Vehicle brake system

A vehicle brake system including: a brake operation member to be operated by a driver; a brake device configured to generate a braking force in accordance with an operation of the brake operation member; an operation amount sensor configured to detect an operation amount of the brake operation member; and a controller configured to control the braking force generated by the brake device, wherein the controller determines whether the brake operation member is in an operating state or in a non-operating state and controls, based on the determination, the braking force, and wherein the controller determines that the brake operation member is in the operating state when the operation amount exceeds an operating-state determining threshold and determines that the brake operation member is in the non-operating state when a time not less than a first set time elapses with the operation amount kept less than a non-operating-state determining threshold.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2016-180149, which was filed on Sep. 15, 2016, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND

Technical Field

The following disclosure relates to a vehicle brake system configured to control a braking force generated by a brake device.

Description of Related Art

A known vehicle brake system includes (A) a brake operation member to be operated by a driver, (B) a brake device configured to generate a braking force in accordance with an operation of the brake operation member, (C) an operation amount sensor for detecting an operation amount of the brake operation member, and (D) a controller configured to control the braking force generated by the brake device. In the known brake system, the controller has a state determining portion configured to determine whether the brake operation member is in an operating state (ON state) or in in a non-operating state (OFF state) based on the operation amount detected by the operation amount sensor. The controller controls the braking force based on the determination. In the thus constructed brake system, a threshold for determining the ON state and a threshold for determining the OFF state are preferably made considerably small for generating, without delay, the braking force having an appropriate magnitude in response to the brake operation. In this case, however, with a decrease in the thresholds, hunting tends to occur due to noise of the operation amount sensor, for instance. In a system disclosed in JP-A-2015-63151, hysteresis is provided between a braking force command value used when a stroke amount increases and a braking force command value used when the stroke amount decreases, for preventing hunting.

SUMMARY

In the above system in which hysteresis is provided as described above, the threshold for determining the ON state needs to be made larger than the threshold for determining the OFF state. This results in an increase in idle stroke when the braking operation is started, undesirably influencing an operation feeling in the brake operation as felt by a driver. Thus, the utility of the vehicle brake system is improved by solving the problem. An aspect of the disclosure relates to a vehicle brake system having a high degree of utility.

In one aspect of the disclosure, a vehicle brake system is configured such that (i) a brake operation member is determined to be in an operating state when an operation amount exceeds an operating-state determining threshold and (ii) the brake operation member is determined to be in a non-operating state when a first set time elapses with the operation amount kept less than a non-operating-state determining threshold.

The vehicle brake system constructed as described above effectively prevents or reduces an occurrence of hunting and enables an operating-state determining threshold to be made comparatively small for shortening idle stroke, thereby ensuring a good operation feeling.

FORMS OF THE INVENTION

There will be exemplified and explained various forms of an invention that is considered claimable. (The invention will be hereinafter referred to as “claimable invention” where appropriate). Each of the forms is numbered like the appended claims and depends from the other form or forms, where appropriate. This is for easier understanding of the claimable invention, and it is to be understood that combinations of constituent elements that constitute the invention are not limited to those described in the following forms. That is, it is to be understood that the claimable invention shall be construed in the light of the following description of various forms and embodiments. It is to be further understood that, as long as the claimable invention is construed in this way, any form in which one or more constituent elements is/are added to or deleted from any one of the following forms may be considered as one form of the claimable invention.

In the following forms, the form (1) corresponds to claim1. A form in which technical features of the form (2) are added to claim1corresponds to claim2. A form in which technical features of the form (3) are added to claim1or2corresponds to claim3. A form in which technical features of the form (5) are added to claim3corresponds to claim4. A form in which technical features of the form (6) are added to claim4corresponds to claim5. A form in which technical features of the form (7) are added to claim3corresponds to6.

(1) A vehicle brake system including: (A) a brake operation member to be operated by a driver; (B) a brake device configured to generate a braking force in accordance with an operation of the brake operation member; (C) an operation amount sensor configured to detect an operation amount of the brake operation member; and (D) a controller configured to control the braking force generated by the brake device,

wherein the controller determines whether the brake operation member is in an operating state or in a non-operating state and controls, based on the determination, the braking force generated by the brake device, and

wherein the controller determines that the brake operation member is in the operating state when the operation amount exceeds an operating-state determining threshold and determines that the brake operation member is in the non-operating state when a time which is not less than a first set time elapses with the operation amount kept less than a non-operating-state determining threshold.

The vehicle brake system according to this form may have any configuration as long as the braking force generated by the brake device is controllable. For instance, the vehicle brake system may be configured to control the braking force generated by the brake device without depending on an operation force applied to the brake operation member or may be configured to control the braking force generated by the brake device in dependence on both of the operation force applied to the brake operation member and a force generated by an actuator (assist force).

The brake system of this form is based on a configuration in which it is determined whether the brake operation member is in the operating state or in the non-operating state and the braking force is controlled based on the determination. Specifically, the brake system may be configured to control the braking force generated by the brake device or the assist force, during a time period in which the brake operation member is determined to be in the operating state, for instance. Further, the brake system may be configured to generate the braking force determined based on the operation amount, during a time period in which the brake operation member is determined to be in the operating state.

