Vehicle braking system with brake pedal unit

Abstract

A brake pedal unit includes a housing defining a hole formed therein. An input piston is slidably disposed in the hole. The input piston is connected to a brake pedal so that engagement of the brake pedal causes the input piston to move within the hole of the housing of the brake pedal unit. The brake pedal unit is defined to be in an idle position when the brake pedal is not engaged causing the input piston to move. The primary piston is slidably disposed in the hole to pressurize a primary chamber. The primary passage allows fluid communication between the primary chamber and a reservoir, wherein when the brake pedal unit is in the idle position, fluid flow through the primary passage is blocked. The secondary piston is slidably disposed in the hole to pressurize the secondary chamber. The secondary passage allows fluid communication between the secondary chamber and the reservoir, wherein when the brake pedal unit is in the idle position, fluid flow through the secondary passage is blocked.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 592,929, filed on November 30, 2017, the disclosure of which is incorporated herein by reference. Background of the Invention

[0004] The present invention generally relates to vehicle braking systems. Vehicles are typically slowed and stopped using hydraulic braking systems. These systems vary in complexity, but a basic braking system typically includes a brake pedal, a tandem master cylinder, fluid conduits arranged in two similar but independent brake circuits, and wheel brakes in each circuit. The driver of the vehicle operates a brake pedal connected to the master cylinder. When the brake pedal is depressed, the master cylinder generates hydraulic pressure in the two brake circuits by pressurizing the brake fluid. The pressurized fluid travels through the fluid conduits in the two circuits to actuate the brake cylinders at the wheels, thereby slowing the vehicle.

[0005] Basic braking systems typically use a brake booster that provides force to the master cylinder to assist the pedal force generated by the driver. The booster can be vacuum or hydraulically operated. A typical hydraulic booster senses the movement of the brake pedal and produces a pressurized fluid that is introduced into the master cylinder. The fluid from the booster assists the pedal force on the pistons of the master cylinder, which produce pressurized fluid in conduits that are in fluid communication with the wheel brakes. As a result, the pressure generated by the master cylinder is increased. The hydraulic booster is usually located adjacent to the master cylinder piston and uses a booster valve to control the pressurized fluid applied to the booster.

[0006] Braking a vehicle in a controlled manner under adverse conditions requires the driver to apply the brakes precisely. Under these conditions, the driver may tend to apply excessive brake pressure, thereby causing one or more wheels to lock, resulting in excessive slip between the wheels and the road surface. Such wheel locking conditions may result in greater stopping distances and possible loss of directional control.

[0007] Advances in braking technology have led to the introduction of anti-lock braking systems (ABS). ABS systems monitor wheel rotation behavior and selectively apply and release brake pressure in corresponding wheel brakes to maintain wheel speed within a selected slip range to achieve maximum braking force. While such systems are typically adapted to control the braking of each braked wheel of a vehicle, some systems have been developed to control the braking of only a portion of the multiple braked wheels.

[0008] Electronically controlled ABS valves (including apply valves and dump valves) are located between the master cylinder and the wheel brakes. The ABS valves regulate the pressure between the master cylinder and the wheel brakes. Typically, when enabled, these ABS valves operate in the following three pressure control modes: pressure apply, pressure dump, and pressure hold. The apply valve allows the pressurized brake fluid of the corresponding wheel brake of these wheel brakes to increase in pressure during the apply mode, while the dump valve releases the brake fluid from its associated wheel brake during the dump mode. During the hold mode, the wheel brake pressure is kept constant by closing both the apply valve and the dump valve.

[0009] In order to achieve maximum braking force while maintaining vehicle stability, it is desirable to achieve an optimal slip level at the wheels of both the front and rear axles. During vehicle deceleration, different braking forces are required at the front and rear axles to achieve the desired slip level. Therefore, the brake pressure should be distributed proportionally between the front and rear brakes to achieve the maximum braking force at each axle. ABS systems with such capabilities, known as Dynamic Rear Proportional (DRP) systems, use ABS valves to control the brake pressure on the front and rear wheels separately, thereby dynamically achieving the best braking performance at the front and rear axles under the current conditions.

[0010] Further developments in braking technology have led to the introduction of traction control (TC) systems. Typically, valves have been added to existing ABS systems to provide a braking system that controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration can cause wheel slip and loss of traction. The electronic control system senses this condition and automatically applies brake pressure to the wheel cylinders of the slipping wheel to reduce slip and increase available traction. To achieve optimal vehicle acceleration, pressurized brake fluid is made available to the wheel cylinders even if the driver has not actuated the master cylinder.

[0011] During vehicle manoeuvres such as cornering, dynamic forces are generated which may reduce the stability of the vehicle. The Vehicle Stability Control (VSC) brake system improves the stability of the vehicle by counteracting these forces with selective brake actuation. These forces and other vehicle parameters are detected by sensors which send signals to the electronic control unit. The electronic control unit automatically operates a pressure control device to regulate the amount of hydraulic pressure applied to specific individual wheel brakes. For optimum vehicle stability, a brake pressure greater than the master cylinder pressure must always be rapidly available.

[0012] The braking system can also be used for regenerative braking to regain energy. The electromagnetic force of the electric motor / generator is used for regenerative braking to provide a portion of the braking torque to the vehicle to meet the braking needs of the vehicle. The control module in the braking system communicates with the powertrain control module to provide coordinated braking during regenerative braking and braking for wheel locking and slip conditions. For example, when the operator of the vehicle starts braking during regenerative braking, the electromagnetic energy of the motor / generator will be used to apply braking torque to the vehicle (that is, electromagnetic resistance is used to provide torque to the powertrain). If it is determined that there is no longer a sufficient amount of storage device to store the energy recovered from regenerative braking, or if regenerative braking cannot meet the operator's requirements, hydraulic braking is enabled to complete all or part of the braking action required by the operator. Preferably, the hydraulic brake operates in a regenerative braking compound mode so that the compound is effectively and insignificantly implemented in the case of insufficient electromagnetic braking (left off). It is expected that the vehicle movement should have a smooth transition change to hydraulic braking so that the conversion is not noticed by the driver of the vehicle.

[0013] The braking system may also include autonomous braking capabilities, such as adaptive cruise control (ACC). During an autonomous braking event, various sensors and systems monitor the traffic conditions in front of the vehicle and automatically enable the braking system as needed to slow the vehicle. Autonomous braking can be configured to respond quickly to avoid emergency situations. The braking system can be enabled without the driver depressing the brake pedal or even if the driver fails to apply sufficient pressure to the brake pedal. Advanced autonomous braking systems are configured to operate the vehicle without any driver input and rely solely on various sensors and systems that monitor the traffic conditions around the vehicle.

[0014] exist Figure 1 1 shows a schematic diagram of a prior art master cylinder or brake pedal unit (generally indicated by 10). The brake pedal unit 10 is an example of a prior art brake pedal unit used in the above-mentioned braking system. The brake pedal unit 10 includes a housing having a multi-step bore 12 formed therein. An input piston 14, a primary piston 16, and a secondary piston 18 are slidably disposed in the bore 12. The input piston 14 is connected to a brake pedal (not shown) via a link arm 20. Under certain conditions, leftward movement of the input piston 14, the primary piston 16, and the secondary piston 18 can cause pressure increases in the input chamber 24, the primary chamber 26, and the secondary chamber 28, respectively. The various seals of the brake pedal unit 10 and the structure of the housing and the pistons 14, 16, and 18 define the chambers 24, 26, and 28. For example, the input chamber 24 is generally defined between the input piston 14 and the primary piston 16. The primary chamber 26 is generally defined between the primary piston 16 and the secondary piston 18. The secondary chamber 28 is generally defined between the secondary piston 18 and an end wall 30 of the housing formed by the bore 12 .

[0015] The input chamber 24 is selectively in fluid communication with a pedal simulator (not shown) to simulate force feedback on the brake pedal to the vehicle driver. The cylindrical outer wall of the input piston 14 engages a lip seal 32 and a pair of seals 34 and 36. The seal 36 acts as a secondary seal with the seal 34 and provides an additional layer of leakage protection so that fluid does not leak out of the brake pedal unit 10 from the input chamber 24. A fluid passage 38 (or multiple passages) is formed through the wall of the input piston 14. Figure 1 As shown, when the brake pedal unit 10 is in its rest position (the driver is not depressing the brake pedal), the passage 38 is located between the lip seal 32 and the seal 36. In the rest position, the passage 38 permits fluid communication between the input chamber 24 and the fluid reservoir (not shown). It should be noted that in the rest position of the brake pedal unit 10, the lip seal 32 is located to the left of the passage 38, thereby permitting fluid communication between the input chamber 24 and the fluid reservoir. During initial operation of the brake pedal unit 10, as observed Figure 1 , sufficient leftward movement of the input piston 14 will cause the passage 38 to move past the lip seal 32, thereby preventing fluid from flowing into the reservoir from the input chamber 24. Further leftward movement of the input piston 14 will pressurize the input chamber 24, thereby causing fluid to flow into the pedal simulator.

