Master cylinder apparatus

A master cylinder apparatus includes: an input piston that can be moved forward by operating a brake operating member; a pressure piston provided in front of the input piston to be capable of moving relative to the input piston; and a stroke velocity ratio modification device capable of modifying a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages within a range not greater than a predetermined value larger than 1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a master cylinder apparatus that has a master cylinder and is included in a hydraulic brake system.

2. Description of Related Art

Japanese Patent Application Publication No. 2008-24098 (JP 2008-24098 A) discloses a master cylinder apparatus. The master cylinder includes an input piston and a pressure piston, and the pressure piston can be caused to advance relative to the pressure piston by fluid pressure from a rearward back surface chamber.

SUMMARY OF THE INVENTION

The present invention provides an improved master cylinder apparatus having a master cylinder that includes an input piston and a pressure piston. According to the invention, a brake operating member operated by a driver, for example, is improved in operability.

A master cylinder apparatus according to a first aspect of the invention includes: an input piston that is configured to move forward by operating a brake operating member; a pressure piston provided in front of the input piston and configured to move relative to the input piston; and a stroke velocity ratio modification device configured to modify a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages within a range no greater than a predetermined value larger than 1 while the input piston moves from a retreat end position to a advancement end position.

According to the aspect described above, a brake operating member operated by a driver can be improved in operability.

In the aspect described above, the stroke velocity ratio modification device may include a normal use region velocity ratio reduction unit that sets the stroke velocity ratio small when a stroke of the input piston is large, compared to the stroke velocity ratio when the stroke of the input piston is small. In an initial stage of a brake operation, the stroke velocity ratio is increased, and therefore an initial response delay in the brake can be suppressed favorably.

In the aspect described above, the input piston may be disposed opposite the pressure piston via an inter-piston chamber, the pressure piston may include a large diameter portion, and a front small diameter portion that is provided in front of the large diameter portion and has a smaller diameter than the large diameter portion, and a surface area of the large diameter portion of the pressure, piston on which pressure is received from a front side may be smaller than a surface area of the pressure piston on which pressure is received from the inter-piston chamber side. When the pressure piston is caused to advance by fluid pressure in a back surface chamber in a condition where an opposing chamber and the inter-piston chamber communicate with each other but are cut off from a reservoir, working fluid is supplied from the opposing chamber to the inter-piston chamber. When, in this case, an effective pressure receiving surface area a1of a part of the pressure piston that receives fluid pressure from the opposing chamber is smaller than an effective pressure receiving surface area a2of a part of the pressure piston that receives fluid pressure from the inter-piston chamber, advancement of the input piston is permitted, and therefore the stroke velocity ratio falls to or below a set value. Note that the effective pressure receiving surface area is the surface area of a part that actually receives fluid pressure, and takes a value (q/s) obtained by dividing volumetric change q in a space of the inter-piston chamber (the opposing chamber) capable of housing working fluid when the input piston (the pressure piston) moves by a set stroke s by the set stroke.

In the aspect described above, the input piston may be disposed opposite the pressure piston via an inter-piston chamber, the pressure piston may include a large diameter portion, a front small diameter portion that is provided in front of the large diameter portion and has a smaller diameter than the large diameter portion, and a step constituted by the large diameter portion and the front small diameter portion, and the stroke velocity ratio modification device may include a communication condition control device provided between an opposing chamber, which is provided in front of the step, and the inter-piston chamber and a reservoir, the communication condition control device being configured to control communication conditions therebetween, the communication condition control device being configured to switch between an inter-chamber connection condition in which the opposing chamber and the inter-piston chamber communicate with each other but are cut off from the reservoir, and an inter-chamber cutoff condition in which the opposing chamber is cut off from the inter-piston chamber, the inter-piston chamber is cut off from the reservoir, and the opposing chamber communicates with the reservoir. A stroke velocity ratio γa (vout/vin), which is a ratio between a stroke velocity vout of the pressure piston and a stroke velocity yin of the input piston in an inter-chamber connection condition of (i), is determined by the effective pressure receiving surface area a1of the part of the pressure piston that receives fluid pressure from the opposing chamber, the effective pressure receiving surface area a2of the part that receives fluid pressure from the inter-piston chamber, and an effective pressure receiving surface area a3of a part of the input piston that receives fluid pressure from the inter-piston chamber.
γa=a3/(a2−a1)
A stroke velocity ratio γb in an inter-chamber cutoff condition of (ii) is determined by the effective pressure receiving surface area a3of the part of the input piston that receives fluid pressure from the inter-piston chamber and the effective pressure receiving surface area a2of the pressure piston.
γb=a3/a2

A master cylinder apparatus according to a second aspect of the invention includes: an input piston that is configured to move forward by operating a brake operating member; a pressure piston that is provided coaxially with the input piston and configured to move relative to the input piston, disposed opposite the input piston via an inter-piston chamber, and has a stepped shape including a large diameter portion and a front small diameter portion that has a smaller diameter than the large diameter portion and is provided in front of the large diameter portion; and a stroke velocity ratio modification device that modifies a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages while the input piston moves from a retreat end position to an advancement end position, and includes a communication condition control device provided between an opposing chamber, which is provided in front of a step surface between the large diameter portion and the front small diameter portion, and the inter-piston chamber and a reservoir, the communication condition device being configured to control communication conditions therebetween, wherein the communication condition control device that is configured to switch between an inter-chamber connection condition in which the opposing chamber and the inter-piston chamber communicate with each other but are cut off from the reservoir, and an inter-chamber cutoff condition in which the opposing chamber is cut off from the inter-piston chamber, the inter-piston chamber is cut off from the reservoir, and the opposing chamber communicates with the reservoir, and an effective pressure receiving surface area of the pressure piston on which fluid pressure is received from the opposing chamber is smaller than an effective pressure receiving surface area of the pressure piston on which fluid pressure is received from the inter-piston chamber. According to the aspect described above, the brake operating member operated by the driver can be improved in operability.

DETAILED DESCRIPTION OF EMBODIMENTS

A hydraulic brake system including a master cylinder according to an embodiment of the invention will be described in detail below on the basis of the drawings. The hydraulic brake system includes a master cylinder apparatus according to an embodiment of the invention.

The hydraulic brake system is provided in a vehicle.FIG. 1shows an example of the hydraulic brake system according to the invention. The hydraulic brake system includes (i) brake cylinders12FL,12FR,12RL,12RR of hydraulic brakes that are provided respectively on front, rear, left, and right wheels10FL,10FR,10RL,10RR and operated by fluid pressure to suppress rotation of the respective wheels, (ii) a master cylinder apparatus13, and so on. The master cylinder apparatus13includes (a) a master cylinder14that supplies fluid pressure to the brake cylinders12FL,12FR,12RL,12RR, (b) a communication condition control device15that controls communication conditions between a reservoir and an opposing chamber and an inter-piston chamber of the master cylinder14, to be described below, (c) a servo pressure supply device18serving as a back surface fluid pressure control device that supplies regulatory fluid pressure (also referred to as servo pressure hereafter) to a back surface chamber16of the master cylinder14, and so on. Note that the communication condition control device15may be provided separately to the master cylinder14or as part of the constituent elements of the master cylinder14.

