Electric power steering device

Abstract: An electric power steering device (1) includes an output shaft (4) connected to a steering mechanism of a steering system for power assisting the steering operation. The EPS (1) includes a motor (5) and a control circuit (9) accommodated in a common motor housing (10). The motor housing (10) comprises an end frame (11) and a cup-shaped motor case (12) hermetically covering the motor (5) and the control circuit (9). The end frame (11) is secured to a rack housing (13) accommodating the steering mechanism, so that heat generated by the motor (5) and the control circuit (11) is transmitted to the rack housing (13).

Description: The present invention relates to an electric power steering device for assisting the steering operation. Nowadays, an electric power steering device (hereinafter abbreviated as EPS) has been put into practical use, according to which the steering operation is assisted by the rotational output of a motor. The EPS is constituted by a motor for assisting the steering power, a torque sensor for detecting the steering torque, an electromagnetic clutch for disconnecting the motor from the steering mechanism under abnormal condition, and a control circuit for controlling the motor current based upon signals detected by the torque sensor, a vehicle speed sensor and other sensors. For example, EP-A-0535422 discloses an electric power steering apparatus for a vehicle having a steering system including a steering mechanism accommodated in a rack shaft case, the electric power steering apparatus comprising: an output shaft which is connected to the steering apparatus; a motor for torque-assisting the steering apparatus via the output shaft; a signal processing circuit to provide signals to a control circuit which controls the motor; and a motor housing for accommodating the motor, said motor housing being arranged in thermal contact with the rack housing. The EPS is mounted to a speed sensor and other sensors. The EPS is mounted to a vehicle at a lower position in the engine room, and is exposed to undesirable environmental conditions so that it is submerged in the water depending upon the running speed even though the depth of water may be small and it is subjected to muddy water and snow adhered thereto. Therefore, the control circuit of the EPS is usually arranged inside the passenger compartment where the environment is favorable. The conventional electric power steering devices, however, involve the following problems. (1) The control circuit has many signal lines that are connected to the motor, the electric clutch, the rotational angle sensor for detecting the steering angle, and the like units. Therefore, many connectors are used for connecting these signal lines, causing the control circuit to become bulky. Furthermore, defective contact of the connectors may occur with an increase in the number of the connectors. (2) In order to mount the control circuit in the passenger compartment, the signal lines must pass through a partitioning wall between the compartment and the engine room, so an increased number of assembling steps are required for accomplishing the connection. In particular, the wiring for flowing a large motor current therethrough is thick from the standpoint of decreasing wiring resistance, and is not easy to handle. (3) In the EPS, the motor current is controlled by the switching operation of transistors, so electromagnetic noise occurs due to the switching operation of the transistors. The present invention was accomplished in view of the above-mentioned circumstances, and its object is to provide an electric power steering device which is small in size, requires a simplified wiring, generates less electromagnetic noise, and features high reliability. According to the present invention, there is provided an electric power steering device for an automobile having a steering system including a steering mechanism accommodated in rack housing, the electric power steering device comprising an output shaft coupled to the steering mechanism, a motor for torque-assisting the steering mechanism via said output shaft, a control circuit for controlling the motor, and a motor housing for accommodating the motor, said motor housing being arranged in thermal contact with the rack housing; characterised in that: the motor housing also accommodates the control circuit therewithin; the motor housing is secured to the rack housing; and the motor and the control circuit are in thermal contact with the rack housing via the motor housing. In this arrangement, the control circuit and the motor can be accommodated in the common motor housing, and it is possible to electrically connect the control circuit to the motor within the motor housing. By connecting the control circuit to the motor within the motor housing, it is not necessary to connect the control circuit to external units by wirings, so that the number of the necessary harnesses can be decreased. Accordingly, it is possible to reduce defective