Patent Publication Number: US-2013236337-A1

Title: Solenoid actuators using embedded printed circuit coils

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims filing date priority based on Provisional Applications No. 61/685,003, filed Mar. 9, 2012, and No. 61/686,305, filed Apr. 3, 2012. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     SEQUENCE LISTING, ETC ON CD 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to linear electromagnetic motors and, more particularly, to solenoid actuators used for driving switches, valves, pumps, and similar loads. 
     2. Description of Related Art 
     Traditional motors and solenoids use loops of insulated copper magnet-wire wound (or ‘turned’) around a bobbin or similar hollow structure to create a magnetic field that provides motive force to a moving core of ferromagnetic material when the coil is energized. Bobbins of magnet wire have been in wide use since the publication of Michael Faraday&#39;s research in 1831 and are used for motors, solenoids, and countless other applications. Now available in massive quantities as commodities from low-cost suppliers, wire-wound coils provide the backbone of the electromagnetic actuation industry. 
     But wound wire coils are not without their drawbacks and limitations. Their form factor defines the shape and scale of the device (much like a spool of thread), requiring hand assembly operations at several points in the manufacturing process. Mechanical &amp; electrical (solder) connections must be made to the delicate, hair-thin wires, and mounting features and magnetic-circuit-confining iron components are built up around the bobbin. The mass of magnet wire, together with the mass of the ferromagnetic core, determines that solenoids have a large mass relative to the force that is developed and the stroke that is provided. 
     From an operational standpoint, motors and solenoids are prone to failure due to thermal cycling or mechanical stress on the fragile connections within the coil. Tiny copper wires, thermal cycling, heavy iron assemblies, and hand-assembly processes eventually lead to failure of the device at the weakest points. 
     Although solenoid construction has not changed significantly since Faraday, electronic circuit technology has progressed rapidly, particularly in the late 20 th  and early 21 st  century. Printed circuit techniques have enabled the creation of complex circuit connections using printed lines on a robust circuit board, resulting in radically reduced costs for constructing electronic circuits. Indeed, these printed circuit techniques have been used to form printed coils that are embedded in a multilayer circuit board. For example, U.S. Pat. No. 6,664,883 describes a printed circuit board (hereinafter, PCB) that has multiple layers, each layer hosting at least one printed conductor in the form of a loop or multiple loop. The loops are electromagnetically interactive, so that they may be used as an inductance or a voltage transformer in a circuit. 
     It is significant to note that this printed coil approach has not been applied to solenoid actuator design. Thus none of the benefits of modern PCB techniques and their economies of scale have been directed to ameliorate the drawbacks of traditional solenoid actuator designs. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention generally comprises a method and apparatus that applies modern PCB techniques to the construction of solenoid actuators and similar electromagnetic motor devices. A fundamental feature of the invention is that the typical wire wound electromagnetic coil is eliminated, and replaced functionally by printed coil structures that are embedded in multilayer circuit boards. The most significant advantages of the invention are the elimination of a great amount of mass (the mass of the wire winding), and the provision of coil connections that are integral to the printed circuit and therefore much more robust than prior art solenoid actuator construction. 
     PCBs can be manufactured with up to thirty layers of copper in a wide range of copper/insulator thicknesses. As is described in the prior art, a copper-trace spiral may be printed on each layer, resulting in very thin, lightweight coils. It is relatively easy to generate complex patterns on each layer to optimize the resultant magnetic field (shape and strength), and internal thermal planes can also be included to optimize heat rejection. A PCB bearing a large plurality of layers in surprisingly thin, a flat board in the range of 0.1 inch, with a mass that is a small fraction of the mass of wire in a comparable wirewound electromagnet. 
     In one aspect the invention comprises an electromagnetic coil formed of multiple printed conductor segments on multiple lamina of a multilayer PCB. The conductor segments are loops or spirals that are all disposed about a common axis and interconnected to form an embedded electromagnet in which the field contributions of each conductor segment are oriented for mutual reinforcement. A shaft extends through an opening formed coaxially in the PCB, and a permanent magnet with axially opposed poles is secured to the shaft in proximity to the PCB. Applying current to the embedded electromagnet generates a magnetic field that may attract or repel the permanent magnet, depending on the direction of the current and the resulting magnetic field. The permanent magnet thus drives the shaft axially to do useful work. A spring may be secured to one of the embedded PCB coils and connected to the shaft so that the shaft is resiliently biased axially with respect to the PCB, thus to establish a normal quiescent state. 
