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This invention relates to electromagnetic vibratory motors for providing
the motive force for vibratory feeders, conveyors, separators, screens and
other vibratory equipment used in materials handling and related fields.
Vibratory motors of both the electromechanical and electromagnetic type are
used to provide the motive force for vibratory equipment used in materials
handling and related fields. Each has its own advantages and
disadvantages. The present invention is concerned only with vibratory
motors of the electromagnetic type. These have in the past been driven by
an electromagnet with an air gap. Part of the electromagnet is usually in
the shape of an `E` and is fitted with electrical coil or coils, and the
other part of the magnetic circuit is in the shape of an `I` fitted to the
other part of the machine with an air gap between them, so that the two
may vibrate relative to one another with a consequent oscillatory increase
and decrease in the size of the air gap.
Examples of vibratory conveyors and feeders using electromagnetiic
vibratory motors are shown in Australian Patent Specifications Nos.
140,193; 204,274 and 245,317; and in U.S. Pat. Nos. 2,163,249; 2,654,466
and 2,694,156.
Many variations to the basic arrangement described above using magnetic
circuits made up of E and I parts are known, some employing permanent
magnets to permit operation with full wave alternating current while
others operate from half wave alternating current by use of rectifiers.
Almost all of these machines run at the frequency of the electricity
supply, in general either at 50 or 60 Hz. The machines may be made to
operate at a lower frequency, again usually some sub-multiple of the
supply frequency, such as 25 or 30 Hz, but the machines are not suited to
provide a large vibrational amplitude and thus are quite inefficient at a
lower frequency than the full mains frequency. The air gap is in the
direction of motion of the moving parts and is subject to drift in size.
Striking of the two parts of the magnetic circuit may occur due to
expansion or contraction caused by temperature variation or by
fluctuations in supply voltage and frequency, or changes in the natural
frequency of vibration of the resonant system. If striking does occur, the
machine may be very rapidly damaged and this is the most important
disadvantage of the electromagnetic vibratory feeder, either preventing
its use in many applications or requiring extremely complicated control
equipment to prevent this striking taking place.
Linear motors have been known for some years and their application to
passenger trains and other similar vehicles is well known. Several
attempts have been made in the past to adapt a linear motor to provide the
motive force for vibratory feeders and similar machines, but each attempt
has failed due to a design fault in the suggested method. Most linear
motors constructed have been of the travelling flux wave type where
electrical coils are energized in sequence to provide a tractive force
moving along the motor. This type of motor is directly applicable to
vehicle transportation on rails where the vehicle length is only a very
small part of the track length, but a similar principle used in machines
for materials handling means that a large proportion of the materials
making up the motor are not in use at any one time and the design is
therefore inefficient.
Another type of linear motor which has been adapted to vibratory equipment
is that which takes the form of a double solenoid. An electrical coil,
when energized, draws a piece of ferrous material, usually soft iron, into
a magnetic path. When the coil is de-energized, the armature is forced out
of the magnetic path. The motor is usually double ended in that while one
end of the armature is being drawn into a magnetic path by one coil being
energized, the other end of the armature is forced out of a similar
magnetic path as its coil is deenergized.
FIG. 1 illustrates this type of motor where the soft iron armature 1 is
attached to a frame 2 and a guiding bar 3 moving through bearings 4. A
magnetic path is formed by the structure 5. For the particular motor
shown, A.C. current of a particular frequency passes through coils 6 and
7.
This type of motor will provide an oscillatory action particularly when
used on machines which utilize resonance. The design is not efficient as
much of the iron making up the magnetic circuit is not fully utilized. Of
more importance, however, when used with machines that do employ
resonance, the motor is not inherently self limiting in stroke. The force
of traction on the soft iron armature poles 1 decreases as these poles
enter the magnetic path formed by the structure 5, but the tractive force
does not drop to zero. Even with the pole pieces 1 touching the electrical
coil 6 there would still be a tractive force drawing the armature frame 2
to the right in the figure when the coil 6 was energized, so that damage
could occur.
An example of a further type of linear motor which has been adapted to
vibratory machines is that shown in U.S. Pat. No. 2,444,134 by Hittson.