The operating state may be interpreted as a state in which the brake operation is being made by a driver while the non-operating state may be interpreted as a state in which the brake operation is not being made by the driver. Alternatively, the operating state may be interpreted as a state in which generation of the braking force or the assist force is allowed while the non-operating state may be interpreted as a state in which generation of the braking force or the assist force is prohibited. In the brake system constructed as described above, the determination as to whether the brake operation member is in the operating state or in the non-operating state can be appropriately made, so that the braking force that is actually generated can be controlled to have an appropriate magnitude for the operation amount.

The vehicle brake system of this form is characterized by the method for determining whether the brake operation member is in the operating state or in the non-operating state. In the vehicle brake system, even when the operation amount becomes less than the non-operating-state determining threshold, it is not determined that the brake operation member is in the non-operating state until a time which is not less than the first set time elapses, in order to prevent hunting. The first set time is desirably a comparatively long time, as compared with a time pitch of execution of a program, a frequency of noise of the operation amount sensor, and so on. Specifically, the first set time is about 1.0 second, for instance. There has been known a method in which a threshold for determining the operating state (operating-state determining threshold) is made larger than a threshold for determining the non-operating state (non-operating-state determining threshold) in order to prevent hunting, namely, there has been known a method in which hysteresis is provided as described above. In contrast, it is not necessary in the brake system of this form to limit a relationship in level between the operating-state determining threshold and the non-operating-state determining threshold. It is thus possible to make the operating-state determining threshold smaller than that in the system using the conventional method, resulting in shortened idle stroke in the brake operation. That is, the vehicle brake system of this form ensures improved operation feeling, as compared with the system using the conventional method.

As described above, the relationship in level between the operating-state determining threshold and the non-operating-state determining threshold is not limited. For example, a case is considered in which the operating-state determining threshold is smaller than the non-operating-state determining threshold. In this case, the operating state and the non-operating state are repeatedly switched in a situation in which the detected operation amount fluctuates between the operating-state determining threshold and the non-operating-state determining threshold. To prevent such repeated switching, the operating-state determining threshold is preferably not smaller than the non-operating-state determining threshold.

(2) The vehicle brake system according to the form (1), wherein the operating-state determining threshold and the non-operating-state determining threshold are the same value.

In this form, the relationship in level between the operating-state determining threshold and the non-operating-state determining threshold is limited, in other words, the two thresholds are the same value. As mentioned above, in the case Where the operating-state determining threshold is smaller than the non-operating-state determining threshold, the operating state and the non-operating state are repeatedly switched in a situation in which the detected operation amount fluctuates between the operating-state determining threshold and the non-operating-state determining threshold due to noise of the operation amount sensor, for instance. From the viewpoint of preventing such repeated switching, the operating-state determining threshold is preferably not smaller than the non-operating-state determining threshold. From the viewpoint of shortening idle stroke in the brake operation, the operating-state determining threshold is preferably made small. That is, this form is a desirable form that considers those viewpoints.

(3) The vehicle brake system according to the form (1) or (2),

wherein the controller shortens the first set time when the operation amount exceeds a threshold for shortening the first set time which is set to be larger than the operating-state determining threshold, and

wherein the controller returns the first set time to its original value when a time which is not less than a second set time elapses with the operation amount kept less than the non-operating-state determining threshold, the second set time being set to be longer than the shortened first set time.

A case is considered in which the first set time is not shortened. In this case, even if the brake operation made by the driver is already finished, the controller does not determine that the brake operation member is in the non-operating state until the time which is not less than the first set time elapses. In other words, even after the brake operation by the driver was actually finished, the control of the braking force is being executed such that the braking force becomes equal to 0 as a target. Thus, it takes a time before the braking force actually becomes equal to 0 due to the execution of the control. In an instance where it is confirmed that the brake operation was made positively or definitely by the driver and thereafter the brake pedal is returned so as to finish the brake operation, it is preferable to make determination of the non-operating state so as to decrease the braking force quickly to 0 by completing the control of the braking force. According to this form, when the operation amount becomes comparatively large and it is accordingly confirmed that the driver made the braking operation positively, the controller quickly makes determination of the non-operating state in response to returning of the brake operation member, so that the braking force can be quickly made equal to 0.

In the case in which the first set time is not shortened, if the driver again operates the brake operation member before the first set time elapses after the brake operation by the driver was finished, the operating state continues while not being determined to be the non-operating state at all. According to this form, even when the driver again operates the brake operation member after a short time interval, the determination as to whether the brake operation member is in the operating state or in the non-operating state can be appropriately made, so that the braking force is appropriately controlled.

In this form, when the time which is not less than the second set time elapses in a state in which the operation amount is kept less than the non-operating-state determining threshold, the shortened first set time is returned to its original value, and the brake system is returned to a state in which hunting is prevented. The second set time is set to be longer than the shortened first set time, so that the first set time is not returned to its original value before the time which is not less than the second set time elapses and it can be appropriately determined that the brake operation member is in the non-operating state when the shortened first set time elapses.

