US 3743898 A
Latching devices which utilize the residual magnetism retained in the relatively soft magnetic materials of the magnetic curcuit for its latching force. The actuators are proportioned so as to have a minimum effective air gap in the actuated position to achieve a high latching force without the use of a permanent magnet in the magnetic circuit. A means is provided for demagnetizing the magnetic circuit to release the actuator from the actuated position. Various configurations are disclosed, including some ideally suited for use with a simple remote switching means.
Claims available in
Description (OCR text may contain errors)
United States Patent 1191 3,743,898 Sturman July 3, 1973 LATCHING ACTUATORS 3,349,356 10/1967 Shinohara 335/266 x 3,126,501 3/1964 Fl 335/254 x [761 invent l l m! 18643 2,375,017 5/1945 M xison 335/230 Kirkcolm Lane, NErtIiridge ICaIif. 9 l 3 24 Primary Examiner-George Harris  Filed; M 27, 1972 Attorney-Spensley, Horn and Lubitz Related US. Application Data Latching devices which utilize the residual magnetism  Continuation-impart of er, N 24,232, March 31 retained in the relatively soft magnetic materials of the 1970, abandoned. magnetic curcuit for its latching force. The actuators are proportioned so as to have a minimum effective air  U.S. Cl 317/154, 335/254, 335/268 gap in the actuated position to achieve a high latching  Int. Cl. H01h 47/04 force without the use of a permanent magnet in the  Field of Search 335/230, 253, 254, magnetic circuit. A means is provided for demagnetiz- 335/256, 266, 268; 317/123, 154 ing the magnetic circuit to release the actuator from I a the actuated position. Various configurations are dis-  References Cited closed, including some ideally suited for use with a sim- UNITED STATES PATENTS ple remote switching means. 3,202,886 8/1965 Kramer 335/254 X 19 Claims, 15 Drawing Figures f 2 1 4 i ii i 1 I i i E\ SCONDAPYCOAL r 1 3 \7 L .J 5 5 I g f g P/P/MA/Q Y 60/4 5 5 i 2 A 1 5 1 14 J 5 I -21 )4 m '-I2 l6 1 I v25 *16 i2. 3 g 2 Z "(I x E 1 a E, 1 1
I 1 P/P/MAPS (0/4 2 ;A r i k 1 1, Siam/049V 06/2 12 1r v T, I i 1 I 52 1 j; K
Patented July 3, 1973 4 Sheets-Sheet l 4 4 m w k w w y m W W m V P r m D R A N A M N w M m E H 2 Z W 5 S P l 2 S 6 m r 2 M N 4 @2 mwwwwmw ro s r! I I 5- M I. m V w m 5 M 1 MAGA/ET/Z/A/G ERCE- 0526/7506 Patented July 3, 1973 3,743,898
4 Sheets-Sheet 5 ISO LATCHING ACTUATORS CONTINUATION IN PART This application is a continuation in part of my application for patent entitled Latching Actuator, Ser. No. 24,282 now abandoned.
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to solenoid actuators of the latching type.
2. Prior Art Presently the great majority of electrical latching devices are solenoids of the on-of type, where current is required continuously in order to keep the armature at the on position. This type of device consumes extremely large amounts of electrical energy whenever the on time is larger than the time of armature movement. This excessive energy must be dissipated as heat, and as a consequence, conventional solenoids are relatively large and heavy.
To overcome the basic limitations of solenoids, various latching devices have been constructed. However, all such devices require energy storing members such as mechanical biasing elements or permanent magnets. Such energy storing members, however, have inherent disadvantages which have prevented general replacement of the conventional solenoid. Those devices utilizing mechanical biasing elements must be supplied with energy prior to the point at which the actuator is ready for operation. This greatly complicates the mechanism and generally necessitates a substantial increase in the size of the latching device.
Some of the permanent magnet types of latching actuators have proved to be more efficient, smaller and lighter than conventional solenoids. Thus, in my application for patent entitled Self-Latching Solenoid Actuator, Ser. No. 153,939, now US. Pat. No. 3,683,239, simple and efficient solenoids of this type, particularly suited for operation through a simple control circuit, are disclosed. However, the cost of incorporating apermanent magnet into the solenoid structure is significant, and if the use of a permanent magnet could be eliminated while still maintaining the simplicity of structure and operation, the manufacturing cost of such devices could be further reduced. Similarly, as shall be subsequently discussed more fully, power consumption in operation may actually be reduced by elimination of permanent magnet.
To more fully highlight the limitations of devices using permanent magnets, it is to be noted that the saturation flux density of permanent magnet materials is substantially lower than the saturation density of typical soft magnetic materials. Furthermore, to efficiently utilize permanent magnet materials, the magnet proportions must be designed so that the permanent magnet operates well down on its demagnetization curve. By way of example, for Alnico V, the maximum external energy is achieved with an operating flux density of the permanent magnet of approximately 8,500 Gauss. Since the saturation flux density of soft magnetic materials which may be used for the other parts ofa solenoid may range upwards of 20,000 Gauss, and since the solenoid force is proportional to BA, where B is the flux density and A is the effective area of the field, maximum solenoid force will be achieved if the solenoid area is substantially less than the cross sectional area of the permanent magnet so as to increase B to the higher saturation density.
Also, it should be noted that a permanent magnet requires a high coercive force to either magnetize or demagnetize the magnet. Thus, for actuation ofa solenoid device having a permanent magnet, the solenoid coil must create a sufficient MMF to create a high flux density, both in the air gap in the magnetic circuit, and in the permanent magnet forming a part of the magnetic circuit. Consequently, to achieve the maximum solenoid force while the air gap in the solenoid is at or near its maximum, a substantially higher MMF is required to be generated by the solenoid coil than in solenoids not having a permanent magnet, thereby increasing the energy input to the coil and the FR loss therein. Also, once the solenoid is actuated and latched the unlatching of the solenoid creates no external mechanical work in that the return of the solenoid moving member to the unactuated position will be accomplished by a return spring (or other means). However, to demagnetize a permanent magnet, a significant electrical energy input to the solenoid coil is required to generate the demagnetizing MMF, thereby causing the expenditure of further electrical energy without obtaining any useful work out of the solenoid.
Since a permanent magnet used in the solenoid should have a cross-sectional area substantially larger than the cross-sectional area of the solenoid air gap, and further, since permanent magnets tend to be brittle, easily chipped, etc., permanent magnets should not be used as the pole face of the solenoid, but instead must be buried" in another part of the magnetic circuit. This characteristically requires a substantial increase in a number of parts making up the magnetic circuit, thereby increasing manufacturing costs, assembly time and tolerance accumulation.
