Patent Publication Number: US-2019193775-A1

Title: Electromotive Drive Device and Electrically-Powered Steering Device

Description:
TECHNICAL FIELD 
     The present invention relates generally to an electric drive device and an electric power steering device, and particularly to an electric drive device and an electric power steering device in which an electronic control unit is provided. 
     BACKGROUND ART 
     In a general field of industrial machinery, a controlled object of a mechanical system is driven by an electric motor. In recent years, employment of an electric drive device of mechatronical integration type has been started, wherein the electric drive device includes both of an electric motor and an electronic control unit in a package, and wherein the electronic control unit includes semiconductor elements and others for controlling rotational speed and torque of the electric motor. 
     As an example of electric drive device of mechatronical integration type, an electric power steering device for an automotive vehicle includes an electric motor, and an electronic control unit (ECU) for controlling the electric motor, wherein the electronic control unit is configured to sense a direction and a torque of rotation of a steering shaft rotated by driver&#39;s operation of a steering wheel, and drive the electric motor based on these sensed values, to produce a steering assist torque to rotate the steering shaft in the direction of rotation of the steering shaft. 
     Japanese Patent Application Publication No. 2015-134598 (patent document 1) discloses a known conventional electric power steering device composed of an electric motor section and an electronic control section. In the electric motor section, an electric motor is housed in a motor housing, wherein the motor housing has a cylindrical part made of an aluminum alloy or the like. In the electronic control section, a board provided with electrical components is attached to a heat sink serving as an ECU housing, wherein the ECU housing is arranged at a side of the motor housing opposite to an output shaft of the electric motor in its axial direction. The board attached to the heat sink is provided with a power supply circuit part, a power conversion circuit part, and a control circuit part, wherein the power conversion circuit part includes power switching elements such as MOSFETs or IGBTs for driving and controlling the electric motor, and wherein the control circuit part is configured to control the power switching elements. Output terminals of the power switching elements and input terminals of the electric motor are connected electrically via a bus bar. 
     This electronic control part attached to the heat sink is supplied with electric power from a power supply via a connector case made of synthetic resin, and also supplied with a sensing signal indicating operating states and others from sensors and others. The connector case serves as a cover fixed to hermetically cover the heat sink, and is fixed to a surface of an outer periphery of the heat sink by fixing bolts. 
     Other known examples of electric drive device where an electronic control device is integrally provided include an electric brake device, and an electric hydraulic pressure control device for control of various hydraulic pressures. The following describes an electric power steering device as a representative example. 
     PRIOR ART DOCUMENT(S) 
     Patent Document(s) 
     Patent Document 1: Japanese Patent Application Publication No. 2015-134598 
     SUMMARY OF THE INVENTION 
     Problem(s) to be Solved by the Invention 
     In an electric power steering device as disclosed in patent document 1, a motor housing made of metal, a heat sink made of metal, and a connector case made of synthetic resin are fixed together by fixing bolts each of which extends through a fixing portion of each component, wherein the fixing portion projects radially outwardly. For prevention of entrance of water, O rings are disposed between the motor housing and the heat sink and between the heat sink and the connector case, respectively. 
     However, the provision of the fixing portions and fixing bolts at outer peripheries of the motor housing, the heat sink, and the connector case, causes an adverse effect of causing an enlargement in exterior shape and an increase in weight. The accompanying provision of the O rings for water tightness in addition to the provision of the fixing bolts, causes an adverse effect of causing an increase in number of components and an increase in manufacturing unit cost. Furthermore, although the motor housing is in intimate contact with the heat sink, the configuration that a part of intimate contact and an electronic control part are hermetically covered by the connector case made of synthetic resin that has a large thermal resistance and fails to allow preferable heat transfer and is therefore not preferable in heat dissipation property, causes an adverse effect that the connector case fails to serve for heat dissipation, and the device does not have a preferable heat dissipation property. Therefore, it is desired to provide an electric drive device and an electric power steering device where these problems are solved. 
     From a further auxiliary viewpoint, in an electric power steering device as disclosed in patent document 1, a heat sink member is arranged between a motor housing and an ECU housing for dissipating heat especially from a power supply circuit part and a power conversion circuit part to the outside. The provision of the heat sink member leads to enlarging the axial length of the electric power steering device. Moreover, since electrical components constituting the power supply circuit part and the power conversion circuit part generate a large quantity of heat, it is required to effectively dissipate the heat to the outside, especially when the electric power steering device is made compact. Accordingly, it is desirable to provide an electric drive device which is made as compact in the axial direction as possible and in which heat is effectively dissipated from a power supply circuit part and a power conversion circuit part to the outside. 
     It is a main object of the present invention to provide a new electric drive device and a new electric power steering device each of which is compact in exterior shape, and is improved in weight and number of components, and has a preferable heat dissipation property. 
     Means for Solving the Problem(s) 
     The present invention is characterized in that: a motor housing is made of aluminum-based metal, and includes an end face part opposite to an output part of a rotating shaft of an electric motor; a metal cover is made of aluminum-based metal and structured to cover an electronic control part configured to control the electric motor; one of the metal cover and the end face part of the motor housing includes an outer peripheral surface including a stepped portion having a radially inward recess form extending annularly; the stepped portion includes a fit portion where an opening portion of the metal cover is fitted; and the fit portion is formed with a friction stir welding portion where the motor housing and the metal cover are welded together. 
     Effect(s) of the Invention 
     According to the present invention, the feature that the end face part of the motor housing made of aluminum-based metal includes the outer peripheral surface including the stepped portion, and the stepped portion is engaged with and joined to the opening portion of the metal cover made of aluminum-based metal by friction stir welding, serves to cause a decrease in exterior shape and a decrease in weight and a decrease in number of components, by omission of fixing bolts and O rings. Moreover, the feature that the motor housing and the metal cover are welded together, serves to cause a decrease in thermal resistance and further cause the metal cover to serve for heat dissipation, and thereby cause an improvement in heat dissipation property. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a whole perspective view of a steering device as an example of device to which the present invention is applied. 
