JP2006306272A - Brake control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a brake control device capable of preventing sudden increase of pedal reaction force in the communication of a hydraulic pressure passage. <P>SOLUTION: In normal time, a master cylinder 2 and wheel cylinders 6a, 6b are shut off by shut-off valves 3a, 3b, and the hydraulic pressure is generated by a pump 9 based on a stroke sensor detection value to control a wheel cylinder pressure. The brake control device directly controls the wheel cylinder pressure with the liquid pressure of the master cylinder by communicating the master cylinder 2 with the wheel cylinders 6a, 6b in an abnormal time. The brake control device is equipped with a pressure reduction control part 15 having a piston 15b where a pressure receiving area A2 receiving the pressure of the master cylinder side is set larger than the pressure receiving area A1 receiving the pressure of the wheel cylinder side. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a brake control device capable of controlling the working fluid pressure to the brake cylinder of each wheel separately from a master cylinder that boosts the working fluid pressure by depressing a brake pedal.

  In general, a brake control device (brake-by-wire system) using a hydraulic circuit transmits hydraulic pressure according to brake operation to a wheel cylinder by a pump or the like in a state where the hydraulic pressure path from the master cylinder to the wheel cylinder is cut off in a normal state. When an abnormality occurs, the hydraulic pressure path from the master cylinder to the wheel cylinder is communicated, and the hydraulic pressure corresponding to the brake operation is transmitted from the master cylinder to the wheel cylinder.

In such a brake control device, when an abnormal wheel is detected due to a leakage of hydraulic fluid in the wheel cylinder, the hydraulic pressure supply from the hydraulic pressure source to the wheel cylinder of the wheel in which the abnormality is detected is interrupted and It is known to suppress a reduction in braking performance by controlling the hydraulic pressure applied to the wheel cylinder of a wheel in which no wheel is detected (see, for example, Patent Document 1).
JP 2000-168536 A

By the way, in the conventional brake control device, when the brake-by-wire system is normal, the hydraulic pressure supplied to the wheel cylinder is, for example, 7 to 8 times the master cylinder pressure with respect to the driver's brake operation. The hydraulic pressure corresponds to the boosted hydraulic pressure.
However, in the brake control device described in Patent Document 1, for example, when an abnormality occurs during braking in the normal state (the wheel cylinder pressure is equivalent to the hydraulic pressure obtained by boosting the master cylinder pressure) The shut-off valve that shuts off the master cylinder and the wheel cylinder is opened, and the hydraulic pressure path from the master cylinder to the wheel cylinder communicates, so that the high pressure on the wheel cylinder side flows into the low pressure on the master cylinder side, Since the pedal reaction force increases rapidly, there is an unsolved problem that the driver may feel uncomfortable.
SUMMARY OF THE INVENTION An object of the present invention is to provide a brake control device that can prevent a sudden increase in pedal reaction force during communication of a hydraulic pressure path.

  In order to achieve the above object, a brake control device according to the present invention is interposed between a master cylinder and an on-off valve, and the pressure receiving area for receiving pressure on the master cylinder side receives the pressure on the wheel cylinder side. A pressure reducing cylinder having a larger piston is provided.

  According to the present invention, during braking in which the wheel cylinder side is at a higher pressure than the master cylinder side, a valve that is abnormal and shuts off the master cylinder and the wheel cylinder is opened, and the master cylinder to the wheel cylinder is opened. Even when the communication path is connected, the wheel cylinder side pressure is controlled by the balance between the force received by the small-diameter piston and the force received by the large-diameter piston. Since the pressure is reduced and transmitted to the master cylinder side, an abrupt increase in the master cylinder pressure can be suppressed, and an effect of preventing a sudden increase in the pedal reaction force can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a schematic configuration diagram of a brake control device according to the first embodiment. In the figure, reference numeral 1 denotes a brake pedal, and reference numeral 2 denotes a master cylinder whose pressure is increased according to the depression amount of the brake pedal 1. .
The master cylinder 2 has two output systems, and outputs the working fluid pressure to these two output systems. Hereinafter, one output system is referred to as a primary system, and the other is referred to as a secondary system. These working fluid pressures are supplied via a pressure reduction control unit 15 as a pressure reducing cylinder, shut-off valves (open during standby) 3a and 3b as open / close valves, and normally open solenoid valves (open during standby) 4a and 4b. The wheel cylinders 6a and 6b generate braking force on the wheels 5a and 5b.

