Brake system

Abstract

To provide a brake actuator which can save the battery power.SOLUTION: A brake system includes a primary brake actuator configured to increase and decrease the hydraulic pressure generated in the master cylinder, a secondary brake actuator connected in series with and downstream of the master cylinder and in series with and upstream of the primary brake actuator, and configured to increase and decrease the hydraulic pressure generated in the master cylinder, and at least one wheel cylinder configured to apply brake torque to a wheel of a vehicle based on the hydraulic pressure generated by the master cylinder and increased or decreased by the primary actuator or the secondary actuator. An automatic brake hold control method includes sequentially performing automatic brake hold control on at least one wheel cylinder by the primary brake actuator and then by the secondary brake actuator.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention generally relates to a brake actuator connected in series. brake system with Regarding. [Background technology]

[0002] In the era of electric and hybrid electric vehicles, demands on vehicle batteries are increasing dramatically. Advances in other onboard electrical systems are also contributing to the increased demand. Furthermore, critical vehicle systems, such as brake actuators, continue to operate on battery power. Therefore, there is a need for brake actuator configurations that are configured to conserve battery power. Summary of the Invention

[0003] The brake system is At least one wheel cylinder and an adjustable hydraulic pressure supplied to the at least one wheel cylinder a primary brake actuator configured a brake actuator that is disposed on the opposite side of the at least one wheel cylinder with the primary brake actuator interposed therebetween, has the same structure as the primary brake actuator, and is capable of adjusting the hydraulic pressure supplied to the at least one wheel cylinder; a secondary brake actuator configured wherein the at least one wheel cylinder is configured to apply a brake torque to a wheel of the vehicle based on the hydraulic pressure adjusted by the primary brake actuator or the secondary brake actuator, and the primary brake actuator and the secondary brake actuator each have two input ports and four output ports, and two of the four output ports of the secondary brake actuator are connected to one of the two input ports of the primary brake actuator, and the remaining two output ports of the four output ports of the secondary brake actuator are connected to the other of the two input ports of the primary brake actuator. .

[0004] The automatic brake hold control method includes performing automatic brake hold control on at least one wheel cylinder by a primary brake actuator, and after performing automatic brake hold control on at least one wheel cylinder by the primary brake actuator, performing automatic brake hold control on at least one wheel cylinder by a secondary brake actuator. [Brief explanation of the drawings]

[0005] [Figure 1] 1 is a general schematic diagram of a braking system according to an embodiment;

[0006] [Figure 2A] FIG. 1 is a schematic diagram of a series-connected dual brake actuator according to an exemplary embodiment. [Figure 2B]FIG. 1 is a schematic diagram of a series-connected dual brake actuator according to an exemplary embodiment.

[0007] [Figure 3] 1 illustrates a first aspect of a method for automatic brake hold control according to an exemplary embodiment.

[0008] [Figure 4] FIG. 4 illustrates a second aspect of a method for automatic brake hold control according to an exemplary embodiment.

[0009] [Figure 5] FIG. 6 illustrates a third aspect of a method for automatic brake hold control according to an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010]

[0023] Referring now to the accompanying drawings, detailed descriptions of embodiments of a series-connected dual brake actuator and automatic brake hold method are provided below, which represent examples of the series-connected dual brake actuator and automatic brake hold method of the present invention.

[0011] FIG. 1 illustrates an overall schematic diagram of a vehicle braking system according to an exemplary embodiment. As illustrated, a vehicle's main ECU, configured as, for example, a programmed microcomputer operatively connected to various on-board sensors, is operatively connected in an embodiment to a primary brake actuator BRKp and a secondary brake actuator BRKs by a Controller Area Network (CAN bus), a known vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications. The main ECU may also be operatively connected by the CAN bus to, for example, engine-specific control units, among other components external to the braking system. Furthermore, a primary brake control unit ECUbp of the primary brake actuator BRKp is powered by the vehicle's primary battery BATTp, and a secondary brake control unit ECUbs of the secondary brake actuator BRKs is powered by the vehicle's secondary battery BATTs.