         FIG. 2  is a whole perspective view of an electric power steering device according to an embodiment of the present invention. 
         FIG. 3  is an exploded perspective view of the electric power steering device shown in  FIG. 2 . 
         FIG. 4  is a perspective view of a motor housing shown in  FIG. 3 . 
         FIG. 5  is a cutaway view of the motor housing shown in  FIG. 4 , where the motor housing is cut by a plane containing a central axis of the motor housing. 
         FIG. 6  is a perspective view of the motor housing shown in  FIG. 4  where a power conversion circuit part is mounted and fixed to the motor housing. 
         FIG. 7  is a perspective view of the motor housing shown in  FIG. 4  where a power supply circuit part is mounted and fixed to the motor housing. 
         FIG. 8  is a perspective view of the motor housing shown in  FIG. 4  where a control circuit part is mounted and fixed to the motor housing. 
         FIG. 9  is a perspective view of the motor housing shown in  FIG. 4  where a connector terminal assembly is mounted and fixed to the motor housing. 
         FIG. 10  is a longitudinal sectional view of a part including a place where the motor housing is joined to a metal cover. 
         FIG. 11  is a sectional view of a part where the joint between the motor housing and the metal cover shown in  FIG. 11  is implemented by friction stir welding. 
         FIG. 12  is a longitudinal sectional view of a part including a place where a motor housing is joined to a metal cover, according to another embodiment. 
         FIG. 13  is a sectional view of a part where the joint between the motor housing and the metal cover shown in  FIG. 12  is implemented by friction stir welding. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     The following details an embodiment of the present invention with reference to the drawings. However, the present invention is not limited to the embodiment, but includes various modifications and applications belonging to technical conception of the present invention. 
     The following briefly describes configuration of a steering device as an example of device to which the present invention is applied, with reference to  FIG. 1 , prior to description of the embodiment of the present invention. 
     First, the following describes a steering device for steering front wheels of an automotive vehicle. Steering device  1  is configured as shown in  FIG. 1 . A steering shaft  2  is connected to a steering wheel not shown, and includes a lower end formed with a pinion not shown, wherein the pinion is in mesh with a rack not shown, wherein the rack extends in a vehicle body lateral direction. The rack includes ends linked to respective tie rods  3  for steering the front wheels leftward and rightward, and is housed by a rack housing  4 . A rubber boot  5  is provided between rack housing  4  and each tie rod  3 . 
     An electric power steering device  6  is provided for producing an assist torque while the steering wheel is being turned. Specifically, electric power steering device  6  includes a torque sensor  7 , an electric motor section  8 , and an electronic control section or unit (ECU)  9 , wherein torque sensor  7  is structured to sense a direction of rotation of steering shaft  2 , and a rotating torque applied to steering shaft  2 , wherein electric motor section  8  is structured to apply a steering assist force to the rack via a gear  10  depending on a sensed value from torque sensor  7 , and wherein electronic control section  9  is configured to control an electric motor arranged in electric motor section  8 . Electric motor section  8  of electric power steering device  6  is connected to gear  10  by three bolts not shown at three spots of an outer peripheral part of an output shaft side of electric motor section  8 . Electronic control section  9  is arranged at a side of electric motor section  8  opposite to an output shaft of electric motor section  8 . 
     Electric power steering device  6  operates as follows. As the steering wheel is turned to rotate steering shaft  2  in one direction, torque sensor  7  then senses the direction of rotation of steering shaft  2 , and the rotating torque applied to steering shaft  2 . A control circuit part calculates a quantity of operation of the electric motor, based on a sensed value from torque sensor  7 . Power switching elements of a power conversion circuit part are controlled to drive the electric motor based on the calculated quantity of operation, so that an output shaft of the electric motor is rotated to drive the steering shaft  2  in the same direction as the direction of operation of the steering wheel. The rotation of the output shaft of the electric motor is transferred to the rack via the pinion and gear  10 , thereby steering the automotive vehicle. Further description is omitted because its configuration and operation are well known. 
     As described above, in an electric power steering device as disclosed in patent document 1, a motor housing made of metal, a heat sink made of metal, and a connector case made of synthetic resin are fixed together by fixing bolts each of which extends through a fixing portion of each component, wherein the fixing portion projects radially outwardly. For prevention of entrance of water, O rings are disposed between the motor housing and the heat sink and between the heat sink and the connector case, respectively. 
     The provision of the fixing portions and fixing bolts at outer peripheries of the motor housing, the heat sink, and the connector case, causes an adverse effect of causing an enlargement in exterior shape and an increase in weight. The accompanying provision of the O rings for water tightness in addition to the provision of the fixing bolts, causes an adverse effect of causing an increase in number of components and an increase in manufacturing unit cost. Furthermore, although the motor housing is in intimate contact with the heat sink, the configuration that a part of intimate contact and an electronic control part are hermetically covered by the connector case made of synthetic resin that has a large thermal resistance and fails to allow preferable heat transfer and is therefore not preferable in heat dissipation property, causes an adverse effect that the connector case fails to serve for heat dissipation, and the device does not have a preferable heat dissipation property. 
     In view of the foregoing background, according to the present embodiment, an electric power steering device is proposed which is configured as follows. Specifically, according to the present embodiment: a motor housing is made of aluminum-based metal, and includes an end face part opposite to an output part of a rotating shaft of an electric motor; a metal cover is made of aluminum-based metal and structured to cover an electronic control part configured to control the electric motor; the end face part of the motor housing includes an outer peripheral surface including a stepped portion having a radially inward recess form extending annularly; the stepped portion includes a fit portion where an opening portion of the metal cover is fitted; and the fit portion is formed with a friction stir welding portion where the motor housing and the metal cover are welded together. 
