Patent Publication Number: US-2009230805-A1

Title: Motor for electric power steering apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a motor used as a driving source in electric power steering apparatuses, more particularly, the invention relates to a technique that is useful when applied to an electric power steering apparatus of rack-assist type in which a rack shaft of a vehicle is inserted into the center part of the motor. 
     BACKGROUND ART 
     In recent years, a so-called power steering apparatus has been used in many vehicles, assisting the steering force of vehicle wheels such as automobiles. At present, more and more vehicles have an electrically driven steering apparatus (so-called electric power steering apparatus), in order to reduce the load on the engine or decrease the weight of the vehicle and the like. The electric power steering apparatus (hereinafter abbreviated to EPS) is generally applied to a rack-and-pinion type steering apparatus and EPSs now available are classified roughly into three types, in accordance with the motor position. Namely, from the side near a driver, the three types such as the column assist type in which the motor is arranged on the steering shaft; the pinion assist type in which the motor is arranged at the junction between the steering shaft and the rack shaft; and the rack assist type in which the motor is arranged coaxial with the rack shaft are known. 
     EPS disclosed in Patent Document 1 is an apparatus of rack assist type, in this apparatus, the motor arranged coaxial with the rack shaft exerts a steering assist force.  FIG. 3  is a sectional view showing the configuration of such a rack assist type EPS as is disclosed in Patent Document 1. In the EPS  51  of  FIG. 3 , the motor  53  arranged coaxial with the rack shaft  52  generates a steering assist force, and this force is transmitted to the rack shaft  52  by a ball-screw mechanism  54 . The rack shaft  52  is linked, at the both ends thereof, to a steering control wheel by a tie rod (not shown), a knuckle arm (not shown) and the like, and also coupled to a steering shaft  55  by way of a rack-and-pinion gear. The rack shaft  52  is moved in its axial direction (to the left or the right, in  FIG. 3 ) as the driver operates the steering wheel. The motor  53  has a cylindrical yoke  56 , a magnet  57 , a cylindrical rotor shaft  58  and a rotor core  59 , the magnet  57 , rotor shaft  58  and rotor core  59  are coaxially inserted in the yoke  56  and the rack shaft  52  is inserted in the rotor shaft  58 . 
     In the EPS  51 , when the steering shaft  55  rotates as the steering wheel is rotated, the rack shaft  52  moves to the direction in accordance with the rotation to perform the steering operation. Activating a steering torque sensor (not shown) by the operation, appropriate power is supplied to the motor  53  based on the detected torque. When the motor  53  is thereby driven, the ball-screw mechanism  54  transmits the rotation of the motor to the rack shaft  52 . In other words, the ball-screw mechanism  54  converts the rotation of the motor  53  to an axial motion of the rack shaft  52 , a steering assist force is given to the rack shaft  52 . The steering control wheels are turned by the steering assist force and manual steering force to reduce steering loads of the driver.
     Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 10-152058   Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 2004-180449   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     On the other hand, in most cases, a strict limitation is imposed on the physical size (particularly, the outer dimensions) of such a rack assist type EPS as shown in  FIG. 3  so that the EPS may be laid out compact (thin and short) in the engine room. An EPS for use in, for example, small cars, is commercially unsuccessful if its outside diameter exceeds 100 mm, the motor should therefore satisfy optimal specification so that its outside diameter may be 100 mm or less. On the other hand, the rack shaft itself that passes through the motor has an outside diameter of about 20 to 30 mm, the rotor shaft, in which the rack shaft is inserted, should therefore has an inside diameter of about 20 to 40 mm. Accordingly, the outside diameter of any motor for the lack assist type EPS should be 100 mm or less, though the rack shaft lies, extending in the center part, further, it is demanded that the motor should have a desired output with the limited size, and low heat generation, low friction and low cost are required. 
     However, to determine the specification of a motor for the EPS that can be small, perform well and be manufactured at low cost, very complex and intricate design adjustments are required. More specifically, various parameters concerning the magnets and windings are involved in structural designing such a motor, and many of them are in trade-off relation. In many cases, it is difficult, even for skilled and experienced designers, to determine the specification of a motor, which satisfies all requirement described above, therefore, some design guidelines are required that would help the designers to provide the optimal motor easily. 
     An object of the present invention is to design a motor for the EPS easily, which can bring out desired performance as to output, heat generation and the like, while satisfying the dimensional restriction imposed on it. 
     Means for Solving the Problems 
     A motor for use in an electric power steering apparatus, according to the present invention, is configured to be mounted on, and arranged coaxial with, a rack shaft coupled to a steering wheel. The winding of the motor has a ratio between the number of turns and the wire diameter (number of turns/wire diameter) set in the range of 15 to 25. 
     In the present invention, since the winding of the motor has the ratio between the number of turns and the wire diameter set in the range of 15 to 25, it is possible to obtain an EPS motor that satisfies items of requirements such as size, output, heat generation, feeling of steering and cost, in a well-balanced manner. Further, if the design specification of the components of the motor is determined in accordance with the numerical value specified above, the specification setup of the motor will be fit for the EPS. 
     In the motor for use in the electric power steering apparatus, the ratio between the number of turns and the wire diameter may preferably be set in the range of 18 to 22. Further, the motor for use in the electric power steering apparatus may be a brushless motor that has six-pole, nine-slot. 
     The motor may have a stator comprising a housing, a stator core secured to an inner circumferential surface of the housing, and a winding wound around the stator core; and a rotor comprising a hollow-cylindrical rotor shaft in which the rack shaft of the steering apparatus is inserted, a hollow-cylindrical rotor core mounted on an outer circumferential surface of the rotor shaft, a magnet mounted on an outer circumferential surface of the rotor core, and a magnet cover fitted on the outside of the magnet. In this case, the above-described outside diameter may range from 85 mm to 100 mm. 
     ADVANTAGES OF THE INVENTION 
     The motor for use in an electric power steering apparatus, according to the present invention, is configured to be mounted on, and arranged coaxial with, a rack shaft coupled to a steering wheel and since the winding of the motor has a ratio between the number of turns and the wire diameter set in the range of 15 to 25, it is possible to obtain a suitable EPS motor that satisfies items of requirements such as size, output, heat generation, feeling of steering and cost, in a well-balanced manner. Further, it is possible to obtain a specification setup fit for the EPS concerning the ratio between the number of turns and the wire diameter, which is one of the parameters that poses a problem in designing the motor structure, if the specification of each part of the motor is determined according to said value, thus, the optimal design of the motor for EPS can be attained. Moreover, since the number of designing steps can be reduced, the cost of developing the product can be reduced and the cost of the product can be also lowered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing a motor for EPS according to the present invention; 
         FIG. 2  is an explanatory diagram showing the relation between the ratio of the number of turns and the wire diameter, the resistance of the motor and the length of the magnet circuit; and 
         FIG. 3  is a sectional view showing an EPS of rack assist type. 
     
