 pole, samarium/cobalt
alloy permanent magnet rotor of approximately 1-3/16 inch (3 centimeters) diameter;
its axial length (taken as the combined length of the stator and end caps), was
2 inches (5.1 centimeters).
In operation, the motor exhibited a position resolution of 0.02 degree;
with suitably modified software for the microprocessor, it is expected that the
angular resolution could be made even more precise. This level of accuracy can
be compared with the excellent value of 0.15625 degree, which is reported in the
above-mention Horber patent for an otherwise comparable motor having only four
sensor poles. The instant motor also exhibits a peak torque (system) value, at
the specified operating temperature of 85° Centigrade, of 105 ounce-inches, a
no-load speed of 6000 RPM (with 150 volts DC applied to the H-bridges, and an average
terminal inductance of 0.05 millihenry), a rotor inertia value of 0.00176 ounce-inch-second,
and a terminal resistance of 4.0 ohm.
Finally, it will be appreciated that the control functions for the
motor will generally be entirely digital and integrated into the systems. In addition
to the other self-evident benefits of such construction, the fully integrated construction
facilitates self-tuning or "expert" operation, thereby rendering the system of
the invention especially well suited for certain applications, such as robotics
and the like.
Thus, it can be seen that the present invention provides a novel
and improved brushless DC motor which is relatively incomplex and inexpensive to
manufacture, which affords a highly desirable balance of accuracy, speed and torque
characteristics for precise positioning capability, in which chopping noise and
induced voltages are minimized, and which is highly efficient, effective and reliable
for its intended purposes. The motor operates in a closed loop feedback mode, employing
internal features that are capable of providing signals of large magni- tude and
exceptional accuracy, the latter being due primarily to the effective averaging
of feedback signals over multiple poles, thus compensating for magnet heterogeneity,
and to the increased correspondence that is achieved between the mechanical and
magnetic motor axes. In addition, the motor is highly responsive to commands,
it enables extraction of exact rotor position information at standstill and has
a large number of angular resolution points to provide very precise position information,
and it exhibits increased torque and very smooth running, and full power starting,