This application claims priority to Japanese Patent Application No. P2000-275783 filed on Sep. 11, 2000 in Japan. The contents of the aforementioned application are hereby incorporated by reference.
The present invention relates to a brake fluid pressure retaining apparatus for continuously retaining brake fluid pressure within a wheel cylinder after releasing the brake pedal. The brake fluid pressure retaining apparatus is mounted on a vehicle, which is equipped with an automatic transmission and a driving force control unit for making a creep driving force smaller at a depression of the brake pedal than at a release of the brake pedal.
Various brake fluid pressure retaining apparatus are known. The brake fluid pressure retaining apparatus continuously retains brake fluid pressure within a wheel cylinder after releasing the brake pedal so that the braking force continuously acts on the vehicle. For example, the applicants disclose a brake fluid pressure retaining apparatus in Japanese Patent Application No. Hei-11-164621 (unpublished), in which brake fluid pressure passages are provided with a solenoid valve and a bypass passage bypassing the solenoid valve is provided with a restriction. The brake fluid pressure retaining apparatus gradually decreases and releases the retained brake fluid pressure through the restrictions when a certain time (about 1 second) passes after the driving force is changed into a greater condition. For this reason, the brake fluid pressure retaining apparatus ensures a starting operation of the vehicle on an up slope as well as a smooth starting operation of the vehicle on a down slope, at which a displacement force due to the vehicle""s own weight affects on, without a sudden feel of the driver.
However, there is a possibility that the vehicle undesirably moves or displaces backwards when starting from an up slope, because of the relations between the inclination of the slope, the retained brake fluid pressure and the like. If the driver releases the retained brake fluid pressure while the vehicle rolling down the slope, the backward displacement amount of the vehicle is increased and the driver experiences an awkward feel. Therefore, a smooth starting operation of the vehicle is required on an up slope. Meanwhile, a smooth start without sudden impacts is also required on a down slope.
Accordingly, the object of the present invention is to provide a brake fluid pressure retaining apparatus, which ensures a smooth starting operation of the vehicle on an up slope with decreased backward displacement amount of the vehicle and also ensures a smooth starting operation on a down slope.
The term xe2x80x9cbackward displacementxe2x80x9d is also referred to as a xe2x80x9cbackward movementxe2x80x9d throughout the specification and the claims.
According to a first aspect of the present invention, there is provided a brake fluid pressure retaining apparatus in combination with a vehicle equipped with a driving force control unit, the driving force control unit which transmits a driving force from a driving motor to driving wheels when a transmission is set in a driving range even after a depression of an accelerator pedal is released at a certain or lower vehicle speed, and which switches the magnitude of the driving force transmitted to the driving wheels between a greater condition and a smaller condition in accordance with a depression of a brake pedal, so that the driving force is made smaller at a depression of the brake pedal than at a release of the brake pedal, the brake fluid pressure retaining apparatus comprising:
a brake fluid pressure passage connecting a master cylinder and a wheel cylinder;
a solenoid valve provided in the brake fluid pressure passage and switchable between a communicating position, where the brake fluid pressure passage communicates, and a shut-off position, where the brake fluid pressure passage is shut off;
a bypass passage equipped with a restriction for communicating the master cylinder and the wheel cylinder respective of the shut-off position of the solenoid valve; and
a control unit for switching the solenoid valve between the communicating position and the shut-off position, wherein said control unit is constituted such that the solenoid valve is switched to the shut-off position for retaining brake fluid pressure within the wheel cylinder when the vehicle is stopped with the brake pedal depressed, and that the solenoid valve is switched to the communicating position for releasing the retained brake fluid pressure when a first setting time passes after the driving force is increased to the greater condition due to the release of the brake pedal,
and wherein said brake fluid pressure retaining apparatus further comprises a backward movement state determination device for determining a backward movement state of the vehicle, and wherein said control unit is constituted such that when the vehicle is determined as being in a backward movement state, said solenoid valve is switched to the communicating position after a second setting time passes, so as to retain brake fluid pressure for a period longer than the first setting time.
With such a constitution of the brake fluid pressure retaining apparatus, because brake fluid pressure is continuously retained within the wheel cylinder after releasing the brake pedal, the vehicle is prevented from undesirably rolling down a slope upon starting from the slope. The control means releases the retained brake fluid pressure when the setting time passes after the driving force is increased to the greater condition due to release of the brake pedal. When doing so, the brake fluid pressure is not retained constantly. The retained brake fluid pressure is gradually decreased by the flow rate control of the bypass passage equipped with a restriction. For this reason, the braking force is released gradually and slowly, which leads to a smooth starting operation of the vehicle on both up and down slopes without any sudden impacts.
