BRAKING FORCE CONTROL DEVICE

Abstract: A braking force control device for generating, when emergency braking is required, a braking force greater than normally required, and realizing a natural and smooth operation feeling, wherein whether or not an emergency braking operation has been performed based on both the master cylinder pressure P M/C and its changing rate Δ is judged, wherein a plurality of initiating conditions (100, 112, 118) are set on the assumption of a variety of situations, and wherein braking assisting control is initiated (114) when an initiating condition selected based on the moving condition of the vehicle is met.

Description: TECHNICAL FIELD The present invention relates to a brake force control apparatus and, more particularly, to a brake force control apparatus which generates, when an emergency braking is required, a brake force greater than that generated in an ordinary time. BACKGROUND ART Conventionally, for example, as disclosed in Japanese Laid-Open Patent Application 4-121260, a brake force control apparatus which generates, when an emergency braking is required, a brake force greater than that generated in a normal time is known. The above-mentioned conventional apparatus comprises a control circuit which generates a drive signal corresponding to an operational speed of a brake pedal and a fluid pressure generating mechanism which generates a brake fluid pressure corresponding to the drive signal generated by the control circuit. The control circuit determines that, when an operational speed of a brake pedal is less than a predetermined value, the brake pedal is normally operated. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure corresponding to a brake pressing force is generated. Hereinafter, this control is referred to as a normal control. Additionally, the control circuit determines that, when an operational force of the brake pedal exceeds a predetermined value, an emergency braking is required by the driver. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure is maximized. Hereinafter, this control is referred to as a brake assist control. Thus, according to the above-mentioned conventional apparatus, a brake force corresponding to a brake pressing force can be generated in a normal time, and a large brake force can be immediately generated in an emergency. In the above-mentioned conventional apparatus, a normal braking operation and an operation requiring an emergency braking are discriminated in accordance with an operational speed of the brake pedal. Generally, the operational speed of the brake pedal when an emergency braking is required is higher than that of the normal braking operation. Thus, according to the above-mentioned discriminating method, the operation requiring an emergency braking and the operation requiring a normal brake can be discriminated with high accuracy. However, for the purpose of obtaining a suitable deceleration, depending on travel circumstances, the brake pedal may be slightly pressed at a high speed without an intention to rapidly decelerate the vehicle. (Hereinafter, this operation is referred to as a small high-speed operation). In an apparatus in which the emergency braking and the normal brake are discriminated based on only an operational speed of the brake pedal such as in the above-mentioned apparatus, when the above-mentioned small high-speed operation is performed, it is possible that an erroneous determination is made that an emergency braking is required. Additionally, in the above-mentioned apparatus, when the brake pedal is pressed at an operational speed exceeding a predetermined value, the fluid pressure generating mechanism is switched from a state for realizing the normal control to a state for realizing the brake assist control. Such a switching operation requires a certain time delay. Accordingly, when a brake fluid pressure at a high-pressure level can be obtained by continuing the normal control when a driver is highly skilled, it is preferred that the switching to the brake assist control not be performed. However, in the above-mentioned conventional apparatus, when an operational speed of the brake pedal exceeds a predetermined speed, the switching to the brake assist control is always performed. In this regard, the above-mentioned conventional apparatus may give an unpleasant feel to the driver due to that control when the driver's skill level is high. Additionally, depending on travel circumstances of the vehicle, there may be a case in which a braking operation is started gently and, thereafter, the brake pedal is pressed at a high speed, due to an emergency braking being required. (Hereinafter, such an operation is referred to as a spurt operation.) When the above-mentioned spurt operation is performed, a brake fluid pressure has already been increased to a certain level at a stage in which the brake pedal is pressed at a high speed. Accordingly, the operational speed of the brake pedal in the spurt operation is not as high as the operational speed of the brake pedal in an ordinary emergency braking. However, in the above-mentioned conventional apparatus, it is always determined whether the braking operation by the driver is a normal braking operation or an operation requiring an emergency braking based on the determination as to whether or not the operational speed of the brake pedal exceeds the constant threshold value. Accordingly, the above-mentioned conventional apparatus has a characteristic in which the switching from the normal control to the brake assist control tends not to be performed when the brake pedal is subjected to the spurt operation. As mentioned above, the above-mentioned conventional apparatus may generated a difference between a driver's intention and contents of the control to be performed, since the switching between the normal control and the brake assist control is performed based on the determination as to whether or not the operational speed of the brake pedal exceeds the constant threshold value. JP-A-07-76267 discloses a brake force control apparatus in accordance with the features of the preamble of claim 1. DISCLOSURE OF INVENTION The present invention is invented in view of the above-mentioned point, and it is an object of the present invention to provided a brake force control apparatus which generates an appropriate brake force conforming to the driver's intention without an incongruous feel in practice under a condition in which the normal brake is required and a condition in which an emergency braking is required. This object is solved by the subject matter of claim 1. Preferred embodiments are the subject matters of the subclaims. In the present invention, the start time of the brake assist control is determined based on the operational speed and the amount of operation of the brake pedal. When the driver requires an emergency braking, the brake pedal is operated at a high speed with a large travel. Accordingly, by rendering both the operational speed and the amount of operation of the brake pedal as parameters, the driver's intention can be detected with good accuracy. Thus, according to the brake force control apparatus of the present invention, the brake assist control can be appropriately performed when the driver is actually requiring an emergency braking. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a system structure diagram of a brake force control apparatus according to an embodiment of the present invention; Fig. 2 is an illustration for showing a change in a brake pressing force achieved under various circumstances; Fig. 3 is an illustration for showing a start condition used for determining whether brake assist control is started in the brake force control apparatus shown in Fig. 1; Fig. 4 is a flowchart of an example of a control routine performed in the brake force control apparatus shown in Fig. 1; Fig. 5 is a flowchart of an example of another control routine performed in the brake force control apparatus shown in Fig. 1; Fig. 6, is a system structure diagram of a brake force control apparatus according to a second embodiment of the present invention; Fig. 7 is an illustration for showing a vacuum booster used in the brake force control apparatus shown in Fig. 6 and a peripheral structure thereof; and Fig. 8 is a system structure of a brake force control apparatus according to a third embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION Fig. 1 is a system structure diagram of a brake force control apparatus according to an embodiment of the present invention. The brake force control apparatus shown in Fig. 1 is controlled by an electronic control unit 10 (hereinafter, referred to as ECU 10). The brake force control apparatus comprises a pump 12. The pump 12 has a motor 14 as a power source thereof. An inlet port 12a of the pump 12a communicates with a reservoir tank 16. An accumulator 20 communicates with an outlet port 12b of the pump via a check valve 18. The pump 12 delivers brake fluid in the reservoir tank 16 from the outlet port 12b so that a predetermined pressure is always accumulated in the accumulator 20. The accumulator 20 communicates with a high-pressure port 24a of a regulator 24 via a high-pressure passage 22, and communicates with a regulator switching solenoid 26 (hereinafter, referred to as STR 26). The regulator 24 has a low-pressure port 24b and a control fluid pressure port 24c. The low-pressure port 24b communicates with the reservoir tank 16 via a low-pressure passage 28. The control fluid pressure port 24c communicates with the STR 26 via a control fluid pressure passage 29. The STR 26 is a two-position solenoid valve which selectively set one of the control fluid pressure passage 29 and the high-pressure passage 22 in a conductive state, and sets the control fluid pressure passage 29 in a conductive state and sets the high-pressure passage 22 in a closed state in a normal state. A brake pedal 30 is connected to the regulator 24, and a master cylinder is mounted to the regulator 24. The regulator 24 has a fluid pressure chamber therein. The fluid pressure chamber always communicates with the control fluid pressure port 24c, and selectively communicates with the high-pressure port 24a or the low-pressure port 24b in accordance with an operational state of the brake pedal 30. The regulator 24 is configured so that a pressure inside the fluid pressure chamber is adjusted to a fluid pressure corresponding to a brake pressing force F P exerted on the brake pedal 30. Accordingly, the fluid pressure corresponding to the brake pressing force F P always appears at the control fluid pressure port 24c of the regulator 24. Hereinafter, this fluid pressure is referred to as a regulator pressure P RE . The brake pressing force F P exerted on the brake pedal 30 is mechanically transmitted to the master cylinder 32 via the regulator 24. Additionally, a force corresponding to the fluid pressure inside the fluid pressure chamber of the regulator 24, that is, a force corresponding to the regulator pressure P RE , is transmitted to the master cylinder 32. The master cylinder 32 is provided with a first fluid pressure chamber 32a and a second fluid pressure chamber 32b therein. A master cylinder pressure P M/C corresponding to a resultant force of the brake pressing force F P and a brake assist force F A is generated in the first fluid pressure chamber 32a and the second fluid pressure chamber 32b. Both the master cylinder pressure P M/C generated in the first fluid pressure chamber 32a and the master cylinder pressure P M/C generated in the second fluid pressure chamber 32b are supplied to a proportioning valve 34 (hereinafter, referred to as P valve 34). The P valve 34 communicates with a first fluid pressure passage 36 and a second fluid pressure passage 38. The P valve 34 supplies the master cylinder pressure P M/C to the first fluid pressure passage 36 and the second fluid pressure passage 38 without change in a range where the master cylinder pressure P M/C is less than a predetermined value. Additionally, the P valve 34 supplies the master cylinder pressure P M/C to the first fluid pressure passage 36 without change and supplies a fluid pressure obtained by decreasing the master cylinder pressure P M/C by a predetermined ratio to the second fluid pressure passage 38 in a range where the master cylinder pressure P M/C is less than a predetermined value. A hydraulic pressure sensor 40, which outputs an electric signal corresponding to the master cylinder pressure P M/C , is provided between the second fluid pressure chamber 32b of the master cylinder 32 and the P valve 34. An output signal of the hydraulic pressure sensor 40 is supplied to the ECU 10. The ECU 10 detects the master cylinder pressure P M/C generated in the master cylinder 32 based on the output signal of the hydraulic pressure sensor 40. The above-mentioned STR 26 communicates with a third fluid pressure passage 42. The third fluid pressure passage 42 communicates with one of the control fluid pressure passage 29 and the high-pressure passage 22 in accordance with a state of the STR 26. In the present embodiment, wheel cylinders 44FL and 44FR provided to left and right front wheels FL and FR are provided with a brake fluid pressure from the first fluid pressure passage 