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Description  |
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FIELD OF THE INVENTION
The present invention pertains to the field of electric motors, and more
particularly, to linear motors.
BACKGROUND OF THE INVENTION
Electric motors for converting electrical energy into rotary motion are
highly efficient devices, particularly in the larger sizes, but much of
this efficiency is lost in the process of converting this rotary motion
into linear or reciprocating motion as is necessary in some machines, such
as reciprocating pumps and positioning equipment.
As used herein, the term linear pump or motor means a device having a
piston that translates along its axis rather than rotating about its axis;
the axis along which the piston travels may be a straight line, or curved
line, or a combination of both. The two principal obstacles to the
development of an efficient linear motor have been the difficulty of
establishing tight flux linkages between the stator and armature, and the
complexity of the control circuitry needed to drive the motor.
Previous efforts have incorporated permanent magnets with poles facing
axially toward the ends of the cylinders. In order to establish adequate
coupling in this configuration pole pieces have sometimes been used in the
ends of the working cylinder, in which case stroke length is severely
limited as shown in U.S. Pat. No. 2,701,331 to Holst and U.S. Pat. Nos.
3,754,154, 3,846,682 and 3,884,125 to Massie. The concept of using a
plurality of coils has been investigated as shown in U.S. Pat. No.
4,541,787 to Delong, but at the cost of good magnetic coupling between
drive coils and piston. These designs also called for relatively thick
metallic cylinders, further reducing magnetic coupling and introducing the
additional complication of induced eddy currents in the cylinder caused by
the electrical current in the drive coils.
Rotating synchronous electrical machinery has an advantage over linear
equipment in that some slippage is allowable. If magnetic coupling between
stator and armature poles is lost in a rotating magnetic field such as is
encountered in ordinary induction motors it is quickly reestablished with
the following armature pole, and the result is only a minor loss of
efficiency. In a linear motor, loss of magnetic coupling results in
erratic behavior or complete motor stoppage. In the prior art, drive coil
currents were controlled by passive circuitry such as mechanical switching
or electronic oscillators, and currents were applied without reference to
the actual position of the piston. Such a motor would operate efficiently
only in a very narrow range of conditions, and would not operate at all if
momentarily overloaded. In the present invention this difficulty has been
overcome by the use of electronic circuitry capable of sensing piston or
armature position and supplying current pulses so as to eliminate
slippage.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide an improved
linear motor.
It is also an object of the present invention to provide a linear motor
incorporating a highly efficient flux linkage between piston and enclosing
cylinder.
It is another object of the present invention to provide a linear motor
having drive coils energized in accordance with signals derived from motor
piston position and acceleration.
It is also an object of the present invention to provide a linear motor
adaptable for use in a variety of applications such as a pump or
compressor wherein a working fluid is transported through the motor by
pistons whose position and velocity are controlled by the motor.
It is also an object of the present invention to provide a linear motor
that is adaptable for efficient use in many applications heretofore
inappropriate for prior art linear motors.
These and other objects of the present invention will become apparent to
those skilled in the art as the description thereof proceeds.
SUMMARY OF THE INVENTION
Briefly, in accordance with one embodiment chosen for illustration, a
linear pump or compressor is provided with a plurality of drive coils
arranged circumferentially about a cylinder with the coils displaced with
respect to each other longitudinally along the cylinder. The coils are
imbedded in a matrix of plastic material which forms the body of a
cylinder to provide a passageway for a piston positioned therein. The
piston is provided with a plurality of arcuate permanent magnets each of
which is permanently magnetized to provide an exposed north and south pole
adjacent the outer surface of the piston. In this manner, the flex
emanating from the permanent magnets is directed perpendicularly to the
cylinder wall.