(4) The vehicle brake system according to the form (3), wherein the second set time is shorter than the first set time before shortened.

As described above, it is desirable that the brake system be quickly returned to the state in which hunting is prevented after the first set time has been shortened. It is thus desirable that the second set time be shorter than the first set time before shortened. In view of this, appropriate limitation as the second set time is added in this form.

(5) The vehicle brake system according to the form (3) or (4),

wherein, where the operating-state determining threshold is defined as a first operating-state determining threshold, a second operating-state determining threshold which is larger than the first operating-state determining threshold is set,

wherein, when the operation amount exceeds a prohibition threshold, the controller prohibits the determination of the operating state that is to be made on the condition that the operation amount exceeds the first operating-state determining threshold, the prohibition threshold being set to be larger than the first operating-state determining threshold, and

wherein, when the operation amount exceeds the second operating-state determining threshold, the controller determines that the brake operation member is in the operating state.

This form is based on the form in which the first set time is shortened when the operation amount exceeds the threshold for shortening the first set time. In such a form, in a situation in which the operation amount detected by the operation amount sensor fluctuates in the neighborhood of the first operating-state determining threshold, for instance, there may be caused a risk that the brake operation member is again determined to be in the operating state immediately after determination of the non-operating state has been made. According to this form, when the operation amount exceeds the prohibition threshold for prohibiting determination at the first operating-state determining threshold and it is confirmed that the brake operation was again made positively or definitely by the driver, for instance, determination of the operating state, which is to be made on the condition that the operation amount exceeds the first operating-state determining threshold, is prohibited. It is consequently possible to obviate the risk that the brake operation member is again determined to be in the operating state immediately after determination of the non-operating state has been made.

In this form, the brake operation member is determined to be in the operating state immediately when the operation amount exceeds the second operating-state determining threshold. That is, this form prevents an occurrence of hunting and, at the same time, enables quick determination of the operating state when it is confirmed that the brake operation was made again positively or definitely by the driver.

(6) The vehicle brake system according to the form (5), wherein the controller cancels the prohibition of the deter ruination of the operating state when a time which is not less than a set time elapses with the operation amount kept less than the non-operating-state determining threshold after the controller has prohibited the determination.

This form limits timing of cancellation of the prohibition of the determination of the operating state which is to be made on the condition that the operation amount exceeds the first operating-state determining threshold. In a state in which the determination is prohibited, idle stroke is inevitably long. It is thus desirable to cancel the prohibition at an early stage while preventing the brake operation member from being again determined to be in the operating state immediately after determination of the non-operating state has been made. Here, a case is considered in which the driver again makes the brake operation immediately after determination of the non-operating state has been made. In an instance where a speed of the brake operation made by the driver is high, the driver is less likely to feel that the braking force is insufficient in an idle stroke range of the brake operation. On the contrary, in an instance where the speed of the brake operation made by the driver is low, the driver is likely to feel that the braking force is insufficient in the idle stroke range of the brake operation. In view of this, the “set time” in this form may be set such that determination of the operating state is made when the operation amount exceeds the second operating-state determining threshold, only for the braking operation whose speed is comparatively high.

(7) The vehicle brake system according to the form (3) or (4),

wherein, where the operating-state determining threshold is defined as a first operating-state determining threshold, a second operating-state determining threshold which is larger than the first operating-state determining threshold is set, and

wherein, even when the operation amount exceeds the first operating-state determining threshold immediately after it has been determined that the brake operation member is in the non-operating state, the controller does not make the determination of the operating state until a time which is not less than a set time elapses, but determines that the brake operation member is in the operating state when the operation amount exceeds the second operating-state determining threshold.

Like the above-indicated form in which the determination of the operating state, which is to be made on the condition that the operation amount exceeds the first operating-state determining threshold, is prohibited, this form obviates the risk that the brake operation member is again determined to be in the operating state immediately after determination of the non-operating state has been made, in the situation in which the operation amount detected by the operation amount sensor fluctuates in the neighborhood of the first operating-state determining threshold. In this form, the brake operation member is determined to be in the operating state immediately when the operation amount exceeds the second operating-state determining threshold. In other words, this form prevents an occurrence of hunting and, at the same time, enables quick determination of the operating state when it is confirmed that the brake operation was made again positively or definitely by the driver.

As in the above-indicated form in which the prohibition of the determination of the operating state is cancelled, it is preferable to early return the brake system to the state in which determination of the operating state is made when the operation amount exceeds the first operating-state determining threshold while preventing determination of the operating state immediately after determination of the non-operating state has been made. As in the above-indicated form in which the prohibition of the determination of the operating state is cancelled, the “set time” in this form may be set such that determination of the operating state is made when the operation amount exceeds the second operating-state determining threshold, only for the braking operation whose speed is relatively high.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to the drawings, there will be explained below in detail embodiments of the claimable invention. It is to be understood that the claimable invention is not limited to the details of the following embodiments and the forms described in Forms of the Invention, but may be changed and modified based on the knowledge of those skilled in the art. It is to be further understood that modifications of the following embodiments can be provided utilizing technical features described in the Forms of the Invention.

