U.S. patent application number 10/305789 was filed with the patent office on 2004-05-27 for moving coil linear actuator.
This patent application is currently assigned to LEVRAM MEDICAL SYSTEMS, LTD.. Invention is credited to Livshitz, Leonid M., Vaknin, Yuval.
Application Number | 20040100344 10/305789 |
Document ID | / |
Family ID | 32312189 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040100344 |
Kind Code |
A1 |
Livshitz, Leonid M. ; et
al. |
May 27, 2004 |
MOVING COIL LINEAR ACTUATOR
Abstract
A moving coil linear actuator has a bobbin or coil wound as
close packed turns of rectangular - cross section wire and radially
polarized circumferentially continuous ring magnets which are
provided with flux focusing cladding. The result is a light-weight
high power linear actuator.
Inventors: |
Livshitz, Leonid M.;
(Nesher, IL) ; Vaknin, Yuval; (Nesher,
IL) |
Correspondence
Address: |
THE FIRM OF KARL F ROSS
5676 RIVERDALE AVENUE
PO BOX 900
RIVERDALE (BRONX)
NY
10471-0900
US
|
Assignee: |
LEVRAM MEDICAL SYSTEMS,
LTD.
|
Family ID: |
32312189 |
Appl. No.: |
10/305789 |
Filed: |
November 27, 2002 |
Current U.S.
Class: |
335/220 |
Current CPC
Class: |
H01F 7/066 20130101;
H01F 7/081 20130101; H02K 41/0356 20130101; H04R 9/00 20130101 |
Class at
Publication: |
335/220 |
International
Class: |
H01F 007/08 |
Claims
We claim:
1. A moving linear coil actuator comprising: a ferromagnetic core;
a coil axially movable on said core and surrounding said core, said
coil being comprised of a multiplicity of closely packed turns of a
rectangular cross section conductor; at least one radially
polarized circumferentially continuous ring magnet surrounding said
coil and magnetically interacting with said coil upon electrical
energization thereof to produce a force axially displacing said
coil; a ferromagnetic sleeve open at one axial end, receiving said
ring magnet, and connected to said core at an opposite axial end;
and a flux-confining cladding on said magnet and extending over an
end of said magnet at said one axial end.
2. The moving linear coil actuator defined in claim 1 wherein a
plurality of said radially polarized circumferentially continuous
ring magnets are provided around said coil in said sleeve.
3. The moving linear coil actuator defined in claim 2 wherein said
ferromagnetic sleeve and said core are composed of vanadium
permendur.
4. The moving linear coil actuator defined in claim 3 wherein said
coil has a packing factor ratio of volume of said conductor to
total volume of at least 80%.
5. The moving linear coil actuator defined in claim 4 wherein said
conductor is copper.
6. The moving linear coil actuator defined in claim 5 wherein said
packing factor ratio is about 85%.
7. The moving linear coil actuator defined in claim 5 wherein said
cladding is magnetic and is poled transversely to the polarization
of said magnet.
8. The moving linear coil actuator defined in claim 1 wherein said
ferromagnetic sleeve and said core are composed of vanadium
permendur.
9. The moving linear coil actuator defined in claim 1 wherein said
coil has a packing factor ratio of volume of said conductor to
total volume of at least 80%.
10. The moving linear coil actuator defined in claim 1 wherein said
conductor is copper.
11. The moving linear coil actuator defined in claim 1 wherein said
packing factor ratio is about 85%.
12. The moving linear coil actuator defined in claim 1 wherein said
cladding is magnetic and is poled transversely to the polarization
of said magnet.
Description
FIELD OF THE INVENTION
[0001] Our present invention relates to a moving coil linear
actuator and, more particularly, to a linear actuator of the type
which utilizes an electromagnetic force generated by the
interaction of a coil with a field of a permanent magnet.
BACKGROUND OF THE INVENTION
[0002] Moving coil actuators, also termed "voice coil actuators",
can be utilized to provide direct drive in control systems as
servomotors with linear characteristics and for the most part are
traditional servo devices since the force which is produced by the
interaction of the coil and the field of the permanent magnet is
proportional to the current applied to the coil. Such devices can
also be referred to as two-terminal devices since the current is
applied to the two conductors of a coil and the direction of
movement of the coil is a function of the direction of current flow
through the latter. Such devices can be used in applications
requiring high acceleration, high frequency actuation and a flat
force versus displacement characteristic.
