 |
Claims  |
|
|
What is claimed is:
1. An electromagnetic actuator, comprising:
a permanent magnet structure mounted on a shaft, the shaft being mounted at
each end within a housing constructed of material having high magnetic
permeability and open at both ends along the axis of the shaft, and
wherein the permanent magnet structure is magnetized in an axial
direction;
a plurality of coils would around a non magnetic bobbin mounted coaxially
within the housing, the magnetic structure being situated within the
bobbin with each pole of the magnet in close proximity to a coil arranged
so that when the coils are energized by an electrical input signal an
axial force is applied to the permanent magnet structure in accordance
with the polarity and amplitude of the input signal;
nonmagnetic endcaps at each end of the housing within which are mounted an
end of the shaft, the endcaps keeping the magnetic field of the permanent
magnet structure directed more perpendicularly toward the coils;
wherein the coils are electrically connected together so that adjacent
coils produce opposing magnetic fields when energized to thereby present a
low inductive load to an electrical input signal;
wherein the coils are located adjacent to each pole of the permanent magnet
structure, the dimensions of the coils and permanent magnetic structure
being such that the magnetic flux impinging on the coils from the
permanent magnet structure remains relatively constant as the magnet
structure moves in an axial direction; and
wherein said permanent magnet structure is a magnet stack which contains
one or more coaxially positioned permanent magnets with pole plates at
each end of the magnet stack and between each magnet.
2. The actuator as set forth in claim 1 wherein the permanent magnet
structure is slidably mounted on the shaft with the latter stationarily
mounted within the housing.
3. The actuator as set forth in claim 1 wherein the shaft is slidably
mounted within the housing with the permanent magnet structure being
stationarily mounted on the shaft.
4. The actuator as set forth in claim 1 further comprising means for
driving the actuator with an input signal so as to oppose unwanted
vibrations.
5. An electromagnetic actuator, comprising:
a cylinder mounted coaxially on a shaft mounted at each end within a
housing constructed of material having high magnetic permeability and open
at both ends along the axis of the shaft;
a plurality of coils wound around the cylinder;
an annular permanent magnet corresponding to each coil mounted coaxially on
the inner periphery of the housing and magnetized in a radial direction
with each permanent magnet positioned in close proximity to a coil and
arranged so that when the coils are energized
by an electrical input signal an axial force is applied to the coils in
accordance with the polarity and amplitude of the input signal;
nonmagnetic endcaps at each end of the housing within which are mounted an
end of the shaft, the endcaps keeping the magnetic field of the annular
permanent magnets directed more perpendicularly toward the coils;
wherein the coils are electrically connected together so that adjacent
coils produce opposing magnetic fields when energized to thereby present a
low inductive load to an electrical input signal; and,
wherein an annular magnet is located adjacent to each of the coils, the
dimensions of the coils and annular magnets being such that the magnetic
flux impinging on the coils from the annular magnets remains relatively
constant as the coils move in an axial direction.
6. The actuator as set forth in claim 5 wherein the cylinder is slidably
mounted on the shaft with the latter stationarily mounted within the
housing.
7. The actuator as set forth in claim 5 wherein the shaft is slidably
mounted within the housing with the cylinder being stationarily mounted on
the shaft.
8. The actuator as set forth in claim 5 further comprising a non-magnetic
core occupying the center of the cylinder and with the field coils being
separated from each other on the cylinder by a non-magnetic material.
9. The actuator as set forth in claim 5 further comprising means for
driving the actuator with an input signal so as to oppose unwanted
vibrations.
10. An electromagnetic actuator for active vibration control applications,
comprising:
a housing adapted to be mounted between a vibrating surface and a
nonvibrating surface;
a moving element slidably mounted within the housing;
means for causing the moving element to undergo reciprocal linear motion in
a direction perpendicular to the two surfaces in response to an electrical
input signal;
means for sensing the vibrations of the vibrating surface and generating
the electrical input signal fed to the motion causing means in a manner so
as to reduce the sensed vibrations; and,
wherein the motion of the moving element creates reaction forces on the
housing due to the mass of the moving element and conservation of momentum
which can be used to reduce the vibrations of the vibrating surface.
11. The actuator as set forth in claim 10 further comprising means for
mounting the actuator between a vibrating surface and a non vibrating
surface.
12. The actuator as set forth in claim 10 further comprising springs within
the housing and acting on the moving element so as to alter the natural
frequency of vibration of the moving element and thereby also alter the
frequency response to the actuator.