The design shown in this patent again suffers from the disadvantage that
the arrangement is not inherently self limiting. Movement of the elongated
armature 25 shown in FIG. 3 of that patent where poles 26 on the armature
are attracted to poles 20, initially slightly offset from poles 26, on a
field structure 16 is limited in one direction as should the poles 26
begin to pass the pole 20, attraction in one direction will cease and
attraction in the other direction will begin, halting the motion of the
armature. However, no such arrangements exist in the other direction as
movement in the other direction is not under control of the magnetic
circuit, the electrical coils (22) being not energized. The basic design
fault of the motor shown in that patent is that for operation very close
to or at resonance, the maximum movement of the armature is not limited by
the motor.
U.S. Pat. No. 1,490,155 on "Electromagnetic Power Device" issued to Zbinden
is another example of a linear motor which has an inherent, although in
this case weak, halting motion of the armature. For instance, note that in
Zbinden the breadth of the pole rings is approximately equal to the
breadth of the gap between the pole pieces so that the tractive force of
the motor will not exhibit a sinusoidal function because the magnetic path
as seen by the motor elements does not have a constant reluctance
throughout the path of motion. This is further accentuated by the filling
of non-magnetic material placed between the pole pieces in Zbinden to
create an insulating bridge annuli between stator fields. The halting on
limiting motion of the motor is weak due to the large gap between the pole
pieces. Also a basic design fault of the motor shown in Zbinden is that
the efficiency of the motor is reduced because of the varying reluctance.
A motor with constant reluctance through its motion will appear to be
almost entirely resistive and be able to transfer power very efficiently
from the electrical power supply to the driven load, whereas a motor with
varying reluctance will appear inductive and be inefficient in power
transfer.
The prior art exemplified by U.S. Pat. No. 1,490,155 and issued to Zbinden
and Trench, U.S. Pat. No. 3,149,255 respectively incorporate armatures
which are solids of rotation with their longitudinal axis coincident with
the longitudinal axis of the stators. This cylindrical form is inefficient
when large pole areas are used because the volume within the circle is
wasted. The tractive force in these motors is small so it is essential to
have large pole areas. In the present invention the poles form planes and
thus large pole areas can be realized without lost volume.
Departing from the cylindrical form prevents the use of simple circular
bushings as guides for the moving armature and for withstanding the large
tractive forces between armature and stator. This penalty does not exist
in the present invention because the inventive concept includes the
elimination of bearings and bushings as can be seen in the illustrations
described further on in this specification.
In the construction of vibratory machines employing a resonant phenomenon
for handling materials, the cost of the springs or other energy storage
devices that provide the resonant action is a significant part of the cost
of the machine. Machines employing linear motors are normally operated
very close to or at resonance and if operated at resonance there must be
some limit on the motion that can occur. With efficient use of materials
the limit on this motion should lie only just above the maximum vibratory
movement to be usefully obtained from the machine. This means a linear
motor should have an inherently self limiting action very close to the
maximum amplitude to be used. The self limiting action should also act as
a rigid boundary on the possible amplitude of vibration as a self limiting
action which permits, say, half as much amplitude again as the maximum
useful amplitude means that the cost of the energy storage devices such as
springs will also be at least half as much again as the cost need be.
FIG. 2 shows the relationship between tractive forces and displacement from
rest position for the type of linear motion shown in FIG. 1. The upper
curve in the figure shows the tractive force generated when the armature 1
in FIG. 1 is to be driven to the right and the lower curve applies when
the armature is being driven to the left. It can be seen that for motion
away from the rest position the tractive force increases in value to a
maximum and then falls but does not reverse direction before a mechanical
limit to motion is encountered.
FIG. 3 shows the relationship between the tractive force and displacement
from rest position for the vibratory conveyor motor shown in U.S. Pat. No.
2,444,134. It can be seen that for motion in one direction from the rest
position the tractive force increases to a maximum, falls and then
reverses direction. This force reversal will effectively limit motion in
one direction, but no such provision is made for motion in the other
direction from the rest position as the coils, Item 22 in FIG. 2 of
subject patent, are not energized for motion in the other direction.
While there are many ways to limit the motion of the armature by electrical
means, such as reducing the current flow through the electrical coils, it
is much more preferable that the motor be inherently self limiting to
prevent damage to the springs or other parts of the machine as when the
machine is operating at resonance without being required to convey bulk
materials, only very small forces are required to generate extremely large
amplitudes of oscillation.