signal contacts of the connectors or damages to harnesses in the whole system, to thereby greatly improve reliability. In addition, the motor and the control circuit with the motor housing are arranged in thermal contact with the rack housing. The rack housing is preferably made of aluminum and has a large thermal capacity and a small heat resistance. Therefore, the rack housing suitably absorbs heat transmitted from the motor housing, so that heat generated by the motor and the control circuit is efficiently radiated through the motor housing to the rack housing and the temperature in the motor housing is maintained relatively low. Accordingly, the stable operation of the electric power steering device is guaranteed. Preferably, the motor housing comprises an end frame having the motor and the control circuit assembled thereonto, and a cup-shaped motor case covering the motor and the control circuit and hermetically coupled to the end frame, the end frame being fixed to the rack housing. As a result, the whole motor housing is of a water-proof structure, and the connector for connection to external units can be hermetically passed through the motor case without impairing waterproof characteristics. Preferably, a coating material having a small thermal resistance and a good sealing property is applied between adjoining surfaces of the end frame and the rack housing. Therefore, heat is transferred from the end frame to the rack housing while the internal space in the motor housing is in an air-tight condition. Preferably, the end frame is integrally provided with a thin heat-radiating plate, and the control circuit is assembled so that it thermally contacts the heat-radiating plate. Therefore, the heat resistance between the control circuit and the end frame is decreased and it possible to suppress the rise of temperature of the control circuit. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more apparent from the following description of the preferred embodiments, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view illustrating the motor housing and the rack housing of the electric power steering device according to the embodiment of the present invention; Fig. 2 is an enlarged cross-sectional view of a portion of the motor housing and the rack housing of Fig. 1; Fig. 3 is a diagram illustrating the steering system including the electric power steering device; Fig. 4 is a block diagram illustrating the internal constitution of the control circuit; Fig. 5 is a perspective view illustrating connection terminals of the control circuit; Fig. 6 is a side cross-sectional view illustrating connection terminals of the control circuit; Fig. 7 is a model diagram illustrating the flow of heat; Fig. 8 is a plan view illustrating the control circuit and the brushes; Fig. 9 is a side view illustrating the pigtails; Fig. 10 is a cross-sectional view illustrating the brush holder; Fig. 11 is a perspective view illustrating the brush holder; and Fig. 12 is a cross-sectional view illustrating the mounting of the control circuit. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the electric power steering device (EPS) of the present invention will now be described with reference to the drawings. Figures 1 and 2 show the internal structure of the electric power steering device (EPS), and Fig. 3 shows the steering system including electric power steering device of Figs. 1 and 2. As shown in Fig. 3, the steering system of an automobile has a steering wheel 8, and a steering mechanism 3 coupled to the steering wheel 8 via a steering column for moving wheels 54 of the automobile. The EPS 1 includes an output shaft 4 coupled to the steering mechanism 3 via a reduction gear 2, a motor 5 for producing a rotational output, an electromagnetic clutch 6 for transmitting the rotational output of the motor 5 to the output shaft 4, a steering angle sensor 7 for detecting the steering angle (rotational angle of the output shaft 4), and a control circuit 9 for controlling the motor current depending upon the steering force of a steering wheel 8 and the vehicle speed. These constituent parts (motor 5, electromagnetic clutch 6, steering angle sensor 7, control circuit 9) other than the output shaft 4 are constituted in a unitary structure and are accommodated in a motor housing 10 (see Fig. 1). Motor Housing 10 Referring to Fig. 1, the motor housing 10 comprises an end frame 11 to which the above-mentioned parts are mounted, and a motor case 12 hermetically fitted to the end frame 11 for enclosing the constituent parts. The end frame 11 is intimately attached to the end surface of a rack housing 13 which accommodates the steering mechanism 3 and is made of a material (e.g., aluminum) having good heat conductivity. The end frame 11 is then secured to the rack housing 13 by tightening bolts 14. The motor case 12 is made of a magnetic material (e.g., steel), shaped in a cylindrical shape with one closed end (upper end in Fig. 1), i.e., in the shape of a cup, and secured at its fixing