     The feature that the end face part of the motor housing made of aluminum-based metal includes the outer peripheral surface including the stepped portion, and the stepped portion is engaged with and joined to the opening portion of the metal cover made of aluminum-based metal by friction stir welding, serves to cause a decrease in exterior shape and a decrease in weight and a decrease in number of components, by omission of fixing bolts and O rings. Moreover, the feature that the motor housing and the metal cover are welded together, serves to cause a decrease in thermal resistance and further cause the metal cover to serve for heat dissipation, and thereby cause an improvement in heat dissipation property. 
     The following details specific configuration of the electric power steering device according to the embodiment of the present invention with reference to  FIGS. 2 to 10 .  FIG. 2  shows whole configuration of the electric power steering device according to the present embodiment.  FIG. 3  shows components of the electric power steering device shown in  FIG. 2  in disassembled state as viewed diagonally.  FIGS. 4 to 9  show states of assembling when the components are assembled in an assembling order.  FIG. 10  is a longitudinal sectional view of a part including a place where the motor housing is joined to a metal cover. The following description refers to these drawings as appropriate. 
     As shown in  FIG. 2 , the electric power steering device includes electric motor section  8  and electronic control section  9 . Electric motor section  8  includes a motor housing  11  and an electric motor not shown. Motor housing  11  includes a cylindrical part made of an aluminum-based metal such as aluminum or an aluminum alloy. The electric motor is housed in motor housing  11 . Electronic control section  9  includes a metal cover  12 , and an electronic control assembly not shown housed in metal cover  12 . Metal cover  12  is made of an aluminum-based metal such as aluminum or an aluminum alloy, and is arranged at a side of motor housing  11  opposite to the output shaft in the axial direction. 
     Motor housing  11  and metal cover  12  are fixed to each other by friction stir welding in a circumferential fit region EA of their end faces facing each other, wherein circumferential fit region EA extends circumferentially, as detailed below. Metal cover  12  includes an accommodation space inside thereof, which accommodates the electronic control assembly. The electronic control part includes a power supply circuit part for supplying electric power as required, and a power conversion circuit part having power switching elements such as MOSFETs or IGBTs for driving and controlling the electric motor of electric motor section  8 , and a control circuit part for controlling the power switching elements. Output terminals of the power switching elements and input terminals of a coil of the electric motor are connected electrically via a bus bar. 
     At an end face of metal cover  12  opposite to motor housing  11 , a connector terminal assembly  13  is exposed through a hole of metal cover  12 . Connector terminal assembly  13  is fixed to a fixing portion of motor housing  11  by fixing bolts. Connector terminal assembly  13  includes a connector terminal forming part  13 A for power supply, a connector terminal forming part  13 B for sensors, and a connector terminal forming part  13 C for sending a state of control to external devices. 
     The electronic control assembly housed in metal cover  12  is supplied with electric power from a power supply via the connector terminal forming part  13 A made of synthetic resin, and is supplied with sensing signals indicative of operating states from sensors and others via the connector terminal forming part  13 B, and sends a present control state of the electric power steering device via the connector terminal forming part  13 C. 
       FIG. 3  shows electric power steering device  6  in exploded perspective view. Inside of motor housing  11 , a side yoke not shown is fitted, wherein the side yoke has an annular shape and is made of iron. The electric motor not shown is mounted inside of the side yoke. The electric motor includes an output part  14  structured to apply a steering assist force to the rack via the gear. Description of specific configuration of the electric motor is omitted because it is well known. 
     Motor housing  11  is made of an aluminum alloy, thereby serving as a heat sink member for dissipating heat to outside atmosphere, wherein the heat is generated by the power conversion circuit part and the power supply circuit part described below. The electric motor and motor housing  11  form the electric motor section. 
     Electronic control part EC is attached to an end face part  15  of motor housing  11  opposite to the output part  14  of electric motor section  8 . Electronic control part EC includes power conversion circuit part  16 , power supply circuit part  17 , control circuit part  18 , and connector terminal assembly  13 . The end face part  15  of motor housing  11  is formed integrally with motor housing  11 , but may be formed separately from motor housing  11  and bolted or welded to motor housing  11 . 
     Power conversion circuit part  16 , power supply circuit part  17 , and control circuit part  18  form redundant systems, namely, a main electronic control system and an auxiliary electronic control system. Normally, the main electronic control system is employed to drive and control the electric motor, and when an abnormality or failure occurs in the main electronic control system, the control is switched from the main electronic control system to the auxiliary electronic control system so that the auxiliary electronic control system drives and controls the electric motor. 
     Accordingly, as detailed below, heat of the main electronic control system is normally transferred to motor housing  11 . When the main electronic control system is failed or abnormal, operation of the main electronic control system is stopped and the auxiliary electronic control system is operated so that heat of the auxiliary electronic control system is transferred to motor housing  11 . 
     However, although not adopted by the present embodiment, there is an alternative configuration that both of the main and auxiliary electronic control systems are simultaneously employed to form a normal electronic control system, and when one of the main and auxiliary electronic control systems is failed or abnormal, only the other electronic control system is employed to drive and control the electric motor with half of full performance. This ensures a limp-home function, although the performance of the electric motor is only half. Accordingly, the heat of the main electronic control system and the auxiliary electronic control system is normally transferred to motor housing  11 . 
     Electronic control part EC is composed of power conversion circuit part  16 , power supply circuit part  17 , control circuit part  18 , and connector terminal assembly  13 , which are arranged in this order away from end face part  15  of motor housing  11 . Control circuit part  18  is configured to generate control signals for driving the switching elements of power conversion circuit part  16 , and includes a microcomputer and a peripheral circuit. Power supply circuit part  17  is configured to supply electric power to power conversion circuit part  16 , and includes a capacitor, a coil, switching elements, and others. Power conversion circuit part  16  is configured to regulate electric power flowing through the coil of the electric motor, and includes switching elements and others forming three-phase upper and lower arms. 
