Permanent magnet bistable solenoid actuator

A bistable actuator comprising a permanent magnet assembly secured to an armature shaft and a pair of core elements axially disposed on either side of the permanent magnet assembly. The cores have axially opposed inner and outer annular extensions defined in each core by a central axial opening which supports the armature shaft and an annular recess in which is received an electrical coil. The permanent magnet assembly comprises inner and outer annular axially magnetized permanent magnets radially spaced by a ferromagnetic ring so as to be aligned with the inner and outer core extensions.

This invention relates to a solenoid actuator and more particularly to an 
actuator having permanent magnets for maintaining the armature thereof in 
either of two positions. 
Actuators of the above type are generally referred to as bistable actuators 
since the armature is stable in either of its positions without the 
consumption of externally supplied electrical power. Power is only 
consumed in moving the armature from one position to the other. Although 
such actuators would seem ideal in applications where low power 
consumption is particularly important, they usually suffer from slow 
response and limited armature travel. 
To improve the armature travel aspect, it has been proposed, as for example 
in the U.S. Pat. No. 3,202,886, to Kramer, issued Aug. 24, 1965, to 
configure the armature and core such that armature movement is effected in 
response to a combination of repulsive and attractive magnetic forces. The 
repulsive force is dominant during initial armature movement and the 
attractive force becomes dominant as the armature approaches its final or 
target position. However, such an effect has not been achieved in the 
prior art without worsening the actuator speed of response. 
The object of the present invention is to provide an improved bistable 
actuator having an armature and core geometry that results in relatively 
fast speed of response as well as relatively large armature travel. 
The core of the actuator is comprised of two axially spaced ferromagnetic 
elements having inner and outer annular extensions defined in each element 
by a central axial opening for receiving the armature shaft and an axial 
annular recess in which an electrical coil is mounted. The armature 
assembly comprises inner and outer axially magnetized rare-earth ring 
magnets of opposite polarity which are radially spaced by a ferromagnetic 
ring and attached to the armature shaft for axial movement therewith. The 
width of the ring magnets corresponds to the width of the core element 
annular extensions, and the ferromagnetic ring spaces the ring magnets so 
that they are axially aligned with the respective annular core element 
extensions. When the armature assembly is in a first position, the ring 
magnets are in close proximity to one of the core elements and magnetic 
flux lines are established through each of the magnets and the 
intermediate ferromagnetic ring which develop a retaining force for 
maintaining the armature assembly in such position. When the armature 
assembly is in the other of its positions, the ring magnets are in close 
proximity to the other core element, and magnetic flux lines are 
established through the magnets and the intermediate ferromagnetic ring 
which develop a retaining force that maintains the armature assembly in 
such position. When the electrical coils of each core element are 
energized with electrical current, the armature shifts position in 
response to the repulsive magnetic force between the permanent magnets and 
the core element in close proximity thereto and the attractive magnetic 
force between the permanent magnets and the opposing core element. 
The novel armature and core configuration of this invention yields improved 
speed of response in two ways. First, the core design, while capable of 
generating attractive and repulsive forces to move the armature is 
amenable to lamination, if desired. As a result, eddy current losses in 
the core are reduced and speed of response is increased. Second, the 
armature assembly is configured to take advantage of the small size and 
high flux density properties of the rare-earth magnets. As a result, the 
armature assembly is lightweight and contributes to improved speed of 
response. 
In a preferred embodiment, the magnets are secured to the ferromagnetic 
ring with a suitable adhesive to form an assembly which is then potted 
with epoxy or a similar substance, and encapsulated in a container of 
stainless steel or other nonmagnetic material. The container, in turn, is 
then welded or otherwise secured to the armature shaft for movement 
therewith.

