Electromagnetic actuator having preloaded spring means

An actuator comprising a coil, an armature moveable along a path toward and away from the coil, and a spring for resiliently urging the armature in one direction along the path. The coil is energizable to urge the armature in the other direction along the path against the biasing action of the spring. The spring rate of the spring progressively increases as the armature is moved by electromagnetic forces. This is accomplished by using an appropriately configured leaf spring and a a ramp for progressively supporting the leaf spring as it is deflected. The leaf spring is also preloaded.

BACKGROUND OF THE INVENTION 
One kind of actuator typically includes an armature, a spring for urging 
the armature in one direction, and electromagnetic means energizable to 
move the armature against the biasing force of the spring. The 
electromagnetic means may include a coil and a core. By repeatedly 
energizing the electromagnetic means, the armature can be driven through 
many cycles of reciprocation each second. Actuators of this type can be 
used to power various devices, such as positive displacement pumps. 
Actuators of this type must necessarily have a short stroke. One problem 
with these actuators is in properly limiting the length of the stroke. The 
obvious way to limit the stroke of the armature as it moves toward the 
coil is with a positive stop. Unfortunately, however, the rapid and 
repeated contact of the armature and/or the associated structure against a 
positive stop produces a noise which makes the actuator unsuited for many 
applications. Moreover, the positive stops tend to wear and, therefore, 
permit an increase in the length of the stroke with time. Overstroking as 
a result of the action of the spring in moving the armature could damage 
the diaphragm. 
SUMMARY OF THE INVENTION 
This invention provides a very quiet actuator which does not require the 
usual positive stops for limiting travel of the armature toward the coil. 
Overstroking as a result of the spring moving the armature is also 
substantially eliminated. Moreover, the actuator uses a very short stroke 
which minimizes inertia problems and facilitates high-speed operation. 
With a short stroke, the armature can be very close to the core of the 
coil where the electromagnetic forces are greatest. The actuator is 
constructed so that tolerances are easily controlled. 
To substantially eliminate noise without employing conventional stops, this 
invention provides for progressively increasing the spring rate of the 
spring as the armature is moved by the coil. By progressively stiffening 
the spring in this manner, the spring serves not only to power the return 
stroke, but also to terminate, or assist in terminating, the stroke 
provided by the force from the coil. This can be advantageously 
accomplished by utilizing a leaf spring and one or more ramps for 
progressively supporting the leaf spring as the leaf spring is deflected 
under the influence of the electromagnetic force. This reduces the 
effective length of the leaf spring as the spring is deflected. 
To further provide for increasing the spring rate as a function of spring 
deflection, the leaf spring may have at least one dimension which 
increases as the leaf spring extends from one end thereof toward a central 
region of the leaf spring. With this construction, the unsupported length 
of the leaf spring progressively widens as the leaf spring is deflected to 
increase the stiffness of the unsupported length of the leaf spring. 
In use, the armature is mounted for movement along a path between first and 
second positions. The spring resiliently urges the armature toward the 
first position, and the electromagnetic means is energizable to apply a 
force to the armature to move the armature along the path to the second 
position against the biasing action of the spring. 
Another feature of this invention is to preload the spring in the first 
position so that the electromagnetic means must provide a force which 
exceeds the preload on the spring before the spring allows the armature to 
move along the path toward the second position. With a preloaded spring, 
the stroke can be relatively short, and the armature can be close to the 
core at all positions so that the electromagnetic force is relatively high 
throughout the full stroke. Preloading the spring also tends to avoid 
overstroking because, upon the return stroke, the spring runs out of 
powering force for the armature the instant the first or preloaded 
position is reached. In addition, preloading the spring provides a higher 
spring force for the return stroke. 
This invention uses the armature to provide the preload. This can be 
accomplished, for example, by providing the armature with a first surface 
having a configuration different from the configuration of the leaf spring 
in its relaxed condition and mounting the armature on the leaf spring with 
the leaf spring being held against the first surface. For example, the 
first surface may be concave. In this event, the leaf spring is deformed 
into the concavity of the armature to preload the leaf spring. 