The master cylinder 2 has pressure generation chambers 2a and 2b, and these pressure generation chambers 2a and 2b are formed by a primary piston 2c and a secondary piston 2d in the cylinder. A spring 2e is installed in the pressure generation chamber 2a, and a spring 2f is installed in the pressure generation chamber 2b.
That is, when the brake pedal 1 is depressed, the input rod 17 moves forward by the depression force to move the pistons 2c and 2d, and the working fluid pressure is output from the pressure generating chambers 2a and 2b, respectively. Here, the output system from the pressure generating chamber 2a corresponds to the primary system, and the output system from the pressure generating chamber 2b corresponds to the secondary system.

FIG. 2 is a configuration diagram illustrating details of the decompression control unit 15. In FIG. 2, only the primary system pressure reduction control unit 15 of the master cylinder 2 will be described, but the secondary system pressure reduction control unit 15 also has the same configuration.
The pressure reduction control unit 15 includes a cylinder 15a and a piston 15b that is slidably disposed in the cylinder 15a and has different pressure receiving areas on the master cylinder 2 side and the shutoff valve 3b side. The pressure receiving area A ₂ for receiving the pressure on the master cylinder 2 side of the piston 15b is set larger than the pressure receiving area A ₁ for receiving the pressure on the shutoff valve 3b side (wheel cylinder 6b side).

In FIG. 1, a pump 9 is connected to a reservoir 8 arranged in parallel with the master cylinder 2. The pump 9 includes an electric motor 9a and a hydraulic pump 9b that is rotationally driven by the electric motor 9a. A braking pressure output from the pump 9 is a fluid path between the shut-off valve 3a and the normally-open electromagnetic valve 4a. To be supplied. The fluid path between the shut-off valve 3a and the normally open solenoid valve 4a, and the fluid path between the shut-off valve 3b and the normally open solenoid valve 4b are normally closed (standby closed) electromagnetic on-off valves. Are connected via a system shut-off valve 10.
Further, the wheel cylinders 6a and 6b and the reservoir 8 are connected via normally closed solenoid valves (closed during standby) 11a and 11b, and control the discharge of hydraulic pressure from the wheel cylinders 6a and 6b to the reservoir 8. It is configured as follows.

  A stroke simulator 12 is provided on the upstream side of the shutoff valve 3b and the pressure reduction control unit 15 on one output side of the master cylinder 2 via a stroke simulator cut valve 13 which is a normally closed (closed during standby) electromagnetic on-off valve. Is connected. When the brake control device (brake-by-wire system) is activated and normal, the shut-off valves 3a and 3b are closed to shut off the master cylinder 2 and the wheel cylinders 6a and 6b, and the stroke simulator cut valve 13 is energized. The open state is established, and the output side and the stroke simulator 12 are connected. When the stroke simulator cut valve 13 is in the open state, the stroke simulator 12 is configured to absorb the working fluid pressure output from the master cylinder 2 in response to the depression of the brake pedal 1.

The stroke simulator 12 includes a cylinder, a piston 12a slidably disposed in the cylinder, and a coil spring 12b that urges the piston 12a, and a pedal corresponding to the stroke of the brake pedal 1. Generate reaction force.
The brake control device further includes a stroke sensor 18 for detecting the amount of depression of the brake pedal 1 and wheel cylinder pressure sensors 19a and 19b for detecting the working fluid pressure of the wheel cylinder.

  The shutoff valves 3a and 3b, normally open solenoid valves 4a and 4b, the system shutoff valve 10, the normally closed solenoid valves 11a and 11b, and the stroke simulator cut valve 13 are each supplied from the control unit 30 supplied to the solenoid. The open / close state is controlled by a control signal, and when the control signal is in an off state (non-energized state), it is in a normal state, and when it is in an on state (energized state), it is configured to switch to an offset position.

  The control unit 30 is configured via an arithmetic processing device such as a microcomputer, for example. In the control unit 30, the master cylinder 2 and the wheel cylinders 6a and 6b are shut off by closing the shut-off valves 3a and 3b between the master cylinder 2 and the wheel cylinders 6a and 6b in a normal state. By driving the pump 9 in accordance with the stroke of the brake pedal 1 detected by the stroke sensor 18, the hydraulic pressure supplied to the wheel cylinders 6a and 6b is generated. At this time, the stroke simulator cut valve 13 is opened, and the stroke simulator 12 absorbs the depression of the brake pedal 1.