[A] Structure of Vehicle Brake System

Structure of Hydraulic Brake System

A vehicle brake system according to one embodiment of the claimable invention is a hydraulic brake system and includes a brake circuit shown inFIG. 1. The vehicle brake system includes a brake pedal10as a brake operation member, a manual hydraulic-pressure source device12configured to pressurize a working fluid in dependence on a depression force (pedaling force) applied to the brake pedal10, a power hydraulic-pressure source device14configured to pressurize the working fluid in dependence on a force generated by a power source, four hydraulic brake devices18provided for respective four wheels16, and a brake actuator20disposed between the two hydraulic pressure source devices12,14and the brake devices18and configured to adjust a pressure of the working fluid generated by the power hydraulic-pressure source device14. Where it is necessary to distinguish the four wheels16in terms of “front”, “rear”, “right”, and “left”, the four wheels16are indicated as a front right wheel16FR, a front left wheel16FL, a rear right wheel16RR, and a rear left wheel16RL, respectively. Where it is necessary to similarly distinguish constituent elements, the same affixes “FL”, “FR”, “RL”, “RR” as used for the wheels16are used to indicate to which one of the four wheels16each of the constituent elements corresponds.

Each of the four brake devices18includes a brake cylinder22and is configured to operate by a fluid pressure in the brake cylinder22, so as to give a braking fierce to a corresponding one of the four wheels16. In the present embodiment, the brake device18is a disc brake in which brake pads, each as a friction member, held by a non-rotary body are pressed onto a disc rotor, as a brake rotary body, configured to rotate with the wheel16, by the fluid pressure in the brake cylinder22.

The manual hydraulic-pressure source device12includes a hydraulic booster30and a master cylinder32. The hydraulic booster30includes: a regulator (indicated as “reg” inFIG. 1)34configured to generate a fluid pressure higher than that corresponding to an operation force applied to the brake pedal10; a power piston36that operates in coordination with the brake pedal10; and a booster chamber38disposed rearward of the power piston36. While not illustrated, the regulator34includes a spool valve and a pressure regulating chamber. The regulator34is connected to the power hydraulic-pressure source device14and to a reservoir40. When the spool valves moves by an operation of the brake pedal10, the regulated-pressure chamber is selectively brought into communication with one of the power hydraulic-pressure source device14and the reservoir40, so that a pressure of the working fluid in the regulated-pressure chamber is regulated to a level in accordance with a magnitude of the operation force. The working fluid in the regulated-pressure chamber (i.e., the working fluid whose pressure has been regulated by the regulator34) is supplied to the booster chamber38, so that there is applied, to the power piston36, a force that moves the power piston36forward and the operation force is thus assisted.

The master cylinder32includes a pressurizing piston50that operates in coordination with the power piston36and a pressurizing chamber52disposed forward of the pressurizing piston50. The forward movement of the power piston36causes the pressurizing piston50to move forward, so that a fluid pressure is generated in the pressurizing chamber52.

When the brake pedal10is depressed, the power piston36is moved forward to cause the pressurizing piston50to move forward. The fluid pressure regulated by the regulator34in accordance with the operation force is supplied to the booster chamber38. The pressurizing piston50is moved forward by a force that is a sum of the operation force and the assist force (corresponding to the fluid pressure in the booster chamber38), so that there is generated, in the pressurizing chamber52, a fluid pressure whose level corresponds to the force. In the present embodiment, the fluid pressure in the regulator34and the fluid pressure in the pressurizing chamber52are made substantially equal to each other.

The power hydraulic-pressure source device14incudes: a pump60for pumping up the working fluid from the reservoir40; a pump motor62, as the power source, for driving the pump60; and an accumulator64that stores, in a pressurized state, the working fluid ejected from the pump60. The pump motor62is controlled such that a pressure of the working fluid stored in the accumulator64is held within a predetermined range. The power hydraulic-pressure source device14further includes a relief valve66that restricts an ejection pressure of the pump60to not higher than a set value, so as to prevent the ejection pressure of the pump60from becoming excessively high.

The power hydraulic-pressure source device14, the regulator34of the hydraulic booster30, and the pressurizing chamber52of the master cylinder32are connected to the brake actuator20. More specifically, the power hydraulic-pressure source device14is connected to a control pressure passage70, the regulator34of the hydraulic booster30is connected to a booster passage72, and the pressurizing chamber52of the master cylinder32is connected to a master passage74. The control pressure passage70, the booster passage72, and the master passage74are connected to a common passage76of the brake actuator20. The common passage76is connected to brake cylinders22FR,22FL,22RR,22RL respectively provided for the wheels16FR,16FL,16RR,16RL via respective individual passages78FR,78FL,78RR,78RL. The common passage76is connected also to the reservoir40via a low-pressure passage80.