portions 12a to the end frame 11 by tightening bolts 15. A passage opening 17 (Fig. 2) is formed in the open end portion of the motor case 12 and the end frame 11 to allow a connector 16 to extend therethrough for electrically connecting the control circuit 9 to external units (not shown). Motor 5 The motor 5 comprises a field device (e.g., a cylindrical magnet shown in Fig. 1) 18, an armature 19 that revolves along the inner periphery of the magnet 18, a commutator 21 provided on one end of a shaft 20, brushes 22 (see Figs. 2 and 8) that are brought into sliding contact with the outer peripheral surface of the commutator 21, and a brush holder 23 for holding the brushes 22. The armature 19 is mounted in an upright position with respect to the end frame 11 in Fig. 1. As shown in Fig. 1, the armature 19 is rotatably supported by a bearing 24 arranged in a housing 25 of the electromagnetic clutch 6 on one end of the shaft 20 adjacent to the commutator 21, and by a bearing 26 arranged in a bearing portion 12c of the bottom surface of the motor case 12 on the other end of the shaft 20. Electromagnetic Clutch 6 The electromagnetic clutch 6 is provided between the motor 5 and the output shaft 4, and has a coil 27 controlled by the control circuit 9, the housing 25 secured to the end frame 11 for holding the coil 27, a rotor 28 coupled to one end of the shaft 20 for rotation therewith, and an armature 29 coupled to the output shaft 4 for rotation therewith. When an electric current is supplied to the coil 27 through the control circuit 9, the armature 29 is attracted to the rotor 28 that is magnetized by the magnetic force of the coil 27, whereby the armature 29 rotates together with the rotor 28, and the rotational output of the motor 5 is transmitted to the output shaft 4 through the rotor 28 and the armature 29. However, when the operation of the EPS 1 is abnormal, no electric current is supplied to the coil 27 through the control circuit 9, so that the rotational output of the motor 5 will not be transmitted to the output shaft 4. Thus, the shaft 20 is disconnected from the output shaft 4. Steering Angle Sensor 7 As shown in Fig. 2, the steering angle sensor 7 comprises a magnetic member 30 (e.g., magnet) secured to the outer peripheral surface of the armature 29 of the electromagnetic clutch 6, and a magnetism detecting means 31 (e.g., Hall IC) disposed on the outside of the. armature 29 opposite to the magnetic member 30. A change in the magnetic flux during the rotation of the magnetic member 30 with the armature 29 is detected by the magnetism detecting means 31. The magnetism detecting means 31 is directly soldered to the substrate 9a (see Fig. 6) of the control circuit 9. Control Circuit 9 The control circuit 9 is disposed in the motor housing 10 in a side-by-side relationship with the motor 5, and the connector 16 is coupled to the control circuit 9 for connection to external units. As shown in Figs. 1 and 2, the connector 16 hermetically extends through the passage 17 formed between the motor case 12 and the end frame 11 via a sealing member 32. Referring to Fig. 4, the control circuit 9 comprises four switching elements 33 (33a to 33d) forming an H-bridge circuit, an I/F circuit 36 converting input signals delivered from a torque sensor 34 that detects the steering torque and a vehicle speed sensor 35 that detects the vehicle speed into digital signals, a microcomputer (CPU) 37 for setting a motor current required for assisting the torque depending upon the steering force converted by the I/F circuit 36 and the vehicle speed, an I/F circuit 38 converting a digital signal output from the microcomputer 37 into an analog value, a current detecting unit 39 for detecting a current flowing into the motor 5, a current control unit 40 for forming a duty signal for the switching elements 33 based on an analog current instruction value converted by the I/F circuit 38 and a current value detected by the current detecting unit 39, a gate drive circuit 41 for driving the switching elements 33 upon receiving the duty signal formed by the current control unit 40 and a direction for assisting the torque determined by the microcomputer 37, and a capacitor 42 for smoothening a battery current that undergoes fluctuation during the switching operation of the switching elements 33. The control circuit 9 further has a temperature sensor (not shown) near the switching element 33 mounted to the substrate 9a for limiting the current depending upon the operation temperature of the switching element 33. That is, the EPS 1 permits the flow of a large current within short periods of time, and overheating may result depending upon the conditions of use. By limiting the current in response to the operation temperature of the switching elements 33 detected by the temperature sensor, which the operation temperature is usually set to to a value lower than the operation temperature of the winding of the motor, it is possible to use the motor 5 within a permissible temperature range at all