In summary of the above, there is need for a latching actuator of the solenoid type which utilizes a minimum number of parts, fabricatable through mass production methods of inexpensive materials, and readily assembleable without substantial machining to achieve a reliable latching actuator with a high latching force, and which may be easily and efficiently actuated and unlatched in response to an electrical signal.
BRIEF SUMMARY OF THE INVENTION Latching devices which utilize the residual magnetism retained in the relatively soft magnetic materials in the magnetic circuit for the latching force. The actuators are proportioned so as to have a minimum effective air gap in the actuated position to achieve a high latching force without the use of permanent magnets in the magnetic circuit. Means are also disclosed for applying power to the actuators so as to assure the maintenance of at least a substantial magnetizing current until the moving member of the solenoid reaches the actuated position, whereafter the retentivity of the soft magnetic materials achieves a high latching force even after the magnetizing current decreases to zero. Means are also provided for demagnetizing the magnetic circuit to release the actuator from the actuated position. Various configurations are disclosed including some ideally suited for use with a simple remote switching means. One such configuration includes a switch mechanically coupled to the moving member of the solenoid so as to switch one solenoid coil lead between first and second actuator terminals. Various means are also disclosed for maintaining the required magnetizing current during switching of the switch coupled to the moving member so that a momentary open of the switch does not result in the collapse of the magnet field in the magnetic material before the moving member reaches the actuated position. The latching devices of the present invention may be fabricated from substantially any magnetic material, particularly relatively soft magnetic materials selected substantially entirely based upon the .cost of fabrication thereof, and fabricated in simple configurations which may be assembled with a minimum of individual parts to achieve a latching actuator with a very high latching force.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a typical latching actuator of the present invention.
FIG. 2 is the hysteresis loop for Ingot Iron.
FIGS. 3a through 3d are schematic representations of the magnetic components of the actuator of FIG. 1 illustrating the intensity of the magnetic fields therein under various actuation conditions.
FIG. 4 is a schematic diagram of a typical means for applying electrical power to the actuator of FIG. 1.
FIG. 5 is a cross-section of an alternate embodiment latching actuator of the present invention.
FIG. 6 is a schematic illustration of the interconnection and means of applying electrical power to the actuator of FIG. 5.
FIGS. 7a through 7c are schematic diagrams of the instantaneous circuit for the circuit of FIG. 6 at various stages of operation of the actuator.
FIG. 8 is a cross-sectional view of a further alternate embodiment of the present invention.
FIG. 9 is a diagram illustrating the interconnection of and the means of applying power to the solenoid actuator of FIG. 8.
FIG. 10 is a schematic diagram for an alternate means of applying electrical power to the solenoid actuator of the present invention.
DETAILED DESCRIPTION OF THE INVENTION Referring now in detail to FIG. 1, the latching actuator device of one embodiment is comprised of a housing, generally designated 10, which is constructed of a magnetic material. The housing has outer walls 12, and inner walls 14, joined by wall 16. Housing 10 is bordered on opposite ends by pole faces 18 and 20. Above wall 16, and between walls 14 and 12, is positioned a first primary coil 21. Below wall 16 and between walls 14 and 12 is positioned a second primary coil 22. A first secondary coil 24 is positioned within the housing between inner walls 14 and 12 and above primary coil 21 and below first pole face 18. A second secondary coil 26 is positioned within said housing between inner wall 14 andouter wall 16, below second primary coil 22 and above pole face 20.
An armature 28 is slidably mounted within housing 10 and between inner walls 14. It is free to move linearly within walls 14 and between pole faces 18 and 20. The armature is constructed so that there is virtually no air gap between the side of the armature and inner walls 14. Extending vertically from armature 28 is a plunger 30. Pole face 20 has a hole 32 therein to allow plunger 30 to extend therethrough. Plunger 30 then moves with armature 28, as the armature moves up and down within housing l0.
Armature 28 may also have a hole 34 therein to correspond with a hole 36 within pole face 18. Such an embodiment, would allow for a member (not shown) to extend into armature 28 and be acted upon by the armature in its linear movement.
In operation, when a current is run through primary coil 21, a magnetic circuit is energized creating magnetic flux paths which pull armature 28 to pole face 18. When the coil current is turned off, a portion of the magnetic energy will be retained by the magnetic circuit in the form of residual magnetism. The invention provides for essentially no air gap in the latched position and consequently the full residual induction of the material can be utilized. As an example, the hysteresis loop for Alnico Magnetic Ingot Iron is shown in FIG. 2. When the current is turned off, and the magnetizing force is at zero the residual induction has a value of 10 kilo-gauss. Since the latching force is proportional to the square of the induction, it is consequently quite high. Therefore, once a current is passed through the primary coil, a very strong latching force can be maintained long after the current is turned off.
To move the armature in the opposite direction, primary coil 22 is energized simultaneously with secondary coil 24. Energizing secondary coil 24, creates a temporary magnetic field which cancels the retained magnetic fields set up by primary coil 21, thus eliminating this residual magnetism effect leaving the armature free to move. Energizing primary coil 22 then creates a magnetic field which causes armature 28 to move downward and against pole face 20. The residual magnetism effect works as before, holding armature 28 against pole face 20.
Thus, in operation, the latch actuating device is able to maintain a high latching force without the need for mechanical energy storing devices, such as springs or permanent magnets.
