Fluid-pressure and analogous brake systems – With pump
Reexamination Certificate
1999-11-08
2001-11-13
Butler, Douglas C. (Department: 3613)
Fluid-pressure and analogous brake systems
With pump
C303S116100
Reexamination Certificate
active
06315369
ABSTRACT:
This application is based on Japanese Patent Application No. 10-343773 filed on Dec. 3, 1998, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydraulically operated braking system including a hydraulic pump operated by an electric motor.
2. Discussion of the Related Art
One example of a hydraulically operated braking system of the type indicated above is disclosed in Japanese Patent Application No. 10-243036 which was filed by the assignee of the present application and has not been laid open at the time the present invention was made. This braking system includes a wheel brake cylinder operated by a pressurized fluid to brake a vehicle wheel, a hydraulic pump for delivering the pressurized fluid to the wheel brake cylinder, an electric motor for operating the hydraulic pump, and a motor control device for controlling an electric current applied to the electric motor. In this braking system, the fluid pressure in the wheel brake cylinder is controlled by controlling the amount of the electric current applied to the electric motor. The amount of the electric current applied to the electric motor is increased to increase the output of the electric motor, when the required fluid pressure in the wheel brake cylinder or the required rate of increase of the fluid pressure is relatively high, for applying an abrupt brake to the vehicle wheel, for example.
A PWM (pulse width modulation) control is known as a common method of controlling the amount of the electric current to be applied to the electric motor. In the PWM control, a non-contact switching element included in the motor control device is alternately turned ON and OFF at a controlled duty ratio to control the amount of the electric current to be applied to the electric motor. The duty ratio is a ratio of the ON time during which the switching element is held ON, to the cycle time which is a sum of the ON and OFF times. The amount of the electric current applied to the electric motor is increased with an increase in the duty ratio. For minimizing the operating noise of the electric motor, it is generally known that the switching frequency of the switching element (namely, the PWM control frequency) is desirably higher than the upper limit of the audible range, that is, higher than the upper audibility limit. It is also known that the amount of heat generated by the switching operation of the switching element increases with an increase in the duty ratio and an increase in the switching frequency, and that the expected service life of the switching element is prolonged with a decrease in the amount of the generated heat.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a hydraulically operated braking system which includes a hydraulic pump operated by an electric motor and which has a reduced operating noise and a prolonged service life.
The above object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, so as to indicate various technical features and possible combination of those features within the principle of the present invention. However, it is to be understood that the present invention is not limited to those features and combinations.
(1) A hydraulically operated braking system including a hydraulic pump operated by an electric motor, characterized by comprising:
a motor control device which controls the electric motor and controls an electric current to be applied to the electric motor, by pulse width modulation, such that a PWM frequency at which the electric current is controlled by the pulse width modulation is set at a relatively high value higher than an upper audibility limit while the braking system is placed in a normal braking operation during which the generation of an operating noise of the braking system is not allowed, and at a relatively low value not higher than the upper audibility limit while the braking system is placed in a noise-tolerable braking operation during which the generation of the operating noise is allowed.
In the hydraulically operated braking system of the present invention constructed as described above, the PWM frequency at which the electric current to be applied to the electric motor for driving the hydraulic pump is made higher than the upper audibility limit (upper limit of the audible range) while the braking system is in the normal braking operation. Accordingly, the generation of the operating noise can be prevented during the normal braking operation. Usually, the operating noise generated by the electric motor is most uncomfortable to the vehicle operator and passengers, in the hydraulic braking system. In this sense, preventing the generation of the operating noise of the electric motor is valuable and significant. While the braking system is placed in the noise-tolerable braking operation, on the other hand, the PWM frequency is lowered below the upper audibility limit, the amount of heat generated by a switching operation to effect the pulse width modulation for controlling the electric current to the electric motor is restricted
(2) A hydraulically operated braking system according to the above mode (1), wherein the motor control device includes a switching element which is alternately turned on and off at a controlled switching frequency as the PWM frequency.
The expected service life of the switching element is relatively long when it has a relatively large capacity. However, the switching element having a large capacity is expensive and is not desirable in this respect. Further, the service life of the switching element usually affects that of the motor control device. In this sense, restricting the amount of heat generated by the switching element according to the present invention is effective to prolong the service life of the motor control device.
The present invention was made for the purpose of reducing the operating noise of the electric motor and the service life of the motor control device in the hydraulically operated braking system as disclosed in the Japanese Patent Application No. 10-243036 referred to above. As discussed below, a hydraulic pump is used for various purposes in a hydraulically operated braking system. Such a hydraulic pump is commonly driven by an electric motor. The principle of the present invention is applicable to such a hydraulically operated braking system equipped with a hydraulic pump and an electric motor for operating the hydraulic pump.
The electric motor is generally operated when the braking system is in a braking operation, which is either the normal braking operation described above, or the noise-tolerable braking operation also described above. Usually, whether the braking system is placed in the normal braking operation or the noise-tolerable braking operation is determined by the presence or absence of a command to initiate a braking operation, and by the desired degree or manner of braking. A braking operation is initiated in response to an operation of a brake operating member by the vehicle operator, or automatically irrespective of the operation of the brake operating member. In the former case, the desired degree or manner of braking is determined by the manner in which the brake operating member is operated by the vehicle operator. In the latter case, the braking operation is effected for effecting a traction control of a drive wheel of the vehicle, or for improving the running stability of the vehicle, for example. In this latter case, the desired degree or manner of braking is determined by the specific requirements for the traction control or vehicle stability control.
(3) A hydraulically operated braking system according to the above mode (1) or (2), wherein the motor control device includes an abrupt-braking-operation frequency lowering device which lowers the PWM frequency to the relatively low value when the braking system is
Butler Douglas C.
King Bradley
Oliff & Berridg,e PLC
Toyota Jidosha & Kabushiki Kaisha
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