The brake actuator20includes an output-pressure control valve device90for adjusting a pressure of the working fluid in the accumulator64, namely, a pressure of the working fluid output from the power hydraulic-pressure source device14. The output-pressure control valve device90includes: a pressure-increase linear valve92provided in the control pressure passage70connecting the power hydraulic-pressure source device14and the common passage76; and a pressure-decrease linear valve94provided in the low-pressure, passage80connecting the common passage76and the reservoir40. The pressure-increase linear valve92is configured to control an inflow of the working fluid from the power hydraulic-pressure source device14into the common passage76while the pressure-decrease linear valve94is configured to control an outflow of the working fluid from the common passage76into the reservoir40. For each of the pressure-increase linear valve92and the pressure-decrease linear valve94, a predetermined relationship is set between: a pressure difference between a high-pressure side and a low-pressure side of each valve; and a supply current to be supplied to each valve. A valve-opening pressure of each valve is changeable in accordance with a decrease or an increase of the supply current. Thus, each of the pressure-increase linear valve92and the pressure-decrease linear valve94can continuously change a supply pressure which is a pressure of the working fluid to be supplied to the common passage76, by controlling the supply current, whereby the supply pressure can be easily controlled to a desired level.

The brake actuator20includes an individual-pressure control valve device110configured to adjust a fluid pressure in each of the four brake cylinders22FR,22FL,22RR,22RL. The individual-pressure control valve device110includes: four pressure holding valves112FR,112FL,112RR,112RL provided in the respective individual passages78FR,78FL,78RR,78RL; and four pressure-decrease valves114FR,114FL,114RR,114RL each of which is provided between a corresponding one of the four pressure holding valves112and the reservoir40. Each pressure holding valve112is a normally-opened electromagnetic control valve which is placed in an open state when no electric current is supplied to its solenoid and which is configured to increase or hold the fluid pressure in the corresponding brake cylinder22. Each pressure-decrease valve114is a normally-closed electromagnetic control valve which is placed in a closed state when no electric current is supplied to its solenoid and which is configured to decrease the fluid pressure in the corresponding brake cylinder22.

A communication valve120is provided in the common passage76between a portion at which the common passage76is connected to the right and left front wheels16FR,16FL and a portion at which the common passage76is connected to the right and left rear wheels16RR,16RL. In other words, the communication valve120is configured to switch a state of communication of the brake cylinders22RR,22RL which respectively correspond to the right and left rear wheels16RR,16RL and the brake cylinders22FR,22FL which respectively correspond to the right and left front wheels16FR,16FL between a communicating state and a shut-off state. The communication valve120is a normally-closed electromagnetic control valve which is placed in a closed state when no electric current is supplied to its solenoid. In a normal condition, the communication valve120is placed in an open state so as to allow communication between the brake cylinders22RR,22RL which respectively correspond to the right and left rear wheels16RR,16RL and the brake cylinders22FR,22FL which respectively correspond to the left and right front wheels16FR,16FL, such that the working fluid from the pressure-increase linear valve92is supplied to not only the brake cylinders22RR,22RL but also the brake cylinders22FR,22FL.

The brake actuator20further includes a regulator cut valve130provided in the booster passage72and a master cut valve132provided in the master passage74. Each of the regulator cut valve130and the master cut valve132is a normally-opened electromagnetic control valve which is placed in an open state when no electric current is supplied to its solenoid. A stroke simulator134is connected to the master passage74via a simulator cut valve136which is a normally-closed electromagnetic control valve.

The present brake system includes a brake electronic control unit150as a controller. The brake electronic control unit150will be hereinafter referred to as “brake ECU150” or “ECU150” where appropriate. To the ECU150, the pump motor62of the power hydraulic-pressure source device14, and the electromagnetic control valves92,94,112,114,130,132,136of the brake actuator20are connected. The ECU150controls the pump motor62and the electromagnetic control valves, so as to control the fluid pressure in the brake cylinder22of each brake device18. The ECU150includes driver circuits for controlling operations of the pump motor62and the electromagnetic control valves.

The hydraulic brake system includes sensors each as a device for obtaining parameters for the control. The sensors are connected to the brake ECU150. Specifically, the sensors connected to the ECU150include a stroke sensor160as an operation amount sensor for detecting an operation amount of the brake pedal10, an accumulator pressure sensor162provided in the control pressure passage70for detecting the pressure of the working fluid in the accumulator64, a regulator pressure sensor164provided in the booster passage72for detecting the fluid pressure in the regulator34of the hydraulic booster30generated in accordance with the operation force applied to the brake pedal10by the driver, and a brake cylinder pressure sensor166provided in the common passage76for detecting the fluid pressure in each brake cylinder22by detecting the fluid pressure in the common passage76.

[B] Control in Brake System

In the present hydraulic brake system, at least one of the power hydraulic-pressure source device14, the hydraulic booster30of the manual hydraulic-pressure source device12, and the master cylinder32of the same12is selectively brought into communication with the common passage76by controlling the pressure-increase linear valve92and the pressure-decrease linear valve94of the output-pressure control valve device90, the regulator cut valve130, and the master cut valve132.

