Abstract:
A magnetically latching solenoid and method of determining a position of a plunger contained therein. The solenoid includes a frame, a plunger configured to move through the frame between a first stable position and a second stable position, and at least one magnet mounted near the center of the frame such that a first and second magnetic fields are produced by the magnet through the frame and the plunger, wherein each of the first and second magnetic fields drive a separate portion of the frame into magnetic saturation depending on the position of the plunger. The solenoid also includes a first and second sensors mounted on the frame at different locations configured to detect and measure the first and second magnetic fields. The detected and measured magnetic fields are then used to determine the position of the plunger in the solenoid.

Description:
RELATED APPLICATION AND CLAIM OF PRIORITY 
     This application claims the priority of U.S. Provisional Application No. 61/117,819, filed Nov. 24, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     Not Applicable 
     BACKGROUND 
     This document relates to a magnetically latching solenoid, and more particularly, to a position sensor for detecting the position of a plunger in a magnetically latching solenoid. 
     A magnetically latching solenoid has an advantage over conventional solenoids in that no control power is required to maintain a plunger of a magnetically latching solenoid in either of two possible stable positions. Magnetically latching solenoids are described in detail in U.S. Pat. No. 3,022,450 to Chase. By contrast, the plunger of a conventional non-latching solenoid is held by a spring in a first position when no current is applied to coil in the solenoid, and is driven to a second position by magnetic forces whenever sufficient current is applied to the coil. Such current must be continuously maintained as long as it is desired for the solenoid plunger to occupy the second position. 
       FIGS. 1-3  illustrate a magnetically latching solenoid, such as those described in the U.S. Pat. No. 3,022,450. It should be noted that the solenoid illustrated in  FIGS. 1-3  has a box frame with open sides rather than the closed tubular frame shown in the U.S. Pat. No. 3,022,450. It should be noted that the position sensor disclosed in this document will work with either type of frame. 
     In  FIG. 1 , magnetically latching solenoid  100  may include a nonmagnetic shaft  102  that projects out of the frame  104  at one or both of opposing sides  105 A and  105 B. One purpose of the shaft  102  may be to guide and support the internal moving components of the solenoid  100 . The shaft  102  also can be attached to external components (not shown), so that the external components can be caused to move by the solenoid  100 . Solenoid  100  also includes two coils  106  inside the frame  104 , and a permanent magnet structure including two magnets  108  between the coils  106 . 
       FIG. 2A  shows solenoid  100  with both coils  106  made invisible for added clarity. With coils  106  invisible, additional components of solenoid  100  may be seen. Cylindrical steel anvils  110 A and  110 B are attached to the inside surface of the left and right end of the frame  104 . In this example, anvil  110 A is attached to the left side  105 A of solenoid  100  and anvil  110 B is attached to the right sides  105 B. Between the anvils  110 A and  110 B is a cylindrical steel plunger  112 , which may be attached to the shaft  102  so that the plunger and the shaft can both slide left or right together, thus moving the shaft through an opening in one or both anvils until the plunger strikes one of the anvils. This motion of the plunger  112  and of the shaft  102  together is called the stroke of the solenoid  100 . In  FIG. 2  the plunger  112  and the shaft  102  are at the left end of their stroke. 
     With coils  106  made invisible,  FIG. 2A  also shows that the outer surfaces of the magnets  108  make contact with the inside surfaces of the top and the bottom of the frame  104 , and that the inner surfaces of the permanent magnets make contact with coupler  114 . The coupler  114  may be made from a magnetic material (such as steel) and has a large hole through which the plunger  112  passes, with a small clearance so that the coupler does not touch the plunger. One purpose of the coupler  114  is to conduct the magnetic flux from the magnets  108  into the plunger  112 , thereby facilitating the latching of the plunger  112 , and thus, the shaft  102 , at either end of its stroke. An alternative construction may avoid the coupler  114  by vertically extending the magnets  108  toward a center-plane of the solenoid  100 . Semicircular notches may be added to the extended magnets  108  such that the magnets deliver any magnetic flux directly to the plunger  112  across a small air gap. 
     Similar to  FIG. 2A ,  FIG. 2B  shows solenoid  100  with frame  104  and coils  106  made invisible. 