13. A system for actively cancelling primary vibrations emanating from a
vibration source by generating secondary vibrations, comprising:
means for sensing residual vibrations remaining after the interaction of
the primary and secondary vibrations;
means for generating an electrical signal in accordance with a signal
received from the vibrations sensing means in order to drive an actuator
in such a manner so as to reduce the sensed vibrations; and,
an electromagnetic actuator for producing secondary vibrations comprising:
a permanent magnet structure mounted on a shaft, the shaft being mounted at
each end within a housing constructed of material having high magnetic
permeability and open at both ends along the axis of the shaft, and
wherein the permanent magnet structure is magnetized in an axial
direction;
a plurality of coils wound around a non magnetic bobbin mounted coaxially
within the housing, the magnetic structure being situated within the
bobbin with each pole of the magnet in close proximity to a coil arranged
so that when the coils are energized by an electrical input signal an
axial force is applied to the permanent magnet structure in accordance
with the polarity and amplitude of the input signal;
nonmagnetic endcaps at each end of the housing within which are mounted an
end of the shaft, the endcaps keeping the magnetic field of the permanent
magnet structure directed more perpendicularly toward the coils;
wherein the coils are electrically connected together so that adjacent
coils produce opposing magnetic fields when energized to thereby present a
low inductive load to an electrical input signal; and,
wherein coils are located adjacent to each pole of the permanent magnet
structure, the dimensions of the coils and permanent magnetic structure
being such that the magnetic flux impinging on the coils from the
permanent magnet structure remains relatively constant as the magnet
structure moves in an axial direction.
14. A system for actively cancelling primary vibrations emanating from a
vibration source by generating secondary vibrations, comprising:
means for sensing residual vibrations remaining after the interaction of
the primary and secondary vibrations;
means for generating an electrical signal in accordance with a signal
received from the vibrations sensing means in order to drive the actuator
in such a manner so as to reduce the sensed vibrations; and,
an electromagnetic actuator for producing secondary vibrations, comprising:
a cylinder mounted coaxially on a shaft mounted at each end within a
housing constructed of material having high magnetic permeability and open
at both ends along the axis of the shaft;
a plurality of coils wound around the cylinder;
an annular permanent magnet corresponding to each coil mounted coaxially on
the inner periphery of the housing and magnetized in a radial direction
with each permanent magnet positioned in close proximity to a coil and
arranged so that when the coils are energized by an electrical input
signal an axial force is applied to coils in accordance with the amplitude
and polarity of the input signal;
nonmagnetic endcaps at each end of the housing within which are mounted an
end of the shaft, the endcaps keeping the magnetic field of the annular
permanent magnets directed more perpendicularly toward the coils;
wherein the coils are electrically connected together so that adjacent
coils produce opposing magnetic fields when energized to thereby present a
low inductive load to an electrical input signal; and,
wherein an annular magnet is located adjacent to each of the coils, the
dimensions of the coils and annular magnets being such that the magnetic
flux impinging on the coils from the annular magnets remains relatively
constant as the coils move in an axial direction. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
FIELD OF THE INVENTION
The present invention relates to linear actuators generally and, in
particular, actuators suitable for the use in an active vibration control
system.
BACKGROUND
Numerous methods and techniques have been devised in the past to alleviate
the problem of unwanted vibrations produced by such sources as vibrating
machinery. Although such vibrations cannot usually be completely
eliminated by any means, any attenuation of the vibrations is desirable in
order to both lessen their destructive effects and increase human comfort.
The earliest attempts to control vibration represent what may be termed
"passive" vibration control systems. In such a system, energy damping
elements or materials are arranged so as to absorb the unwanted vibrations
and dissipate them as heat. Sound insulation and automobile
shock-absorbers are two common examples. Other passive vibration control
systems include energy-storing elements such as springs which present a
reactive load to the vibrating source. For example, large motors are
sometimes mounted on springs which have a spring constant chosen in
accordance with the frequency at which the motor is known to vibrate. Such
springs, by storing and re-emitting the vibrational energy, essentially
serve as mechanical filters to lessen the vibrational force transmitted to
the surface upon which the motor is mounted.
More recently, improved systems for vibration control have appeared which
may be termed "active" vibration control systems. Such systems utilize an
actuator for producing vibrations to cancel out the unwanted vibrations,
the actuator being under the control of a feedback control system with a
vibration sensing means for deriving the error signal used to drive the
actuator. Examples of such active systems are found in the U.S. Pat. Nos.