[0016] Under certain conditions, the primary chamber 26 and the secondary chamber 28 are each in fluid communication with a pair of wheel brakes (not shown), and when a pressure buildup occurs in the first pressure chamber 26 and the second pressure chamber 28, these chambers provide pressurized fluid to the wheel brakes. The outer wall of the primary piston 16 engages with a lip seal 40 and a seal 42 mounted in a groove formed in the housing. A fluid passage 44 (or multiple passages) is formed through the wall of the primary piston 16. When the primary piston 16 is in its rest position, the passage 44 is located between the lip seal 40 and the seal 42. It should be noted that in the rest position of the brake pedal unit 10, the lip seal 40 is located to the left of the passage 44, thereby permitting fluid communication between the primary chamber 26 and the reservoir. The outer wall of the secondary piston 18 engages with a lip seal 46 and a seal 48 mounted in a groove formed in the housing. A fluid passage 50 (or multiple passages) is formed through the wall of the secondary piston 18. When the secondary piston 18 is in its rest position, the passage 50 is located between the lip seal 46 and the seal 48. It should be noted that in the rest position of the brake pedal unit 10, the lip seal 50 is located to the left of the passage 50, thereby permitting fluid communication between the secondary chamber 28 and the fluid reservoir.

[0017] The brake pedal unit 10 includes an input spring assembly 52 that is generally disposed between the input piston 14 and the primary piston 16. The main spring assembly 54 is disposed between the primary piston 16 and the secondary piston 18. The secondary spring assembly 56 is disposed between the secondary piston 18 and the bottom wall of the housing. The input spring assembly 52, the main spring assembly 54, and the secondary spring assembly 56 serve as a cage spring assembly to bias the pistons 14, 16, and 18 away from each other and act to properly position the pistons 14, 16, and 18 within the housing of the brake pedal unit 10. The cage spring assembly includes a structure that limits and defines the length of the cage spring assembly, thereby positioning the pistons at a predetermined distance relative to each other. A spring preload may be present in the spring member of the cage spring assembly such that an initial force is required to compress the spring member of the cage spring assembly. The brake pedal unit 10 further includes a return spring 58, as observed Figure 1 , the return spring biases the input piston 14 in the right direction. It should be noted that the caged secondary spring assembly 56 is provided with a gap 90 between the enlarged head portion 62 of the post 64 and the retainer 66 in the rest position of the brake pedal unit 10.

[0018] In the idle position of the brake pedal unit 10, all three chambers 24, 26 and 28 are in fluid communication with the fluid reservoir via corresponding passages 38, 44 and 50. During operation of the brake pedal unit 10, the driver depresses the brake pedal, causing the input piston 14 to move to the left. Fluid from the input chamber 24 is discharged to the fluid reservoir until the passage 38 slides past the lip seal 32. Further leftward movement of the input piston 14 causes the pressure in the input pressure chamber 24 to increase, causing actuation of the pedal simulator. It should be noted that when the input piston 14 moves, the fluid from all three pressure chambers 24, 26 and 28 to the reservoir is cut off at the same time because the corresponding passages 38, 44 and 50 slide past the corresponding lip seals 32, 40 and 46. Under normal braking conditions, pressure from the primary chamber 26 and the secondary chamber 28 is prevented from causing any fluid to flow to the wheel brakes, thereby hydraulically locking the primary chamber 26 and the secondary chamber 28 and preventing further movement of the primary piston 16 and the secondary piston 18. Instead, another source of pressurized fluid is utilized to provide controlled fluid pressure to actuate the wheel brakes. However, under certain fault conditions of the braking system, the brake pedal unit 10 can be used to provide pressurized fluid to the wheel brakes by permitting fluid from the primary chamber 26 and the secondary chamber 28 to be directed to the wheel brakes.

[0019] Although the design of the brake pedal unit 10 functions properly, a relatively large initial force is required by the driver to actuate the brake pedal unit. Large forces from the spring arrangement and seal friction must first be overcome. This may give the driver undesirable pedal feel characteristics. Although the brake pedal unit 10 may be suitable for trucks or larger vehicles, it may be undesirable for smaller passenger vehicles. Summary of the invention

[0020] The present invention relates to a master cylinder or brake pedal unit, which is connected to a brake pedal and is in selective fluid communication with a fluid reservoir. The pedal unit includes a housing, which defines a hole formed therein. An input piston is slidably disposed in the hole. The input piston is connected to the brake pedal so that engagement of the brake pedal causes the input piston to move within the hole of the housing of the brake pedal unit. The brake pedal unit is defined to be in an idle position when the brake pedal is not engaged to cause the input piston to move. The primary piston is slidably disposed in the hole to pressurize a primary chamber. A primary passage allows fluid communication between the primary chamber and the reservoir, wherein when the brake pedal unit is in the idle position, fluid flow through the primary passage is blocked. A secondary piston is slidably disposed in the hole to pressurize a secondary chamber. A secondary passage allows fluid communication between the secondary chamber and the reservoir, wherein when the brake pedal unit is in the idle position, fluid flow through the secondary passage is blocked.

[0021] Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic diagram of a prior art brake pedal unit.

[0023] Figure 2 is a schematic diagram of an embodiment of a brake system according to the present invention.

[0024] Figure 3 is Figure 2 Schematic cross-sectional view of a brake pedal unit of a braking system.

[0025] Figure 4 yes Figure 2 Schematic cross-sectional view of a pedal simulator for a braking system.

[0026] Figure 5 yes Figure 2 Schematic cross-sectional view of a plunger assembly of a braking system.

[0027] Figure 6 yes Figure 1 and Figure 5 A graphical representation of the pedal force versus pedal travel of a brake pedal unit during its operation. DETAILED DESCRIPTION

[0028] Now referring to the accompanying drawings, Figure 2 An embodiment of a vehicle braking system (generally indicated by 100) is schematically illustrated. The braking system 100 is a hydraulic power-assisted braking system in which power-assisted fluid pressure is utilized to apply braking force to the braking system 100. The braking system 100 can be suitably used on a ground vehicle, such as a motor vehicle having four wheels and wheel brakes associated with each wheel. In addition, the braking system 100 can be provided with other braking functions, such as anti-lock braking (ABS) and other anti-skid control features to effectively brake the vehicle, as discussed below. The components of the braking system 100 can be housed in one or more blocks or housings. The blocks or housings can be made of a solid material, such as aluminum, which has been drilled, machined or otherwise formed to accommodate various components. Fluid conduits can also be formed in the blocks or housings.

[0029] In the illustrated embodiment of the braking system 100, there are four wheel brakes 120a, 120b, 120c and 120d. The wheel brakes 120a, 120b, 120c and 120d can have any suitable wheel brake structure operated by applying a pressurized brake fluid. The wheel brakes 120a, 120b, 120c and 120d can include, for example, brake calipers, which are mounted on the vehicle to engage friction elements (such as brake discs) that rotate with the vehicle wheels to achieve braking of the associated vehicle wheels. The wheel brakes 120a, 120b, 120c and 120d can be associated with any combination of the front wheels and rear wheels of the vehicle on which the braking system 100 is installed. For example, the braking system 100 can be configured as a vertical split system, so that the front pressure circuit is associated with providing fluid to the wheel brakes 120a and 120b, and the rear pressure circuit is associated with providing fluid to the wheel brakes 120c and 120d. In this example, wheel brake 120a may be associated with the left front wheel of the vehicle on which brake system 100 is installed, and wheel brake 120b may be associated with the right front wheel. Wheel brake 120c may be associated with the left rear wheel, and wheel brake 120d may be associated with the right rear wheel. Alternatively, brake system 100 may be configured as a diagonally split brake system such that wheel brakes 120a and 120d are associated with wheels at opposite corners of the vehicle, while wheel brakes 120b and 120c are associated with other opposite corners of the vehicle.

[0030] The braking system 100 generally includes a brake pedal unit (generally indicated by 130), a pedal simulator (generally indicated by 132), a plunger assembly (generally indicated by 134), and a fluid reservoir 136. The reservoir 136 stores and contains hydraulic fluid for the braking system 100. The fluid in the reservoir 136 is preferably maintained at atmospheric pressure or approximately atmospheric pressure, but the fluid can be stored at other pressures if desired. The reservoir 136 is schematically shown, and the reservoir has three tanks or sections and three fluid conduit lines connected thereto. These sections can be separated by several inner walls within the reservoir 136, and are configured to prevent the reservoir 136 from being completely emptied when one of the sections is exhausted due to leakage through one of the three lines connected to the reservoir 136. Alternatively, the reservoir 136 may include multiple separate housings. The reservoir 136 may include a fluid level sensor 138 for detecting the fluid level of one or more sections of the reservoir 136.

[0031] As will be discussed in more detail below, the plunger assembly 134 of the brake system 100 is used as a pressure source to provide a desired pressure level to the wheel brakes 120a, 120b, 120c, and 120d during a typical or normal brake application. After the brakes are applied, the fluid from the wheel brakes 120a, 120b, 120c, and 120d can be returned to the plunger assembly 134 and / or diverted to the reservoir 136. In a preferred embodiment, the plunger assembly 134 is a double-acting plunger assembly, so that it is configured to provide boost pressure to the brake system 100 when the piston of the plunger assembly 134 completes the stroke both backward and forward, as will be described in more detail below.

[0032] The braking system 100 further includes an electronic control unit or ECU 140. The ECU 140 may include a microprocessor and other electrical circuitry. The ECU 140 receives various signals, processes the signals, and controls the operation of various electrical components of the braking system 100 in response to the received signals. The ECU 140 may be connected to various sensors, such as a reservoir fluid level sensor 138, a pressure sensor, a travel sensor, a switch, a wheel speed sensor, and a steering angle sensor. The ECU 140 may also be connected to an external module (not shown) to receive information related to the yaw rate, lateral acceleration, and longitudinal acceleration of the vehicle, such as for controlling the braking system 100 during vehicle stability operations. In addition, the ECU 140 may be connected to a dashboard to collect and provide information related to warning indicators (e.g., ABS warning lights, brake fluid level warning lights, and traction control / vehicle stability control indicator lights).

