Abstract:
A DC motor is commutated by rotating a magnetic rotor to induce a magnetic field in at least one magnetostatic relay in the motor. Each relay is activated in response to the magnetic field to deliver power to at least one corresponding winding connected to the relay. In some cases, each relay delivers power first through a corresponding primary winding and then through a corresponding secondary winding to a common node. Specific examples include a four-pole, three-phase motor in which each relay is activated four times during one rotation of the magnetic rotor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/080,063, filed on Mar. 31, 1998. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title. 
    
    
     TECHNOLOGICAL FIELD 
     The invention relates to miniaturization of electronic components and, in particular, to using a micromachined magnetostatic relay in commutating a DC motor. 
     BACKGROUND 
     Manufacturers and users of electrical and electronic components strive to reduce the size and increase the reliability of these components and the systems in which they are used. Miniaturization of components leads to more compact and lightweight systems, which increases the range of uses for these systems and decreases the costs associated with transporting and using these systems. Improving component reliability lengthens the lifespan and enhances the performance of systems in which the components are used. 
     Miniaturization and reliability improvements are particularly important in areas such as space exploration and satellite communications. The cost of launching equipment from the Earth&#39;s surface is directly related to the size and weight of the equipment, and even modest reductions in equipment size produce large reductions in cost. Likewise, improving the reliability of components used in spaceborne systems extends and improves the performance of these systems, thus reducing the associated costs. In general, each newly developed generation of space oriented components and systems must meet or exceed the performance and cost standards set by previous generations. 
     One example of commonly used components for which size and reliability are particularly important is DC electric motors. DC motors are used widely as motive devices for linear and rotary drives in spaceborne applications. As gains have been made in the miniaturization of DC motors, the size, weight, and complexity of DC motor systems have become dominated by the commutation and control electronics that drive the motors. The disparity between the size of the motor and the size of its control electronics is particularly noticeable in a highly miniaturized motor, such as a commercially available 3-mm diameter motor, the commutation and control electronics of which are more than ten times larger than the motor itself. Even modest reductions in the power budget, complexity, mass, and volume of components such as these produce tremendous gains in the cost and reliability of spaceborne systems. 
     SUMMARY 
     In recognition of the above, the inventors have developed micromachined magnetostatic relays or switches that are highly miniaturized and highly reliable. The switches are made very small using micromachining fabrication techniques, and the materials are carefully selected to provide high reliability. The switches are useful in a wide variety of microelectronic mechanical system (MEMS) applications, particularly in the miniaturization of DC electric motors. For example, in one embodiment of the invention, the switches are used as relays in a MEMS circuit that replaces the conventional commutation and control electronics in a DC motor. This MEMS circuit is much smaller than the DC motor itself, so the size of the motor, not the size of the commutation electronics, is most critical in space constrained applications. The magnetostatic switch requires no biasing current or voltage and is useful in directly switching loads. 
     In one aspect, the invention features a DC motor having a plurality of windings and at least one magnetostatic relay positioned to activate in the presence of a magnetic field. Each relay is connected electrically to at least one corresponding winding and to power. The motor also includes a magnetic rotor having at least one pole positioned to induce a magnetic field in each magnetostatic relay when passing by the relay. 
     In some embodiments, the windings are arranged in pairs of primary and secondary windings, and each relay connects to a corresponding one of the pairs of windings. In some cases the secondary windings all connect to a common node, and each of the primary windings connects to the corresponding relay. In one implementation, the motor is a four-pole, three-phase motor that includes three relays separated from each other by approximately 120°. 
     In another aspect, the invention features a DC motor having a plurality of windings and at least one magnetostatic relay connected electrically to at least one of the windings and to power. Each relay has at least one substrate formed from a non-conductive or semiconductive material, a springing beam formed on the substrate, and two electrically conductive elements, one of which is formed on the springing beam. The electrically conductive-elements together define at least two-switching states, including an open state in which the conductive elements are physically separated from each other, and a closed state in which the conductive elements physically contact each other. The springing beam includes a magnetic material which, in the presence of a magnetic field, creates an actuation force that causes the electrically conductive elements to apply power to or remove power from at least one of the windings by switching from one of the switching states to another of the switching states. The motor also includes a magnetic rotor having at least one pole positioned to induce a magnetic field in each magnetostatic relay when passing by the relay. 