  Further, when a predetermined condition is satisfied (when an abnormality occurs), the master cylinder 2 and the wheel cylinders 6a and 6b are communicated with each other by opening the shut-off valves 3a and 3b, and directly by the hydraulic pressure of the master cylinder 2. The hydraulic pressure of the wheel cylinders 6a and 6b is generated. At this time, by closing the system shut-off valve 10, the primary and secondary systems of the master cylinder generate hydraulic pressure independently.

Further, when this abnormality occurs, the stroke simulator cut valve 13 is closed to prevent the brake fluid from flowing into the stroke simulator 12, thereby reducing the loss stroke caused by the stroke simulator 12.
Here, in the present embodiment, the occurrence of an abnormality means a state in which an abnormality (failure) has occurred in any of the system power supply, the pump 9 and the stroke sensor 18.

Next, the operation of this embodiment will be described.
It is assumed that the driver depresses the brake pedal 1 and the brake operation is performed when the system is normal. In this case, the input rod 17 moves forward by the depression force of the brake pedal 1, and the depression amount of the brake pedal 1 is detected by the stroke sensor 18. Then, the pump 9 is driven in accordance with the depression amount, whereby the working fluid pressure in the reservoir 8 is sucked, and the hydraulic pressure in the wheel cylinders 6a and 6b is increased by the discharge pressure. At this time, as shown by a dotted line in FIG. 3, the wheel cylinder pressure Pw is a hydraulic pressure corresponding to a hydraulic pressure boosted, for example, 7 to 8 times the master cylinder pressure Pm. In this way, a braking force according to the brake depression amount of the driver is generated.

In this state, if an abnormality such as power shut-off occurs at time t ₀ , the shut-off valves 3a and 3b are opened, and the master cylinder 2 and the wheel cylinders 6a and 6b are in communication.
At this time, in the conventional apparatus, the high pressure on the wheel cylinder side flows into the low pressure on the master cylinder side, and the master cylinder pressure suddenly increases to the wheel cylinder pressure as shown by the broken line P ₀ in FIG. There was a problem that power increased rapidly.
In contrast, in the present embodiment, a rapid increase in the master cylinder pressure is prevented by providing the pressure reduction control unit 15 between the master cylinder 2 and the shutoff valves 3a and 3b.

As shown in FIG. 4, when an abnormality occurs, the shut-off valve 3b is opened and the master cylinder 2 and the wheel cylinder 6b are communicated with each other, and the piston 15b of the pressure reducing control unit 15 causes the wheel cylinder side force Fw (= A ₁ × Pw) stops when the master cylinder side force Fm (= A ₂ × Pm) is balanced.
In other words, the wheel cylinder side pressure is reduced and transmitted to the master cylinder side by the balance between the force Fw that receives the boosted wheel cylinder pressure with the small-diameter piston and the force Fm that receives the master cylinder pressure with the large-diameter piston. Will do. Here, the master cylinder pressure Pm = Pw / (A ₂ / A ₁ ), and the wheel cylinder side pressure is reduced according to the area ratio A ₂ / A ₁ .

For example, when A ₂ = 2A ₁ (area ratio A ₂ / A ₁ = 2), as shown by the solid line P ₁ in FIG. Without changing the amount of change.
Further, the larger the area ratio A ₂ / A ₁ , the smaller the change amount of the master cylinder pressure, and the area ratio A ₂ / A ₁ is greater than or equal to the ratio Pw / Pm between the master cylinder pressure Pm and the wheel cylinder pressure Pw (for example, If set to 8), as shown in solid lines P ₂ in FIG. 3, it is possible to eliminate the variation at the time of occurrence of an abnormality in the master cylinder pressure.

  Thus, in the first embodiment, a piston having a different pressure receiving area is provided between the master cylinder and the shutoff valve, and the pressure receiving area for receiving pressure on the master cylinder side is larger than the pressure receiving area for receiving pressure on the wheel cylinder side. Therefore, the valve that shuts off the master cylinder and the wheel cylinder is opened during braking, where the wheel cylinder side is higher than the master cylinder side, and the master cylinder and the wheel cylinder are in communication. Even in this case, the wheel cylinder pressure is reduced by the balance between the force received by the small-diameter piston and the force received by the large-diameter piston. Since it is transmitted to the master cylinder side, it is possible to suppress a sudden increase in the master cylinder pressure and prevent a sudden increase in the pedal reaction force. It is possible to prevent that give a sense of discomfort to.