The master cylinder14includes (1) a housing20, and (2) an input piston22and two pressure pistons24,25fitted to the housing20to be fluid-tight and capable of sliding. The input piston22and the two pressure pistons24,25are disposed on an identical axis (Lm) to be capable of moving relative to each other. A brake pedal26serving as a brake operating member is linked to the input piston22via an operation rod27to be capable of advancing in response to a depression operation of the brake pedal26. Further, a return spring27ris provided between a member capable of moving integrally with the input piston22and the housing20. Pressure chambers28,29are formed respectively in front of the pressure pistons24,25. The brake cylinders12FL,12FR of the left and right front wheels10FL,10FR are connected to the pressure chamber28, and the brake cylinders12RL,12RR of the left and right rear wheels10RL,10RR are connected to the pressure chamber29. Furthermore, return springs29r,28rare provided respectively between the pressure pistons24,25and between the pressure piston24and the housing20. An inter-piston chamber30is provided between the pressure piston25and the input piston22to the rear thereof. Hence, in the hydraulic brake system according to this embodiment, the master cylinder14is a tandem type master cylinder having front and rear systems.

In the pressure piston25, a front portion is constituted by a front small diameter portion32, an intermediate portion is constituted by an intermediate large diameter portion33, and a rear portion is constituted by a rear small diameter portion34having a smaller diameter than the front small diameter portion32. The pressure piston25is formed in a stepped shape by the front small diameter portion32and the intermediate large diameter portion33. The pressure chamber29is provided in front of the front small diameter portion32. An opposing chamber38is formed in front of a step surface36between the front small diameter portion32and the intermediate large diameter portion33. The back surface chamber16is provided rearward of a step surface42serving as a pressure receiving surface between the intermediate large diameter portion33and the rear small diameter portion34. Further, the front small diameter portion32, the intermediate large diameter portion33, and the rear small diameter portion34are respectively fitted in a fluid-tight fashion to the housing20. As a result, the opposing chamber38, the back surface chamber16, the inter-piston chamber30, and the pressure chamber29are cut off from each other so as to be fluid-tight. In other words, fluid pressure can be generated individually and independently in each of the opposing chamber38, the back surface chamber16, the inter-piston chamber30, and the pressure chamber29.

In this embodiment, an effective pressure receiving surface area a1(=a1x−a1y) of the step surface36of the pressure piston25that opposes the opposing chamber38is smaller than an effective pressure receiving surface area a2of a part of the rear small diameter portion34positioned in the inter-piston chamber30(a1<a2), and the effective pressure receiving surface area a2is smaller than an effective pressure receiving surface area a3of a part of the input piston22positioned in the inter-piston chamber30(a3>a2). The effective pressure receiving surface area is a surface that substantially receives fluid pressure. More specifically, when a stroke of a piston (here, the pressure piston25and the input piston22) is set as s and volumetric change in a space of a fluid pressure chamber (here, the opposing chamber38and the inter-piston chamber30) housing working fluid is set as q, the effective pressure receiving surface area takes a value (q/s) obtained by dividing the volumetric change q by the set stroke s.

The communication condition control device15controls communication conditions between the inter-piston chamber30, the opposing chamber38, and a reservoir50. The communication condition control device15includes (i) a reservoir passage54connecting the opposing chamber38, to the reservoir50, (ii) an inter-chamber connection passage56connecting the opposing chamber38to the inter-piston chamber30, (iii) a reservoir connection valve58provided in the reservoir passage54, and (iv) an inter-chamber connection cutoff valve60provided in the inter-chamber connection passage56. The reservoir connection valve58and the inter-chamber connection cutoff valve60are respectively constituted by normally open solenoid valves that are open when a current is not supplied to respective solenoids thereof.

The servo pressure supply device18includes a regulator90, a high pressure source92, a linear valve device94, and so on. As shown inFIG. 2, the regulator90is capable of controlling the fluid pressure (servo pressure) supplied to the back surface chamber16to a magnitude corresponding to an operating force (also referred to as a brake operating force hereafter) applied to the brake pedal26using fluid pressure from the high pressure source92. The regulator90includes a housing100, a spool102fitted to the housing100to be capable of sliding, an advancement driving member104that applies force to the spool102in an advancement direction, and a retreat driving member106that applies force to the spool102in a retreat direction. The spool102, the advancement driving member104, and the retreat driving member106are respectively disposed on an identical axis (Ls) to be capable of moving relative to each other. The housing100is provided with an output port110to which the back surface chamber16is connected, an input port112to which the inter-piston chamber30is connected, a master pressure port114to which the pressure chamber29is connected, a low pressure port118to which the reservoir50is connected via a pressure reducing linear valve116, a high pressure port120to which the high pressure source92is connected, a linear pressure port124to which the high pressure source92is connected via a pressure increasing linear valve122, and a feedback pressure port126to which the back surface chamber16is connected. These ports are provided in the housing100at intervals from each other in a radial direction or a direction of the axis (Ls). An annular communication groove130extending in the axis (Ls) direction is formed in an outer peripheral portion of an intermediate portion of the spool102. The communication groove130is formed in a position and a size whereby the output port110and the linear pressure port124are open normally, the low pressure port118opens when the spool102is in a retreat end position, and the high pressure port120opens when the spool102is in an advancement end position. Fluid pressure in the output port110is controlled by moving the spool102relative to the housing100so that either the low pressure port118or the high pressure port120is connected selectively to the output port110. A return spring132is provided between the spool102and the housing100to bias the spool102in a retreat direction. Further, a rear end surface133of the spool102receives fluid pressure from the input port112.

The advancement driving member104is disposed to the rear of the spool102, and fluid pressure from the master pressure port114is received by a rear end surface134thereof. The advancement driving member104can be caused to advance by advancement direction force generated by the fluid pressure of the master pressure port114, and applies the advancement direction force generated by the master pressure to the spool102. Further, the advancement driving member104has a stepped shape including a small diameter portion and a large diameter portion, and the retreat end position is defined by contact between a step portion formed between the small diameter portion and the large diameter portion and the housing100. In this condition, a front end surface of the advancement driving member104functions as a stopper that determines the retreat end position of the spool102.

The retreat driving member106is disposed in front of the spool102via a gap, and fluid pressure in the feedback pressure port126is received by a front end surface136thereof. An elastic member140made of rubber or the like is provided on a rear portion (a main body rear portion) of the retreat driving member106, and a retainer141having a stopper function is provided in an intermediate portion so as to project in the radial direction. The advancement end position is defined by contact between the retainer141having a stopper function and the housing100. Meanwhile, a return spring142is provided between the retainer141having a stopper function and the housing100. The return spring142biases the retreat driving member106in the advancement direction. A set load Fset of the return spring142is set at a comparatively large value. The retreat driving member106can be caused to retreat by retreat direction force of a magnitude obtained by subtracting an elastic force of the return spring142from the fluid pressure of the feedback pressure port126, and applies the retreat direction force to the spool102.

