Patent Publication Number: US-10770994-B2

Title: Variable torque electric motor assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/794,183 filed Oct. 26, 2017, the contents of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to electric machines. More specifically, the subject disclosure relates to magnetic flux regulation of electric machines. 
     Many electrical actuators employ a DC electric motor, such as a Permanent Magnet DC Motor (PMDCM) to move a surface or object. Typically, the size of the actuator is related to the available power that must be delivered. In some cases, electrical actuators are needed to be able to deliver maximum power under a wide range of conditions and faults. 
     A PMDCM is limited in power by the available flux developed by the magnets on the rotor of the motor. In conventional PMDCMs, the magnetic field strength dictates the size of the rotor, because the rotor holds the permanent magnet(s). The higher the power, the more magnetic flux is needed, either through larger size or stronger magnets. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention an actuator assembly that includes an electric motor including a rotor assembly and a stator assembly configured to be actuated to cause the rotor assembly to rotate based on an amount of magnetic flux in the rotor assembly is disclosed. The assembly also includes a controllable magnetic device coupled to the rotor assembly, an actuator coupled to the rotor assembly; and a controller configured to apply electric current to the controllable magnetic device to adjust an amount of torque provided by the electric motor by adjusting the magnetic flux in the rotor assembly. 
     According to any aspect disclosed herein, the electric motor is configured to generate an amount of torque that is less than or equal to a maximum amount of torque, and the controllable magnetic device is configured to increase the maximum amount of torque based on a change in a condition that affects the electric motor. 
     According to any aspect disclosed herein, the controller is configured to apply the electric current in a first direction to increase the amount of torque by increasing the magnetic flux in the rotor assembly. 
     According to any aspect disclosed herein, the controller is configured to apply the electric current in a second direction to decrease the amount of torque by decreasing the magnetic flux in the rotor assembly. 
     According to any aspect disclosed herein, the controller includes field coupler coupled to the rotatable member and configured to rotate with the rotor assembly. 
     According to any aspect disclosed herein, the field coupler is configured to be energized by stationary windings. 
     According to any aspect disclosed herein, the controller includes a rectifier device configured to convert alternating current generated by the field coupler to direct current and apply the direct current to the controllable magnetic device. 
     According to any aspect disclosed herein, the controllable magnetic device includes one or more windings coupled to the rotatable member. 
     According to any aspect disclosed herein, the rotor assembly includes a permanent magnet. 
     According to any aspect disclosed herein, the rotor assembly and the stator assembly form at least part of a brushless direct current (DC) motor. 
     According to one aspect, a method of controlling an actuator assembly is disclosed. The method includes: actuating an electric motor by applying a first electric current to a stator assembly to cause rotation of a rotor assembly, the rotor assembly configured to rotate based on an amount of magnetic flux in the rotor assembly; causing movement of an actuator coupled to the rotor assembly; and applying a second electric current to a controllable magnetic device coupled to the rotor assembly, the second electric current causing an amount of torque provided by the electric motor to change by adjusting the magnetic flux in the rotor assembly. 
     According to any aspect disclosed herein, the electric motor is configured to generate an amount of torque that is less than or equal to a maximum amount of torque, and the controllable magnetic device is configured to increase the maximum amount of torque based on a change in a condition that affects the electric motor. 
     According to any aspect disclosed herein, the first electric current is applied in a first direction to increase the amount of torque by increasing the magnetic flux in the rotor assembly. 
     According to any aspect disclosed herein, the first electric current is applied in a second direction to decrease the amount of torque by decreasing the magnetic flux in the rotor assembly. 
     According to any aspect disclosed herein, the second electric current is applied via a field coupler coupled to the rotatable member and configured to rotate with the rotor assembly. 
     According to any aspect disclosed herein, the field coupler is configured to be energized by stationary windings. 
     According to any aspect disclosed herein, the field coupler is connected to a rectifier device configured to convert alternating current generated by the field coupler to direct current and apply the direct current to the controllable magnetic device. 
