Patent Publication Number: US-10315640-B2

Title: Vehicle having brake system and method of operating

Description:
BACKGROUND 
     The present invention relates to vehicles having brake systems. It is known to provide a vehicle with a full-power brake system (also referred to as a “decoupled” or “brake-by-wire” system) in which driver applied force does not propagate to produce the actual braking force to the brake devices. Instead, fluid is pushed from the master cylinder into a simulator circuit while another mechanism provides the actual braking force. Although satisfactory for the intended purpose, a great number of valves and sensors must all be in working order to provide brake-by-wire operation, and it can be difficult to diagnose faults within the system. 
     SUMMARY 
     In one aspect, the invention provides a vehicle including a master cylinder having an input side configured to receive an input from a brake pedal and an output side configured to provide a master cylinder output. At least one braking circuit of the vehicle has at least one wheel cylinder and a brake pressure generator, including a strokable piston, separate from the brake pedal. A simulator circuit includes a pedal feel simulator coupled to the master cylinder output side through a switchable simulator valve, the pedal feel simulator providing a reaction force to the brake pedal when the switchable simulator valve is in an open position. At least one normally-open isolation valve is operable to close and isolate the at least one braking circuit from the master cylinder and the simulator circuit. A pressure sensor is operable in at least one vehicle configuration to be in fluid communication with both the simulator circuit and the brake pressure generator. A controller is programmed to, at a designated diagnostic time when no input is received from the brake pedal, establish a diagnostic circuit connecting the simulator circuit with an output of the brake pressure generator, and to stroke the piston in an advancing, pressure-generating direction while observing a resulting increase in brake fluid pressure with the pressure sensor. The controller is further programmed to check whether the relationship between the observed brake fluid pressure increase and the piston stroke is within a predetermined acceptable range for continued operation of a brake-by-wire vehicle braking mode in which the master cylinder is coupled to the simulator circuit and decoupled from the at least one braking circuit, while brake fluid pressure is generated solely by the brake pressure generator. 
     In another aspect, the invention provides a method of operating a vehicle utilizing a controller. The vehicle is operated in a primary brake-by-wire braking mode including: A) receiving an input from a brake pedal at an input side of a master cylinder and providing a master cylinder output corresponding to the brake pedal input at an output side of the master cylinder output, B) closing, by a controller signal, at least one normally-open isolation valve to isolate the output side of the master cylinder from at least one braking circuit having at least one wheel cylinder, C) sending a controller signal to a switchable simulator valve to open a fluid connection between the master cylinder output side and a simulator circuit including a pedal feel simulator to provide a reaction force to the brake pedal, D) generating a braking request signal with a primary pressure sensor responsive to the input from the brake pedal, the braking request signal being sent to the controller, and E) driving a brake pressure generator of the at least one braking circuit with the controller responsive to the braking request signal to achieve braking at the at least one wheel cylinder, the brake pressure generator having a strokable piston separate from the brake pedal. A controller signal is sent, at a designated diagnostic time when no input is received from the brake pedal, to open the simulator valve to place the pedal feel simulator in fluid communication with an output of the brake pressure generator. At the designated diagnostic time, the piston of the brake pressure generator is stroked in an advancing, pressure-generating direction while observing a resulting increase in brake fluid pressure with a secondary pressure sensor. The controller determines whether the relationship between the observed brake fluid pressure increase and the piston stroke is within a predetermined acceptable range for continued operation of the primary brake-by-wire braking mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a vehicle braking system according to one aspect of the present invention. A diagnostic system configuration during non-braking is illustrated. 
         FIG. 2  is a graph of sensed pressure versus piston stroke, including a separation curve distinguishing normal response curves from hard pedal response curves. 
