Abstract:
A brake system includes an electric brake force generator for braking a wheel of a vehicle by a driving force of an electric motor. A problem determination device is provided in which the problem determination device sends an electrical signal to the electric motor such that the electric motor rotates in a direction opposite that of the direction of rotation to generate a brake force. If a rotation of the electric motor is not detected, then the problem determination device determines that a problem has occurred.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority under 35 USC §119 based on Japanese patent application No. 2007-11108 filed 22 Jan. 2007. The subject matter of this priority document is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a brake system comprising an electric brake force generator or generating means that brakes a wheel by a driving force of an electric motor. 
         [0004]    2. Description of the Related Art 
         [0005]    Japanese Patent Application Laid-open No. 2005-343366 discloses a brake system of the type referred to as a brake by wire (BBW) brake system, which converts a braking operation of a driver into an electrical signal used to operate an electric brake force generator or generating means, such as a motor cylinder, and operates a wheel cylinder by brake fluid pressure generated by the motor cylinder. 
         [0006]    When a problem occurs that renders inoperable an electric motor of an electric brake force generating means (slave cylinder) of the BBW brake system, or a problem that causes a seizure of a ball screw mechanism for converting a driving force of an electric motor into advance and retreat movements of a piston, it is required to perform a failsafe or backup process for braking by using brake fluid pressure generated by a master cylinder instead of the brake fluid pressure generated by the slave cylinder. 
         [0007]    In order to determine whether the slave cylinder is functioning normally, it may be determined whether the electric motor is rotating normally or whether the piston is moving normally forward when the driver applies a pushing force to a brake pedal. However, in the system where the problem determination is performed based on the pushing force applied to the pedal by the driver, it is impossible to determine the occurrence of a problem before the driver applies the pushing force to the brake pedal to operate the slave cylinder, leading to a possibility that the backup process cannot be quickly performed upon the occurrence of the problem. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention has been made in view of the above-described circumstances, and has an object to reliably determine occurrence of a problem in an electric brake force generating means or generator. 
         [0009]    To achieve the above object, according to a first aspect of the present invention, there is provided a brake system comprising: an electric brake force generator that brakes a wheel by a driving force of an electric motor; and a problem determining device that supplies an electrical current so as to rotate the electric motor in a direction opposite to a brake force generating direction, and determines that a problem has occurred when the rotation of the electric motor is not detected. 
         [0010]    A slave cylinder  23  of an exemplary embodiment of the present invention corresponds to the electric brake force generator of the present invention, and an electronic control unit U corresponds to the problem determining device of the present invention. 
         [0011]    With the first aspect of the present invention, an electrical current is supplied so as to rotate the electric motor of the electric brake force generator in the direction opposite to the brake force generating direction, and it is determined that a problem has occurred when the rotation of the electric motor is not detected. Therefore, even if the electric brake force generator is not being operated, it is possible to reliably perform a problem determination process. Also, the problem determination process can be performed before the electric brake force generator actually operates, so that it is possible to quickly perform a backup process when it is determined that a problem has occurred. 
         [0012]    According to a second aspect of the present invention, in addition to the first aspect, the problem determining device performs the determination process when a driver is applying a pushing force to an accelerator pedal. 
         [0013]    With the second aspect of the present invention, when a driver is applying a pushing force to the accelerator pedal, that is, when it is ensured that a predetermined time exists until the driver performs the next braking operation, the problem determination process is performed with respect to the electric brake force generator. Therefore, it is possible to avoid a situation where the electric brake force generator is operated during the problem determination process to affect the problem determination. 
         [0014]    According to a third aspect of the present invention, in addition to the first aspect, the problem determining device performs the determination process when the electric brake force generator is not generating brake fluid pressure. 
         [0015]    With the third aspect of the present invention, the problem determination process is performed when the electric brake force generator is not generating brake fluid pressure. Therefore, it is possible to prevent a situation where the problem determination process renders the electric brake force generator incapable of generating a braking force. 
         [0016]    According to a fourth aspect of the present invention, in addition to the third aspect, the brake system comprises: a master cylinder that generates brake fluid pressure by a braking operation by a driver; a wheel cylinder that brakes the wheel by the brake fluid pressure generated by the master cylinder or the electric brake force generator, wherein the electric brake force generator generates brake fluid pressure by a driving force of the electric motor, and the electric brake force generator is arranged between the master cylinder and the wheel cylinder; and a controller which controls the master cylinder and the wheel cylinder to communicate with each other when the electric brake force generator is not generating brake fluid pressure. 