The present invention provides a means for permitting large amplitudes to
be obtained from electromagnetic vibratory motors and eliminates the air
gap in the direction of motion of the vibrating parts, thus eliminating
the need for constant adjustment of the air gap, and care in ensuring that
striking does not take place. A further object of the invention is to
provide an inherently self limiting operation of the vibratory motor so
that the vibratory machine is not damaged when operated at or near
resonance and at the maximum amplitude which the motor is capable of
generating when unloaded.
According to this invention an electromagnetic vibratory motor comprises an
armature having a plurality of magnetic poles spaced apart along the
direction of action of the motor, the spacing between said poles forming
an air gap having a distance between individual pole pieces less than the
width along the direction of motion of the motor of the individual pole
pieces, said poles having pole faces generally parallel to said direction
of action; spring means attached to said armature; a stator having a
plurality of magnetic poles spaced apart along the direction of action of
the motor, the pole pitch of the stator being the same as that of the
armature, the poles of the armature and stator being closely adjacent and
one set of poles being either of fixed magnetic polarity or of
predetermined alternating character, and the stator poles being arranged
symmetrically about the centres of respective armature poles in the rest
position; first electrical windings about the poles of the other set of
poles; and means for feeding alternating current through said windings.
References will now be made to the further accompanying drawings in which:
FIG. 4 shows a vibratory conveyor employing an electromagnetic vibratory
motor according to this invention;
FIG. 5 shows, partly in section, the stator, armature, spring assembly of
the electromagnetic motor used in the conveyor of FIG. 4;
FIG. 6 is a detail showing the pole arrangement used for staggered poles;
FIG. 7 shows the forces acting on a single pole in the initial position;
FIG. 8 shows cessation of motion of pole;
FIG. 9 shows motion of pole in other direction;
FIG. 10 is similar to FIG. 6 but shows diagrammatically use of windings on
both stator and armature for the pole arrangement;
FIG. 11 shows diagrammatically an embodiment in which the armature windings
take a form analogous to those of the rotor winding of a conventional
squirrel cage induction motor;
FIG. 12 shows the relationship between tractive forces and displacement
from rest position;
FIG. 13 is a graph showing the variation of resonant natural frequency with
vibrational amplitude;
FIG. 14 is a graph showing a family of curves of vibrational amplitude
plotted against the applied current for variation in drive frequency;
FIG. 15 is a graph showing a method of control used;
FIG. 16 shows the vibratory motor applied to a machine using coil springs
as the resonant spring system;
FIG. 17 is a detail showing the pole arrangement used for poles in
opposition;
FIG. 18 shows the forces acting on a pole in opposition;
FIG. 19 shows a combination of staggered poles and poles in opposition;
FIG. 20 shows tractive forces for various pole arrangements;
FIG. 21 shows the power that may be drawn from various pole arrangement.
Referring now to FIG. 4, a conveyor trough 28 to be vibrated is bolted to
an armature bracket 18 by means of bolts 27. The armature bracket 18 in
turn is connected to spring means 23 which couples the armature and trough
to the stator 8 of the motor and a base or reaction mass 14 enclosing the
motor. The use of leaf springs 23 as shown in a known means for providing
resonant tuning between the two masses to be vibrated. The leaf springs
could be replaced by coil springs or other resilient means, as is well
known. A cover 25 protects the driving means from the entry of dust or
water. The whole machine is resiliently supported on springs 26, 29 to
permit free vibration. The ends of these springs are attached to a rigid
supporting structure by spring seat plates 38 and 39. Ribs 30 on the
trough provide structural stiffness.
The vibratory motor is shown in more detail in FIG. 5. The armature bracket
18 is connected to an armature having a group of magnetic poles 9 and 10.
These magnetic poles are of constant polarity, the magnetizing force being
generated by permanent magnets 11. A clamp 31 joins the poles to the
armature bracket through a spacer 17.
