U.S. patent number 3,889,219 [Application Number 05/412,012] was granted by the patent office on 1975-06-10 for solenoid actuator with magnetic latching.
This patent grant is currently assigned to Fluid Devices Limited. Invention is credited to Donald Alexander Larner.
United States Patent |
3,889,219 |
Larner |
June 10, 1975 |
Solenoid actuator with magnetic latching
Abstract
It is known to use a radially-magnetised annular permanent
magnet for magnetic latching in a moving armature electromagnetic
actuator of the pulse-operated type. I provide a composite annular
permanent magnet formed of at least two individual, preferably
substantially sector-shaped, permanent magnets which are magnetised
so as to have one pole face radially inwards of the other pole
face.
Inventors: |
Larner; Donald Alexander
(Kingston-upon-Thames, EN) |
Assignee: |
Fluid Devices Limited
(Kingston-upon-Thames, Surrey, EN)
|
Family
ID: |
10456667 |
Appl.
No.: |
05/412,012 |
Filed: |
November 1, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Nov 2, 1972 [GB] |
|
|
50624/72 |
|
Current U.S.
Class: |
335/234;
335/253 |
Current CPC
Class: |
F16K
35/16 (20130101); F16K 31/082 (20130101) |
Current International
Class: |
F16K
31/08 (20060101); F16K 35/00 (20060101); F16K
35/16 (20060101); H01f 007/08 () |
Field of
Search: |
;335/229,230,234,253 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Harris; G.
Claims
I claim:
1. An electromagnetic actuator comprising:
an armature displaceable between first and second axially spaced
positions;
a pair of spaced pole-pieces defining said first and second axial
positions to which said armature is displaceable, each of said pair
of spaced pole-pieces being disposed at one of said first and
second axially spaced positions to limit the axial displacement of
said armature, said armature and one of said pair of spaced
pole-pieces forming a low reluctance magnetic path at each of said
first and second axially spaced positions;
winding means for developing, when energized, lines of flux in said
axial direction for displacing said armature between said first and
second axially spaced positions; and
a plurality of permanent magnet means forming an annular array for
latching said armature in said first and second axially spaced
positions, each of said plurality of permanent magnet means being
disposed in a plane which is transverse to said axial direction and
exhibiting a magnetic polarity which is transverse to said axial
direction, and each of said plurality of permanent magnet means
being separated from an adjacent one of said plurality of permanent
magnet means by a plane along said axial direction and extending
radially outward therefrom.
2. An actuator as claimed in claim 1, wherein each of said
plurality of permanent magnet means is magnetised parallel to the
mean radius of said magnet.
3. An actuator as claimed in claim 1 further comprising radial webs
separating and holding in position each of said plurality of
permanent magnet means.
4. An actuator as claimed in claim 1 and comprising a bobbin upon
which is wound said winding means and which mounts said plurality
of permanent magnet means forming said annular array.
5. An actuator as claimed in claim 4, wherein said bobbin comprises
two portions, each portion having an individual winding of said
winding means wound thereon, said portions being spaced apart
axially and interconnected by radially-extending webs, and
individual ones of said plurality of permanent magnet means being
disposed between said bobbin portions and retained in position by
said webs.
6. An actuator as claimed in claim 4 additionally comprising
simple, releasable fixing means for removably securing said bobbin
on said actuator.
7. An actuator as claimed in claim 1 further comprising a
non-ferromagnetic tube disposed intermediate said spaced
pole-pieces, said spaced pole-pieces being formed of ferromagnetic
material, closing the ends of the tube and defining therein an
enclosed space, said armature being disposed in said space and
movable between the two end positions whereat said armature abuts a
respective one of said pole-pieces and forms part of a
substantially closed magnetic circuit, said plurality of permanent
magnet means and said winding means being stationary and disposed
outside said tube; and a ferromagnetic outer tube surrounding said
winding means and said plurality of permanent magnet means, said
ferromagnetic pole-pieces additionally closing the ends of said
ferromagnetic outer tube.