times. By mounting the temperature sensor directly onto the substrate 9a, the wiring can be omitted. Referring to Figs. 5 and 6, a motor wiring connecting the control circuit 9 (switching elements 33) to the motor 5 (brushes 22) is realized by using connection terminals 43 provided on the control circuit 9. That is, pigtails 44 (see Figs. 2 and 8) leading from the brushes 22 are usually made of a copper mesh wire to provide lower resistance, and on the other hand, the connection of the switching elements 33 is carried out by bonding wires 45. Thus, the brushes 22 and the switching elements 33 are coupled in a different manner. According to this embodiment, therefore, an external case 9b (made of a resin) of the control circuit 9 is provided with connection terminals 43 (insert molding), so that bonding wires 45 leading from the switching elements 33 are connected to the connection terminals 43, and pigtails 44 of the brushes 22 are connected (welded) to the connection terminals 43, to thereby realizes the motor wiring (see Fig. 8). The above-mentioned parts of the EPS 1, i.e., the motor 5, the control circuit 9, electromagnetic clutch 6 and steering angle sensor 7 are accommodated in the motor as a unit. Therefore, the layout arrangement of the parts will become an important problem for improving the mounting density, without impairing functions of the constituent parts. Moreover, since the heat generating units such as the motor 5 and the control circuit 9 are concentratedly arranged, it is important to ensure a good heat-radiating performance. Therefore, this embodiment employs the layout structure shown in Fig. 1 in order to obtain good heat-radiating performance without impairing functions of the constituent parts. Basically, from the standpoint of designing the heat radiation, heat generated by the motor 5 and the control circuit 9 is transmitted to the end frame 11 as shown in Fig. 7 (model diagram illustrating the flow of heat), and is radiated from the end frame 11 to the rack housing 13 (made of aluminum) which contains the steering mechanism 3. Therefore, it becomes necessary to contrive an arrangement in which the heat generated from the armature 19 and brushes 22 of the motor 5, and from the switching elements 33 (heat generated accompanying the switching operation) and from the capacitor 42 (loss due to reactive current) which are major sources of heat, is efficiently transmitted to the end frame 11. (a) The arrangement of the motor 5 is first described. The motor 5 is so disposed that the shaft 20 is concentrically arranged with the output shaft 4 extending through the end frame 11, via the electromagnetic clutch 6. Therefore, in order to efficiently transmit the heat generated from the brushes 22 to the end frame 11, it is advisable to arrange the commutator 21 near the end frame 11. That is, the brushes 22 are arranged near the end frame 11, the heat resistance between the brushes 22 and the end frame 11 can be decreased.From the functional point of view, furthermore, it is desired that the brushes 22 are arranged on the opposite side of the control circuit 9 from the motor 5 with the commutator 21 in the center (see Fig. 8). That is, the brushs 22 is maintained in a sliding contact with the outer peripheral surface of the commutator 21 by a spring 46 (see Fig. 2) so that a contact pressure against the commutator 21 is obtained at all times despite it is worn out. When the brushs 22 are disposed near the control circuit 9 with the commutator 21 in a center, the shortened pigtails 44 are connected to the brushs 22 and stress is exerted on the brushs 22 to prevent its desired motion. When the brush 22 is disposed on the opposite side of the control circuit 9 with the commutator 21 in a center, as shown in Fig. 8, elongated pigtails 44 can be connected to the brushs 22, so freedom of motion is imparted to the brushs 22 and the motion of the brushs 22 is not hindered.In order that the heat generated from the brushs 22 is efficiently transmitted to the end frame 11, the pigtails 44 may be covered with sleeves 47 made of an insulating member having a small heat resistance, as shown in Fig. 9, and may be clamped as designated at 48 to the end frame 11 or to a metal member (e.g., housing 25 of the electromagnetic clutch 6) that is thermally contacted to the end frame 11. Accordingly, the heat generated from the brushs 22 is transmitted to the end frame 11 or to the metal member (hereinafter referred to as housing 25) through the pigtails 44. By treating the surface of the end frame 11 or the housing 25 with alumite or the like to impart electric insulation thereto, the pigtail 44 may be secured in its bare form to the end frame 11 or to the housing 25 without being covered with the above-mentioned sleeve 47.Moreover, heat generated from the brushs 22 may be radiated into the end frame 11 via the brush holder 23. In particular, the brush holder 23 is made of a material (e.g., aluminum) having good heat conductivity. An alumite layer lla having a small