The spool102, the advancement driving member104, and the retreat driving member106are respectively fitted to the housing100to be fluid-tight. As a result, the master pressure port114, the input port112, and the feedback pressure port126are cut off from each other in a fluid-tight manner. Further, a surface area of the rear end surface133of the spool102is set as Aio, a surface area of a part144obtained by subtracting a surface area of a contact portion contacting the advancement driving member104from the rear end surface133(a surface area of an annular part, or in other words a surface area of a part that receives the fluid pressure of the input port112in a condition where the spool102contacts the advancement driving member104) is set as Ai, a surface area of the rear end surface134of the advancement driving member104is set as Am, and a surface area of the front end surface136of the retreat driving member106is set as As. Furthermore, in a condition where the spool102is in the retreat end position (the spool102is located at a rearward end portion of a movable range thereof) and the retreat driving member106is in the advancement end position (a rearward end portion of a range in which the retreat driving member106can move toward the spool102), a gap x1between a rear end surface of the elastic member140provided on the retreat driving member106and a front end surface of the spool102equals or exceeds a distance x2between a rear end surface of the communication groove130and the low pressure port114(x1≧x2), and a gap x3between a main body rear end surface146of the retreat driving member106and the front end surface of the spool102equals or exceeds a distance x4between a front end surface of the communication groove130of the spool102and the high pressure port112(x3≧x4), wherein the distance x1is equal to or shorter than the distance x4(x1≦x4). The distances x1to x4are designed so that the spool102can move to a pressure increasing position in which the output port110communicates with the high pressure port120via the communication groove130before the front end surface of the spool102contacts the main body rear end surface146of the retreat driving member106, and so that in the pressure increasing position, the spool102contacts (and, in certain cases, elastically deforms) the elastic member140.

The high pressure source92includes a pump device163having a pump160and a pump motor162, an accumulator164, and an accumulator pressure sensor166that detects fluid pressure in the accumulator164. The pump160is a plunger pump, for example. The pump motor162is controlled to keep the accumulator pressure within a set range. As described above, the linear valve device94includes the pressure increasing linear valve122provided between the high pressure source92and the linear pressure port124, and the pressure reducing linear valve116provided between the low pressure port118and the reservoir50. Respective front-rear differential pressures of the pressure increasing linear valve122and the pressure reducing linear valve116can be controlled to magnitudes corresponding to amounts of current supplied to respective solenoids thereof. Further, the pressure increasing linear valve122and the pressure reducing linear valve116are normally open valves that are open when no current is supplied to the solenoids. The linear valve device94is used during an automatic brake operation such that when the brake pedal26is operated, the pressure increasing linear valve122is kept closed and the pressure reducing linear valve116is kept open. Note that, the pressure increasing linear valve122may be a normally closed valve.

Furthermore, a slip control valve device182F including at least one solenoid valve is provided between the pressure chamber28and the brake cylinders12FL,12FR of the left and right front wheels. Moreover, a slip control valve device182R including at least one solenoid valve is provided between the pressure chamber29and the brake cylinders12RL,12RR of the left and right rear wheels.

The hydraulic brake system is provided with brake ECU200(seeFIG. 1) having a computer as a main body. The brake ECU200includes an execution unit, an input/output unit, and a storage unit. The accumulator pressure sensor166, a stroke sensor210that detects an operating stroke of the brake pedal26, a depression force sensor212that detects a depression force as the operating force applied to the brake pedal26, an input fluid pressure sensor214that detects the fluid pressure in the inter-piston chamber30, and so on are connected to the input/output unit together with the reservoir connection valve58, the connection cutoff valve60, the linear valve device94, the pump motor162, and so on. A large number of programs and tables, including a solenoid valve control program, are stored in the storage unit of the brake ECU200.

An operation of this hydraulic brake system will now be described.

When a depression operation has not been performed on the brake pedal26(in a non-brake operation condition), the master cylinder14, the communication condition control device15, and the regulator90are in origin positions shown in the drawing. In the master cylinder14, the input piston22and the pressure pistons24,25are in the retreat end position, whereby the inter-piston chamber30and the pressure chambers28,29communicate with the reservoir50. In the regulator90, the output port110communicates with the low pressure port118, and the back surface chamber16communicates with the reservoir50.

[Initial Stage of Brake Operation]

When the brake pedal26is depressed, the reservoir connection valve58and the inter-chamber connection cutoff valve60of the communication condition control device15are respective set in a closed condition and an open condition, as shown inFIG. 5. In the master cylinder14, the input piston22advances, thereby cutting off the inter-piston chamber30from the reservoir50, and as a result, fluid pressure is generated. The fluid pressure of the inter-piston chamber30is supplied to the regulator90.

In the regulator90, the fluid pressure of the inter-piston chamber30is supplied from the input port112such that advancement direction force acts on the spool102. When the advancement direction force exceeds a set load of the return spring132, the spool102advances relative to the advancement driving member104. The output port110is cut off from the low pressure port118and connected to the high pressure port120. As a result, fluid pressure starts to be supplied to the back surface chamber16(a point As inFIG. 3). Since the high pressure port120communicates with the output port110, the fluid pressure in the back surface chamber16increases on a large gradient in a region RAs inFIG. 3. A position in which the output port110and the high pressure port120of the spool102communicate is available as the pressure increasing position. As described above, x1≧x2, x3≧x4, and x4≧x1are established, and therefore, when the advancement direction force acting on the spool102equals or exceeds a sum (F1+F2) of a force F1by which the return spring132can be elastically deformed by a displacement amount x4and a force F2by which the elastic member140can be elastically deformed by a displacement amount (x4−x1), the spool102is moved to the pressure increasing position {when x4=x1, F2is zero}. Further, in the pressure increasing position of the spool102, the spool102contacts the elastic member140. Note that in this embodiment, the set load and a spring constant of the return spring132and a set load and a spring constant of the elastic member140are set at small values, and therefore the spool102is moved to the pressure increasing position when the advancement direction force acting on the spool102, or in other words the fluid pressure in the inter-piston chamber30(corresponding to the brake operating force) is small.

When the spool102is in the pressure increasing position, retreat direction force Fb having a magnitude indicated by a following equation is applied to the retreat driving member106by a fluid pressure Ps of the back surface chamber16.
Fb=Ps×As−Pi×Aio(1)

In the above equation, a fluid pressure Pi is the fluid pressure of the inter-piston chamber30. The spool102contacts the retreat driving member106, and therefore advancement direction force generated by the fluid pressure in the input port112acts on the retreat driving member106via the spool102. When the retreat direction force Fb acting on the retreat driving member106exceeds the set load Fset of the return spring142(Fb>Fset), the retreat driving member106is moved in the retreat direction, and as a result, the spool102retreats. The high pressure port120is disconnected from the communication groove130, and the high pressure port120is cut off from the output port110(a point Bs inFIG. 3). A fluid pressure Psa of the back surface chamber16at this point has a magnitude indicated by a following equation.
Psa=(Fsets+Pi×Aio)/As(2)
Further, a brake operating force Fps at this point has a magnitude corresponding to the fluid pressure Pi of the inter-piston chamber30, and can be obtained in advance (hereafter, the operating force Fps will also be referred to as an initial operation completion determination operating force Fpb).

In the master cylinder14, when the advancement direction force acting on the pressure piston25exceeds a set load of the return spring29r, the pressure pistons25,24start to advance (a point Af inFIG. 4). When the pressure pistons25,24advance, the pressure chambers29,28are cut off from the reservoir50, and as a result, fluid pressure is generated. Further, the opposing chamber38and the inter-piston chamber30are in a communicative condition, and therefore, as the pressure piston25advances, working fluid is supplied from the opposing chamber38to the inter-piston chamber30. In this embodiment, the effective pressure receiving surface area a1of the pressure piston25relative to the opposing chamber38is smaller than the effective pressure receiving surface area a2thereof relative to the inter-piston chamber30(a1<a2), and therefore advancement of the input piston22is permitted even when the working fluid is supplied to the inter-piston chamber30from the opposing chamber38. As shown inFIG. 4, in a region RAf, a ratio γa (=vin/vout) between a stroke velocity yin of the input piston22and a stroke velocity vout of the pressure piston25takes a magnitude expressed by a following equation.
γa=a3/(a2−a1)  (3)
This embodiment is designed such that a3>a2>a1and a difference (a2−a1) is small. The ratio γa therefore takes a large value. Note that both a force corresponding to the fluid pressure in the inter-piston chamber30and a force corresponding to the fluid pressure in the back surface chamber16are exerted on the pressure pistons25,24, and therefore fluid pressure corresponding to the advancement direction force acting on the pressure pistons25,24is generated in the pressure chambers29,28. This embodiment is designed such that when the fluid pressure in the back surface chamber16reaches the magnitude indicated by Equation (2), the fluid pressure of the pressure chambers28,29, or in other words a fluid pressure of the brake cylinder12, reaches a set pressure Pma that equals or exceeds a fluid pressure at which a first fill is completed.