In the normal condition, the regulator cut valve130and are the master cut valve132are closed to shut off a supply of working fluid from the manual hydraulic-pressure source device12, and the working fluid is supplied from the power hydraulic-pressure source device14by controlling a supply current to the solenoid of each of the pressure-increase linear valve92and the pressure-decrease linear valve94. In this instance, the communication valve120is opened, and the simulator cut valve136is opened. By placing the four pressure holding valves112in the open state and placing the four pressure-decrease valves114in the closed state, the brake cylinders22operate by the working fluid supplied from the power hydraulic-pressure source device14. The control executed in the normal condition is well known in the art and explained here briefly. When the driver operates the brake pedal10, a target braking force is determined based on the detection results of the stroke sensor160and the regulator pressure sensor164, and the output-pressure control valve device90is controlled to attain the target braking force.

In the brake system, it is determined whether the brake pedal10is in an operating state (ON state) or in a non-operating state (OFF state) based on a stroke St of the brake pedal10detected by the stroke sensor160. When the brake pedal10is in the ON state, the control described above is executed. While not explained in detail, the target braking force when the brake pedal10is depressed is determined in accordance with an increase amount as measured from the operation amount at a time point when the brake pedal10is placed in the ON state, and the target braking force when the brake pedal10is returning is determined such that the target braking force is decreased according to the relationship between the operation amount and the target braking force when the brake pedal10is depressed.

[C] Determination of Operating State and Non-Operating State

There will be next explained in detail a method of determining whether the brake pedal10is in the ON state or in the OFF state. In a state in which the brake pedal10is not operated, it is determined that the brake pedal10is in the OFF state. As shown inFIG. 2A, when the brake pedal10is depressed and the stroke St detected by the stroke sensor160exceeds a first threshold St1(e.g., about 0.2 mm), it is determined that the brake pedal10is in the ON state. On the other hand, when the brake pedal10is returned, the stroke St accordingly becomes smaller than the first threshold St1, and a first set time T1(=T1L, e.g., 1.0 sec) elapses in this state, it is determined that the brake pedal10is in the OFF state. In the brake system of the present embodiment, the first threshold St1functions as an operating-state determining threshold (first operating-state determining threshold) and functions also as a non-operating-state determining threshold.

A case is considered in which the detection results of the stroke sensor160suffer from noise, as shown inFIG. 2B. Even if the stroke St becomes smaller than the first threshold St1, it is not determined that the brake pedal10is in the OFF state unless the first set time T1Lelapses with the stroke St kept less than the first threshold St1. It is thus possible, in the present brake system, to prevent the ON state and the OFF state from being alternately switched at short time intervals, namely, the so-called hunting is prevented.

Some of conventional brake systems are configured to prevent hunting by setting the operating-state determining threshold to be larger than the non-operating-state determining threshold. In such systems, however, idle stroke increases due to the operating-state determining threshold that is set to be larger, undesirably deteriorating an operation feeling as felt by the driver. In the present brake system, determination of the OFF state is made when the first set time T1Lelapses with the stroke St kept less than the first threshold St1, allowing the operating-state determining threshold to be freely set. In the present brake system, the operating-state determining threshold is made equal to the non-operating-state determining threshold and is made smaller than the operating-state determining threshold in the conventional systems.

In this respect, the operating-state determining threshold may be made smaller than the non-operating-state determining threshold. In this case, however, there may arise a risk that the OFF state and the ON state are alternately determined due to noise of the stroke sensor160in a situation in which the stroke fluctuates within a range smaller than the non-operating-state determining threshold. In the brake system, the operating-state determining threshold and the non-operating-state determining threshold are the same value, obviating such alternate determination of the OFF state and the ON state.

In an instance where it is confirmed that the brake operation was made positively or definitely by the driver and thereafter the brake pedal10is returned and the brake operation is accordingly finished, it is preferable to quickly complete the control of the braking force. In view of this, a process explained below is executed in the present brake system. Thus, determination of the OFF state is not actually made in the manner shown inFIG. 2Ain the present brake system. The process will be hereinafter explained in detail.

In the present brake system, when the brake pedal10is depressed and it is confirmed that the stroke St exceeds a second threshold St2(e.g., about 1.0 mm), the first set time T1is shortened from T1Lto T1S(e.g., 36 msec). The second threshold St2is larger than the first threshold St1and is a value by which it can be determined that the brake operation was made positively or definitely by the driver. Consequently, it is quickly determined that the brake pedal10is in the OFF state as shown inFIG. 3Awhen the brake operation by the driver is finished.

As indicated by the long dashed short dashed line inFIG. 3A, however, there is a risk that hunting occurs when the stroke St exceeds the first threshold St1immediately after the OFF state has been determined, in an instance where the driver rests his/her foot on the brake pedal10, noise is generated in the stroke sensor160, or the brake pedal10rebounds. In the present brake system, therefore, when the stroke St exceeds the second threshold St2, determination of the operating state that is to be made on the condition that the stroke St exceeds the first threshold St1is prohibited. According to this configuration, even when the stroke St exceeds the first threshold St1immediately after determination of the OFF state has been made, it is not determined that the brake pedal10is in the ON state, thus preventing an occurrence of hunting.