     In another aspect the invention comprises a pair of embedded PCB coils described above and assembled in parallel, spaced apart, coaxial relationship. A shaft extends through the central openings of each embedded coil, and the permanent magnet is disposed intermediate the two embedded PCB coils. The coils may be driven so that one repels the permanent magnet while the other attracts it, whereby the shaft may be driven reversibly to do useful work. The assembly may be augmented with a ferromagnetic detent component secured to one or both of the pair of embedded PCB coils. When no current is applied to the coils, the permanent magnet will be attracted preferentially to the nearest ferromagnetic detent component, thereby moving to a defined position adjacent the PCB coil. Powering the coils repels the permanent magnet away from the ferromagnetic detent component and attracts it toward the opposed end of the assembly. If both ends are provided with ferromagnetic detent components, the shaft will be magnetically latched at each end of its reversible axial motion in bistable fashion; if only one end has the detent, the shaft will return toward that one end whenever the coils are deactivated, in monostable motion. The ferromagnetic detent may comprise a strip or washer containing nickel, iron, steel, or the like. 
     The method and apparatus are suitable for devices of a size that is generally termed “micro”; that is, a dimension range of approximately 5-20 mm, though these figures are not necessarily size limitations. The micro-actuators described herein may be used to drive fluid pumping devices, fluid valves, electrical relay contacts, latch mechanisms, and the like. 
     In any of the aspects described above, the invention may include measures to guide the flux lines of the PM and the embedded electromagnets. The axially extending shaft is a key flux guide, and a metal or ferromagnetic frame or housing may extend between the PCBs that host the embedded electromagnetic coils. This increases the reluctance of the assembly and the efficiency of the device. 
     In a further aspect of the invention, a plurality of embedded electromagnetic coils may be arrayed in a common plane about a main axis transverse to the plane. A rotor is mounted on a shaft extending coaxially, and the rotor supports a plurality of PM having magnetic axes parallel to the main axis. The embedded coils are stationary, and are driven serially and sequentially to attract the PM in the rotor, so that the rotor is driven stepwise or continuously and useful work may be transferred through the shaft to a load. 
     In all of the embodiments and aspects of the invention, it is significant that most or all of the components may be assembled using established PCB fabrication processes and pick-and-place techniques that are easily accomplished in very high volume automated assembly lines. Thus these devices may be manufactured far more inexpensively than comparable prior art devices. Moreover, in comparison to existing solenoid actuators, the mass of wirewound coils is eliminated, and the fragile electrical connections of the fine wires of existing solenoids is replaced by fixed, robust connections of PCB construction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIGS. 1 and 2  are perspective views of the top and bottom surfaces, respectively, of a single layer of the multilayer PCB with embedded electromagnetic coils of the invention. 
         FIGS. 3 and 4  are plan views of the top and bottom surfaces, respectively, of a single layer of the multilayer PCB with embedded electromagnetic coils of the invention. 
         FIG. 5  is an exploded perspective view of a portion of the multilayer PCB with embedded electromagnetic coils of the invention. 
         FIG. 6  is a perspective view of one embodiment of a solenoid actuator using the multilayer PCB with embedded electromagnetic coils of the invention. 
         FIG. 7  is a plan view of the solenoid actuator depicted in  FIG. 6 . 
         FIGS. 8-10  are schematic views of the magnetic field lines of the embedded electromagnetic coils and the PM in different embodiments of a solenoid actuator. 
         FIG. 11  is a schematic view of a fluid pump device employing a solenoid actuator arrangement of the invention. 
         FIG. 12  is a schematic view of a fluid valve device employing a solenoid actuator arrangement of the invention. 
         FIG. 13  is an end view of the fluid valve device depicted in  FIG. 12 . 
         FIG. 14  is a bottom view of a brushless DC motor device employing the embedded PCB electromagnetic coils of the invention. 
         FIG. 15  is bottom view of a brushless DC motor device shown in  FIG. 14 . 
         FIG. 16  is a cross-sectional elevation of the brushless DC motor device shown in  FIGS. 14 and 15 . 