By way of further illustration of the operation of the latching actuator of FIG. 1, reference is now made to FIGS. 3a through 3d. These figures are schematic representations of the magnetic components of the solenoid (and further including the nonmagnetic plunger 30) and are for the purpose of illustrating the primary disposition and intensity of the magnetic fields within the solenoid throughout different phases of operation thereof. FIG. 3a shows the solenoid armature 28 latched in the upper position as a result of the relatively high magnetic field, generally indicated by the heavy lines 40. It will be noted that the air gap between the top surface of armature 28 and the lower surface of the first pole face 18 is substantially zero. Furthermore, the area of inner wall 14 is very much larger than the area of contact between the first pole face 18 and the top of armature 28, and further, the gap between the armature and the inner walls is chosen by design to be as small as possible and still provide for free motion of the armature. Thus, because of'the large area, the radial flux density between the armature 28 and inner walls 14 is relatively low, and this together with the small air gap therebetween results in only a small demagnetizing force for the otherwise magnetized material of the solenoid. Because of the finite air gap between the armatureand the inner walls (as well as a less than perfect surface contact between the armature 28 and the first pole face 18) the actual operating point of the magnetic material will be determined by the intersection of line 42 with the demagnetization curve 44 for the particular magnetic material used. The slope of line 42 is a measure of the air gaps (and other nonmagnetic ma' terials) in the magnetic circuit, weighted by the effective crosssectional areas of the particular air gaps. For a circuit having no air gaps line 42 would be vertical, and the flux density in the material would be the flux density at point 46. On the other hand, if the air gaps in the circuit were large and/or the cross-sectional area of the air gap was small, line 42 would have a very low slope, such as that of line 48. In this case, the flux density would be that of point 50, a negligible flux density for purposes of latching (B is very low at point 50). However, by keeping the air gaps as small as possible, and further by keeping the cross-sectional area of all air gaps as large as possible (with the exception of the air gap across which the primary solenoid force is to be developed) the slope of line 42 may be kept large so as to result in a relatively large field intensity in the magnetic material, indicated by point 52. Thus, even soft magnetic materials will retain a substantial flux density after having been magnetized if the air gaps are kept very small.
To actuate the solenoid from the position shown if FIG. 3a, a current is applied to primary coil 22 and secondary coil 24 (FIG. 1), preferably by connecting these two coils in series and passing a current through the series combination. The number of turns on each secondary coil is selected in proportion to the primary coil so that the secondary coil will demagnetize the previously magnetized material while the primary coil will magnetize the surrounding magnetic material and further will create a high flux density in the air gap so as to encourage the armature to the opposite position. By way of example, coil 22 and the current applied thereto are selected so as to create a very high field, indicated by the lines 54 in FIG. 3b, preferably nearly saturating the magnetic material. At the same time, secondary coil 24 ideally should create a demagnetizing force on the associated magnetic material of approximately 1 Oersted (see FIG. 2). This will substantially demagnetize the associated magnetic material, as shown by the light field lines 40 in FIG. 3b. While perfect demagnetization is in general not possible, at least on a repeatable basis, the flux density in the upper portion of the solenoid may be so reduced by this demagnetizing force as to cause negligible upward force on the armature 28. (In this regard, it is to be noted that if the flux density in the upper portion of the solenoid is reduced to 20 percent of its value at point 52 (FIG. 2), the force urging the armature to the upper position will be only 4 percent of the value experienced when the fields are the condition shown in FIG. 3a). Thus, the high magnetic field in the lower portion of the solenoid, indicated by lines 54, cause a high force between armature 28 and the lower pole face 20 so as to urge the armature to the lower position. Consequently, the force on the armature will reverse, as shown in FIG. 3b, and the armature will accelerate toward the lower position. When the armature reaches the lower position, as shown in FIG. 30, and current is terminated, the lower portion of the solenoid will remain magnetized in the same manner as heretofore described with respect to the upper portion of the solenoid as described in relation to FIG. 3a. At this time, the upper portion of the solenoid will have substantially zero flux therein since it was substantially demagnetized as shown in FIG. 3b before the armature moved to the lower position, and further because the large air gap between the armature 28 and the upper pole face 18 causes a very high demagnetizing force on the magnetic materials.
To return the armature to the upper position, a magnetizing current is applied to the primary coil 21 and a demagnetizing current to secondary coil 26. Thus, the upper portion of the solenoid is strongly magnetized as shown by the heavy field lines 40 in FIG. 3d, and the lower portion of the solenoid is substantially demagnetized as indicated by the light field lines 34. Thus, the armature will accelerate toward the upper position and will be latched in the upper position upon termination of the current, as originally shown with respect to FIG. 3a. It should be noted that when the solenoid is unlatched in one position and forced to the opposite position, the solenoid current inducing such action must not be terminated before the armature arrives at its final position. If the current is terminated before the armature reaches its final position, the magnetizing force on the associated portion of the solenoid magnetic material will fall to zero while there is a substantial air gap, and thus a very large demagnetizing influence on the solenoid, so that there will be no substantial residual magnetic field in the solenoid to latch the solenoid at either position, Thus, removal of the solenoid actuating current before the armature reaches a steady state posi tion, will result in the solenoid being substantially unlatched in either position.
A suitable circuit for applying power to the solenoid of FIG. 1 is shown in FIG. 4. In this circuit, a spring biased switch, generally indicated by the numeral 60, is connected to a source of DC power 62. The moving element of the switch 64 is spring biased to a central off position. (schematically represented by springs 66). When the moving element 64 is moved to the upper position, power supply 62 is connected through switch 60 and line 68 to primary coil 21 and secondary coil 26 to unlatch the armature, force it to the upper position and latch it at the new position. When finger pressure is removed from the switch, power is disconnected from the solenoid, though the solenoid will remain in the upper position because of the latching action thereof. When the moving member 64 of switch 60 is moved to the lower position, power is applied through coils 22 and 24 to unlatch the solenoid, move it to the lower position and latch it at that position. Thus, the switch provides a convenient means for actuating the solenoid between the upper and lower positions, and because the solenoid response is much faster than the manual switching of switch 60, maintenance of actuating current for at least as long as it takes the armature to reach a new position is assured.
Now referring to FIG. 5, an alternate embodiment of the present invention may be seen. This embodiment is substantially the same as the first embodiment shown and described in my copending application entitled Self-Latching Solenoid Actuator, Ser. No. 153,939, with the exception that the permanent magnet has been removed and the proportions of the device changed slightly in accordance with the present invention. Thus, an outer nonmagnetic enclosure 70 provides the housing within which the various parts of the solenoid may be assembled and which may be adapted as desired for mounting of the solenoid. A plunger 72 is disposed adjacent one end of the case, with an integral plunger rod 74 projecting through an opening in the end of the case for attachment to the mechanism to be actuated by the solenoid. A magnetic inner case member 76 has an inner diameter 78 forming a loose slip fit with the outer diameter of plunger 72, and has an integral upward projecting cylindrical member 80 forming a portion of the magnetic circuit and providing an inner diameter for location of the solenoid coil 82. Thus, fitting in the upward projecting member 80 is a solenoid coil 82 wound on a plastic bobbin 84. Also integral with magnetic member 76 is an inner upward projecting cylindrical member 86 projecting upward but terminating substantially short of upper pole piece 88, which completes the magnetic circuit and retains the various components of the solenoid in cooperative disposition. Thus, the magnetic members of the solenoid comprise the upper pole piece 88, the plunger 72, and the inner frame member 76, all of which are of simple physical configuration so as to be readily fabricatable by mass production methods.