[0003] The moving coil linear actuator is a direct-drive linear
motor and utilizes the principles described in "The Straight
Attraction", Part One, Tony Morcos, Motion Control June 2000, pages
29-33, "The Straight Attraction", Part Two, Tony Morcos,
July/August 2000, Motion Control, pages 24-28, U.S. Pat. No.
5,345,206, U.S. Pat. No. 4,808,955.
[0004] In all of these systems, the moving coil linear actuator may
suffer from a wasting of air gap space because of a low packing
factor of the conductive material of the coil, a greater weight of
the unit than may be necessary, problems with flux leakage at an
open end of the coil and flux irregularity which may result because
of segmentation of the magnets as will be described in greater
detail hereinafter.
OBJECTS OF THE INVENTION
[0005] It is the principal object of the present invention to
provide a moving coil linear actuator which is free from at least
some of the drawbacks of earlier actuators and has the advantage
thereover of being of reduced weight and greater magnetic flux
efficiency.
[0006] Another object of the invention is to provide an improved
moving coil actuator in which there is better utilization of the
air gap within the actuator than has been the case heretofore.
[0007] Another object of this invention is to provide a moving coil
linear actuator which has all of the advantages of prior art
actuators but is of reduced weight and improved response in terms
of the utilization of the magnetic field.
SUMMARY OF THE INVENTION
[0008] These objects and others which will become apparent
hereinafter are attained, in accordance with the invention by
providing a flux cladding at the open end of the coil which reduces
flux leakage and, in addition, by increasing the conductor density
of the coil.
[0009] More particularly, the moving coil actuator of the invention
can comprise:
[0010] a ferromagnetic core;
[0011] a coil axially movable on the core and surrounding the core,
the coil being comprised of a multiplicity of closely packed turns
of a rectangular cross section conductor;
[0012] at least one radially polarized circumferentially continuous
ring magnet surrounding the coil and magnetically interacting with
the coil upon electrical energization thereof to produce a force
axially displacing the coil;
[0013] a ferromagnetic sleeve open at one axial end, receiving the
ring magnet, and connected to the core at an opposite axial end;
and
[0014] a flux-confining cladding on the magnet and extending over
an end of the magnet at the one axial end.
[0015] According to the invention, therefore, the coil provides a
conductor of rectangular cross section, which, when wound in a
close packed relationship of the turns, can have a packing factor
(ratio of conductive material to total coil volume) which is at
least 80% and can be as great as 85% or more (preferably at least
90%). Of course, the usual conductor is copper.
[0016] It has been found to be advantageous to provide a plurality
of the radially polarized circumferentially continuous ring magnets
in axially-spaced relationship in the sleeve or even axially
contiguous with one another. The circumferential continuity of the
ring magnets ensures that flux leakage, as can occur with segmented
magnets, is eliminated. The cladding itself for the coil can be
magnetic and poled transversely to the polarization of the ring
magnet.
[0017] It has been found to be advantageous, moreover, to make the
core and sleeve assembly of a high permeability ferromagnetic
material such as vanadium permadur.
[0018] The moving coil linear actuator of the invention thus has
the advantage that there is a uniform magnetic field in the air
gaps because of the whole ring radial magnetization and flux
cladding at the open end. The shape of the core assembly, generated
by computer as described below, results in a substantial reduction
of weight and hence a reduced need for expensive high-permeability
ferromagnetic material such as the vanadium permendur.
[0019] The packing factor is also high, as noted, being 85% or
better.
[0020] The flux focusing which results from the cladding allows
radially magnetized magnets which are embedded in the interior of
the ferromagnetic sleeve or cylinder, to have an optimum thickness
which, in accordance with the graphs provided below, will produce
maximum force per unit of ohmic power dissipated in the coil and
optimal magnet volume. The magnetic field intensity as a result of
flux focussing increases away from the magnets and for a fixed
power dissipation, the current density decreases with the winding
thickness.
[0021] The inner core of the ferromagnetic material is joined at
the closed end to the permanent magnet assembly of the sleeve.