     In electronic control part EC, power conversion circuit part  16  and power supply circuit part  17  generate more quantities of heat than others. The generated heat of power conversion circuit part  16  and power supply circuit part  17  is dissipated via motor housing  11  made of the aluminum alloy. This configuration is detailed below. 
     Connector terminal assembly  13 , which is made of synthetic resin, is arranged between control circuit part  18  and metal cover  12 , and is connected to a vehicle battery (power supply) and external control devices not shown. Connector terminal assembly  13  is also connected to power conversion circuit part  16 , power supply circuit part  17 , and control circuit part  18 . 
     Metal cover  12  functions to house and seal liquid-tightly the power conversion circuit part  16 , power supply circuit part  17 , and control circuit part  18 . In the present embodiment, metal cover  12  is fixed to motor housing  11  by friction stir welding. 
     This feature serves to allow the exterior shape to be made compact by omission of fixing bolts, and allow fixing bolts and O rings for water tightness to be omitted. Moreover, the feature that motor housing  11  and metal cover  12  are welded together, serves to cause a decrease in thermal resistance and thereby enhance heat transfer capability between motor housing  11  and metal cover  12 . The further feature that metal cover  12  is made of metal serves to allow generated heat of power conversion circuit part  16 , power supply circuit part  17 , etc. to the outside. 
     The following describes configuration of the components and a process of assembling the components with reference to  FIGS. 4 to 9 .  FIG. 4  shows an exterior view of motor housing  11 , and  FIG. 5  shows its axial sectional view. As shown in  FIGS. 4 and 5 , motor housing  11  is cylindrically shaped and includes a lateral peripheral surface part  11 A, end face part  15 , and an end face part  19 . The end face part  15  closes a first end of lateral peripheral surface part  11 A, whereas the end face part  19  closes a second end of lateral peripheral surface part  11 A. In the present embodiment, lateral peripheral surface part  11 A and end face part  15  are formed integrally such that motor housing  11  has a cylindrical shape having a bottom. The end face part  19  serves as a cover for covering the second end of lateral peripheral surface part  11 A after the electric motor is mounted inside the lateral peripheral surface part  11 A. 
     End face part  15  includes an end portion including an outer peripheral surface including a stepped portion  35  having a radially inward recess form extending annularly. Stepped portion  35  is fitted with an opening portion of metal cover  12 . The fitting is shown as a circumferential fit region EA in  FIG. 2 . The form of fitting between stepped portion  35  and the opening portion of metal cover  12  is referred to as “spigot engagement” or “spigot fitting”. 
     As shown in  FIG. 5 , a stator  21  is fitted inside the lateral peripheral surface part  11 A, wherein stator  21  is formed by winding the coil  20  around an iron core. A rotor  22  is rotatably mounted inside the stator  21 , wherein a permanent magnet is embedded in rotor  22 . A rotating shaft  23  is fixed to rotor  22 . One end of rotating shaft  23  forms the output part  14 , whereas the other end of rotating shaft  23  forms a rotation-sensing target part  24  serving as a target for sensing the rotational phase and speed of rotating shaft  23 . Rotation-sensing target part  24  is provided with a permanent magnet, extending through a through hole  25  formed in end face part  15 , and projecting to the outside. The rotational phase and speed of rotating shaft  23  is sensed by a magnet-sensing part such as a GMR element or the like not shown. 
     Referring back to  FIG. 4 , the surface of end face part  15  opposite to the output part  14  of rotating shaft  23  is formed with heat dissipation regions  15 A and  15 B for power conversion circuit part  16  (see  FIG. 3 ) and power supply circuit part  17  (see  FIG. 3 ), which is a characterizing feature. Four corners of end face part  15  are formed integrally with board-connector-fixing projecting parts  26 , each of which extends perpendicularly from end face part  15 . Each board-connector-fixing projecting part  26  is formed with a threaded hole inside. Board-connector-fixing projecting parts  26  are configured to fix a board of control circuit part  18  described below and connector terminal assembly  13 . Each board-fixing projecting part  26  projecting from power conversion part heat dissipation region  15 A described below is formed with a board-receiving part  27  having the same height as power supply part heat dissipation region  15 B described below in the axial direction. Each board-receiving part  27  is configured to mount and fix a glass epoxy board  31  of power supply circuit part  17  described below. The flat area forming the end face part  15  and extending in the radial direction and perpendicular to rotating shaft  23  is divided into two regions, namely, power conversion part heat dissipation region  15 A and power supply part heat dissipation region  15 B. Power conversion circuit part  16  is attached to power conversion part heat dissipation region  15 A. Power supply circuit part  17  is attached to power supply part heat dissipation region  15 B. In the present embodiment, the area of power conversion part heat dissipation region  15 A is set larger than that of power supply part heat dissipation region  15 B, for ensuring more space for mounting the power conversion circuit part  16 , because power conversion circuit part  16  includes redundant systems as described above, and thereby requires a sufficient mounting space. 
     There is a step between power conversion part heat dissipation region  15 A and power supply part heat dissipation region  15 B such that power conversion part heat dissipation region  15 A and power supply part heat dissipation region  15 B have different heights in the axial direction (the direction in which rotating shaft  23  extends). Namely, power supply part heat dissipation region  15 B is formed with an outward step away with respect to power conversion part heat dissipation region  15 A in the axial direction of rotating shaft  23  of the electric motor. This step is set to have a height enough to prevent interference between power conversion circuit part  16  and power supply circuit part  17  when power supply circuit part  17  is assembled after power conversion circuit part  16  is assembled. 