Located above the upper pole piece 88 is a nonmagnetic spacer 90, and located thereabove at the top of the outer case member 70 is a single pole, double throw switch 92 having a centrally disposed actuating member 94. The switch 92, of the type referred to as micro switches, is retained in position by cementing the switch in place in the case member 30 (as are the other parts of the solenoid).
The plunger 72 has a cylindrical depression 96 extending downward from the top face 98 of the plunger, and is adapted to receive the switch actuating pin 100. Switch actuating pin 100, which is a nonmagnetic pin, has an enlarged head 102 at the lower end thereof, fitting within cylindrical depression 96 in the plunger 72, and extends upward through clearance holes in pole piece 88 and the spacer 90 to a position adjacent switch actuating member 94. A coil spring 104 disposed between pole piece 88 and the enlarged head 102 on the switch actuating pin 100, urges the switch actuating pin and plunger 72 to the downward position shown in FIG. 5.
Now referring to both FIGS. and 6, the electrical connection of the solenoid may be seen. One lead of the coil 82 is connected to the moving member 104 of the switch 92 through line 106. The other lead 108 of the solenoid coil is connected to a remote switching means, generally indicated by the numeral 110, which may be schematically representable as a single pole double throw switch having a moving member 112. This switch may be a mechanical or electronic single pole double throw switching means, and may further be biased (mechanically or electrically) to a central off position, since, as it shall be subsequently more fully described, the switch need only provide a momentary switching signal at either of the two positions. One connection of the switch 110 is coupled through a current limiting means, specifically resistor 114, to the negative side of DC power supply 116. The other contact of switch 110 is coupled to the positive side of the DC power supply 116. Similarly, one contact of the switch 92 is coupled through line 118 to the negative side of the power supply and the other contact is coupled through line 120 to the positive side of the power supply.
Initially both switch contacts 104 and 112 may be in the upper position, as shown in FIG. 6. In this position, both ends of the solenoid coil 82 are connected to the negative power supply terminal so that no power is applied to the solenoid coil. When the moving member 112 of remote switch 110 is moved to the lower position, line 108 of the solenoid coil 82 is connected to the positive power supply terminal while line 122 remains coupled to the negative power supply terminal through line 1 18. Thus, power of a first polarity is applied to the solenoid coil 82 causing the solenoid to move toward the actuated position. To latch at the actuated position it is necessary for the current in solenoid coil 82 to persist until the plunger 72 reaches the upper position (FIG. 5) and come to rest against the pole face 88. Consequently, some means must be provided to maintain a current in coil 82 until the plunger reaches the actuated position. One means of achieving the desired result is to provide a time delay between the motion of plunger 72 and switching of the switch 92. Switches of this type have a certain time delay which may be further enchanced by separating the actuating pin 100 from plunger 72 by a coil spring between these two members (not shown). However, it has been found that a time delay in the switching action of switch 92 for commercially available switches is generally inadequate to assure proper latching of the solenoid, and additional mechanical means for increasing the time delay as a result of indirect coupling between actuating pin 100 and plunger 72 unnecessarily complicates the structure. Consequently, it is generally preferred to provide some other means for maintaining the current.
One such means is provided by diode 124 which is coupled between the moving member 104 of switch 92 and the stationary contact of the switch connected to line 120. When moving member 104 is in the upper position, diode 124 is back biased and therefore nonconductive. When the moving member 112 of remote switch 110 is moved to the lower position, the full voltage of power supply 116 is applied across solenoid coil 82, thereby resulting in a very high flux in the magnetic circuit of the solenoid causing the rapid acceleration of solenoid plunger 72 toward the actuated position. Upon initial application of power to the solenoid coil 82, the electrical connection of a solenoid coil is effectively as shown in FIG. 7a. As the plunger moves toward the actuated position, switch 92 is actuated with moving member 104 going through an open condition before ultimately making contact with the fixed contact coupled to line 120. During the time of the open condition, the circuit is as shown in FIG. 7b. Of course, before the moving member 104 moves to the open position, a relatively high current was caused to flow through solenoid coil 82, thereby causing a high magnetic field in the magnetic structure of the solenoid,
preferably approaching the saturation flux density of the magnetic members of the solenoid. When the moving member 104 moves to the open position, the magnetic field in the solenoid iron begins to collapse and creates a back EMF, that is, a voltage of reverse polarity compared to that of FIG. 7a,-determined by the equation E Nd/dt, where N is the number of turns in the solenoid coil and d/dt is the rate of collapse of the magnetic field in the solenoid iron. The back EMF, while being physically most readily appreciated as a measure of field collapse given by the above equation, it may be expressed in a different form by noting that qb LI/N where L is the inductance of the solenoid coil and I is the current therethrough. Thus, Ndldt LdI/dt IdL/dt and the entire loop equation for the instantaneous circuit of FIG. 7 becomes L dI/dt I dL/dt Vd [R where R is the resistance in the circuit primarily in the coil 82, and Vd is the diode voltage drop of diode 124. It is to be noted that the value of L is constantly changing (increasing) as the plunger moves toward the stationary portion of the solenoid, and further, because of the acceleration, the velocity of the plunger is not uniform and therefore dL/dt is also changing. However, at any point along the trajectory of the plunger, approximate values for L and dL/dt may be substituted into the above equation to determine the approximate equation form for the current in the coil at that time. Thus, rearranging the above equation there results The instantaneous solution of the above equation, making the assumptions as herebefore stated, is an exponential decay of the current from an initial value to a value of I=Vd/R (this of course is a gross approximation achieved by assuming a constant value for the time varying perameters within the parenthesis but is sufficient to illustrate the operation of the circuit. Also, the current could never go negative because of the diode 124, and therefore the equation and the approximate solution thereof are only applicable for positive currents). The above equation and the highly approximate solution thereof are presented merely to illustrate the fact that the current in the solenoid coil and the field in the solenoid iron do not immediately decrease to zero, but instead merely start to decay at a rate determined by the various components of the system.