36 communicating with the P valve 34 or the third fluid pressure passage 42 communicating with the STR 26. Additionally, wheel cylinders 44RL and 44RR provided to left and right rear wheels RL and RR are provided with a brake fluid pressure from the second fluid pressure passage 38 communicating with the P valve 34 or the third fluid pressure passage 42 communicating with the STR 26. The first fluid pressure passage 36 communicates with a first assist solenoid valve 46 (hereinafter referred to as SA -1 46) and a second assist solenoid valve 48 (hereinafter, referred to as SA -2 ). On the other hand, the third fluid pressure passage 42 communicates with a right front holding solenoid valve 50 (hereinafter, referred to as SFRH 50), a left front holding solenoid valve 52 (hereinafter, referred to as SFLH 52) and a third assist solenoid valve 54 (hereinafter, referred to as SA -3 54). The SFRH 50 is a two-position solenoid valve which maintains an open state in a normal state. The SFRH 50 communicates with the SA -1 46 and a right front wheel pressure decreasing solenoid valve 58 (hereinafter, referred to as SFRR 58) via a pressure adjusting fluid pressure passage 56. A check valve 60 permitting a fluid flow only in a direction from the pressure adjusting fluid pressure passage 56 to the third fluid pressure passage 42 is provided, in parallel, between the third fluid pressure passage 42 and the pressure adjusting fluid pressure passage 56. The SA -1 46 is a two-position solenoid valve which selectively renders one of the first fluid pressure passage 36 and the pressure adjusting fluid pressure passage 56 to be communicated with the wheel cylinder 44FR, and renders the first fluid pressure passage 36 and the wheel cylinder 44FR to be in a communicating state in a normal state (OFF state). On the other hand, the SFRR 58 is a two-position solenoid valve which renders the pressure adjusting fluid pressure passage 56 and the reservoir tank 16 to be in a connected state or a disconnected state. The SFRR 58 renders the pressure adjusting fluid pressure passage 56 and the reservoir tank 16 to be in a disconnected state in a normal state (OFF state). The SFLH 52 is a two-position solenoid valve which maintains an open state in a normal state. The SFLH 52 communicates with the SA -2 48 and a left front wheel pressure decreasing solenoid valve 64 (hereinafter, referred to as SFLR 64) via a pressure adjusting fluid pressure passage 62. A check valve 66 permitting a fluid flow only in a direction from the pressure adjusting fluid pressure passage 62 to the third fluid pressure passage 42 is provided, in parallel, between the third fluid pressure passage 42 and the pressure adjusting fluid pressure passage 62. The SA -2 48 is a two-position solenoid valve which selectively renders one of the first fluid pressure passage 36 and the pressure adjusting fluid pressure passage 62 to be communicated with the wheel cylinder 44FL, and renders the first fluid pressure passage 36 and the wheel cylinder 44FL to be in a communicating state in a normal state (OFF state). On the other hand, the SFLR 64 is a two-position solenoid valve which renders the pressure adjusting fluid pressure passage 62 and the reservoir tank 16 to be in a connected state or a disconnected state. The SFLR 64 renders the pressure adjusting fluid pressure passage 62 and the reservoir tank 16 to be in a disconnected state from each other in a normal state (OFF state). The second fluid pressure passage 38 communicates with the above-mentioned SA -3 54. The downstream side of the SA -3 54 communicates with a right rear wheel holding solenoid valve 68 (hereinafter, referred to as SRRH 68) provided in correspondence with a wheel cylinder 44RR of the right rear wheel RR and a left rear wheel holding solenoid valve 70 (hereinafter, referred to as SRLR 70) provided in correspondence with a wheel cylinder 44RL of the left rear wheel RL. The SA -3 54 is a two-position solenoid valve which selectively selectively renders one of the second fluid pressure passage 38 and the third fluid pressure passage 42 to be communicated with the SRRH 68 and the SRLR 70, and renders the second fluid pressure passage 38, the SRRH 68 and the SRLR 70 in a communicating state in a normal state (OFF state). The downstream side of the SRRH 68 communicates with the wheel cylinder 44RR and a right rear wheel pressure decreasing solenoid valve 74 (hereinafter, referred to as SRRR 74) via a pressure adjusting fluid pressure passage 72. The SRRR 74 is a two-position solenoid valve which renders the pressure adjusting fluid pressure passage 72 and the reservoir tank 16 in a communicating state or a disconnected state, and renders the pressure adjusting fluid pressure passage 72 and the reservoir tank 16 in the disconnected state in a normal state (OFF state). Additionally, a check valve 76 permitting a fluid flow only in a direction from the pressure adjusting fluid pressure passage 72 to the SA -3 54 is provided, in parallel, between the SA -3 54 and the pressure adjusting fluid pressure passage 72. Similarly, the downstream side of the SRLH 70 communicates with the wheel cylinder 44RL and a left rear wheel pressure decreasing solenoid valve 80 (hereinafter, referred to as SRLR 80) via a pressure adjusting fluid pressure passage 78. The SRLR 80 is a two-position solenoid valve which renders the pressure adjusting fluid pressure passage 78 and the reservoir tank 16 in a communicating state or a disconnected state, and renders the pressure adjusting fluid pressure passage 78 and the reservoir tank 16 in the disconnected state in a normal state (OFF state). Additionally, a check valve 82 permitting a fluid flow only in a direction from the pressure adjusting fluid pressure passage 78 to the SA -3 54 is provided, in parallel, between the SA -3 54 and the pressure adjusting fluid pressure passage 78. In the system according to the present embodiment, a brake switch 84 is provided near the brake pedal 30. The brake switch 84 is a switch that generates an ON output when the brake pedal 30 is pressed. The output signal of the brake switch 84 is supplied to the ECU 10. The ECU 10 determines whether or not a braking operation is performed by the driver based on the output signal of the brake switch 84. Additionally, in the system according to the present embodiment, wheel speed sensors 86FL, 86FR, 86RL and 86RR (hereinafter, these are referred to as 86** as a whole) are provided near the left and right front wheels FL and FR and the left and right rear wheels RL and RR, each of the sensors generating a pulse signal when the respective wheel rotates a predetermined angle. The output signals of the wheel speed sensors 86** are supplied to the ECU 10. The ECU 10 detects a wheel speed of each of the wheels FL, FR, RL and RR based on the output signals of the wheel speed sensors 86**. The ECU 10 supplies, if necessary, drive signals to the above-mentioned STR 26, SA -1 46, SA -2 48, SA -3 54, SFRH 50, SFLH 52, SFRR 58, SFLR 64, SRRH 68, SRLH 70, SRRR 74 and SRLR 80 based on the output signal of the brake switch 84. A description will now be given of an operation of the brake force control apparatus according to the present embodiment. The brake force control apparatus according to the present embodiment performs the normal control for generating a brake force corresponding to the brake pressing force F P exerted on the brake pedal 30 when the vehicle is in a stable state. The normal control can be achieved, as shown in Fig. 1, by turning off all of the STR 26, SA -1 46, SA -2 48, SA -3 54, SFRH 50, SFLH 52, SFRR 58, SFLR 64, SRRH 68 SRLH 70, SRRR 74 and SRLR 80 based on the output signal of the brake switch 84. That is, in the state shown in Fig. 1, the wheel cylinders 44FR and 44FL communicate with the first fluid pressure passage 36, and the wheel cylinders 44RR and 44RL communicate with the second fluid pressure passage 38. In this case, the brake fluid flows between the master cylinder 32 and the wheel cylinders 44FR, 44FL, 44RL and 44RR (hereinafter, these may be referred to as 44** as a whole), and a brake force corresponding to the brake pressing force F P is generated in each of the wheels FL, FR, RL and RR. In the present embodiment, when a possibility for shifting to a locked state is detected in one of the wheels, it is determined that a condition for performing an antilock brake control (hereinafter, referred to as ABS control) is established. The ECU 10 calculates wheel speeds V WFL , V WFR , V WRL and V WRR (hereinafter, these are referred to as V W ** as a whole) of the wheels based on output signals of the wheel speed sensors 86**, and calculates an assumed value V SO (hereinafter, referred to as an assumed vehicle speed V SO ) of a speed of the vehicle according to a publicly known method. Then, when the vehicle is in a braking state, a slip rate S of each wheel is calculated according to the following equation so as to determine that the wheel may shift to a locked state when the slip rate S exceeds a predetermined value. S = (V SO - V W **)·100/V SO When the condition for performing the ABS control is established, the ECU 10 outputs the drive signals to the SA -1 46, SA -2 48 and SA -3 54. As a result, when the SA -1 46 is turned on, the wheel cylinder 44FR is disconnected from the first fluid pressure passage 36 and connected to the pressure adjusting fluid pressure passage 56. Additionally, when the SA -2 48 is turned on, the wheel cylinder 44FL is disconnected from the first fluid pressure passage 36 and connected to the pressure adjusting fluid pressure passage 62. Further, when the SA -3 54 is turned on, the upstream side of the SRRH 68 and the SRLH 70 is disconnected from the second fluid pressure passage 38 and connected to the third fluid pressure passage 42. In this case, all wheel cylinders 44** communicate with respective holding solenoid valves SFRH 50, SFLH 52, SRRH 68 and SRLH 70 (hereinafter, these are referred to as holding solenoid S**H) and respective pressure decreasing solenoid valves SFRR 58, SFLR 64, SRRR 74 and SRLR 80 (hereinafter, these are referred to as pressure decreasing solenoid S**R), and a regulator pressure PRE is introduced to the upstream side of each of the holding solenoids S**H via the third fluid pressure passage 42 and the STR 26. In the above-mentioned condition, a wheel cylinder pressure P W/C of the respective wheel cylinders 44** is increased with the regulator pressure P RE as an upper limit by the holding solenoids S**H being in an open state and the pressure decreasing solenoids S**R being in a closed state. Hereinafter, this state is referred to as a pressure increasing mode 1 ○. Additionally, the wheel cylinder pressure P W/C of the respective wheel cylinders 44** is maintained without being increased or decreased by the holding solenoids S**H being in a closed state and the pressure decreasing solenoids S**R being in the closed state. Hereinafter, this state is referred to as a holding mode 2 ○. Further, the wheel cylinder pressure P W/C of the respective wheel cylinders 44** is decreased by the holding solenoids S**H being in the closed state and the pressure decreasing solenoids S**R being in the open state. Hereinafter, this state is referred to as a pressure decreasing mode 3 ○. The ECU 10 achieves, if necessary, the above-mentioned pressure increasing mode 1 ○, holding mode 2 ○ and pressure decreasing mode 3 ○ so that a slip rate S of each wheel during a braking time becomes an appropriate value, that is, so that each wheel does not shift to the locked state. When a depression of the brake pedal 30 is released by the driver during execution of the ABS control, the wheel cylinder pressure P W/C must be immediately decreased. In the system according to the present embodiment, the check valves 60, 66, 76 and 82 are provided in hydraulic pressure paths corresponding to each of the wheel cylinders 44**, each of the check valves 60, 66, 76 and 82 permitting a fluid flow only in the directions from the wheel cylinders 44** to the third fluid pressure passage 42. Thus, according to the system of the present embodiment, the wheel cylinder pressures P W/C of all of the wheel cylinders 44** can be immediately decreased after the depression of the brake pedal 30 is released. In the system according to the present embodiment, when the ABS control is performed, the wheel cylinder pressure P W/C is increased by the brake fluid being supplied from the regulator 24 to the wheel cylinders 44**, that is, by the brake fluid being supplied from the pump 12 to the wheel cylinders 44**, and is decreased by the brake fluid in the wheel cylinders 44** flowing to the reservoir tank 16. When the increase in the wheel cylinder pressure P W/C is performed by using the master cylinder 32 as a fluid pressure source and if the pressure increasing mode and the pressure decreasing mode are repeatedly performed, the brake fluid in the master cylinder 32 gradually decreases and a so-called bottoming of the master cylinder may occur. On the other hand, if the pump 12 is used as a fluid pressure source so as to increase the wheel cylinder pressure P W/C , as in the system according to the present embodiment, such a bottoming can be prevented. Thus, in the system according to the present embodiment, a