In the above configuration, when the driver again makes the brake operation immediately after the brake pedal10has been returned and the OFF state is accordingly determined as shown inFIG. 3B, the ON state is determined as follows. For example, as indicated by the long dashed short dashed line inFIG. 3B, in an instance where the driver again makes the brake operation and the stroke St exceeds the second threshold St2in a time period during which determination of the operating state at the first threshold St1is prohibited, it is determined that the brake pedal10is in the ON state.

The shortening of the first set time and the prohibition of determination of the operating state at the first threshold St1are cancelled when a second set time T2(e.g., 200 msec) elapses in a state in which the stroke St is kept less than the first threshold St1. That is, the first set time T1is returned to T1L, and determination of the operating state that is to be made on the condition that the stroke St exceeds the first threshold St1is allowed. In the brake system of the present embodiment, the shortened first set time is returned to its original value and the prohibition is cancelled under the same condition, namely, when the same length of set time elapses. The length of the set time for returning the shortened first set time and the length of the set time for cancelling the prohibition may be mutually different.

In an instance where the operation speed of the brake operation is comparatively low as indicated by the solid line inFIG. 3B, determination of the operating state at the first threshold St1is allowed before the stroke St reaches the second threshold St2, and the ON state is determined when the stroke St exceeds the first threshold St1.

With an increase in idle stroke upon depression of the brake pedal10, the driver is more likely to feel the existence of idle stroke. On the other hand, with an increase in the operation speed of the brake operation, the driver is less likely to feel the existence of idle stroke. In view of this, the second set time T2is set in consideration of the operation speed of the brake operation. More specifically, only for the brake operation whose operation speed is comparatively high, the ON state is determined because of an increase in the stroke St beyond the second threshold St2in a time period during which determination of the operating state at the first threshold St1is prohibited. As indicated by the lone dashed double-short dashed line inFIG. 3B, in an instance where the stroke St becomes less than the first threshold St1and thereafter becomes larger than the first threshold St1before the second set time T2elapses, the ON state is determined when the stroke St exceeds the second threshold St2.

As explained above, the second threshold St2functions as a second operating-state determining threshold, also as a threshold for shortening the first set time, and further as a prohibition threshold for prohibiting determination at the first operating-state determining threshold. These three thresholds may be made mutually different values.

[D] Control Program

The process for determining the ON state or the OFF state is executed such that the ECU150executes a program for state determining process indicated by a flowchart ofFIG. 4. This program is repeatedly executed at a short time pitch Δt (e.g. several milliseconds).

This program uses a determined-state flag FL0indicative of whether the brake pedal10is in the ON state or in the OFF state and an operation confirmation flag FL1indicative of whether the brake operation was made positively or definitely by the driver. The flag value of the determined-state flag FL0is 0 when it is determined that the brake pedal10is in the OFF state while the flag value thereof is 1 when it is determined that the brake pedal10is in the ON state. The flag value of the operation confirmation flag FL1is 0 when the stroke St is 0 or small while the flag value thereof is 1 when the stroke St becomes comparatively large and it is accordingly confirmed that the brake operation was made positively or definitely by the driver.

In the program for state determining process, step1(hereinafter “step” is abbreviated as “S”) is implemented to detect the stroke St by the stroke sensor160. At S2, the flag value of the determined-state flag FL0is confirmed. When the flag value is 0, S3is implemented to execute a determining process in the non-operating state for determining whether the current OFF state is to be continued or to be switched to the ON state. When the flag value is 1, S4is implemented to execute a determining process in the operating state for determining whether the current ON state is to be continued or to be switched to the OFF state.

The determining process in the non-operating state is executed by executing a subroutine for the determining process in the non-operating state indicated by a flowchart ofFIG. 5. In this subroutine, S11is initially implemented to confirm the flag value of the operation confirmation flag FL1. When the flag value is 0, it is determined at S12whether the stroke St is larger than the first threshold St1. When the stroke St is larger than the first threshold St1, it is determined at S13that the brake pedal10is in the ON state, and the flag value of the determined-state flag FL0is set to 1. When the stroke St is not larger than the first threshold St1, execution of the subroutine ends.

When it is determined at S11that the flag value of the operation confirmation flag FL1is 1, it is determined at S14whether the stroke St is larger than the second threshold St2. When the stroke St is larger than the second threshold St2, it is determined at S13that the brake pedal10is in the ON state, and the flag value of the determined-state flag FL0is set to 1.

When it is determined at S14that the stroke St is not larger than the second threshold St2, S15is implemented to determine whether the stroke St is smaller than the first threshold St1. When the stroke St is smaller than the first threshold St1, S16is implemented to obtain an elapsed time t after the stroke St has become smaller than the first threshold St1. At S17, it is determined whether the elapsed time t is not less than the second set time T2. When the elapsed time t is not less than the second set time T2, the flag value of the operation confirmation flag, is set to 0 at S18and the elapsed time t is reset to 0 at S19. When the elapsed time t is less than the second set time T2, S18and a subsequent step are skipped. When it is determined at S15that the stroke St is not smaller than the first threshold St1, the elapsed time t is reset to 0 at S19. Thus, execution of the subroutine ends.