     Power conversion part heat dissipation region  15 A is formed with three heat dissipation projecting parts  28 , wherein each heat dissipation projecting part  28  has a narrow rectangular shape. Heat dissipation projecting parts  28  are configured to mount power conversion circuit part  16  thereon, wherein power conversion circuit part  16  described below has redundant systems. Each heat dissipation projecting part  28  projects away from the electric motor in the direction of rotating shaft  23  of the electric motor. 
     Power supply part heat dissipation region  15 B is generally flat and is configured to mount power supply circuit part  17  thereon, where power supply circuit part  17  is described below. Accordingly, each heat dissipation projecting part  28  serves as a heat dissipation part to transfer heat from power conversion circuit part  16  to end face part  15 , whereas power supply part heat dissipation region  15 B serves as a heat dissipation part to transfer heat from power supply circuit part  17  to end face part  15 . 
     Each heat dissipation projecting part  28  may be omitted so that power conversion part heat dissipation region  15 A serves as a heat dissipation part to transfer heat from power conversion circuit part  16  to end face part  15 . However, in the present embodiment, each metal board of power conversion circuit part  16  is welded and securely fixed to heat dissipation projecting part  28  by friction stir welding. 
     At end face part  15  of motor housing  11  according to the present embodiment described above, the axial size can be made compact because there is no heat sink member. Moreover, since motor housing  11  has a sufficient thermal capacity, the heat generated in power supply circuit part  17  and power conversion circuit part  16  can be dissipated to the outside effectively. 
       FIG. 6  shows a state where power conversion circuit part  16  is placed on heat dissipation projecting parts  28  (see  FIG. 4 ). As shown in  FIG. 6 , power conversion circuit part  16  composed of redundant systems is placed on heat dissipation projecting parts  28  (see  FIG. 4 ) formed in power conversion part heat dissipation region  15 A. The switching elements constituting the power conversion circuit part  16  are placed on a metal board, which is made of an aluminum-based metal material in this example, allowing their generated heat to be dissipated effectively. The metal board is welded to heat dissipation projecting part  28  by friction stir welding. 
     In this way, the metal board is securely fixed to heat dissipation projecting parts  28  (see  FIG. 4 ), to allow generated heat of the switching elements to be transferred to heat dissipation projecting parts  28  (see  FIG. 4 ) effectively. The heat transferred to heat dissipation projecting parts  28  (see  FIG. 4 ) is dissipated to power conversion part heat dissipation region  15 A, and then to lateral peripheral surface part  11 A of motor housing  11 , and finally to the outside. As described above, power conversion circuit part  16  is prevented from interfering with power supply circuit part  17  described below, because the height of power conversion circuit part  16  is shorter than that of power supply part heat dissipation region  15 B in the axial direction. 
     In this way, power conversion circuit part  16  is placed on heat dissipation projecting parts  28  of power conversion part heat dissipation region  15 A. This allows the generated heat of the switching elements of power conversion circuit part  16  to be transferred to heat dissipation projecting parts  28  effectively. The heat transferred to heat dissipation projecting parts  28  is dissipated to power conversion part heat dissipation region  15 A, and then to lateral peripheral surface part  11 A of motor housing  11 , and finally to the outside. 
       FIG. 7  shows a state where power supply circuit part  17  is placed over power conversion circuit part  16 . As shown in  FIG. 7 , power supply part heat dissipation region  15 B is covered by power supply circuit part  17 . Power supply circuit part  17  includes glass epoxy board  31 , and capacitors  29 , coils  30  and others placed on glass epoxy board  31 . Power supply circuit part  17  includes redundant systems, each of which includes a power supply circuit composed of capacitors  29  and coil  30  and arranged symmetrically with each other as shown in  FIG. 7 . 
     The surface of glass epoxy board  31  facing the power supply part heat dissipation region  15 B (see  FIG. 6 ) is fixed to end face part  15  in contact with power supply part heat dissipation region  15 B. As shown in  FIG. 7 , this fixing is implemented by bolting with a fixing bolt not shown through a threaded hole formed in each board-receiving part  27  of board-fixing projecting part  26 , and also with a fixing bolt not shown through a threaded hole formed in power supply part heat dissipation region  15 B (see  FIG. 6 ). 
     The configuration that power supply circuit part  17  is based on glass epoxy board  31  allows the components of power supply circuit part  17  to be mounted on both sides of the power supply circuit part  17 . The surface of glass epoxy board  31  facing the power supply part heat dissipation region  15 B (see  FIG. 6 ) is provided with a sensing part for sensing the rotational phase and speed of rotating shaft  23  (see  FIG. 5 ) in cooperation with rotation-sensing target part  24  (see  FIG. 5 ) of rotating shaft  23 , wherein the sensing part includes a GMR element and a sensing circuit not shown. 
     The configuration that glass epoxy board  31  is fixed to power supply part heat dissipation region  15 B (see  FIG. 6 ) in contact with power supply part heat dissipation region  15 B as described above, allows the generated heat of power supply circuit part  17  to be transferred to power supply part heat dissipation region  15 B effectively. The heat transferred to power supply part heat dissipation region  15 B (see  FIG. 6 ) is transferred and spread into lateral peripheral surface part  11 A of motor housing  11 , and then dissipated to the outside. In order to enhance the thermal conductivity, an adhesive agent or dissipation grease or dissipation sheet having a high thermal conductivity may be disposed between glass epoxy board  31  and power supply part heat dissipation region  15 B (see  FIG. 6 ). 
     In this way, power supply circuit part  17  is placed on the upper side of power supply part heat dissipation region  15 B. The surface of glass epoxy board  31  of power supply circuit part  17  facing the power supply part heat dissipation region  15 B, on which the circuit elements of power supply circuit part  17  are placed, is fixed to end face part  15  in contact with power supply part heat dissipation region  15 B. This allows the generated heat of power supply circuit part  17  to be transferred to power supply part heat dissipation region  15 B effectively. The heat transferred to power supply part heat dissipation region  15 B is transferred to and spread in lateral peripheral surface part  11 A of motor housing  11 , and dissipated to the outside. 