         [0017]    With the fourth aspect of the present invention, the master cylinder and the wheel cylinder communicate with each other when the electric brake force generator arranged between the master cylinder and the wheel cylinder is not generating brake fluid pressure, that is, when there is a possibility that the problem determination process is performed. Therefore, when it is determined that a problem has occurred, it is possible to easily perform the backup braking process by transmitting the brake fluid pressure generated by the master cylinder to the wheel cylinder without shutting off the brake fluid pressure at the electric brake force generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a fluid-pressure circuit diagram of a vehicle brake system according to an embodiment of the preset invention under normal operating conditions; 
           [0019]      FIG. 2  is a fluid-pressure circuit diagram of a vehicle brake system corresponding to  FIG. 1  under abnormal operating conditions; 
           [0020]      FIG. 3  is a block diagram of a control system of the vehicle brake system according to an embodiment of the present invention; and 
           [0021]      FIG. 4  is a flowchart showing a problem determination process of an embodiment of the present invention, with respect to a slave cylinder. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    An embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . 
         [0023]    As shown in  FIG. 1 , a tandem master cylinder  11  has two fluid pressure chambers  13 A and  13 B which output brake fluid pressure according to a pushing force applied to a brake pedal  12  by a driver depressing the brake pedal  12 . One of the fluid pressure chambers  13 A is connected to wheel cylinders  16  and  17  of disc brake devices  14  and  15  for braking, for example, a left front wheel and a right rear wheel through fluid passages Pa, Pb, Pc, Pd, and Pe. The other first fluid pressure chamber  13 B is connected to wheel cylinders  20  and  21  of disc brake devices  18  and  19  for braking, for example, a right front wheel and a left rear wheel through fluid passages Qa, Qb, Qc, Qd, and Qe. 
         [0024]    A shutoff valve  22 A, which is a normally open solenoid valve, is provided between the fluid passages Pa and Pb. A shutoff valve  22 B, which is a normally open solenoid valve, is provided between the fluid passages Qa and Qb. A slave cylinder  23  is provided between the fluid passages Pb, Qb and the fluid passages Pc, Qc. An anti-lock brake system (ABS)  24  is provided between the fluid passages Pc, Qc and the fluid passages Pd, Pe; Qd, Qe. 
         [0025]    A reaction force permission valve  25 , which is a normally closed solenoid valve, is connected between a fluid passage Ra branching from the fluid passage Qa and a fluid passage Rb. A stroke simulator  26  is connected to the fluid passage Rb. The stroke simulator  26  has a cylinder  27  and a piston  29  slidably fitted in the cylinder  27  while being urged by a spring  28 . A fluid chamber  30 , formed on the side of the piston  29  opposite from the spring  28 , communicates with the fluid passage Rb. 
         [0026]    An actuator  51  of the slave cylinder  23  has a drive bevel gear  53  provided on the rotating shaft of an electric motor  52 , a follower bevel gear  54  meshing with the drive bevel gear  53 , and a ball screw mechanism  55  operated by the follower bevel gear  54 . A sleeve  58  is rotatably supported in an actuator housing  56  via a pair of ball bearings  57 . An output shaft  59  is coaxially arranged on an inner periphery of the sleeve  58 . The follower bevel gear  54  is arranged on an outer periphery of the sleeve  58 . 
         [0027]    A pair of pistons  38 A and  38 B urged in a retreat direction by a pair of return springs  37 A and  37 B are slidably disposed in a cylinder body  36  of the slave cylinder  23 . A pair of fluid pressure chambers  39 A and  39 B are defined on the front faces of the pistons  38 A and  38 B, respectively. A front end of the output shaft  59  abuts on a rear end of the rear piston  38 A. One of the fluid pressure chamber  39 A communicates with the fluid passages Pb, Pc via ports  40 A,  41 A, while the other fluid pressure chamber  39 B communicates with the fluid passages Qb, Qc through ports  40 B,  41 B. 