In the communication condition control device15, as shown inFIG. 5, when the brake operating force Fp detected by the depression force sensor212reaches the initial operation completion determination operating force. Fps, the inter-chamber connection cutoff valve60is closed and the reservoir connection valve58is opened. Note that a control timing of the communication condition control device15may be determined on the basis of the fluid pressure in the inter-piston chamber30, detected by the input fluid pressure sensor214, and the operating stroke of the brake pedal26, detected by the stroke sensor210, instead of the brake operating force. The fluid pressure Pi of the inter-piston chamber30, which corresponds to the servo pressure Psa, can be obtained from Equation (2). Further, the operating stroke of the brake pedal26, which corresponds to the initial operation completion determination operating force Fps, can be obtained in advance.

In the regulator90, the fluid pressure in the pressure chambers28,29increases, and when a fluid pressure Pm supplied to the master pressure port114increases, the advancement driving member104advances so as to contact the spool102. In a condition where the spool.102, the advancement driving member104, and the retreat driving member106contact each other, a force expressed by a following equation acts on the spool102.
Ps×As−(Ks×Δ+Fsets)=Pi×Ai+Pm×Am(4)
In the above equation, Pm is the fluid pressure of the pressure chamber29, Ks is a modulus of elasticity of the return spring142, and Δ is a displacement amount of the return spring142. According to the above equation, when the retreat direction force on the left side and the advancement direction force on the right side are counterbalanced, the spool102moves in the direction of the axis Ls such that the output port110communicates selectively with the high pressure port120or the low pressure port118. As a result, an increase gradient of the servo pressure Ps relative to the brake operating force. Fp (corresponding to the fluid pressure Pi of the inter-piston chamber30and the fluid pressure Pm of the pressure chamber29) is smaller in a region RBs ofFIG. 3than in the region RAs. In the master cylinder14, the inter-piston chamber30is cut off from the opposing chamber38and the reservoir50, whereas the opposing chamber38communicates with the reservoir50. As indicated by a following equation, a stroke velocity ratio γb (vout/vin) in this case is γb=a3/a2. The ratio γb is greater than 1. Note that since a force corresponding to the fluid pressure in the inter-piston chamber30and a force corresponding to the fluid pressure in the back surface chamber16act on the pressure pistons25,24, the magnitude of the fluid pressure in the pressure chambers29,28is determined by these forces. Meanwhile, the fluid pressure in the back surface chamber16has a magnitude corresponding to the brake operating force, and therefore the fluid pressure in the pressure chambers29,28also has a magnitude corresponding to the brake operating force.

[When Abnormality Occurs in Electrical System]

In the communication condition control device15, as shown inFIG. 5, when the current supply to the solenoids is stopped, the reservoir connection valve58and the connection cutoff valve60are opened. Accordingly, both the opposing chamber38and the inter-piston chamber30communicate with the reservoir50. In the regulator90, no fluid pressure is generated in the inter-piston chamber30, and therefore, in the initial stage of the brake operation, the spool102is in the position shown in the drawing. When fluid pressure is subsequently generated in the pressure chamber28such that the advancement direction force increases, the advancement driving member104advances, thereby causing the spool102to advance. The output port110is cut off from the low pressure port118and connected to the high pressure port120. The fluid pressure of the output port110is controlled while fluid pressure remains in the accumulator164, and therefore the servo pressure Ps can be supplied to the back surface chamber16. Further, even when fluid pressure can no longer be supplied from the accumulator164, working fluid can be supplied from the reservoir50to the output port110via the high pressure port120and the linear valve port124(the pressure increasing linear valve122of which is open) by an action of a check valve (a discharge valve, an intake valve) provided in the plunger pump160. In the master cylinder14, when the brake pedal26is depressed (caused to perform an advancement operation), the input piston22advances so as to contact the pressure piston25. The input piston22and the pressure piston25advance integrally, and therefore a stroke velocity ratio γc is 1. Further, by supplying the servo pressure Ps to the back surface chamber16, the fluid pressure in the pressure chambers28,29can be increased correspondingly.

[Execution of Solenoid Valve Control Program]

The reservoir connection valve58and the inter-chamber connection cutoff valve60of the communication condition control device15are controlled by executing a solenoid valve control program illustrated on a flowchart shown inFIG. 5B. In step1(abbreviated hereafter to S1; likewise for all other steps), a determination is made as to whether or not an operation to depress the brake pedal26has been performed. In this embodiment, a depression operation can be detected by determining whether or not a detection value of the stroke sensor210equals or exceeds an operation start threshold (a stroke) at which it may be determined that the brake pedal26has been depressed, whether or not a detection value of the depression force sensor212equals or exceeds an operation start threshold (an operating force) at which it may be determined that the brake pedal26has been depressed, and so on. Further, a brake switch may be provided, and the depression operation may be detected on the basis of an ON/OFF condition of the brake switch. When the depression operation of the brake pedal26is not detected, a current is not supplied to the solenoids of the reservoir connection valve58and the inter-chamber connection cutoff valve60in S2. Hence, the reservoir connection valve58and the inter-chamber connection cutoff valve60are kept open. When the depression operation of the brake pedal26is detected, a determination is made in S3as to whether or not the detection value of the depression force sensor212equals or exceeds the initial operation completion determination operating force Fps. When the detection value is smaller than the initial operation completion determination operating force Fps, the reservoir connection valve58is closed and the connection cutoff valve60is opened in S4. This condition is maintained as long as the brake operating force remains smaller than the initial operation completion determination operating force Fps, and when the brake operating force reaches or exceeds the initial operation completion determination operating force Fps, the reservoir connection valve58is opened and the connection cutoff valve60is closed in S5. Note that when an abnormality occurs in the electrical system, a current is not supplied to the solenoids, and therefore the reservoir connection valve58and the inter-chamber connection cutoff valve60remain open. Hence, in this embodiment, the communication conditions between the inter-piston chamber30, the opposing chamber38, and the reservoir50are controlled by controlling the two solenoid valves58,60.

When it is necessary to operate an automatic brake, for example during traction control, vehicle stability control, inter-vehicle control, and so on, the linear valve device94(the pressure increasing linear valve122and the pressure reducing linear valve116) of the servo pressure supply device18is controlled. The fluid pressure controlled by the linear valve device94is supplied to the back surface chamber16via the output port110, and as a result, the pressure pistons25,24advance relative to the input piston22such that fluid pressure is generated in the pressure chambers29,28.