     In another aspect, the invention features a method for use in commutating a DC motor. The method includes rotating a magnetic rotor to induce a magnetic field in at least one magnetostatic relay in the motor. Each relay is activated in response to the magnetic field to deliver power to at least one winding in the motor. 
     In some embodiments, each relay first delivers power through a corresponding primary winding and then through a corresponding secondary winding to a common node. Other embodiments include activating each relay four times during one rotation of the magnetic rotor. 
     Other embodiments and advantages will become apparent from the following description and from the claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are simplified diagrams of a normally-open magnetostatic switch. 
     FIGS. 2A and 2B are simplified diagrams of a normally-closed magnetostatic switch. 
     FIGS. 3A,  3 B, and  3 C are diagrams illustrating, in cross-section, the fabrication of a magnetostatic switch micromachined from two substrates. 
     FIGS. 4A,  4 B,  4 C,  4 D, and  4 E are perspective views of a substrate at several steps of a two-substrate switch fabrication process. 
     FIGS. 5A,  5 B, and  5 C are plan views of substrates in a three-substrate switch fabrication process. 
     FIG. 6 is a plan view of a DC motor having a MEMS commutation circuit that uses micromachined magnetostatic switches. 
     FIG. 7 is a schematic diagram of the motor windings for the DC motor of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A and 1B show a normally-open microelectronic mechanical system (MEMS) relay or switch  100 . The switch  100  includes a cantilever beam  105  mounted on a substrate  110 . For convenience, the substrate  110  is made from a substrate material, such as silicon, that is plentiful and relatively inexpensive. The substrate  110  includes an electrical contact  125  made of an electrically .conductive material, such as gold or silver, with a relatively low contact resistance at modest contact forces. The cantilever beam  105  includes a magnetic actuation plate  120  which, in many embodiments, is made of a soft magnetic material with high permeability, such as permalloy (Ni 80 Fe 20 ). The cantilever beam  105  also includes an electrical contact  115 , which may or may not be made of the same material that forms the contact  125  on the substrate  110 . 
     As shown in FIG. 1A, the cantilever beam  105  keeps the electrical contacts  115 ,  125  separated when the switch  100  is inactive, i.e., when no magnetic field is present. When an external magnetic field H appears, magnetic forces attempt to align the magnetic actuation plate  120  with the magnetic field H, causing the cantilever beam  105  to bend toward the substrate. If the strength of the magnetic field exceeds the design threshold of the switch, the electrical contacts  115 ,  125  touch, as shown in FIG. 1B, completing an electrical circuit through bond wires  130 ,  135 . The electrical circuit is broken when the magnetic field disappears and the restoring force of the cantilever beam  105  separates the electrical contacts  115 ,  125 . In alternative implementations, the cantilever beam  105  is designed to separate the contacts  115 ,  125  when the direction or the magnitude of the magnetic field changes. 
     FIGS. 2A and 2B show a normally-closed MEMS switch  200  of similar structure. The cantilever beam  205  in this switch is mounted on the substrate  210  so that the electrical contacts  215 ,  225  of the beam  205  and the substrate  210  are held together when the switch  200  is inactive. Applying a magnetic field to the magnetic actuator plate  220  causes the beam  205  to bend away from the substrate  210 , thus separating the contacts  215 ,  225 . The contacts  215 ,  225  come together again when the magnetic field disappears or, alternatively, when the direction or the magnitude of the magnetic field changes. 
     An alternative design for the normally closed switch resembles the normally open switch of FIG. 1A, except that the cantilever beam  105  is formed such that residual stress imparts ID curvature to the beam  105 , holding the tip of the beam  105  against the lower electrical contact  125 . In this embodiment, subjecting the beam  105  to a magnetic field creates a  105  force that opposes the residual stress in the beam  105 , pulling the contacts  115 ,  125  apart. 
     Several design parameters are considered when designing micromachined magnetostatic switches like these. For a normally-open switch, these parameters include load voltage, maximum current through the switch, operating force (i.e., the force between the contacts when the switch is closed), contact closing time, and lifetime operations. Table I below shows typical values for these parameters in three types of switches: conventional electrostatic microswitches, conventional electromagnetic microswitches, and the micromachined magnetostatic switch described here. This table shows, among other things, that the micromachined magnetostatic switch produces much larger contact forces than the conventional microswitches produce, which reduces contact resistance and thus supports much larger operating currents. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                   
                   