8. An actuator as claimed in claim 1 wherein said winding means
includes two separate windings, said windings being axially spaced
in a symetrical manner about said annular array formed by said
plurality of permanent magnetic means and mounted in a coaxial
relationship therewith, the actuator further comprising means for
applying current to said windings for the passage of equal currents
through each said winding, whereby induced in said plurality of
permanent magnet means due to said windings substantially
cancel.
9. An actuator as claimed in claim 1 wherein said plurality of
permanent magnet means are each configured substantially in the
form of a sector.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic actuator
comprising a moving armature, at least one winding for electrical
energisation and moving the armature, and an annular permanent
magnet for providing magnetic latching, the permanent magnet being
magnetised so as to have one pole face radially inwards of the
other pole face. The annular magnet will normally extend over the
path of movement of the centre of the armature, the armature being
movable between two end positions. The latching effect is
particularly useful with pulse-operated type actuators, and causes
the armature to be positively retained in the end position it has
reached until such time as the energisation of the winding(s) is
reversed; this enables the actuator to be actuated by a short
current pulse without requiring continuous energisation to retain
the armature in its proper position.
FIG. 1 of British Pat. No. 1,089,596 discloses such an
electromagnetic actuator.
A high coercivity magnet should be used for the annular magnet as
the annular magnet must resist the coercive effect of the actuator
windings, and this magnet can for instance be made of ceramic
material. The magnet is difficult to fabricate and difficult to
magnetise. A central pole-piece (usually in the form of a mandrel)
must be used to obtain the radial magnetising field required, and
the field strength of the magnetising field must be very high. As
this pole-piece must be of reasonable size to obtain a strong
enough magnetising field, the annular magnet could be made with an
oversize bore, an annular iron packing piece being inserted into
the bore to provide a central bore of suitably small size for the
actuator. However, accurate machining is required to avoid an air
gap between the annular magnet and the iron packing piece; in
addition, in order to obtain sufficient volume of permanently
magnetised material, the annular magnet must be relatively long or
of large diameter.
The permanent magnet could be used for a purpose other than that of
providing latching, for instance as described in relation to FIG. 1
of German Pat. No. 2,013,051.
It is desirable to provide an annular permanent magnet which is
easy to fabricate and to magnetise without using an excessive
amount of magnetised material.
According to the present invention, the annular permanent magnet is
a composite magnet formed of at least two individual magnets which
are magnetised so as to have one pole face radially inwards of the
other pole face.
Although the individual magnets can be for instance bar magnets
arranged radially around the axis of movement of the armature, the
individual magnets are preferably substantially in the shape of
sectors.
The magnetised sectors can be cheap to make and magnetise. Each
individual sector can be formed for instance by a sintering
process, and the effective radial thickness of the composite magnet
can be easily determined by grinding or otherwise machining the
inner arcuate surface of each sector in a jig which locates on the
outside diameter (or outer arcuate surface) of the sector; the
grinding can be performed using a formed peripheral grinding wheel
with the sectors clamped in line. It is possible also to grind or
otherwise machine the outside surfaces of the sectors if so
desired, either as an alternative to machining the inside surfaces,
or in addition to such machining; machining the inside, however,
can result in the removal of less material for the same effect.
As no iron packing piece is required, the necessary magnetised
volume can be obtained while keeping the composite magnet
relatively short; for instance, its length (axial thickness) can be
1/8 to 1/2 of its internal diameter, preferably about 1/2 of its
internal diameter. Its external diameter can be from 11/2 times to
4 times its internal diameter, and is preferably about double its
internal diameter.
During magnetisation, the pole-piece adjacent the radially inner
surface can be as large as is desired, within normal practical
limits. The magnetisation of each individual permanent magnet can
be parallel to the mean radius of the individual magnet, but it is
possible to obtain more nearly radial magnetisation; near radial
magnetisation is more desirable the smaller the number of
individual magnets, but in general terms, the field of the
composite magnet does not have to be strictly uniform as long as it
is generally radial.
There may be only two individual permanent magnets, which may be
each approximately half annular, but there are preferably three or
more individual permanent magnets, four individual magnets being
found to be a useful number which can give the advantages of the
invention without requiring too many parts to fabricate, magnetise
and assemble; nonetheless, more than four permanent magnets may be
used.