According to this embodiment, therefore, the stroke velocity ratio in the master cylinder14while the brake pedal26moves from the retreat end position to the advancement end position takes a value larger than 1. As a result, an operating stroke by which a driver operates the brake pedal26can be reduced. Further, the stroke velocity ratio can be modified in at least two stages, i.e. the initial stage of the brake operation and the normal use region, and therefore the stroke velocity ratio is greater in the initial stage of the brake operation than in the normal use region. As a result, the operating stroke in the initial stage of the brake operation can be reduced favorably while favorably suppressing an initial response delay. Furthermore, by adjusting the operating stroke in the normal use region, the fluid pressure of the pressure chambers28,29can be regulated easily, leading to an improvement in an operating feeling experienced by the driver. Moreover, when an abnormality occurs in the electrical system, the inter-piston chamber30and the opposing chamber38can both be connected to the reservoir50, and in so doing, the stroke velocity ratio can be set at 1. As a result, an increase in the operating stroke of the driver can be suppressed even when an abnormality occurs in the electrical system.

As is evident from the above description, a stroke velocity ratio modification device is constituted by the communication condition control device15, the pressure piston25, the input piston22, parts of the brake ECU200for storing and executing the solenoid valve control program, and so on. The communication condition control device15also serves as a normal use region velocity ratio reduction unit. Further, a solenoid valve control unit is constituted by the parts of the brake ECU200for storing and executing the solenoid valve control program, and so on. Note that there are no limitations on respective structures of the regulator90and the servo pressure supply device18. Moreover, the regulator90does not necessarily have to be provided, and the fluid pressure of the back surface chamber16may be controlled by control performed by the linear valve device94. Furthermore, in the first embodiment, the stroke velocity ratio is modified between the initial stage of the brake operation and normal use region, but a modification timing is not limited thereto. For example, the stroke velocity ratio may be modified at a timing where the brake operating force reaches or exceeds a set force at which it may be determined that a large braking force is required.

The structure of the communication condition control device is not limited to the structure described in the above embodiment, and a structure shown inFIGS. 6 and 7, for example, may be employed instead. All other parts are identical to the first embodiment, and therefore description thereof has been omitted. In this embodiment, as shown inFIGS. 6 and 7A, a communication condition control device300includes (a) the reservoir connection valve58, and (b) a connection cutoff mechanism302that switches the communication conditions between the inter-piston chamber30, the opposing chamber38, and the reservoir50mechanically. The connection cutoff mechanism302includes a housing310, and a movable member312provided to be capable of sliding relative to the housing310in a direction of an axis Lt. An inter-piston chamber connection port313to which the inter-piston chamber30is connected, an opposing chamber connection port314to which the opposing chamber38is connected, a pilot pressure port316to which the fluid pressure in the back surface chamber16is supplied as pilot pressure, and a reservoir connection port318to which the reservoir50is connected are provided in the housing310at intervals in the axis Lt direction.

The movable member312has a stepped shape in which an intermediate large diameter portion330having a large diameter is provided in an intermediate portion in the axis (Lt) direction, and a first small diameter portion332and a second small diameter portion334respectively extending in the axis (Lt) direction are provided on either side of the intermediate large diameter portion330. The first small diameter portion332extends in a T direction inFIG. 7A, while the second small diameter portion334extends in a TR direction (an opposite direction to the T direction). A communication chamber340is formed on the TR direction side (the second small diameter portion side) of the intermediate large diameter portion330, and the inter-piston chamber connection port313and the opposing chamber connection port314are opened on this side. Further, an elastic member (a blocking member)342made of rubber or the like is disposed around an opening of the inter-piston chamber connection port313formed in the housing310to open onto the communication chamber340. When the second small diameter portion334contacts the elastic member342, the opening of the inter-piston chamber connection port313into the communication chamber340is blocked such that the inter-piston chamber30is cut off from the opposing chamber38. In this sense, it may be considered that the second small diameter portion334, the opening of the inter-piston chamber connection port313formed in the housing310, the elastic member342, and so on together constitute an inter-chamber connection cutoff valve.

A pilot pressure chamber343into which the pilot pressure port316opens is formed on the opposite side of the intermediate large diameter portion330to the communication chamber340(i.e. on the T direction side). Further, a step surface344between the intermediate large diameter portion330and the first small diameter portion332of the movable member312receives fluid pressure from the pilot pressure chamber343. Furthermore, a low pressure chamber346into which the reservoir connection port318opens is formed in a position opposing a T direction end surface345of the first small diameter portion332, and a connection passage348capable of connecting the low pressure chamber346to the communication chamber340(i.e. having openings into both the low pressure chamber346and the communication chamber340) is formed in the movable member312. Meanwhile, an elastic member (a blocking member)350is disposed in a position on the end surface345of the first small diameter portion332of the housing310that opposes an opening of the connection passage348. When the first small diameter portion332is separated from the elastic member350, the low pressure chamber346is connected to the communication chamber340by the connection passage348. When the first small diameter portion332contacts the elastic member350, on the other hand, the connection passage348is blocked such that the low pressure chamber346is cut off from the communication chamber340. Hence, it may be considered that the first small diameter portion332, the connection passage348, the elastic member350, and so on together constitute a reservoir cutoff valve. Note that the movable member312is fitted to the housing310to be fluid-tight by the intermediate large diameter portion330and the first small diameter portion332, and therefore the low pressure chamber346, the pilot pressure chamber343, and the communication chamber340are cut off from each other in a fluid-tight fashion. Further, a return spring352is provided between the intermediate large diameter portion330and the housing310in order to bias the movable member312in the T direction.

As shown inFIG. 7B, in the non-brake operation condition, the reservoir connection valve58is open. The movable member312is in an origin position (a T direction movement end position) shown in the drawing, and therefore the connection passage348is blocked. The communication chamber340is cut off from the low pressure chamber342, while the inter-piston chamber30and the opposing chamber38communicate via the communication chamber340. This position of the movable member312will be referred to as an inter-chamber connection position. Further, the opposing chamber38and the inter-piston chamber30communicate with the reservoir50via the reservoir connection valve58.

[Initial Stage of Brake Operation]

When the brake pedal26is depressed, the reservoir connection valve58is closed. In the connection cutoff mechanism302, the fluid pressure of the back surface chamber16is supplied to the pilot pressure chamber344such that TR direction force acts on the movable member312. As long as the TR direction force is smaller than a set load of the return spring352, the movable member312remains in the inter-chamber connection position shown in the drawing. The opposing chamber38and the inter-piston chamber30communicate with each other but are cut off from the reservoir50. This condition corresponds to the region RAf inFIG. 4. In the master cylinder14, the stroke velocity ratio γa takes a large value.

When the fluid pressure of the back surface chamber16increases such that the TR direction force exerted on the movable member312is increased beyond the set load of the return spring352by fluid pressure in the pilot pressure chamber344, the movable member312is moved in the TR direction. When the first small diameter portion332separates from the elastic member350and the second small diameter portion334contacts the elastic member342, the opening of the inter-piston chamber connection port313is blocked such that the inter-piston chamber30is cut off from the opposing chamber38. Further, the low pressure chamber346communicates with the communication chamber340via the connection passage348. As a result, the opposing chamber38communicates with the reservoir50via the connection passage348. This position of the movable member312will be referred to as an inter-chamber cutoff position. This condition corresponds to the region RBf inFIG. 4. The stroke velocity ratio in the master cylinder14shifts to γb.

[When Abnormality Occurs in Electrical System]

When the current supply to the solenoid is stopped, the reservoir connection valve58is opened. Further, when an abnormality occurs in the electrical system, the fluid pressure in the back surface chamber16cannot be raised sufficiently. Therefore, the TR direction force exerted on the movable member312cannot be increased beyond the set load of the return spring352by the fluid pressure in the back surface chamber16, and as a result, the movable member312stays in the inter-chamber connection position. The opposing chamber38and the inter-piston chamber30communicate with each other, and communicate with the reservoir50via the reservoir connection valve58. Similarly to the first embodiment, the stroke velocity ratio γc in the master cylinder14reaches 1.