heat resistance and insulating property is formed on the surface of the end frame 11 or of the housing 25 (treatment with alumite as described above), and the brush holder 23 is secured from the upper side of the alumite layer lla using screws 50 or the like (see Fig. 10). Accordingly, heat generated from the brushs 22 is transmitted to the end frame 11 or to the housing 25 via the brush holder 23. In Figs. 1 and 2, the brush holder 23 is secured to the housing 25 of the electromagnetic clutch 6. Here, however, since the housing 25 is thermally contacted to the end frame 11, heat transmitted to the housing 25 is further transmitted to the end frame 11 favorably.Referring to Fig. 11, furthermore, the contact surfaces between the brushs 22 and the brush holder 23 may be formed rugged to increase the contact areas therebetween, so as to improve heat conducting property. In this case, an insulator (insulating member having a small heat resistance) may be interposed between the brush holder 23 and the end frame 11 or the housing 25, or the surface of the end frame 11 or the housing 25 may be treated with alumite as described above.Alternatively, an insulating film (coating or the like) having a negligibly small heat resistance may be formed on the inner surface of the brush holder 23, i.e., on the surface that comes in contact with the brushs 22 to maintain insulation relative to the brushs 22 and to directly secure the brush holder 23 without effecting insulation treatment onto the surface of the end frame 11 or of the housing 25 (or without interposing an insulator). (b) The arrangement of the control circuit 9 is then described. As shown in Fig. 1, the control circuit 9 is disposed by the motor 5 in parallel with the magnet 18 of the motor 5, so that the connector 16 is formed integrally with the outer case 9b of the control circuit 9 and is passed through the passage 17. Owing to this arrangement, the switching elements 33 and the capacitor 42 are disposed near the end frame 11, enabling the heat generated due to the switching operation of the switching elements 33 and the heat generated from the capacitor 42 to be efficiently transmitted to the end frame 11.In order to decrease the heat resistance relative to the end frame 11, the substrate 9a (metal base) in the control circuit 9 may be secured by using a screw 52 to a heat-radiating plate llb provided on the end frame 11.The operation of the EPS 1 according to this embodiment is now described.When the steering wheel 8 is operated by a driver, the steering force is detected by the torque sensor 34 and is input to the control circuit 9. The control circuit 9 calculates the assisting torque based upon an input signal from the torque sensor 34 and a vehicle speed signal input from the vehicle speed sensor 35, and sets a motor current for producing the torque. A duty signal for driving the switching elements 33 is formed depending upon a current instruction value for instructing the motor current and a current that flows into the motor 5, the switching elements 33 are turned on and off depending upon the duty signal and the direction for assisting the torque, and a voltage applied to the motor 5 from a battery 53 (see Figs. 3 and 4) is varied to control the current that flows into the motor 5.The torque produced by the motor current is transmitted to the output shaft 4 through the electromagnetic clutch 6 and is further transmitted to the reduction gear 2 that is in mesh with the output shaft 4 to assist the torque of the steering mechanism 3. As a result, the driver needs to exert a decreased steering force, the output of the torque sensor 34 decreases, and the steering operation is completed. Here, however, in case the computer 37 in the control circuit 9 diagnoses that the EPS 1 is abnormally operating, no current is supplied to the coil 27 of the electromagnetic clutch 6, and the motor 5 is disconnected from the steering mechanism 3. Therefore, the steering operation is manually executed by the driver without relying upon the motor 5, and safety is maintained.When turning the vehicle, the tires 54 (see Fig. 3) produce a self-aligning torque (SAT: force that works to bring the steering wheel 8 back to its neutral position). When the steering wheel 8 that is turned is released from the hands, therefore, SAT acts upon the steering wheel 8 so that it returns back to its neutral steering angle position at all times. When the EPS 1 is mounted, however, the frictional torque (load torque of when no assisting torque is produced) of the motor 5 is amplified by the reduction gear 2 and becomes greater than SAT. Accordingly, the steering wheel 8 does not return back to its neutral steering angle position despite it is released from the hands. Therefore, the steering angle is detected by the steering angle sensor 7 and the motor current is controlled based upon the detected value, so that the steering wheel 8 that is released from the hands returns back to the neutral steering angle position. Features and Effects of