Meanwhile, when the backward movement state determination device determines that the vehicle is undesirably rolling down a slope (backward movement detected) after the depression of the brake pedal is released and during the first setting time, the control means releases the retained brake fluid pressure when the second setting time passes, so as to retain the brake fluid pressure longer than the first setting time and continuously restrict the backward movement of the vehicle. The second setting time is for extending or delaying the first setting time. Because the second setting time is set only when the vehicle undesirably rolls down a slope, unnecessary brake dragging does not arise at a start of the vehicle unless the vehicle moves backwards.
Herein, the term xe2x80x9ca certain vehicle speedxe2x80x9d indicates that the vehicle is stopped (or the vehicle speed=0 km/h) or the vehicle runs at a vehicle speed just before stopping. Therefore, if the certain vehicle speed is set to be 0 km/h, the term xe2x80x9cat a certain or lower vehicle speedxe2x80x9d only indicates the vehicle speed of 0 km/h. Alternatively, as an example of the preferred embodiment to be described later, if the certain vehicle speed is set to be 5 km/h, the term xe2x80x9cat a certain or lower vehicle speedxe2x80x9d indicates a vehicle speed range from 0 km/h to 5 km/h.
Further, the term xe2x80x9csmaller conditionxe2x80x9d or xe2x80x9cmade smallerxe2x80x9d includes an instance when the driving force is decreased to zero (in the preferred embodiment to be described later, the instance when the engine is stopped). Therefore, the term xe2x80x9cthe driving force is increased to the greater conditionxe2x80x9d includes (1) an instance that the driving force arises from zero and thereafter increases (in the preferred embodiment, the instance that the driving force is increased from the engine stop condition to the strong creep condition) and (2) an instance that the existing driving force further increases (in the preferred embodiment, the instance that the driving force is increased from the weak creep condition to the strong creep condition).
As to the xe2x80x9cfirst setting timexe2x80x9d, a sufficient time (for example 1 second) is required for decreasing the brake fluid pressure to such an extent that the driver does not experience any sudden feels when the solenoid valve is switched to the communicating position and the retained brake fluid pressure is instantly released. However, other optional values may be set as the first setting time in accordance with the flow rate control of the restriction.
As mentioned above, the xe2x80x9csecond setting timexe2x80x9d is set for decreasing the backward movement of the vehicle. However, the xe2x80x9csecond setting timexe2x80x9d is also set for eliminating unnecessary brake dragging. As long as substantially extending the xe2x80x9cfirst setting timexe2x80x9d, the xe2x80x9csecond setting timexe2x80x9d may be set desirably with the use of any means or manners.
According to this brake fluid pressure retaining apparatus, if the vehicle does not undesirably move backwards, a smooth starting operation of the vehicle can be carried out without a sudden impact. This can be achieved by the braking force that is based on the brake fluid pressure gradually decreasing through the restriction during the first setting time.
Meanwhile, when the vehicle undesirably moves backwards, the brake fluid pressure is retained longer by the second setting time in comparison with the situation when the vehicle does not move backwards. Thereby, backward displacement amount of the vehicle becomes small. Further, because of the restriction D, the longer the brake fluid pressure retaining apparatus retains the brake fluid pressure, the smaller the retained brake fluid pressure becomes at the time of releasing the retained brake fluid pressure. Therefore, the release of the brake fluid pressure is carried out smoothly. As a result, even if the vehicle undesirably moves backwards, a smooth starting operation of the vehicle can be achieved with the backward displacement amount decreased.
Preferably, in the aforementioned brake fluid pressure retaining apparatus, the second setting time becomes shorter as the load of the driving motor increases.
With such a constitution of the brake fluid pressure retaining apparatus, as the load of the driving motor increases, the second setting time becomes shorter. In other words, when the load of the driving motor is greater, it is assumed that the backward movement restriction force derived from the driving force is also greater. Therefore, the backward movement of the vehicle can be prevented even if the second setting time becomes shorter. Further, shortening the second setting time prevents unnecessary brake dragging. Meanwhile, when the load of the driving motor is smaller, it is assumed that the backward movement restriction force derive from the driving force is smaller. Therefore, in order to decrease the backward displacement amount of the vehicle, the second setting time is not made shorter.
Herein, the term xe2x80x9cthe load of the driving motorxe2x80x9d includes those understood as a rotational speed (Ne) of the engine or the motor, a driving force transmission capacity (starting torque) of the starting clutch, a throttle angle (xcex8TH) and the like.
According to this brake fluid pressure retaining apparatus, it is possible to decrease unnecessary brake dragging because the second setting time becomes shorter in accordance with the increment of the load of the driving motor. Decreasing the retention time of the brake fluid pressure corresponds with the driver""s intention of quickly starting the vehicle with the increased load of the driving motor.