  In addition, by changing the pressure receiving area ratio of the piston of the pressure reduction control unit, it is possible to change the amount of change in the master cylinder pressure when the shut-off valve is opened at the time of occurrence of an abnormality. By setting it to be equal to or higher than the boost ratio of the wheel cylinder pressure boosted with respect to the master cylinder pressure, it is possible to eliminate the change in the master cylinder pressure when an abnormality occurs.

Next, a second embodiment of the present invention will be described.
In the second embodiment, in the first embodiment described above, a bypass having an orifice is provided by bypassing the pressure reduction control unit so as to communicate the master cylinder and the shutoff valve.
That is, as shown in FIG. 5 which is a configuration diagram illustrating details of the second embodiment, a fluid path between the master cylinder 2 and the pressure reduction control unit 15, and between the pressure reduction control unit 15 and the shutoff valve 3 b. Since it has the same configuration as that of the first embodiment except that the fluid path is communicated with the bypass 21 as the first bypass and the orifice 22 is provided on the bypass 21, the detailed description thereof is omitted. To do. Thus, by providing the orifice 22 on the bypass 21, the bypass 21 is configured to increase the flow path resistance with respect to the fluid path in which the decompression control unit 15 is interposed.

As a result, in the state where the brake operation is performed by the driver and the braking force is generated when the system is normal, an abnormality such as power interruption occurs at time t ₀ in FIG. 6, and the master cylinder 2 and the wheel cylinder 6a, shut-off valve 3a that cut off and 6b, when the 3b is opened, as described above, according to the ratio a ₂ / a ₁ of the pressure receiving area of the piston 15b of the pressure reduction control section 15, a rapid master cylinder pressure Prevent the rise.

That is, when the area ratio A ₂ / A ₁ = 2 is set, the amount of change in the master cylinder pressure when an abnormality occurs is suppressed from α to β.
Thereafter, the pressure of the wheel cylinder side flows gradually master cylinder through the bypass 21 through the orifice 22, as shown by curve P ₃ in FIG. 6, the master cylinder pressure is increased gradually until the wheel cylinder pressure .
For example, when the area ratio A ₂ / A ₁ = 8 is set, the amount of change in the master cylinder pressure when an abnormality occurs is eliminated (suppressed from α to γ), and then the curve P ₄ in FIG. As shown, the master cylinder pressure gradually increases to the wheel cylinder pressure.

  As described above, in the second embodiment, the fluid path between the master cylinder and the pressure reduction control unit and the fluid path between the pressure reduction control unit and the shutoff valve are communicated with each other by bypass, and an orifice is provided on the bypass. Therefore, when the shut-off valve is opened when an abnormality occurs, the master cylinder pressure is gradually increased to the wheel cylinder pressure to prevent a sudden pressure change, so that a sudden increase in the pedal reaction force can be prevented.

  In the second embodiment, the case where the orifice is provided on the bypass has been described. However, the present invention is not limited to this, and the bypass has a flow resistance higher than that of the fluid path in which the pressure reduction control unit is interposed. A large configuration may be used, and even if an orifice is not provided, the bypass passage itself is made narrower or longer than the fluid passage in which the pressure reduction control unit is interposed, or the bypass passage itself is configured by a choke valve. May be.

In the first and second embodiments, the piston 15b of the decompression control unit 15 may be biased by a spring 15c as a biasing means. At this time, even if the piston 15b is urged toward the shut-off valve 3b as shown in FIG. 7A, or the piston 15b is urged toward the master cylinder 2 as shown in FIG. 7B. Good.
As shown in FIG. 7 (a), when energized to the shut-off valve 3b side, the piston 15b can be moved to the shut-off valve 3b side in a normal state, so there was a brake operation by the driver. Sometimes, the piston 15b can be prevented from being moved by the working fluid pressure generated by the master cylinder 2. As a result, the loss of the stroke simulator 12 can be reliably prevented.

  Further, as shown in FIG. 7B, when the piston 15b is urged toward the master cylinder 2, the piston 15b can be moved toward the master cylinder 2 in a normal state. It is possible to play the role of a stroke simulator that absorbs the working fluid pressure generated by the master cylinder 2 during the brake operation. Therefore, the stroke simulator 12 as shown in FIG. 7A can be eliminated, and the number of parts can be reduced to reduce the cost.