    
    
     EXPLANATION OF REFERENCE SYMBOLS 
       
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 1: 
                 Motor 
                 2: 
                 Lack shaft 
               
               
                 3: 
                 Ball-screw mechanism 
                 11: 
                 Stator 
               
               
                 12: 
                 Housing 
                 13: 
                 Stator core 
               
               
                 14: 
                 Winding 
                 15: 
                 Power supplying wires 
               
               
                 21: 
                 Rotor 
                 22: 
                 Rotor shaft 
               
               
                 23: 
                 Rotor core 
                 24: 
                 Magnet 
               
               
                 25: 
                 Magnet cover 
                 31: 
                 Housing 
               
               
                 32: 
                 Bearing 
                 33: 
                 Resolver 
               
               
                 34: 
                 Resolver stator 
                 35: 
                 Resolver rotor 
               
               
                 36: 
                 Coil 
                 41: 
                 Housing 
               
               
                 42: 
                 Nut section 
                 43: 
                 Screw section 
               
               
                 44: 
                 Ball 
                 45: 
                 Angular bearing 
               
               
                 46a, 46b: 
                 Bearing-holding ring 
                 47: 
                 Stepped section 
               
               
                 48: 
                 Bearing-holding ring 
                 49. 
                 Stepped section 
               
               
                 51: 
                 Electric power steering 
                 52: 
                 Rack shaft 
               
               
                   
                 apparatus 
               
               
                 53: 
                 Motor 
                 54: 
                 Ball-screw mechanism 
               
               
                 55: 
                 Steering shaft 
                 56: 
                 Yoke 
               
               
                 57: 
                 Magnet 
                 58: 
                 Rotor shaft 
               
               
                 59: 
                 Rotor core 
               
            
           
           
               
               
            
               
                 R: 
                 Ratio between number of turns and wire diameter 
               
               
                   
                 (number of turns/wire diameter) 
               
               
                   
               
            
           
         
       
     