     According to any aspect disclosed herein, the controllable magnetic device includes one or more windings coupled to the rotatable member. 
     According to any aspect disclosed herein, the rotor assembly includes a permanent magnet. 
     According to any aspect disclosed herein, the rotor assembly and the stator assembly form at least part of a brushless direct current (DC) motor. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts a rotor assembly of an embodiment of an electric motor assembly; 
         FIG. 2  depicts the electric motor assembly of  FIG. 1  including the rotor assembly and a stator assembly; 
         FIG. 3  depicts a rotor assembly of an embodiment of an electric motor assembly, which includes a field coupler connected to a controllable magnetic device by rotating diodes; and 
         FIG. 4  depicts an embodiment of an actuator assembly including an electric motor assembly; 
         FIG. 5  depicts an embodiment of an electric motor system having a primary electric motor and a redundant secondary electric motor; and 
         FIG. 6  is a flow diagram depicting an embodiment of manufacturing and/or operating an electric motor assembly. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Devices, systems and methods are provided herein for control of an electric motor. An embodiment of a motor assembly includes an electric motor (e.g., a permanent magnet DC motor) having a stator assembly and a rotor assembly. A controllable magnetic device is coupled to the rotor assembly and can be actuated and/or controlled to control an amount of magnetic flux in the rotor assembly. In one embodiment, the controllable magnetic device is an electromagnet formed by one or more windings that are mounted on a rotor shaft or otherwise coupled to the rotor assembly so that the windings rotate concurrently with the rotor assembly. The controllable magnetic device may be configured to increase the magnetic flux to, for example, increase the torque of the motor assembly and/or increase the power of an actuator coupled to the motor assembly. The controllable magnetic device can also be configured to decrease the magnetic flux. 
     In one embodiment, the controllable magnet device is incorporated into a primary motor and/or a secondary (back-up or redundant) motor in a redundant motor topology. The secondary motor can be used to supplement the torque and power of the primary motor and/or be used in the event of a failure of the primary motor. The controllable magnetic device can be actuated to reduce the back electromotive force (BEMF) that can be generated by the failed primary motor when the secondary motor is in operation. 
       FIG. 1  illustrates aspects of an embodiment of an electric motor assembly, such as a permanent magnet DC (PMDC) motor assembly. The electric motor assembly  10  includes a stator assembly (not shown) and a rotor assembly  12 . The rotor assembly  12  includes a permanent magnet  14  (or a plurality of permanent magnets) mounted on a rotor shaft  16  that extends longitudinally along a motor assembly axis  18 . The permanent magnet  14  may be made of a rare-earth material or other suitable magnetic material. The rotor assembly  12  is configured to be disposed in a stator assembly (not shown), so that application of current to the stator assembly generates a current-induced magnetic field. The permanent magnet  14  couples with the current-induced magnetic field to cause rotation of the permanent magnet  14  and the rotor shaft  16 . 
     The motor assembly  10  also includes a controllable magnetic device that is configured to adjust the magnetic flux density in the rotor assembly  12  (i.e., in the gap between the stator and rotor assemblies). In one embodiment, the controllable magnetic device is an electromagnet  20  formed by windings  22  mounted on the rotor assembly  12  so that the windings  22  rotate with the rotor assembly  12 . A motor control device  24  controls the magnetic field generated by the electromagnet  20  via any suitable coupling device. The motor control device  24  includes components for performing functions related to control of the motor assembly  10  and the controllable magnetic device, such as a processor, a memory, a power supply, an input/output device and others. 
     In one embodiment, the motor assembly  10  includes a field coupler  26  connected to a conversion device  28 . The field coupler  26  provides alternating current that is converted to a direct current. The direct current is then applied to the electromagnet  22  to adjust the magnetic field flux density in the rotor assembly  12 . 
     The motor control device  24  and the electromagnet  20  provide a mechanism to vary the magnetic flux density in the air gap between the rotor and stator assemblies, thereby providing a variable flux motor assembly. By applying current in a first direction, the electromagnet  20  can generate a magnetic field that increases the magnetic flux density and thereby increases motor power. By applying current in a reverse direction, the electromagnet  20  can generate a magnet field that reduces the magnetic flux density (field weakening) and thereby reduces motor power. 