Next, a third embodiment of the present invention will be described.
In the third embodiment, in the second embodiment described above, a bypass having a check valve is provided so as to bypass the pressure reduction control unit and connect the master cylinder and the shutoff valve.
That is, as shown in FIG. 8, a configuration diagram for explaining details of the third embodiment is configured such that the piston 15 b of the pressure reduction control unit 15 is biased toward the shutoff valve 3 b, and the master cylinder 2 and the pressure reduction control unit 15 are Except that a fluid path between the decompression control unit 15 and the shutoff valve 3b is communicated by a bypass 23 as a second bypass, and a check valve 24 is provided on the bypass 23. Since the configuration is the same as that of the second embodiment, detailed description thereof is omitted.

The check valve 24 allows the working fluid to pass only in one direction and blocks the working fluid from the reverse direction. In this embodiment, only the passage from the master cylinder side to the wheel cylinder side is permitted. .
With such a configuration, when an abnormality occurs and the shut-off valves 3a and 3b are opened and the master cylinder 2 and the wheel cylinders 6a and 6b are in communication with each other, the driver depresses the brake pedal 1 quickly. Assuming that the working fluid passes through the bypass 23 via the check valve 24, the hydraulic pressure of the wheel cylinder can be increased without delay with respect to the brake operation by the driver.

  As described above, in the third embodiment, the fluid path between the master cylinder and the pressure reduction control unit and the fluid path between the pressure reduction control unit and the shutoff valve are communicated by bypass, and the check is performed on the bypass. When a brake operation is performed by the driver when an abnormality occurs and the shut-off valve is open, the working fluid pressure from the master cylinder quickly passes through the check valve. The hydraulic pressure of the wheel cylinder can be increased without delay with respect to the brake operation by the driver, and the response of the brake operation can be improved.

  In the third embodiment, the case where the piston 15b of the pressure reduction control unit 15 is biased toward the shut-off valve 3b has been described. However, the present invention is not limited to this and is biased toward the master cylinder 2 side. You may do it. In this case, as shown in FIG. 7B described above, the stroke simulator can be eliminated (also used), and the number of parts can be reduced to reduce the cost.

Next, a fourth embodiment of the present invention will be described.
In the fourth embodiment, the decompression control unit is integrally formed with the master cylinder.
That is, as shown in FIG. 9, the configuration diagram for explaining the details of the fourth embodiment is formed integrally with the decompression control unit 15 and the master cylinder 2, and the piston 15 b of the decompression control unit 15 is used as the secondary fluid chamber of the master cylinder 2. Except that the bypass 21 and the bypass 23 are formed so as to communicate with the pressure generation chamber 2b of the master cylinder 2 and the upstream side of the shutoff valve 3a. Since the configuration is the same as that of the first embodiment, detailed description thereof is omitted.

The cylinder 15a of the decompression control unit 15 is formed integrally with the tip of the secondary piston 2d side (opposite side of the brake pedal 1) in the cylinder body of the master cylinder 2, and the secondary piston 2d and the piston 15b of the decompression control unit 15 Thus, the pressure generating chamber 2b is formed. Then, the piston 15b is different pressure receiving area between the shut-off valve 3a side and the pressure generating chamber 2b side, the pressure receiving area A ₂ of the pressure generating chamber 2b side, larger than the pressure receiving area A ₁ of the shut-off valve 3a side Has been. The piston 15b is biased toward the pressure generating chamber 2b by a spring 15c.

  With such a configuration, when the brake pedal 1 is not depressed during normal operation of the system, the state shown in FIG. 9 is maintained, and when a brake operation is performed by the driver, the brake pedal 1 is depressed. The input rod 17 and the pistons 2c and 2d move forward to move the piston 15b of the decompression control unit 15. Thus, it plays the role of a stroke simulator that absorbs depression of the brake pedal 1.

If an abnormality such as power interruption occurs from this state and the shut-off valves 3a and 3b that shut off the master cylinder 2 and the wheel cylinders 6a and 6b are opened, as described above, the piston 15b of the pressure-reducing control unit 15 is opened. The master cylinder pressure is prevented from abruptly increasing according to the pressure receiving area ratio A ₂ / A ₁ .
At this time, since the pressure reduction control unit 15 is provided only in the secondary system and not in the primary system, the pressure reduction control unit 15 can also serve as a stroke simulator although the effect of suppressing the pedal reaction force is halved. Therefore, the number of parts can be reduced and the cost can be reduced.