stable operational state can be maintained if the ABS control is continued for a long time. In the system according to the present embodiment, the ABS control is started when a possibility for shifting to the locked state is detected in one of the wheels. Accordingly, in order to start the ABS control, as a precondition, a braking operation having a level at which a large slip rate S is generated in one of the wheels must be performed. Fig. 2 shows changes in the brake pressing force F P applied to the brake pedal 30 with respect to time under various conditions. Curves indicated by 1 ○ and 2 ○ in Fig. 2 represent changes in the pressing force F P when an emergency braking is performed by a highly skilled driver (hereinafter, referred to as a high-grade driver) and an unskilled driver or a driver lacking (hereinafter, referred to as a beginner-grade driver), respectively. The emergency braking operation is an operation performed when is it desired to rapidly decelerate a vehicle. Accordingly, the brake pressing force associated with the emergency braking operation is preferably a force sufficiently large as the ABS control is performed. As shown by the curve 1 ○, when the driver of the vehicle is a high-grade driver, the brake pressing force F P is immediately and rapidly increased in response to establishment of a condition in which an emergency braking is required, and a large brake pressing force F P can be maintained for a long time. If such a brake pressing force F P is exerted on the brake pedal 30, a sufficiently high brake fluid pressure can be provided from the master cylinder 32 to each of the wheel cylinders 44** so as to start the ABS control. However, as shown by the curve 2 ○ when the driver of the vehicle is a beginner-grade driver, the brake pressing force F P may not be increased to a sufficiently high value in response to the condition in which an emergency braking is required. If the brake pressing force F P exerted on the brake pedal 30 is not sufficiently increased as shown by the curve 2 ○ after an emergency braking is required, the wheel cylinder pressure P W/C in each of the wheels 44** is not sufficiently increased, which results in a possibility that the ABS control is not started. As mentioned above, when the driver of the vehicle is a beginner-grade driver, the braking ability of the vehicle may not be sufficiently performed even when an emergency braking operation is performed despite that the vehicle has a good braking ability. Accordingly, the system according to the present embodiment is provided with a brake assist function for sufficiently increasing the wheel cylinder pressure P W/C even if the brake pressing force F P is not sufficiently increased when the brake pedal is operated with an intention to perform an emergency braking. Hereinafter, a control performed by the ECU 10 to achieve such a function is referred to as a brake assist control. In the system according to the present embodiment, when performing the brake assist control, an accurate determination must be made as to whether, when the brake pedal 30 is operated, the operation is intended to perform an emergency braking operation or to perform a regular braking operation. Curves indicated by shown 3 ○ and 4 ○ in Fig. 2 show changes in the brake pressing force F P when the driver operates the brake pedal with an intention to perform a normal braking operation under various conditions. As shown by the curves 1 ○ to 4 ○, a change in the brake pressing force F P associated with the normal braking operation is gentle as compared to a change in the brake pressing force F P associated with an emergency braking operation. Additionally, a convergent value of the brake pressing force F P associated with the normal braking operation is not so large as a convergent value of the brake pressing force F P associated with an emergency braking operation. Giving attention to those differences, when the brake pressing force F P is increased to a sufficiently large value at a rate of change exceeding a predetermined value after a braking operation is started, that is, when the brake pedal 30 is operated so that the brake pressing force F P reaches an area indicated by (I) in Fig. 2, it can be determined that an emergency braking is performed. Additionally, when the rate of change of the brake pressing force F P is smaller than the predetermined value or when the convergent value of the brake pressing force F P is smaller than the predetermined value, that is, when the brake pedal 30 is operated so that the brake pressing force F P always changes within an area indicated by (II) in Fig. 2, it can be determined that a normal braking operation is performed. Accordingly, in the system according to the present embodiment, an operational speed and an amount of operation of the brake pedal are detected or assumed, and, then, it is determined whether or not the operational speed exceeds a predetermined value and whether or not the amount of operation exceeds a predetermined value, and, thereby, it can be determined whether or not the operation on the brake pedal 30 is intended to perform an emergency braking. In the brake force control apparatus according to the present embodiment, the brake pedal 30 is moved by an increase or decrease in the brake pressing force F P . At this time, a larger operational speed is generated in the brake pedal 30 as the brake pressing force shows a steep slope, and an amount of operation substantially corresponding to the brake pressing force F P is generated. Accordingly, the operational speed and the amount of operation of the brake pedal 30 can be accurately assumed from the brake pressing force F P . When the brake pressing force F P is exerted on the brake pedal 30, a stroke L corresponding to the brake pressing force F P is generated in the brake pedal 30. Additionally, when the stroke L is generated in the brake pedal 30, a master cylinder pressure P M/C corresponding to the stroke L, which corresponds to the brake pressing force F P is generated in the master cylinder 32. When the master cylinder pressure P M/C corresponding to the brake pressing force F P is generated, a vehicle deceleration G corresponding to the brake pressing force F P is generated in the vehicle. Accordingly, an operational speed and an amount of operation of the brake pedal 30 can be assumed from parameters including 2 ○ the pedal stroke L, 3 ○ the master cylinder pressure P M/C , 4 ○ the vehicle deceleration G, 5 ○ the assumed vehicle speed V SO and 6 ○ the wheel speed V W **, other than the above-mentioned 1 ○ brake pressing force F P . In order to accurately assume an operational speed and an amount of operation of the brake pedal 30, that is, in order to accurately discriminate an emergency braking and a normal brake, preferred parameters of the above-mentioned parameters (hereinafter, referred to as basic parameters) are those detected at positions closest to the foot of the driver. According to such a point of view, the parameters 1 ○ to 6 ○ have a superiority in the order of 1 ○ → 6 ○ when used as the basic parameters. In order to detect 1 ○ the brake pressing force F P , it is required to provide (i) a pressing force sensor. Additionally, in order to detect 2 ○ the pedal stroke L, it is required to provide (ii) a stroke sensor. Similarly, in order to detect 3 ○ the master cylinder pressure P M/C and 4 ○ the vehicle deceleration G, it is required to provide a (iii) a hydraulic pressure sensor and (iv) a deceleration sensor, respectively. Further, in order to detect 5 ○ the assumed vehicle speed V SO and 6 ○ the wheel speed V W **, it is required to provide (v) a wheel speed sensor. The (v) wheel speed sensor and the (iv) deceleration sensor among the above-mentioned sensors (i) to (v) are conventionally and widely used sensors for a vehicle. On the other hand, the (ii) stroke sensor and the (i) pressing force sensor are not popular sensors for a vehicle. Accordingly, considering a cost merit of a sensor due to a mass production effect, the above-mentioned sensors (i) to (v) have a superiority in the order of (v) → (i). In the system according to the present embodiment, considering the above-mentioned merit and demerit, the hydraulic pressure sensor 40 is used as a sensor for detecting the basic parameters so as to discriminate an emergency braking operation and a normal braking operation by using the master cylinder pressure P M/C as a basic parameter. A description will now be given of an operation of the system according to the present embodiment when it is determined by the ECU 10 that an emergency braking is performed. The ECU 10 determines that an emergency braking is performed when the master cylinder pressure P M/C exceeding the predetermined value is detected and a rate of change ΔP M/C is detected after the brake pedal 30 is pressed. When it is determined that an emergency braking is performed, the ECU 10 outputs the drive signals to the STR 26, the SA -1 46, the SA -2 48 and the SA -3 54. When the STR 26 is turned on upon receipt of the above-mentioned drive signal, the third fluid pressure passage 42 and the high-pressure passage 22 are directly connected to each other. In this case, an accumulator pressure P ACC is introduced into the third fluid pressure passage 42. Additionally, when the SA -1 46 and the SA -2 48 are turned on upon receipt of the drive signals, the wheel cylinders 44FR and 44FL communicate with the pressure adjusting fluid pressure passages 56 and 62, respectively. Further, when the SA -3 54 is turned on upon receipt of the above-mentioned drive signal, the upstream side of the SRRH 68 communicates with the third fluid pressure passage 42. In this case, a state is established in which all of the wheel cylinders 44** communicate with the respective holding solenoids S**H and the respective pressure decreasing solenoids S**R and the accumulator pressure P ACC is introduced to the upstream side of each of the holding solenoids S**H. In the ECU 10, all of the holding solenoids S**H and all of the pressure decreasing solenoids S**R are maintained in the OFF state immediately after execution of an emergency braking is detected. Accordingly, as mentioned above, when the accumulator pressure P ACC is introduced to the upstream side of the holding solenoids S**H, the fluid pressure is provided to the wheel cylinders 44** without being changed. As a result, the wheel cylinder pressure P W/C of all of the wheel cylinders 44** is increased toward the accumulator pressure P ACC . As mentioned above, according to the system of the present embodiment, when an emergency braking is performed, the wheel cylinder pressure P W/C of all of the wheel cylinders 44** can be immediately increased irrespective of a magnitude of the brake pressing force F P . Thus, according to the system of the present embodiment, a large brake force can be generated immediately after establishment of a condition in which an emergency braking is required, even if the driver is a beginner-grade driver. When the accumulator pressure P ACC begins to be supplied to the wheel cylinders 44**, as mentioned above, a slip rate S of each of the wheels FL, FR, RL and RR is rapidly increased, and the condition for performing the ABS control is finally established. When the condition for performing the ABS control is established, the ECU 10 achieves, if necessary, the above-mentioned pressure increasing mode 1 ○, holding mode 2 ○ and pressure decreasing mode 3 ○ so that the slip rate S of each of the wheels becomes an appropriate value, that is, so that each of the wheels does not shift to the locked state. It should be noted that when the ABS control is performed subsequent to an emergency braking operation, the wheel cylinder pressure P W/C is increased by using the pump 12 and the accumulator 20 as a fluid pressure source, and is decreased by the brake fluid in the wheel cylinders 44** flowing to the reservoir tank 16. Accordingly, if the pressure increasing mode and the pressure decreasing mode are repeated, a so-called bottoming of the master cylinder 32 does not occur. When the brake assist control is started as mentioned above by execution of an emergency braking operation, the brake assist control must be ended when a press of the brake pedal 30 is released. In the system according to the present invention, as mentioned above, the STR 26, the SA -1 46, the SA -2 48 and the SA -3 54 are maintained to be in the ON state. When the STR 26, the SA -1 46, the SA -2 48 and the SA -3 54 are in the ON state, each of the fluid pressure chamber in the regulator 24 and the first fluid pressure chamber 32a and the second fluid pressure chamber 32b becomes substantially a closed space. In this case, the accumulator pressure P ACC is supplied to the wheel cylinder of each of the wheels but the master cylinder pressure P M/C corresponding to the brake pressing force F P is supplied to the hydraulic pressure sensor 40. Accordingly, the ECU can accurately determine whether or not the press of the brake pedal 30 is released based on the detected value of the hydraulic pressure sensor 40. When the release of the press of the brake pedal 30 is detected, the ECU 10 stops the supply of the drive signals to the STR 26, the SA -1 46, the SA -2 48 and the SA -3 54 so as