     By creating a controllable magnetic device or devices on the rotor assembly  12 , the available flux of a brushless permanent magnet DC motor or other motor (e.g., a brushed motor) can be increased to gain power in critical envelopes of operation. The size of the motor assembly  10  can also be reduced (e.g., the size of a typical rare earth magnet rotor can be reduced) as the peak power required can be accommodated by the use of additive flux provided by the controllable magnetic device or devices. 
       FIG. 2  shows an example of the motor assembly  10 , and illustrates an example of the field coupler  26  and the conversion device  28 . In this example, the motor assembly  10  includes a stator assembly that is mounted in a stationary housing  30 . The housing  30  may also house the permanent magnet  14 , the electromagnet  20 , the field coupler  26  and/or the conversion device  28 . The stator assembly includes motor control stator windings  32  mounted in the housing  30 . 
     In one embodiment, the field coupler  26  is configured to be inductively powered by stator windings  34 , which may also be disposed in the housing  30 . The field coupler  26  includes a plurality of windings or coils configured to rotate with the rotor assembly. The field coupler coils may be mounted on the rotor shaft  16 , mounted on another shaft or member connected to the rotor shaft  16  or in any manner so that the field coupler coils rotate with the permanent magnet  14 . 
     The controller  24  may be a single controller or other processing device, or may including multiple controllers that operate different components of the motor assembly  10 . In the example of  FIG. 2 , the controller  24  is configured to control both the rotor assembly  12  and the electromagnet  20 , and is operably connected to the stator windings  32  and the field coupler stator windings  34  by respective three-phase circuits  36  and  38 . 
     In the example of  FIG. 2 , the conversion device  28  includes a plurality of rotating diodes configured to rotate with the rotor assembly  12 . The rotating diodes act as a rectifier to convert AC current from the field coupler coils to DC current that is applied to the electromagnet  20 . 
       FIG. 3  is a schematic diagram of aspects of an embodiment of the motor assembly  10 . In this embodiment, the electromagnet  20  is electrically connected to a series of rotating diodes  40  for each current phase, which convert the AC induced current into a DC current. This current is then used to energize the coils  22  of the electromagnet  20 . As shown, the amount of current transferred from the field coupler  26  to the electromagnet  20  directly control the strength of the electromagnets affecting the total power of the motor assembly. 
       FIG. 4  shows an example of an actuator assembly  50  that can incorporate the motor assembly  10 . The actuator assembly  50  may be a linear actuator, a rotary actuator or any suitable type of actuator or other device that utilizes torque generated by the motor assembly  10 . As discussed above, the electromagnet  20  can be energized to either strengthen or weaken the primary magnetic field generated by the stator windings  32  and the permanent magnet  14 . 
     In this example, the rotor shaft  16  is connected via a gear train  52  to a ball screw  54  that is turned by the motor assembly to generate linear motion. The ball screw  54  can be coupled to an actuator rod or other mechanism for moving an object or surface. The actuator assembly includes various other components, such as but not limited to, an internal anti-rotation post  56 , load cell  58 , a ball nut  60  and a seal tube  62 . 
     Referring to  FIG. 5 , in one embodiment, the rotor assembly  12  is incorporated into redundant motor topography, in which the rotor assembly  12  forms part of a primary motor that is used, e.g., to control an actuator. A secondary motor is operably connected to the primary motor, e.g., by a common rotor shaft or by respective rotor shafts joined with a differential. Sharing a common shaft can be useful for, e.g., weight reduction. 
     In this embodiment, the primary motor includes the permanent magnet  14 , the electromagnet  20 , the conversion device  28  and the field coupler  26 . The secondary motor includes a secondary permanent magnet  70  and may also include a controllable magnetic device such as a secondary electromagnet  72 , which can be operably connected to a secondary field coupler  74  via a conversion device  76 . The primary and secondary motors may be controlled via a single controller  24 , or by separate controllers. For example, the primary motor is controlled by the controller  24  and the secondary motor is independently controlled by a secondary controller  78 . 