4,566,118 and 4,490,841, the disclosures of which are hereby incorporated
by reference.
It is imperative in active vibration control systems that the actuator be
capable of producing vibrations in strict accordance with the input signal
used to drive the actuator. That is, the preferred relationship between
the input signal and the vibrational force produced by the actuator should
be a linear one. One type of electromagnetic actuator which is very
suitable for vibration control applications is the moving coil type of
arrangement commonly used to drive loudspeakers. In such an actuator, an
input current is applied to a solenoidal coil which is subjected to a
constant magnetic field directed perpendicularly to the direction of the
current within the coil. The coil thus experiences a force which is
proportional to the input current and directed perpendicularly to the
direction of both the magnetic field and current. In the case of
loudspeakers, the moving coil (a.k.a. the voice coil) is attached to a
diaphragm for producing sound waves in accordance with the input signal.
In the case of a motor or actuator, on the other hand, the moving coil is
mounted around a slidably mounted shaft which undergoes reciprocating
motion in correspondence to the input current. A variation of the moving
coil type of actuator involves the placement of one or more stationary
coils around a slidably mounted shaft upon which is mounted a permanent
magnet. The N-S pole of the magnet is coaxial with the slidably mounted
shaft and the surrounding coil. Applying current to the stationary coil
causes a magnetic field to be produced which moves the shaft in one
direction or the other with a force proportional to the input current
applied to the coil.
As aforesaid, for active vibration control applications, an actuator needs
to respond in as linear a fashion as possible to its driving input signal.
To obtain such a linear response, several design considerations arise.
First, the force experienced by the moving element (i.e., the moving coil
or moving permanent magnet) should be the same for a given amount of input
current irrespective of the moving element's position along its line of
travel. This means that ideally the magnetic field produced by the
permanent magnet should be uniform all along the stroke of the actuator.
Second, the coil current should follow the input voltage signal as closely
as possible. The coil to which the input voltage signal is applied,
however, is an inductive load which means that the coil current cannot
change instantaneously in response to a change in the input signal
voltage. Rather, the coil current responds with a time constant
proportional to the inductance of the coil. For linear operation, this
time constant should be as small as possible which means the inductance
should be minimized.
Third, physical law says that a current-carrying conductor subjected to a
constant magnetic field experiences a force proportional to the magnitude
of the current. In order to increase the efficiency of the actuator, a
suitable flux path should be provided which concentrates the constant
magnetic field produced by the permanent magnet and causes it to be
directed perpendicularly toward the coil. This maximizes the force
experienced by the coil (or magnet in the case of a moving magnet type
actuator) for a given amount of coil current and permanent magnet
strength.
The current which flows through the actuator coil, however, itself produces
a magnetic field which changes the field produced by the permanent magnet.
In order to maintain the proportionality between input current and force,
the change of the field produced by the magnet should be eliminated,
resulting in a force per unit of current which is independent of the
amount and direction of the current.
Several prior devices described in the literature represent attempts to
partially solve the problems mentioned above. For example, U.S. Pat. No.
4,692,999, issued to Frandsen, describes an electromagnetic actuator of
the moving coil variety which uses dual coils and permanent magnets in
order to more nearly maintain the uniformity of the magnetic field
irrespective of coil position. Japanese Patent Application No. W081/02501
discloses an electromagnetic transducer wherein a compensating coil is
mounted coaxially with the moving coil in order to produce a magnetic
field in opposition to that produced by the moving coil. U.S. Pat. No.
3,202,886, issued to Kramer, discloses an actuator of the moving permanent
magnetic type for two-position operation which makes use of two oppositely
energized coils. None of these references, however, teach or suggest an
actuator which overcomes the aforementioned problems to the extent
necessary for active vibration control applications.
SUMMARY OF THE INVENTION
The present invention is an electromagnetic actuator for achieving the
design objectives enumerated above. In a first embodiment, the actuator
comprises a permanent magnet structure mounted on a shaft which is
situated within a magnetically permeable housing which is open at both
ends along the axis of the shaft. The shaft is mounted at each end within
nonmagnetic endcaps mounted at each open end of the housing. The permanent
magnet structure is magnetized in the axial direction. Two or more field
coils are wound around a non-magnetic bobbin mounted coaxially within the
housing. The coils are electrically connected so that, when driven by an
input current, the coils produce magnetic fields in opposition to one
another. The resulting forces cause the permanent magnet structure to be
driven in one direction or the other, depending upon the polarity of the
input current. Each pole of the permanent magnet structure is placed in
close proximity to a field coil so as to leave a small magnetic air gap.