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is a sectional view showing the configuration of a motor for EPS, according to the present invention. The motor  1  of  FIG. 1  is used as driving source for use in such a rack assist type EPS as shown in  FIG. 3 , and a rack shaft  2  passes through the inside of the motor  1 . The motor  1  of  FIG. 1  is a brushless motor, unlike the motor  53  of  FIG. 3 , and the rotation of the motor  1  is transmitted to the rack shaft  2  by a ball-screw mechanism  3  so as to be a steering assist force. 
     The motor  1  has an inner rotor type structure, comprising a stator  11  outside and a rotor  21  inside. The stator  11  comprises a housing  12 , a stator core  13  secured to an inner circumferential side of the housing  12 , and a winding  14  wound around the stator core  13 . The housing  12  is made of iron etc. and an outside diameter thereof is kept at 100 mm or less. The stator core  13  is composed of many steel plates laid one on another, and a plurality of teeth (nine, in this embodiment) protrude from the inner circumferential side of the stator core  13 . A plurality of slots (nine, in this embodiment) are provided between the teeth, and a coil is wound around the slot formed between the teeth, to form the winding  14 . The winding  14  is connected to a battery (not shown) via power supplying wires  15 . 
     The rotor  21  is arranged inside the stator  11  and it includes a rotor shaft  22  shaped like a hollow cylinder, a rotor core  23 , a magnet  24  and a magnet cover  25  arranged coaxial with one another. The rack shaft  2  is inserted in the rotor shaft  22 . The cylindrical rotor core  23  is mounted on the outer circumferential surface of the rotor shaft  22 . On the outer circumferential surface of the rotor core  23 , the magnet  24  having six poles structure is fixed. 
     As the magnet  24 , a rare-earth magnet, for example, neodymium-iron magnet is used, which is small and provides high flux density. By the use of a rare-earth magnet for the magnet  24 , the motor  1  can be miniaturized, as well as the inertia of the rotor  21  decreases, improving the feeling of steering. The magnet  24  is shaped like a ring, and has N poles and S poles arranged alternately in the circumferential direction. Note that, the magnet  24  may be replaced by a plurality of segment magnets. The magnet cover  25  is fitted on the outside of the magnet  24 , even if the magnet  24  is broken, the motor  1  is prevented from being locked by the resulting fragments. 
     To the right side end of the housing  12  ( FIG. 1 ), there is secured a housing  31  that is an aluminum die-cast product. A bearing  32  supporting the right end of the rotor  21  and a resolver  33  detecting the rotation of the rotor  21  are held in the housing  31 . The resolver  33  comprises a resolver stator  34  secured to the housing  31  side and a resolver rotor  35  secured to the rotor  21  side. Around the resolver stator  34 , a coil  36  having an exciting coil and a detecting coil is wound. The resolver rotor  35  secured to the rotor shaft  22  is arranged inside the resolver stator  34 . The resolver rotor  35  is composed of many metal plates laid on one another, and has projections protruding in three directions. 
     As the rotor shaft  22  rotates, the resolver rotor  35  rotates inside the resolver stator  34 . A high frequency signals is applied to the exciting coil of the resolver stator  34 . As the projections approach and leave the detecting coil, a phase of the signal output from the detecting coil changes. By comparing the signal output from the detecting coil with a reference signal, the rotation position of the rotor  21  is detected. Based on the rotation position of the rotor  21  thus detected, the current supplied to the winding  14  is switched appropriately, as a result, the rotor  21  is driven and rotated. 
     To the left side end of the housing  12  ( FIG. 1 ), there is secured a housing  41  that is an aluminum die-cast product. The housing  41  holds the ball-screw mechanism  3 . The ball-screw mechanism  3  has a nut section  42 , a screw section  43  provided at the outer circumferential surface of the rack shaft  2 , and a number of balls  44  arranged between the nut section  42  and the screw section  43 . The rack shaft  2  is supported by the nut section  42  in such a way that its rotary motion around the axis of rotation is restricted but it is reciprocated right and left direction as the nut section  42  is rotated. 
     The nut section  42  is fixed to the left end of the rotor shaft  22  and supported by an angular bearing  45  secured to the housing  41 , and can rotate. The angular bearing  45  is secured, restricted in its axial motion, between bearing-holding rings  46   a ,  46   b  screwed into an opening formed in the housing  41  and a stepped section  47  formed in the housing  41 . The relative axial movement of the nut section  42  and the angular bearing  45  is restricted by another bearing holder ring  48  screwed into the left end of the nut section  42  and another stepped section  49  formed on the outer peripheral wall of the nut section  42 . 
     In the EPS having the motor  1  thus configured, the steering shaft is rotated when the steering wheel is operated, and the rack shaft  2  is moved in the direction corresponding to the sense of rotation of the steering shaft to carry out a steering operation. A steering torque sensor (not shown) is actuated by the operation, then, electric power is supplied from the battery to the winding  14  through the power supplying wires  15  in accordance with the detected torque. When the power is supplied to the winding  14 , the motor  1  is activated and the rotor shaft  22  is thereby driven. As the rotor shaft  22  is so driven, the nut section  42 , which is coupled to the rotor shaft  22 , is rotated, the steering assist force is transmitted to the rack shaft  2  under the effect of the ball-screw mechanism  3 . As a result, the movement of the rack shaft  2  is promoted and the steering power is assisted. 