[0033] The brake system 100 further includes a first isolation valve 150 and a second isolation valve 152. The isolation valves 150 and 152 may be solenoid-actuated three-way valves. The isolation valves 150 and 152 are generally operable to two positions, such as Figure 2 Schematically illustrated. The first isolation valve 150 and the second isolation valve 152 each have a port in selective fluid communication with an output conduit 154, which is generally in communication with an output end of the plunger assembly 134, as discussed below. The first isolation valve 150 and the second isolation valve 152 also include ports in fluid communication with conduits 156 and 158, respectively, when the first isolation valve 150 and the second isolation valve 152 are de-energized, and the conduits 156 and 158 are connected to the brake pedal unit 130, as discussed below. Figure 2 The first isolation valve 150 and the second isolation valve 152 further include ports in fluid communication with conduits 160 and 162, respectively, which provide fluid to and from the wheel brakes 120a, 120b, 120c, and 120d.

[0034] In a preferred embodiment, the first isolation valve 150 and / or the second isolation valve 152 can be mechanically designed so that when in its de-energized position, flow is permitted to flow in the opposite direction (from the output conduit 154 to the conduits 156 and 158, respectively), and can bypass the normally closed valve seats of the valves 150 and 152. Therefore, although the 3-way valves 150 and 152 are not schematically shown to indicate such a fluid flow position, it should be noted that the valve design can permit such a fluid flow. This can facilitate performing self-diagnostic testing of the brake system 100.

[0035] The system 100 further includes a plurality of different solenoid actuated valves (slip control valve arrangements) to permit controlled braking operations such as ABS, traction control, vehicle stability control, dynamic rear ratio, regenerative brake compounding, and autonomous braking. A first set of valves includes a first apply valve 170 and a first dump valve 172 in fluid communication with conduit 160 to cooperatively supply fluid received from the first isolation valve 150 to the front wheel brakes 120a and cooperatively release pressurized fluid from the wheel brakes 120a to a reservoir conduit 173 in fluid communication with the reservoir 136. A second set of valves includes a second apply valve 174 and a second dump valve 176 in fluid communication with conduit 160 to cooperatively supply fluid received from the first isolation valve 150 to the wheel brakes 120b and cooperatively release pressurized fluid from the wheel brakes 120b to the reservoir conduit 173. The third set of valves includes a third apply valve 178 and a third dump valve 180 in fluid communication with conduit 162 to cooperatively supply fluid received from the second isolation valve 152 to wheel brake 120c and cooperatively release pressurized fluid from wheel brake 120c to reservoir conduit 173. The fourth set of valves includes a fourth apply valve 182 and a fourth dump valve 184 in fluid communication with conduit 162 to cooperatively supply fluid received from the second isolation valve 152 to wheel brake 120d and cooperatively release pressurized fluid from wheel brake 120d to reservoir conduit 173. It should be noted that in normal braking events, fluid flows through the de-energized open apply valves 170, 174, 178, and 182. In addition, dump valves 172, 176, 180, and 184 are preferably in their de-energized closed positions to prevent fluid from flowing to reservoir 136.

[0036] The brake pedal unit 130 is connected to the brake pedal 190 and is actuated by the driver when the driver of the vehicle depresses the brake pedal 190. A brake sensor or switch 192 may be connected to the ECU 140 to provide a signal indicating that the brake pedal 190 is depressed. As will be discussed below, the brake pedal unit 130 may be used as a backup source of pressurized fluid to substantially replace the normal supply source of pressurized fluid from the plunger assembly 134 under certain fault conditions of the brake system 100. This situation is referred to as an artificial push event. The brake pedal unit 130 may supply pressurized fluid to conduits 156 and 158 (conduits 156 and 158 are normally closed at the first isolation valve 150 and the second isolation valve 152 during normal brake application) to the wheel brakes 120a, 120b, 120c, and 120d as needed.

[0037] As schematically shown in FIG. 3 , the brake pedal unit 130 includes a housing having a multi-step hole 200 formed therein for slidably receiving various cylindrical pistons and other components therein. It should be noted that the housing is not specifically schematically shown in FIG. 3 , but the wall of the hole 200 is shown. The housing can be formed as a single unit, or include two or more separately formed parts connected together. An input piston 202, a primary piston 204, and a secondary piston 206 are slidably disposed in the hole 200. The input piston 202 is connected to the brake pedal 190 via a connecting rod arm 208. Under certain conditions, the leftward movement of the input piston 202, the primary piston 204, and the secondary piston 206 can cause the pressure in the input chamber 210, the primary chamber 212, and the secondary chamber 214 to increase, respectively. The various seals of the brake pedal unit 130 and the structure of the housing and the pistons 202, 204, and 206 define the chambers 210, 212, and 214 accordingly. For example, the input chamber 210 is generally defined between the input piston 202 and the primary piston 204. The primary chamber 212 is generally defined between the primary piston 204 and the secondary piston 206. The secondary chamber 214 is generally defined between the secondary piston 206 and an end wall 216 of the housing formed by the bore 200. The primary piston 204 and the secondary piston 206 define a pair of output pistons of the brake pedal unit 130. The primary chamber 212 and the secondary chamber 214 define a pair of output ends of the brake pedal unit 130.

[0038] The input chamber 210 is in fluid communication with the pedal simulator 132 via a conduit 218, for reasons that will be explained below. The input piston 202 is slidably disposed in a bore 200 of the housing of the brake pedal unit 130. The cylindrical outer wall 219 of the input piston 202 engages a lip seal 220 and a seal 222 mounted in a groove formed in the housing. A fluid passage 224 (or passages) is formed through the wall of the input piston 202. Figure 2 3, when the brake pedal unit 130 is in its rest position (the driver is not depressing the brake pedal 190), the passage 224 is located between the lip seal 220 and the seal 222. In the rest position, the passage 224 permits fluid communication between the input chamber 210 and the reservoir 136 via the conduit 226.

[0039] Return to reference Figure 1 The brake system 100 may further include an optional solenoid-actuated simulator test valve 227 that may be used in Figure 2The simulator test valve 227 is electronically controlled between the open position shown and the closed position achieved with power. The simulator test valve 227 is not required during normal brake application or for the manual boost mode. The simulator test valve 227 can be actuated to the closed position during various test modes to determine the correct operation of other components of the brake system 100. For example, the simulator test valve 227 can be actuated to the closed position to prevent discharge to the reservoir 136 via the conduit 226, so that the pressure established in the brake pedal unit 130 can be used to monitor fluid flow to determine whether leakage through the seals of the various components of the brake system 100 may occur.

[0040] During initial operation of the brake pedal unit 130, as viewed in Figure 3, sufficient leftward movement of the input piston 202 will cause the passage 224 to move past the lip seal 220, thereby preventing fluid from flowing from the input chamber 210 into the conduit 226 and into the reservoir 136. Further leftward movement of the input piston 202 pressurizes the input chamber 210, thereby causing fluid to flow into the pedal simulator 132 via the conduit 218. As the fluid is diverted into the pedal simulator 132, the pedal simulator 132 is actuated to provide a feedback force to the driver of the vehicle via the brake pedal 190 that simulates the force felt by the driver at the brake pedal 190, for example, in a conventional vacuum-assisted hydraulic braking system.

[0041] Now refer to Figure 4, an embodiment of the pedal simulator 132 includes a housing defining a hole 230. A cup-shaped piston 232 is slidably disposed in the hole 230. The piston 232 is sealingly engaged with the wall of the hole 230 by a seal 234. A pressure chamber 235 is defined by the hole 230 and the piston 232. The pressure chamber 235 is in fluid communication with the input chamber 210 of the brake pedal unit 130 via a conduit 218. The pedal simulator 132 may include a first spring 236 and a second spring 238, the second spring having a relatively low spring stiffness compared to the first spring 236. In the example shown, the second spring 238 has a preload value of about 3N. A tubular retainer 240 is disposed between the first spring 236 and the second spring 238, so that the first spring and the second spring act on each other via the retainer 240. The pedal simulator 132 may further include a spring washer assembly 242. The spring washer assembly 242 may include one or more conical washer springs having a relatively high spring stiffness. Of course, spring washer assembly 242 can include any suitable type of spring design, such as wave spring, Belleville washer or (a plurality of) elastomer pads. In the illustrated embodiment, spring washer assembly 242 includes a pair of conical washers 243 and 244. After first spring 236 is fully compressed and retainer 240 has moved enough distances to close the gap, the right end of retainer 240 will begin to compress conical spring washer assembly 242, compressing springs 236 and 238 simultaneously. This arrangement helps to cause nonlinear gradual spring stiffness characteristics, to obtain the desired force feedback to the driver.

[0042] As discussed above, the simulation pressure chamber 235 of the pedal simulator 132 is in fluid communication with the conduit 218, which is in fluid communication with the input chamber 210 of the brake pedal unit 130. Figure 2 As shown, a solenoid-actuated simulator valve 246 is positioned within conduit 218 for selectively preventing fluid from flowing from input chamber 210 to simulation pressure chamber 235, such as during a fault condition in which brake pedal unit 130 is being used to provide a pressurized fluid source to wheel brakes 120a, 120b, 120c and 120d.