         FIG. 3  is a flow diagram illustrating steps of a method according to one aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     The braking system  20  of  FIG. 1  includes a master cylinder  24  having an input side coupled with an input rod  25  to a brake pedal  28  to pressurize hydraulic fluid therein. The master cylinder  24  includes a first piston  26   1  that is coupled to the brake pedal  28  to move directly therewith. The first piston  26   1  pressurizes hydraulic fluid in a first chamber of the master cylinder  24  to be output from the first chamber at a first outlet  40   1 . A second piston  26   2  of the master cylinder  24  can be moved under the influence of fluid pressurized in the first chamber by the first piston  26   1 , without the second piston  26   2  having any direct connection to the first piston  26   1  or the brake pedal  28 . The second piston  26   1  pressurizes hydraulic fluid in a second chamber of the master cylinder  24  to be output from the second chamber at a second outlet  40   2 . A fluid reservoir  32  is in fluid communication with the first and second chambers of the master cylinder  24  until the brake pedal  28  is initially actuated, at which time the pistons  26   1 ,  26   2  block off the master cylinder chambers from the reservoir  32 . A normally-open solenoid valve  34  can selectively establish a connection between the reservoir  32  and the first master cylinder chamber. A pedal travel sensor  36  is coupled to the brake pedal  28  and is operable to detect an amount of travel of the brake pedal  28 , so that a corresponding signal can be sent to a controller (not shown). The controller can be a computer electrically coupled with each sensor and each electrically-operable valve of the braking system  20 , to send signals thereto and/or receive signals therefrom to establish communication and control necessary to operate the braking system  20 . 
     The dual outputs  40   1 ,  40   2  of the master cylinder  24  are selectively in fluid communication with a first braking circuit and a second braking circuit, respectively. In the illustrated construction, each of the braking circuits includes a pair of brake devices or wheel cylinders WC operable to slow down the wheels of a vehicle on which the braking system  20  is provided. The wheel cylinders WC of a particular circuit can be associated with a set of front vehicle wheels, a set of rear vehicle wheels, or a set of diagonal vehicle wheels. Each braking circuit includes an inlet valve  44  and an outlet valve  48  associated with each respective wheel cylinder WC. The inlet valves  44  are normally-open and can be electrically closed by the controller to stop or limit pressurized hydraulic fluid supplied to the wheel cylinder WC. The outlet valves  48  are normally-closed and can be electrically opened by the controller to relieve pressurized hydraulic fluid at the wheel cylinder WC, to the reservoir  32 . Each of the master cylinder outlets  40   1 ,  40   2  is coupled to one of the braking circuits through a normally-open isolation valve  52   1 ,  52   2 . Each of the isolation valves  52   1 ,  52   2  is operable to be closed by the controller to fluidly separate or isolate the master cylinder  24 , and thus the brake pedal  28 , from the braking circuits having the wheel cylinders WC. 
     Although the master cylinder  24  is capable of providing mechanical braking from the brake pedal  28  to the wheel cylinders WC of the two braking circuits, the system  20  can be provided with an alternate or auxiliary device, separate from the brake pedal  28  and referred to herein as a brake pressure generator  60 , for generating hydraulic fluid pressure to the wheel cylinders WC for the requisite braking need. The brake pressure generator  60  can include a plunger or piston  62  drivable in a cylinder by an actuator such as an electric motor  64  operated by the controller. As such, the brake pressure generator  60  is operable to drive pressurized hydraulic fluid to the wheel cylinders WC of the first and second braking circuits. For example, an outlet  68  of the brake pressure generator  60  can be coupled, in parallel, to the first and second braking circuits through respective apply pressure control valves  72   1 ,  72   2 . Each of the apply pressure control valves  72   1 ,  72   2  can be a controller-modulated solenoid valve (e.g., having a range of open positions, or receiving a pulse-width modulation signal to achieve a similar effect) operable to control the pressure supplied from the brake pressure generator  60  to the wheel cylinders WC of the given braking circuit. The apply pressure control valves  72   1 ,  72   2  can be coupled to respective brake fluid supply lines or passages, each of which extends between one of the isolation valves  52   1 ,  52   2  and the respective inlet valves  44  of the braking circuit. One or more pressure sensors  76  can be positioned along the fluid path between the brake pressure generator outlet  68  and the respective inlet valves  44  and operable to report the fluid pressure to the controller. The pressure sensor  76  can be referred to as an “active circuit” pressure sensor as it senses and reports the fluid pressure in the passage(s) coupled to the wheel cylinders WC, as contrasted with fluid pressure in the master cylinder  24  or a simulator circuit, which are not part of an active braking circuit during brake-by-wire operation. Additional sensors may be provided to monitor parameters of the piston  62  and/or the electric motor  64 , and may include any one or more of: linear or angular position, electrical current, electrical voltage, force, torque, or temperature. 