The stator 8 includes a plurality of magnetically variable poles disposed
on either side of the group of magnetically constant poles, 9 and 10, and
is fastened to the base mass 14 by suitable brackets 13. A small constant
air gap 42 is provided between the poles 9 and 10 and the fixed poles on
stator 8. This air gap is adjustable by suitable clamping of the fixing
bolts 24. The leaf springs 23 with spacers 22 of some suitable material
comprise the resonant spring system, and clamping screws 19, 20 and
clamping blocks 21, 32 prevent relative axial movement between springs and
spacers. The side covers 33 and rear cover 25 protect the driving means
from the entry of dust or water. Electrical windings 12 are wound on the
fixed poles on stator 8 in such a way tht adjacent poles have opposite
polarities. An electric cable 15 provides connection of the windings to a
suitable source of alternating current, now shown, and is supported by a
gland 16.
The vibratory motor of this embodiment has a pole arrangement as detailed
in FIG. 6 in which the poles in the rest position are arranged in a
staggered fashion such that the pole gaps 43 on the armature are
approximately opposite the centres of the poles on the stator. The stator
carries an electrical energizing winding 12 wound on the stator poles 48
and the armature poles 10 are energized by permanent magnets 11.
The pole gaps 43 form air gaps along the face of the poles 11 which are
approximately equal to the air gaps between the stator poles 48. These air
gaps are small relative to the width of the stator and armature poles so
the magnetic path approaches a constant reluctance throughout the motion
of the motor. This constant reluctance tends to maximize the efficiency of
the motor and result in a more powerful linear motor for a given dimension
and current input. In a preferred embodiment such as illustrated in FIG.
6, the air gaps 43 are less than half the width of the pole pieces 10 and
18 as taken along the direction of motion of the motor. This provides for
considerable overlapping the poles from one set to the other so that the
desired constant reluctance may be approached. No filler material is
utilized between pole pieces primarily because it may have a tendency to
create an insulating effect with respect to the magnetic path and thus
prevent approaching a constant reluctance in the system.
FIG. 7 shows the forces of attraction and repulsion action on an armature
pole for a particular polarity of stator poles resulting from energization
of the electrical windings 12. The armature pole shown is of South
polarity and moves to the left, being attracted to the North pole of the
stator and repelled by the South pole of the stator.
FIG. 8 shows the armature pole 10 having moved fully to the left and being
directly under the North pole of the stator. The armature pole is now
repelled equally by South poles on either side of the North pole but
movement further to the left would be also impeded by the attraction force
between the armature pole and the North pole on the stator.
FIG. 9 shows the reversal of polarity of the stator effected by change in
direction of the current flow through electrical windings 13. The armature
pole will now move to the right.
The use of permanent magnets is not essential to provide magnet poles of
constant polarity and FIG. 10 illustrates a case where electrical windings
are present on both the stator and armature poles. In this figure, Item 37
is an assembly of moving poles with an electric coil 36 providing magnetic
energization. The stator poles 48 have an electrical winding 12 providing
varying polarity. A D.C. current flowing in winding 36 will cause the pole
37 to maintain a constant polarity. D.C. current is not necessary,
however, as current in winding 36 varying at a frequency twice that of the
frequency of alternation of current flowing in winding 12 and having a
suitable phase relationship between the two alternating currents, will
give a similar effect.
Alternatively, the windings can take a form similar to that of the rotor
winding of a conventional squirrel cage induction motor in that no
external connections are necessary, induction providing the necessary flow
for magnetization of the armature poles. This is shown in FIG. 11, where
44 is the armature pole and the electrical windings are shown as Item 45.
Each electrical winding is a short circuit around one pole and the current
flow in the short circuit winding caused by polarity reversal from
electrically driven poles 48, provides the driving force.
The operation of the motor in this manner is analogous to but different
from the operation of a squirrel cage induction motor. For the rotary
motor a slip frequency is set up in the squirrel cage due to the
difference in rotational frequency of the travelling magnetic wave and the
rotor, and current is induced in the rotor by transformer action at the
slip frequency. Here, no such phenomenon exists as motion is constantly at
one frequency and it is the reduced amplitude of vibration of the armature
compared to the motion of the reciprocating flux wave that causes current
to flow in the armature winding 45.