The drive coils are energized in a sequential manner in accordance with a
control circuit that is utilized to detect several parameters within the
system. The detected parameters include piston position as well as
cylinder pressure. The coils are energized to create force upon the piston
causing the piston to move along the cylinder axis to compress a fluid
therein.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may more readily be described by reference to the
accompanying drawings in which:
FIG. 1 is a schematic illustration of the present invention in the form of
a compressor incorporating a single piston.
FIG. 2 is a cross-sectional configuration of a portion of the cylinder and
piston, slightly modified, shown in FIG. 1.
FIG. 3 is an illustration of suitable permanent magnets for use in the
embodiment of FIGS. 1 and 2.
FIG. 4 is a cross-sectional configuration of another embodiment of a piston
and cylinder, similar to that shown in FIG. 2, incorporating permanent
magnets of different configuration as well as magnetic flux directing iron
plates.
FIG. 4a is a side elevational view of the embodiment shown in FIG. 4.
FIG. 4b is a perspective view of a permanent magnet suitable for use in the
embodiment shown in FIG. 4.
FIG. 5 is a schematic diagram of a suitable circuit for use in the
embodiment of FIG. 1 useful for explaining the operation of the system.
FIG. 6 is a schematic illustration of another embodiment of the linear
motor of the present invention showing the system formed into a torus with
multiple pistons that may be used for pumping or compressing a working
fluid.
FIG. 7 is a schematic illustration of an application of the system of the
present invention showing an embodiment incorporating plural curved and
plural straight sections of the cylinder.
FIG. 8 is another embodiment of the present invention showing the system
used as a linear motor for use in applications to provide
electromechanical displacement to an external apparatus or device attached
to the linear motor.
FIGS. 9a and 9b are schematic representations of pistons and cylinders
constructed in accordance with the prior art and in accordance with the
invention respectively, useful for illustrating the flux pattern
differences between prior art devices and the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, a number of drive coils 1 are arranged along a
central axis so as to describe a cylinder and are then embedded or potted
in a matrix of epoxy, ceramic, or thermoplastic which forms the body of
the device 2. It may be noted that the cross-section of the "cylinder"
need not be circular. That is, there may be applications wherein an
elliptical, rectangular, or even an asymmetrical cross-section would be
appropriate. Therefore, as used herein, the term "cylindrical" or
"cylinder" means a chamber having any of those cross-sections or any other
cross-sectional configuration perpendicular to a defined axis. A thin,
non-structural cylinder liner 3 of non-magnetic, non-conductive material
such as Teflon.RTM., ceramic material or Mylar.RTM. is used to form a
smooth inner surface for sealing against the piston rings (not shown in
FIG. 1) and to provide electrical insulation between the piston 4 and the
drive coils 1. The piston and cylinder form an annular gap; magnets or
conductive coils 5 within the piston interact with the magnetic field
produced by currents flowing in the drive coils 1 in such a way as to
produce forces acting on the piston 5 which cause the piston to move.
Intake 6 and exhaust 7 valves are located in end caps or heads 8 which are
affixed to the body of the compressor. These heads contain inlet and
outlet ports 10, 11 which direct a working fluid to the appropriate
location. Alternately, as is well understood in the prior art relating to
compressors, valves may be installed in the piston.
The logic control circuitry 12, comprises a microprocessor and other
electronic devices as appropriate to the specific application. This
control circuitry receives input from a variety of sensors 13 which may
include piston position sensors, thermostats, pressure transducers, remote
computers, and timers. The microprocessor performs calculations based on
algorithms appropriate to the specific application and delivers current
pulses to selected drive coils 1 in such a way as to produce the desired
piston motion. Electrical energy is drawn from the power supply 14 which
may be line current, battery, or other source of electricity.