                 Micromachined 
               
               
                   
                   
                 Electrostatic 
                 Electromagnetic 
                 Magnetostatic 
               
               
                 Parameter 
                 Unit 
                 Microswitch 
                 Microswitch 
                 Switch 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 load voltage 
                 volts 
                 20 
                 20 
                 36 
               
               
                 maximum 
                 mA 
                 0.1 
                 100 
                 &gt;500 
               
               
                 current 
               
               
                 operating 
                 mN 
                 0.001 
                 0.1 
                 &gt;1 
               
               
                 force 
               
               
                 contact gap 
                 μm 
                 2 
                 &gt;5 
                 &gt;5 
               
               
                 contact 
                 μsec 
                 20 
                 200 
                 &lt;100 
               
               
                 closing time 
               
               
                 lifetime 
                 cycle 
                 &gt;10 million 
                 N/A 
                 &gt;100 million 
               
               
                 operations 
               
               
                   
               
             
          
         
       
     
     In most situations, the micromachined magnetostatic switches and the systems in which they are used are designed to produce large actuation forces, which leads to several additional benefits. Larger actuation forces are present allow a stiffer s cantilever beam, which leads to shorter switching time, higher g-force tolerance, and greater contact breaking force. Greater contact breaking force in turns leads to increased switching lifetime. Large actuation forces also provide the large contact forces, typically between 100 μN and 1 mN, required to yield an acceptable contact resistance when common contact materials, such as silver and gold, are used. The presence of large actuation forces also allows the switches to be designed with large gap distances between contacts, which increases device breakdown voltage. 
     The force generated at the free end of the cantilever beam is represented by the equation: 
     