       FIG. 8  shows a state where control circuit part  18  is placed over the power supply circuit part  17 . As shown in  FIG. 8 , electric motor section  8  is arranged over power supply circuit part  17 . Microcomputers  32  and peripheral circuits  33  constituting the control circuit part  18  are placed on glass epoxy board  34 . Control circuit part  18  includes redundant systems, each of which includes a control circuit composed of microcomputer  32  and peripheral circuits  33  and arranged symmetrically with each other as shown in  FIG. 8 . 
     Microcomputers  32  and peripheral circuits  33  may be placed on the surface of glass epoxy board  34  facing the power supply circuit part  17 . 
     As shown in  FIG. 8 , glass epoxy board  34  is fixed by fixing bolts not shown through the threaded holes formed in the top portions of board-fixing projecting parts  26  (see  FIG. 7 ), wherein glass epoxy board  34  is sandwiched between board-fixing projecting parts  26  and connector terminal assembly  13 . 
     The space between glass epoxy board  31  of power supply circuit part  17  (see  FIG. 7 ) and glass epoxy board  34  of control circuit part  18  is used for arrangement of capacitors  29 , coils  30  and others of power supply circuit part  17  shown in  FIG. 7 . 
       FIG. 9  shows a state where connector terminal assembly  13  is placed over the control circuit part  18 . As shown in  FIG. 9 , connector terminal assembly  13  is arranged over control circuit part  18 . Connector terminal assembly  13  is fixed by fixing bolts  36  through the threaded holes formed in the top portions of board-fixing projecting parts  26 , sandwiching the control circuit part  18 . Under this condition, connector terminal assembly  13  is connected to power conversion circuit part  16 , power supply circuit part  17 , and control circuit part  18 , as shown in  FIG. 3 , and opening portion  37  of metal cover  12  is fitted with stepped portion  35  of motor housing  11  by spigot fitting or the like, and is welded to stepped portion  35  of motor housing  11  in circumferential fit region EA by friction stir welding, thereby sealing liquid-tightly power conversion circuit part  16 , power supply circuit part  17 , and control circuit part  18 . 
       FIG. 10  shows a part including the circumferential fit region EA of motor housing  11  and metal cover  12  in its longitudinal sectional view. In  FIG. 10 , electronic control part EC is arranged adjacent to end face part  15  of motor housing  11 , and is covered by metal cover  12 , and is thereby accommodated in an accommodation space Sh formed by metal cover  12  and end face part  15 . A magnet hold part  38  is fixed to the end part of rotating shaft  23  opposite to output part  14 , wherein a permanent magnet (sensor magnet)  39  is housed in and fixed to magnet hold part  38 , wherein permanent magnet  39  forms the rotation-sensing target part. 
     The end part of rotating shaft  23 , magnet hold part  38 , and permanent magnet  39  project toward the electronic control part EC with respect to end face part  15  of motor housing  11 . A magnetic sensor  40  such as a GMR element is fixed to the surface of glass epoxy board  31  of power supply circuit part  17  facing the motor housing  11 , wherein power supply circuit part  17  is arranged in electronic control part EC. Magnetic sensor  40  has a magnet-sensing function and is configured to sense the rotational phase or the like of rotating shaft  23  based on rotation of permanent magnet  39 . A ball bearing  42  is provided in a through hole  41  and is structured to support the rotating shaft  23  rotatably, wherein through hole  41  is formed at or near a center of end face part  15 , and wherein rotating shaft  23  extends through the through hole  41 . 
     As shown in  FIGS. 10 and 11 , stepped portion  35  formed in the outer peripheral surface of end face part  15  includes a stepped portion side wall  35 S and a stepped portion bottom wall  35 B, wherein stepped portion side wall  35 S is formed by radially inwardly recessing, and wherein stepped portion bottom wall  35 B connects stepped portion side wall  35 S to lateral peripheral surface part  11 A of end face part  15 . Stepped portion  35  composed of stepped portion bottom wall  35 B and stepped portion side wall  35 S is fitted with opening portion  37  of metal cover  12  by spigot fitting. The portion of contact between stepped portion side wall  35 S and metal cover  12  forms the circumferential fit region EA. 
     As shown in  FIG. 11 , a process of friction stir welding is applied to a part including a central region where stepped portion bottom wall  35 B is in contact with opening portion  37  of metal cover  12  (i.e. where stepped portion bottom wall  35 B is butted with opening portion  37  of metal cover  12 ). Specifically, a region of contact between stepped portion bottom wall  35 B and a distal end of opening portion  37  of metal cover  12 , and a region of contact between stepped portion side wall  35 S and a part of an inner periphery of opening portion  37  of metal cover  12  are welded together by friction stir welding, thereby forming a friction stir welding portion FSW. 
     In  FIG. 11 , the welded portion extends deeply and includes the region of contact between stepped portion side wall  35 S and the inner periphery of opening portion  37  of metal cover  12 , but this configuration may be modified such that the welded portion extends shallowly and includes only the region of contact between stepped portion bottom wall  35 B and the distal end of opening portion  37  of metal cover  12 . 
     In general, friction stir welding is implemented by: pressing, with great effort, a tool onto a joint portion between members to be joined, wherein the tool has a cylindrical shape having a distal end including a projecting portion, while rotating the tool; thereby causing the projecting portion of the tool to intrude into the joint portion; generating frictional heat and soften workpieces; causing a plastic flow of the joint portion and its surroundings by power of rotation of the tool; and thereby mixing and integrating the members together. 