The housing, with its nonmagnetic endcaps, provides a flux path for the
magnet structure which concentrates the field and causes it to be directed
perpendicularly within the air gap toward the field coils.
In a second embodiment, a plurality of field coils are wound around a
magnetically permeable cylinder mounted coaxially on a shaft. The shaft is
mounted within a magnetically permeable housing which is open at both ends
along the axis of the shaft, with the shaft mounted within nonmagnetic
endcaps as in the first embodiment. Mounted on the inner periphery of the
housing are annular permanent magnets corresponding to each field coil and
magnetized in the radial direction. As in the first embodiment, the field
coils are electrically connected so that, when driven by an input current,
the coils produce opposing magnetic fields. The coils and shaft are moved
in one direction or the other depending upon the polarity of the input
current. The housing, with its nonmagnetic endcaps, and cylinder provide a
flux path which serves to concentrate and direct perpendicularly the
permanent magnet field within the air gap between the cylinder and the
housing, with the field coils situated within that air gap.
Either of the embodiments described above is particularly suitable for
active vibration control when the housing of the actuator is mounted
stationarily between a vibrating and a non-vibrating surface. The actuator
is then driven with an input current signal so as to oppose the vibrations
of the vibrating surface by means of a feedback control system. When the
actuator shaft is connected to the vibrating surface, the operating force
is directly applied to that surface. When the actuator shaft is left free
to slide in reciprocal fashion, on the other hand, the mass reaction force
opposes the vibrations. If the total mass mounted on (or integral with)
the actuator shaft within the housing is large enough, the reciprocating
motion of the moving element causes a significant reaction force on the
actuator housing due to conservation of momentum. This reaction force is
then applied to the surfaces in a manner which tends to prevent the
transmission of vibrations from the vibrating to the non-vibrating
surface.
Other objects, features, and advantages of the invention will become
evident in light of the following detailed description considered in
conjunction with the referenced drawings of a preferred exemplary
embodiment according to the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of a moving magnet embodiment.
FIG. 2 is a sectional view of the embodiment shown in FIG. 1.
FIG. 3 is an exploded view of a moving coil embodiment.
FIG. 4 is a sectional view of the embodiment shown in FIG. 3.
FIG. 5 shows an embodiment of the invention particularly suitable for
vibration control.
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIGS. 1 and 2, there is shown an embodiment of the
present invention which is of the moving magnet type. A permanent magnet
structure or armature M1 is mounted on an actuator shaft S1 and magnetized
in an axial direction. The shaft S1 is slidably mounted at its ends within
a magnetically permeable housing H1 which is open at both ends along the
axis of the shaft S1 The armature M1 is a magnet stack which contains one
or more coaxially positioned permanent magnets and two or more pole plates
PP1. A plate PP1 is placed between each magnet and at either end of the
magnet stack. In case of more than one magnet, the magnets are stacked on
the shaft S1 in such a way that equal magnetic poles face each other. The
armature M1 in this embodiment is cylindrical with a circular
cross-section but could be of any arbitrary shape.
Two or more solenoidal coils C1 are wound on a non magnetic bobbin B1
mounted within the housing H1. The coils C1 are spaced from each other in
an axial direction so as to form rings on the bobbin B1. The inside
diameter fits around the magnet structure M1 with some minimal diametrical
clearance so as to allow free axial motion of the armature structure M1.
The outside diameter of the bobbin B1 fits snugly inside the cylindrical
steel housing H1. The coils C1 are electrically connected with each other
in such a way that an electrical current flowing through adjacent coils C1
will create opposing magnetic fields.
The open ends of the cylindrical outer housing H1 are closed off with non
magnetic endcaps 15 at each end with a sleeve bearing 20 in the center of
each endcap 15. The housing H1 is made of magnetically soft steel while
the endcaps 15 are made of aluminum or other non-magnetic material. The
bearings 20 guide the actuator shaft S1 and center the armature M1. When
an electrical input current is applied to the coils C1, the armature M1 is
subjected to reciprocal forces in accordance with the input current
signal.
The magnetically permeable housing H1, nonmagnetic endcaps 15, and
nonmagnetic bobbin B1 cause the field produced by the magnet stack to be
concentrated and directed more perpendicularly toward the coils C1.