     On the other hand, in such a motor for EPS, it is a problem how the specification of the winding (i.e., number of turns and wire diameter) should be determined in order to hold the motor size and to obtain a high output when the specification items is determined in order to satisfy the required performance.  FIG. 2  is a diagram explaining the relation between the ratio of the number of turns and the wire diameter, the resistance of the motor and the length of the magnet circuit (here, the axial direction length of the magnet  24 .) As seen from  FIG. 2 , the resistance of the motor and the length of the magnet circuit have a trade-off relation with the ratio between the number of turns and the wire diameter. If the ratio between the number of turns and the wire diameter is small, the length of the magnet circuit will increase and the total length of the motor will also increase, inevitably making the motor larger or heavier, raising the cost, lowering the productivity, and increasing the inertia to worsen the feeling of steering. If the ratio is large, the resistance of the motor will increase, inevitably reducing the performance or durability due to the generation of heat or the reduction in magnetic flux density. 
     In view of the problems specified above, the inventors hereof studied for an optimal value for the ratio between the number of turns and the wire diameter in six-pole, nine-slot motors suitable for EPS to obtain a torque required, suppress the heat generation and improve the feeling of steering, notwithstanding the severe dimensional restriction imposed on the motor. As the result of this, according to the inventors&#39; experiment, an EPS motor that satisfies size, output, heat generation, feeling of steering, cost and the like, in a well-balanced manner, can be provided if the ratio R between the number of turns and the wire diameter (R=number of turns/wire diameter) is 15 to 25, preferably 18 to 22, the ratio R being in the vicinity of the intersection of two curves, indicating the number of turns and the wire diameter respectively, shown in  FIG. 2 . If the ratio R approaches 15, the motor will become a high-speed type, if the ratio R approaches 25, the motor will become a high-torque type. 
     In the inventors&#39; experiment, a six-pole (P=6), nine-slot motor was made, which has a winding made by winding a wire having a diameter of 1.4 mm 27 times (thus, R=19.3) and which had an outside diameter (i.e., that of the housing  12 ) of 100 mm or less. This motor was heated to only 180° C. at most, when driven at output of 750 W and input voltage of 12 V. In this case, it is extremely difficult to hold the outside diameter of the motor to less than 85 mm, because of the outside diameter of the rack shaft and the like, therefore, in the motor according to this invention, the outside diameter of the motor is set to 90 to 100 mm, preferably 85 to 95 mm. The condition of 15≦R≦25, applied to this invention, proved to be effective, particularly to six-pole, nine-slot motors. 
     Since a motor that has ratio R exceeding 25 (i.e., the number of turns is large and the wire is thin), much heat is generated, causing problems such as demagnetization etc., some measures should be taken to cool the motor and the cost will also increase according to it. On the other hand, in any motor that has ratio R less than 15 (i.e., the number of turns is large and the wire is thin), the length of the magnet increases as the length of the magnet circuit increases. This runs counter to the demand that the motor should be small, as well as increases the amount of the rare-earth magnet that is expensive, it will be disadvantageous to the cost. Moreover, since the distance for which the magnet  24  faces the stator teeth becomes longer, the inertia or the motor friction increase. 
     As mentioned above, the motor for EPS according to this invention exhibits optimal characteristics for use in the EPS, and it satisfies size, output, heat generation, feeling of steering and cost, in a well-balanced manner. This small, high-output motor ultimately serves to save fuel in the vehicle. In addition, its rotor is small, decreasing the inertia and, hence, improving the feeling of steering. Furthermore, in the motor according to the invention, since appropriate parameters for the EPS are preset with respect to the ratio R of the number of turns and the wire diameter, which is one of the parameters that poses a problem in designing the motor structure, the designer only needs to determine the specification of each motor component in accordance with the corresponding parameter on the structure design. That is, the present invention provides design guidelines optimal for the EPS. An EPS motor can therefore be easily provided, which is smaller and can produce a high output, generate less heat, make smaller friction and lower cost than the conventional EPS motors and it is possible to realize optimal design and to reduce the number of designing steps. Therefore, the cost of developing the product can be proportionally reduced, and the product cost can be also lowered. 
     Needless to say, the present invention should not be limited to the embodiment described above, and may be variously modified within the scope not departing from the gist. 
     For example, the motor described above, in which R=19.3, is merely an example, and needless to say, motors of any other specifications can be manufactured.