to return the brake force control apparatus to a state (hereinafter, referred to as a normal brake state) in which the normal control is performed. As for the basic parameters which are the basis of discrimination between an emergency braking and a normal brake, 1 ○ the brake pressing force F P , 2 ○ the pedal stroke L, 4 ○ the vehicle deceleration G, 5 ○ the assumed vehicle speed V SO and 6 ○ the wheel speed V W ** other than the above-mentioned 3 ○ master cylinder pressure P M/C may be applicable. Among those parameters, the 1 ○ brake pressing force F P and 2 ○ the pedal stroke L are parameters that are sensitive to a change in the brake pressing force F P , similar to 3 ○ the master cylinder pressure P M/C . Accordingly, when 1 ○ the brake pressing force F P or 2 ○ the pedal stroke L is used as a basic parameter, it can be easily determined whether or not the press of the brake pedal 30 is released by monitoring the parameter. On the other hand, 4 ○ the vehicle deceleration G and 6 ○ the wheel speed V W ** are parameters that are changed by a change in a brake force. In other words, during execution of the brake assist control, the brake pressing force F P is hardly reflected in those parameters. Accordingly, when the parameters of 4 ○ to 6 ○ are used as the basic parameter, it is effective to perform a determination for a termination of the brake assist control based on the output state of a pressing force switch that is provided for outputting different signals according to whether the brake pressing force F P is applied or released. An apparatus, such as the brake force control apparatus according to the present embodiment, which generates a brake force larger than that of a normal braking operation when an emergency braking operation is performed is effective for providing a superior braking ability to the vehicle when the driver is a beginner-grade driver. However, in such an apparatus, it is important to achieve the above-mentioned functions without giving an incongruous feel to the driver. The brake force control apparatus according to the present embodiment has a feature in that the brake assist control can be started without giving an incongruous feel to the driver by changing the condition for performing the brake assist control, if necessary, in accordance with a state of the vehicle or an operational state of the brake pedal 30. A description will now be given, with respect to Fig. 3, of contents of a process performed by the ECU 10 to achieve the above-mentioned functions. Fig. 3 shows a map of start conditions of the brake assist control used by the ECU 10. The start conditions (I), (II) and (III) shown in Fig. 3 can be represented as follows. P 1 < P M/C and ΔP 2 < ΔP M/C < ΔP 4 P 2 < P M/C and ΔP 1 < ΔP M/C < ΔP 3 P 2 < P M/C and ΔP 2 < ΔP M/C < ΔP 3 The ECU 10 selects an optimum condition from among the above mentioned start conditions (I) to (III) in accordance with the assumed vehicle speed V SO and an elapsed time T after the brake pedal 30 is pressed so as to start the brake assist control when the master cylinder pressure P M/C and the rate of change ΔP M/C satisfy the selected condition. As mentioned above, each of the start conditions (I) to (III) used in the present embodiment is two-dimensionally set according the master cylinder pressure P M/C and the rate of change ΔP M/C . Accordingly, if any one of the start conditions is used, the brake assist control is not started by the brake pedal 30 being slightly operated at a high-speed, that is, the brake pedal 30 being slightly pressed at a high-speed. Thus, according to the brake force control apparatus of the present embodiment, when the driver operates the brake pedal 30 at a high-speed without an intention to rapidly decelerate the vehicle, the brake assist control is prevented from being erroneously started. In the system according to the present embodiment, there is a certain time delay until the wheel cylinder pressure P W/C begins to be increased by execution of the brake assist control after an emergency braking is detected. Therefore, when the master cylinder pressure P M/C is increased at a high speed, the master cylinder pressure P M/C can be rapidly increased by continuing the normal control rather than starting the brake assist control. In each of the start conditions (I) to (III) used in the present embodiment, an upper limit value with respect to the rate of change ΔP M/C of the master cylinder pressure P M/C is set. Accordingly, even if any one of the start conditions is used, the brake assist control is not started when the driver is a high-grade driver and the master cylinder pressure P M/C is increased at a sufficiently high speed. Thus, according to the brake force control apparatus of the present embodiment, a brake force can be rapidly increased by performing the brake assist control when the driver is a beginner-grade driver. Additionally, when the driver is a high-grade driver, the brake force can be rapidly raised by prohibiting execution of the brake assist control. The ECU 10 selects the start condition (I) or (II) when the assumed vehicle speed V SO is greater than a predetermined speed V H , that is, when the vehicle is moving at a high or middle speed. On the other hand, the ECU 10 selects the start condition (III) when the assumed vehicle speed V SO is smaller than the predetermined speed V H , that is, when the vehicle is moving at a low speed. A deceleration feel given to the driver when a full-braking is performed in the vehicle is smaller as the vehicle moves faster, and is larger as the vehicle moves slower. Accordingly, if the brake assist control is performed at a frequency similar to that of the vehicle moving at a high speed when the vehicle is moving at a low speed, a riding quality at a low speed is deteriorated. In the present embodiment, the start condition (III) which is selected when the vehicle is moving at a high speed is narrower and harder to establish as compared to the start condition (I) or (II) which is selected when the vehicle is moving at a middle or high speed. Thus, according to the brake force control apparatus of the present embodiment, when the vehicle is moving at a low speed, the brake assist control is hardly started as compared to a case in which the vehicle is moving at a middle or high speed. Therefore, according to the brake force control apparatus of the present embodiment, both a superior braking ability and a superior riding quality can be obtained during the entire vehicle speed area. Additionally, the ECU 10 selects the start condition (I) immediately after the brake pedal 30 is pressed. On the other hand, the ECU 10 selects the start condition after a predetermined period T 0 passes after the brake pedal 30 is pressed during the middle or high-speed movement. When the driver presses the brake pedal 30 with an intention to perform an emergency from the beginning, the master cylinder pressure P M/C and the rate of change ΔP M/C thereof start to rapidly increase immediately after the brake pedal 30 is pressed. Accordingly, considering such a condition, it is appropriate to determine whether or not the braking operation being performed is an emergency braking operation based on P M/C and ΔP M/C obtained immediately after the brake pedal is pressed. Additionally, when an emergency braking is intended from the beginning as mentioned above, the master cylinder pressure P M/C starts to increase from an atmospheric pressure. In this case, the master cylinder pressure P M/C shows a rapid increase in a relatively low-pressure area. Accordingly, in such a case, a threshold value with respect to the master cylinder pressure P M/C should be set to a relatively small value and a threshold value with respect to the rate of change ΔP M/C should be set to a relatively large value. On the other hand, if the driver intends to perform an emergency braking after the brake pedal has been pressed, the master cylinder pressure P M/C and the rate of change ΔP M/C start to increase after a certain period passes after the brake pedal is pressed. Thus, a determination should be made that an emergency braking is intended to be performed after the brake pedal 30 is pressed when the master cylinder pressure P M/C starts to increase after a predetermined time T 0 has been passed after the brake pedal is pressed. As mentioned above, when an emergency braking is intended to be performed after the brake pedal 30 was pressed, the master cylinder pressure P M/C is further increased after increasing to a certain level. In this case, the master cylinder pressure P M/C shows a rapid increase in a relatively high-pressure area. However, in such a condition, the rate of change ΔP M/C as large as that generated when the master cylinder pressure P M/C is increased from an atmospheric pressure is not generated. Accordingly, in such a case, the threshold value with respect to the master cylinder pressure P M/C should be set to a relatively large value and the threshold value with respect to the rate of change ΔP M/C should be set to a relatively small value. In the present embodiment, the start conditions (I) and (II) are set so as to satisfy the above-mentioned conditions, respectively. Thus, according to the brake force control apparatus of the present embodiment, the brake assist control can be started along with the driver's intention both when the brake pedal 30 is pressed with an intention to perform an emergency braking from the beginning and when an emergency braking is intended to be performed after the brake pedal 30 is pressed. Fig. 4 is a flowchart of an example of a control routine performed by the ECU 10. It should be noted that the routine shown in Fig. 4 is a periodic interruption routine which is started at every predetermined time. When the routine shown in Fig. 4 is started, the process of step 100 is performed first. In step 100, it is determined whether or not the master cylinder pressure P M/C is larger than a predetermined value α. The predetermined value α is a value which is not output when the hydraulic pressure sensor 40 is normally operated. Accordingly, if it is determined that P M/C > α is established, it can be determined that an abnormality occurs in the hydraulic pressure sensor 40. In this case, the process of step 102 is performed subsequently. On the other hand, if it is determined that P M/C > α is not established, the process of step 104 is performed. In step 102, execution of the brake assist control is prohibited. Accordingly, when an abnormality occurs in the hydraulic pressure sensor 40, the control is not continued based on an abnormal master cylinder pressure P M/C . After the process of step 102 is completed, the routine at this time is ended. In step 104, it is determined whether or not the rate of change ΔP M/C of the master cylinder pressure P M/C is greater than a predetermined value β. The predetermined value β is a value which is not generated when the hydraulic pressure sensor 40 normally outputs the master cylinder pressure P M/C . Accordingly, if it is determined that ΔP M/C > β is established, it can be determined that a noise is superimposed on the output signal of the hydraulic pressure sensor 40. In this case, the process of step 102 is performed subsequently. Thus, according to the brake force control apparatus of the present embodiment, an improper control is not performed due to an influence of a noise. On the other hand, if it is determined that ΔP M/C > β is not established, the process of step 106 is performed next. In step 106, it is determined whether or not the assumed vehicle speed V SO is greater than the predetermined speed V H . As a result, if it is determined that V SO ≧ V H is established, it can be determined that the vehicle is moving at a middle or high speed. In this case, the process of step 108 is performed next. In step 108, it is determined whether or not the elapsed time T after the brake pedal 30 is pressed, that is, after an ON signal starts to be output from the brake switch 84 is smaller than the predetermined time T 0 . As a result, if it is determined that T < T 0 is established, the process of step 110 is performed so as to proceed with the process by using the start condition (I) shown in Fig. 3. On the other hand, if it is determined that T < T 0 is not established, the process of step 112 is performed so as to proceed with the process by using the start condition (II) shown in Fig. 3. In step 110, it is determined whether or not the master cylinder pressure P M/C and the rate of change ΔP M/C thereof satisfy the start condition (I), that is, it is determined whether or not P 1 < P M/C and ΔP 2 < ΔP M/C < ΔP 4 are established. As a result, if the above-mentioned condition is established, it is determined that an emergency braking operation is performed by the driver, and, then, the process of step 114 is performed. On the other hand, if the above-mentioned condition is not established, the process is not continued and the routine at this time is ended. In step 114, execution of the brake assist control is started. Thereafter, the brake assist control is continued until the press of the brake pedal 30 is released and the master cylinder pressure P M/C is decreased. After the process of step 114 is ended, the routine at this time