[0043] The braking system 100 may further include a check valve 247 that is in a parallel path arrangement with a flow restriction orifice 248 in the conduit 118. The check valve 247 and the flow restriction orifice 248 may be integrally constructed or formed in the simulator valve 246, or may be formed separately from the simulator valve. The flow restriction orifice 248 provides damping during the peak application of the driver's rapid and forceful depressing of the brake pedal 190. The damping provides force feedback so that depressing the brake pedal 190 feels more like a traditional vacuum booster, which may be a desirable characteristic of the braking system 100. By substantially avoiding too much brake pedal travel for the vehicle deceleration that can be delivered by the braking system 100, the damping may also provide a more accurate relationship between the brake pedal travel and the vehicle deceleration. The check valve 247 provides an easy flow path and allows the brake pedal 190 to return quickly, which allows the associated brake pressure to be quickly reduced according to the driver's intention.

[0044] The main chamber 212 of the brake pedal unit 130 is in fluid communication with the second isolation valve 152 via the conduit 158. The main piston 204 is slidably disposed in the bore 200 of the housing of the brake pedal unit 130. The outer wall 249 of the main piston 104 engages with a lip seal 250 (main seal) and a seal 252 mounted in a groove formed in the housing. One or more passages 254 are formed through the wall of the main piston 204. Unlike the arrangement of the input piston 202, when the brake pedal unit 130 is in its idle position (the driver is not depressing the brake pedal 190), the passage 254 is just to the left of the lip seal 250. As shown in FIG. 3, the passage 254 is spaced apart from the lip seal 250 by a relatively small distance D of approximately 1. P This position prevents fluid communication between the main chamber 212 and the reservoir 136 via the conduit 265. Note that with respect to the input piston 202, the passage 224 is spaced apart from the lip seal 220 by a slightly greater distance D I It should be noted that the schematic diagram of FIG. 3 is not drawn to scale and that dimensions are shown for ease of illustration.

[0045] The secondary chamber 214 of the brake pedal unit 130 is in fluid communication with the first isolation valve 150 via the conduit 156. Referring now to FIG. 3 , the secondary piston 206 is slidably disposed in the bore 200 of the housing of the brake pedal unit 130. The outer wall 259 of the secondary piston 206 engages a lip seal 260 (secondary seal) and a seal 262 mounted in a groove formed in the housing. One or more passages 264 are formed through the wall of the secondary piston 206. Similar to the arrangement of the primary piston 204, when the brake pedal unit 130 is in its rest position (the driver is not depressing the brake pedal 190), the passage 264 is just to the left of the lip seal 260. As shown in FIG. 3 , the passage 264 is spaced apart from the lip seal 260 by a relatively small width D of approximately 1.5 mm.S This position prevents fluid communication between the secondary chamber 214 and the reservoir 136 via the conduit 266 .

[0046] Lip seals 220, 250 and 260 may have any suitable sealing structure. Lip seals 250 and 260 may be designed so that when the pressure within chambers 212 and 214 drops below atmospheric pressure (the pressure within reservoir 136), fluid may flow into primary chamber 212 and secondary chamber 214, respectively, via conduits 265 and 266 in a direction away from reservoir 136. This may occur even if brake pedal unit 130 is in its idle position. This may be caused by a slight leak or a change in fluid volume in the wheel brakes.

[0047] In an alternative embodiment, lip seals similar to lip seals 220, 250 and / or 260 may be mounted on pistons 202, 204 and / or 206, respectively. Then, passages similar to passages 224, 254 and 264 will be formed in the housing of the brake pedal unit and in fluid communication with reservoir 136.

[0048] If necessary, the primary piston 204 and the secondary piston 206 can be mechanically connected with limited movement therebetween. The mechanical connection of the primary piston 204 and the secondary piston 206 prevents a large gap or distance between the primary piston 204 and the secondary piston 206. This prevents pedal travel loss by preventing the primary piston 204 and the secondary piston 206 from having to advance a relatively large distance without increasing pressure during a fault system event. For example, if the brake system 100 is in manual boost mode and fluid pressure is lost in the front circuit (such as in the conduit 156) relative to the secondary piston 206 (secondary chamber 214), the secondary piston 206 will be forced or biased in the left direction due to the greater pressure in the primary chamber 212. If the primary piston 204 and the secondary piston 206 are not connected together, the secondary piston 206 will freely travel to its leftmost position, as when observing Figure 3, and the driver will have to press the pedal 190 for a distance to compensate for this travel loss. However, since the primary piston 204 and the secondary piston 206 are connected together, such movement of the secondary piston 206 is prevented, and the stroke loss that occurs in this type of failure is relatively small.

[0049] Any suitable mechanical connection between the primary piston 204 and the secondary piston 206 may be used. For example, as schematically shown in FIG3 , the right end of the secondary piston 206 includes an outwardly extending flange 270 that extends into a groove 272 formed in an inner wall 274 of the primary piston 204. The width of the groove 272 is greater than the width of the flange 270, thereby providing a relatively small amount of travel between the primary piston 204 and the secondary piston 206 relative to each other.

[0050] The brake pedal unit 130 further includes a return spring 286 that biases the input piston 202 in the right direction when viewing FIG. 3 . The input spring 288 is disposed around an axial rod column 290 formed in the input piston 202 and engages with a washer 292 that is in direct contact with a shoulder 294 formed in the right end of the primary piston 212. The axial rod column 290 extends into a hole 296 formed in the right end of the primary piston 212. An elastomeric pad 298 is disposed in the hole 296 and when the input piston 210 moves a sufficient distance toward the primary piston 212, the elastomeric pad will engage with an enlarged head 300 formed at the end of the axial rod column 290. The compression of the elastomeric pad 298 by the head 300 of the rod column 290 provides a desired spring rate characteristic. The enlarged head 300 is spaced apart from the washer 292 and the shoulder 294 by a certain gap 302. It should be noted that the washer 292 actually engages and contacts the shoulder 294 and is not spaced apart from the shoulder as shown in the schematic diagram of FIG3. For clarity, the schematically shown very small gap between the washer 292 and the shoulder 294 is shown so that the lines are not drawn on top of each other. This schematic diagram is correct for other components of the brake pedal unit 130 that actually contact each other but are shown as slightly out of contact with each other.

[0051] The brake pedal unit 2702 further includes a main spring 304 generally disposed between the secondary piston 206 and the primary piston 204. The main spring 304 is disposed within the bore 274 and engages with a retainer 306 that forms a cage spring assembly configuration in which an axial rod post 308 extends from the bottom of the bore 274 of the primary piston 204. The retainer 306 is constrained by an enlarged head 310 formed on the end of the axial rod post 308.

[0052] The brake pedal unit 2702 further includes a secondary spring 312 generally disposed between the secondary piston 206 and the bottom wall 216 of the bore 200. The secondary spring 312 is disposed within a bore 314 formed in the left end of the secondary piston 206 and engages with a retainer 316 that forms a cage spring assembly configuration in which an axial rod post 318 extends from the bottom of the bore 314 of the secondary piston 206. The retainer 316 is restrained by an enlarged head 320 formed on the end of the axial rod post 318. It should be noted that in the idle position, the enlarged head 320 contacts the end of the retainer 316 so that there is substantially no gap therebetween.

[0053] like Figure 2As shown, the brake system 100 may further include a pressure sensor 330 in fluid communication with the conduit 156 for detecting the pressure within the secondary pressure chamber 214 of the brake pedal unit 130 and transmitting a signal indicating the pressure to the ECU 140. The brake system 100 further includes a pressure sensor 332 in fluid communication with the output conduit 154 for transmitting a signal indicating the pressure at the output end of the plunger assembly 134. The ECU 140 utilizes the signals from the pressure sensors 330 and 332 to actuate the brake system 100 under various braking events.

[0054] In a preferred embodiment of the brake system 100 , the brake pedal unit 130 includes a pair of travel sensors 340 (one redundant) for generating and providing signals indicative of the stroke length and / or stroke rate of the input piston 202 to the ECU 140 .

[0055] Now refer to Figure 5 , schematically illustrates an enlarged view of the plunger assembly 134. The plunger assembly 134 includes a housing having a multi-step bore 400 formed therein. Figure 5 400. The housing is not specifically shown schematically, but the wall of the bore 400 is shown. The bore 400 includes a first portion 402 and a second portion 404. A piston 406 is slidably disposed within the bore 400. The piston 406 includes an enlarged end portion 408 connected to a central portion 410 of smaller diameter. The piston 406 has a second end 411 connected to a ball screw mechanism (generally indicated by 412). The ball screw mechanism 412 is provided for moving the piston 406 in the bore 400 of the housing along the axis defined by the bore 400 in a forward direction (as viewed from above). Figure 5 left) and backward (when observing Figure 5 In the illustrated embodiment, the ball screw mechanism 412 includes an electric motor, schematically and generally indicated at 414, which is electrically connected to the ECU 140 for actuation thereof. The motor 414 rotatably drives the screw shaft 416. The motor 414 generally includes a stator 415 and a rotor 417. Figure 5 In the illustrated exemplary embodiment, the rotor 417 and the shaft 416 are integrally formed together. The second end 411 of the piston 406 includes a threaded hole 420 and serves as a driven nut of the ball screw mechanism 412. The ball screw mechanism 412 includes a plurality of balls 422, which are retained in a spiral raceway 423 formed in the screw shaft 416 and in the threaded hole 420 of the piston 406 to reduce friction.

[0056] Although the ball screw mechanism 412 is shown and described with respect to the plunger assembly 134, it should be understood that other suitable mechanical linear actuators may be used to cause the piston 406 to move. It should also be understood that although the piston 406 serves as a nut for the ball screw mechanism 412, the piston 406 may be configured to serve as a screw shaft for the ball screw mechanism 412. Of course, in this case, the screw shaft 416 is configured to serve as a nut having an internal spiral raceway formed therein.