       FIG. 3  shows the same components as  FIG. 2B , but with the anvils  110 A and  110 B made invisible, and viewed from the right end.  FIG. 3  also shows that both magnets  108  are oriented so that their north poles are in contact with the coupler  114 , and their south poles are in contact with the frame  104  (not shown in  FIG. 3 ). The solenoid  100  would also work as well if the poles of both magnets  108  were reversed. The arrows represent the magnetic flux inside the magnets  108 , the steel coupler  114 , or the steel plunger  112 . In this example, both magnets  108  drive magnetic flux into the coupler  114 , from which the magnetic flux crosses the clearance gap  120  into the plunger  112 . In an alternative construction with notched magnets such as the embodiment illustrated in  FIG. 3A , the magnets  108  may direct magnetic flux directly across the gap  120  into plunger  112 . 
     A magnetically latching solenoid latches because most of the magnetic flux tends to follow the path of least reluctance, which is the path that includes the largest portion in a high permeability material such as steel, and the least portion in air. When the plunger is at or near one end of its stroke, most of the flux from the magnets tends to pass through the shorter air gap, with very little passing through the longer air gap at the other end of the plunger. 
     The attractive forces produced on the flat ends of the plunger  112  are proportional to the square of the magnetic flux density there. Therefore, the attractive force across the shorter air gap will be much greater than the attractive force across the longer air gap. The difference between these forces will tend to hold or latch the plunger at the end of its stroke, without any current in the coil or coils. 
     A magnetically latching solenoid may be caused to change position by energizing one or both coils with a polarity such that the flux from the coil surrounding the shorter air gap tends to oppose the flux created in the shorter air gap by the magnets. When the attractive force in the shorter air gap becomes weak enough, the attractive force in the longer air gap may overcome it and cause the plunger to move. Once the plunger nears the opposite end of its stroke, the opposite air gap will become the shorter one, and the solenoid will latch in its new position. 
     The magnets  108  have a characteristic maximum flux density, which depends on the material from which the magnets are made. For example, Neodymium-Iron-Boron magnets have a maximum flux density of about 1.2 Tesla. By comparison, steel is capable of conducting a flux density of about 2.0 Tesla before it saturates. 
     To obtain large latching forces it is desirable to maximize the flux density at the flat ends of the plunger  112 . The magnets  108  may be chosen to have a cross-sectional area larger than the cross-sectional area of the plunger  112 . When the coupler  114  conducts the magnetic flux from the magnets  108  into the plunger  112 , the magnetic flux is concentrated into a smaller cross-sectional area, and the flux density in the plunger is thereby increased over the flux density in the magnets, thereby increasing the latching forces that may be produced on the plunger. The maximum possible latching forces may be achieved when the plunger  112  reaches a flux density where its steel is saturated. 
     With a conventional solenoid it is possible to deduce the position of the plunger of the solenoid by detecting the presence or absence of sufficient current in the solenoid coil. This method is not feasible with a magnetically latching solenoid because the plunger may occupy either position when the coils are not energized. Therefore it is generally necessary to add extra components to a magnetically latching solenoid for the purpose of detecting the plunger position. Such extra components could include a micro-switch mounted on the stationary portion of the solenoid, with an actuator mounted on the moving portion of the solenoid. Depending on the position of the plunger, and thus the actuator, the switch would indicate whether the plunger is in a first or second position. Other possible extra components could include an optical sensor or a magnetic proximity sensor, but all share the drawback that an extra moving component is required, which decreases the reliability of the solenoid. For the case of a micro-switch, reliability is further decreased because the electrical contacts inside the micro-switch may become contaminated or corroded. 
     SUMMARY OF THE INVENTION 
     The invention described in this document is not limited to the particular systems, methodologies or protocols described, as these may vary. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present disclosure. 
     It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. As used herein, the term “comprising” means “including, but not limited to.” 
     In one general respect, the embodiments disclose a magnetically latching solenoid. The solenoid includes a frame, a plunger configured to move through the frame between a first stable position and a second stable position, at least one magnet mounted on the frame configured to produce a magnetic field through the plunger and the frame, wherein the magnetic field varies throughout the frame based upon the position of the plunger, and at least one sensor mounted to the frame configured to detect and measure the magnetic field at a selected location. 
     In another general respect, the embodiments disclose a magnetically latching solenoid. The solenoid includes a frame, a plunger configured to move through the frame between a first stable position and a second stable position, at least one magnet mounted near the center of the frame such that a first magnetic field and a second magnetic field are produced by the magnet through the frame and the plunger, wherein each of the first and second magnetic fields drive a separate portion of the frame into magnetic saturation depending on the position of the plunger, a first sensor mounted on the frame at a first location configured to detect and measure the first magnetic field at the first location of the frame, and a second sensor mounted on the frame at a second location configured to detect and measure the second magnetic field at the second location of the frame. 