The actuator of this invention can be used to power various different 
devices, such as a pump. When used with a pump, the electromagnetic force 
and the spring force are used to power the intake and discharge strokes, 
respectively. With this arrangement, if the pump is shut off, the spring 
cannot expand against the force of the liquid being pumped. Consequently, 
the air gap between the armature and the core is minimal, and under these 
conditions, only a small operating current is required and heat loss is at 
a minimum. If the spring powers the intake stroke, the air gap would be 
larger with the pump shut off, and the heat losses would be greater. 
The invention can best be understood by reference to the following 
illustrative description taken in connection with the accompanying 
illustrative drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an apparatus 11 which generally includes an actuator 13 and a 
pump 15 driven by the actuator. The actuator 13 may be used to power many 
different devices, and the pump 15 is merely illustrative. Moreover, the 
actuator 13 may be used to power many different kinds of positive 
displacement pumps. In the embodiment illustrated, the pump 15 includes a 
chamber 17 having a diaphragm 19 reciprocable within the chamber. On the 
intake stroke, the diaphragm 19 draws in a fluid, such as water, through 
an inlet check valve 21, and on the discharge stroke, water is forced out 
of the chamber 17 under pressure through an outlet check valve 23. The 
diaphragm 19 is suitably coupled to the actuator 13 and is reciprocated 
thereby. 
The actuator 13 includes electromagnetic means which, in the embodiment 
illustrated, includes a core 25 and a coil 27 wound on the core. The core 
25 may be, for example, a conventional E laminated core having three legs 
29. The coil 27 can be repeatedly energized by a suitable ac source 31. A 
diode 33 is coupled in series between the ac source 31 and the coil 27 so 
that only half of each cycle of ac is applied to the coil. For example, 
when 60-cycle alternating current is used, the coil 27 is energized sixty 
times each second. 
The core 25 and the coil 27 are suitably mounted within a housing section 
35 which forms a heat sink. The housing section 35 closes one end of the 
apparatus 11 and is releasably coupled to a housing member 37 by threaded 
fasteners 39. The housing member 37 can be advantageously integrally 
molded from a plastic material, and the housing section 35 is preferably 
constructed of metal. 
Also mounted on the fasteners 39 and sandwiched between the housing section 
35 and the housing member 37 are identical ramps 41 and 41a and a leaf 
spring 43. The ramps 41 and 41a lie in a groove 42 of the housing member 
37, and the ends of the leaf spring are held in a fixed position by the 
fasteners 39. An armature 45 of magnetic material is mounted on the leaf 
spring 43 by a screw 47. The core 25 has a cavity 49 to allow the head of 
the screw 47 to move into close proximity to the core. 
The armature 45 in the embodiment illustrated, includes a plurality of 
plates 51 suitably held together as by a pair of rivets 53. The armature 
45 has a concave surface 55 which faces away from the core 25 and which 
forms a segment of a cylinder. 
The threaded end of the screw 47 is received in a coupling 57 which passes 
through an opening 59 in the housing member 37 and attaches to the 
diaphragm 19 in any suitable manner. The inner end of the coupling 57 
bears against the leaf spring 43 so that the leaf spring is deformed into 
conformity with the concave surface 55. In the unstressed condition, the 
leaf spring 43 is planar, and so by deforming the leaf spring as shown in 
FIG. 1, the leaf spring is preloaded. Accordingly, in order to move the 
leaf spring 43 toward the core 25 from the position shown in FIG. 1, a 
force in excess of the preload force on the spring must first be applied 
to the spring. Also, the preload on the leaf spring 43 enables the 
armature 45 to be very close to the core 25 in the position of FIG. 1. For 
example, the stroke of the armature 45 may take it from about 0.050 to 
0.060 inch from the core 25 to about 0.005-0.010 inch from the core 25. 
In the embodiment illustrated, the leaf spring 43 is elongated and has 
openings at its opposite ends through which the threaded fasteners 39 
project to firmly mount the leaf spring at its opposite ends to the 
housing. The leaf spring is resiliently deflectable and so it mounts the 
armature 45 for movement along a linear path toward and away from the core 
25. As shown in FIG. 2, the leaf spring 43 has inclined edges 61 and 63 
adjacent its opposite ends so that the leaf spring progressively widens as 
it extends from its opposite ends toward a central region of the leaf 
spring. The inclined edges 61 and 63 are joined by parallel edges 65. The 
leaf spring 43 may be made of any suitable resilient material, such as 
steel. 