The micro switches of the type commercially available for use as switch 92 are generally designed to rapidly switch between the two alternate switching positions, and therefore the moving member 104 will very rapidly make contact with the fixed contact coupled to line 120. This motion will be further enhanced, in general, by motion of the actuating member 94 as a result of rapid motion of the plunger by the time the actuating member is depressed thereby. Thus, when the switch completes the switching motion, the circuit will be as shown in FIG. 7c. This figure is substantially the same as that of FIG. 7b, though the diode 124 is shorted out and therefore removed from the circuit as a result of the switch closure. Thus, the equation for the rate of decay of current for this circuit is given as follows:
which in general is similar to the above equation but with a somewhat lower rate of decay of current, and therefore field strength, in the solenoid. When the plunger arrives at the actuated position, the inductance is at a maximum and is constant thereafter so that the equation defining the conditions of the circuit of FIG. 7c further simplifies to the following:
dI/dt IR/L O The solution of this last equation, of course, is a true exponential decay in current to zero.
The purpose of the above equations is merely to illustrate the manner in which the diode allows the maintance of a substantial magnetizing current to remain in the solenoid coil, even when switch 92 is moving through the open condition, so as to prevent the collapse of the magnetic field in the solenoid before the solenoid plunger reaches the actuated position. Once in the actuated position the further decay of the magnetizing current to zero is of little consequence, inasmuch as the residual flux in the solenoid iron maintains a high latching force even after the current decays essentially to zero. Thus, it may be seen that the diode prevents an open circuit as switch 92 switches from a first position to a second position so as to maintain a substantial magnetizing current in the solenoid coil, at least until the plunger arrives at the latched position. Furthermore, it should be noted that in most applications, a solenoid is actuating a load which is substantially inertial, and though actuation of the solenoid generally requires at least a few milliseconds, most of the actuation time is used in establishing the magnetic field in the solenoid and in initially accelerating the plunger toward the actuated position. Thus, by the time switch 92 first starts switching and moves into the open position, the plunger is rapidly moving toward the latched position so that the magnetizing current need only be maintained as hereabove described for a very short time. Thus, the flux in the solenoid will decrease only slightly in order to create the back EMF to maintain the magnetizing current until latching is achieved.
To maintain the latching force, even in the presence of this slight decrease in flux, the contacting pole faces may be chamfered so as to concentrate the flux and therefore increase the flux density over the pole face to achieve higher latching forces. (Since the solenoid force is proportional to B A or dfi/A, where is the total flux and A is the area over which the flux is distributed, the latching force for a given flux may be increased by decreasing the area over which theflux is distributed). Thus, chamfers 150 and 152 are provided in the solenoid of FIG. 5. It should be further noted, referring again to FIG. 2, that the residual flux density in the exemplary material is approximately 10,000 Gauss. The saturation flux density for the same material is approximately 15,000 Gauss, and is approached with a magnetizing force of only approximately 8 Oersteds. Thus, the latching force may be further increased by further decreasing the contact area of the plunger with the pole 88, thereby further concentrating the flux to approach the saturation flux density for the material at that point. This may be achived with only a very slight drop in total fiux in the solenoid, since the concentration of flux is very local in nature and thus the HL drop (coercive force in Oersted inches) due to the concentration is very low because of the very short effective length L in the region of concentration, and may easily be made up by the remaining 95 percent of the magnetic circuit as a result of a slightly higher demagnetizing force thereon. By way of specific example, 95 percent of the iron in the magnetic circuit may be operated at a flux density, at point 150, of approximately 7,000 Gauss instead of approximately 8,500 Gauss at point 52. Since the demagnetizing force of point is approximately one fourth of an oersted greater than at point 52 and represents the demagnetizing force of approximately 95 percent of the circuit, the additional demagnetizing force in this part of the circuit results in a magnetizing force per unit length of approximately 20 times one fourth or approximately 5 oersteds in the area of flux concentration. Thus, it may be seen in FIG. 2 that the flux density for latching may be approximately 14,000 Gauss achieved as a result of proportioning the magnetic circuit and chamfering the plunger pole face so as to reduce the plunger contact area in the latched condition to approximately one half that of the average cross sectional area of the solenoid iron. Further, if the chamfering (or other method of reducing the pole face area of contact such as by putting a groove pattern on the pole face, etc.) is slight, the gap in the chamfered area may be relatively large when the solenoid is in the latched position but may be small in comparison with the air gap when the solenoid is in the unactuated or only partially actuated position. Thus, the effective pole face area of the unactuated position may be approximately twice that of the actuated position. Consequently, assuming the initial solenoid current to be adequate to substantially saturate the solenoid iron, approximately twice the total flux will exist in the solenoid during the initial motion of the plunger and a high initial force of the solenoid will be achieved. Furthermore, the total flux may decay by a factor of approximately 50 percent while maintaining a substantial magnetizing current ashereabove explained with respect to FIGS. 70 through 7d, and result in a very high latching force as a result of the near saturation of the plunger pole face in the actuated position, even in the absence of any continuous latching current.
To unlatch the solenoid, the moving member 112 of the remote switch 110 is again moved to the upper position. Thus, coil 82 is again coupled across power supply 116 with an opposite polarity from that heretofore described, and with current limiting means, namely resistor 114, in series with the circuit. By properly selecting the resistor 114 in conjunction with the voltage of the power supply 116 and the characteristics and dimensions of the solenoid members, the current through the solenoid coil 82 may be caused to provide a demagnetizing force on the solenoid iron to reduce the flux density at the pole face of the plunger to approximately zero. For the exemplary material of FIG. 2, approximately one oersted demagnetizing force is required to achieve this object. This magnetizing force, of course, is very low in comparison to the required magnetizing force for normal permanent magnet materials (by way of example the demagnetizing force of Alnico V is approximately 700 Oersteds). Thus, with the expenditure of only a very slight amount of electrical power, the solenoid iron is substantially demagnetized and the return spring 104 will accelerate the solenoid plunger toward the unactuated position again.
Switch 92, when actuated by the downward motion of the plunger, will go through an open position. However, assuming the solenoid iron to be fairly soft, a significant air gap will result in such a demagnetizing force on the solenoid materials so as to prevent a significant force to result from the recovery of the field upon removal of the demagnetizing current. (If the demagnetizing current is terminated when there are no other demagnetizing influences in the solenoid material e.g. the airgap is still substantially zero, the field will recover from substantially zero at point 160, along a line 162 approximately parallel to the slope of the demagnezation curve at point 46, to point 164, thereby providing a sufficient flux to relatch the solenoid). If the solenoid iron is somewhat magnetically harder than the exemplary material of FIG. 2, there may be a substantial recovery in the flux density in the air gap when the switch moves to the open position, the effects of which may again be minimized by the use of a second diode 166 to maintain a substantial demagnetizing current in the solenoid coil until the plunger moves to the fully extended position, in much-the same manner as that heretofore described with respect to operation of diode 124.