       
           F   bending   =M   s ( WT ) H  cos θ, 
       
     
     where T=the thickness of the magnetic actuation plate  120 , W=the width of the plate  120 , L=the length of the plate  120 , θ=the deflection angle of the beam  105  (θ=0 when the switch is inactive), H=the magnitude of the external magnetic field, and M s =the saturation magnetization of the magnetic material. This equation shows that the bending force is greatest when the values of M s , W, T, and H are large and the deflection angle (θ) is small. In a DC motor, the magnitude of magnetic field (H) is determined by the motor itself, and the deflection angle is determined by the desired gap distance between the contacts in the switch. In most embodiments, the gap distance between contacts and the rotation of the beam are very small, so θ≈0. 
     A soft magnetic material such as permalloy has a high saturation magnetization (M s  greater than 0.8 Tesla), has thick plating capability, and automatically magnetizes with the desired magnetization orientation when actuated. Therefore, materials such as permalloy can be advantageous for constructing the magnetic actuation plate. Forces in excess of 5 mN are easily obtained with a permalloy actuation plate having a width of 3 mm and a thickness of 10 μm in a DC motor that produces a magnetic field strength of approximately 2500 gauss. 
     FIGS. 3A,  3 B, and  3 C show a magnetostatic switch micromachined from two rigid substrates  306 ,  305 , each of which is made from a material such as silicon. The first substrate  300  (FIG. 3A) includes a magnetic actuation plate  310  formed on a surface  315  of the substrate  300 . The size of the plate  310  and the materials used to form the plate  310  are determined by the factors discussed above. The plate  310  is formed over a sacrificial spacing layer (not shown here) that is deposited on a portion of the surface  315  of the substrate  300 , as discussed in more detail below. After the spacing layer is removed, the plate  310  forms a cantilevered beam, a portion of which contacts the substrate  300 , and the rest of which is separated from the substrate  300  by the void left by the spacing layer. An optional contact layer  320  appears on the cantilever portion of the plate  310 . 
     The second substrate  305  (FIG. 3B) includes a contact layer  325 . A permanent spacing layer  330  is deposited and patterned over a portion of the contact layer  325 . Alternatively, the spacing layer  330  is formed directly on the substrate  300 . The height of this spacing layer  330  is determined by the desired gap distance between the contact layers  320 ,  325 . As shown in FIG. 3C, the substrates  300 ,  305  are bonded or clipped together to form a switch. One or more bond wires  335 ,  340  are connected to the magnetic actuation plate  310  on the first substrate  300  and to the contact layer  325  on the second substrate  305 . 
     FIGS. 4A through 4E show one technique for creating the magnetic actuation plate on a substrate  400 . First, a sacrificial spacing layer  405  is deposited onto the substrate  400  (FIG.  4 A). The spacing layer  405  is formed from an etchable material, such as photoresist. In highly miniaturized switches, the spacing layer  405  typically has a thickness of between 2 μm and 20 μm. The spacing layer  405  is patterned to form anchor holes  410 , which allow the magnetic material forming the actuation plate to bond with the substrate  400  as described below. In many switches, a very thin electroplating seed layer is deposited over the spacing layer to facilitate formation of the magnetic actuation plate. 
     A photoresist plating mold layer  415  then is deposited over the spacing layer  405  and patterned to form a mold cavity  420  (FIG.  4 B). The mold cavity  420  exposes most of the spacing layer  405 , including the anchor holes  410 . A magnetic material  425 , such as permalloy, is deposited onto the spacing layer  405 , filling the mold cavity  420  (FIG.  4 C). The magnetic material  425  also fills the anchor holes  410  in the spacing layer  405 , forming anchors (discussed below) that contact the substrate  400  directly. The magnetic material  425  is deposited to a thickness of between 10 μm and 20 μm in many highly miniaturized switches. 
     A layer of contact material  435  then is deposited over the layer of magnetic material  425  (FIG.  4 D). The contact material  435  is selected from a wide range of materials with good electrical contact properties, including evaporated metals such as gold and silver. A typical thickness for the contact layer  435  is between 0.1 μm and 10 μm. 
     An etchant then is used to remove the photoresist mold layer  415  and spacing layer  405  from the substrate  400 , leaving a magnetic actuation plate  440  mounted to the substrate  400  by anchors  445 . The magnetic actuation plate  440 , which includes the layers of magnetic material  425  and contact material  435 , is spaced above the substrate  400  by the thickness of the stripped spacing layer  405 . 
     Fabrication of the second substrate is carried out as shown in FIG. 3B. A layer of contact material is deposited onto a rigid substrate. A permanent spacing layer then is deposited over the contact material and patterned to avoid inhibiting the operation of the magnetic actuation plate. A wide variety of materials, such as photoresist, glass, plated metals, and plastic, are used to form the permanent spacing layer. A typical thickness for this layer is between 10 μm and 200 μm, depending on the desired operating characteristics of the switch. The two substrates then are bonded together to form an operational switch. 
     In other embodiments, the magnetic material is deposited is onto a cantilevered beam formed in the silicon substrate. One fabrication technique uses an anisotropic silicon etchant to produce a cavity in a silicon substrate frame. Etching stops just short of the opposing surface of the substrate, creating a thin silicon membrane at the bottom of the cavity. A photoresist layer then is deposited onto the membrane and patterned to form the shape of the cantilevered beam. The substrate undergoes an etching process, such as reactive ion etching (RIE), to remove all exposed portions of the membrane, leaving only a cantilevered beam connected to the substrate frame, similar to that shown in FIG.  5 A and discussed below. In some cases, the cantilever beam is formed into complex shapes. For example, in one implementation the plate is attached to the substrate via torsional beams. In another implementation, one end of the plate is shaped into multiple independent fingers, as shown in FIG.  5 A. The magnetic material is deposited onto the cantilevered beam using standard techniques, such as permalloy electroplating. This process allows single crystal silicon to serve as the mechanical spring material. Single crystal silicon has strength properties similar to steel without the plastic deformation limitations. 
     FIGS. 5A through 5C show the components of a switch fabricated from three substrates. The first substrate  500  (FIG. 5A) includes the magnetic actuation plate  515 , which is formed on the surface of or as a cantilevered beam  520  in the substrate  500 . Electrical contacts  525  are molded at the free end of the cantilevered beam  520 . At least a portion of the substrate  500  includes a conductive layer  535  that allows electrical connection between the contact points at the end of the magnetic actuation plate  515  and at least a portion of the surrounding frame. In many switches, this conductive layer  535  is the magnetic plate itself. Alternatively, the conductive layer  535  is formed by depositing an electrical contact material, such as silver or gold, over the surface of the substrate  500 . Holes or recesses  530   a-d  are formed in the substrate  500 , including in the conductive area  535 , to allow alignment and, in some cases, electrical contact with the other substrates. 