this Embodiment (1) In the EPS 1 according to this embodiment, the control circuit 9, the electromagnetic clutch 6 and the steering angle sensor 7 are accommodated in the motor housing 10 together with the motor 5 as a unitary structure in a compact form, enabling the electric connection from the control circuit 9 to the motor 5, to the electromagnetic clutch 6 and to the steering angle sensor 7 to be accomplished inside the motor housing 10. This makes it possible to decrease the number of signal lines (i.e., number of harnesses) to the external units and to decrease the number of connectors provided for the control circuit 9 for connection to the external units. As a result, it is allowed to realize the whole system in a small size and in a reduced weight, to greatly decrease the probability of developing defective signal transmission caused by poor connector contact and biting of harnesses, and to improve reliability. (2) In general, furthermore, when the switching elements 33 are operated to control the motor current, electromagnetic noise is radiated in larger amounts from the harness to the motor 5 than from the power-source line of the battery 53. In this EPS 1 in which the control circuit 9 is connected to the motor 5 in the motor housing 10, however, electromagnetic noise generated by the switching operation of the switching elements 33 is decreased by the electromagnetic shielding effect of the motor case 12 and does not cause a problem. (3) Owing to the structure in which the heat generated from the motor 5 and the control circuit 9 is efficiently radiated into the end frame 11, furthermore, the heat is less radiated by the convection of the air inside the motor housing 10 making it possible to suppress the rise of temperature of the internal atmosphere and to guarantee stable operation. Moreover, heat transmitted to the end frame 11 is radiated to the open air via the rack housing 13 of the steering mechanism 3. The rack housing 13 is made of aluminum having good heat radiating property, has a large thickness to meet the requirement of mechanical strength and has a large heat capacity. With the end frame 11 being intimately adhered and secured to the rack housing 13, therefore, the heat capacity of the rack housing 13 can be equivalently included in the heat capacity of the end frame 11. The steering mechanism 3 is mounted at a lower position of the vehicle where it is subjected to a temperature lower than that in the engine room where the temperature is high. It is, however, considered that the steering mechanism 3 is heated by the exhaust pipe of a high temperature. However, heat can be easily shielded by interposing a shielding plate between the steering mechanism and the exhaust pipe, and temperature rise due to the radiant heat is suppressed.Accordingly, heat that is generated in large amounts from the motor 5 and the control circuit 9, flows from the end frame 11 to the rack housing 13 making it possible to suppress the temperature of the end frame 11 from rising. Here, in order to intimately adhere and secure the end frame 11 to the rack housing 13, it is desired to apply a heat-conducting grease (coating material of the invention) having small heat resistance and high sealing property onto the surface of the rack housing 13 onto which the end frame 11 is secured.The EPS 1 does not assist the torque when the vehicle is running straight but assists the torque only when the vehicle is turning consuming energy. However, the turn running is less frequent than the straight running; i.e., energy is little consumed and heat is generated in small amounts in average from the motor 5 and the control circuit 9. Therefore, the heat transmitted to the end frame 11 from the motor 5 and the control circuit 9, is easily radiated into the air from the rack housing 13.When moving the vehicle out of a narrow parking area, on the other hand, steering operation while the vehicle is at rest may occur for a brief period of time to produce large torque assistance. Therefore, heat is generated in large amounts from the motor 5 and the control circuit 9 though only for a short period of time. Even though the heat is generated in large amounts, however, the total heat capacity inclusive of that of the rack housing 13 is large, and the temperature of the end frame 11 does not rise so much.Besides, the armature 19 of the motor 5 that most generates the heat is constituted by a core of a magnetic material and copper wires, has a large heat capacity and is not heated to a high temperature within a short period of time.Accordingly, the temperature at each of the portions is determined depending upon the average temperature of the end frame 11, i.e., depending upon the average temperature of the rack housing 13. (4) In the EPS 1 of this embodiment in which the steering angle sensor 7 is provided at a position closer to the motor 5 than the reduction gear 2, higher resolution is obtained, i.e., higher detection precision is obtained than the actual steering angle. Therefore, even