One further embodiment for a latching actuator which latches at both extremes of its stroke, using the principles of the present invention, is shown in FIG. 8, and a suitable means for applying power to the actuator of FIG. 8 is shown in FIG. 9. The actuator of this embodiment is comprised of an upper magnetic member 200a, a lower magnetic member 200 b, both of which may be identical members, a magnetic plunger 202 and a nonmagnetic actuating pin 204 for connection to the load to be actuated by the actuator. A first coil 206 is located in member 200a and second coil 208 is located in member 200b, coils 206 and 208 being identical coils. When a magnetizing current is applied through coil 208, the plunger 202 will be magnetically attracted to the top face of member 200b, and because of the flat surfaces of member 202 and of 200a and 200b, will lie flat against member 200b, thereby resulting in substantially zero air gap therebetween. Thus, there will be a very high magnetic field, indicated by lines 210, through the magnetic components due to the retentivity of the soft magnetic material and the extremely small demagnetizing force thereon (e.g. substantially zero). To actuate the solenoid to the upper position and latch the solenoid plunger 202 against the bottom face of member 200a, a high magnetizing current is applied through coil 206 while at the same time a small demagnetizing current is applied to coil 208. This demagnetizes the lower portion of the solenoid while magnetizing the upper portion, causing the plunger 202 to move to the upper position and be latched thereat in a manner heretofore described.
The excitation of the coils of the solenoid of FIG. 8
may be accomplished by any suitable means, such as by way of example the means shown in FIG. 9. Here the coils 206 and 208 each have one lead thereof connected to a ground terminal. The moving member of a spring biased switch 212 may be used to momentarily couple line 214 to either the positive or the negative side of a balanced power supply 216. Each of coils 206 and 208 is coupled to line 214 through a parallel connection of a resistor and a diode. Thus, resistor 218 and diode 220 are coupled with coil 206 to line 214, with the diode providing substantially direct connection of line 214 to the coil 206 when line 214 is positive, and the resistor 218 providing a current limiting means (with the diode back biased) when line 214 is negative. Similarly, resistor 222 and diode 224 couple coil 208 to line 214, substantially directly when line 214 is negative, and through the current limiting resistor 222 when line 214 is positive. Thus, when switch 212 couples line 214 to the positive terminal, the high magnetizing current is caused to flow in coil 206, whereas a small, predetermined, demagnetizing current is caused to flow in coil 208. Thus, the lower portion of the solenoid will be demagnetized and the upper portion magnetized so as to force the plunger 202 to the upper position and latch it at that position. When line 214 is connected to the negative terminal, a demagnetizing current is caused to occur in coil 206 and a strong magnetizing current is caused to occur in coil 207, thereby demagnetizing the upper portion of the solenoid and strongly magnetizing the lower portion, thereby actuating and latching the solenoid.
The solenoid of FIG. 8 utilizes only three magnetic members, specifically plunger 202 and members 200a and 200b, two of which are identical. In the embodiment shown, the various members of the solenoid may be assembled with a non-magnetic spacer 230, which may be an aluminum or a plastic ring. This assembly may be placed in the housing in which it is to be used, or as an intermediate assembly may be retained in the assembled position by suitable means such as a nonmagnetic outer case member 232, which may be an aluminum tubular member rolled or otherwise crimped at the ends thereof to retain the assembly. Of course, as an alternate, one of the members 200a and 200b and in the coil therein might be replaced with a nonmagnetic member, and a return spring coupled between the plunger and the remaining members so as to return the plunger to an unlatched condition when the magnetic members are demagnetized. Such a return spring is typically selected to provide a return force of approximately one half of the gross solenoid force so that the net solenoid forces during actuation and when unlatched are approximately one half of the magnetic force generated.
The latching actuators previously described herein all achieve a high latching force without requiring any continuous power applied thereto. Such actuators are ideally suited for battery operation as power dissipation therein is held to a minimum to result in the maximum battery life. In such applications, the source of excitation such as, by way of example, the power source 116 of FIG. 6 might be comprised of a battery adapted to charge a capacitor through a current limiting resistor so that the charge on the capacitor (or more typically a small percentage of such charge) may be used to actuate or unlatch the actuator without requiring a high current from the battery. In other applications however, it may be desired to actuate and latch the actuator on a single electrical input, and then unlatch upon the removal of that input. Thus, by way of example, solenoid actuators are commonly used in both AC and DC devices to actuate and remain actuated upon the application of power thereto, and to unlatch upon removal of the power. Thus, by way of example, in appliances such as washing machines, a solenoid valve will be used for control of the water input. Such solenoids are generally adapted to operate from an AC power source and generally are connected so as to receive the entire line voltage not only during actuation, but throughout the time period in which the solenoid is maintained in the actuated position. Thus, by far the greatest energy is dissipated in the solenoid not during actuation, but rather during the prolonged time period thereafter in which the solenoid is retained in the actuated position. The problem is even somewhat worse in the case of DC actuation, since at least in AC actuation the increased inductance of the solenoid .when in the actuated position tends to reduce the current input thereto, though in both AC and DC solenoids which are to actuate and remain in the actuated position by full application of power thereto, the size of the solenoid is determined more by the heat dissipation after actuation than during actuation. I
For such applications, the actuator of FIG. may be used with only minor modification thereto so as to achieve the latching actuator which, when connected to a power supply will actuate with a relative high magnetizing current therein, and will automatically switch so as to draw a greatly reduced current to remain in the latched condition. Since actuation requires only a fraction of a second to achieve (typically as in the order of milliseconds) the amount of energy dissipated at the higher current is small, and the small retaining current results in little power dissipation in the solenoid. Thus solenoids substantially smaller than heretofore used in dishwashers and the like may be used, thereby resulting in smaller and less expensive solenoid devices for such applications. This may be achieved by using the solenoid which is basically the device of FIG. 5 connected to a source'of power as shown in FIG. 10. Thus, in this figure, the source of power 304, which may be an AC or a DC power source, is generally controlled through a switch 302, which may be a mechanical or an electronic switch, and characteristically for appliances of the type heretofore mentioned is a mechanical switch operated by a mechanical timer mechanism. The solenoid, generally located within the indicated enclosure, has a solenoid coil 82 connected to switch 302 and also connected to a switch 92a. The switch 92a is generally similar in arrangement and function as the switch 92 in FIG. 5, with the exception that only a single pole, single throw switch is required. Thus, when switch 92a is in the position shown, one end of the solenoid coil is coupled through the switch to the power supply. A resistor 300 is also coupled between the power supply lead and the solenoid coil lead, so that when switch 92a is switched to the open position, the solenoid coil is cou pled through the resistor 300 to the power supply.