[0012] 2A and 2B are schematic diagrams of a series-connected dual brake actuator according to an exemplary embodiment. A brake operating member BP (e.g., a brake pedal) is operably connected to a master cylinder MC and a brake booster VB. When the brake operating member BP is depressed, a master piston provided in the master cylinder MC is compressed so that the generated depression force is boosted by the booster VB, generating equal master cylinder hydraulic pressure in first and second chambers defined within the master cylinder MC. During normal operation, the hydraulic pressure of the master cylinder is applied to wheel cylinders WCfl, WCfr, WCrl, and WCrr of corresponding front left, front right, rear left, and rear right wheels WHfl, WHrl, and WHrr of the vehicle via a primary brake actuator BRKp and a secondary brake actuator BRKs, which will be described in detail below.

[0013] The primary brake actuator BRKp includes differential pressure control valves SS1 and SS2, and the secondary brake actuator BRKs includes differential pressure control valves SS3 and SS4. The differential pressure control valves SS1, SS2, SS3, and SS4 can be independently controlled to be in a communication state or a differential pressure control state. In the communication state, brake fluid flows through the conduit in which the differential pressure control valve SS1, SS2, SS3, or SS4 is located without generating a differential pressure. In other words, the level of hydraulic pressure in the conduit upstream of the differential pressure control valve SS1, SS2, SS3, or SS4 is the same as the level of hydraulic pressure downstream of the differential pressure control valve SS1, SS2, SS3, or SS4. On the other hand, in the differential pressure generating state, a differential pressure is generated between the hydraulic pressure in the conduit upstream of the differential pressure control valve SS1, SS2, SS3, or SS4 and the hydraulic pressure in the conduit downstream of the differential pressure control valve SS1, SS2, SS3, or SS4.

[0014] Furthermore, the primary brake actuator BRKp includes pressure increase control valves SZfl, SZfr, SZrl, and SZrr, and the secondary brake actuator BRKs includes pressure increase control valves SZfls, SZfrs, SZrls, and SZrrs. The pressure increase control valves SZfl, SZfr, SZrl, SZrr, SZfls, SZfrs, SZrls, and SZrrs are two-position solenoid valves that can be independently controlled between a communication established state (i.e., a state in which brake fluid can flow through the two-position solenoid valve) and a communication cut-off state (i.e., a state in which the flow of brake fluid is cut off). In particular, when the current supplied to the pressure increase control valves SZfl, SZfr, SZrl, SZrr, SZfls, SZfrs, SZrls, or SZrrs is zero (0), in other words, when electricity is not supplied to the valve, the communication established state is established (i.e., the valve is controlled to be in an open state). On the other hand, when current is supplied to the pressure increase control valves SZfl, SZfr, SZrl, SZrr, SZfls, SZfrs, SZrls, and SZrrs, in other words, when the valves are energized, a communication cut-off state is established (i.e., the valves are controlled to be closed). In other words, each of the pressure increase control valves SZfl, SZfr, SZrl, SZrr, SZfls, SZfrs, SZrls, and SZrrs is configured as a so-called normally open valve.

[0015] Additionally, the primary brake actuator BRKp includes pressure-reducing control valves SGfl, SGfr, SGrl, and SGrr, and the secondary brake actuator BRKs includes pressure-reducing control valves SGfls, SGfrs, SGrls, and SGrrs. The pressure-reducing control valves SGfl, SGfr, SGrl, SGrr, SGfls, SGfrs, SGrls, and SGrrs are two-position solenoid valves that can be independently controlled between a communication-established state (i.e., a state in which brake fluid is allowed to flow through the two-position solenoid valve) and a communication-blocked state (i.e., a state in which the flow of brake fluid is blocked). In particular, when the current supplied to the pressure-reducing control valve SGfl, SGfr, SGrl, SGrr, SGfls, SGfrs, SGrls, or SGrrs is zero (0), in other words, when electricity is not supplied to the valve, the communication-blocked state is established (i.e., the valve is controlled to be closed). On the other hand, when current is supplied to the pressure reducing control valve SGfl, SGfr, SGrl, SGrr, SGfls, SGfrs, SGrls, or SGrrs, a communication established state is established (i.e., the valve is controlled to be in an open state). In other words, each of the pressure reducing control valves SGfl, SGfr, SGrl, SGrr, SGfls, SGfrs, SGrls, and SGrrs is configured as a so-called normally closed valve.

