JP2006027385A - Motor-driven power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a motor-driven power steering device assuring a good layout performance and good feeling of operation. <P>SOLUTION: The motor-driven power steering device works with a motor 5 to drive a rack shaft 4. The motor 5 has a ring-shaped stator fixed to a housing and a rotor 13 installed rotatably inside the stator. The rotor 13 has a rotor shaft 14 in cylindrical shape, and a magnet 17 subjected to Halbach magnetization is adhered to the outside surface of the rotor shaft 14. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric power steering apparatus.

  Vehicles such as automobiles are often equipped with an electric power steering device that assists the driver's steering operation. As an electric power steering device, there is one called a coaxial type in which a motor is arranged along the length direction of a rack shaft. The motor has a housing, and a cylindrical rotor shaft and a stator that rotationally drives the rotor shaft are provided in the housing. A magnet is bonded to the outer peripheral surface of the rotor shaft, and a ball screw nut is attached to the inner peripheral surface. A ball screw of a rack shaft is screwed into the ball screw nut, and the rack shaft is connected to a wheel via a rack arm or the like (see, for example, Patent Document 1).

Here, it is known that a segment magnet or an anisotropic sintered ring magnet is used as the magnet fixed to the rotor shaft (see, for example, Patent Document 2). The segment magnet is formed by bonding a plurality of magnets so that the magnetic poles are arranged in the radial direction along the outer periphery of the rotor shaft. The anisotropic sintered ring magnet is obtained by bonding a ring-shaped magnet to the outer peripheral surface of the rotor shaft, and is magnetized so as to obtain a sinusoidal surface magnetic flux density distribution. Regardless of which magnet is used, the rotor shaft is used as the magnetic path.
In such an electric power steering apparatus, attempts have been made to reduce the physique by using a magnet having a large magnetic flux, to improve the layout, and to improve the fuel efficiency of the automobile.
JP 2003-111313 A JP 2002-354721 A

However, since the motor uses the rotor shaft as a magnetic path, magnetic flux may leak from the rotor shaft to the inside, and a magnetic path may be formed in the ball screw. When a magnetic path is formed in the ball screw, a force acting on the ball screw is generated. However, if this force acts unevenly, the life of the ball screw and the rack shaft may be reduced.
In order to prevent the magnetic path from being formed in the ball screw, the rotor shaft can be made thicker or the distance (air gap) between the ball screw and the rotor shaft can be increased. This is not preferable because the outer diameter of the motor increases. In addition, when a magnetic path is formed on the rotor shaft, dust such as metal powder easily adheres to the inside of the rotor shaft during assembly, and the dust may deteriorate the movement of the rack shaft.
The present invention has been made in view of such problems, and an object of the present invention is to provide an electric power steering apparatus having a good layout and a good feeling of operation.

  The invention according to claim 1 of the present invention for solving the above-mentioned problems has a cylindrical motor stator provided in a housing, and a cylindrical motor rotor is rotatable inside the motor stator. A motor that is inserted into the outer periphery of the motor rotor, rotates the motor rotor about an axis based on a current flowing through a winding wound around the motor stator, and the motor rotation A pinion that is attached to the rotor and rotates together with the motor rotor, and a rack shaft that passes through the motor rotor, the ball screw meshing with the pinion, and the magnet is Halbach magnetized. It was set as the electric power steering device characterized by being.

  The invention according to claim 2 has a cylindrical motor stator provided in the housing, and a cylindrical motor rotor is rotatably inserted inside the motor stator, A magnet is disposed on the outer periphery, and a motor that rotates the motor rotor around an axis based on a current flowing through a winding wound around the motor stator, and a motor rotor that is attached to the motor rotor, A pinion that rotates together with the pinion and a rack shaft that penetrates the motor rotor, and the magnet is a ring-shaped magnet having polar anisotropy. The electric power steering device.

  In this electric power steering apparatus, the motor rotor of the motor is disposed on the outer peripheral side of the rack shaft so that the rotation center of the motor rotor coincides with the axis of the rack shaft. Further, the motor rotor is provided with a magnet on the outer peripheral surface. The magnet is magnetized so that a sinusoidal magnetic flux density distribution is formed on the outer surface thereof. Since the magnet magnetized in this way does not generate magnetic flux on its inner peripheral surface, magnetic flux does not leak from the motor rotor to the rack shaft.