FIG. 12 shows the relationship between tractive force and displacement from
rest position for the type of vibratory motor described in this patent
application, with the pole arrangement shown in FIG. 6. The upper curve
shown in the figure shows the tractive force generated when the armature
is to be driven to the right and the lower curve in the figure shows the
tractive force generated when the armature is to be driven to the left. It
can be seen that for motion on either side of the rest position the
tractive force increases, rises to a maximum and then falls, reversing in
sign. Passage of the armature through this point of force reversal retards
further motion in this direction. It is this symmetrical action on either
side of the rest position which provides the motor with its mose useful
feature, that of inherent self limitation of amplitude of vibration, at a
maximum safe stroke set by design, a feature not shared by other linear
motors with a reciprocating flux action up till now.
For the vibratory conveyor shown in FIG. 4, using leaf springs as the
resonant spring system, the vibrating linear motor is able to take
advantage of the non-linearity of the spring system.
FIG. 13 plots the change in natural frequency of the vibrating leaf spring
system shown in FIG. 5 as the amplitude of vibration increases. For small
amplitudes of vibration the natural frequency is that of the leaf springs
in bending but as the amplitude increases the natural frequency rises,
approaching the natural frequency of the leaf springs in tension. The
behaviour of the vibrating machine when operating with this non-linear
resonant system is that, for any driving frequency between the two natural
frequencies of the leaf spring system, the amplitude of vibration will
remain stable at a value corresponding to the deflection where the natural
frequency of the springs is combined bending and tension equals that
particular driving frequency. By use of this phenomenon, stable
oscillation may be obtained irrespective of loading.
FIG. 14 shows the operation of the vibratory machine employing leaf springs
in a non-linear manner where vibrational amplitudes (d) is plotted against
alternating current applied to the electrical windings. If the machine is
excited at the natural frequency of the spring system in bending the
amplitude will initially rise rapidly with current, the rate of increase
changing until the amplitude does not increase further with increase in
current. For drive frequencies higher than the bending natural frequency,
the amplitude will initially rise slowly with increasing current until a
point is reached when the amplitude will jump to a larger stable value,
this amplitude being that which causes the natural frequency to equal the
driving frequency. This amplitude will remain stable for a large range of
applied current. If the current reduces to a very low value the
vibrational amplitude will suddenly decrease to a very small value, the
motor behaving as though stalled.
FIG. 15 shows a graph of the method of control that may be employed when
the vibratory linear motor is applied to machines using leaf springs or
other energy storage elements which display a non-linear characteristic
with amplitude. The applied current at a frequency corresponding to the
natural frequency of the system for small amplitudes is increased until a
point (D) is reached and then both the applied current and the applied
frequency are increased to provide increase in amplitude of vibration.
FIG. 16 illustrates one method of applying the vibratory linear motor to a
machine using coil springs as the resonant spring system. The basic
principle of operation of the motor is the same as that shown in FIG. 5,
but the arrangement is sufficiently different to warrant description. A
vibrating trough 50 supported by isolating means 51 carries a beam 52
between wing plates 53 connected to the trough bottom. Coil springs 54
provide a resonant connection between driving mass 55 and the vibrating
trough. Stator poles 56 are mounted above and below an armature 57 coupled
to the beam 52. The armature is made up of permanently magnetized poles 9,
10, magnetization being generated by permanent magnets 11. Electrical
windings 58 on the stator poles are provided with alternating current
through electrical cable 59 from a suitable electrical source not shown. A
small constant air gap 60 is provided between the faces of the armature
poles and the stator poles, the gaps between the armature poles
corresponding to the centres of the stator poles in the rest position as
shown in FIG. 6. Stabilization means 61 are provided between the stator
and armature to maintain the air gap 60 at a constant value while the
machine is vibrating.
FIG. 17 shows an alternative arrangement for the vibratory motor where the
armature poles 10 have the same pitch as the stator poles 48 but the gaps
between poles are opposite each other, and the poles are in direct
opposition in the rest position. This arrangement has no starting force
but once in motion provides large driving forces. One method of starting
such a vibratory motor would be a conventional electromagnet as is
described in lines 10-17 of Page 2 but with a much larger air gap to
eliminate the possibility of striking. This electromagnet could then
provide initial vibration until the linear motor was able to continue of
its own accord. The high current necessary for the electromagnet could be
tolerated if this current were only used infrequently for starting
purposes.
For the arrangement of poles in opposition the driving forces on the poles
are quite different to the driving forces on the staggered pole
arrangement shown in FIG. 6.