[0016] The primary brake actuator BRKp is a front-rear circuit system including brake circuits HP1 and HP2 and a primary brake control unit ECUbp, which is implemented, for example, as a programmed microcomputer. The brake circuit HP1 includes a conduit LA with a differential pressure control valve SS1. During normal braking, the master cylinder hydraulic pressure generated by actuation of the brake pedal BP is transmitted via this conduit to the wheel cylinders WCfl and WCfr via the secondary brake actuator BRKs. The differential pressure control valve SS1 is controlled to an open position, and a communication state is established at the differential pressure control valve SS1 when brake control of the wheel cylinders WCfl and WCfr is not performed by the primary brake actuator BRKp. When power is supplied to the differential pressure control valve SS1 by the primary brake control unit ECUbp, the differential pressure control valve SS1 is controlled to a closed position, and a differential pressure generation state is established at the differential pressure control valve SS1. A check valve CV1 is provided in parallel with the differential pressure control valve SS1 to bypass the differential pressure control valve SS1 when the hydraulic pressure upstream of the differential pressure control valve SS1 becomes greater than the hydraulic pressure downstream of the differential pressure control valve SS1.

[0017] The conduit LA branches into a conduit LAfl and a conduit LAfr downstream of the differential pressure control valve SS1. The conduit LAfl is provided with a pressure increase control valve SZfl for controlling the pressure increase of the brake hydraulic pressure supplied to the wheel cylinders WCfl. The conduit LAfr is provided with a pressure increase control valve SZfr for controlling the pressure increase of the brake hydraulic pressure supplied to the wheel cylinders WCfr. The pressure increase control valves SZfl and SZfr can be operated individually by the primary brake control unit ECUbp. Check valves cv2 and cv3 are provided in parallel with the pressure increase control valves SZfl and SZfr, respectively, to bypass the respective pressure increase control valves SZfl and SZfr when the hydraulic pressure downstream of the respective pressure increase control valves SZfl and SZfr becomes greater than the hydraulic pressure upstream of the respective pressure increase control valves SZfl and SZfr.

[0018] A conduit LB, used to reduce the brake hydraulic pressure, extends from a regulating reservoir R1 and branches into conduits LBfl and LBfr. The conduit LBfl connects the regulating reservoir R1 to a portion of the conduit LAfl extending between the pressure-increase control valve SZfl and the wheel cylinder WCfl, and the conduit LBfr connects the regulating reservoir R1 to a portion of the conduit LAfr extending between the pressure-increase control valve SZfr and the wheel cylinder WCfr. A pressure-reducing control valve SGfl is provided in the conduit LBfl, and a pressure-reducing control valve SGfr is provided in the conduit LBfr. The pressure-reducing control valves SGfl and SGfr can be independently operated by the primary brake control unit ECUbp.

[0019] A conduit LC is provided between the regulating reservoir R1 and the conduit LA, and connects to the conduit LA upstream of the branch or branch of the conduit LA into LAfl and LAfr, and downstream of the differential pressure control valve SS1. A conduit LD is provided between the regulating reservoir R1 and the master cylinder MC (via the secondary brake actuators BRKs). A hydraulic pump OP1 operatively connected to the electric motor MT1 is provided in the conduit LC. When the electric motor MT1 is energized by the primary brake control unit ECUbp and the hydraulic pump OP1 is driven by the electric motor MT1, brake fluid drawn from the regulating reservoir R1 by the hydraulic pump OP1 is discharged (via the secondary brake actuators BRKs) to the master cylinder MC or to one or both of the wheel cylinders WCfl and WCfr.

[0020] Brake circuit HP2 is similar to brake circuit HP1 except that it supplies brake fluid to wheel cylinders WCrl and WCrr. Differential pressure control valve SS2, pressure increase control valves SZrl and SZrr, pressure reduction control valves SGrl and SGrr, check valves cv4, cv5, and cv6, regulating reservoir R2, hydraulic pump OP2, and conduits LE, LErl, LErr, LF, LFrl, LFrr, LG, and LH of brake circuit HP2 are configured and function in a manner corresponding to that of differential pressure control valve SS1, pressure increase control valves SZfl and SZfr, pressure reduction control valves SGfl and SGfr, check valves cv1, cv2, and cv3, regulating reservoir R1, hydraulic pump OP1, and conduits LA, LAfl, LAfr, LB, LBfl, LBfr, LC, and LD of brake circuit HP1.