         FIG. 17  is a bottom view of a diaphragm pump or valve employing the embedded PCB electromagnetic coils of the invention. 
         FIGS. 18 and 19  are cross-sectional elevations of the diaphragm pump/valve of  FIG. 17 , showing it in the quiescent position and full stroke position, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally comprises a method and apparatus for construction of solenoid actuators and similar electromagnetic motor devices that employ printed coil structures that are embedded in multilayer circuit boards. With regard to  FIGS. 1-6 , a significant feature of the invention is the use of one or more embedded printed circuit electromagnetic coils  21  as a driver element for electromagnetic linear and rotary motors. Each embedded coil  21  is comprised of a plurality of individual lamina  22 , each having a spiral conductor  23  printed on one surface and a spiral conductor  27  printed on the reverse side. A central opening  33  extends coaxially through the coil  21 , and may be lined with a bushing (not shown). Conductor  23  terminates at its outer extent at contact pad/via  24  and at its inner extent at contact pad/via  26 , while conductor  27  terminates at its outer extent at contact pad/via  28  and at its inner extent at contact pad/via  29 . Each conductor may include as many as 10 or more concentric “turns” arranged in an Archimedean spiral in which the conductor curves in the plane of the lamina surface about a fixed central axis and increases smoothly in radial distance from the axis. These printed conductor formats of the preferred embodiment are not limiting factors for the invention in general. 
     The two spiral conductors are designed to proceed in opposite rotational directions, in the nature of left-hand and right-hand threads. The contact pad  24  of spiral conductor  23  is connected to a current source, and the inner contact pad/via  26  is connected to the inner contact pad/via of spiral conductor  27 . The outer contact pad/via  28  is connected to the next adjacent lamina  22 . Due to the fact that the coils  23  and  27  are reverse-handed, the magnetic fields created by the current flow through the two coils  23  and  27  are oriented in the same general direction and are additive, generating a strong local magnetic field that is polarized along the central axis. 
     With regard to  FIG. 5 , the lamina  22  are stacked together in coaxial alignment, with an insulating binder layer  31  interposed between each two adjacent lamina  22 . Vias  32  are provided so that the contact  28  of one lamina  22  may be connected to the contact  24  of the next adjacent lamina  22 . The processes involved in printing the spiral conductors, forming the contact pads and vias, and laminating the layers together are all well-known in the printed circuit industry, and are reliable and inexpensive. PCB&#39;s having 20 or more layers are commonplace, and may be compressed into a multilayer board that is approximately 0.1 inch thick. As an example, providing twenty lamina  22 , each having two printed coils with 10 turns each yields a combined coil of 400 turns in a very thin space, and the result is a surprisingly strong magnetic field. It appears that the current density (the radial and axial copper density or packing fractions) may be as important as the number of turns, and that there is an opportunity for significant optimization of embedded coils by modifying packing fractions within the laminated assembly. 
     The embedded coils  21  described herein may be employed in a variety of magnetomotive applications. With reference to  FIG. 6 , a solenoid actuator may be formed by a pair of embedded coils  21  that are disposed parallel, spaced apart, and coaxial. In this embodiment the coils  21  are embedded in square plates  41  formed by cutting the coil  21  from a larger circuit board assembly. Other perimeter shapes such as rectangular, circular, hexagonal, and the like may be employed. A plurality of struts  34  are secured between the two plates  41  to maintain their spacing and rigid connection, the struts  34  having opposed ends that are secured adjacent respective vertices of the plates  41 . A shaft  36  extends coaxially and is received through the central openings  33  of the plates  21 , and a disk-like permanent magnet  37  is secured coaxially to a medial portion of the shaft  36 . The magnet  37  is preferably a rare earth, high strength magnet, although other permanent magnets or ferromagnetic materials may suffice for some uses that require a less forceful device. 
