Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-05-18
2001-07-31
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S07500D, C310S06800R, C310S06800R, C310SDIG006, C310S089000, C310S091000, C180S443000, C701S041000
Reexamination Certificate
active
06268669
ABSTRACT:
TECHNICAL FIELD
This invention relates to an improvement in an electric motor for an electric power steering system for applying an assisting force to a steering of the electric motor.
BACKGROUND ART
There has been known an electric power steering system for applying a proper assisting force to a steered member such as wheels from an electric motor in accordance with a torque in-putted from an input portion such as a steering wheel. The structure of the electric motor to be used in such electric power steering system has been proposed by us in Unexamined Published Japanese Patent Application No. 7-203888, for example.
FIG. 18
shows this electric motor of the electric power steering system of the prior art. As shown, an electric motor
201
is fixed in an assembling hole
204
which is formed in a wall
203
of a board case
202
. In the case
202
, a circuit board
205
is fitted to the wall
203
. On this circuit board
205
, a drive circuit comprising a plurality of FETs (Field Effect Transistors)
206
acting as switching elements for driving the electric motor
201
, and a control circuit comprising a CPU
207
are installed.
An output shaft
210
extending from the electric motor
201
further extends through the assembling hole
204
. It is supported on a cover
211
of the board case
202
via a bearing
212
. And associated with a worm shaft
214
accommodated in a gear case
213
. The worm shaft
214
is associated with a steered member, not shown, to transmit the assisting force from the electric motor
201
to the steered member.
Brushes
216
in sliding contact with a commutator
215
on the outer circumference of the output shaft
210
are held by brush holders
217
, which are supported on an insulating spacer
218
mounted in the assembling hole
204
. Pigtail wires (or lead wires)
219
for conducting the brushes
216
and the circuit board
205
are directly mounted on the circuit board
205
.
The electric motor
201
of the prior art is thus constructed, but this structure has the following problems.
First of all, this electric motor
201
of the prior art cannot retain a sufficient heat releasing path from the plurality of FETs
206
composing the drive circuit and from the output shaft
210
.
The plurality of FETs
206
composing the drive circuit on the circuit board
205
are usually arranged in the vicinity of the brushes
216
, i.e., in the vicinity of the output shaft
210
from conveniences of the circuit construction. Since these FETs
206
generate heat the releasing path for this heat has to be retained. According to this prior art motor, however, the brushes
216
are located in the assembling hole
204
of the electric motor
201
and the FETs are also located in the vicinity of the hole
204
, so the heat releasing path from the FETs
206
and the output shaft
210
is restricted to the path through the circuit board
205
. Even if the board case
202
and the circuit board
205
are made of aluminum or the like having a high thermal conductivity, therefore, the heat release from the FETs
206
and the output shaft
210
may not be sufficient.
Secondly, this prior art motor has a problem that the temperature of the circuit board
205
becomes high when the pigtail wires
219
are fixed thereon. Specifically, the pigtail wires
219
extending from the brushes
216
are directly fixed on the circuit board
205
, and this fixing operation is made by the hot-soldering or ultrasonic-welding work. As a result, the heat generated by this hot-soldering or ultrasonic-welding operation is hard to be released from the circuit board
205
. In the worst case, the circuit pattern of the circuit board
205
may be separated by that heat.
Thirdly, this prior art motor is not convenient for carrying the dynamic characteristic tests of the electric motor
201
by itself. In this structure of the prior art motor, more specifically, the output shaft
210
of the electric motor
201
extends downward of the board case body
203
holding the circuit board
205
and it is supported on the cover
211
via the bearing
212
. As a result, the dynamic characteristic tests of the electric motor
201
in the state detached from the case
202
have required a special jig for supporting the electric motor
201
.
Fourthly, a torque limiter has to be provided for the electric motor
201
. In this prior art motor, the torque limiter has to be arranged outside of the board case
202
which makes the structure of the entire system large-sized.
This invention has been conceived noting those problems and has an object to provide an electric motor for an electric power steering system, which has an excellent heat releasing property from the circuit board (especially, FETs composing the drive circuit) and the output shaft.
Another object of this invention is to provide an electric motor for an electric power steering system, which is not adversely affected by the high temperature of the circuit board at a step of electrically connecting the brushes and the circuit board of the electric motor.
Still another object of this invention is to provide an electric motor for an electric power steering system, in which the dynamic characteristic tests of the electric motor can be easily performed.
Still another object of this invention is to provide an electric motor for an electric power steering system, which can accommodate a torque limiter compactly in the electric motor thereby to reduce the size of the entire system.
DISCLOSURE OF THE INVENTION
This invention provides an electric motor for an electric power steering system for applying an assisting force to a member to be steered, in accordance with an input torque inputted to an input member. The motor comprises a motor case for accommodating an electric motor body and a circuit board on which a drive circuit for driving the electric motor body and a control circuit for controlling the drive circuit are installed, and a board mounting member formed across an inner space of the motor case as a wall partitioning the electric motor body and the board. The board mounting member is made of a material having a high thermal conductivity and supports a bearing of an output shaft of the electric motor body which projects through the board mounting member toward the board.
Even if the parts mounted on the board is caused to generate the heat by the drive of the electric motor, this heat is smoothly released through the board mounting member, because the entire circuit board is in contact with the board mounting member. As a result, the stability of the operation of the circuits on the circuit board is ensured. Since the bearing of the output shaft is supported on the board mounting member, the heat of the output shaft is also smoothly released via the board mounting member. Especially when the drive circuit employs the field effect transistors as the switching elements, the heat from the field effect transistors is effectively released via the board mounting member, so the field effect transistors do not easily reach the junction temperature and the operation performance of the drive circuit is maintained stable. Moreover, the output shaft of the electric motor body is supported by the board mounting member
23
via the bearing, so the dynamic characteristic tests of the electric motor in a state detached from the board case can be executed without supporting the electric motor with a special jig, and the tests can be executed more efficiently.
According to an aspect of this invention, the motor further comprises a brush lead extending from a brush in sliding contact with a commutator of the electric motor body to the circuit board through the board mounting member. The brush lead has a wide surface area on the side of the brush. An ECU lead extending from the circuit board is connected electrically with the brush lead, and a lead wire extending from the brush is welded to a part corresponding to the wide surface area of the brush lead. As a result, the high heat generated at the time of welding the lead wire is efficiently released through the
Endo Akira
Kawaguchi Wataru
Nakamura Takenobu
Wakao Hirokazu
Kayaba Kogyo Kabushiki Kaisha
Mullins Burton S.
Rabin & Champagne, P.C.
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