[0021] The secondary brake actuator BRKs is a front-rear circuit system including brake circuits HP3 and HP4 and a secondary brake control unit ECUbs, which is implemented, for example, as a programmed microcomputer. The brake circuit HP3 includes a conduit LI with a differential pressure control valve SS3. During normal braking, the master cylinder hydraulic pressure generated by actuation of the brake pedal BP is transmitted to the brake circuit HP1 of the primary brake actuator BRKp via this conduit. The differential pressure control valve SS3 is controlled to an open position, and a communication state is established at the differential pressure control valve SS3 when brake control of the wheel cylinders WCfl and WCfr is not performed by the secondary brake actuator BRKs. When power is supplied to the differential pressure control valve SS3 by the secondary brake control unit ECUbs, the differential pressure control valve SS3 is controlled to a closed position, and a differential pressure generation state is established at the differential pressure control valve SS3. A check valve CV7 is provided in parallel with the differential pressure control valve SS3 to bypass the differential pressure control valve SS3 when the hydraulic pressure upstream of the differential pressure control valve SS3 becomes greater than the hydraulic pressure downstream of the differential pressure control valve SS3.

[0022] Downstream of the differential control valve SS3, the conduit LI branches into conduits Llfls and Llfrs, which rejoin the circuit HP1 upstream. The conduits Llfl and Llfr are provided with pressure increase control valves SZfls and SZfrs for controlling the pressure increase of the brake hydraulic pressure supplied to the brake circuit HP1. The pressure increase control valves SZfls and SZfrs are provided in parallel and are therefore energized in tandem by the secondary brake control unit ECUbs. Check valves cv8 and cv9 are provided in parallel with the pressure increase control valves SZfls and SZfrs, respectively, to bypass the respective pressure increase control valves SZfls and SZfrs when the hydraulic pressure downstream of the respective pressure increase control valves SZfls and SZfrs becomes greater than the hydraulic pressure upstream of the respective pressure increase control valves SZfls and SZfrs.

[0023] The conduit LJ, used to reduce the brake hydraulic pressure, extends from the regulating reservoir R3 and branches into conduits LJfl and LJfr. The conduit LJfl connects the regulating reservoir R3 to a portion of the conduit Llfl extending between the pressure increase control valve SZfls and the rejoining of the conduits Llfl and LIfr, and the conduit LJfr connects the regulating reservoir R3 to a portion of the conduit Llfr extending between the pressure increase control valve SZfrs and the rejoining of the conduits Llfl and Llfr. A pressure reduction control valve SGfls is provided in the conduit LJfl, and a pressure reduction control valve SGfrs is provided in the conduit LJfr. The pressure reduction control valves SGfls and SGfrs are provided in parallel and are therefore energized in tandem by the secondary brake control unit ECUbs.

[0024] A conduit LK is provided between the regulating reservoir R3 and the conduit Ll, and connects to the conduit Ll upstream of the branch or branch of the conduit Ll into Llfl and Llfr, and downstream of the differential pressure control valve SS3. A conduit LL is provided between the regulating reservoir R3 and the master cylinder MC. A hydraulic pump OP3 operatively connected to an electric motor MT2 is provided in the conduit LK. When the electric motor MT2 is energized by the secondary brake control unit ECUbs and the hydraulic pump OP3 is driven by the electric motor MT2, brake fluid drawn from the regulating reservoir R3 by the hydraulic pump OP3 is discharged to the master cylinder MC or to the brake circuit HP1 of the primary brake actuator BRKp.

[0025] Brake circuit HP4 is similar to brake circuit HP3 except that it supplies brake fluid to brake circuit HP2 of primary brake actuator BRKp. Differential pressure control valve SS4, pressure increase control valves SZrls and SZrrs, pressure reduction control valves SGrls and SGrrs, check valves cv10, cv11, and cv12, regulating reservoir R4, hydraulic pump OP4, and conduits LM, LMrl, LMrr, LN, LNrl, LNrr, LO, and LP of brake circuit HP4 are configured and function in a manner corresponding to that of differential pressure control valve SS3, pressure increase control valves SZfls and SZfrs, pressure reduction control valves SGfls and SGfrs, check valves cv7, cv8, and cv9, regulating reservoir R3, hydraulic pump OP3, and conduits Ll, ​​Llfl, Llfr, LJ, LJfl, LJfr, LK, and LL of brake circuit HP3.