The individual magnets (preferably sectors) forming the composite
magnet need not be in contact with each other and can be separated
by radial webs which hold the magnets in position. In a convenient
arrangement, the winding is or windings are wound on a bobbin which
also mounts the magnets. The bobbin can thus have two portions on
which two respective windings are wound, the portions being spaced
apart axially and interconnected by radially-extending webs; the
magnets are positioned between the bobbin portions and retained by
the webs. An outer sleeve can surround the bobbin and hold the
magnets in position. The bobbin can be arranged such that it can be
removed from the actuator by releasing a simple, releasable fixing
means, such as a coaxial nut, for easy removal of the bobbin,
winding(s) and magnets.
Preferably, the winding(s) and the armature are coaxial with the
path of movement of the armature and the individual permanent
magnets are symmetrically arranged about the axis of the winding(s)
and armature. This enables all the main parts (with the exception
of the individual magnets and the separating webs) to be coaxial
solids of rotation and enables the actuator to be cheap to
manufacture.
The armature preferably moves in a space which is limited at each
end by stationary ferromagnetic pole-pieces and whose sides are
limited by a non-ferromagnetic tube interconnecting the
pole-pieces, the composite annular magnet and the winding(s) being
stationary and outside the tube, a ferromagnetic tube (acting as a
magnetic yoke) surrounding the winding(s) and the composite annular
magnet, and the ends of the ferromagnetic tube being closed by
ferromagnetic end pieces. This is a useful constructional
arrangement. In addition, by having the permanent magnets
stationary, i.e., fixed, they are not subject to mechanical shock
during the operation and there is thus considerably less tendency
to demagnetise, or, if they are sintered, to break up.
The armature is preferably movable between two end positions, in
each of which it abuts a third magnetic pole-piece and forms part
of a substantially closed magnetic circuit. By having the magnetic
circuit substantially closed, there is very little reluctance and a
greater latching force. This reduces the susceptibility of the
actuator to shocks of the kind causing it to change over when
de-energised.
There are preferably two axially-spaced, coaxial windings with the
composite annular magnet coaxially therebetween, the windings being
dimensioned and electrically interconnected so that both are
energised for change-over of the actuator and so that the induced
fluxes in the composite annular magnet substantially cancel each
other out, and this is most simply arranged by having the windings
equi-spaced from the composite annular magnet and of the same size,
and passing the same current through each winding. In this way,
there can be little or no induced flux through the individual
permanent magnets, and thus there is less tendency to de-energize
the individual permanent magnets.
The actuator can in particular be used for actuating a valve such
as a pilot valve, and the valve may be a valve as described and
claimed in U.S. Pat. No. 3,760,843 filed Feb. 1, 1972.
The armature can if desired be a movable valve member, for instance
carrying rubber seats, forming an electromagnetically-actuated
valve.
The actuator may be in the form of a bistable,
electromagnetically-actuated valve comprising three pressure fluid
connections and two valve orifices connected to two respective
pressure fluid connections, the armature being movable between two
end positions such that a first said pressure fluid connection is
connected to the third said pressure fluid connection when the
armature is at one end position, and the second said pressure fluid
connection is connected to the third pressure fluid connection when
the armature is at the other end position. This bistable,
electromagnetically-actuated valve provides a very simple bistable
valve, particularly for pilot valve use, and although it is
preferred that the annular permanent magnet is a composite annular
permanent magnet, this need not necessarily be so.
In a preferred embodiment, the bistable valve has an enclosed space
in which the armature moves, the third connection being permanently
connected to the enclosed space, the first and second valve
orifices being at opposite ends of the enclosed space and the
armature carrying, e.g., rubber valve seats for closing the
respective valve orifices. One of the end faces of the enclosed
space may have a hole therein, a tube being mounted in the hole and
spaced from at least one side thereof to leave a gap outside the
tube, and the third pressure fluid connection being connected to
the gap, the opening in the end of the tube providing a said valve
orifice; the hole and tube are preferably coaxial with the enclosed
space, the gap being an annular gap. This provides a simple
solution for connecting the third pressure fluid connection to the
enclosed space while providing a relatively large flow passage
which does not occupy too much of the end face or pole-piece area.