     In another general respect, the embodiments disclose a method for determining a position of a plunger in a magnetically latching solenoid. The method includes producing, by at least one magnet, a magnetic field through a plunger and a frame of a magnetically latching solenoid; detecting and measuring, at least one sensor mounted on the frame, the magnetic field at a selected location on the frame; and determining, by a processor operably connected to the sensor, the location of the plunger based upon the magnetic field detected and measured by the at least one sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects, features, benefits and advantages of the present invention will be apparent with regard to the following description and accompanying drawings, of which: 
         FIG. 1  illustrates various embodiments of a magnetically latching solenoid; 
         FIGS. 2A and 2B  illustrate various embodiments of a magnetically latching solenoid; 
         FIG. 3  illustrates various embodiments of a permanent magnet structure for use in a magnetically latching solenoid; 
         FIG. 3A  illustrates various embodiments of an alternative permanent magnet structure for use in a magnetically latching solenoid; 
         FIG. 4  illustrates an exemplary magnetizing curve for steel; 
         FIG. 5  illustrates various embodiments of an exemplary circuit board for use with a magnetically latching solenoid; 
         FIG. 5A  illustrates various embodiments of the exemplary circuit board of  FIG. 5 ; 
         FIG. 6  illustrates various embodiments of a magnetically latching solenoid having the exemplary circuit board of  FIG. 5 ; 
         FIG. 6A  illustrates various embodiments of a magnetically latching solenoid having the exemplary circuit board of  FIG. 5A ; 
         FIG. 7  illustrates additional various embodiments of a magnetically latching solenoid having the exemplary circuit board of  FIG. 5 ; and 
         FIG. 8  illustrates an exemplary chart illustrating magnetic fields produced during a stroke of a solenoid plunger. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of solenoid  100 , such as is discussed above, the flux density inside the magnets  108  may be about 1.2 Tesla, if, for example, the magnet material is Neodymium-Iron-Boron. In such a case, the flux density inside the portion of the plunger  112  to one side of the magnets may be saturated at about 2.0 Tesla. This difference in flux densities may be due to the relative difference in total cross-sectional areas of the magnets  108  and the plunger  112 . The reason only a portion of the plunger has a high flux density is because most of the magnetic flux tends to follow the path of least reluctance, which is the path that includes the largest portion in steel and the least portion in air. In  FIG. 2A  the path of least reluctance is toward the left side  105 A of frame  104  when the plunger is positioned as shown, because the air gap (if any) between the plunger  112  and the left anvil  110 A is small as compared to the air gap between the plunger and the right anvil  110 B. The magnetic flux tends to avoid paths of high reluctance, such as the path which crosses the larger air gap between the plunger  112  and the right anvil  110 B, as can be seen in  FIG. 2B . 
     The flux density in the left side  105 A of frame  104  may also be saturated at about 2.0 Tesla, provided the frame has a similar total cross-sectional area to the plunger  112 , and is made from a similar material (in this example steel). The saturation may also be due to the closeness of the plunger  112  to the left anvil  110 A. The flux density in the portions of the plunger  112  and of the right side  105 B of frame  104  to the right of the magnets  108  may be much less than saturation due to the larger air gap between the plunger and the right anvil  110 B, causing a path of high reluctance. 
     If the plunger  112  was at the right end of its stroke in  FIG. 2A , the flux densities discussed above would essentially be reversed, the flux density of the right end of the plunger and the right side  105 B of frame  104  may be saturated, while the left end of the plunger and the left side  105 A of the frame may be much less than saturation. 
       FIG. 4  shows an exemplary magnetization curve for a type of steel, in this example Armco M6 steel. The X-axis is the Magneto-Motive Force (hereafter MMF) in amp-turns per meter and the Y-axis is the magnetic field strength in Tesla. The inset shows an expanded view of the first 2% of the main chart. 
     The magnetization curve is extremely non-linear. Note that to achieve a magnetic flux density of 0.5 Tesla, an MMF of only 6 amp-turns per meter may be required. To achieve a magnetic flux density of 1.7 Tesla, an MMF of 110 amp-turns per meter may be required. To achieve a magnetic flux density of 1.9 Tesla, an MMF of 2000 amp-turns per meter may be required. 