[0026] In an embodiment, the secondary brake actuators BRKs have the same physical structure as the primary brake actuators BRKp. This allows economies of scale to be achieved by manufacturing both brake actuators BRKp and BRKs of the brake system together, thereby reducing costs. However, in an alternative embodiment, the secondary brake actuators may have a different structure in which the conduits corresponding to conduits LI, LJ, LM, and LN are not branched and only have a single pressure increase control valve and pressure decrease control valve per brake circuit.

[0027] The main ECU is operatively connected to the primary brake control unit ECUbp and causes the primary brake actuator BRKp to individually regulate the hydraulic pressure in the wheel cylinders WHfl, WHfr, WHrl, and WHrr, for example, during anti-skid control (ABS control), traction control (TCS control), or electronic stability control (ESC control) in a known manner by providing appropriate currents to the differential pressure control valves SS1 and SS2, the pressure increase control valves SZfl, SZfr, SZrl, and SZrr, the pressure decrease control valves SGfl, SGfr, SGrl, and SGrr, and the motor MT1 for driving the hydraulic pumps OP1 and OP2. Furthermore, the main ECU is operatively connected to both the primary brake control unit ECUbp and the secondary brake control unit ECUbs and causes the primary brake actuators BRKp and the secondary brake actuators BRKs to perform automatic brake hold, as discussed in detail below.

[0028] Additionally, while one of the primary or secondary actuators BRKp or BRKs is in use, the other can perform self-diagnosis. Also, if one of the primary or secondary actuators BRKp or BRKs fails, the other actuator can be used for emergency braking of the vehicle, if necessary. Additionally, the primary or secondary actuators BRKp or BRKs can be used in place of an electronic parking brake (EPB) or as a backup parking brake function in the event of an EPB failure. Furthermore, the secondary actuator BRKs can perform a basic form of ABS control, for example, where hydraulic pressure is reduced at the front wheels as a pair or at the rear wheels as a pair if the primary actuator BRKp is disabled.

[0029] The main ECU monitors the need for automatic brake hold control by monitoring the vehicle's operation. When the vehicle brakes to a stop and the driver releases the brake pedal, the main ECU determines to automatically hold the vehicle stationary. When the driver takes action to resume operation (for example, by pressing the accelerator pedal), the brake torque is released and full driver control is restored.

[0030] When the primary brake actuator BRKp starts automatic brake hold control, the differential pressure control valves SS1 and SS2 enter the differential pressure control state, and the corresponding pressure increase control valves SZfl, SZfr, SZrl, or SZrr in each wheel cylinder WCfl, WCfr, WCrl, and WCrr are energized, and the hydraulic pumps OP1 and OP2 (and motor MT1) operate until the wheel cylinder hydraulic pressure reaches an appropriate level to maintain the wheel cylinder hydraulic pressure.

[0031] Similarly, when the secondary brake actuator BRKs starts automatic brake hold control, the differential pressure control valves SS3 and SS4 enter the differential pressure control state, and the corresponding pressure increase control valve pairs (SZfls / SZfrs or SZrls / SZrrs) in each wheel cylinder pair WCfl / WCfr and WCrl / WCrr are energized, and the hydraulic pumps OP3 and OP4 (and motor MT2) operate until the wheel cylinder hydraulic pressure reaches an appropriate level to maintain the wheel cylinder hydraulic pressure.

[0032] As discussed above, it is advantageous to distribute the use of the primary battery BATTp and the secondary battery BATTs. Thus, in an exemplary embodiment, when automatic brake-hold control is executed, the main ECU determines which brake actuator last executed the automatic brake-hold control and selects another brake actuator to initiate the automatic brake-hold control. Furthermore, automatic brake-hold switching is performed by the main ECU, where after a predetermined time (e.g., two or three minutes) during which one actuator is used for automatic brake-hold control, the other actuator is used for automatic brake-hold control.