     In addition to the active braking components in the system  20 , a simulator circuit is provided in fluid communication with the output side of the master cylinder  24 . The simulator circuit is provided upstream of the isolation valves  52   1 ,  52   2 , meaning the side nearer the master cylinder and remote from the braking circuits so that the simulator circuit is kept in fluid communication with the master cylinder  24  when the isolation valves  52   1 ,  52   2  are closed. The simulator circuit includes a pedal feel simulator  80  coupled to an outlet of the master cylinder  24  (e.g., the first outlet  40   1 ) through a switchable simulator valve  84 . The simulator valve  84  can be a normally-closed switchable solenoid valve operable to be opened by the controller to establish fluid communication between the master cylinder outlet  40   1  and the pedal feel simulator  80 . When the simulator valve  84  is open, fluid pushed out of the master cylinder chamber through the outlet  40   1  is passed into the pedal feel simulator  80 , which has a biasing mechanism that provides a feedback force to the brake pedal  28 . Thus, the simulator circuit mimics the feel of actuating the wheel cylinders WC when in fact the brake pedal  28  is decoupled by the isolation valves  52   1 ,  52   2  from the actual braking pressure activating the wheel cylinders WC in the braking circuits. A pressure sensor, referred to herein as the primary pressure sensor  88 , is provided in fluid communication with the master cylinder  24  to detect a fluid pressure generated in one of the master cylinder chambers. For example, the primary pressure sensor  88  can be coupled to the second master cylinder outlet  40   2 , upstream of the isolation valve  52   2 . The primary pressure sensor  88  is operable to generate a braking request signal responsive to an input force from the brake pedal  28 . 
     Though not conducive to labeling in  FIG. 1 , it will be understood that each braking circuit extends from one of the isolation valves  52   1 ,  52   2  to the respective wheel cylinder(s) WC, and further includes the passages connecting to the brake pressure generator  60 , and the respective passages connecting to the fluid reservoir  32 , while the simulator circuit is a separate circuit, not part of either of the braking circuits, since fluid in the simulator circuit is not conveyed to contribute to actual braking force at the wheel cylinders WC. 
     During normal operation of the braking system  20 , the brake pedal  28  is decoupled from the wheel cylinders WC so that braking occurs fully in a primary brake-by-wire mode. When the driver depresses the brake pedal  28 , the isolation valves  52   1 ,  52   2  are actuated to a closed position (opposite the position shown in  FIG. 1 ) so that the master cylinder  24  and the simulator circuit are cut-off or isolated from the braking circuits. The simulator valve  84  is also switched open by the controller upon initial actuation of the brake pedal  28 , which can be detected by the pedal travel sensor  36 . A pressure increase occurs in the second master cylinder chamber and between the second outlet  40   2  and the second isolation valve  52   2  since the pedal  28  urges the pistons  26   1 ,  26   2  to move toward the closed-off second isolation valve  52   2 . The pressure increase is measured or detected by the primary pressure sensor  88  and conveyed as a signal to the controller, which is programmed to use the information to determine the degree of actuation of the brake pressure generator  60  to achieve a target brake force as requested by the driver&#39;s actuation of the brake pedal  28 . In some constructions, an output of the pedal travel sensor  36  is also considered by the controller along with the primary pressure sensor  88  in quantifying the driver&#39;s braking request. The controller can also provide variable manipulation of the apply pressure control valves  72   1 ,  72   2  to achieve a desired brake force and brake force balance in the braking circuits. Thus, in the illustrated construction, the motor  64  is energized as programmed by the controller to drive the piston  62  forward in the cylinder toward the outlet  68  so that fluid pressure is generated at the outlet and hydraulic fluid is moved from the brake pressure generator  60  toward the wheel cylinders WC, which may include one or more pistons incorporated into brake calipers so that the hydraulic fluid from the generator  60  causes the wheel cylinders WC to squeeze onto a brake disc. As can be interpreted from this description, the brake pressure generator  60  is controlled to achieve an amount of braking according to the driver&#39;s request, which is interpreted at least in part from an output of the primary pressure sensor  88 , which continuously measures how hard the driver is applying pressure to the brake pedal  28 . In the event of a component failure or abnormality, the braking system  20  is designed to provide a back-up mode of operation in which the isolation valves  52   1 ,  52   2  return to their normally-open positions to allow the brake pedal  28  to actuate the wheel cylinders WC through the master cylinder  24 . However, the invention provides a diagnostic routine to determine whether to transition to a direct mechanical push-through back-up mode or to a secondary brake-by-wire mode when a malfunction occurs in the primary, normal mode of brake-by-wire operation. 