     Stated differently, a span of steel one inch long (0.0254 meters) containing a magnetic flux density of 0.5 Tesla may represent 0.1524 amp-turns of effective MMF, but the same span of steel containing a magnetic flux density of 1.9 Tesla may represent 50.8 amp-turns of effective MMF. These magnetic field values are typical of the right and left sides respectively of the frame  102  as discussed above. 
     The embodiments described in this document use the difference in effective MMF between steel at differing flux densities (such as 0.5 Tesla vs. 1.9 Tesla) to detect the position of the plunger  112  in a magnetically-latching solenoid such as solenoid  100 . If the effective MMF is included within a closed secondary path of steel containing a small air gap, the included MMF may create a secondary magnetic field in the air gap. The strength of the secondary magnetic field may be measured to determine whether a portion (e.g., side  105 A or  105 B) of steel frame  104  is saturated or not, which may provide an indication of the position of the plunger  112 . 
       FIGS. 5 and 5A  illustrate an exemplary circuit board (CB)  500 . CB  500  may be designed to include two Hall Effect sensors  502  mounted near two corners of the CB, represented by small boxes. Hall Effect sensors are specialized integrated circuits which respond to the presence of a magnetic field. One example of a Hall Effect sensor is a Binary Hall Effect sensor. A Binary Hall Effect sensor produces a digital signal indicating whether a detected magnetic field is above or below a threshold value. Another example of a Hall Effect sensor is a Linear Hall Effect sensor. A Linear Hall Effect sensor produces an analog signal proportional to the strength of a detected magnetic field. Any other device which responds to a magnetic field may be used, but Hall Effect sensors are mass-produced by many suppliers and are therefore very inexpensive. As shown in  FIG. 5A , CB  500  may also contain a connector  504  to receive power from and to return signals to other remote circuits. There may also be conductive traces  506  on CB  500  which connect the Hall Effect sensors  502  to the connector. 
     By positioning CB  500  in a location on a magnetically latching solenoid where any magnetic saturation in the frame of the solenoid may be detected by the Hall Effect sensors  502  on the CB, the position of the plunger of the solenoid may be determined.  FIG. 6  illustrates one exemplary magnetic latching solenoid  600  with CB  500  attached to detect magnetic flux densities that may be used to determine plunger location. 
     Solenoid  600  includes similar components to solenoid  100  discussed above. Shaft  602  may pass through frame  604  and may include a plunger (not visible in  FIG. 6 ). It should be noted that frame  604  may be made from a magnetic material, such as steel. Frame  604  may include two anvils (not visible) constructed from a magnetic material such as steel. Coils  606  may be placed around the plunger on shaft  602 . A permanent magnet structure may also be attached to frame  604  and may include magnets  608  and coupler  614 . 
     CB  500  may be mounted parallel to and close to the upper (or, conversely, lower) surface of the frame  604  of solenoid  600 , near the center, and secured by non-magnetic (for example brass) fasteners such as screws  616  and spacers  617 . In addition, the same non-magnetic screws  616  may secure one or more magnetic brackets  618  above CB. The magnetic brackets  618 , which may be L-shaped (as shown) or of another suitable shape, may extend left and right nearly to the sides  605 A and  605 B of the frame  604 , where they are further secured by magnetic (for example steel) fasteners, such as screws  620  and spacers  621 . For example, magnetic brackets  618  may be a ferro-magnetic bracket positioned such that any magnetic field produced by the magnets  608  may be conducted to the CB  500 .  FIG. 6A  illustrates an alternative exemplary embodiment of solenoid  600  having the embodiment of CB  500  as described in  FIG. 5A , as well as showing alternative configurations for spacers  617 , magnetic brackets  618  and spacers  621 . 
       FIG. 7  shows a close-up view of the configuration of magnetic brackets  618  and the CB  500  with the Hall Effect sensors  502 . Each magnetic bracket  618  may be attached to the frame  604  in two locations. The first location may be near the center of the frame  604 . In this first location, CB  500  is also attached with non-magnetic screws  616  and spacers  617 . The second location where each magnetic bracket  618  may be attached is near the outside edge of frame  604 . Here, magnetic screws  620  and spacers  621  may be used. By using magnetic screws  620  and spacers  621 , any MMF due to saturation present in the outer portions of frame  604  may be conducted though each magnetic bracket  618  to the air gap containing the Hall Effect sensors  502  where any MMF will create a magnetic flux through the air gap and hence through the sensor. Non-magnetic screws  616  and spacers  617  may be used near the Hall Effect sensors to avoid diverting any magnetic flux away from the sensors. 