FIG. 2 is a detailed cross-sectional view of the pump body 2 and piston 4
showing how permanent magnets 5 may be arranged in order to maximize air
gap flux density and magnetic linkage with the drive coils 1. In this
illustration soft iron filings 15 are shown mixed into the epoxy potting
medium. The drive coils 1 have been energized while the potting medium
cured, causing the filings to align in the optimum configuration to
concentrate the magnetic flux produced by the drive coils into the annular
air gap formed between the cylinder liner and the magnets or conductive
coils 5. Also shown in this view are reinforcing circumferential 16 and
axial (or longitudinal) fibers 17 of glass, Kelvar.RTM., or other
appropriate material. Near either end of the piston, grooves such as that
shown at 18 may be formed into the body of the piston to retain a seal or
piston ring 19. A further detail shown in this view is the presence of a
"back" iron 20 (a soft iron strap) which may be incorporated into the
design to furnish a flux path when separate magnets ar used to provide
North and South poles as shown.
FIG. 3 shows a detail of how permanent magnets used in the piston may be
formed. Each individual magnet is made in the form o an arc and radially
magnetized; that is, the outer face 21 carries one pole and the inner face
22 carries the opposite pole. A number of such arc shaped magnets
sufficient to encircle the piston are set into a groove formed in the
piston as shown in FIG. 2. Two such sets of magnets with opposite
orientation along with a back iron formed into a mating arc is set into
each groove. An appropriate number of such sets is placed in the groove to
fill the circumferential extent of the groove. Neither permanent magnets
nor back irons should form an electrically continuous ring; such
continuity would allow eddy currents to be generated when the drive coils
7 are pulsed. To minimize such eddy currents, a thin spacer such as that
shown at 22a electrically insulates adjacent sets of magnets.
FIGS. 4 and 4a show an alternate embodiment in which thin iron plates 23 of
soft iron are used to direct magnetic flux, as is well understood in the
prior art relating to electric motors. However, in the present invention,
such iron plates extend longitudinally of the cylinder and extend radially
outwardly from the cylinder's inner surface. In this illustration the iron
plates have been left to protrude outside of the potting medium 24 so that
they may be used to radiate excess heat. Here again, circumferential
reinforcing fibers 25 are used, but in this case longitudinal
reinforcement is provided by the iron plates 23. Also shown in this
embodiment are permanent magnets 26 with a horseshoe or "U" shaped
cross-section and magnetization. These permanent magnets are also shown in
FIG. 4b. In this case no "back" iron is required. It is to be understood
that these magnets have essentially the same arc shape as the combination
of magnets and back iron described in FIG. 3.
FIG. 5 is a schematic diagram of a suitable electronic control for use in
the system of the present invention when the system is applied to fluid
pump environments such as compressor applications. A power supply 27
accepts AC line current and provides low voltage DC output for the
microprocessor 36 and a high voltage DC output to power the drive coils
28. When an electrical pulse is applied to a drive coil 28 through a power
transistor 29 (or other power switching device) voltage sensor 30 and
current sensor 31 measure the instantaneous voltage across and the
instantaneous current through the coil; the microprocessor 36 uses this
information to compute the voltage-current lag and hence the total
inductance of the drive coil self-inductance and the mutual inductance
between the drive coil and the piston at the instant of measurement. Since
the measurement of inductance in this manner is a function of piston
position, the position and the rate of change of position (velocity) is
readily calculated. High pressure side and low pressure side temperature
sensors 32 and pressure sensors 33 as well as an ambient temperature
sensor 34 and control thermostat 35 feed information to the microprocessor
36 which uses this information to determine the optimum speed and stroke
length for the compressor. Additional interfaces are provided for
controlling fans 37, valves 38, and warning devices 39. Alternately, a
measurement of the current required to move the piston may be used to
calculate the pressure.