     In this way, according to the present embodiment, the outer peripheral surface of end face part  15  of motor housing  11  made of aluminum-based metal includes the stepped portion  35  having a radially inward recess form, and opening portion  37  of metal cover  12  made of aluminum-based metal is fitted with stepped portion  35 , and this circumferential fit region EA is formed with friction stir welding portion FSW where motor housing  11  and metal cover  12  are welded together. 
     The feature that stepped portion  35  of the outer peripheral surface of end face part  15  of the motor housing is fitted with and joined to opening portion  37  of metal cover  12  by friction stir welding, serves to cause a decrease in exterior shape and a decrease in weight and a decrease in number of components, by omission of fixing bolts and O rings. Moreover, the feature that motor housing  11  and metal cover  12  are welded together, serves to cause a decrease in thermal resistance and further cause the metal cover to serve for heat dissipation, and thereby cause an improvement in heat dissipation property. The integration of metal cover  12  and motor housing  11  serves to provide a large heat capacity, and thereby cause a further improvement in heat dissipation property. 
     In addition, according to the present embodiment, power conversion circuit part  16  is placed on the upper side of heat dissipation projecting part  28  formed in power conversion part heat dissipation region  15 A. This allows the generated heat of the switching elements of power conversion circuit part  16  to be transferred to heat dissipation projecting part  28  effectively. Furthermore, the heat transferred to heat dissipation projecting part  28  is spread in power conversion part heat dissipation region  15 A, and transferred to lateral peripheral surface part  11 A of motor housing  11 , and dissipated to the outside. 
     Similarly, power supply circuit part  17  is placed on the upper side of power supply part heat dissipation region  15 B. The surface of glass epoxy board  31  of power supply circuit part  17  facing the power supply part heat dissipation region  15 B, on which the circuit elements of power supply circuit part  17  are placed, is fixed to end face part  15  in contact with power supply part heat dissipation region  15 B. This allows the generated heat of power supply circuit part  17  to be transferred to power supply part heat dissipation region  15 B effectively. The heat transferred to power supply part heat dissipation region  15 B is transferred to and spread in lateral peripheral surface part  11 A of motor housing  11 , and dissipated to the outside. 
     With the configuration described above, the heat generated in power supply circuit part  17  and power conversion circuit part  16  is transferred to end face part  15  of motor housing  11 , allowing to omit a heat sink member, and thereby shorten the axial size. Moreover, since motor housing  11  has a sufficient thermal capacity, the heat generated in the power supply circuit part and the power conversion circuit part can be dissipated to the outside effectively. 
     As described above, the present invention is exemplified by a configuration: a motor housing is made of aluminum-based metal, and includes an end face part opposite to an output part of a rotating shaft of an electric motor; a metal cover is made of aluminum-based metal and structured to cover an electronic control part configured to control the electric motor; the end face part of the motor housing includes an outer peripheral surface including a stepped portion having a radially inward recess form; an opening portion of the metal cover is fitted with the stepped portion; and this portion of fitting is formed with a friction stir welding portion where the motor housing and the metal cover are welded together. 
     The feature that the stepped portion of the motor housing is engaged with and joined to the opening portion of the metal cover by friction stir welding, serves to cause a decrease in exterior shape and a decrease in weight and a decrease in number of components, by omission of fixing bolts and O rings. Moreover, the feature that the motor housing and the metal cover are welded together, serves to cause a decrease in thermal resistance and further cause the metal cover to serve for heat dissipation, and thereby cause an improvement in heat dissipation property. 
       FIGS. 12 and 13  show another embodiment. This embodiment differs from the foregoing embodiment in that the stepped portion is formed in the metal cover, wherein the remaining configuration is the same as in the foregoing embodiment. As shown in  FIGS. 12 and 13 , metal cover  12  includes an outer peripheral surface including a stepped portion  35 , wherein stepped portion  35  includes a stepped portion side wall  35 C and a stepped portion connection wall  35 D, wherein stepped portion side wall  35 C is formed by radially inwardly recessing, and wherein stepped portion connection wall  35 D connects stepped portion side wall  35 S to a lateral peripheral surface part  12 A of metal cover  12 . Stepped portion  35  composed of stepped portion connection wall  35 D and stepped portion side wall  35 C is fitted with an opening portion  43  of motor housing  11  by spigot fitting. The portion of contact between stepped portion side wall  35 C and opening portion  43  of motor housing  11  forms a circumferential fit region EA. 
     As shown in  FIG. 13 , a process of friction stir welding is applied to a part including a central region where stepped portion connection wall  35 D is in contact with opening portion  43  of motor housing  11  (i.e. where stepped portion connection wall  35 D is butted with opening portion  43  of motor housing  11 ). 
     Specifically, a region of contact between stepped portion connection wall  35 D and a distal end of opening portion  43  of motor housing  11 , and a region of contact between stepped portion side wall  35 C and a part of an inner periphery of opening portion  43  of motor housing  11  are welded together by friction stir welding, thereby forming a friction stir welding portion FSW. The present embodiment produces similar advantageous effects, similar to the first embodiment. 
     The present invention is not limited to the embodiment described above, but includes various modified embodiments. The described embodiment is detailed merely for easy understanding of the present invention, and the present invention is not limited to a form including all of the features described above, for example. Part of features of one of the embodiments may be replaced with features of another one of the embodiments. Features of one of the embodiments may be additionally provided with features of another one of the embodiments. Part of features of each of the embodiments may be additionally provided with other features or removed or replaced. 
     The electric drive device according to the embodiment described above may be exemplified as follows. 
     According to one aspect, an electric drive device includes: a motor housing made of aluminum-based metal and structured to house an electric motor, wherein the motor housing includes an end face part opposite to an output part of a rotating shaft of the electric motor, and wherein the electric motor is structured to drive a controlled object of a mechanical system; an electronic control part arranged at the end face part of the motor housing, and configured to drive the electric motor, wherein the electronic control part includes a control circuit part, a power supply circuit part, and a power conversion circuit part; and a metal cover made of aluminum-based metal and structured to cover the electronic control part; wherein one of the metal cover and the end face part of the motor housing includes an outer peripheral surface including a stepped portion having a radially inward recess form extending annularly; the stepped portion includes a fit portion where an opening portion of the metal cover is fitted; and the fit portion is formed with a friction stir welding portion where the motor housing and the metal cover are welded together. 