Referring next to FIGS. 3 and 4, there is shown another embodiment of the
present invention which is of the moving coil type. Two or more annular
magnet segments M2 are attached on the inside of an outer housing H2 made
of highly magnetically permeable material such as magnetically soft steel.
The housing H2 is closed at each end with non-magnetic endcaps 15. The
magnets M2 are magnetized in a radial direction. Adjacent magnets M2 have
their N-S poles oriented in opposite directions.
Two or more field coils C2 are wound on a thin walled magnetic steel
cylinder 25. A non-magnetic core 26 occupies the center of the cylinder
25. The coils C2 are spaced from each other with a non-magnetic material
27. An actuator shaft S2 is mounted in the center of the core 27 and shaft
S2 is slidably mounted within the housing H2 by means of sleeve bearings
20 in each endcap 15. The coils C2 are electrically connected so that an
input current applied to the coils creates opposing magnet fields in each
adjacent coil C2 which also causes the field coils C2 to undergo
reciprocal motion in accordance with the input signal. The housing H1
cylinder 25, and nonmagnetic endcaps 15 provide a magnetic flux path for
the magnets M2 which concentrates and directs perpendicularly their fields
toward the coils C2.
In either of the embodiments described above, by enclosing the magnets with
a material having high magnetic permeability (i.e., housing H1 or H2),
most of the magnetic flux is guided through the magnetic gap in which the
coils C1 or C2 are placed. Only very little flux will leak past this gap,
thus creating a highly efficient electromagnetic actuator. In the case of
the first embodiment, all the flux in the pole plates PP1 is guided
through the coils. In the second embodiment, the cylinder 25 provides a
low reluctance pathway for the flux produced by the magnets M2. In both
embodiments, the nonmagnetic endcaps 15 help to direct the permanent
magnet flux perpendicularly toward the coils C1 or C2.
Each of the embodiments of the present invention also provides an actuator
which presents a low inductance load to the electrical input signal. Since
the change in input current due to a change in the input voltage signal
occurs with a time constant proportional to this inductance, reducing this
inductance results in a more linear relationship between the input signal
and the force applied to the moving element. The inductance is reduced by
electrically connecting adjacent coils in series so as to create opposing
magnetic fields. An inductive coupling between adjacent coils occurs due
to a linkage of magnetic fields created when a current runs through the
coils. When the direction of the current in two adjacent coils is the
same, the resulting magnetic fields are added and the total inductance of
the combination is double the inductance of a single coil. When the
currents run in opposite directions, the induced magnetic fields oppose
each other which reduces the total inductance. Since the electrical time
constant is proportional to the total inductance, it follows that the time
constant is also thereby reduced. This effective reduction in total
inductance is enhanced the better is the flux linkage between the coils.
The above-described embodiments achieve a high degree of flux linkage
between adjacent coils by providing a low reluctance pathway inside the
coils.
The embodiments described above also provide an actuator where the force on
the moving element (i.e., either the coil or the magnet) due to a given
input current is independent of the position of the moving element along
its stroke path. This is achieved in the first embodiment by providing
coils C1 adjacent to each pole of the magnets making up the armature M1.
Similarly, in the second embodiment, an annular magnet M2 is provided
adjacent each of the coils C2. By properly manipulating the dimensional
relations of the coils and the pole plates the magnetic flux flowing
through the coils can be made constant over the length of the moving
element's stroke.
Referring next to FIG. 5, there is shown an embodiment of the present
invention which is especially suitable for active vibration control
applications. The actuator shown is of the moving magnet type as described
earlier with reference to FIGS. 1 and 2. Accordingly, the actuator
comprises a housing H3 having mounted there within field coils C3
electrically connected so as to create opposing magnetic fields when an
electrical input signal is applied to the coils. The magnetic armature M3
is magnetized in the axial direction so that the magnetic fields produced
by the current-carrying coils C3 produce a force on the armature M3. As in
the earlier embodiment, the housing H3 and pole plates PP3 provide a low
reluctance pathway for the magnetic flux. In this embodiment, however, the
shaft S3 is mounted stationarily within the housing H3 while the armature
M3 is slidably mounted on the shaft. The armature M3 thus undergoes
reciprocal motion in accordance with the input current applied to the
coils C3 and is unimpeded by any mechanical load other than minimal
frictional resistance. The housing H3 is shown as being stationarily
mounted between a vibration source VS and a mounting surface MS. The
actuator is physically connected to the vibration source by means of
amount 50 made of rubber or similar material attached to a metal plate 51.
| | |