Hence, according to the communication condition control device300according to the second embodiment, in the non-brake operation condition and when an abnormality occurs in the electrical system, the opposing chamber38and the inter-piston chamber30communicate with the reservoir50via the reservoir connection valve58, while in the normal use region, the opposing chamber38communicates with the reservoir50via the connection cutoff mechanism302. During the brake operation, therefore, the opposing chamber38and the reservoir50can be switched between a communicative condition and a cutoff condition without controlling the solenoid of the reservoir connection valve58. As a result, when a brake operation is performed while the electrical system is normal, the stroke velocity ratio in the master cylinder14can be modified in two stages.

The communication condition control device may also be structured as shown inFIGS. 8 and 9. All other structures are identical to the first embodiment, and therefore description thereof has been omitted. In this embodiment, a communication condition control device380includes a connection cutoff mechanism382, and a flow limitation device384provided between the reservoir50and the opposing chamber38. As shown inFIG. 9A, in the connection cutoff mechanism302according to the second embodiment, the back surface chamber16is connected to the pilot pressure port316, whereas in the connection cutoff mechanism382, the pressure chamber29is connected to the pilot pressure port316. The flow limitation device384includes (i) a check valve392that allows the working fluid to flow from the reservoir50into the opposing chamber38but prohibits the working fluid from flowing in reverse, and (ii) a relief valve390that allows the working fluid to flow from the opposing chamber38into the reservoir50when the fluid pressure in the opposing chamber38exceeds the fluid pressure in the reservoir50by at least a set relief pressure, wherein the check valve392and the relief valve390are provided in parallel. The check valve390is provided to prevent negative pressure in the opposing chamber38, and returns working fluid to the opposing chamber38from the reservoir50when the operation of the brake pedal26is released.

As shown inFIG. 9B, in the non-brake operation condition, the inter-piston chamber30and the opposing chamber38communicate with each other and are connected to the reservoir50via the flow limitation device384. Hence, the inter-piston chamber30, the opposing chamber38, and the reservoir50are substantially communicative.

[Initial Stage of Brake Operation]

Even when the brake pedal26is depressed, the movable member312stays in the inter-chamber communication condition shown in the drawing as long as the fluid pressure in the pressure chamber29remains low. Since the opposing chamber38and the inter-piston chamber30are communicative, the fluid pressure in the opposing chamber38is supplied to the inter-piston chamber30. As a result, the fluid pressure in the opposing chamber38does not increase beyond the set relief pressure, and therefore the opposing chamber38is substantially cut off from the reservoir50. This condition corresponds to the region RAf inFIG. 4.

When the fluid pressure in the pressure chamber29increases such that the TR direction force exerted on the movable member312increases beyond the set load of the return spring352, the movable member312is moved to the inter-chamber cutoff position. The opposing chamber38is cut off from the inter-piston chamber30, but communicates with the reservoir50via the connection passage348. This condition corresponds to the region RBf inFIG. 4.

[When Abnormality Occurs in Electrical System]

Even when an abnormality occurs in the electrical system, fluid pressure is generated in the pressure chambers28,29of the master cylinder14by a manual operation. As long as the fluid pressure in the pressure chamber29remains low, the movable member312stays in the inter-chamber connection position, but when the fluid pressure in the pressure chamber29increases as a result of the manual operation, the fluid pressure in the pilot pressure chamber343increases. When the TR direction force exerted on the movable member312increases beyond the set load (a set value) of the return spring352, the movable member312is moved to the inter-chamber cutoff position, and as a result, the inter-piston chamber30is cut off from the opposing chamber38. The opposing chamber38communicates with the reservoir50via the connection passage348. Since the inter-piston chamber30is closed, the stroke velocity ratio γc reaches (a3/a2), which is larger than the values thereof in the first and second embodiments.

Hence, in this embodiment, it is possible during the brake operation to switch between a condition in which the opposing chamber38and the inter-piston chamber30communicate with each other but are cut off from the reservoir50and a condition in which the opposing chamber38communicates with the reservoir50while the inter-piston chamber30is cut off from both the reservoir50and the opposing chamber38even though the communication condition control device380does not include a solenoid valve. Further, when an abnormality occurs in the electrical system, the inter-piston chamber30is cut off, and therefore the stroke velocity ratio can be set at a value larger than 1, enabling a reduction in the operating stroke of the brake pedal26. Note that the check valve392does not necessarily have to be provided, and a cap seal provided between the reservoir port of the master cylinder14and the opposing chamber38may be used instead. An example of this will be described as a fourth embodiment.

The communication condition control device may also be structured as shown inFIG. 10. As shown conceptually inFIG. 11A, a communication condition control device400includes (i) a reservoir connection valve410constituted by a solenoid valve provided between the inter-piston chamber30and the reservoir50, (ii) a connection valve412that is provided between the inter-piston chamber30and the opposing chamber38, and is switched to an open condition when the fluid pressure in the opposing chamber38is higher, thereby permitting a bidirectional flow, and switched to a closed condition when the fluid pressure in the inter-piston chamber30is higher, and (iii) a flow limitation device414provided between the opposing chamber38and the reservoir50. The flow limitation device414includes (a) a check valve416that allows the working fluid to flow from the reservoir50into the opposing chamber38but prohibits the working fluid from flowing in reverse, and (b) a relief valve418that allows the working fluid to flow from the opposing chamber38into the reservoir50when the fluid pressure in the opposing chamber38exceeds the fluid pressure in the reservoir50by at least a set relief pressure, wherein the check valve416and the relief valve418are provided in parallel. The reservoir connection valve410is a normally open valve that is open when no current is supplied to the solenoid thereof. Further, as shown inFIG. 10, in this embodiment, the connection valve412and the check valve416are provided in an interior of a master cylinder420. The check valve416serves as a cap seal provided between the reservoir50and the opposing chamber38, while the connection valve412is provided in a connection passage424formed in a rear small diameter portion422of a pressure piston421to connect the inter-piston chamber30and the opposing chamber38. All other parts are identical to the first embodiment, and therefore description thereof has been omitted.

As shown inFIG. 11B, in the non-brake operation condition, the reservoir connection valve410is open, and therefore the inter-piston chamber30communicates with the reservoir50. Further, the opposing chamber38communicates with the reservoir50either via the check valve416or via the connection valve412, the inter-piston chamber30, and the reservoir connection valve410, and therefore the opposing chamber38and the reservoir50are substantially communicative.

[Initial Stage of Brake Operation]

When the brake pedal26is depressed, the reservoir connection valve410is closed, and therefore the inter-piston chamber30is cut off from the reservoir50. Meanwhile, the fluid pressure in the back surface chamber16increases, causing the advancement direction force exerted on the pressure piston25to increase, and when the fluid pressure in the opposing chamber38increases, the working fluid is permitted to flow from the opposing chamber38into the inter-piston chamber30through the connection valve412. Accordingly, advancement of the pressure piston25is permitted. The fluid pressure in the opposing chamber38does not increase beyond the set relief pressure, and therefore the opposing chamber38is substantially cut off from the reservoir50. This condition corresponds to the region RAf inFIG. 4.