The result of this circuit is as follows. When switch 302 is open, the solenoid is generally in the unlatched position. When switch 302 is closed, the full voltage of the power supply 304, whether AC or DC, is applied across coil 82, thus causing a high MMF in the solenoid to cause the actuation thereof. As the moving member 72 of the solenoid moves toward the actuated position, and preferably as it approaches the actuated position, (or even after it reaches the actuated position, if sufficient time lag or hysteresis is built into the switching mechanism) switch 92a moves to the open position. Thus the current in coil 82 decreases from the high initial actuating current to a substantially reduced current, determined by the resistor 300 and the other circuit parameters. However, since actuation of the switch only occurs as the moving member 72 approaches the actuated position, the magnetizing current required to continue the operation of the solenoid and to latch the moving member in the actuated position is greatly reduced, and thus by proper selection of the value of resistor 300, the magnetizing current may be reduced as a result of the switching of switch 92a to a value which is still adequate to latch the solenoid.
By way of specific example, assume that the solenoid is intended to operate from a DC power supply, and generally operates an inertial load. The switch 92a may be adjusted to switch to the open position, at least statically, when the moving member has traveled percent of the stroke toward the actuated position. Thus, neglecting any time lag or mechanical hysteresis in switch 92a, and further neglecting the fact that the high initial magnetizing current will not immediately decay to a value determined by resistor 300, but will only change to the lower value with some characteristic time constant, resistor 300 may be chosen so as to reduce the magnetizing current in the solenoid coil to approximately 20 percent of the initial value and still substantially saturate the solenoid iron. Thus, the total power dissipated has been reduced by a factor of five, and further, the power actually dissipated in coil 82 to achieve latching, that is the PR loss in the coil, is only approximately for percent of the PR loss during initial actuation.
When switch 302 is opened, the magnetizing current drops to zero. This causes a substantial decrease in the field strength in the solenoid, and by proper selection of the return spring (or other return means) the solenoid may be caused to unlatch and return to the unactuated position when magnetizing current is removed. (To assure a sufficient drop in the field strength of the solenoid to unlatch the solenoid, it may be desirable to plate with a nonmagnetic material, by way of example, the moving member surfaces so as to result in a controlled minimum air gap in the solenoid to assure a sufficient effective demagnetizing force when in the actuated position to cause a substantial drop in the field upon complete termination of the magnetizing current. In this regard however, it should be noted that a drop in field strength of only 50 percent, say from substantial saturation to point 150 in FIG. 2, will cause a drop in solenoid force of 75 percent, allowing selection of the return means to provide the desired return force).
There has been described in detail herein a number of embodiments of the present invention Latching Actuator, as well as a number of alternate means for properly applying electrical power to the actuator so as to achieve the desired results of the present invention. In part, the present invention comprises solenoids designed to have a minimum effective air gap when in the actuated position, either or both by minimizing the length of the air gap or gaps and by maximizing the effective cross sectional area of the air gaps. The solenoids may be assembled from a minimum member of parts of simple geometry and of any magnetic material, typically relatively soft magnetic material, which may be selected primarily for ease of fabrication directly to the desired configuration with a minimum or absence of machining after basic forming. By way of example, the magnetic components may be formed by well known powder metallurgy methods so as to result in magnetic parts of finished dimension which will provide the high latching force in the latched condition. Of course, machined parts may also be used, fabricated by way of example from low carbon steel, such as, C1010 and C1020, etc., either hot rolled or cold rolled, and used directly as obtained from an automatic turning machine,
The term soft magnetic material" and equivalent terms as used herein have no specific quantitative definition, but is used herein to distinguish from materials commonly used for permanent magnets because of their high retentivity While solenoids having very high latching forces may be fabricated from even the softest materials such as annealed hydrogenizedirons, such irons are expensive and difficult to fabricate,an'd thus in general, somewhat harder materials are most practical because of overall cost considerations, though in general materials having a coercive force of less than Oersteds are probably the most practical materials. Also, it should be noted that the effective air gap that may be used in a solenoid of the present invention and still achieve the high latching forces will depend on the coercive force of the material used. For annealed hydrogenized irons, the tolerable air gap is very small, and configurations such as that of FIG. 8 are best, while slightly harder materials will provide higher latching forces for configurations having some gap in the actuated position, such as that of FIGS. 1 and 5, (a high radial force will cause the plunger to lie against one side of the stationary member, thereby effectively reducing the clearance or air gap-therebetween). Thus the design criteria is that the length of any air gaps be held sufficiently small and the area of any air gaps (the word air including all nonmagnetic gaps in the magnetic circuit) be maintained sufficiently large, (except for the primary solenoid pole faces on which the solenoid force is created) so that the total demagnetization force (Oersted inches) of such gaps is only a fraction of the coercive force (Oersted inches) of the materials used even with relatively high flux densities (preferably at least percent of the residual density of point 46, FIG. 2) in the magnetic circuit when the plunger is in the latched position.
Also included as part of the present invention are the various means for applying electrical power to the actuator to assure proper latching with a high latching force. Thus, the actuator of the present invention may be fabricated from a minimum number of parts selected substantially entirely-upon the cost of the fabricated part, to result in an extremely low cost latching actuator having a latching force which may equal or exceed that of an actuator having a permanent magnet therein, and which may be easily actuated without complicated actuation circuits. Thus, while the invention has been particularly shown and described with reference to preferred embodiment thereof, it will be understood by those skilledv in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
1. A latching solenoid actuator having a stationary member and a moving member adapted for motion between first and second orientations with respect to said stationary member, and a solenoid coil, said stationary member and said moving member forming a magnetic circuit and both being fabricated of magnetic materials exhibiting the magnetic characteristics of generally soft magnetic materials, said solenoid coil being disposed with respect to said magnetic circuit so as to cause a magnetizing force in said circuit in response to a current therethrough, said stationary member and said moving member being adapted to magnetically urge said moving member toward said first orientation with respect to said stationary member in response to the establishment of a magnetic field in said magnetic circuit, said stationary member and said moving member having cooperatively disposed surfaces so as to result in a minimum air gap in said circuit when in said first orientation, whereby the retentivity of said magnetic materials causes a high latching force in said first orientation whenever said moving member reaches said first orientation before the current in said coil causing said magnetizing force is terminated.