     According to a preferable aspect, the electric drive device is configured such that: the end face part of the motor housing includes the outer peripheral surface including the stepped portion; the stepped portion includes a stepped portion side wall and a stepped portion bottom wall, wherein the stepped portion side wall is recessed radially inwardly, and wherein the stepped portion bottom wall connects the stepped portion side wall to a lateral peripheral surface part of the end face part; the opening portion of the metal cover is fitted with the stepped portion by spigot fitting; and the friction stir welding portion has a central region where the stepped portion bottom wall and the opening portion of the metal cover are in contact with each other. 
     According to another preferable aspect, the electric drive device according to one of the foregoing aspects is configured such that the friction stir welding portion extends in a region of contact between the stepped portion bottom wall and a distal end of the opening portion of the metal cover, and in a region of contact between the stepped portion side wall and part of an inner periphery of the opening portion of the metal cover. 
     According to a further preferable aspect, the electric drive device according to one of the foregoing aspects is configured such that: the end face part of the motor housing includes a power conversion part heat dissipation region and a power supply part heat dissipation region; the power conversion circuit part is mounted to the power conversion part heat dissipation region in a manner to allow generated heat of the power conversion circuit part to be transferred to the motor housing via the power conversion part heat dissipation region; and the power supply circuit part is mounted to the power supply part heat dissipation region in a manner to allow generated heat of the power supply circuit part to be transferred to the motor housing via the power supply part heat dissipation region. 
     According to a further preferable aspect, the electric drive device according to one of the foregoing aspects is configured such that the end face part of the motor housing includes a step between the power supply part heat dissipation region and the power conversion part heat dissipation region such that the power supply part heat dissipation region projects away from the electric motor in an axial direction of the electric motor with respect to the power conversion part heat dissipation region. 
     According to a further preferable aspect, the electric drive device according to one of the foregoing aspects is configured such that the power conversion part heat dissipation region includes a heat dissipation projecting part projecting away from the electric motor in the axial direction of the electric motor. 
     According to a further preferable aspect, the electric drive device according to one of the foregoing aspects is configured such that the power conversion circuit part, the power supply circuit part, and the control circuit part of the electronic control part are arranged in this order away from the electric motor in the axial direction of the electric motor. 
     The electric power steering device according to the embodiment described above may be exemplified as follows. 
     According to one aspect, an electric power steering device includes: an electric motor structured to apply a steering assist force to a steering shaft, depending on an output from a torque sensor, wherein the torque sensor is structured to sense a direction of rotation of the steering shaft and a rotating torque applied to the steering shaft; a motor housing structured to house the electric motor, wherein the motor housing includes an end face part opposite to an output part of a rotating shaft of the electric motor; an electronic control part arranged at the end face part of the motor housing, and configured to drive the electric motor, wherein the electronic control part includes a control circuit part, a power supply circuit part, and a power conversion circuit part; a metal cover made of aluminum-based metal and structured to cover the electronic control part; wherein one of the metal cover and the end face part of the motor housing includes an outer peripheral surface including a stepped portion having a radially inward recess form extending annularly; the stepped portion includes a fit portion where an opening portion of the metal cover is fitted; and the fit portion is formed with a friction stir welding portion where the motor housing and the metal cover are welded together. 
     According to a preferable aspect, the electric power steering device is configured such that: the end face part of the motor housing includes the outer peripheral surface including the stepped portion; the stepped portion includes a stepped portion side wall and a stepped portion bottom wall, wherein the stepped portion side wall is recessed radially inwardly, and wherein the stepped portion bottom wall connects the stepped portion side wall to a lateral peripheral surface part of the end face part; the opening portion of the metal cover is fitted with the stepped portion by spigot fitting; and the friction stir welding portion has a central region where the stepped portion bottom wall and the opening portion of the metal cover are in contact with each other. 
     According to another preferable aspect, the electric power steering device according to one of the foregoing aspects is configured such that the friction stir welding portion extends in a region of contact between the stepped portion bottom wall and a distal end of the opening portion of the metal cover, and in a region of contact between the stepped portion side wall and part of an inner periphery of the opening portion of the metal cover. 
     According to a further preferable aspect, the electric power steering device according to one of the foregoing aspects is configured such that: the end face part of the motor housing includes a power conversion part heat dissipation region and a power supply part heat dissipation region; the power conversion circuit part is mounted to the power conversion part heat dissipation region in a manner to allow generated heat of the power conversion circuit part to be transferred to the motor housing via the power conversion part heat dissipation region; and the power supply circuit part is mounted to the power supply part heat dissipation region in a manner to allow generated heat of the power supply circuit part to be transferred to the motor housing via the power supply part heat dissipation region. 
     According to a further preferable aspect, the electric power steering device according to one of the foregoing aspects is configured such that the end face part of the motor housing includes a step between the power supply part heat dissipation region and the power conversion part heat dissipation region such that the power supply part heat dissipation region projects away from the electric motor in an axial direction of the electric motor with respect to the power conversion part heat dissipation region. 
     According to a further preferable aspect, the electric power steering device according to one of the foregoing aspects is configured such that the power conversion part heat dissipation region includes a heat dissipation projecting part projecting away from the electric motor in the axial direction of the electric motor. 
     According to a further preferable aspect, the electric power steering device according to one of the foregoing aspects is configured such that the power conversion circuit part, the power supply circuit part, and the control circuit part of the electronic control part are arranged in this order away from the electric motor in the axial direction of the electric motor.