When the fluid pressure in the back surface chamber16increases such that the fluid pressure in the opposing chamber38rises beyond the set relief pressure, the working fluid flows from the opposing chamber38into the reservoir50through the relief valve418. When the fluid pressure in the inter-piston chamber30increases beyond the fluid pressure in the opposing chamber38, the connection valve412is closed, and therefore the inter-piston chamber30is cut off from both the opposing chamber38and the reservoir50. This condition corresponds to the region RBf inFIG. 4.

[When Abnormality Occurs in Electrical System]

When the current supply to the solenoid is stopped, the reservoir connection valve410is opened. Hence; the inter-piston chamber30and the opposing chamber38both communicate with the reservoir50. As a result, the input piston22and the pressure piston25are moved integrally such that the stroke velocity ratio γc reaches 1.

In this embodiment, therefore, the stroke velocity ratio can be switched in two stages in the master cylinder420using a simple structure without a connection cutoff mechanism.

The structure of the servo pressure supply device is not limited to that of the embodiments described above, and a structure shown inFIGS. 12 and 13may be employed instead. All other parts are identical to the first embodiment, and therefore description thereof has been omitted. As shown inFIGS. 12 and 13A, a servo pressure supply device450includes a regulator460, the high pressure source92, a linear valve device462, a servo fluid pressure sensor464that detects the fluid pressure in the back surface chamber16, and so on. The regulator460is provided between the back surface chamber16, the high pressure source92, the linear valve device462, and the reservoir50, and in the regulator460, the servo pressure supplied to the back surface chamber16is controlled by control performed by the linear valve device462using the fluid pressure of the high pressure source92. The regulator460includes a housing500, and a plurality of movable members502to506fitted to the housing500in series so as to be fluid-tight and capable of sliding. An output port510connected to the back surface chamber16, a high pressure port512connected to the high pressure source92, a low pressure port514connected to the reservoir50, a linear pressure port516connected to the linear valve device462, and a pilot pressure port518connected to the pressure chamber29are provided in the housing500at intervals in a direction of an axis (Lr).

The movable member502can be moved by the fluid pressure of the pilot pressure port518. A movable member504has a stepped shape including a small diameter portion520and a large diameter portion522, wherein a large diameter portion side end surface serves as a pressure receiving surface for receiving fluid pressure from the linear pressure port516, or in other words fluid pressure controlled by the linear valve device462. Thus, the movable member504can be moved by the fluid pressure controlled by the linear valve device462. An axial direction passage524and an output passage526serving as a radial direction passage are formed in a mutually communicative condition in the movable member506. The output passage526communicates with the output port510. Further, the movable member506has a stepped shape including a small diameter portion528and a large diameter portion530, wherein an annular groove portion532provided in an outer peripheral surface of the small diameter portion528to extend in a parallel direction to the axis Lr communicates with the high pressure port512. A step portion (a valve element)534between the small diameter portion528and the large diameter portion530and a step portion (a valve seat)536provided in the housing500together constitute a high pressure supply valve538. By opening and closing the high pressure supply valve538, the annular groove portion532is connected to and cut off from the output port510. The high pressure supply valve538is biased to a closed condition by a spring540provided between the movable member506and the housing500. Further, the small diameter portion520of the movable member504is positioned inside the axial direction passage524of the movable member506, whereby a step portion (a valve element)544between the small diameter portion520and the large diameter portion522of the movable member504and an opening edge portion (a valve seat)546of the axial direction passage524of the movable member506together constitute a low pressure cutoff valve548. By opening and closing the low pressure cutoff valve548, the low pressure port514is connected to and cut off from the output port510. The low pressure cutoff valve548is biased to an open condition by a spring550provided between the movable member504and the movable member506. An elastic member (a member formed from rubber, for example)552is provided between an end portion of the movable member506on an opposite side to the movable member504and the housing500. When the elastic member552undergoes elastic deformation, the movable member506is permitted to move in a direction of an arrow P (movement in a direction for switching the high pressure supply valve538to an open condition).

The linear valve device462includes a pressure increasing linear valve570provided between the high pressure source92and the linear pressure port516, and a pressure reducing linear valve572provided between the linear pressure port516and the reservoir50. Respective front-rear differential pressures of the pressure increasing linear valve570and the pressure reducing linear valve572can be controlled to magnitudes corresponding to amounts of current supplied to respective solenoids thereof. Further, the pressure Increasing linear valve570is a normally closed valve that is closed when no current is supplied to the solenoid thereof, while the pressure reducing linear valve572is a normally open valve that is open when no current is supplied to the solenoid thereof. By controlling the pressure increasing linear valve570and the pressure reducing linear valve572, the fluid pressure of the linear pressure port516is controlled to a desired magnitude. Furthermore, the fluid pressure in the pressure chamber29is supplied to the pilot pressure port518.

In the servo pressure supply device450, the currents supplied to the solenoids of the linear valve device462are controlled such that the servo pressure, or in other words the fluid pressure actually output from the output port510, which is detected by the servo pressure sensor464, approaches a target fluid pressure. By controlling the fluid pressure of the linear pressure port516, the high pressure supply valve538and the low pressure cutoff valve548are opened and closed, and as a result, the servo pressure approaches the target fluid pressure. In this embodiment, as shown inFIG. 13B, the target fluid pressure of the servo pressure is determined such that in the initial stage of the brake operation, a gain takes a large value relative to the brake operating force, whereas in the normal use region, the gain takes a small value relative to the brake operating force. An increase gradient of the fluid pressure actually output from the output port510is therefore large in the initial stage of the brake operation and smaller in the normal use region.

Note that in the regulator460, the inter-piston chamber30may be connected to the pilot pressure port518. Either the fluid pressure of the pressure chamber29or the fluid pressure of the inter-piston chamber30may be used as the pilot pressure, and in both cases, fluid pressure corresponding to the brake operating force can be used.

In this embodiment, as shown inFIG. 14, in a master cylinder600, the effective pressure receiving surface area a2of the pressure piston25relative to the inter-piston chamber30and the effective pressure receiving surface area a3of the input piston22are substantially identical (a2=a3). Therefore, as shown inFIG. 15, the stroke velocity ratio in the region RAf is smaller than that of the first embodiment, while the stroke velocity ratio the region RBf is 1. By modifying the respective magnitudes of the effective pressure receiving surface areas of the pressure piston25and the input piston22relative to the inter-piston chamber30in this manner, the stroke velocity ratio can be modified appropriately.

In this embodiment, as shown inFIG. 16, in a master cylinder650, the surface area of the step surface36of the pressure piston25, or in other words the effective pressure receiving surface area a1of the part opposing the opposing chamber38, and the effective pressure receiving surface area a2of the part opposing the inter-piston chamber30are substantially identical (a1≅a2). Therefore, as shown inFIG. 17, in the initial stage of the brake operation, the working fluid is supplied from the opposing chamber38to the inter-piston chamber30such that advancement of the input piston22is suppressed. As a result, the ratio between the stroke velocity of the input piston22and the stroke velocity of the pressure piston25becomes extremely large in a region FAf so as to be theoretically infinite. By greatly increasing the stroke velocity ratio in the initial stage of the brake operation in this manner, an initial response delay in the brake can be suppressed even further.

There are no limitations on the structure of the hydraulic brake circuit and so on, and in addition to the embodiments described above, the invention may be implemented in various other modified and amended embodiments on the basis of the knowledge of persons skilled in the art.