[0033] 3-5 illustrate an exemplary control method according to the foregoing. For example, normal braking mode, in which brake pressure is determined by brake pedal force / master cylinder hydraulic pressure, is entered in step S2 after a normal vehicle start (step S1) or after automatic brake-hold is stopped by the primary actuator in step S9 or by the secondary actuator in step S15, as discussed below. If a command to start automatic brake-hold is not received (step S3), normal brake-hold is maintained; otherwise, a countdown timer (a predetermined time, e.g., two or three minutes) is started (step S4), and it is determined whether the last automatic brake-hold control was performed by the primary brake actuator (step S5). If the last automatic brake-hold control was not performed by the primary brake actuator, automatic brake-hold by the primary actuator is initiated (step S6). Otherwise, automatic brake-hold by the secondary actuator is initiated (step S7).

[0034] After the primary actuator initiates automatic brake-hold (step S6, or step S18 discussed below), it is determined whether a command to stop automatic brake-hold has been received (step S8) or whether the countdown timer determines that a predetermined time has elapsed (step S10). If it is determined that a command to stop automatic brake-hold has been received (step S8), automatic brake-hold by the primary actuator is stopped (step S9) and normal braking mode is resumed (step S2). If the countdown timer determines that the predetermined time has elapsed (step S10), the countdown timer is restarted (step S11), automatic brake-hold by the secondary actuator is initiated (step S12), and automatic brake-hold by the primary actuator is stopped (step S13).

[0035] After the automatic brake-hold by the secondary actuator is initiated (step S7 or step S12), it is determined whether a command to stop the automatic brake-hold has been received (step S14) or whether the countdown timer has determined that a predetermined time has elapsed (step S16). If it is determined that a command to stop the automatic brake-hold has been received (step S14), the automatic brake-hold by the secondary actuator is stopped (step S15), and the vehicle returns to normal braking mode (step S2). If the countdown timer determines that the predetermined time has elapsed (step S16), the countdown timer is restarted (step S17), the automatic brake-hold by the primary actuator is initiated (step S18), and the automatic brake-hold by the secondary actuator is stopped (step S19).

[0036] The above detailed description describes embodiments of a series-connected dual brake actuator and an automatic brake hold method. However, the present invention is not limited to the exact embodiments and variations described. Various changes, modifications, and equivalents may be made by those skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims. All such changes, modifications, and equivalents that fall within the scope of the claims are expressly intended to be embraced by the claims.

Claims

1. A braking system comprising: at least one wheel cylinder; a primary brake actuator configured to adjust the hydraulic pressure supplied to the at least one wheel cylinder; a secondary brake actuator that is disposed on the opposite side of the at least one wheel cylinder across the primary brake actuator, has the same structure as the primary brake actuator, and is configured to be able to adjust the hydraulic pressure supplied to the at least one wheel cylinder; the at least one wheel cylinder is configured to apply a brake torque to a wheel of the vehicle based on the hydraulic pressure adjusted by the primary brake actuator or the secondary brake actuator; the primary brake actuator and the secondary brake actuator each have two input ports and four output ports; two of the four output ports of the secondary brake actuator are connected to one of the two input ports of the primary brake actuator, and the remaining two of the four output ports of the secondary brake actuator are connected to the other of the two input ports of the primary brake actuator. Brake system.

2. 2. The braking system of claim 1, wherein the primary brake actuator is powered by a first battery and the secondary brake actuator is powered by a second battery different from the first battery.

3. The braking system of claim 1 , wherein the primary brake actuator and the secondary brake actuator are each configured to implement automatic brake hold control.

4. 2. The braking system of claim 1, wherein the primary brake actuator and the secondary brake actuator each include a pump configured to pump brake fluid to increase or decrease the hydraulic pressure applied to the at least one wheel cylinder.

5. 2. The braking system of claim 1, wherein the at least one wheel cylinder comprises four wheel cylinders, and the primary brake actuator is configured to individually increase or decrease the hydraulic pressure applied to each of the four wheel cylinders.

6. 2. The braking system of claim 1, wherein the at least one wheel cylinder comprises two pairs of two wheel cylinders, and the secondary brake actuator is configured to individually increase or decrease the hydraulic pressure applied to each of the two pairs of two wheel cylinders.

7. 2. The braking system of claim 1, wherein the primary brake actuator and the secondary brake actuator each comprise at least one differential pressure control valve.

8. 2. The braking system of claim 1, wherein said primary brake actuator and said secondary brake actuator each include at least one normally open pressure increase control valve.

9. 2. The braking system of claim 1, wherein said primary brake actuator and said secondary brake actuator each include at least one normally closed pressure reducing control valve.

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