is ended. In step 112, it is determined whether or not the master cylinder pressure P M/C and the rate of change ΔP M/C thereof satisfy the start condition (II), that is, it is determined whether or not P 2 < P M/C and ΔP 1 < ΔP M/C < ΔP 3 are established. As a result, if the above-mentioned condition is established, it is determined that an emergency braking operation is performed by the driver, and, then, the process of step 114 is performed. On the other hand, if the above-mentioned condition is not established, the process is not continued and the routine at this time is ended. If it is determined, in step 106, that the assumed vehicle speed V SO is lower than the predetermined speed V H , it is then determined, in step 116, whether or not the assumed vehicle speed V SO is greater than a predetermined speed V L (< V H ). The brake assist control is a process for rapidly decelerating a vehicle. Accordingly, if the vehicle can be easily stopped without performing such a control, the brake assist control is not necessarily performed. The predetermined speed V L is a minimum speed of the vehicle at which the brake assist control can provide a merit. Accordingly, if it is determined that V SO ≧ V L is not established, it can be determined that the brake assist control is not needed to be performed. In this case, the routine at this time is ended without performing any process thereafter. On the other hand, if it is determined that V SO ≧ V L is established, the step of 118 is performed next. In step 118, it is determined whether or not the master cylinder pressure P M/C and the rate of change ΔP M/C thereof satisfy the start condition (II), that is, it is determined whether or not P 2 < P M/C and ΔP 2 < ΔP M/C < ΔP 3 are established. As a result, if the above-mentioned condition is established, it is determined that an emergency braking operation is performed by the driver, and, then, the process of step 114 is performed. On the other hand, if the above-mentioned condition is not established, the process is not continued and the routine at this time is ended. In the above-mentioned embodiment, when setting the start conditions (I) to (III), although an upper limit value is provided to only the rate of change ΔP M/C , the present invention is not limited to this and an upper limit value may be provided to a condition of the master cylinder pressure P M/C . As mentioned above, according to the routine shown in Fig. 4, the brake assist control is continued until the master cylinder pressure P M/C is decreased after the condition for execution of the brake assist control is established. However, depending on moving circumstances of the vehicle, the braking operation may reach an area where the brake assist is not needed after a braking operation satisfying the condition for executing the above-mentioned (I) to (III). In such a case, it is appropriate to restart the normal control by ending the brake assist control so as to maintain a sufficiently large brake force without giving an incongruous feel to the driver. Accordingly, in the brake force control apparatus of the present embodiment, the master cylinder pressure P M/C and the rate of change ΔP M/C are continuously monitored after the brake assist control is started so that the execution of the brake assist control is canceled when it is determined that the braking operation by the driver has reached an area in which a response can be made to a request for an emergency braking. Fig. 5 is a flowchart of an example of a control routine performed by the ECU 10 so as to achieve the above-mentioned function. The routine shown in Fig. 5 is a periodic interruption routine which is started at every predetermined time. When the routine shown in Fig. 5 is started, the process of step 120 is performed first. In step 120, it is determined whether or not the brake assist control is being performed. This routine is a routine for canceling an execution of the brake assist control under a predetermined condition. Accordingly, if the brake assist control is not being performed, there is no merit to continue the subsequent process. Thus, if it is determined that the brake assist control is not being performed, the process is not continued and the routine at this time is ended. On the other hand, if it is determined that the brake assist control is being performed, the process of step 122 is performed next. In step 122, it is determined whether or not the master cylinder pressure P M/C exceeds a predetermined threshold value α. As a result, if it is determined that P M/C > A is established, the process of step 124 is performed. In step 124, it is determined whether or not the rate of change ΔP M/C of the master cylinder pressure P M/C exceeds a predetermined threshold value B. The above-mentioned threshold values A and B are threshold values which are set so as to determine whether or not the braking operation by the driver has reached the area in which the brake assist control is unnecessary. Accordingly, if it is determined, in step 122, that P M/C > A is established, or if it is determined, in step 124, that ΔP M/C > B is established, it can be determined that a condition in which the brake assist control is not necessarily performed is established. In such cases, the process of step 126 is performed subsequently. In step 126, a process for canceling the execution of the brake assist control is performed. Specifically, a process of turning off the STR 26, the SA -1 46, SA -2 48 and SA -3 54 is performed. It should be noted that after the process of step 126 is completed, the routine at this time is ended. After the above-mentioned process is performed, the wheel cylinders 44** communicate with the master cylinder 32, and the normal control is restarted. On the other hand, if both the condition of step 122 and the condition of step 124 are not established, it can be determined that the condition in which the brake assist control is required is maintained. In this case, the routine at this time is ended without performing any process. According to the above-mentioned process, the execution of the brake assist control, which has been started, can be canceled when the emergency braking operation is changed to a sharp operation after it is started with a gentle change. Thus, according to the brake force control apparatus of the present embodiment, a good operational feel which does not give an incongruous feel to a driver can be achieved. A description will now be given, with reference to Fig. 6 and Fig. 7, of a second embodiment according to the present invention. Fig. 6 shows a system structure diagram of a brake force control apparatus according to the present invention. It should be noted that, in Fig. 6, only a part of the brake force control apparatus corresponding to one wheel is shown for the sake of convenience. The brake force control apparatus shown in Fig. 6 is controlled by an ECU 200. The brake force control apparatus according to the present embodiment has a brake pedal 202. A brake switch 203 is provided near the brake pedal 202. The brake switch 203 is a switch which generates an ON output when the brake pedal 202 is pressed. The output signal of the brake switch 203 is supplied to the ECU 200. The ECU 200 determines whether or not a braking operation is being performed based on the output signal of the brake switch 203. The brake pedal 202 is connected to a vacuum booster 204. The vacuum booster 204 is an apparatus which assists a brake pressing force by using an intake negative pressure of an internal combustion engine as a power source. The brake force control apparatus according to the present embodiment has a feature to generate an assist power having a predetermined power ratio with respect to a brake pressing force F P when a normal braking operation is performed, and generate a maximum assist power irrespective of the brake pressing force F P when an emergency braking is performed. A structure of the vacuum booster 204 will be described later. A master cylinder 206 is fixed to the vacuum booster 204. The master cylinder 206 has a fluid pressure chamber therein. Additionally, a reservoir tank 208 is provided above the master cylinder 206. The fluid pressure chamber of the master cylinder and the reservoir tank 208 communicate with each other when a press of the brake pedal 202 is released, whereas they are disconnected from each other when the brake pedal is pressed. Accordingly, brake fluid is supplied to the fluid pressure chamber each time the press of the brake pedal 202 is released. The fluid pressure chamber of the maser cylinder 206 communicates with a fluid pressure passage 210. The fluid pressure passage 210 is provided with a hydraulic pressure sensor 212 which outputs an electric signal corresponding to a pressure inside the fluid pressure passage 210. The output signal of the hydraulic pressure sensor 212 is supplied to the ECU 200. The ECU 200 detects a fluid pressure generated by the master cylinder 206, that is, the master cylinder pressure P M/C based on the output signal of the hydraulic pressure sensor 212. The fluid pressure passage 210 is provided with a holding solenoid 216 (hereinafter, referred to as SH 216). The SH 216 is a two-position solenoid valve which maintains an open state in a normal state (OFF state). The SH 216 is set to be in an ON state (closed state) by a drive signal being supplied by the ECU 200. The downstream side of the SH 216 communicates with a wheel cylinder 218 and a pressure decreasing solenoid 220 (hereinafter, referred to as SR220). The SR 220 is a two-position solenoid valve which maintains a closed state in a normal state (OFF state). SR 220 is set to be in an ON state (open state) by a drive signal being supplied by the ECU 200. Additionally, a check valve 222 which permits a fluid flow only in a direction from the wheel cylinder 218 to the fluid pressure passage 210 is provided between the wheel cylinder 218 and the fluid pressure passage 210. It should be noted that a wheel speed sensor 219 which generates a pulse signal each time the wheel rotates a predetermined angle is provided near the wheel cylinder 218. An output signal of the wheel speed sensor 219 is supplied to the ECU 200. The ECU 200 detects a wheel speed based on the output signal of the wheel speed sensor 219. A reservoir 224 is provided on the downstream side of the SR 220. The brake fluid flowing out of the SR 220 when the SR 220 is set to be in the ON state (open state) is stored in the reservoir 224. It should be noted that the reservoir previously stores a predetermined amount of brake fluid. The reservoir 224 communicates with an inlet port 226a of a pump 226. Additionally, an outlet port 226b of the pump 226 communicates with the fluid pressure passage 210 via a check valve 228. The check vale 228 is a one-way valve which permits a fluid flow only in a direction from the pump 226 to the fluid pressure passage 210. A description will now be given of a structure of the vacuum booster 204 and a structure of a periphery thereof. Fig. 7 shows a structure of the vacuum booster 204 and a structure of a periphery thereof. It should be noted that, in Fig. 7, the master cylinder 206 is fixed to the vacuum booster 204 on the left side thereof. Additionally, the brake pedal 202 is connected to the vacuum booster 204 on the right side thereof. The vacuum booster 204 includes a housing 234 which comprises a front shell 230 and a rear shell 232. A diaphragm 236 and a cylinder member 238 are provided inside the housing 234. The cylinder member 238 is a cylindrical, elastic member having a side surface formed in bellows so that the cylinder member can be elongated and compressed in leftward and rightward directions in Fig. 7. An inner space of the housing 234 is divided into a negative pressure camber 240, a first pressure changing chamber 242 and a second pressure changing chamber 244 by the diaphragm 236 and the cylinder member 238. The front shell 230 is provided with a negative pressure introducing port 246 which communicates with the negative pressure chamber 240. The negative pressure introducing port 246 communicates with a negative pressure passage 248 which communicates with a negative pressure source such as, for example, an intake passage of an internal combustion engine. The front shell 230 is also provided with a adjusting pressure introducing port 250 which communicates with the second pressure changing chamber 244. The adjusting pressure introducing port 250 communicates with a negative pressure introducing valve 252 and an adjusting pressure passage 256 which is communicated to an atmospheric pressure introducing valve 254. The negative pressure introducing valve 252 is a two-position solenoid valve which is positioned between the adjusting pressure passage 256 and the negative pressure passage 248, and maintains an open state in a normal state (OFF state). On the other hand, the atmospheric pressure introducing valve 254 is a two-position solenoid valve which controls communication between the adjusting pressure passage 256 and an atmosphere, and maintains a closed state in a normal state (OFF state). The negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254 are rendered to be in the ON state (closed state or open state, respectively) by