     The second substrate  505  (FIG. 5B) includes a conductive contact plate  540  that connects electrically to the magnetic actuation plate  515  only when the switch is active. The contact plate  540  often is formed from the same material as the electrical contacts  525  on the magnetic actuation plate  515 , but other contact materials also are used. The second substrate  505  also includes spacers  545 ,  550   a-d  that provide the required physical separation between the magnetic actuation plate  515  and the contact plate  540 . The spacers  545 ,  550   a-d  can be formed in place from nearly any material, either conductive or insulative. The spacers may also be placed manually, if desired. One approach uses a low-resistance conductive material, such as copper, that is electrodeposited onto the surface of the substrate  505  through a mold. In this implementation, at least one spacer  545  connects electrically to the contact plate  540 . The remaining spacers  550   a-d  may or may not connect electrically to the magnetic actuation plate  515 . Each spacer  545  that connects to the contact plate  540  is isolated electrically from the spacers that connect to the magnetic actuation plate  515 . Holes  555   a-c ,  560   a-d  in the spacers allow alignment and, in some cases, electrical connectivity with the other substrates. 
     The third substrate  510  (FIG. 5C) serves as an output and protective layer for the switch. This substrate  510  includes two conductive areas  565 ,  570   a-b  that are electrically isolated from each other. One of these areas  570   a-b  connects electrically to the magnetic actuation plate  515 . The other area  565  connects to the spacer  545 , which is connected to the contact plate  540 . The two areas  565 ,  570   a-b  connect electrically to each other only when the switch is active, i.e., only when the magnetic actuation plate  515  and the contact plate  540  are in contact. The conductive areas  565 ,  570   a-b  terminate in conductive pads  575   a-b ,  580   a-b  that allow the switch to connect to outside circuitry. 
     Several alignment pegs  585   a-c ,  590   a-d  extend from the conductive areas  565 ,  570   a-b  on this substrate  505 . These pegs allow alignment with the other substrates and, in some cases, are electrically conductive to ensure electrical connectivity with the other substrates. The first substrate  500  rests directly on the third substrate, with four of the pegs  590   a-d  protruding through the holes  530   a-d  in the first substrate  500 . In some cases, the pegs bond to the conductive surface  535  of the substrate  500  through a conductive bonding material, such as solder, thus connecting the magnetic actuation plate  515  to the corresponding conductive area  570   a-b  of the third substrate  510 . In other applications, an insulative adhesive, such as epoxy, is used. 
     The second substrate  505  sits directly over the first substrate  500 . One set of pegs  585   a-c  protrudes into the holes  555   a-c  in the spacer  545  that connects to the contact plate  540 , thus bonding the contact plate  540  to the corresponding conductive area  565  on the third substrate  510 . The other set of pegs  590   a-d  protrudes into the holes  560   a-d  in the other spacers  550   a-d . The pegs and the spacers usually are bonded using a conductive bonding material, such as solder. The first substrate  500  and the second substrate  505  are oriented so that the magnetic actuation plate  515  touches the contact plate  540  when the switch is active. 
     FIG. 6 shows a DC motor  600  having a commutation circuit that includes micromachined magnetostatic relays  602 ,  604 ,  606  like those described above. In this example, the motor  600  is a four-pole, three-phase brushless motor having three pairs of primary and secondary windings A-A′, B-B′, C-C′. The windings in each pair are positioned on opposite sides of the motor housing  608  and are separated by a magnetic rotor having four poles. The relays  602 ,  604 ,  606  here are shown in relative positions in which they are spaced by angles of 120° and are placed in close proximity to stator poles. Absolute positioning of the relays  602 ,  604 ,  606 , and even the number of relays, depends on the particular motor and wiring implementation with which they are used. More complex commutation techniques involving micromachined relays include H-bridge circuits, zener diode shunts, and other electronics. The particular commutation circuit used depends on the desired performance and lifetime characteristics for the motor in a particular application. 
     FIG. 7 is a schematic diagram showing the windings of the DC motor of FIG. 6 wired into a common “Y” or “star” configuration. The circuit  700  includes three branches  702 ,  704 ,  706  extending from a common node  708 . Each of the branches includes one of the pairs of primary and secondary windings A-A′, B-B′, C-C′, connected in series between the common node  708  and one of three power nodes  710 ,  712 ,  714 . The common node  708  connects to ground. Each of the power nodes  710 ,  712 ,  714  connects to a power supply line (PWR) through one of the magnetostatic relays  602 ,  604 ,  606 . The magnetostatic relays  602 ,  604 ,  606  close, and therefore apply power to the corresponding branches  702 ,  704 ,  706  of the circuit  700 , each time the magnetic rotor  610  induces a magnetic field in the relays. 
     Other embodiments are within the scope of the following claims. For example, some embodiments of the micromachined magnetostatic switch are produced using a single-substrate fabrication technique, instead of the two-substrate and three-substrate techniques described above. Also, in many applications the switch is formed from magnetic and electrically conductive materials having properties different than the properties of those materials described above. For example, magnetic materials other than permalloy are used in applications requiring higher or lower values for the saturation magnetization of the material. Also, the magnetic material itself may be used instead of a traditional contact material to form the electrical contact when the switch is closed. In some embodiments, a springing element such as a torsional beam or a helical spring is used instead of or in addition to the cantilever beam. 
     The micromachined magnetostatic switch is suitable for a wide range of applications other than commutation of DC motors, including virtually any application for which traditional relays and switches are used. Examples include telecommunication switching, household electronics and appliances, computers, and handheld electronics. The magnetostatic switch also is useful as a magnetic field sensor. An array of these switches designed to respond to magnetic fields of varying strengths provides very sensitive magnetic field measurements. As a result, these switches also are useful in applications previously reserved for traditional magnetic devices, such as Hall Effect sensors. The switches are useful as rotational and linear encoders, as well as in applications previously requiring reed relays.