[0057] The piston 406 may include structure that engages with cooperating structure formed in the housing of the plunger assembly 134 to prevent the piston 406 from rotating when the screw shaft 416 rotates around the piston 406. For example, the piston 206 may include outwardly extending splines or tabs 426 (see Figure 5 ), these splines or tabs are arranged in longitudinally extending grooves 428 formed in the housing of the plunger assembly 134, so that these tabs 426 slide in the grooves 428 when the piston 406 travels in the hole 400.

[0058] As will be discussed below, the plunger assembly 134 is preferably configured to provide pressure to the output conduit 154 when the piston 406 moves in the forward or rearward direction. The plunger assembly 134 includes a seal 430 mounted on the enlarged end portion 408 of the piston 406. When the piston 406 moves in the bore 400, the seal 430 is slidably engaged with the cylindrical inner surface of the first portion 402 of the bore 400. The seal 434 and the seal 436 are mounted in a groove formed in the second portion 404 of the bore 400. The seals 434 and 436 are slidably engaged with the cylindrical outer surface of the central portion 410 of the piston 406. The first pressure chamber 440 is generally defined by the first portion 402 of the bore 400, the enlarged end portion 408 of the piston 406, and the seal 430. An annular second pressure chamber 442 located generally rearward of the enlarged end portion 408 of the piston 406 is generally defined by the first and second portions 402, 404 of the bore 400, seals 430 and 434, and the central portion 410 of the piston 406. The seals 430, 434, and 436 may have any suitable sealing structure.

[0059] Although the plunger assembly 134 can be configured to any suitable size and arrangement, in one embodiment, the effective hydraulic area of ​​the first pressure chamber 440 is greater than the effective hydraulic area of ​​the annular second pressure chamber 442. The first pressure chamber 440 generally has an effective hydraulic area corresponding to the diameter of the central portion 410 of the piston 406 (the inner diameter of the seal 434) because the fluid is diverted through the output conduit 154 and conduits 443 and 454 when the piston 406 advances in the forward direction. The second pressure chamber 442 generally has an effective hydraulic area corresponding to the diameter of the first portion 402 of the bore 400 minus the diameter of the central portion 410 of the piston 406. Generally, if the annular area is smaller than the diameter of the central portion 410, this configuration provides that during the rear stroke of the piston 406 moving backward, the motor 414 requires less torque (or power) to maintain the same pressure as during its forward stroke. In addition to using less power, the motor 414 can also generate less heat during the rearward stroke of the piston 406. In the case of high brake pressure desired, the plunger assembly 134 can be operated from a forward stroke to a rearward stroke. Thus, although the forward stroke is used in most braking applications, a rearward pressure stroke can be utilized. In addition, in the case of a driver depressing the pedal 190 for a long duration, the brake system 10 can be operated by controlling the first plunger valve 450 and the second plunger valve 452 (as discussed below) to a closed position to maintain brake pressure (rather than continuously energizing the plunger assembly 134), and then shutting down the motor or plunger assembly 134.

[0060] The plunger assembly 134 preferably includes a sensor (schematically shown as 418) to indirectly detect the position of the piston 406 within the bore 400. The sensor 418 communicates with the ECU 140. In one embodiment, the sensor 418 detects the rotational position of the rotor 417, which may have a metal element or a magnetic element embedded therein. Since the rotor 417 is integrally formed with the shaft 416, the rotational position of the shaft 416 corresponds to the linear position of the piston 406. Therefore, the position of the piston 406 can be determined by sensing the rotational position of the rotor 417 via the sensor 418.

[0061] The piston 406 of the plunger assembly 134 includes a passage 444 formed therein. The passage 444 defines a first port 446 extending through the cylindrical outer wall of the piston 406 and in fluid communication with the secondary chamber 442. The passage 444 also defines a second port 448 extending through the cylindrical outer wall of the piston 406 and in fluid communication with a portion of the bore 400 located between the seals 434 and 436. The second port 448 is in fluid communication with a conduit 449, which is in fluid communication with the reservoir 136. When in an idle position (e.g., Figure 2 and Figure 5443). This helps ensure proper release of pressure at the output end of the plunger assembly 34 and within the pressure chambers 440 and 442 themselves. After the piston 406 initially moves forward from its rest position, the port 448 moves past the lip seal 434, thereby disconnecting the pressure chambers 440 and 442 from fluid communication with the reservoir 136, thereby permitting the pressure chambers 440 and 442 to build pressure as the piston 406 moves further.

[0062] Return to reference Figure 2 The braking system 100 further includes a first plunger valve 450 and a second plunger valve 452. The first plunger valve 450 is preferably a normally closed solenoid actuated valve. Therefore, in the unpowered state, the first plunger valve 450 is in a closed position, such as Figure 2 As shown. The second plunger valve 452 is preferably a normally open solenoid actuated valve. Therefore, in the unpowered state, the second plunger valve 452 is in an open position, such as Figure 2 As shown. A check valve may be arranged within the second plunger valve 452 so that when the second plunger valve 452 is in its closed position, fluid may still flow through the second plunger valve 452 in a direction from the first output conduit 454 (from the first pressure chamber 440 of the plunger assembly 134) to the output conduit 154, thereby leading to the isolation valves 150 and 152. It should be noted that during the rearward stroke of the piston 406 of the plunger assembly 134, pressure may be generated within the second pressure chamber 442 to be output into the output conduit 154. The brake system 100 further includes a check valve 451 that permits fluid to flow in a direction away from the conduit 449 (away from the reservoir 136) to the conduit 454 and into the first pressure chamber 440 of the plunger assembly 134, such as during the rearward pressure generating stroke of the piston 406.

[0063] In general, the first plunger valve 450 and the second plunger valve 452 are controlled to allow fluid flow at the output end of the plunger assembly 134 and to allow discharge to the reservoir 136 through the plunger assembly 134 when necessary. For example, the first plunger valve 450 is preferably energized to its open position during normal braking events. In addition, it is preferred that the first plunger valve 450 and the second plunger valve 452 are both kept open (this can reduce noise during operation). Preferably, the first plunger valve 450 is almost always energized during the ignition cycle when the engine is running. Of course, the first plunger valve 450 and the second plunger valve 452 can be purposefully operated to their closed positions, such as during the backward pressure generating stroke of the plunger assembly 134 or during the ramp anti-skid braking operation. When the piston 406 of the plunger assembly 134 operates in its forward stroke to maximize the flow, the first plunger valve 450 and the second plunger valve 452 are preferably in their open positions. When the driver releases the brake pedal 190, the first plunger valve 450 and the second plunger valve 452 preferably maintain their open positions. However, in some cases, such as during slip control and during controlled low pressure, when the driver pushes the brake pedal 190 hard and then the driver releases the brake pedal 190 halfway, it may be desirable to operate the first plunger valve 450 and the second plunger valve 452 to their closed positions. It should be noted that fluid can flow through the check valve within the closed second plunger valve 452, as well as through the check valve 451 from the reservoir 136, depending on the direction of travel of the piston 406 of the plunger assembly 134 and the state of the first plunger valve 450 and the second plunger valve 452.

[0064] It may be desirable that the first plunger valve 450 is configured to have a relatively large orifice passing through it when in its open position. The relatively large orifice of the first plunger valve 450 helps provide a simple flow path through it. The second plunger valve 452 can be provided with a much smaller orifice than the first plunger valve 450 when in its open position. One reason for doing this is to help prevent the piston 406 of the plunger assembly 134 from being quickly reversed in a fault event caused by fluid rushing through the first output conduit 454 into the first pressure chamber 440 of the plunger assembly 134, thereby preventing damage to the plunger assembly 134. Since the fluid is restricted when it flows through the relatively small orifice, dissipation occurs due to some energy being converted into heat. Therefore, the orifice should be of a sufficiently small size to help prevent the piston 406 of the plunger assembly 134 from being suddenly and catastrophically reversed when the brake system 100 fails, such as when power to the motor 414 is interrupted or lost and the pressure in the output conduit 154 is relatively high. The plunger assembly 134 may include an optional spring member, such as a spring washer 419, to assist in damping such rapid rearward reverse driving of the piston 406. The spring washer 419 may also assist in damping any such speed movement of the piston 406 as it approaches its most retracted position within the bore 400 as it approaches its rest position. It should be noted that although the isolation valves 150 and 152 may be shuttled to their original positions during a power failure, the isolation valves 150 and 152 may be shuttled to their original positions during a power failure. Figure 2 The position shown, but the presence of the spring washer 419 enables the isolation valves 150 and 152 to be manufactured cheaply with smaller solenoids, wherein these isolation valves can be hydraulically locked and cannot shuttle, thereby allowing the piston 406 to be quickly driven backward. The spring washer 419 can also be used as a parking element, so that the piston 406 can gently hit the spring washer 419 on the return stroke to determine its home, starting or idle position. The home position can be determined when it is detected that the piston 406 has stopped moving by hitting the spring washer 419.

[0065] The first plunger valve 450 and the second plunger valve 452 provide an open parallel path between the pressure chambers 440 and 442 of the plunger assembly 134 during normal braking operation (when the first plunger valve 450 is energized). Although a single open path may be sufficient, the advantage of having both the first plunger valve 450 and the second plunger valve 452 is that the first plunger valve 450 can provide an easy flow path through its relatively large orifice, while the second plunger valve 452 can provide a restricted flow orifice path during certain fault conditions (when the first plunger valve 450 is de-energized to its closed position). It should be noted that a single normally open valve with a relatively large orifice may be sufficient to replace the two plunger valves 450 and 452, however, a relatively large solenoid may be required for the single valve, and during de-energization, the single valve may be closed, which may result in locking the isolation valves 150 and 152.