The determining process in the operating state is executed by executing a subroutine for the determining process in the operating state indicated by a flowchart ofFIG. 6. In this subroutine, S21is initially implemented to determine whether the stroke St is larger than the second threshold St2. When the stroke St is larger than the second threshold St2, the flag value of the operation confirmation flag FL1is set to 1 at S22.

When it is determined at S21that the stroke St is not larger than the second threshold St2, S23is implemented to determine whether the stroke St is smaller than the first threshold St1. When the stroke St is smaller than the first threshold St1, S24is implemented to obtain an elapsed time t after the stroke St has become smaller than the first threshold St1. At S25, the flag value of the operation confirmation flag FL1is confirmed. When the flag value is 0, it is determined at S26whether the elapsed time t is not less than the first set time T1L, which is not shortened. When the elapsed time t is not less than the non-shortened first set time T1L, the elapsed time t is reset to 0 at S27. At S28, it is determined that the brake pedal10is in the OFF state, and the flag value of the determined-state flag FL0is set to 0. When it is determined at S26that the elapsed time t is less than the non-shortened first set time T1L, S27and subsequent steps are skipped.

On the other hand, when it is determined at S25that the flag value of the operation confirmation flag FL1is 1, it is determined at S29whether the elapsed time t is not less than the shortened first set time T1S. When the elapsed time t is not less than the shortened first set time T1S, it is determined at S28that the brake pedal10is in the OFF state, and the flag value of the determined-state flag FL0is set to 0. When it is determined at S29that the elapsed time t is less than the shortened first set time T1S, execution of the subroutine ends. When it is determined at S23that the stroke St is not smaller than the first threshold St1, the elapsed time t is reset to 0 at S30. Thus, execution of the subroutine ends.

When execution of the subroutine for the determining process in the non-operating state or execution of the subroutine for the determining process in the operating state ends, one execution of the program for state determining process ends.

[E] Features of Brake System

In the brake system according to the present embodiment, the ECU150as the controller includes a state determining portion200configured to determine whether the brake pedal10is in the operating state or in the non-operating state. In principle, the state determining portion200determines that the brake pedal10is in the operating state when the stroke St exceeds the operating-state determining threshold (the first threshold St1) and determines that the brake pedal10is in the non-operating state when a time which is not less than the first set time T1Lelapses with the stroke St kept less than the non-operating-state determining threshold (the first threshold St1). According to the configuration, the brake system of the present embodiment effectively prevents an occurrence of hunting and ensures a good operation feeling by making the operating-state determining threshold comparatively small so as to shorten idle stroke.

The brake system of the illustrated embodiment is configured such that, when the stroke St exceeds the second threshold St2by which it can be determined that the brake operation was made positively or definitely by the driver, the first set time T1is shortened, whereby it is quickly determined that the brake pedal10is in the OFF state when the brake operation by the driver is finished. To prevent an occurrence of hunting that may result from the configuration, the brake system of the illustrated embodiment is configured such that determination of the operating state at the first operating-state determining threshold (the first threshold St1) is prohibited and such that the prohibition of determination of the operating state at the first operating-state determining threshold is canceled when the time which is not less than the set time elapses with the stroke St kept less than the non-operating-state determining threshold (the first threshold St1).

A brake system according to a modified embodiment differs from the brake system of the illustrated embodiment in a specific method to be taken after the first set time T1has been shortened for preventing an occurrence of hunting. In the modified embodiment, it is determined that the brake pedal10is in the ON state when a time which is not less than a set time TONelapses in a state in which the stroke St exceeds the first operating-state determining threshold (the first threshold St1). As shown inFIG. 7A, however, the set time TONis a considerably short time, i.e., 6 msec. (The set time TONmay be 0 sec.) Thus, it may be considered that the set time TONis not substantially set. The set time TONis changed to TON′ (e.g., 100 msec) when the stroke St becomes less than the first threshold St1. The changed set time TON′ continues for a set time T3(e.g., 300 msec).

In an instance indicated by the long dashed double-short dashed line inFIG. 7B, it is determined that the brake pedal10is in the ON state when a time which is not less than the set time TON′ elapses with the stroke St exceeding the first operating-state determining threshold (the first threshold St1). In an instance indicated by the solid line inFIG. 7B, the set time for determining the ON state is changed from TON′ back to TONwhen the set time T3elapses after the stroke St has become less than the first threshold St1. Because the stroke St already exceeds the first threshold at this time, it is determined that the brake pedal10is in the ON state after the set time TONhas elapsed, namely, immediately when the set time T3has elapsed. An instance indicated by the long dashed short dashed line inFIG. 7Bis considered. In this instance, before the set time T3elapses after the stroke St has become less than the first threshold St1and before the time which is not less than the set time TON′ elapses with the stroke St exceeding the first threshold St1, the stroke St exceeds the second threshold St2. In this instance, it is determined that the brake pedal10is in the ON state immediately when the stroke St exceeds the second threshold St2.

Also in the modified embodiment, even when the driver again operates the brake pedal10after a short time interval, the determination as to whether the brake pedal10is in the operating state or in the non-operating state can be appropriately made, so that the braking force is appropriately controlled.