     The secondary motor can be activated in the event of a failure of the primary motor or other condition for which the primary motor is insufficient for a given application. When the secondary motor is in use, the permanent magnet  14  of the primary motor produces a back electromotive force (BEMF) which can impede movement of the secondary motor. To reduce or eliminate the BEMF, the electromagnet  20  and/or the electromagnet  74  can be actuated and controlled to generate a magnetic field that reduces or eliminates the BEMF. In this way, in a dual redundant motor system, a failed motor can be effectively disengaged so that motors do not have to be sized up to drive through the BEMF. 
       FIG. 6  illustrates a method  90  of manufacturing and/or operating an electric motor. The method  90  is discussed in conjunction with the motor assembly  10  and the rotor assembly  12 , although the method  90  may be utilized in conjunction with any suitable device or system that can utilize an electric motor. The method  90  includes one or more stages  91 - 94 . In one embodiment, the method  90  includes the execution of all of stages  91 - 94  in the order described. However, certain stages may be omitted, stages may be added, or the order of the stages changed. 
     In the first stage  91 , the motor assembly  10  is assembled and installed as part of, e.g., an actuator device, e.g., the actuator assembly  50 . For example, the actuator assembly  50  is installed as part of an aircraft control system and is controlled by a pilot, operator and/or processing device (e.g., aircraft computer). 
     In the second stage  91 , electric current is applied to the motor control stator windings  32 , e.g., via three-phase current. The electric current flows circumferentially through the windings  32  around the permanent magnet  14 , causing rotation of the permanent magnet  14  and the rotor shaft  16 . 
     In the third stage  93 , a controllable magnet such as the electromagnet  20  is actuated and controlled as discussed above. For example, the electromagnet  20  is energized by applying DC current via the field coupler  26  and the conversion device. The electromagnet  20  generates a magnetic field that interacts with the magnetic field in the rotor assembly  12  to either increase the magnetic flux density in the rotor assembly  12  or decrease the magnetic flux density. 
     In the fourth stage  94 , the torque generated by the assembly  10  may be transferred or translated to an actuator. The actuator may be a linear actuator and/or a rotary actuator used in an aircraft or in any other suitable environment. For example, the torque is transferred to a component, e.g., an aileron or other moveable component of an aircraft. Other examples of the component include any moveable component or load (e.g., a pump). 
     Embodiments described herein provide a number of advantages and technical effects. The motor assemblies described herein provide an electric motor assembly and/or an electric actuator that can change its performance curves to match the needs of a system more closely than conventional motors and actuators, and thus be more efficient relative to conventional motors and actuators, while being more fault tolerant of environment conditions and degraded components. In addition, the embodiments can produce a relatively high power density without having to increase the size of an electric motor or actuator and/or require stronger and more costly magnets. 
     Modern aircraft are becoming more electric in an effort to reduce weight and increase performance. Many aircraft systems in safety critical functions involve full redundancy to achieve required safety levels. There are several disadvantageous of such an architecture including high weight and cost. Embodiments described herein allow for the construction of motor assemblies that do not require full redundancy while satisfying desired safety criteria. For example, a conventional technique of redundancy reduction includes ganging lower reliability components together (such as a motor) to a single high reliability function (such as a linkage or pump). To do this, one must ensure that when a motor fails it does not inhibit other components. In the case of motors, this is accomplished through torque summing or disengagement clutches or release mechanisms, which adds weight cost and complexity back. By using a motor assembly with active field generation used to counteract permanent magnets as described herein, a clutch or other mechanism is not needed. 
     As the use of electric motors becomes more prevalent in aircraft, the need for a variable power motor is also growing. Sizing a motor for peak loads that may be seen only during a small part (e.g., 5%) of the total operating time imposes a huge burden on the size and cost of the motor. Embodiments described herein provide the ability to supplement a motor&#39;s native power without having to significantly increase the size or cost of the motor. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.