FIG. 18 illustrates this, where the motion of a moving pole 10, shown as
having for this example a constant North polarity, is depicted. 48 is a
magnetic pole which alternates in polarity, depending on the direction of
current flow in the electrical winding 12. In the first position FIG. 18
(a), the two poles are of the same polarity and a repulsion force causes
motion of the pole 10 to the right relative to the pole 48. When the
current in the winding reverses, the two poles become of opposite polarity
as shown in FIG. 18 (b) and are attracted to each other, causing relative
movement of the pole 10 to the left. The current in the winding again
reverses, causing repulsion as shown in FIG. 18 (c) and the movement
continues to the left, passing through the position where the poles are
exactly opposite each other. At the end of the motion to the left, the
current reverses as shown in FIG. 18 (d), causing the poles to again be
attracted to each other and the pole 10 returns to the right. The result
is that the poles vibrate at exactly half the applied frequency and there
is a highly efficient energy transfer as direct magnetic force is
available for each part of the movement of the poles.
Use of the two types of pole arrangement together gives the advantage of
both high starting forces and driving forces remaining high close to the
maximum amplitude of vibration. FIG. 19 illustrates the use of the two
pole arrangements together. Armature poles 10 are energized by permanent
magnets 11 and have the same pitch as the stator poles 48. Some of the
armature pole 10 align with the stator poles as shown in the top of the
figure, while other poles 10 have their centres aligning with the gaps
between poles 48 as shown at the bottom of the figure. An armature pole 40
of different width is used to change from one pole arrangement to the
other. Electrical windings 12 on the part of the stator having its pole
gaps opposite the centres of the armature poles in the rest position are
supplied with alternating current. Electrical windings 41 on the part of
the stator where the centres of the stator poles align with the centres of
the armature poles are supplied with alternating current of a frequency
twice that supplied to electrical windings 12 and with a suitable phase
between the two currents so that the driving forces generated by both
currents are additive.
FIG. 20 shows several tractive force graphs for movement to the right. The
figure would show forces symmetrical but of opposite sign for forces
causing movement to the left but these are omitted for clarity. Case A
shows the tractive force available from poles where the centres of the
stator poles are opposite the gaps between armature poles, as shown in
FIG. 6. Case B shows the tractive force available from poles where the
centre of the stator pole is opposite the centre of the armature pole as
shown in FIG. 17. It can be seen that the peak for Case B occurs further
from the zero point for movement to the right. Case C represents a
composite of Case A and Case B where the two forces have been added
together, half the stator poles being opposite the armature poles and the
other half of the stator poles having the pole centres opposite the
armature pole gaps.
Case E illustrates where all the stator poles have their centres opposite
the gaps on the armature poles. It can be seen that the maximum force for
Case E is greater than the maximum force available for Case C but the
maximum force for Case E occurs nearer to the zero point of motion.
FIG. 21 compares the power available from Case C and Case E plotted against
vibrational amplitude. The power required by a load carried by a
vibrational machine is zero for zero vibrational amplitude and increases
as the vibrational amplitude increases, usually in a non-linear manner,
increasing more rapidly as the amplitude increases. Case E, where all of
the stator poles have their centres opposite the gaps of the armature
poles, while having a higher maximum tractive force than Case C, has a
power curve which is not as high as that for Case C, near the maximum
possible vibrational amplitude. When the vibratory motor is driving the
load the maximum amplitude that could be obtained from Case E is shown as
F, whereas for Case C a much greater amplitude, G, could be obtained. The
maximum amplitude H will be the same for either Case C or E when the
machine is unloaded. The machine which has its maximum load amplitude
closest to its maximum unloaded amplitude will be the most efficient in
design and it is shown here that the vibratory motor having poles of two
types, that is, poles in opposition and staggered poles, will be the most
efficient.
FIG. 21 relating power to vibration amplitude is analogous to that of the
torque speed curve for induction motors where motors having the smallest
slip or reduction in speed from their full speed to their loaded speed are
the most efficient in design.
The most important feature of the linear motor shown adapted to resonant
vibratory machines is that the amplitude of vibration is inherently self
limiting preventing damage to the machine when operated at resonance.
Obviously those skilled in the art may make various changes in the details
and arrangement of parts without departing from the spirit and scope of
the invention as defined by the claims hereto appended and we therefore
wish not to be restricted to the precise construction herein disclosed.
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