FIG. 6 illustrates an embodiment in which the straight cylinder previously
described has been formed into a torus. The configuration offers
advantages for the movement of fluids in that flow is continuous and in a
uniform direction. The essential elements of the drive coils 40, pump body
41, cylinder liner 42, pistons 43, permanent magnets or coils 44 and logic
control circuitry 45 are all unchanged except for minor differences in
geometries. No distinction is made between inlet and outlet ports because
the direction of flow is dependent only upon the direction of piston
motion, and may easily be reversed by changing the timing of the
electrical pulses supplied to the drive coils 40 The pump will work
equally well in either direction. Pumping is accomplished by varying the
speed with which the pistons 43 are caused to travel through various
sections of the torus by the currents in the drive coils 40. In order to
induce flow in the direction indicated by the arrows in FIG. 6, one of the
pistons 43a is accelerated rapidly through a section of the toroidal
cylinder 46a, creating a partial vacuum and drawing in the working fluid
through port 47a. Velocity is maintained through sections 46b.
Deceleration of the piston occurs in section 46c of the toroidal cylinder
and as the piston approaches the position shown in 43c the working fluid
in front of the piston is forced out through port 47b. As the originally
designated piston reaches the position shown in 43d it is decelerated by
the action of the drive coils 40 to a near stop, and the following piston,
now at or near the position shown in 43c assists i- forcing the working
fluid which the originally designated piston drew in at port 47a out of
the pump body through port 47b. At least two pistons are required in this
embodiment, but smoother pumping action may be accomplished by using more
pistons.
FIG. 7 shows an embodiment of the present invention as a distributed force
pump of arbitrary length. Again, the essential features of radially
oriented magnetic poles in the piston, close magnetic coupling, and
microprocessor control remain the same as that described above, and only
the treatment of the ends of the pump body are altered. Here the pump body
comprises a combination o.+-.adjacent straight 48 and curved 49 sections
extending along a desired path and closing on itself. A number of pistons
similar in design to those previously disclosed move within the pump
according to the electrical pulses supplied by the control circuitry to
the drive coils. Inlet 50 and outlet 5 of the pump may be as shown in FIG.
6. The operation of the embodiment shown in FIG. 7 is similar to that
described in connection with FIG. 6. The low pressure inlet such as the
inlet 50 supplies the fluid to the system at low pressure; since several
pistons may be utilized in the embodiment of FIG. 7 between the inlet 50
and outlet 51, the pressure of the fluid can be significantly increased
since the pressure drop across any one piston is only an increment of the
pressure drop between the inlet and the outlet. Thus, the disadvantages of
the utilization of a single piston within a cylinder to compress a fluid
from a preselected minimum pressure to a predetermined high pressure are
avoided.
FIG. 8 illustrates an embodiment of the present invention for use as a
linear motor. In this instance the piston 52 is connected to a drive rod
53 having a means of attachment 54 suitable for the particular
application. The motor body 55 is also fitted with an attachment means 56.
While rings are shown in this illustration it will be obvious that any
suitable attachment means, such as threaded fittings, plates or weldments
may also be used. A simple electromagnetic brake 57 or other means may be
used as desired to prevent unwanted motion when the drive coils 58 are not
energized.
Referring now to FIG. 9a, a schematic representation of a prior art linear
motor is shown. A cylinder 58 and drive coils 59, enclose a piston 62
having a permanent magnet 64 disposed therein. The permanent magnet, or in
some instances ferromagnetic material without permanent magnetism, is
generally aligned longitudinally with respect to the cylinder and the
piston. The lines of flux 65 generally emanate from the ends of the piston
62 as shown. In contrast, referring to FIG. 9b, the piston 67 of the
present invention is provided with the radially magnetized permanent
magnets 69 as shown and described in connection with FIGS. 4, 4a and 4b.
It may be seen that the lines of flux 70 are primarily radially directed
perpendicular to the annular gap between the piston 67 and cylinder 71;
further, it may be seen that the flux concentration is significantly
greater in FIG. 9b as a result of the close proximity of the permanent
magnet pole faces to the cylinder inner surface. It may also be s-en that
the ability of the present invention to concentrate magnetic flux
perpendicularly through the annular gap between the cylinder and piston
greatly increases the efficacy of the interaction of the flux emanating
from the piston and the flux provided by the drive coils. Therefore,
greater force is available using the technique of the present invention.
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