         [0028]    The structure of the ABS  24  is of a well-known type. The ABS  24  has two streams structurally identical to each other: a stream including the disc brake devices  14  and  15  for braking the left front wheel and the right rear wheel; and a stream for the disc brake devices  18  and  19  for braking the right front wheel and the left rear wheel. Of these streams, the stream for the disc brake devices  14  and  15  will be described as a representative, with the understanding that the stream for disk brakes  18  and  19  works in a similar fashion. A pair of in-valves  42  comprising normally open solenoid valves are provided between the fluid passage Pc and the fluid passages Pd, Pe. A pair of out-valves  44  comprising normally closed solenoid valves are provided between the fluid passages Pd, Pe on the downstream side of the in-valves  42  and a reservoir  43 . A fluid pressure pump  47  interposed between a pair of check valves  45  and  46  is provided between the reservoir  43  and the fluid passage Pc. The fluid pressure pump  47  is driven by an electric motor  48 . 
         [0029]    As shown in  FIG. 3 , connected to an electronic control unit or controller U for controlling the operation of the shutoff valves  22 A and  22 B, the reaction force permission valve  25 , the electric motor  52  of the slave cylinder  23  and the ABS  24 , are a fluid pressure sensor Sa for detecting the brake fluid pressure generated by the master cylinder  11 , a fluid pressure sensor Sb for detecting the brake fluid pressure transmitted to the disc brake devices  18  and  19 , vehicle wheel speed sensors Sc for detecting the vehicle wheel speeds of the vehicle wheels, an accelerator pedal switch Sd for detecting operation of an accelerator pedal (not shown), and a motor rotational position sensor Se for detecting the rotational position of the electric motor  52 . 
         [0030]    The operation of an exemplary embodiment of the present invention having the above-described arrangement will now be described. 
         [0031]    When the system is operating under normal conditions, the shutoff valves  22 A and  22 B, comprising normally open solenoid valves, are demagnetized so as to be in an open state, and the reaction force permission valve  25 , comprising a normally closed solenoid valve, is magnetized so as to be in an open state. In this state, when the fluid pressure sensor Sa provided in the fluid passage Qa detects a pushing force on the brake pedal  12  by the driver, the actuator  51  of the slave cylinder  23  is operated. That is, when the electric motor  52  is driven in one direction, the output shaft  59  is advanced by the drive bevel gear  53 , the follower bevel gear  54  and the ball screw mechanism  55 , so that the pair of the pistons  38 A and  38 B urged by the output shaft  59  are advanced. Because the ports  40 A and  40 B leading to the fluid passages Pb and Qb are closed quickly after the pistons  38 A and  38 B begin to advance, a brake fluid pressure is generated in the fluid pressure chambers  39 A and  39 B. This brake fluid pressure is transmitted to the wheel cylinders  16 ,  17 ,  20 , and  21  of the disc brake devices  14 ,  15 ,  18 , and  19 , respectively, through the opened in-valves  42  of the ABS  24 , thereby braking the vehicle wheels. 
         [0032]    Because the ports  40 A and  40 B leading to the fluid passages Pb and Qb are closed by the pistons  38 A and  38 B, the brake fluid pressure generated by the master cylinder  11  is not transmitted to the disc brake devices  14 ,  15 ,  18 , and  19 . At this time, the brake fluid pressure generated in the other fluid pressure chamber  13 B of the master cylinder  11  is transmitted to the fluid chamber  30  of the stroke simulator  26  through the opened reaction force permission valve  25  to move the piston  29  against the spring  28 , thereby generating a pseudo pedal reaction force while permitting the stroke of the brake pedal  12  to eliminate an uncomfortable feeling to the driver. 
         [0033]    The operation of the actuator  51  for the slave cylinder  23  is controlled so that the brake fluid pressure generated by the slave cylinder  23  and detected by the fluid pressure sensor Sb provided in the fluid passage Qc has a value corresponding to the brake fluid pressure generated by the master cylinder  11  and detected by the fluid pressure sensor Sa provided in the fluid passage Qa, thereby generating the braking force in the disc brake devices  14 ,  15 ,  18 , and  19  according to the pushing force applied to the brake pedal  12  by the driver. 