a drive signal being supplied by the ECU 200. The rear shell 232 is provided with an atmospheric pressure introducing port 258 which communicates with the first pressure changing chamber 242. The atmospheric pressure introducing port 258 communicates with the adjusting pressure passage 256 via a check valve 260. The check valve 260 is a one-way valve which permits a fluid flow only in a direction from the adjusting pressure passage 256 to the atmospheric pressure introducing port 258. Accordingly, air flows through the atmospheric pressure introducing port 258 only when a pressure higher than a pressure in the first pressure changing chamber 242 is generated in the adjusting pressure passage 256. A booster piston 262 is fit in the center of the diaphragm 236. The booster piston 262 is slidably supported by the rear shell 232 so that an end thereof is exposed in the second pressure-changing chamber 244. Additionally, the booster piston 262 is urged toward an original position, that is, in a rightward direction in Fig. 7, by a spring 263 provided within the second pressure-changing chamber 244. An inner space 264 is formed in a center of the booster piston 262, the inner space extending in a radial direction of the booster piston 262. Additionally, the booster piston 262 is provided with a negative pressure passage which connects the second pressure changing chamber 244 to the internal space 264 and a pressure changing passage 268 which connects the internal space 264 and the first pressure changing chamber 242. The internal space 264 of the booster piston 262 is provided with a pressing force transmitting member 270 which is slidable in an axial direction thereof. The pressing force transmitting member 210 has an annular air valve 272 on an end located on a rearward side of the vehicle, and has a cylindrical pressing force transmitting part 274 on an end located on a forward side of the vehicle. A control valve 276 is provided in the internal space 264 of the booster piston 262. The control valve 276 includes a cylindrical part 278 fixed on an inner wall of the internal space 264 and a flat part 280 formed on an end located on a forward side of the vehicle. The flat portion 280 can move inside the inner space 264 in an axial direction of the control valve 276 with elongation and compression of the cylinder part 278. A through hole 282 is formed in the flat portion 280 of the control valve 276, the through hole 282 extending in the center of the flat portion 280. An input rod 284 is inserted into the through hole 282. The diameter of the through hole 282 is sufficiently larger than the diameter of the input rod 284. Thus, an appropriate clearance is formed between the periphery of the input rod 284 and the through hole 282. An end of the input rod 284 located on the forward side of the vehicle is connected to the pressing force transmitting member 270, and the other end of the input rod 284 located on the rearward side of the vehicle is connected to the brake pedal shown in Fig. 6. An end of a spring 286 is engaged with the input rod 284. The other end of the spring 286 is engaged with the cylindrical part 278 of the control valve 276. The spring 286 urges the input rod 284 and the pressing force transmitting member 270 toward the brake pedal 202 relative to the cylindrical part 278, that is, the booster 262. When a brake pressing force is not input to the input rod 284, the input rod 284 and the pressing force transmitting member 270 are held at a reference point shown in Fig. 1 by the above-mentioned urging force generated by the spring 286. An end of a spring 288 is also engaged with the input rod 284. The other end of the spring 288 contacts the flat part 280 of the control valve 276. An urging force of the spring 288 serves as a force to urge the flat part 280 toward the air valve 272. When the pressing force transmitting member 270 is held at the reference position as shown in Fig. 7, no force against the urging force of the spring 288 is exerted on the flat portion except for a reaction force generated by the air valve 272. Accordingly, when the pressing force transmitting member 270 is located at the reference point, the flat part 280 is maintained to be in contact with the air valve 272. The diameter of the air valve 272 is set to be larger than the diameter of the through hole 282 of the control valve 276. Accordingly, under such a condition, a state in which the through hole 282 is closed by the air valve 272 is established. The booster piston is provided with an annular valve seat 290 at a position opposite to the flat part 280 of the control valve 276. The valve seat 290 is formed so that a predetermined clearance is maintained between the valve seat 290 and the flat part 280 when the input rod 284 and the pressing force transmitting member 270 are located at the reference position. If there is a clearance between the valve seat 290 and the flat part 280, the above-mentioned negative pressure passage 266 communicates with the internal space 264. Additionally, if the valve seat 290 contacts the flat portion 280, the negative pressure passage 266 is disconnected from the internal space 264. Air filters 292 and 294 are provided in the internal space 264 of the booster piston 262. The internal space 264 is open to an atmospheric space via the filters 292 and 294. Accordingly, an atmospheric pressure is always introduced around the through hole 282 of the control valve 276. The booster piston 262 contacts a reaction disc 296 at an end surface located on the forward side of the vehicle. The reaction disc 296 is a disc-like member formed by an elastic material. The other surface of the reaction disc 296 contacts an output rod 298. The output rod 298 is a member which is connected to an input shaft of the master cylinder 206 shown in Fig. 6. When a brake pressing force is exerted on the brake pedal 202, a pressing force corresponding to the brake pressing force is transmitted to the master cylinder via the output rod 298. On the other hand, a reaction force corresponding to the master cylinder pressure P M/C is input to the reaction disc 296. The center of the reaction disc 296 is opposite to the pressing force transmitting part 274 of the pressing force transmitting member 270. The pressing force transmitting member 270 is formed so that a predetermined clearance is formed between the pressing force transmitting part 274 and the reaction disc 296 when the pressing force transmission member 270 is located at the reference position with respect to the booster piston 262. A description will now be given of an operation of the brake force control apparatus according to the present embodiment. In the present embodiment, similar to the ECU 10 of the above-mentioned first embodiment, the ECU 200 determines whether the brake assist control should be started by performing a routine shown in Fig. 4, and determines whether the brake assist control should be continued by performing the routine shown in Fig. 5. That is, the ECU 200 selects an appropriate condition from among the start conditions (I) to (III) shown in Fig. 3 based on the elapsed time T after the brake pedal 202 is pressed and the assumed vehicle speed V SO . Then, the ECU 202 continues the normal control when the master cylinder pressure P M/C detected by the hydraulic pressure sensor 212 and the rate of change ΔP M/C thereof do not satisfy the selected start condition, and, on the other hand, starts the brake assist control when P M/C and ΔP M/C satisfy the selected start condition. Further, when a sufficiently strong braking operation is performed after the brake assist control is started, the ECU 200 cancels the execution of the brake assist control. In the system according to the present embodiment, when the ECU 200 performs the normal control, both the negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254 are maintained to be in the OFF state. In this case, a negative pressure is introduced into the negative pressure chamber 240 of the vacuum booster 204, and a negative pressure is also introduced into the second pressure-changing chamber 244. A description will now be given of an operation of the vacuum booster 204 under such a condition. When the brake pressing force F P is not applied to the brake pedal 202, the input rod 284 and the pressing force transmitting member 270 are held at the reference position (position shown in Fig. 7). In this case, a state in which the air valve 272 is seated on the flat part 280 of the control valve 276, and the flat part 280 is separated from the valve seat 290, that is, a state in which the pressure changing passage 268 is disconnected from the atmospheric space and communicates with the negative pressure passage 266, is formed. Under such a condition, the second pressure-changing chamber 244 communicates with the first pressure-changing chamber 242. Accordingly, a pressure inside the first pressure-changing chamber becomes a negative pressure similar to the pressure inside the second pressure changing chamber 244 and the pressure inside the negative pressure chamber 240. When the pressure inside the first pressure-changing chamber 242 is equal to the pressure inside the second pressure-changing chamber 244, no force caused by the negative pressures is exerted on the diaphragm 236. Therefore, when the brake pressing force F P is not input, a pressing force is not transmitted from the output rod 298 to the master cylinder 206. When the brake pressing force F P is applied to the brake pedal 202, the input rod 284 is moved relative to the booster piston 262 in the forward direction of the vehicle, that is, in the rightward direction in Fig. 7. When a relative displacement of the input rod 284 reaches a predetermined length, an end surface of the pressing force transmitting part 274 contacts the reaction disc 296, and the flat part 280 of the control valve 276 seats on the valve seat 290 of the booster piston 262 so that the negative pressure passage 266 is disconnected from the pressure changing passage 268. If the input rod 284 is further pressed in the direction toward the reaction disc 296, the input rod 284 and the pressing force transmitting member 270 continues to move while elastically deforming the center part of the reaction disc 296, that is, a part of the reaction disc 296 (hereinafter, simply referred to as a center part) which contacts the pressing force transmitting part 274. If the relative displacement of the pressing force transmitting member 270 is increased as mentioned above, a reaction force corresponding to an elastic deformation, that is, an elastic force corresponding to the brake pressing force F P , is transmitted to the input rod 284. Additionally, after a state in which the flat part 280 is seated on the valve seat 290 is established as mentioned above, the displacement of the flat part 280 relative to the booster piston 262 is restricted. Thus, if the input rod 284 is further pressed in the direction toward the reaction disc 296 after such a condition is established, the air valve 272 is separated from the flat part 280 of the control valve 276, and the pressure changing passage 268 communicates with the through hole 282. If such a state is established, an atmospheric air is introduced into the first pressure-changing chamber 242 via the through hole 282 and the pressure changing passage 268. As a result, the pressure inside the first pressure-changing chamber 242 becomes higher than the pressure inside the second pressure-changing chamber 244 and the negative pressure chamber 240. As mentioned above, if a pressure difference ΔP B is generated between the first pressure changing chamber 242 and each of the second pressure changing chamber 244 and the negative pressure chamber 240, a pressing force F A (hereinafter, referred to as brake assist force F A ) which urges the diaphragm 236 in a direction toward the front of the vehicle is exerted on the diaphragm 236. It should be noted that the brake assist force F A can be approximately represented by the following equation by using an effective cross-sectional area S B of the negative pressure chamber 240 and an effective cross-sectional area S C of the second pressure changing chamber 244. F A = (S B + S C )·ΔP B The thus-generated brake assist force F A is transmitted from the diaphragm 236 to the booster piston 262, and further transmitted to a periphery of the reaction disc 296, that is, a part of the reaction disc (hereinafter, simply referred to as a peripheral part) which contacts the booster piston 262. When the brake assist force F A is input from the booster piston to the peripheral part of the reaction disc 296, an elastic deformation is generated in the peripheral part of the reaction disc 296. This elastic deformation increases as a pressure difference ΔP between opposite sides of the diaphragm 236 increases, that is, as the introduction of air into the first pressure changing chamber 242 is continued. In the process in which an amount of elastic deformation in the peripheral part of the reaction disc 296 is increased as mentioned above, the booster piston is moved relative to a reaction force transmitting part 28 in the direction toward the front of the vehicle. Then, if the amount of elastic deformation of the peripheral part