According to a third aspect of the present invention, in the electric power steering apparatus according to the first aspect, the magnet has a plurality of poles, and the poles include five or more segments whose orientations are different in stages. And
In this electric power steering apparatus, when a magnet having a plurality of poles is magnetized by Halbach, one pole is made up of five or more segments having different magnetization orientations. The magnetic flux density distribution on the outer peripheral surface becomes a smooth sine wave. Here, when the number of segments is less than 5, the difference in orientation between the segments becomes large, so that the magnetic flux density distribution approaches a trapezoidal wave shape as in the conventional case, and the cogging torque increases. Note that even if the number of segments is more than five, the shape of the magnetic flux density distribution hardly changes. Therefore, if the number of segments is five or more, the same operation and effect can be obtained.

  According to the present invention, the motor rotor of the motor is provided coaxially with the rack shaft, and the magnet of the motor rotor is a Halbach magnet or a ring-shaped magnet having polar anisotropy. Magnetic flux can be prevented from leaking. For this reason, the magnetization of the rack shaft is prevented, and the life of the rack shaft can be extended. In addition, since the distance between the rack shaft and the rotor rotor can be reduced, the electric power steering apparatus can be reduced in size. When the electric power steering device is downsized, the degree of freedom of layout in the vehicle increases, and for example, a buffer gap is easily formed.

The best mode for carrying out the invention will be described in detail with reference to the drawings.
As shown in FIG. 1, an electric power steering apparatus 1 includes an elongated rack shaft 4 connected to each of a pair of tires 2 via a rack arm and the like, and a motor 5 that moves the rack shaft 4 forward and backward in the axial direction thereof. It is attached to the front side of the vehicle body.
The motor 5 has a substantially cylindrical motor housing 6, and the motor housing 6 is fixed to a rack housing 7. The rack housing 7 accommodates the rack shaft 4 and is fixed to the vehicle body side. A stator 10 that is a motor stator is fixed to the inner peripheral surface of the motor housing 6. The stator 10 has an annular stator core 11 in which electromagnetic steel plates are laminated. A winding 12 is wound around each of a plurality of slots formed in the stator core 11 to form a coil 12. The end of the winding is connected to the terminal 8. The terminal 8 penetrates the motor housing 6 and is drawn out to the outside so that a socket for energization can be connected. A rotor 13 as a motor rotor is inserted on the inner peripheral side of the stator 10.

  The rotor 13 has a cylindrical rotor shaft 14 disposed coaxially with the stator 10. One end of the rotor shaft 14 is rotatably supported on the rack housing 7 by a bearing 15. A magnet 17 is bonded to the outer peripheral surface of the rotor shaft 14. A predetermined gap is formed between the magnet 17 and the stator 10. As shown in FIG. 3, the magnet 17 is a Halbach magnet in which a plurality of segment magnets 18 magnetized so as to have a certain orientation are connected to form an annular shape. Each segment magnet 18 is magnetized in the direction shown by the arrow in FIG. That is, in FIG. 3, the segment magnet 18 is a six-pole ring magnet, and one magnetic pole is composed of five segment magnets 18. The segment magnet 18 is magnetized so that the direction of the magnetic flux differs from that of the adjacent segment magnet 18, and the direction of the magnetic flux is changed stepwise from the S pole or the N pole.

Here, FIG. 4 shows a change in the magnetic flux density distribution with respect to the direction of the magnetic flux of the segment magnet 18. In this figure, the horizontal axis indicates the arrangement of the segment magnets 18 and the direction of the magnetic flux of each segment magnet 18, and the vertical axis indicates the distribution of magnetic flux density on the front or back surface of the magnet 17.
As shown on the upper side in FIG. 4, the magnetic flux on the surface (outer peripheral surface) of the magnet 17 changes smoothly according to the direction in which the segment magnet 18 is magnetized, and has a sinusoidal shape as a whole. The magnetic flux density has a maximum value at a position corresponding to the center of the N pole, and a minimum value at a position corresponding to the center of the S pole. In addition, the broken line in a figure shows the magnetic flux density distribution of the surface in the conventional radial type magnet. In the radial type magnet, the magnetic flux density distribution has a trapezoidal wave shape, and a cogging torque is generated when switching from a curved line portion indicated by a solid line to a straight line portion indicated by a broken line.
On the other hand, as shown on the lower side in FIG. 4, the magnet 17 does not generate magnetic flux on the back surface (inner peripheral surface). In a conventional radial magnet, a trapezoidal magnetic flux density distribution is generated as indicated by a broken line.