         [0034]    If slip ratio of any vehicle wheel is increased and a tendency of locking is detected based on the output from the wheel speed sensor Sc corresponding to a vehicle wheel during the above-described braking, the ABS  24  is operated in a state in which the slave cylinder  23  is maintained in the operating state, thereby preventing locking of the vehicle wheel. 
         [0035]    That is, when any vehicle wheel has a tendency of locking, a pressure reducing operation is performed to release the brake fluid pressure in the wheel cylinder by opening the out-valve  44  such that the transmission of the brake fluid pressure from the slave cylinder  23  is shut off by closing the in-valve  42  communicating with the wheel cylinder; and a pressure maintaining operation is subsequently performed to maintain the brake fluid pressure in the wheel cylinder by closing the out-valve  44 , thereby reducing the braking force to avoid locking of the vehicle wheel. 
         [0036]    When the vehicle wheel speed is recovered to reduce the slip ratio, a pressure increasing operation is performed to increase the brake fluid pressure in the wheel cylinder by opening the in-valve  42 , thereby increasing the braking force for braking the vehicle wheel. The above-described pressure reducing, maintaining and increasing operation is repeatedly performed each time it is detected that the vehicle wheel has a tendency of locking. The operation is repeatedly performed to generate the maximum braking force while preventing locking of the vehicle wheels. The brake fluid flowing into the reservoir  43  during this process is returned by the fluid pressure pump  47  to the fluid passages Pc and Qc on the upstream side. 
         [0037]    During the above-described ABS control, the shutoff valves  22 A and  22 B are magnetized so as to be closed, thereby preventing a fluid pressure fluctuation associated with the operation of the ABS  24  from being transmitted as a kickback from the master cylinder  11  to the brake pedal  12 . 
         [0038]    If the slave cylinder  23  becomes inoperable due to power failure or other problem, the braking control is performed using the brake fluid pressure generated by the mater cylinder  11  in place of the brake fluid pressure generated by the slave cylinder  23 . 
         [0039]    That is, in the event of power failure or other problem, as shown in  FIG. 2 , the shutoff valves  22 A and  22 B, comprising normally open solenoid valves, remain open; the reaction force permission valve  25 , comprising a normally closed solenoid valve is automatically closed; the in-valves  42 , comprising normally open solenoid valves, are automatically opened; and the out-valves  44 , comprising normally closed solenoid valves, are automatically closed. In this state, the brake fluid pressure generated in the fluid pressure chambers  13 A and  13 B of the master cylinder  11  passes the shutoff valves  22 A and  22 B, the fluid pressure chambers  39 A and  39 B of the slave cylinder  23  and the in-valves  42 , without being absorbed by the stroke simulator  26 , and operates the wheel cylinders  16 ,  17 ,  20 , and  21  of the disc brake devices  14 ,  15 ,  18 , and  19 , respectively, for braking the vehicle wheels, thus generating the braking force without any problem. 
         [0040]    When a problem occurs in the electric motor  52  of the actuator  51  of the slave cylinder  23  which renders the electric motor  52  incapable of rotating or that causes a seizure of the drive bevel gear  53 , follower bevel gear  54 , ball screw mechanism  55  or the like of the actuator  51  of the slave cylinder  23 , the slave cylinder  23  becomes incapable of generating brake fluid pressure. Therefore, it is required for the brake system to quickly and reliably detect a problem and perform a backup process. 
         [0041]    Next, the operation of a problem determination process with respect to the slave cylinder  23  will be described in reference to  FIG. 4 . 
         [0042]    First, if it is determined at Step S 1  that the driver is not applying a pushing force to the brake pedal  12  and the fluid pressure sensor Sa does not detect a brake fluid pressure generated by the master cylinder  11 , that is, the brake is not being operated, and if it is detected a pushing force is being applied that the accelerator pedal at Step S 2 , and if the motor rotational sensor Se detects at Step S 3  that the actuator  51  of the slave cylinder  23  is not being operated, a problem determination is performed on the slave cylinder  23  at Step S 4  and the subsequent steps. 
