Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-05-02
2003-06-10
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S071000, C310S084000, C310S064000, C310SDIG006, C318S488000, C318S489000, C318S648000
Reexamination Certificate
active
06577030
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automotive electric power steering apparatus for applying an assisting force to the steering wheel of a vehicle and, more particularly, to an electric power steering apparatus that provides excellent productivity and reliability.
2. Description of the Related Art
FIG. 14
shows the structure of a general electric power steering apparatus.
FIG. 15
is a block diagram of a driver circuit for the electric power steering apparatus. In
FIG. 14
, a vehicle has a steering wheel
1
that is a steering device. A torque sensor
2
detects the steering torque applied to the steering wheel
1
. A vehicle speed sensor
3
detects the speed of the vehicle. An electric motor
4
generates a steering assisting force. A control means
5
receives the output signals from the torque sensor
2
and from the vehicle speed sensor
3
and controls the motor
4
according to the steering torque and the operating conditions of the vehicle to thereby control the value and the direction of the steering assisting torque. A battery
6
is an electric power source for the electric power steering apparatus and supplies a power-supply voltage to the torque sensor
2
, vehicle speed sensor
3
, motor
4
, and control means
5
. The circuit configuration of the electric power steering apparatus made up of these components is shown in FIG.
15
.
FIG. 15
shows an example of a circuit configuration of an electric power steering apparatus using a brushless motor as the motor
4
. An electric motor
4
includes a rotor (not shown) made of a magnet, three-phase armature windings
7
wound around a stator, and an angular position sensor
8
for detecting the angular position of the rotor. A control means
5
includes a microcomputer (CPU)
9
, a driver circuit
10
, a large capacitor
11
for removing ripples from the electric current flowing through the motor
4
, a shunt resistor
12
for detecting the current flowing through the motor
4
, a current-detecting means
13
for detecting the current value from a voltage drop developed across the shunt resistor
12
, and a switching means
14
for switching on or off the current flowing through the motor
4
. The CPU
9
receives the output signals from the torque sensor
2
, from the vehicle speed sensor
3
, and the angular position sensor
8
and controls the motor
4
. The driver circuit
10
controls semiconductor switching elements Q
1
-Q
6
connected in a three-phase bridge form according to a signal from the CPU
9
.
In the general electric power steering apparatus of the construction described above, the CPU
9
receives the output signal from the torque sensor
2
that indicates the steering torque. Also, the CPU
9
receives the output signal from the vehicle speed sensor
3
that indicates the speed of the vehicle. The CPU calculates corresponding assisting torque value and driving direction and delivers output signals indicating the calculated values to the driver circuit
10
. This driver circuit
10
furnishes pulse width-modulated (PWM) signals to the semiconductor switching elements Q
1
-Q
6
and supplies three-phase alternating power to the motor
4
. The CPU
9
receives a current value signal from the current-detecting means
13
and the angular position signal from the angular position sensor
8
to provide feedback. The CPU
9
provides such a control that an appropriate driving current corresponding to both steering torque and vehicle speed is supplied to the motor
4
. The circuit of the switching means
14
controlled by the driver circuit
10
is closed normally. The circuit is opened when an abnormality takes place.
In this electric power steering apparatus, the torque sensor
2
, the vehicle speed sensor
3
, the motor
4
, and the battery
6
are installed inevitably in the automotive engine compartment. On the other hand, the control means
5
is normally mounted in the passenger compartment. Therefore, a long wiring harness is necessary to connect them. This wiring harness increases the weight and cost. In addition, the harness leads to power loss and gives rise to radiation noise due to current flowing through the pulse width-modulated motor
4
. This radiation noise may malfunction other control devices and induces noise in the automotive radio.
In view of these problems, techniques for eliminating a part of the wiring harness to thereby shorten it have been proposed. In particular, the control means
5
and the motor
4
are integrated as a subassembly, which in turn is mounted in the engine compartment. For example, in the technique disclosed in Japanese laid-open patent No. 30434/1997, a metal case is interposed between a DC brush motor and a speed-reduction mechanism for reducing the speed of rotation of the motor. A single metal substrate or board is disposed in this metal case. The control means
5
, the semiconductor switching elements Q
1
-Q
6
, and the brush holder of the motor are mounted on this board. This technique disclosed in this Japanese laid-open patent reduces radiation noise and takes account of heat dissipation. However, the motor itself is inevitably made bulky. Especially, it has an extremely large radial dimension. This deteriorates the ease with which the motor is mounted to the steering mechanism. During mounting operation, the spaces assigned to surrounding parts are inevitably sacrificed.
Furthermore, Japanese laid-open patent No. 192757/1996 discloses a technique having a metal case mounted between a brush DC motor and a speed-reduction mechanism that reduces the rotational speed of a motor. Semiconductor switching elements Q
1
-Q
6
and the brush holder of the motor are received in this case. A casing that houses a control means
5
is mounted radially adjacent to the metal case. In the same way as the previously mentioned known technique, large radial space is necessary. The motor is not easily mounted to the steering mechanism. In addition, the electric power steering apparatus itself is fabricated with decreased productivity.
Additionally, Japanese laid-open patent No. 117093/1997 discloses a technique for use with an electric power steering apparatus using a brush DC motor. This technique uses a substrate or board on which a control means
5
and switching elements Q
1
-Q
6
are mounted. This board is held to an external case on which the brush holder of an electric motor is mounted. The external case holding the board is located at a side of the motor within the housing of the motor. This technique permits simplification of wiring leads among components. Also, increase in size can be suppressed to some extent. However, the semiconductor switching elements are located beside the motor that produces heat. In addition, the switching elements are mounted within the housing of the motor. Therefore, the thermal reliability presents problems. Furthermore, the current-carrying capacity of the semiconductor switching elements is limited. Additionally, limitations are placed on the increase of the output of the motor. Also, the productivity poses problems.
SUMMARY OF THE INVENTION
The present invention has been made to solve the foregoing problems. It is an object of the invention to provide an electric power steering apparatus which reduces radiation noise due to PWM drive and permits higher output power, improvement of productivity, and weight reduction.
An electric power steering apparatus in accordance with the present invention comprises:
an electric motor having an output shaft;
a speed-reduction mechanism for reducing rotational speed of said output shaft of said motor;
a power circuit board having power circuit parts mounted thereon, said power circuit parts including semiconductor switching elements for supplying electric power to said electric motor;
a control circuit board having general circuit parts generating little heat, said general circuit parts including a microcomputer for controlling said semiconductor switching elements according to an output signal from a torque sensor for measuring steering torque
Fujimoto Tadayuki
Tominaga Tsutomu
Aguirrechea J.
Mitsubishi Denki & Kabushiki Kaisha
Ramirez Nestor
Sughrue & Mion, PLLC
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