[0066] The operation of the brake system 100 will now be described. It should be noted that the term "normal braking" or "normal brake application" generally refers to a braking event in which all components of the brake system 100 are functioning normally. Additionally, under normal braking events, the brake system 100 does not suffer from any harmful leaks that could impede normal operation of the brake system 100. Figure 2 3 shows the brake system 100 and brake pedal unit 130 in their rest position. In this condition, the driver is not depressing the brake pedal 190. In a non-autonomous braking event, the driver of the vehicle depresses the brake pedal 190 to indicate his intention to actuate the brake system 100 to decelerate the vehicle. The ECU 140 detects this braking event through a signal from the travel sensor 340 and also through a signal from the pressure sensor 330.

[0067] During a normal brake application operation, the pressurized fluid flow from the brake pedal unit 130 generated by depression of the brake pedal 190 is diverted into the pedal simulator 132. When the input piston 202 moves via the brake pedal 190, the simulation valve 246 is actuated or energized to divert fluid from the input chamber 210 of the brake pedal unit 130 through the simulation valve 246. It should be noted that once the passage 224 in the input piston 202 moves past the seal 220, the flow of fluid from the input chamber 210 to the reservoir 136 is blocked. When the input piston 202 generates fluid pressure within the input chamber 210, the pressurized fluid is diverted into the pressure chamber 235 of the pedal simulator 132. The buildup of pressure within the pressure chamber 235 of the pedal simulator 132 causes the piston 232 to move against the bias of the springs 236 and 238. The compression of the springs 236 and 238 provides force feedback to the vehicle driver as the driver feels resistance on the driver's foot via the brake pedal 190.

[0068] During the duration of normal brake application, the simulation valve 246 remains open, preferably in its energized state. Preferably, the simulation valve 246 is energized for the duration of the entire ignition cycle. In addition, during the normal boost brake application operation, the isolation valves 150 and 152 are energized to the second position to prevent fluid from flowing through the isolation valves 150 and 152 from the conduits 156 and 158, respectively. In one embodiment, the isolation valves 150 and 152 are energized during the duration of the entire ignition cycle, such as when the engine is running, rather than being energized and de-energized. This continuous energization helps minimize noise.

[0069] It should be noted that the primary piston 204 and the secondary piston 206 are not in fluid communication with the reservoir 136 (with the passages 254 and 264 thereof) because they are positioned beyond the lip seals 250 and 260, respectively. Figure 1Preventing fluid from flowing through isolation valves 152 and 150 hydraulically locks primary chamber 212 and secondary chamber 214, thereby preventing further movement of primary piston 204 and secondary piston 206. Thus, no further initial movement of brake pedal 190 is required to close these chambers, as is required with prior art brake pedal 10.

[0070] It is generally desirable to maintain isolation valves 150 and 152 energized during normal braking mode to ensure that fluid is drained through plunger assembly 134 to reservoir 136 when necessary. Figure 5 As shown, the piston 406 of the plunger assembly 134 includes a passage 444 formed therein to permit such venting. However, during a fault condition where the isolation valves 150 and 152 cannot be energized (such as an electrical fault or a fault in the ECU 140), fluid from the wheel brakes 120a, 120b, 120c, and 120d can still be vented via conduits 156 and 158 and through the brake pedal unit 134 when the pressure within the secondary chamber 214 and the primary chamber 212 exceeds a predetermined pressure level. In one embodiment, the predetermined pressure level is approximately 0.65 bar, which is sufficient to move the primary piston 204 and the secondary piston 206 in a rightward direction so that the passages 254 and 264 are to the right of the lip seals 250 and 260, respectively. In this position, fluid can flow from the primary chamber 212 and the secondary chamber 214 into the reservoir 136 via conduits 265 and 266, respectively. The requirement to exceed a predetermined pressure level (e.g., 0.65 bar) is generally only required during certain fault conditions. During a fault condition in which the driver is not applying pressure to the brake pedal 190, this slight pressure (e.g., about 0.65 bar) may cause slight braking of one or more of the wheel brakes 120a, 120b, 120c, and 120d until the increased volume of the brake fluid due to heating thereof can offset the pressure increase, thereby moving the primary piston 204 and the secondary piston 206 to a position that permits discharge through the brake pedal unit 130. It has been found that such minor brake application and heating does not cause excessive brake fade problems at the wheel brakes.

[0071] During normal brake application, when the pedal simulator 132 is actuated by the depression of the brake pedal 190, the ECU 140 operates the brake system 100 to provide actuation of the wheel brakes 120a, 120b, 120c and 120d. The ECU 140 actuates and adjusts the plunger assembly 134 based on information corresponding to the driver's intention from the travel sensor 340 so as to provide pressure to the wheel brakes 120a, 120b, 120c and 120d. In relation to the driver's intention, the plunger assembly 134 is operated to provide the desired pressure level to the wheel brakes 120a, 120b, 120c and 120d. The ECU 140 can also use information from the pressure sensor 332 generally located at the output end of the plunger assembly 134 to adjust the motor 414 of the plunger assembly 134 to obtain the desired pressure level in the output conduit 54.

[0072] To operate the plunger assembly 134, the ECU 140 actuates the motor 414 to rotate the screw shaft 416 in a first rotational direction. The screw shaft 416 rotates in the first rotational direction to rotate the piston 406 in a forward direction (as viewed from above). Figure 2 and Figure 5 443. The pressure in the first pressure chamber 440 is increased and the fluid flows out of the first pressure chamber 440 and into the conduit 454. The fluid can flow into the output conduit 154 via the opened first plunger valve 450 and the second plunger valve 452. It should be noted that as the piston 406 advances in the forward direction, the fluid is permitted to flow into the second pressure chamber 442 via the conduit 443. The pressurized fluid from the output conduit 154 is directed to the conduits 160 and 162 through the isolation valves 150 and 152, respectively. The pressurized fluid from the conduits 160 and 162 can be directed to the wheel brakes 120a, 120b, 120c, and 120d through the opened apply valves 170, 174, 178, and 182, while the dump valves 172, 176, 180, and 184 remain closed. It should be noted that the first plunger valve 450 is energized to its open position during normal braking events, such that both the first plunger valve 450 and the second plunger valve 452 are open. The plunger valve 450 may be energized for the duration of the entire ignition cycle.

[0073] When the driver lifts or releases the brake pedal 190, the ECU 140 can operate the motor 414 to rotate the screw shaft 416 in the second rotational direction, thereby retracting the piston 406, thereby discharging the fluid from the wheel brakes 120a, 120b, 120c and 120d. The speed and distance of the piston 406 retraction are based on the driver's request to release the brake pedal 190 in cooperation with the sensor 418. Of course, if the driver quickly releases the brake pedal 190, the plunger assembly 134 can be operated to avoid this transient pressure drop. Under certain conditions, such as in a non-powered slip control event, the pressurized fluid from the wheel brakes 120a, 120b, 120c and 120d can assist in reverse driving the ball screw mechanism 412, thereby returning the piston 406 to its idle position. It should be noted that when the driver releases the brake pedal 190, the first plunger valve 450 and the second plunger valve 452 preferably maintain their open positions during the non-slip control event.

[0074] During a braking event, the ECU 140 may also selectively actuate the apply valves 170, 174, 178, and 182 and the dump valves 172, 176, 180, and 184, respectively, to provide a desired pressure level to the wheel brakes 120a, 120b, 120c, and 120d. Such selective actuation of the wheel brakes 120a, 120b, 120c, and 120d may even conflict with the driver's intent as sensed by the travel sensor 340. For example, the ECU 140 may control the braking system 100 during ABS, DRP, TC, VSC, regenerative braking, and / or autonomous braking events through general operation of the plunger assembly 134 in conjunction with selective actuation of the apply valves 170, 174, 178, and 182 and the dump valves 172, 176, 180, and 184. Under certain driving conditions, ECU 140 communicates with a powertrain control module (not shown) and other additional brake controllers of the vehicle to provide coordinated braking during these advanced brake control schemes (e.g., anti-lock braking (AB), traction control (TC), vehicle stability control (VSC), and regenerative braking compound, etc.).

[0075] In some cases, such as relatively large brake pressure requirements or extended slip control events, the piston 406 of the plunger assembly 134 can complete its entire stroke length in the bore 400 of the housing, and still expect additional boost pressure to be delivered to the wheel brakes 120a, 120b, 120c, and 120d. The plunger assembly 134 is a double-acting plunger assembly, so that it is configured to provide boost pressure to the output conduit 154 when the piston 406 completes the stroke backward (right) or in the opposite direction. This advantage over conventional plunger assemblies is that first its piston needs to be brought back to its idle or retracted position, and then the piston can move forward again to generate pressure in a single pressure chamber. For example, if the piston 406 completes its entire stroke and still expects additional boost pressure, the second plunger valve 452 is energized to its check valve closed position. The first plunger valve 450 is de-energized to its closed position. ECU 140 actuates motor 414 in a second rotational direction opposite to the first rotational direction to rotate screw shaft 416 in the second rotational direction. Rotation of screw shaft 416 in the second rotational direction causes piston 406 to rotate in a backward direction (as viewed from the left). Figure 2 and Figure 5 The movement of piston 406 causes the pressure in second pressure chamber 442 to increase, and fluid flows out of second pressure chamber 442 and into conduit 443 and output conduit 154. The pressurized fluid from output conduit 154 is directed through isolation valves 150 and 152 to conduits 160 and 162. The pressurized fluid from conduits 160 and 162 can be directed through open apply valves 170, 174, 178, and 182 to wheel brakes 120a, 120b, 120c, and 120d, while dump valves 172, 176, 180, and 184 remain closed.