     It should be noted that the Hall Effect sensors  502  may be positioned directly between the short arms of the magnetic brackets  618  and the center portion of the steel frame  604  of solenoid  600 . Any MMF that may be included in the loop formed by one of the magnetic brackets and the frame may result in a magnetic field across the air gap between the end of the magnetic bracket  618  over the Hall Effect sensor and the steel frame  604 , and part of this magnetic field may pass through the corresponding Hall Effect sensor. By measuring this magnetic field passing through each of the Hall Effect sensors  502 , and comparing the measured values against expect results based upon the magnetic potential of magnets  608  and the material used to construct frame  604 , the position of the plunger of solenoid  600  may be determined. The strength of the magnetic field, for a given MMF, may be controlled to a limited extent by adjusting the height of the spacers, so as to match the sensitivity of the Hall Effect sensors  502 . 
     In an exemplary embodiment, a processor or computing device may be operably connected to the PCB  500  via the connector  504  such that any magnetic field values detected or measured by sensors  502  may be transferred and processed to determine the position of the plunger in the solenoid. The processor or computing device may be operably connected to a computer readable storage device which may include various software and/or algorithms for determining the position of the plunger based upon the detected and measured values of the magnetic field. 
       FIG. 8  illustrates an exemplary chart wherein a plunger of a magnetically latching solenoid is moved through its stroke from left to right, and the corresponding magnetic fields are measured at the locations of Hall Effect sensors mounted similar to those described in  FIGS. 6 and 7 . 
       FIG. 8  illustrates that when the plunger is at the left end of its stroke (the left side of the chart), the magnetic field through the left Hall Effect sensor may have a value of approximately 956 Gauss (0.0956 Tesla), while the magnetic field through the right magnetic field Hall Effect sensor may have a value of approximately −38 Gauss. When the plunger moves 0.1 inch toward the right, the magnetic field through the left Hall Effect sensor may decrease rapidly to approximately 488 Gauss while the magnetic field through the right Hall Effect sensor may increase slightly to approximately 14 Gauss. When the plunger reaches the mid-point of its stroke, the magnetic fields through both Hall Effect sensors may have about the same value of approximately 104 Gauss. When the plunger has moved 0.4 inch, such that it is 0.1 inch from the right end of its stroke, the magnetic field through the left magnetic field sensor may decrease to approximately 15 gauss while the magnetic field through the right magnetic field sensor may increase to approximately 488 Gauss. Finally, when the plunger is at the right end of its stroke, the magnetic field through the left magnetic field sensor may have a value of approximately 69 gauss while the magnetic field through the right magnetic field sensor may have a value of approximately 1143 Gauss. It should be noted that the field strength through the left sensor while at left stroke may differ slightly from the field through the right sensor at right stroke due to unavoidable manufacturing deviations and tolerances. 
     A straight horizontal line has been added to the chart shown in  FIG. 8  representing a possible threshold value of 500 Gauss for a pair of Binary Hall Effect sensors. If the magnetic field strength in the left magnetic field sensors were compared to this threshold value, a signal may be generated that indicates when the plunger has moved within 0.1 inch of the left end of its stroke. If the magnetic field strength in the right magnetic field sensors were compared to this threshold value, a signal may be generated that indicates when the plunger has moved within 0.1 inch of the right end of its stroke. If neither signal was present, it may indicate that the plunger was in the middle portion of its stroke, more than 0.1 inches from either end. In many applications, this would indicate a fault condition in which the movement of the plunger had become blocked or jammed. Thus the position sensing system for a magnetically latching solenoid according to this disclosure may be capable of detecting a mechanical failure. 
     If Linear Hall Effect sensors are used to obtain the position information, the information may be passed to a general purpose computer. The general purpose computer may have software installed that receives this information from the Hall Effect sensors and calculates the position of the plunger. This calculation may be based upon several known factors such as the type of material (e.g., Armco M6 steel) used to manufacture the plunger, the frame, and the brackets; the associated magnetic curve (such as that shown in  FIG. 4 ) for the materials used in the manufacturing process; the strength of the permanent magnets; the strength and accuracy ratings for the Hall Effect sensors; the distance of the stroke of the plunger; and any other relevant information that may factor in to any calculations performed by the software on the general purpose computer. 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.