     The opposite poles of magnet  36  are aligned coaxially with the shaft  36 , and thus are in proximate relationship to respective plates  41  and their embedded coils  21 . The shaft is an important part of the magnetic flux circuit of the device. Each of the coils  21  may be connected to a current source that is selectively directional, so that the each coil  21  may generate an electromagnetic field having opposite polarities that are aligned coaxially with the shaft  36  and the device in general. The polarity of the magnetic field may be reversed by reversing the current, a fundamental principle known in the prior art, to selectively generate magnetic poles that either repel or attract the adjacent poles of the permanent magnet  37 . Thus, for example, as shown in  FIG. 8 , the coil of upper plate  41 ′ is driven to generate a magnetic field that repels the adjacent pole of permanent magnet  37 , while the coil of lower plate  41 ″ is driven to generate a magnetic field that attracts its adjacent pole of magnet  37 . As a result both magnetic fields drive the magnet  37  and shaft  36  linearly along the axis of the device, delivering a stroke of useful length and force. Clearly, the electromagnetic fields may be reversed to drive the shaft reversibly along the axis. The shaft motion may be cyclical, intermittent, sporadic, or continuous, depending on the electrical signals (AC, DC, pulsed) that drive the coils  21 . 
     The solenoid actuator may additionally be provided with a ferromagnetic detent component secured to one or both of the pair of embedded PCB coils. For example, a washer or bushing  30  may be secured in the central opening  33  of one or both plates  41  and dimensioned to allow free translation of the shaft  36 . When no current is applied to the coils, the permanent magnet  37  will be attracted preferentially to the nearest ferromagnetic detent component  30 , thereby moving to a defined position adjacent the respective PCB coil. Powering the coils repels the permanent magnet  37  away from the ferromagnetic detent component and attracts it toward the opposed end of the assembly. If both ends are provided with ferromagnetic detent components, the shaft will be magnetically latched at each end of its reversible axial motion in bistable fashion; if only one end has the detent, the shaft will return toward that one end whenever the coils are deactivated, in monostable motion. This simple latching technique is achieved using very little added mass and no latch assembly. 
     For example, an exemplary device constructed as shown in  FIGS. 6 and 7 , having a total weight of about 5 grams, can produce a useful stroke of 0.25 inches at 8 oz. force. This compares to a solenoid actuator known in the prior art and having similar stroke and force outputs, which weighs on average 50 oz. This is a considerable advance over the prior art. Moreover, the fact that the device may be fabricated virtually entirely using established PCB fabrication methods and pick-and-place devices results in significant savings in production cost. 
     In an alternative embodiment shown in  FIG. 9 , the plate  41 ′ is connected by struts  34  to a plate  42  that does not include an embedded coil  21 . A spring is mounted on the end of shaft  36  and supported to exert a restoring force in response to axial motion of the shaft  36 . When the coil of upper plate  41 ′ is actuated, it will attract the permanent magnet  37 , moving the shaft axially toward the plate  41 ′ and compressing spring  43 . When the coil of upper plate  41 ′ is deactivated, the spring force restores the magnet  37  to a position spaced apart from the plate  41 ′. Thus the shaft  36  has an inherent quiescent position, the electromagnetic drive moves the shaft only when the coil  41 ′ is activate, and the shaft returns to the quiescent position after activation. 
     As noted above, the solenoid actuators described herein may be driven cyclically, intermittently, or continuously. When driven by a low frequency audio signal, the solenoid actuators vibrate perceptibly. They may be installed in a portable consumer product and used to provide haptic feedback to the user. 
     In a further embodiment of the solenoid actuator shown in  FIG. 10 , all the components are assembled as shown and described in  FIG. 8 . However, in this embodiment the magnet  37 ′ is polarized in diametrical opposition rather than axial opposition. When the coils of plates  41 ′ and  41 ″ are activated their magnetic fields interact with magnet  37  to cause it to rotate, thus driving the shaft  36  in a limited rotational excursion. Stops may be provided on the shaft  36  to prevent axial translation, if necessary. 
     There are many possible applications of the embedded coil concept with a moving magnet to simple machines in a small format, and some of them are described below. With regard to  FIG. 11 , the solenoid actuator construction of  FIGS. 6 and 7  may be employed as a simple pump. All of the components described in that solenoid actuator are employed, although the struts  34  may be replaced by a housing  50  that joins to the end plates  41  and encloses the device. In addition, a bladder  51  having a toroidal shape is interposed between the magnet  37  and one of the end plates  41 , and the shaft  36  extends through the central opening of the toroidal bladder. The bladder  51  includes an inlet port  52  and outlet port  53 , and appropriate check valves are provided but not shown. Whenever the device is actuated to drive the magnet  37  toward the bladder  51 , the bladder is compressed and fluid is driven from the bladder; when the magnet  37  moves away from the bladder  51 , the bladder refills due to its natural elasticity. 