BRIEF DESCRIPTION OF THE DRAWING
[0022] The above and other objects, features, and advantages will
become more readily apparent from the following description,
reference being made to the accompanying drawing in which:
[0023] FIG. 1A is a cross sectional view showing one prior art
moving coil linear actuator;
[0024] FIG. 1B is an axial section through a second prior art
moving coil linear actuator;
[0025] FIG. 1C is an axial section through a third moving coil
actuator of the prior art;
[0026] FIG. 1D is an axial section through still a fourth
embodiment of a conventional moving coil linear actuator;
[0027] FIG. 2 is a partly sectioned perspective view of a moving
coil actuator according to the invention;
[0028] FIG. 3 is an axial cross section through this moving coil
actuator;
[0029] FIG. 3A is an axial section through the coil and its bobbin
in the case of a bobbin composed of synthetic resin to reduce eddy
currents;
[0030] FIG. 3B is an axial view of one of the magnet rings for the
system of FIGS. 2 and 3 showing the radial magnetization
thereof;
[0031] FIG. 4A is a graph of the magnetic induction in the air gap
versus the magnetic thickness for inner, middle and outer portions
of the air gap;
[0032] FIG. 4B is a graph of the air gap versus the magnet
thickness;
[0033] FIG. 5A is a graph of force variation with magnet thickness
of a magnet ring or corresponding air gap thickness;
[0034] FIG. 5B is a graph of the variation of the force with the
thickness of the magnet ring under the constraint of constant ohmic
power dissipation, an optimum point being at a magnet thickness of
substantially 0.95 cm;
[0035] FIG. 6A is a graph of coil weight as a function of magnet
thickness;
[0036] FIG. 6B is a graph of coil power dissipation under a
constant current condition;
[0037] FIG. 7 is a diagram illustrating flux leakage at an air gap
contained by transversely magnetized cladding, H.sub.c and H.sub.m
representing the coercivity of the cladding magnet and the main
magnet, respectively;
[0038] FIG. 8 is a graph showing magnetic field flow confinement by
a cladding magnet (FEMlab 2.2 simulation), showing magnetic flux
redirected inward;
[0039] FIG. 9 is a graph showing magnetic field flow (FEMlab 2.2
simulation) without the cladding magnet so that the magnetic flux
leakage through the magnet face is evident; and
[0040] FIG. 10 is a FEM simulation of an optimally designed soft
magnetic core in the case where two radially magnetized magnetic
rings are present.
SPECIFIC DESCRIPTION
[0041] FIGS. 1A-1D show various prior art moving coil linear
actuators. In each of these cases, there is a soft iron core 10
which receives the coil 11 of round wire so that the coil can move
and is coupled to a member which may be actuated by that coil. The
soft ferromagnetic sleeve 12 which surrounds the coil is joined to
the core at the back 13 of the device to form a magnetic circuit.
The actuator includes a magnet such as the magnet 14, made up of a
plurality of angularly-spaced radially magnetized segments as in
FIGS. 1A and 1B, or axially magnetized magnet 15 of FIG. 1C and
axially magnetized segments 16 of FIG. 1D. All of these
two-terminal noncommutated devices can be used in linear
applications requiring high acceleration, high frequency actuation
and flat force versus displacement characteristics.
[0042] They have, however, in common the following drawbacks:
[0043] 1. Nonuniform air gap flux density distribution due to flux
leakage at an open end of the coil and performance degradation
depending upon the number of magnet segments and their spacing.
[0044] 2. They require large amounts of soft magnetic material
which contributes to the high weight of the motor.
[0045] 3. There is a waste of air gap space because of the space
around the circular cross section wires of the coil.
[0046] The linear actuator of the invention, by contrast, comprises
a coil 20, wound on a bobbin 21 from square section wire (see FIG.
3A), and thus with a greater density of copper in the cross section
of the coil, the bobbin being slidable on the core 22 of soft
ferromagnetic material which has a sleeve 23 connected to that core
22 by a back 24 of the actuator. A hole 25 in the back prevents an
air plug from forming in the linear actuator.
[0047] The bobbin 21 can also be formed with a plate 26, likewise
provided with a hole 27 and serving to impart motion to a load.
Members 26 and 21 may be formed in one piece from metal but to
minimize eddy currents may be formed of a synthetic resin material
as shown for the bobbin 21' in FIG. 3A, if desired.
[0048] As can be seen from FIGS. 2 and 3, moreover, the coil 20
cooperates with fixed radially polarized ring magnets 28, 29, 30 .
. . stacked axially adjacent one another and one of which has been
shown at 28 in FIG. 3B where the radial polarization has been
indicated by the arrows B. The thicknesses of these magnets can be
determined from, for example, an optimizing curve as in FIG. 5B,
for example. Of course a single radially magnetized ring magnet may
be used as well.