As described above, in the fourth embodiment, the decompression control unit and the master cylinder are integrally formed, and the piston of the decompression control unit is disposed at the tip of the secondary liquid chamber of the master cylinder. Can be suppressed, and the decompression control unit can also serve as a stroke simulator, so that the number of parts can be reduced.
In the fourth embodiment, the case has been described in which the piston 15b of the decompression control unit 15 is biased toward the pressure generation chamber 2b so that the decompression control unit 15 also serves as a stroke simulator. The stroke simulator 12 as in the first to third embodiments described above may be provided by biasing the piston 15b toward the shutoff valve 3a. Also in this case, a sudden increase in the pedal reaction force can be suppressed by suppressing a rapid increase in the master cylinder pressure when an abnormality occurs.

In the fourth embodiment, the decompression control unit 15 is integrally formed with the cylinder body on the pressure generation chamber 2b side. However, the present invention is not limited to this, and the cylinder on the pressure generation chamber 2a side is not limited thereto. You may make it form integrally with a main body.
Furthermore, in each of the above-described embodiments, the case where the master cylinder in which two liquid chambers are defined by the secondary piston has been described. However, the present invention is not limited to this, and there is one liquid chamber or three liquid chambers. The above can also be applied.

It is a schematic structure figure showing an embodiment of the present invention. It is a figure which shows the detail of the pressure reduction control part in 1st Embodiment. It is a figure explaining the change of the master cylinder pressure at the time of abnormality occurrence. It is a figure explaining the operation | movement in 1st Embodiment. It is a figure which shows the detail of the pressure reduction control part in 2nd Embodiment. It is a figure explaining the change of the master cylinder pressure at the time of abnormality occurrence. It is a figure explaining another example of this invention. It is a figure which shows the detail of the pressure reduction control part in 3rd Embodiment. It is a figure which shows the detail of the pressure reduction control part in 4th Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Brake pedal 2 Master cylinder 3a, 3b Shut off valve 4a, 4b Normally open type solenoid valve 5a, 5b Wheel 6a, 6b Wheel cylinder 8 Reservoir 9 Pump 10 System shut off valve 11a, 11b Normally closed solenoid valve 12 Stroke simulator 13 Stroke simulator cut Valve 15 Pressure reduction control unit 17 Input rod 18 Stroke sensor 19a, 19b Wheel cylinder pressure sensor 21 Bypass 22 Orifice 23 Bypass 24 Check valve 30 Control unit

Claims (8)

  The pressure receiving area that receives the pressure on the master cylinder side of the piston interposed between the master cylinder and the wheel cylinder is set larger than the pressure receiving area that receives the pressure on the wheel cylinder side, and according to the difference in the pressure receiving area A brake control device, wherein the pressure on the wheel cylinder side is reduced and transmitted to the master cylinder side. A master cylinder that outputs a braking fluid pressure according to the depression amount of a brake pedal, a wheel cylinder that generates a braking force for braking a wheel, and an on-off valve that can open and close a communication passage that connects the master cylinder and the wheel cylinder A brake control device comprising:
A pressure reducing cylinder having a piston interposed between the master cylinder and the on-off valve and having a pressure receiving area for receiving pressure on the master cylinder side set larger than a pressure receiving area for receiving pressure on the wheel cylinder side; Brake control device.   The ratio of the pressure receiving area on the master cylinder side to the pressure receiving area on the wheel cylinder side of the piston is set to be equal to or higher than the ratio of the braking fluid pressure of the wheel cylinder to the braking fluid pressure of the master cylinder. The brake control device according to claim 2.   A first bypass that bypasses the pressure reducing cylinder and communicates the master cylinder and the open / close valve is provided, and the first bypass has a flow resistance that is greater than a communication path in which the pressure reducing cylinder is interposed. The brake control device according to claim 2 or 3, wherein the brake control device is large.   A second bypass that bypasses the pressure-reducing cylinder and communicates the master cylinder and the on-off valve is provided, and a check is performed on the second bypass to allow the brake fluid to pass only from the master cylinder to the on-off valve. The brake control device according to any one of claims 2 to 4, wherein a valve is provided.   The brake control device according to any one of claims 2 to 5, wherein the pressure-reducing cylinder includes a biasing unit that biases the piston toward the master cylinder.   A stroke simulator interposed between the pressure reducing cylinder and the master cylinder and capable of generating a reaction force according to a driver's brake operation is provided, and the pressure reducing cylinder attaches the piston to the wheel cylinder side. The brake control device according to claim 2, further comprising an urging unit that urges the brake.   The brake control device according to any one of claims 2 to 6, wherein the pressure reducing cylinder is integrally formed with a cylinder body of a liquid chamber formed in the master cylinder.

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