[0076] In a similar manner as during the forward stroke of the piston 406, the ECU 140 can also selectively actuate the apply valves 170, 174, 178, and 182 and the dump valves 172, 176, 180, and 184 to provide the desired pressure levels to the wheel brakes 120a, 120b, 120c, and 120d, respectively. When the driver fully lifts or releases the brake pedal 190 during the pressurized rearward stroke of the plunger assembly 134, the first plunger valve 450 and the second plunger valve 452 are preferably operated to their open positions (as discussed above), but it is generally sufficient to open only one of the valves 450 and 452. It should be noted that when transitioning out of a slip control event, it is ideal to have the position of the piston 406 and the displaced volume within the plunger assembly 134 almost exactly correlated to a given pressure and fluid volume within the wheel brakes 120a, 120b, 120c, and 120d. However, when the correlation is not certain, fluid can be drawn from the reservoir 136 into the chamber 440 of the plunger assembly 134 via the check valve 451. To drain the plunger assembly 134, fluid can flow through the port 448 to the reservoir 136.

[0077] In the event that portions of the brake system 100 are de-energized, a manual boost or manual application is provided so that the brake pedal unit 130, which is operated by the driver via the brake pedal 190, can supply relatively high pressure fluid to the primary output conduit 158 ​​and the secondary output conduit 156. Thus, the term "manual boost" or "manual boost event or mode" refers to a situation in which the brake pedal 190 is depressed to actuate the brake pedal unit 130, and at least a portion of the brake system 100 is not operating properly, or a harmful leak has occurred within the brake system 100. It should be noted that during a manual boost event, the pedal simulator 132 is not utilized as normally intended.

[0078] During an electrical fault, the motor 2414 of the plunger assembly 134 may cease to operate, thereby failing to generate pressurized hydraulic brake fluid from the plunger assembly 134 to the output conduit 154. During an electrical fault, the ECU 140 may also fail to operate and may fail to actuate the plunger assembly 134 or the solenoid valve of the brake system 100. In this case, the isolation valves 150 and 152 shuttle (or remain) in their positions that permit fluid to flow from the conduits 156 and 158 to the wheel brakes 120a, 120b, 120c, and 120d. The pedal simulator valve 246 will shuttle (or remain) to its de-energized closed position to prevent fluid from flowing out of the input chamber 210 to the pedal simulator 132. The closure of the conduit 218 to the pedal simulator 132 will help to hydraulically lock the input chamber 210. During the manual push application, when the driver continues to depress the brake pedal 190, the input piston 202, the primary piston 204 and the secondary piston 206 will move forward to the left, as shown in FIG. Figure 2 3. This advancement will cause passage 224 to move past seal 220 to prevent fluid from flowing from fluid chamber 210 to reservoir 136. Fluid flows from primary chamber 212 and secondary chamber 214 into conduits 158 and 156, respectively, to actuate wheel brakes 120a, 120b, 120c, and 120d.

[0079] Now returning to FIG. 3 , the brake pedal unit 130 includes various features, such as Figure 1 The prior art brake pedal unit 10 has advantages. One advantage is that less force is required to actuate the brake pedal unit 130 than to actuate the brake pedal unit 10. The brake pedal unit 10 includes springs that have higher preload force values ​​than the brake pedal unit 130. Higher spring preload values ​​are necessary for the proper operation of the brake pedal unit 10. The return spring 58 of the known brake pedal unit 10 has a preload force value of about 40N (Newton). The preload force values ​​of the input spring assembly 52, the main spring assembly 54, and the secondary spring assembly 56 are 100N, 75N, and 50N, respectively. However, for a preferred embodiment of the brake pedal unit 130, the return spring 286 has a preload force value of about 45N, and the preload force values ​​of the input spring 288, the main spring 304, and the secondary spring 312 are about 5N, 80N, and 55N, respectively. The preload value generally indicates the force required to act on the spring to initiate the spring to begin deflection. It should be understood that these values ​​indicate only some examples of desired embodiments of brake pedal unit 130 and are not to be considered limiting of the invention disclosed herein.It should also be noted that each seal of brake pedal units 10 and 130 generally provides a nominal friction force of about 7N to be overcome.

[0080] This design enables a reduction in the force required by the driver on the brake pedal 190 to initiate movement since the primary and secondary pistons 204, 206 of the brake pedal unit 130 do not need to move to a drain position in relation to disconnection from the reservoir 136 as compared to the brake pedal unit 10.

[0081] Due to stack-up tolerances in the manufacture and assembly of the various components of brake pedal units 10 and 130, it is desirable to design clearances between some components while providing contact or no clearance between other components to properly align the piston within the bore of brake pedal units 10 and 130. The design aspects of providing clearances and contact can also help determine the preload spring force within brake pedal units 10 and 130. For example, eliminating clearance 90 (in Figure 1) helps to ensure alignment of the passages 254 and 264 of the primary piston 204 and the secondary piston 206. Since the gap 90 is eliminated in the brake pedal unit 130, it may be desirable to include a gap 302 (as described above) to compensate for any manufacturing tolerances when assembling its components and spring within the housing of the brake pedal unit 130. When designing the brake pedal unit 130, the desired spring force and seal friction are typically determined first, and then the location and spacing of the desired gaps can be appropriately calculated. Since the gap 302 is associated with the spring 288 having a minimum preload force (5N), the compression of the spring 288 during initial actuation of the brake pedal unit 130 is advantageously relatively low.

[0082] When all three pistons (input piston 14, primary piston 16 and secondary piston 18) of the brake pedal unit 10 move to the left (as seen from the Figure 1 ), all three pistons are generally disconnected from the reservoir at the same time. Instead, only the input piston 202 of the brake pedal unit 130 requires initial movement to disconnect from the reservoir 136. The elimination of movement of the primary piston 204 and the secondary piston 206 of the brake pedal unit 130 reduces the force required by the driver during initial operation of the brake pedal unit 130 compared to the brake pedal unit 10.

[0083] Another difference between the brake pedal unit 130 and 10 is that the brake pedal unit 10 includes a redundant seal 36 adjacent to the seal 34. The redundant seal 36 helps prevent a small amount of fluid from leaking out of the brake pedal unit 10 adjacent to the input piston 14 and prevents the opening of the hole in the housing. Since the redundant seal is eliminated from the brake pedal unit 130, a sealing interface (not shown) can be used at the end of the hole 200 in the housing of the brake pedal unit 130 to capture any fluid that bypasses the seal 222. The sealing interface may include an optional sponge (not shown) and is covered by a retainer or cover. Although the addition of the redundant seal 36 can help prevent fluid leakage, the addition of the redundant seal 36 increases the friction resistance of the seal, thereby increasing the force required to initiate the movement of the input piston 14. The addition of the redundant seal 36 also requires the addition of additional spring force to overcome the friction resistance, which further increases the force required to initiate the movement of the input piston 14.

[0084] The operation of a generally lower braking event felt by a driver of a vehicle of a brake pedal as a result of comparing the differences between the prior art brake pedal unit 10 and the brake pedal unit 130 will now be described. Figure 6 As will be seen from the graph of , the structural and operational differences between brake pedal units 10 and 130 (as described above) generally result in a reduction in the pedal force provided to brake pedal unit 130 as compared to prior art brake pedal unit 10 .

[0085] Figure 6 A graphical representation of the low pedal travel application and release occurrence of normal braking events for brake pedal units 10 and 130 is shown. More specifically, the graphical representation is the relationship between pedal force and pedal travel. The pedal travel directly corresponds to the travel of the input pistons 14 and 202 within the brake pedal units 10 and 130, respectively. As described above, movement of the brake pedal 190 causes movement of the corresponding input piston (14 or 202) via the linkage arm 208 of the brake pedal 190. It should be noted that Figure 6 The numerical data for pedal travel application and release occurs in conjunction with a brake pedal having a pedal-to-lever ratio of approximately 3.8. As the driver operates the brake pedal 190, the reaction force from the input piston provides feedback as experienced by the driver's foot. The dashed line 500 is aligned with the brake pedal unit 130 and the brake pedal unit 130 of FIG. 3. Figure 4 The solid line 510 is associated with the operation of the pedal simulator 132. Figure 1 The prior art brake pedal unit 10 is associated with a corresponding pedal simulator (such as a pedal simulator similar to the pedal simulator 132). Figure 6 Appropriate graphical representations are provided that highlight the differences between brake pedal units 10 and 130. However, it should be understood that these values ​​indicate only one example of braking events for brake pedal units 10 and 130 for illustrative purposes and should not be considered limiting of the invention disclosed herein.

[0086] Referring now to the broken or dashed line 500 associated with the brake pedal unit 130, the initial preload force input indicated by the approximately vertical path 500a generally corresponds to the force required to overcome the various preload requirements of the springs within the brake pedal unit 130 and to overcome the various friction forces applied by the various seals of the brake pedal unit 130. Figure 6 In the example shown, the force to overcome these forces is about 16.8 N. This is obtained by adding the preload spring force of return spring 286, 45 N, to the preload spring force of input spring 288, 5 N, to the nominal friction force of each of seals 220 and 22s of input piston 202, about 7 N, and then dividing by the pedal-to-lever ratio of about 3.8 to obtain about 16.8 N.