  FIG. 5 shows the relationship between the number of segment magnets 18 per pole in the six-pole magnet 17 and the content of the fundamental wave component. The content rate of the fundamental wave component is an index indicating that the larger the value is, the more the magnetic flux density distribution becomes sinusoidal. As shown in FIG. 5, when the number of segment magnets 18 is five and the content rate of the fundamental wave component reaches 85%, the content rate of the fundamental wave component is substantially constant even when the number of segment magnets 18 is increased. It has become. That is, it is preferable that the number of the segment magnets 18 when the 6-pole Halbach type is employed as in the present embodiment is 5 or more. If the number is five or more, the magnetic flux density distribution approaches a sine wave and the cogging torque can be reduced. However, if the number is less than five, the content of the fundamental wave component is low, and the magnetic flux density distribution is trapezoidal. This is because the cogging torque increases as the wave approaches. And if it is five or more, magnetic flux density distribution will become substantially constant and the same performance will be acquired.

  Further, as shown in FIG. 2, a ball screw nut 21 that is a pinion is attached to the rotor 13 including the magnet 17. The ball screw nut 21 is fixed to the other end of the rotor shaft 14 and extends along the length direction of the rotor shaft 14. The outer peripheral surface of the ball screw nut 21 is rotatably supported by an angular bearing 16. The angular bearing 16 is fixed to the inner periphery of the rack housing 7 by a bearing nut 16a. Further, a thread groove 22 is carved on the inner periphery of the ball screw nut 21. A ball screw 23 provided on the rack shaft 4 is screwed into the screw groove 22. The rack shaft 4 passes through the ball screw nut 21 and the rotor shaft 14 and is connected to the tire 2 (see FIG. 1).

As shown in FIG. 1, the motor 5 is connected to the control device 30. The control device 30 is configured to control the rotation direction and the rotation amount of the motor 5 based on the result of detecting the rotation direction and the rotation amount of the steering wheel 31 by the resolver 32 (sensor). Here, the resolver 32 is disposed in the vicinity of the bearing 15 in FIG. 2, and includes a resolver rotor that rotates together with the rack shaft 4, and a resolver stator that is fixed to the motor housing 6 and detects the rotational position of the resolver rotor. The rotational direction and amount of rotation of the motor 5 are detected by the induced voltage.
Further, the steering wheel 31 shown in FIG. 1 is rotatably supported by the steering column 33, and the rotation of the steering wheel 31 is mechanically transmitted to the rack shaft 4.

Next, the operation of the electric power steering apparatus 1 will be described with reference to FIGS.
When the steering wheel 31 is operated, the motor 5 is energized from the control device 30 according to the direction and amount of rotation. The current flows through the coil 12 of the stator 10 and generates a magnetic flux therein. The magnetic flux flows between the stator 10 and the magnet 17, and a driving force is generated in the magnet 17. Thereby, the rotor shaft 14 rotates around the central axis, and the ball screw nut 21 attached to the rotor shaft 14 also rotates integrally. When the ball screw nut 21 rotates, the ball screw 23 screwed to the inner surface of the ball screw nut 21 rotates and is moved in the axial direction. As a result, the rack shaft 4 moves while rotating, assisting the movement of the rack shaft 4 that is mechanically moved by the steering wheel 31, and the tire 2 is steered in a predetermined direction.

According to this embodiment, since the magnet 17 used for the rotor 13 of the motor 5 is of the Halbach type, magnetic flux does not leak inside the rotor 13, and a magnetic path is not formed in the ball screw 23. Therefore, unnecessary force does not act on the ball screw 23, and the durability of the ball screw 23 and the rack shaft 4 is improved.
Further, since no magnetic flux is generated inside the magnet 17, it is not necessary to increase the thickness of the rotor shaft 14 so that the magnetic flux does not leak inside the rotor 13, the motor 5 can be downsized, and the inertia of the rotor 13 can be reduced. Can be suppressed. Furthermore, it is not necessary to secure a large gap between the rotor shaft 14 and the ball screw 23. Therefore, the diameter of the motor 5 can be reduced. For these reasons, the electric power steering apparatus 1 can be reduced in size and weight, and the degree of freedom in layout with respect to the vehicle body is increased.
Further, in order to absorb a shock at the time of a vehicle collision and protect an occupant, a buffer gap (crash zone) is often provided between the engine and the electric power steering device 1. By making it small, it becomes easy to ensure such a buffer gap. Furthermore, as shown in FIG. 1, a sufficient distance h from the road surface to the electric power steering apparatus 1 can be secured.