Referring now more particularly to FIGS. 1 and 2, the reference numeral 10 
generally designates the permanent magnet bistable solenoid actuator of 
this invention. Essentially, the actuator 10 comprises two axially spaced 
core elements 12 and 14 disposed within a housing member 16, an armature 
shaft 18 supported for axial movement on a pair of bushings 20 and 22 
disposed within central axial openings 24 and 26 in the core elements 12 
and 14, and a permanent magnet assembly 28 secured to the armature shaft 
18 at a point between the core elements 12 and 14. Leftward movement of 
the armature shaft 18 as shown in FIGS. 1 and 2 is limited by the stop 30 
when engaged by the armature shaft boss 32. Similarly, rightward movement 
of the armature shaft 18 is limited by the stop 34 when engaged by the 
armature shaft boss 36. The relative placement of the stops 30 and 34, the 
bosses 32 and 36, and the permanent magnet assembly 28 is such that the 
permanent magnet assembly is prevented from coming into contact with 
either of the core elements 12 and 14. The armature shaft 18 is rigidly 
secured to or integral with a load device such as the valve 38 which is 
adapted to open or close the port 40 depending on the axial position of 
armature shaft 18. 
The permanent magnet assembly 28 comprises inner and outer axially 
magnetized permanent magnets 42 and 44 of opposite polarity and a 
ferromagnetic ring 46 secured therebetween. The preferred arrangement for 
securing the permanent magnet assembly to the armature shaft 18 is shown 
in FIG. 5. The widths of the magnets 42 and 44 and the ring 46 are such 
that the magnet 44 is in axial alignment with the core element outer 
annular extensions 48 and 50, and the magnet 42 is in axial alignment with 
the core element inner annular extensions 52 and 54. As noted above, the 
magnets 42 and 44 are preferably formed of a high flux density rare-earth 
material such as samarium-cobalt. 
When the armature shaft 18 is in the leftmost (open) position as shown in 
FIG. 1, the permanent magnet assembly 28 is in close proximity to the core 
element 12, and a magnetic flux is generated by magnets 42 and 44 in paths 
through magnets 42 and 44, ferromagnetic ring 46, and the core element 
inner and outer annular extensions 52 and 48 as shown by the lines 56 and 
58. Such flux produces an attractive force between the permanent magnet 
assembly 28 and the core element 12 which opposes any load force urging 
the armature shaft 18 in the other or rightward direction. Similarly, when 
the armature shaft 18 is in the rightmost (closed) position as shown in 
FIG. 2, the permanent magnet assembly 28 is in close proximity to the core 
element 14, and a magnetic flux is generated by magnets 42 and 44 in paths 
through magnets 42 and 44, ferromagnetic ring 46, and the core element 
inner and outer annular extensions 54 and 50 as shown by the lines 60 and 
62. Such flux produces an attractive force between the permanent magnet 
assembly 28 and the core element 14 which opposes any load force urging 
the armature shaft 18 in the other or leftward direction. 
As noted above, the configuration of the core elements 12 and 14 allows 
them to be laminated, thereby reducing eddy current losses and increasing 
the speed of response. The core elements 12 and 14 have complementary 
annular recesses 64 and 66 formed therein for receiving the electrical 
coils 68 and 70, respectively. The coils 68 and 70 are effective when 
concurrently energized with direct current of suitable polarity to develop 
magnetic forces for moving the armature shaft 18 from either of its 
positions to the other position. For example, to move the armature shaft 
18 from its leftmost (open) position depicted in FIG. 1 to its rightmost 
(closed) position depicted in FIG. 2, the coils 68 and 70 are energized 
with current of a polarity that causes the inner annular extensions 52 and 
54 to assume a North (N) magnetic polarity and the outer annular 
extensions 48 and 50 to assume a South (S) magnetic polarity. This 
produces both a repulsive magnetic force between the permanent magnet 
assembly 28 and the core element 12 and an attractive magnetic force 
between the permanent magnet assembly 28 and the core element 14. When the 
permanent magnet assembly 28 is in its leftmost position, the air gap 
between the magnets 42 and 44 and the core element 12 is at a minimum, and 
the air gap between the magnets 42 and 44 and the core element 14 is at a 
maximum. As a result, the repulsive force is at a maximum, and the 
attractive force is at a minimum. As the armature shaft 18 moves 
rightward, the repulsive force decreases and the attractive force 
increases. When the permanent magnet assembly 28 is in its rightmost 
position, the air gap between magnets 42 and 44 and the core element 12 is 
at a maximum, and the air gap between magnets 42 and 44 and core element 
14 is at a minimum. As a result, the repulsive force is at a minimum and 
the attractive force is at a maximum. When the armature shaft 18 reaches 
its new position, the coils 68 and 70 may be de-energized, and the 
permanent magnets 42 and 44 will hold the position as described above in 
reference to FIG. 2. 
Graphic representations of the repulsive and attractive forces described 
above are given in FIGS. 3 and 4. FIGS. 3A-3C show the magnetic flux 
distributions in the permanent magnet assembly 28 and the core elements 12 
and 14 for three different positions of the armature shaft 18. FIG. 3A 
depicts the leftmost position as in FIG. 1, FIG. 3B depicts an 
intermediate position, and FIG. 3C depicts the rightmost position as in 
FIG. 2. 
In FIG. 4, the repulsive and attractive forces are given in Newtons (N) for 
a particular actuator as a function of displacement in millimeters (mm) of 
the armature shaft 18 from a limit position. The repulsive and attractive 
forces are additive and combine to provide a total as depicted by the 
trace 80. 
It will be understood, of course, that the magnetic force characteristics 
described above are developed in equal magnitude and opposite sense when 
the coils 68 and 70 are energized to move the armature shaft 18 from its 
leftmost position shown in FIG. 2 to its rightmost position shown in FIG. 
1. In both cases, the total force acting on the load device 38 is given by 
the trace 80 as a function of armature displacement. 
The preferred construction of the permanent magnet assembly 28 is shown in 
FIG. 5. Essentially, the two magnets 42 and 44 are cemented to the inner 
and outer peripheries respectively, of the ferromagnetic ring 46. That 
assembly, in turn, is encapsulated within a flanged two-piece container 
82, 84 of stainless steel or other nonmagnetic material, and potted in 
epoxy or similar material. The container pieces 82 and 84 are welded 
together at the overlapping portion thereof, as indicated by the reference 
numeral 88, and the container flange 90 in turn is welded to the 
nonmagnetic armature shaft 18. This construction results in a practical 
and rugged assembly, able to withstand repeated cycling operation. 
As noted above, the actuator of this invention provides improved speed of 
response as compared to prior bistable actuators capable of relatively 
large armature travel. The large armature travel characteristic is 
effected by the concurrent generation of attractive and repulsive magnetic 
forces, and the fast speed of response characteristic is effected by an 
armature and core configuration that results in a lightweight armature 
assembly and a low loss magnetic core. 
Although this invention has been described in reference to the illustrated 
embodiment, it will be understood that various modifications will occur to 
those skilled in the art and that actuators incorporating such 
modifications may fall within the scope of this invention, which is 
defined by the appended claims.