of the reaction disc 296 reaches a value almost equal to the amount of elastic deformation of the center part of the reaction disc 296, the flat part 280 of the control valve 276 contacts the air valve 272, and the introduction of atmospheric air to the first pressure changing chamber 242 is stopped. As a result, the pressure difference ΔP generated between opposite sides of the diaphragm 236 is adjusted to a value corresponding to the brake force F P input to the input rod 284. Additionally, the brake assist force F A = (S B + S C )·ΔP B becomes a value corresponding to the brake pressing force F P . At this time, a resultant force of the brake assist force F A and the brake pressing force F P is transmitted to the master cylinder 206. When the resultant force of the brake assist force F A and the brake pressing force F P is transmitted to the master cylinder 206, the master cylinder 206 generates a master cylinder pressure P M/C having a predetermined power ratio with respect to the brake pressing force F P . The ECU 200 turns off the SH 216 and SR 220 so as to set the hydraulic circuit connected to the master cylinder 206 to a normal state. When the hydraulic circuit is set to the normal state, the master cylinder pressure P M/C is introduced into the wheel cylinder 218 as it is. Accordingly, the brake force generated in the wheel cylinder 218 is adjusted to a level corresponding to the brake pressing force F P . If a slip rate S of a wheel exceeds a predetermined value after the braking operation is started, the ECU 200 starts the ABS control similar to the ECU 10 of the above-mentioned first embodiment. The ABS control is performed when the brake pedal 202 is pressed, that is, when the master cylinder pressure P M/C is appropriately increased. Under the condition in which the master cylinder pressure P M/C is appropriately increased, the SH 216 is set to the open state and the SR 220 is set to the closed state, and, thereby, the wheel cylinder pressure P W/C is increased with the master cylinder pressure P M/C as an upper limit value. Hereinafter, this state is referred to as a pressure-increasing mode 1 ○. Additionally, the wheel cylinder pressure P W/C is maintained without being increased or decreased by the SH 216 being set to the closed state and the SR 220 being set to the closed state. Hereinafter, this state is referred to as a holding mode 2 ○. Further, the wheel cylinder pressure P W/C is decreased by the SH 216 being set to the closed state and the SR 220 being set to the open state. Hereinafter, this state is referred to as a pressure decreasing mode 3 ○. The ECU 200 achieves, if necessary, the above-mentioned pressure increasing mode 1 ○, holding mode 2 ○ and pressure decreasing mode 3 ○ so that a slip rate S of the wheel becomes an appropriate value. When a depression of the brake pedal 202 is released by the driver during execution of the ABS control, the wheel cylinder pressure P W/C must be immediately decreased. In the system according to the present embodiment, the check valve 222 is provided in the hydraulic circuit corresponding to the wheel cylinder 218. The check valve 222 permits a fluid flow only in the direction from the wheel cylinder 218 to the master cylinder 206. Thus, according to the system of the present embodiment, the wheel cylinder pressure P W/C of the wheel cylinder 222 can be immediately decreased after the depression of the brake pedal 202 is released. In the system according to the present embodiment, when the ABS control is performed, the wheel cylinder pressure P W/C is increased by the master cylinder 206 as a fluid pressure source. Additionally, the wheel cylinder pressure P W/C is decreased by having the brake fluid in the wheel cylinder flow to the reservoir 224. Accordingly, if the pressure-increasing mode and the pressure-decreasing mode are repeatedly performed, the brake fluid in the master cylinder 206 gradually flows to the reservoir 224. However, in the system according to the present embodiment, the brake fluid in the reservoir 224 is delivered to the master cylinder 206 by the pump 226. Thus, if the ABS control is continued for a long time, a so-called bottoming of the master cylinder does not occur. A description will now be given of an operation achieved by the ECU 200 performing the brake assist control. As mentioned above, when the master cylinder pressure P M/C and the rate of change ΔP M/C thereof satisfy the predetermined start condition, the ECU 200 starts the brake assist control. The brake assist control is achieved by turning on both the negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254, that is, by closing the negative pressure introducing valve 252 and opening the atmospheric pressure introducing valve 254. The ECU 200 maintains both the negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254 to be set to the OFF state until the ECU 200 determines that the start condition of the brake assist control is established after the brake pedal 202 is pressed. Then, if it is determined that the start condition is established, both the negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254 are set to the ON state. Until both the negative pressure-introducing valve 252 and the atmospheric pressure introducing valve 254 are set to the ON state, the input rod 284 moves prior to the booster piston 262. As a result, the control valve 280 sits on the valve seat 290 and the air valve 272 separates from the control valve 276. Thereby, atmospheric air is introduced into the first pressure changing chamber 242, and the brake assist force F A = (S B + S C )·ΔP B is generated. Under such a condition, if the negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254 are set to the ON state, a pressure inside the first pressure changing chamber 242 and the second pressure changing chamber 244 is rapidly increased to an atmospheric pressure. As a result, a pressure difference ΔP AIR is generated between the negative pressure chamber 240 and the first pressure changing chamber 242. In this case, a brake assist force F A represented by the following equation is exerted on the diaphragm 236. F A = S B ·ΔP AIR The brake assist force F A is transmitted from the diaphragm 236 to the booster piston 262, and further transmitted to the peripheral part of the reaction disc 296. Additionally, the brake pressing force F P which is exerted on the brake pedal 202 is also transmitted to the reaction disc 296. Accordingly, thereafter, a resultant force of the brake assist force F A and the brake pressing force F P is transmitted to the master cylinder 206. In the system according to the present embodiment, similar to the above-mentioned first embodiment, the brake assist control is started when the brake pressing force F P is not sufficiently increased, that is, under a condition in which a large brake assist force F A has not been obtained. Accordingly, the brake assist force F A exerted on the booster piston 262 shows a sharp increase before or after the brake assist control is started. If the sharp change occurs in the brake assist force F A as mentioned above, the booster piston 262 is rapidly and relatively moved toward the front of the vehicle immediately after the brake assist control is started. Then, when such a sharp change is generated in the booster piston 262, a phenomenon occurs in which the control valve 276, which was seated on the valve seat 290 before the brake assist control was started is separated from the valve seat 290 when the control is started. When the control valve 276 is separated from the valve seat 290, the second pressure changing chamber 244 communicates with the first pressure changing chamber 242. Accordingly, if a negative pressure is stored in the second pressure changing chamber 244, the negative pressure is provided from the second pressure changing chamber 244 to the first pressure changing chamber 242 after the brake assist control is started. As a result, there is a problem in that the brake assist force F A cannot be raised immediately. However, in the vacuum booster 204 of the present embodiment, atmospheric air is introduced into the second pressure-changing chamber 244 at the same time the brake assist control is started. Thus, according to the system of the present embodiment, if the phenomenon in which the control valve 276 is separated from the valve seat 290 after the brake assist control is started occurs, the brake assist force F A can be raised immediately. The ECU 200 sets the hydraulic circuit connected to the master cylinder 216 to a normal state after the execution condition of the brake assist control is established and until the execution condition of the ABS control is established. In this case, the master cylinder pressure P M/C is introduced to the wheel cylinder 218 without change. Accordingly, the wheel cylinder pressure P W/C is rapidly increased from a pressure corresponding to "(S B + S C )·ΔP B + F P " to a pressure corresponding to "S B ·ΔP AIR + F P " when the brake assist control is started. As mentioned above, according to the system of the present embodiment, when an emergency braking operation is performed, the wheel cylinder pressure P W/C can be rapidly increased to a value sufficiently larger than the brake pressing force F P . Thus, according to the system of the present embodiment, a large brake force can be generated immediately after establishment of a condition in which an emergency braking is required, even if the driver is a beginner-grade driver. After the wheel cylinder pressure P W/C is rapidly increased as mentioned above, the slip rate S of the wheel is rapidly increased, and finally the execution condition of the ABS control is established. After the execution condition of the ABS control is established, the ECU 200 achieves, if necessary, the above-mentioned pressure increasing mode 1 ○, holding mode 2 ○ and pressure decreasing mode 3 ○ so that a slip rate S of the wheel becomes an appropriate value. In the system according to the present embodiment, in a period during which the brake pressing force F P is applied to the brake pedal 202 after the brake assist control is started, the master cylinder pressure P M/C is maintained to be a pressure corresponding to "S B ·ΔP AIR + F P ". On the other hand, if a depression of the brake pedal 202 is released after the brake assist control is started, the master cylinder pressure P M/C is decreased to a pressure corresponding to "S B ·ΔP AIR ". Accordingly, by monitoring the output signal of the master cylinder pressure P M/C detected by the hydraulic pressure sensor 212, the ECU 200 can determine whether or not the depression of the brake pedal 202 is released. Upon detection of the release of the depression of the brake pedal 202, the ECU 200 stops supply of the drive signals to the negative pressure introducing valve 252 and the atmospheric pressure introducing valve 254, and terminates the brake assist control. It should be noted that the brake force control apparatus according to the above-mentioned second embodiment is similar to the brake force control apparatus according to the above-mentioned first embodiment in the following points providing superior effects: 1 ○ when the driver operates the brake pedal 202 at a high speed without intending to rapidly decelerate the vehicle, an erroneous start of the brake assist control can be prevented; 2 ○ the vehicle can be rapidly decelerated by performing the brake assist control when the driver is a beginner-grade driver, and the vehicle can be rapidly decelerated by prohibiting execution of the brake assist control when the driver is a high-grade driver; 3 ○ superior braking ability and superior riding quality can be incompatible with each other in the entire vehicle speed range; 4 ○ the brake assist control can be appropriately started along with the driver's intention both in a case in which the brake pedal 202 is pressed with an intention to perform an emergency braking from the beginning and in a case in which an emergency braking is intended after the brake pedal 202 is pressed; and 5 ○ execution of the brake assist control already started can be appropriately canceled when an emergency braking operation started by a relatively gentle operation is, thereafter, changed to a rapid operation. It should be noted that, in the above-mentioned second embodiment, although the master cylinder pressure P M/C is used as the basic parameter for discriminating between a normal braking operation and an emergency braking operation, the basic parameter is not limited to this, and, similar to the first embodiment, the brake pressing force F P , the pedal stroke L, the vehicle deceleration G, the assumed vehicle speed V SO or the vehicle speed V W ** may be used as the basic parameter. A description will now be given, with respect to Fig. 8, of a third embodiment of the present invention. Fig. 8 shows a system structure diagram of a brake force control apparatus according to the present embodiment. It should be noted that, in Fig. 8, only a part of the brake force control apparatus corresponding to a single wheel is shown. Additionally, in Fig. 8, parts that are the same as the parts shown in Fig. 6 are given the same reference numerals, and descriptions thereof will be omitted. The brake force control