Electricity: motive power systems – Motor-reversing – Armature or primary circuit control
Reexamination Certificate
2001-01-11
2003-04-15
Donels, Jeffrey (Department: 2837)
Electricity: motive power systems
Motor-reversing
Armature or primary circuit control
C361S688000
Reexamination Certificate
active
06548972
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control unit and a method of manufacturing the same for an electric power steering device which is more suitable to be mounted particularly on the relatively large vehicles, or to a control unit for a similar device.
2. Description of the Related Art
Generally, an electric power steering device has a torque sensor for sensing a steering torque produced on a steering shaft by the operation of a handle, and an assist motor (hereinafter simply referred to as a motor) attached to the steering shaft to produce a steering auxiliary torque bypassing an electric current from a battery of a vehicle to the assist motor in accordance with the sensed steering torque. This electric power steering device is difficult to produce a large steering auxiliary torque, in contrast to a hydraulic type, and conventionally was employed mainly in the light cars. However, this electric power steering device has some advantages that electronic control is easy, or the hydraulic pump or hydraulic pipe is unnecessary, and the structure is simple. In recent years, it is examined that the electric power steering device is applied to the small cars having a displacement level of 1800 cc, and will be possibly applied to the large cars in the future.
For the current control of the assist motor in this electric power steering device, a drive circuit having an H bridge circuit composed of normally four (or four pairs) FETs (Field Effect Transistors) is used to drive the assist motor in accordance with a PWM (Pulse Width Modulation) method under the control of a control circuit comprising a microcomputer. Also, the electric power steering device of this kind has a power back-up capacitor normally consisting of an electrolytic capacitor, which is connected to a high potential power line for supplying an electric power from the battery of the vehicle to the drive circuit. The power back-up capacitor resolves the problems with a ripple current or a temporary power voltage drop that may be caused by wiring resistance when a large current flows. Also, the drive circuit, a control circuit, and a large current circuit component for the power back-up capacitor are mounted on a circuit substrate within one unit, referred to as a control unit. This control unit (hereinafter simply referred to as a unit) was hitherto disposed in a space which was hidden from the driver within a compartment of the vehicle.
The prior art control unit for the electric power steering device had a structure of accommodating one sheet of circuit substrate within a metallic case, in which the circuit substrate was constructed as shown in
FIG. 6
, for example. That is, its basic structure is the same as that of a so-called printed board, in which the circuit conductors or passive elements are formed as a wiring pattern by the printed wiring technique on an insulating substrate made of resin. The IC chips or parts such as the FET or power back-up capacitor to constitute the control circuit are mounted thereon.
In this connection, a circuit substrate
1
as shown in
FIG. 6
comprises a FET chip
2
molded by resin, a power back-up capacitor
3
, and an IC chip of a microcomputer
4
constituting the control circuit. The circuit substrate
1
further comprises a bus bar
5
(conductor plate), a heat radiator
6
, and a relay
7
. The bus bar S constitutes a circuit conductor such as a conduction line of the assist motor through which a large current flows. The heat radiator
6
radiates the heat generated mainly in the FET
2
. The relay
7
opens or closes the conduction line of the assist motor under the control of the microcomputer
4
.
Since a current of about 25 A at maximum flows through the assist motor in the light cars, it was practically impossible, due to the reasons of the heat generation or the space on the substrate, to constitute the conduction line of the assist motor using a wiring pattern on the circuit substrate
1
that is limited in the thickness. Therefore, the conduction line was constituted by attaching the bus bar
5
, which was considerably larger in width and thickness than the wiring pattern on the circuit substrate
1
, on the circuit substrate
1
. The quantity of heat generated in the FET
2
is very large depending on the working conditions, and if left alone, the substrate rises in temperature to about several hundreds ° C. in short time. Hence, the radiator
6
was fabricated of aluminum having high heat conductivity, and placed in parallel with each FET
2
at the end of the circuit substrate
1
, while being bonded with a back face of the FET
2
.
By the way, the prior art control unit for the electric power steering device had the following problems with the applicability or on-board capability particularly for the relatively large vehicles.
(i) In prior art, the parts were arranged in plane on one sheet of circuit substrate, so that the size of the circuit substrate became large in a planar direction and the weight was heavy. In particular, in the case of the relatively large vehicles such as the small cars, the current of the assist motor reaches a maximum of 60 to 80 A, making the radiator or bus bar extremely large and heavy. This is directly reflected on the whole size or weight of the unit, resulting in extremely large and heavy unit as a whole. And in the case of the small cars, there is an unnegligible problem with the voltage drop which is caused by the wiring resistance between the unit and the motor owing to increased current of the assist motor. In the situation where the wiring can not be thickened owing to the costs, it is required in practice that the control unit is placed near the assist motor (i.e., within an engine room) However, it is significantly difficult to place it in a space within the engine room, if the unit is large, and further to apply the device itself to the small cars due to the high costs or the limited space.
(ii) To apply the device to the small cars, in the case where the unit is placed within the engine room owing to the necessity as described above, the unit must have a waterproof structure to withstand the submersion under water. However, in prior art, such water resistance was not considered at all, resulting in a problem that such waterproof structure must be constructed inexpensively. Since the water does not normally permeate into the car room, too high water resistance is not required if the unit is disposed within the car room. However, in the case where the unit is placed within the engine room (i.e., beneath the engine room in which the assist motor is placed), the water may possibly permeate into the engine room, whereby the unit must have a water resistance to withstand the submersion under water.
(iii) In prior art, the circuit components such as a power back-up capacitor or a relay through which a large current flows were mounted on one sheet of circuit substrate consisting of a normal printed board typically by soldering the terminals. Particularly in the case where the device is applied to the small cars to increase the current, the solder melts on the connecting portion of this terminal, resulting in a problem that the possibility of improper connection can not be ignored.
In Japanese Patent Unexamined Publication No. Hei. 6-270824 or Japanese Patent Unexamined Publication No. Hei. 8-11732, to solve the above-described problem (i) (larger size of the unit), an electric power steering circuit device (i.e., a control unit for the electric power steering device) was disclosed in which a drive circuit having a bridge circuit comprising FET is mounted on a metal substrate (or a radiating plate), and a control circuit containing a microcomputer is mounted on a separate insulating substrate (i.e., a normal printed board), the metal substrate and the insulating substrate being disposed one on another with a predetermined spacing. With this device, two substrates each having classified circuit components are laminated, giving rise to the effect of smaller size.
However, this devi
Foley & Lardner
Omron Corporation
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