An alternative arrangement is to have a central bore in the end
face to provide a said valve orifice and a parallel bore in the end
face to connect the enclosed space with the third connection, the
latter bore preferably leading into an annular groove in the end
face.
However, in another embodiment, the armature may be connected by a
mechanical linkage such as a push rod to a double-acting valve
member.
In order to ensure good communication between the first or second
pressure fluid connections and the third pressure fluid connection
by way of the respective valve orifice, the armature preferably has
at least one radial slot in at least one of its end faces and at
least one longitudinal slot down its peripheral surface, the
longitudinal slot(s) intersecting the (respective) radial
slot(s).
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described, by way of example, with
reference to the accompanying drawings, which show two actuators in
accordance with the invention, and of which:
FIG. 1 is an elevation of a first actuator, partly in axial section
along the line I--I of FIG. 2;
FIG. 2 is a horizontal section along the line II--II of FIG. 1, but
on a smaller scale;
FIG. 3 is a circuit diagram showing, schematically, one method of
operation of the actuator;
FIG. 4 is a circuit diagram showing, schematically, another method
of operation of the actuator;
FIG. 5 is an elevation of a second actuator, partly in axial
section; and
FIG. 6 is an isometric projection, on a larger scale, of the
armature of the second actuator.
DETAILED DESCRIPTION OF FIRST PREFERRED EMBODIMENT
In the first actuator (FIGS. 1 and 2), a freely slidable armature 1
is enclosed in a tube 6 of dia-magnetic material, the latter being
attached to stationary pole-pieces 2 and 3 and sealed thereto by O
seals 4 so that the actuator can be of the "wet armature" type,
i.e., in fluid communication with a valve being controlled by the
actuator. The movement of the armature 1 is communicated to an
associated valve or other mechanism by a thrust pin 5, also of
dia-magnetic material. A baseplate 7 adapts the assembly for
attachment to the carcase of the associated control valve or other
mechanism. The associated valve may be a valve as described with
reference to the drawings of U.S.A. Pat. application Ser. No.
225,569.
Surrounding the tube and pole-piece assembly is a steel enclosure
formed of an outer sleeve or tubular part 8 and two end pieces 9.
Contained therein is a coil bobbin 10 which carries two stationary
electrical windings 11a and 11b contained in separate parts of the
bobbin which are nevertheless united as a matter of constructional
convenience by four webs or ribs 12. Sandwiched between the two
parts of the bobbin and separated and held by the webs 12 are four
stationary sector-shaped, ceramic, individual permanent magnets 13
which are magnetised along the N-S axes indicated in FIG. 2, thus
generating a total magnetic field in a plane radial to the axis of
the assembly. The magnets 13 are fabricated and magnetised as
described above.
An electrical connector 14 carries terminal pins 15 and an earth
pin 16, for the purpose of connecting the electrical windings to an
external control circuit and providing for electrical safety even
when the coil assembly is removed by hand.
The coil assembly is easily removable for exchange or maintenance
by unscrewing a retaining nut 17, and is also rotatable to any
alternative position around the axis to achieve the most convenient
position for the electrical connector 14.
Insulated cavities 18 formed in the connector 14 provide
accommodation for electrical rectifiers, which are desirable as
integral features where the actuator is to be employed in
conjunction with an alternating electrical supply.
Referring to FIG. 1, if there is no electrical excitation and the
armature is in the upper position shown, the permanent flux
produced by the permanent magnets 13 circulates around the two main
paths shown with double and single arrows; the air gap 20 being
closed, the reluctance of the path with double arrows in FIG. 3 is
relatively low, and this is therefore the preferred path and most
of the flux passes this way. The flux following the path with
single arrows is relatively weak because of the presence of an air
gap 19. The armature 1 is therefore firmly held against the upper
pole-piece 2 by the resultant net attraction thereto.