A period required for the input piston to move from the retreat end position to the advancement end position may be considered to mean “a single continuous operation of the brake operating member”. The stroke may be an amount by which the input piston moves from the retreat end position, and the stroke velocity may be an amount of variation in the stroke within a set time. The stroke velocity ratio may be modified in two stages, three or more stages, or continuously.

The stroke velocity ratio modification device may modify the stroke velocity ratio in the at least two stages described above while the value thereof is at least 1. By allowing the stroke velocity ratio to be modified while the value thereof is at least 1, the stroke velocity ratio may be modified while the value thereof is larger than 1, and when the stroke velocity ratio is modified while the value thereof is larger than 1, the operating stroke of the brake operating member can be reduced in comparison with a normal manual operation.

The input piston may be disposed opposite the pressure piston via the inter-piston chamber such that the effective pressure receiving surface area a3of the part of the input piston that receives the fluid pressure of the inter-piston chamber equals or exceeds the effective pressure receiving surface area a2of the part of the pressure piston that receives the fluid pressure of the inter-piston chamber (a3≧a2). In a case where the effective pressure receiving surface area of the part of the input piston that receives the fluid pressure of the inter-piston chamber exceeds the effective pressure receiving surface area of the part of the pressure piston that receives the fluid pressure of the inter-piston chamber, the stroke velocity ratio (vout/vin) takes an inverse (a3/a2) of the effective pressure receiving surface area ratio when the inter-piston chamber is cut off from both the reservoir and the opposing chamber, and therefore the stroke velocity ratio can be set at 1 or more. Note that the effective pressure receiving surface area a3of the part of the input piston that receives the fluid pressure of the inter-piston chamber may be set to be smaller than the effective pressure receiving surface area a2of the part of the pressure piston that receives the fluid pressure of the inter-piston chamber (a3<a2). In this case, the stroke velocity ratio can be set at a value smaller than 1.

The master cylinder apparatus may include a back surface chamber formed to the rear of the pressure receiving surface of the pressure piston such that the pressure piston can be caused to advance relative to the input piston by the fluid pressure of the back surface chamber. The pressure receiving surface is often provided rearward of the large diameter portion of the pressure piston.

The communication condition control device may include (i) an inter-chamber connection cutoff valve constituted by a normally open solenoid valve provided between the opposing chamber and the inter-piston chamber; (ii) a reservoir connection valve constituted by a normally open solenoid valve provided between the opposing chamber and the reservoir, and (iii) a solenoid valve control unit that controls the communication conditions between the opposing chamber, the inter-piston chamber, and the reservoir by controlling the reservoir connection valve and the inter-chamber connection cutoff valve. The communication conditions between the opposing chamber, the inter-piston chamber, and the reservoir can be controlled by controlling opening and closing of the reservoir connection valve and the inter-chamber connection cutoff valve.

The communication condition control device may include (i) a connection cutoff mechanism that is provided between the opposing chamber and the inter-piston chamber and reservoir and operated by the pilot pressure to be capable of switching between a communicative condition in which the opposing chamber and the inter-piston chamber are connected but cut off from the reservoir, and a cutoff position in which the opposing chamber communicates with the reservoir but the inter-piston chamber is cut off from both the opposing chamber and the reservoir, and (ii) a reservoir connection valve constituted by a normally open solenoid valve provided between the opposing chamber and the reservoir. The connection cutoff mechanism is switched from the communicative condition to the cutoff condition when the pilot pressure is higher than a set pressure. Either the fluid pressure of the back surface chamber or the fluid pressure of the pressure chamber may be used as the pilot pressure.

The communication condition control device may include (a) a flow limitation device having (a-i) a relief valve that allows the working fluid to flow from the opposing chamber into the reservoir when the fluid pressure in the opposing chamber exceeds the fluid pressure in the reservoir by the set relief pressure, but prevents the working fluid from flowing in reverse and (a-ii) a check valve that allows the working fluid to flow from the reservoir into the opposing chamber but prevents the working fluid from flowing in reverse, wherein the relief valve and the check valve are provided in parallel between the opposing chamber and the reservoir, and (b) a connection valve that is provided between the opposing chamber and the inter-piston chamber, switched to an open condition in which a bidirectional flow is permitted when the fluid pressure in the opposing chamber is higher than the fluid pressure in the inter-piston chamber, and switched to a closed condition when the fluid pressure in the inter-piston chamber is higher than the fluid pressure in the opposing chamber. (i) When force generated by the fluid pressure in the back surface chamber acts on the pressure piston, the fluid pressure in the opposing chamber increases. Since the working fluid is allowed to flow from the opposing chamber into the inter-piston chamber, the pressure piston is allowed to advance. Therefore, the opposing chamber and the inter-piston chamber are substantially communicative. Further, when the working fluid flows from the opposing chamber into the inter-piston chamber, the fluid pressure in the opposing chamber does not reach or exceed the set relief pressure. Therefore, the opposing chamber is substantially cut off from the reservoir. Hence, the opposing chamber and the inter-piston chamber are substantially communicative, while the opposing chamber is substantially cut off from the reservoir. (ii) When the fluid pressure in the back surface chamber increases further such that the fluid pressure in the opposing chamber increases beyond the set relief pressure, the working fluid flows out of the opposing chamber into the reservoir via the relief valve. As a result, the fluid pressure in the inter-piston chamber increases beyond the fluid pressure in the opposing chamber, whereby the connection valve is switched to the closed condition. Hence, the inter-piston chamber is substantially cut off from the opposing chamber, while the opposing chamber and the reservoir are substantially communicative. Note that a reservoir connection valve constituted by a normally open solenoid valve may be provided between the inter-piston chamber and the reservoir. Further, the check valve supplies working fluid to the opposing chamber when the operation of the brake operating member is released or the like, for example, and therefore, with the check valve, the opposing chamber is favorably prevented from entering negative pressure.

The master cylinder apparatus may include a back surface fluid pressure control device that controls the fluid pressure in the back surface chamber provided rearward of the pressure receiving surface of the pressure piston, and the back surface fluid pressure control device may include (a) a power fluid pressure source that is operated by a supply of power to be capable of outputting high-pressure fluid pressure, and (b) a regulator that controls the fluid pressure in the back surface chamber to a magnitude corresponding to an operating condition of the brake operating member using the fluid pressure output by the power fluid pressure source. The pressure piston is caused to advance by the fluid pressure in the back surface chamber, and therefore, by controlling the fluid pressure in the back surface chamber to a magnitude corresponding to the operating condition of the brake operating member, the fluid pressure of the pressure chamber can also be controlled to a magnitude corresponding to the operating condition of the brake operating member. The operating condition of the brake operating member may be represented by at least one of the operating force and the operating stroke applied to the brake operating member.

The back surface fluid pressure control device may include (i) a housing in which at least an output port connected to the back surface chamber, a high pressure port connected to the high pressure source, and a low pressure port connected to the reservoir are formed, (ii) a spool that is disposed in the housing to be capable of relative movement and can control fluid pressure output from the output port by connecting the output port selectively to the high pressure port or the low pressure port, and (iii) a regulator having a spool moving device which, when a force that acts on the spool and is determined by the operating condition of the brake operating member reaches or exceeds a predetermined set value while the spool is in a pressure increasing position in which the output port is cut off from the low pressure port and connected to the high pressure port, moves the output port to a non-pressure increasing position in which the output port is cut off from the high pressure port. While the force determined by the operating condition of the brake operating member remains smaller than the set value, or in other words in the initial stage of the brake operation, the spool is in the pressure increasing position, and therefore the fluid pressure in the back surface chamber can be increased on a large gradient.