[0049] The open face of the assembly of the radially polarized
magnets is closed by a cladding magnet 31 which is polarized
transversely to the main magnets 28-30, etc., i.e. in an axial
direction as represented by the arrow 32 in FIG. 3. The effect of
the cladding magnet has been demonstrated in FIGS. 7-9.
[0050] While the specific configuration of the ferromagnetic core
22-24 is not critical, it has been found to be advantageous and
thus, while the inner surface of the sleeve 23 and the outer
surface of the inner member 22 are both cylindrical, the outer
surface of member 23 and the inner wall of member 22 can converge
toward the open side of the chamber 33 in which the bobbin and the
ring magnets are received.
[0051] The two terminals or conductors of the coil have been shown
at 34 and 35 in FIG. 3A and when the coil is energized, depending
upon the polarity of the energizing source, the coil bobbin will
move axially to operate a load as has been described. The advantage
of this design includes:
[0052] 1. A uniform magnetic field in the air gap as a result of
the radial magnetization of the main magnets coupled with the
magnetic cladding of the open end (compare FIGS. 8 and ).
[0053] 2. The assembly permits computer generation of the shape of
the core to ensure a uniform magnetic flux density distribution,
reduced weight and saving in the cost of highly permeable
ferromagnetic material.
[0054] 3. A high packing factor or ratio of copper to total air gap
volume of say at least 90%.
[0055] FIGS. 5A, SB, 6A and 6B show that the linear actuator of the
invention can permit optimization in terms of the coil weight and
magnet thickness. Because of the flux focussing, radially
magnetized magnets have an optimum thickness. Using the graphs of
FIGS. 5A, 5B, 6A and 6B it is possible to find the optimal
thickness of the coil windings to produce a maximum force per unit
of ohmic power dissipated in the armature winding and an optimal
magnetic ring thickness for a given magnetic material volume.
[0056] Since the windings are in the air gap, increasing the
thickness of the windings increases the effective air gap and hence
decreases the magnitude of the air gap flux density. Due to the
effect of flux focusing the magnetic field intensity increases away
from the magnet. For fixed power dissipation in the windings, the
current density decreases with the thickness of the windings.
[0057] As noted, FIGS. 4A shows the magnetic induction B in the air
gap for a magnetic mass which is held constant at various radii of
the air gap. FIG. 4B shows the air gap as a function of the
thickness of the ring magnet.
[0058] FIG. 5A shows the variation of the force with the thickness
of the magnetic ring or the corresponding air gap thickness while
FIG. 5B portrays variation of the force with the thickness of the
magnet ring under the constraint of a constant ohmic power
dissipation. FIG. 6A represents the coil weight as a function of
magnet thickness while FIG. 6B shows the coil power dissipation at
constant current. It may be noted, referring again to FIGS. 7-9
that one of the disadvantages of flux leakage (FIG. 9) is that
sensitive components located outside the actuator may be
detrimentally effected by such leakage.
[0059] Whole ring magnet production has some constraints imposed on
the ratio between inner and outer ring diameters
(Di/Do.apprxeq.0.75-0.95). In this case, two thinner rings can be
used as depicted in FIG. 10. Magnet is rare-earth Neodimium Boron
Ferrum whole ring magnet, with radial magnetization such as
anisotropic magnet NeoMag 42-16, from Magnequench, or HS-38 AR
Hitachi.
[0060] The core carries very little flux near the front and has
little reluctance. The flux-density and the reluctance per unit
length build up is deeper in toward the back of the actuator Using
FEM software we developed an optimally design core, i.e., flux
density distribution is uniform providing significantly reduced
overall actuator weight mass. The core has such form that the steel
mass increases linearly with distance inward (see FIG. 2). Vanadium
Permedur or 400 stainless steel can be used for the core.
[0061] Polymer plastics with high thermal conductivity can be used
for bobbin construction. Such plastics are supplied by Quadrant or
can be thermoplastics from TBA Electro Conductive Products, as
alternatives to aluminum, and will greatly reduce or completely
eliminate eddy currents.
[0062] Rectangular cross section wire (available in Mitsubishi
Cable Industries, LTD Mexel wire) with a ratio of width to
thickness of about 1.5 to 2.5 given packing ratios of more than
90%. Copper (or aluminum, to reduce weight) wire with rectangular
cross section give high "packing factors" (the ratio of copper to
total coil volume).
* * * * *