apparatus shown in Fig. 8 is controlled by an ECU 300. In the brake force control apparatus according to the present embodiment, a vacuum booster 302 is connected to the brake pedal 202. The vacuum booster 302 is an apparatus which assists a brake pressing force by using an intake negative pressure of an internal combustion engine as a power source. The vacuum booster 302 used in the present embodiment is different from the vacuum booster in the second embodiment, and is a general apparatus which always assists the brake pressing force F P with a constant power ratio. In the system according to the present embodiment, the reservoir 224 communicates with a fluid pressure passage 304 which communicates with the reservoir tank 208. The fluid pressure passage 304 is provided with a check valve 306 and a switching solenoid 308 (hereinafter, referred to as SCH 308). The check valve 306 is a one-way valve which permits a fluid flow only in a direction from the reservoir tank 208 to the reservoir 224. Additionally, the SCH 308 is a two-position solenoid valve which maintains a closed state in a normal state (OFF state). The SCH 308 is opened by a drive signal being supplied from the ECU 300. The fluid pressure passage 210 is provided with a fluid pressure cutting solenoid 214 (hereinafter, referred to as SC 214). The SC 214 is two-position solenoid valve which opens and closes the fluid pressure passage 210, and maintains an open state in a normal state (OFF state). The SC 214 is set to an ON state (closed state) by a drive signal being supplied from the ECU 300. A description will now be given of an operation of the brake force control apparatus according the present embodiment. Similar to the ECU 10 of the above-mentioned first embodiment and the ECU 200 of the above-mentioned second embodiment, the ECU 300 of the present embodiment determines whether the brake assist control should be started by performing a routine shown in Fig. 4, and determines whether the brake assist control should be continued by performing the routine shown in Fig. 5. That is, the ECU 300 selects an appropriate condition from among the start conditions (I) to (III) shown in Fig. 3 based on the elapsed time T after the brake pedal 202 is pressed and the assumed vehicle speed V SO . Then, the ECU 300 continues the normal control when the master cylinder pressure P M/C detected by the hydraulic pressure sensor 212 and the rate of change ΔP M/C thereof do not satisfy the selected start condition, and, on the other hand, starts the brake assist control when P M/C and ΔP M/C satisfy the selected start condition. Further, when a sufficiently strong braking operation is performed after the brake assist control is started, the ECU 300 cancels the execution of the brake assist control. In the system according to the present embodiment, when the ECU 300 performs the normal control, all of the SC 214, the SCH 308, the SH 216 ant the SR 220 are maintained to be in the OFF state, and the pump 226 is maintained to be stopped. In such a condition, only the master cylinder 206 can serve as a fluid pressure source, and the master cylinder pressure P M/C generated in the master cylinder 206 is supplied to the wheel cylinder 218. Accordingly, in this case, the wheel cylinder pressure P W/C of the wheel cylinder 218 is adjusted to a fluid pressure having a predetermined power ratio. If the slip rate S of the wheel exceeds a predetermined value, similar to the ECU 200 of the above-mentioned second embodiment, the ECU 300 starts the ABS control. The ABS control can be achieved by operating the pump 226 and by realizing the above-mentioned pressure increasing mode 1 ○, holding mode 2 ○ and pressure decreasing mode 3 ○ so that the slip rate S of the wheel becomes an appropriate value. When the master cylinder pressure P M/C and the rate of change ΔP M/C thereof satisfy a predetermined start condition, the ECU 300 starts the brake assist control. In the system according to the present embodiment, the brake assist control is achieved by turning on both the SC 214 and the SCH 308, that is, by closing the SC 214 and opening the SCH 308, and operating the pump 226. Under such a condition, the master cylinder 206 and the wheel cylinder 218 are disconnected from each other. On the other hand, the pump 226 delivers the brake fluid supplied from the reservoir tank 208 via the fluid pressure passage 304 toward the wheel cylinder 218. Thus, the wheel cylinder pressure P W/C of the wheel cylinder 218 is increased by the pump 226 as a fluid pressure source. The pump 226 is capable of rapidly increasing the wheel cylinder pressure P W/C immediately after the brake assist control is started. Accordingly, when execution of an emergency braking operation is detected by the ECU 300, the wheel cylinder pressure P W/C of the wheel cylinder 218 is rapidly increased irrespective of whether the brake pressing force F P is large or small. As mentioned above, according to the system of the present embodiment, when an emergency braking operation is performed, the wheel cylinder pressure P W/C of the wheel cylinder 218 can be rapidly increase to a sufficiently large value irrespective of the brake pressing force F P . Thus, according to the system of the present embodiment, a large brake force can be generated immediately after establishment of a condition in which an emergency braking is required is established even if the driver is a beginner-grade driver. After the wheel cylinder pressure P W/C is rapidly increased as mentioned above, the slip rate S of the wheel is rapidly increased, and, finally, the execution condition of the ABS control is established. After the execution condition of the ABS control is established, the ECU 300 achieves, if necessary, the above-mentioned pressure increasing mode 1 ○, holding mode 2 ○ and pressure-decreasing mode 3 ○ so that the slip rate S of the wheel becomes an appropriate value. In the system according to the present embodiment, in a period during which the brake assist control is performed, the SC 214 is maintained in the ON state. If the SC 214 is in the ON state, the fluid pressure chamber of the master cylinder 206 and a part of the upstream side of the SC 214 of the fluid pressure passage 210 become substantially a closed space. Under such a condition, the master cylinder pressure P M/C is a value corresponding to the brake pressing force F P . Accordingly, by monitoring the output signal of the master cylinder pressure P M/C detected by the hydraulic pressure sensor 212, the ECU 300 can easily determine whether or not the depression of the brake pedal 202 is released. Upon detection of the release of the depression of the brake pedal 202, the ECU 300 stops supply of the drive signals to the SC 214 and the SCH 308, and terminates the brake assist control. It should be noted that the brake force control apparatus according to the above-mentioned third embodiment is similar to the brake force control apparatus according to the above-mentioned first embodiment in the following points providing superior effects that: 1 ○ when the driver operates the brake pedal 202 at a high speed without intending to rapidly decelerate the vehicle, an erroneous start of the brake assist control can be prevented; 2 ○ the vehicle can be rapidly decelerated by performing the brake assist control when the driver is a beginner-grade driver, and the vehicle can be rapidly decelerated by prohibiting execution of the brake assist control when the driver is a high-grade driver; 3 ○ superior braking ability and superior riding quality can be compatible with each other in the entire vehicle speed range. 4 ○ the brake assist control can be appropriately started along with the driver's intention both in a case in which the brake pedal 202 is pressed with an intention to perform an emergency braking from the beginning and in a case in which an emergency braking is intended after the brake pedal 202 is pressed; and 5 ○ execution of the brake assist control already started can be appropriately canceled when an emergency braking operation started by a relatively gentle operation is, thereafter, changed to a rapid operation. It should be noted that, in the above-mentioned third embodiment, although the master cylinder pressure P M/C is used as the basic parameter for discriminating between a normal braking operation and an emergency braking operation, the basic parameter is not limited to this, and, similar to the first embodiment, the brake pressing force F P , the pedal stroke L, the vehicle deceleration G, the assumed vehicle speed V SO or the vehicle speed V W ** may be used as the basic parameter. It should be noted that, in the above-mentioned first to third embodiments, although the brake assist is always performed when a braking operation satisfying the execution condition is performed, a structure may be used in which the execution of the brake assist control can be prohibited by a manual operation of the driver by providing an on/off switch regarding the brake assist control in the vehicle compartment.

Claims: A brake force control apparatus performing a normal control generating a brake force corresponding to a brake pressing force and a brake assist control generating a brake force larger than that of the normal control, the brake force control apparatus comprising: operational speed detecting means (10, 40) detecting an operational speed (ΔP M/C ) of a brake pedal; operational amount detecting means (10, 40) detecting an operational amount (P M/C ) of the brake pedal; and brake assist control starting means (100, 104, 110, 112, 114, 118) starting said brake assist control when said operational speed (ΔP M/C ) and said operational amount (P M/C ) satisfy a predetermined start condition; characterised in that said predetermined start condition includes that said operational speed (ΔP M/C ) is within a range equal to or greater than a first speed threshold value (ΔP 1 , ΔP 2 ) which is greater than zero, and equal to or less than a second speed threshold value (ΔP 3 , ΔP 4 ) which is greater than said first speed threshold value (ΔP 1 , ΔP 2 ), and said operational amount (P M/C ) is equal to or greater than a first operation threshold value (P 1 , P 2 ) which is greater than zero. The brake force control apparatus as claimed in claim 1, further comprising start condition changing means (106, 108, 110, 112, 118) changing said first speed threshold value in response to an elapsed time after the brake pedal is pressed. The brake force control apparatus as claimed in claim 2, wherein said start condition changing means (106, 108, 110, 112, 118) changes said first speed threshold value (ΔP 1 , ΔP 2 ) so that said first speed threshold value (ΔP 1 , ΔP 2 ) is decreased. The brake force control apparatus as claimed in claim 1, further comprising start condition changing means (106, 108, 110, 112, 118) changing said first operation threshold value (P 1 , P 2 ) in response to an elapsed time after the brake pedal is pressed. The brake force control apparatus as claimed in claim 4, wherein said start condition changing means (106, 108, 110, 112, 118) changes said first operation threshold value (P 1 , P 2 ) so that said first operation threshold value (P 1 , P 2 ) is increased. The brake force control apparatus as claimed in claim 1, further comprising start condition changing means (106, 108, 110, 112, 118) changing said second speed threshold value (ΔP 3 , ΔP 4 ) in response to an elapsed time after the brake pedal is pressed. The brake force control apparatus as claimed in claim 6, wherein said start condition changing means (106, 108, 110, 112, 118) changes said second speed threshold value (ΔP 3 , ΔP 4 ) so that said second speed threshold value (ΔP 3 , ΔP 4 ) is decreased. The brake force control apparatus as claimed in claim 1, further comprising start condition changing means (106, 108, 110, 112, 118) changing said first operation threshold value (P 1 , P 2 ) in response to a vehicle speed (V SO ). The brake force control apparatus as claimed in claim 8, wherein said start condition changing means (106, 108, 110, 112, 118) changes said first operation threshold value (P 1 , P 2 ) so that said first operation threshold value (P 1 , P 2 ) is decreased. The brake force control apparatus as claimed in claim 1, further comprising brake assist start prohibiting means (116) prohibiting a start of said brake assist control when the vehicle speed (V SO ) is smaller than a predetermined value (V L ). The brake force control apparatus as claimed in claim 1, further comprising first control canceling means (122, 126) canceling an execution of said brake assist control when the operational amount (P M/C ) exceeding a predetermined value (A) is detected after said brake assist control is started. The brake force control apparatus as claimed in claim 1, further comprising a second control canceling means (122, 126) canceling an execution of said brake assist control when the operational speed (ΔP M/C ) exceeding a predetermined value (B) is detected after said brake assist control is started.

Inventors: HASHIMOTO YOSHIYUKI, YOSHIDA HIROAKI, HASHIMOTO, YOSHIYUKI, YOSHIDA, HIROAKI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1997
Main classification: B60T13
IPC codes: B60T8, B60T13, B60T7