  In addition, since the magnet 17 is a Halbach type, the magnetic flux density distribution on the magnet surface can be made into a smooth sine wave shape, so that the cogging torque can be reduced as compared with the case of a trapezoidal wave shape like a conventional magnet. be able to. When the cogging torque is reduced in this way, the straightness of the steering wheel 31 is improved, and the steering feeling during high-speed driving of the vehicle can be improved. Furthermore, in the conventional motor, the magnet is sometimes skewed in order to reduce the cogging torque. In this case, although the cogging torque is reduced, the output is reduced. However, in this embodiment, such a decrease in output does not occur. That is, when comparing a magnet 17 that differs only in magnetization and a conventional radial magnet, the magnet 17 has a sinusoidal magnetic flux density distribution as shown in FIG. Since the peak value of the magnetic flux density distribution increases, the output increases. Specifically, the magnet 17 can increase the energy by 20%. For this reason, when the same energy is generated, the motor 5 can be made smaller by using the magnet 17.

  Here, the magnet used as the rotor 13 of the motor 5 may be a magnet 40 having polar anisotropy as shown in FIG. The magnet 40 is a six-pole ring magnet having an anisotropic direction in which adjacent different poles in the magnet 40 are connected by a curve. Such a magnet 40 provides a sinusoidal distribution of magnetic flux density between the magnetic poles, so that the magnetic flux does not leak inside the rotor 13. A large magnetic flux density is obtained at the peak position. Therefore, the motor 5 including the magnet 40 and the electric power steering apparatus 1 can obtain the same effects as described above.

Note that the present invention can be widely applied without being limited to the above-described embodiment.
For example, the number of poles of the magnets 17 and 40 is not limited to six but may be two or eight.
In the electric power steering apparatus 1, the rotor 13 and the rack shaft 4 need only be arranged coaxially, and the arrangement of the ball screw nut 21 and the ball screw 23 is not limited to that shown in FIG.
The electric power steering apparatus 1 may have a configuration in which the steering wheel 31 and the rack shaft 4 are mechanically separated.

It is a figure showing a schematic structure of an electric power steering device in an embodiment of the invention. It is an expanded sectional view showing a schematic structure of a portion concerning a motor. It is sectional drawing perpendicular | vertical to the rotating shaft of a motor, Comprising: It is a figure which shows the structure of the magnet magnetized by Halbach. It is a figure which shows the relationship between orientation of a segment magnet, and distribution of the magnetic flux density of a magnet. It is a figure which shows the relationship between the number of segment magnets per pole, and fundamental wave component content. It is a figure which shows the structure of the 6 pole ring magnet which has polar anisotropy.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electric power steering apparatus 4 Rack shaft 5 Motor 6 Housing 10 Stator (motor stator)
13 Rotor (motor rotor)
17, 40 Magnet 18 Segment magnet 21 Ball screw nut (pinion)
23 Ball screw

Claims (3)

A cylindrical motor stator provided in the housing, a cylindrical motor rotor is rotatably inserted inside the motor stator, and a magnet is disposed on the outer periphery of the motor rotor; A motor for rotating the motor rotor around an axis based on a current flowing through a winding wound around the motor stator;
A pinion attached to the motor rotor and rotating together with the motor rotor;
A rack screw that meshes with the pinion, and a rack shaft that passes through the motor rotor;
An electric power steering apparatus, wherein the magnet is magnetized by Halbach. A cylindrical motor stator provided in the housing, a cylindrical motor rotor is rotatably inserted inside the motor stator, and a magnet is disposed on the outer periphery of the motor rotor; A motor for rotating the motor rotor around an axis based on a current flowing through a winding wound around the motor stator;
A pinion attached to the motor rotor and rotating together with the motor rotor;
A rack screw that meshes with the pinion, and a rack shaft that passes through the motor rotor;
And the magnet is a ring-shaped magnet having polar anisotropy. 2. The electric power steering apparatus according to claim 1, wherein the magnet has a plurality of poles, and the poles include five or more segments whose orientations are different in stages.

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