[0087] Once the preload force of about 16.8 N is overcome, the input piston 202 begins to move toward a position such that the passage 224 moves just past the lip seal 220, as indicated by the inclined path 500 b. It should be noted that during the path 500 b, the primary piston 204 and the secondary piston 206 do not move (unlike the brake pedal unit 10). The gradual rise along the path 500 b is generally attributed to the spring rate of the compression return spring 286 and the input spring 288. Figure 6The graphical representation of may assume that all springs have the same spring rate, for example approximately 2 N / m.

[0088] Once the passage 224 of the input piston 202 has just moved past the lip seal 220, thereby cutting off the fluid communication between the input chamber 210 and the reservoir 136, as indicated at the end of the inclined path 500b, additional force is required to overcome the preload force of the first spring 236 and the second spring 238 of the pedal simulator 132, and to overcome the seal friction of the seals in the pedal simulator 132. This is represented by the nearly vertical path 500c. When the additional force (typically about 20N) is large enough, the input piston 202 continues to move (compressing the input spring 288 and actuating the pedal simulator 132), as represented by the inclined path 500d, until the driver stops applying force to the brake pedal 190, as shown in FIG. Figure 6 . It should be noted that when the driver begins to release the brake pedal 190, the stroke length (about 15mm) is generally meaningless, and the graphical representation is only used to illustrate the situation where the pedal stroke is applied relatively low. During path 500d, the pedal simulator 132 is actuated and its piston 232 moves therein, thereby compressing the first spring 236 and the second spring 238. Paths 500e, 500f and 500g represent the driver releasing the brake pedal 190 back to the idle state. The almost vertical descent of path 500e represents the change in direction of the input piston 202 and the pedal simulator 132 and the associated change in seal friction.

[0089] Referring now to the solid line 510 associated with the prior art brake pedal unit 10, the initial preload force input indicated by the approximately vertical path 510a generally corresponds to the force required to overcome the various preload requirements of the springs within the brake pedal unit 10 and to overcome the various friction forces applied by the various seals of the brake pedal unit 10. Compared to the brake pedal unit 130, a greater force is required because all three pistons (input piston 14, primary piston 16, and secondary piston 18) must now be moved until their respective passages 38, 44, and 50 are blocked, respectively. Figure 6 In the example shown, the force to overcome these forces is about 36.6 N. This is obtained by adding the preload spring force of 40 N of the return spring 58 plus the preload spring force of 50 N of the secondary spring 56, plus the nominal friction force of all seals 32, 24, 26, 40, 42, 46 and 48, each of about 7 N, and then dividing by the pedal-to-lever ratio of about 3.8 to obtain about 36.6 N.

[0090] Once the preload force of about 36.6 N is overcome, all three pistons (input piston 14, primary piston 16, and secondary piston 18) begin to move toward their respective positions, thereby cutting off fluid communication with the reservoir. More specifically, passages 38, 44, and 50 move past lip seals 32, 40, and 46, as indicated by inclined path 510b. The gradual rise along path 510b is generally attributed to the spring rate of compression return spring 58 and secondary spring 56. The gradual rise along path 510b is similar to path 500b.

[0091] Once the passages 38, 44 and 50 move past the lip seals 32, 40 and 46, as indicated at the end of the inclined path 510b, additional force is required to overcome the preload force of the spring of the pedal simulator 132 and the input spring 288 of the brake pedal unit 130 (represented by the nearly vertical path 510c). When the additional force (typically about 44N) is large enough, the input piston 14 continues to move (compressing the input spring 52 and actuating the pedal simulator), as represented by the inclined path 510d, until the driver stops applying force to the brake pedal. During the path 510d, the pedal simulator is actuated.

[0092] Upon release, paths 510e and 510f are similar to paths 500e and 500f until the pedal simulator is emptied of fluid and all three pistons (input piston 14, primary piston 16, and secondary piston 18) begin their rearward movement to their rest positions to about 5.7 mm, as indicated by path 510g. The movement of the three pistons back to their rest positions is indicated by path 510h, and back to the rest state, indicated by path 510i.

[0093] As from Figure 6 As can be seen from the graph of , the differences between the brake pedal units 10 and 130 generally result in a reduced pedal force requirement provided to the brake pedal unit 130 compared to the brake pedal unit 10. The input piston spring 52 (100N preload) of the brake pedal unit 10 has a greater preload value than the input piston spring 288 (5N preload) of the brake pedal unit 130 because the spring 52 is utilized during the initial movement of the primary piston 16 and the secondary piston 18 during simultaneous cut-off. With respect to the brake pedal unit 130, the input spring 288 is not used to move the primary piston 204 and the secondary piston 206. The input spring 288 can provide a much smaller force acting on the primary piston 204 and the secondary piston 206 to prevent movement caused by jolts or motion of the vehicle, which movement would increase the cut-off travel in the next brake application after an unexpected movement of the piston of the brake pedal unit 130.

[0094] With respect to the various valves of the brake system 10, the terms "operate" or "in operation" (or "actuate", "move", "position") as used herein (including in the claims) may not necessarily refer to energizing the solenoid of the valve, but rather to placing or permitting the valve to be in a desired position or valve state. For example, a solenoid-actuated normally open valve may be operated to an open position by simply permitting the valve to remain in its un-energized normally open state. Operating the normally open valve to a closed position may include energizing the solenoid to move the internal structure of the valve to block or prevent fluid flow therethrough. Thus, the term "operate" should not be interpreted to mean moving a valve to a different position, nor should it mean that the valve's associated solenoid is always energized.

[0095] The principles and modes of operation of the present invention have been explained and illustrated in the preferred embodiments thereof. However, it must be understood that the present invention can be practiced in other ways than those specifically explained and illustrated without departing from the spirit or scope of the present invention.

Claims

1. A brake pedal unit connected to a brake pedal and in selective fluid communication with a fluid reservoir, the brake pedal unit comprising: a housing defining an aperture formed therein; an input piston slidably disposed in the bore, wherein the input piston is connected to the brake pedal such that engagement of the brake pedal causes the input piston to move within the bore of the housing of the brake pedal unit, and wherein the brake pedal unit is defined to be in an idle position when engagement of the brake pedal does not cause movement of the input piston, movement of the input piston pressurizing an input chamber in selective fluid communication with a pedal simulator; an input passage permitting fluid communication between the input chamber and the reservoir when the brake pedal unit is in the rest position; a primary piston slidably disposed in the bore to pressurize the primary chamber; a main passage permitting fluid communication between the main chamber and the reservoir, wherein fluid flow through the main passage is blocked when the brake pedal unit is in the rest position; a secondary piston slidably disposed in the bore to pressurize the secondary chamber; as well as a secondary passage permitting fluid communication between the secondary chamber and the reservoir, wherein fluid flow through the secondary passage is blocked when the brake pedal unit is in the rest position, When the brake pedal unit is in the rest position, fluid communication between the primary chamber and the reservoir is blocked via the blocked primary passage, fluid communication between the secondary chamber and the reservoir is blocked via the blocked secondary passage, and the input chamber and the reservoir are in fluid communication via the input passage, and During a manual push event in which the brake pedal is depressed, fluid communication between the primary chamber and the reservoir is blocked via the blocked primary passage, fluid communication between the secondary chamber and the reservoir is blocked via the blocked secondary passage, fluid flow through the input passage is blocked, and fluid communication between the input chamber and the reservoir is blocked via the blocked input passage.

2. The unit according to claim 1, wherein: Movement of the input piston by a predetermined stroke length causes the input passage to be blocked, thereby cutting off fluid communication with the pedal simulator.

3. The unit according to claim 1, wherein: The input chamber is generally defined between the input piston and the primary piston, and wherein the secondary chamber is generally defined between the primary piston and the secondary piston.

4. The unit according to claim 1, wherein: When the brake pedal unit is in the rest position, fluid flow through the primary passage is blocked by a primary seal sealingly engaged between the primary piston and the bore of the housing.

5. The unit according to claim 4, wherein: The primary seal is a lip seal.

6. The unit according to claim 4, wherein: The primary seal is mounted within a groove formed in the bore of the housing and sealingly engages an outer wall of the primary piston.

7. The unit of claim 1, wherein: When the brake pedal unit is in the rest position, fluid flow through the secondary passage is blocked by a secondary seal sealingly engaged between the secondary piston and the bore of the housing.

8. The unit according to claim 7, wherein: The secondary seal is a lip seal.

9. The unit according to claim 7, wherein: The secondary seal is mounted in a groove formed in the bore of the housing and sealingly engages an outer wall of the secondary piston.

10. The unit of claim 1, wherein: The primary passage is formed through the primary piston, and wherein the secondary passage is formed through the secondary piston.

11. The unit of claim 1, wherein: During a normal braking event when the brake pedal is depressed, movement of the primary and secondary pistons is inhibited.

12. The unit according to claim 11, wherein During a manual actuation event in which the brake pedal is depressed, movement of the primary piston and the secondary piston is permitted.

13. The unit of claim 12, wherein: After a manual push event and release of the brake pedal, a pressure slightly above atmospheric pressure is maintained in the primary chamber and the secondary chamber.

14. The unit of claim 1, wherein: The input piston and the primary piston are separated from each other by a first cage spring assembly, and wherein the primary piston and the secondary piston are separated from each other by a second cage spring assembly.

15. The unit of claim 14, wherein: The preload force of the first cage spring assembly is less than the preload force of the second cage spring assembly.

16. The unit of claim 15, wherein: When the brake pedal unit is in the rest position, the first cage spring assembly is slightly compressed.

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