2. The latching actuator of claim 1 for linear actuation wherein said moving member has a generally flat first surface perpendicular to the direction of motion of said moving member and a generally cylindrical second surface parallel to said direction of motion, said stationary member having a first surface parallel to said first surface of said moving member and a second surface adjacent to and providing a clearance with respect to said second surface of said plunger, said first sur-' faces being in substantial contact when said moving member is in said first orientation.
3. The latching actuator of claim 2 wherein said second surfaces have areas which are much larger than said first areas.
4. The latching actuator of claim 1 for linear actuation wherein said moving member and said stationary member each have first and second areas substantially perpendicular to the direction of motion of said moving member, said magnetizing force causing a magnetic field to occur in the air gap between said first surfaces and in the air gap between said second surfaces, said first surfaces and said second surfaces being disposed so as to be in substantial contact when said actuator is in the latched position.
5. The latching actuator of claim 4 wherein the area of at least one pair of said first and second areas is substantially less than the cross-sectional area of said magnetic circuit in regions remote to said first and second areas.
6. The latching actuator of claim 1 further comprised of a means for selectively causing a momentary magnetizing current to flow in said coil for a time period at least as long as the time required for said moving member to move to said first orientation and a demagnetizing means for causing the substantial demagnetization of said magnetic circuit.
7. The latching actuator of claim 6 wherein said demagnetizing means is comprised of a second coil disposed with respect to said magnetic circuit so as to cause a demagnetizing force in said circuit in response to a current therethrough, and a means for selectively causing a demangetizing current in said demagnetizing means.
8. The latching actuator of claim 6 wherein said demagnetizing means comprises a means for selectively causing a predetermined current in said solenoid coil which is substantially less than and of opposite polarity from said magnetizing current.
9. A solenoid actuator having a stationary member and a moving member adapted for motion between first and second orientations with respect to said stationary member, a solenoid coil having first and second leads, a switch and a current maintaining means, said stationary member and said moving member forming a magnetic circuit and both being fabricated of magnetic materials exhibiting the magnetic characteristics of generally soft magnetic materials, said solenoid coil being disposed with respect to said magnetic circuit so as to cause a magnetizing force in said circuit in response to a current therethrough, said stationary member and said moving member having cooperatively disposed surfaces so as to result in a minimum air gap when in said first orientation, and further being adapted to magnetically urge said moving member toward said first orientation with respect to said stationary member in response to the establishment ofa magnetic field in said magnetic circuit,said switch being electrically coupled to said first solenoid coil lead and adapted to switch electrical coupling with said first solenoid coil lead between first and second actuator leads in response to motion between said stationary member and said moving member, said current maintaining means being a means for maintaining a substantial magnetizing current in said solenoid coil during the time of actuation of said moving member from said second orientation to said first orientation.
10. The solenoid actuator of claim 9 wherein said current maintaining means comprises said switch and its mechanical coupling to said stationary and said moving members, whereby switching of said switch is delayed during the motion of said moving member from said second orientation to said first orientation until said moving member comes to rest at said first orientation.
11. The solenoid actuator of claim 9 wherein said current maintaining means comprises a diode coupled between said first solenoid coil lead and said first actuator lead, whereby a magnetizing current may be caused to flow in said solenoid coil when said moving member is in said second orientation by applying power to said second actuator lead and said current may be substantially maintained through said diode during switching of said switch by the back EMF of said solenoid coil.
12. The latching actuator of claim 9 for linear actuation wherein said moving member has a generally flat first surface perpendicular to the direction of motion of said moving member and a generally cylindrical second surface parallel to said direction of motion, said stationary member having a first surface parallel to said first surface of said moving member and a second surface adjacent to and providing a clearance with respect to said second surface of said moving member, said first surfaces being in substantial contact when said moving member is in said first orientation.
13. The latching actuator of claim 12 wherein said second surfaces have areas which are much larger than said first areas.
14. The latching actuator of claim 9 for linear actuation wherein said moving member and said stationary member each have first and second areas substantially perpendicular to the direction of motion of said moving member, said magnetizing force causing a magnetic field to occur in the air gap between said first surfaces and in the air gap between said second surfaces, said first surfaces and said second surfaces being disposed so as to be in substantial contact when said actuator is in the latched position.
15. The latching actuator of claim 14 wherein the area of at least one pair of said first and second areas is substantially less than the cross-sectional area of said magnetic circuit in regions remote to said first and second areas.
16. The latching actuator of claim 9 further comprised of a demagnetizing means for causing the substantial demagnetization of said magnetic circuit.
17. A solenoid actuator having a stationary member and a moving member adapted for motion between first and second orientations with respect to said stationary member, a solenoid coil having first and second leads, a switch arid a current limiting means, said stationary member and said moving member forming a magnetic circuit and both being fabricated of magnetic materials exhibiting characteristics of generally soft magnetic materials, said solenoid coil being disposed with respect to said magnetic circuit so as to cause a magnetizing force in said circuit in response to a current therethrough, said stationary member and said moving member having cooperatively disposed surfaces so as to result in a predetermined air gap when in said first orientation, and further being adapted to magnetically urge said moving member toward said first orientation with respect to said stationary member in response to the establishment of a magnetic field in said magnetic circuit,
said switch and said current limiting means being electrically coupled between said first solenoid coil lead and an actuator connection whereby substantially direct coupling between said actuator connection and said first solenoid coil lead is achieved when said switch is in a first position but said coupling is limited by said current limiting means when said switch is in a second position, said switch being adapted to switch to said second position as said moving member approaches said first position.
18. The solenoid actuator of claim 17 whereby said current limiting means is a resistor and said switch is a single pole single throw switch, said switch and said resistor being coupled in parallel.
I 19. The actuator of claim 17 further comprised of a return means for returning said moving member to said second position, said return means having a predetermined return force when said moving member is in said first position which exceeds the force urging said moving member toward said first position due to the retentivity of said stationary and said moving member, and is less than the force urging said moving member toward said first position due to the combined effects of said retentivity and a magnetizing current in said solenoid coil limited by said current limiting means.