The ramps 41 and 41a are provided for progressively supporting the leaf 
spring 43 as the latter deflects toward the core 25. Portions of the ramp 
41a corresponding to portions of the ramp 41 are designated by 
corresponding reference numerals followed by the letter "a. " The ramp 41 
may be molded from a plastic material and has an opening through which the 
fastener 39 projects to securely mount the ramp. The ramp 41 has a ledge 
67 which houses one end of the leaf spring 43 and an inclined ramp surface 
69 which lies between the leaf spring 43 and the core 25 and which extends 
toward the core 25 as it progresses inwardly. The ledge 67 rests against 
the housing member 37 at the bottom of the groove 42 to position the ramp 
41 on the housing member 37, and the ramp 41 has an upper surface 71 which 
abuts the housing section 35. The ramps 41 and 41a are spaced apart to 
define an opening 73, and the armature 45, in the embodiment illustrated, 
is received in this opening. As a precaution, stops 75 in the form of 
rubber washers may be mounted on the fasteners 39 between the leaf spring 
43 and the housing member 37 to positively limit travel of the spring 43 
in a direction away from the core 25. 
With this construction, the leaf spring 43 mounts the armature 45 for 
movement between a de-energized position shown in FIG. 1 and an energized 
position in which the armature 45 is pulled closer to the core 25. The 
electromagnetic force moves the armature 45 to the energized position 
against the biasing action of the spring 43, and the spring 43 returns the 
armature to the de-energized position when the coil 27 is de-energized. 
Thus, the electromagnetic force and the spring power the intake and 
discharge strokes, respectively, of the diaphragm 19 of the pump 15. 
Because the leaf spring 43 is preloaded, energization of the coil 27 does 
not result in movement of the armature 45 toward the core 25 until the 
preload force on the spring 43 has been overcome by the electromagnetic 
force. Thereafter, the armature 45 moves toward the core 25 against the 
biasing action of the spring 43. However, as the spring 43 deflects to 
allow such movement of the armature, the ramp surfaces 69 and 69a 
progressively support increasing lengths of the leaf spring 43. 
Consequently, the effective length of the leaf spring 43 is progressively 
shortened as the armature 45 moves toward the core 25. This effective 
shortening of the leaf spring stiffens it or increases its spring rate. 
In addition, as best shown in FIG. 2, the opposite end portions of the leaf 
spring 43 progressively widen as they extend toward a central region of 
the leaf spring. Consequently, the effective width of the leaf spring also 
increases as the armature moves toward the core 25. These two factors 
combine to materially increase the spring rate of the spring 43. 
By the time the leaf spring 43 is deflected against the full length of the 
ramp surfaces 69 and 69a, the increased spring force has virtually 
arrested movement of the armature. Additional movement of the spring 
toward the core 25 is essentially prevented because the central region of 
the spring is held against deflection by the concave surface 55, and the 
ramps 41 and 41a retain the end portions of the spring. Thus, the spring 
brings about termination at a precisely known point of movement of the 
armature 45 toward the core 25. In this position, the ramp surfaces 69 and 
69a form a smooth continuation of the concave surface 55. 
When the coil 27 is de-energized, the electromagnetic force is released to 
allow the spring 43 to power the return stroke of the armature. Because 
the spring 43 operates above the preload range applied by the concave 
surface 55 of the armature 45, strong forces are available to power the 
return stroke. In addition, when the initial position shown in FIG. 1 is 
reached, the spring force tending to move the armature 45 away from the 
core 25 instantly terminates so that overstroking is avoided. However, the 
stops 75 can be provided, if desired, as insurance against any 
overstroking that may result due to the relatively small inertia of the 
spring 43 and the armature 45. 
Although an exemplary embodiment of the invention has been shown and 
described, many changes, modifications and substitutions may be made by 
one having ordinary skill in the art without necessarily departing from 
the spirit and scope of this invention.