     With regard to  FIGS. 12 and 13 , a simple valve may be constructed using the same basic solenoid actuator components described in  FIGS. 6 and 7 . A valve element  61  extend diametrically adjacent to one of the plates  41 , and a flow channel  62  extends longitudinally through the valve element. In the center of the valve element  61 , a valve seat  63  (here a cylindrical coaxial bore) extends through the valve element. A post  64  is secured coaxially to the magnet  37  adjacent to the valve element  61 , and is dimensioned to be received in seat  63  in sealing fashion. A fluid source is connected to one end of the channel  62 . When the device is actuated, the magnet is driven in the direction of the motion arrow, and the post  64  is translated into the seat  63  until it bottoms out, whereby the flow channel  62  is blocked. Reversing the movement of the magnet  37  opens the channel for fluid flow. As described previously, the use of a ferromagnetic latching component  30  may impart a normally closed or normally open characteristic to the valve. 
     With reference to  FIGS. 15-17 , a further embodiment for generating rotational motion comprises a brushless DC motor that employs the embedded coils of the invention. A plurality of embedded coils  71  are constructed similarly to embedded coils  21  described previously, and are arrayed at equal angles about a central opening  72 . The coils  71  may be formed individually and assembled a shown (hence the hexagonal perimeter of the coils), or preferably may be formed together on the same PCB  70 . A disk-like armature  73  is directly adjacent to the PCB  70 , and includes an axially extending shaft  76  that extends through opening  72  in freely rotating fashion. The armature  73  includes a plurality of disk-like permanent magnets  74  arrayed at equal angles about the central axis of the assembly. The magnets  74  are polarized along axes parallel to the central axis of the assembly, and are thus oriented to interact with the magnetic field polarities of the coils  71 . As is known in the prior art, the magnetic fields of the coils  71  may be switched sequentially and cyclically to attract the permanent magnets  74  in progressive angular fashion, causing the armature  73  to rotate. The switching of the polarity of the coils  71  is accomplished without brushes, slip-rings, or any other form of moving electrical contacts. A load may be coupled to the rotating shaft  76  to accomplish useful work. 
     With regard to  FIGS. 17-19 , another embodiment of the invention employs an embedded coil  81  formed similarly to the coils  21  and  71  described previously. A central opening  82  extends axially through the coil  81 , and a pin  83  formed of ferromagnetic material is secured in the opening  82 . A pair of ports  84  and  86  also extend through the coil assembly  81  adjacent to the opening  82 . A diaphragm  87  is secured at its perimeter to one surface of the coil  81 , the diaphragm having a diameter sufficient to span and overlap the ports  84  and  86 . Secured to a central portion of the diaphragm  87  is a permanent magnet  88  that is polarized along the axis of the assembly. 
     The ports  84  and  86  may be connected to a source of fluid and a fluid destination, respectively. The magnet  88  is attracted to the ferromagnetic pin  83  and pushes the center of the diaphragm  87  toward the upper surface of the embedded coil  81 , creating a flush impingement of the diaphragm on the upper surface of the coil  81 , as shown  FIG. 18 . As a result, there is no flow space between the diaphragm  87  and the upper surface of coil  81 , and no opportunity for fluid to flow from port  84  to port  86 . When the coil  81  is energized to repel the magnet  88 , the magnet and diaphragm are driven away from the upper surface of the coil  81  ( FIG. 19 ), and the diaphragm forms a flow space  89  between itself and the coil  81 , thereby connecting the ports  84  and  86  for fluid flow therebetween. Thus the device of  FIGS. 17-19  comprises a normally closed fluid valve. 
     The device of  FIGS. 17-19  may be equipped with check valves connected to ports  84  and  86 , in which case the coil  81  may be actuated to expand the diaphragm and draw fluid from inlet port  84  into the flow space  89 . When the coil  81  is deactivated, the attraction of magnet  88  to pin  83  will collapse the diaphragm against the upper surface of the coil  81  and drive the fluid from flow space  89  through outlet port  86 . Thus the device of  FIGS. 17-19  may be configured as a fluid pump. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching without deviating from the spirit and the scope of the invention. The embodiment described is selected to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as suited to the particular purpose contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.