According to another aspect of the present invention, there is provided a brake fluid pressure retaining apparatus in combination with a vehicle equipped with a driving force control unit, the driving force control unit which transmits a driving force from a driving motor to driving wheels when a transmission is set in a driving range even after a depression of an accelerator pedal is released at a certain or lower vehicle speed, and which switches the magnitude of the driving force transmitted to the driving wheels between a greater condition and a smaller condition in accordance with a depression of a brake pedal, so that the driving force is made smaller at a depression of the brake pedal than at a release of the brake pedal, the brake fluid pressure retaining apparatus comprising:
a brake fluid pressure passage connecting a master cylinder and a wheel cylinder;
a solenoid valve provided in the brake fluid pressure passage and switchable between a communicating position, where the brake fluid pressure passage communicates, and a shut-off position, where the brake fluid pressure passage is shut off;
a bypass passage equipped with a restriction for communicating the master cylinder and the wheel cylinder irrespective of the shut-off position of the solenoid valve; and
a control unit for switching the solenoid valve between the communicating position and the shut-off position, wherein said control unit is constituted such that the solenoid valve is switched to the shut-off position for retaining brake fluid pressure within the wheel cylinder when the vehicle is stopped with the brake pedal depressed, and that the solenoid valve is switched to the communicating position for releasing the retained brake fluid pressure when a first setting time passes after the driving force is increased to the greater condition due to the release of the brake pedal,
and wherein said brake fluid pressure retaining apparatus further comprises a backward movement state determination device for determining a backward movement state of the vehicle, and wherein said control unit is constituted such that when the vehicle is determined as being in a backward movement state, said solenoid valve is switched to the communicating position after a second setting time, which decreasingly changes from a time longer than the first setting time in accordance with the increment of the load of the driving motor, passes instead of the first setting time, so as to release the retained brake fluid pressure.
In this brake fluid pressure retaining apparatus, the manner of preventing the backward movement of the vehicle is the same as that of the brake fluid pressure retaining apparatus according to the first aspect of the invention.
However, when the backward movement determination device determines that the vehicle is undesirably rolling down a slope (backward movement detected) after the depression of the brake pedal is released and during the first setting time, the brake fluid pressure retaining apparatus releases the retained brake fluid pressure when the second setting time instead of the first setting time passes. The second setting time decreasingly changes from a time longer than the first setting time in accordance with the increment of the load of the driving motor. In other words, when the load of the driving motor is small, it is assumed that the backward movement restriction force derived from the driving force is also small. Therefore, the second setting time is set to be longer than the first setting time. On the contrary, when the load of the driving motor is great, it is assumed that the backward movement restriction force derived from the driving force is great. Therefore, the second setting time may be set to be shorter. With such a constitution, it is possible to decrease the backward displacement amount of the vehicle and also to decrease the occurrence of brake dragging.
Herein, the terms xe2x80x9ca certain vehicle speedxe2x80x9d, xe2x80x9csmaller conditionxe2x80x9d, xe2x80x9cmade smallerxe2x80x9d, xe2x80x9ca first setting timexe2x80x9d, xe2x80x9cthe load of the driving motorxe2x80x9d are the same as those of the brake fluid pressure retaining apparatus according to the first aspect of the invention.
Likewise the brake fluid pressure retaining apparatus according to the first aspect of the invention, the xe2x80x9csecond setting timexe2x80x9d is set both for decreasing the backward displacement amount of the vehicle and for eliminating unnecessary brake dragging. In the preferred embodiment to be described later, the second setting time is set so as to be decreased from 2 seconds at the maximum to 0 second in accordance with the load of the driving motor. The xe2x80x9csecond setting timexe2x80x9d may be set desirably with the use of any means or manners.
According to this brake fluid pressure retaining apparatus, if the vehicle does not undesirably move backwards, a smooth starting operation of the vehicle can be carried out without a sudden impact. This can be achieved by the braking force that is based on the brake fluid pressure gradually decreasing through the restriction during the first setting time.
Meanwhile, if the vehicle undesirably moves backwards, the release of the brake fluid pressure is carried out when the second setting time passes. The second setting time decreasingly changes from a time longer than the first setting time in accordance with the increment of the load of the driving motor. For this reason, when the load of the driving motor is small (viz. when the driving force is small), the brake fluid pressure is retained longer than the first setting time. Backward displacement amount of the vehicle is therefore decreased. Further, because of the restriction D, the longer the brake fluid pressure retaining apparatus retains the brake fluid pressure, the smaller the retained brake fluid pressure becomes at the time of releasing the retained brake fluid pressure. Therefore, the release of the brake fluid pressure is carried out smoothly. As a result, even if the vehicle undesirably moves backwards, a smooth starting operation of the vehicle can be achieved with the backward displacement amount decreased. Moreover, when the load of the driving motor is great (viz. when the driving force is great), the brake fluid pressure is retained shorter than the first setting time. This can eliminate unnecessary brake dragging. Decreasing the retention time of the brake fluid pressure corresponds with the driver""s intention of quickly starting the vehicle with the increased load of the driving motor.