if the detection precision is lowered per a turn, the precision for detecting the steering angle is little affected; i.e., a cheaply constructed sensor can be used. Moreover, the steering angle sensor 7 utilizes the armature 19 of the electromagnetic clutch 6 as a rotor, and has the magnetism detecting means 31 that is soldered directly onto the substrate 9a of the control circuit 9, making it possible to greatly decrease the number of constituent elements. Modified Embodiments The above-mentioned embodiment uses a DC motor 5. It is, however, also allowable to use a brushless motor which controls the rotational speed relying upon the output frequency of the inverter. The EPS 1 of this embodiment is equipped with an electromagnetic clutch 6 as a protection means and with a steering angle sensor 7. However, the electromagnetic clutch 6 and the steering angle sensor 7 need not necessarily be provided. That is, what is important is that the control circuit 9 and the motor 5 are accommodated in the motor housing 10 as a unitary structure in a compact size. The above-mentioned embodiment has dealt with a rack-mounting system in which the end frame 11 of the EPS 1 was secured onto the rack housing 13 on the outside of the passenger compartment. It is, however, also allowable to employ a column-mounting system in which the EPS 1 is mounted near the column inside the passenger compartment. That is, even the column assist system which assists the torque of the steering axle via a reduction gear, has a metallic housing covering a steering mechanism such as steering gears and having good heat conducting property. Namely, the end frame 11 of the EPS 1 is secured onto the metallic housing, and the heat is radiated from the end frame 11 to the metallic housing.

Claims: An electric power steering device for an automobile having a steering system including a steering mechanism accommodated in rack housing, the electric power steering device comprising: an output shaft (4) coupled to the steering mechanism (3); a motor (5) for torque-assisting the steering mechanism (3) via said output shaft (4); a control circuit (9) for controlling the motor (5); and a motor housing (10) for accommodating the motor (5) therewithin, said motor housing (10) being arranged in thermal contact with the rack housing (13); characterised in that: the motor housing (10) also accommodates the control circuit (9) therewithin; the motor housing (10) is secured to the rack housing (13); and the motor (5) and the control circuit (9) are in thermal contact with the rack housing (13) via the motor housing (10). An electric power steering device according to claim 1, wherein said motor housing (10) comprises an end frame (11) having the motor (5) and the control circuit (13) assembled thereonto, and a cup-shaped motor case (12) covering the motor (5) and the control circuit (9) and hermetically coupled to the end frame (11), the end frame (11) being fixed to the rack housing (13). An electric power steering device according to claim 2, wherein a coating material having small thermal resistance and good sealing properties is applied between adjoining surfaces of the end frame (11) and the rack housing (13). An electric power steering device according to claim 2, wherein the end frame (11) is integrally provided with a thin heat-radiating plate (11b), and the control circuit (9) is assembled so that it thermally contacts the heat-radiating plate (11b). An electric power steering device according to claim 2, wherein a connector (16) hermetically extends through the motor housing (10) for external connection. An electric power steering device according to claim 2, wherein the motor (5) has a rotatable shaft (20) and a commutator (21) on one end of the shaft (20) for supplying a current via a brush (22). An electric power steering device according to claim 6, wherein the brush (22) is arranged on the opposite side of the control circuit (9) from the commutator (21). An electric power steering device according to claim 6, wherein said control circuit (9) has connection terminals (43) attached to an insulation member integrally formed with said connector, the connection terminals (43) being connected to switching elements (33) for controlling the motor current via bonding wires (45) and to pigtail (44) of said brushes (22). An electric power steering device according to claim 6, wherein a brush holder (23) for holding said brush (22) is made of a material (11a) having good heat conductivity, and is secured to said end frame (11) or to a metal member (25) brought into thermal contact with said end frame (11) via an insulating member having small heat resistance.

Inventors: ABE MASAO, HASHIBA YUUCHI, HAYASHI JIROU, KONDO MASANORI, SAWADA TAKESHI, TAKEUCHI KANJI, ABE, MASAO, HASHIBA, YUUCHI, HAYASHI, JIROU, KONDO, MASANORI, SAWADA, TAKESHI, TAKEUCHI, KANJI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1996
Main classification: B62D5
IPC codes: H02K5, H02K9, H02K7, H02K11, H02K23, H01R39, H02K13, B62D5