FIG. 1 also shows a flux which follows a path with triple headed
arrows, right around the actuator; this flux is induced when the
windings 11a and 11b are suitably energised with direct current.
This opposes the double-arrowed holding flux, and complements the
single-arrowed flux, thus eventually impelling the armature 1
downwards to the opposite pole-piece 3. It is believed that two
equal and opposite fluxes are induced in the permanent magnets 13,
giving no net induced flux. This not only avoids any tendency to
demagnetise the permanent magnets 13, but also avoids consumption
of extra power -- extra power would be needed to induce in the
permanent magnets 13 a flux opposing their own flux. When the
excitation is removed, the armature 1 remains in the lower position
because the lower flux is now much stronger than that passing via
the upper pole-piece 2.
Reversal of the armature 1 is later accomplished by exciting the
windings 11a and 11b with reverse polarity, thus inducing a flux of
opposite sense to the triple-arrowed flux.
It has been found that the actuator described above requires a very
low wattage. In an actuator whose height (bottom of baseplate 7 to
top of retaining nut 17) was 45 mm, the power level to switch was 1
watt while the attaching force (force holding the armature 1 to the
respective stationary pole-piece 2 or 3) was 800 grammes.
The previous description of the method of operation of the actuator
supposes that the two windings are connected electrically so that
they are excited simultaneously, and produce magnetomotive forces
which are complementary along the axis of the actuator. Such a
simple arrangement is not inconvenient for use with alternating
current; a suitable arrangement of rectifiers 21 used in this case
facilitates reversal of the current, as shown in FIG. 3. The
rectifiers 21 can be positioned in the cavities 18.
In order to effect reversal on direct current, however, a double
pole changeover switch or an equivalent array of switching means
must be used, which is not very convenient for normal industrial
control purposes. Practical experiment has shown that the device
will work quite satisfactorily if the two windings 11a and 11b are
energised separately, using one only for each sense of movement,
and using it to generate an additional flux across the open air gap
only. Such an arrangement is shown in FIG. 4. With this simpler
arrangement, the power consumed is rather high by comparison with
that first described; however, if this higher loss is tolerable,
the greater simplicity of construction and operation are
desirable.
DETAILED DESCRIPTION OF SECOND PREFERRED EMBODIMENT
In the second actuator (FIGS. 5 and 6), the tube and pole-piece
assembly 8, 9, the bobbin 10, the windings 11a and 11b, the
permanent magnets 13, the electrical connector 14 and the retaining
nut 17, as well as the tube 6 and O seals 4, are exactly as
described with reference to FIGS. 1 and 2.
However, the actuator is formed as a bistable pilot valve, and the
enclosed space within the tube 6 has respective pole-pieces 31 and
32, which are formed with respective valve orifices. The pole-piece
31 has a valve orifice 33 formed by a bore in a central projection
in the pole-piece 31, the bore communicating with a first fluid
pressure connection 34. The pole-piece 32 has an axial bore
containing a coaxial tube 35. The tube is preferably formed of a
ferromagnetic material, but need not be. The lower end portion 36
of the tube 35 is of larger diameter than the remainder, and makes
an interference fit in the hole in the pole-piece 32, the tube 35
being held in position by any suitable adhesive. The top end of the
tube forms a valve orifice, and this valve orifice is connected to
a second pressure fluid connection 37 by way of the interior of the
tube 35 and a transverse bore 38. Around the tube 35, between the
tube 35 and the internal wall of the bore in the pole-piece 32,
there is an annular duct 39 which communicates with a third
pressure fluid connection 40 by way of a transverse bore 41.
The armature 42 has a stepped, central bore containing moulded-in
rubber 43 providing a valve seat at each end of the armature 42. As
shown in FIG. 6, the armature 42 has any suitable number of radial
slots 44 in its end faces and longitudinal slots 45 along its
peripheral surface, the longitudinal slots intersecting respective
radial slots. In this way, good communication is provided between
the annular duct 39 and the enclosed space in which the armature 42
moves, which is particularly important when the armature 42 is in
its lower end position.
* * * * *