         [0043]    If the problem determination is performed while the slave cylinder  23  is generating brake fluid pressure, the generation of brake fluid pressure is interrupted. Therefore, the above-described Steps S 1  to S 3  are performed in order to confirm that the slave cylinder  23  is not being operated at that time or that there is no need to operate the slave cylinder  23  during the problem determination. If it is confirmed that a pushing force is being applied to the accelerator pedal at Step S 2 , it is possible to confirm that the driver has no intention of applying a pushing force to the brake pedal  12 . Thus, it is possible to ensure that a pushing force is not applied to the brake pedal  12  while the problem determination is being performed at Step S 4  and the subsequent steps. 
         [0044]    At Step S 4 , upon a command from the electronic control Unit U, for a very short time an electric current is supplied to electric motor  52 , such that the pistons  38 A,  38 B of the slave cylinder may be driven a microdistance or very small distance in a retreating direction (direction opposite to the brake fluid pressure generating direction) from the standard positions (stop positions when brake fluid pressure is not generated). As a result, when the motor rotational position sensor Se detects at Step S 5  that the electric motor  52  has rotated the pistons  38 A,  38 B in the opposite direction, it is determined that the slave cylinder  23  can be normally operated, and at Step S 6  the pistons  38 A,  38 B are driven forward to the above-described standard positions by driving the electric motor  52  in the brake fluid pressure generating direction. In other words, if the slave cylinder  23  can be normally operated, for a very short time an electric current is supplied to the electric motor  52 , such that pistons  38 A,  38 B of the slave cylinder are driven an equal microdistance or very small distance in the fluid pressure generating direction to offset the prior movement in the opposite direction. 
         [0045]    On the other hand, if the motor rotational position sensor Se does not detect at Step S 5  that the electric motor  52  has rotated in the opposite direction of the pistons  38 A,  38 B, it is determined that there has occurred a problem that makes the electric motor  52  incapable of rotating, or a problem that causes a seizure of the drive bevel gear  53 , the follower bevel gear  54 , ball screw mechanism  55  or the like of the actuator  51  of the slave cylinder  23 , the backup braking process is performed at Step S 7  for braking using the brake fluid pressure generated by the master cylinder  11 . 
         [0046]    The backup braking process is performed, for example, by shutting off supply of the power to the brake system. As a result, the shutoff valves  22 A,  22 B, which are normally-open solenoid valves, are opened and the reaction force permission valve  25 , which is a normally-closed solenoid valve, is closed, whereby the fluid pressure circuit enters an abnormal operation state as shown in  FIG. 2 , thereby enabling braking involving the brake fluid pressure generated by the master cylinder  11 . 
         [0047]    When the above-described problem determination process is performed, the pistons  38 A,  38 B are in the standard positions, and the master cylinder  11  and the wheel cylinders  16 ,  17 ,  20 ,  21  communicate with each other. Therefore, when it is determined that a problem has occurred, it is possible to perform the backup braking process without any problem by transmitting the brake fluid pressure generated by the master cylinder  11  to the wheel cylinders  16 ,  17 ,  20 ,  21  without shutting off the brake fluid pressure at the slave cylinder  23 . 
         [0048]    As described above, an electrical current is supplied so as to rotate the electric motor  52  of the slave cylinder  23  in a direction opposite to the braking force generating direction, and it is determined that a problem has occurred when the rotation of the electric motor  52  is not detected. Therefore, it is possible to reliably perform the problem determination process even if the slave cylinder  23  is not being operated. Also, the problem determination process can be performed before the operation of the slave cylinder  23 , thereby quickly performing the backup braking process when it is determined that a problem has occurred. 
         [0049]    An exemplary embodiment of the present invention has been described above, but various changes in design may be made without departing from the subject matter of the present invention. 
         [0050]    For example, the electric brake force generator of the present invention is not limited to the slave cylinder  23  of the exemplary embodiment. The electric brake force generator may be of a mechanical type (non-fluid-pressure type) that generates a braking force by directly driving a brake pad using the electric motor  52 . 
         [0051]    As another example, in the exemplary embodiment, a problem in the slave cylinder  23  is detected by use of the motor rotational position sensor Se. However, this problem may be detected based on the rotational position of the output shaft  59 , the axial position of the output shaft  59 , and the axial positions of the pistons  38 A,  38 B. 
         [0052]    As a third example, in the exemplary embodiment, the fluid pressure sensor Sa detects a pushing force being applied to on the brake pedal  12  by the driver, however, a pedal switch may be used to detect depression of the brake pedal.