     During operation of the vehicle, at a diagnostic time when the braking system  20  is not being actuated to slow the vehicle, the controller is programmed to put the system into a diagnostic configuration as shown in  FIG. 1  and carry out a diagnostic routine. For the diagnostic configuration, the controller is programmed to open (i.e., not actuate closed) the first isolation valve  52   1  and the controller is programmed to actuate the first apply pressure control valve  72   1  to open. The second isolation valve  52   2  can be actuated closed, and the second apply pressure control valve  72   2  can be left un-actuated to assume the normally-closed position. The open first isolation valve  52   1  places the simulator circuit, in particular the simulator valve  84  and the pedal feel simulator  80 , in fluid communication with the corresponding braking circuit. As such, fluid communication is established between the simulator circuit and the pressure sensor  76  that is positioned in the braking circuit, in particular the pressure sensor  76  (differentiated from the primary pressure sensor  88  as being the “secondary” or “active circuit” pressure sensor) positioned between the brake pressure generator outlet  68  and the apply pressure control valves  72   1 ,  72   2 . The simulator circuit is also fluidly coupled to the outlet  68  of the brake pressure generator  60 . The inlet valves  44  coupled to the brake pressure generator  60  through the first apply pressure control valve  72   1  are actuated closed by the controller as shown on the right side portion of  FIG. 1  so that the associated wheel cylinders WC of the corresponding braking circuit are not exposed to the outlet  68  of the brake pressure generator  60 . The normally-open valve  34  between the input side of the master cylinder  24  and the fluid reservoir  32  is also actuated closed by the controller for the diagnostic configuration. 
     During the diagnostic time, while no application of the brake pedal  28  is detected, the controller is programmed to maintain the simulator valve  84  and the first apply pressure control valve  72   1  open, thus establishing a diagnostic circuit. With the system  20  put into the diagnostic configuration as described above and shown in  FIG. 1 , the controller can evaluate a relationship between displacement of the piston  62  and fluid pressure in the diagnostic circuit, including the simulator circuit. The controller is programmed to stoke the piston  64  in a forward, pressure-generating direction as the resulting fluid pressure is measured by the secondary pressure sensor  76 . The brake pressure generator  60  may be programmed to provide a predetermined actuation amount for the motor  64  to drive the piston  62 , and once complete, the piston  62  can be retracted by reverse operation of the motor  64 . In observing the relationship between piston stroke and the resulting fluid pressure increase, the information observed by the controller is represented by the graph of  FIG. 2 . The x-axis represents actuation amount, or position, of the piston  62 , while the y-axis represents the sensed fluid pressure, in other words, the output of the secondary pressure sensor  76 .  FIG. 2  illustrates two exemplary plots or traces that may be observed by the controller during the diagnostic routine. The controller is programmed with a detection curve  100  that divides the potential graph area. The detection curve  100  may be defined in a particular region of the total available travel range of the piston  62  where the evaluated data points exclude the initial travel range, where the data may overlap for the acceptable and unacceptable conditions. For example, the detection curve  100  can be a vertical line representing a single value of piston position beyond a value where a sharp pressure spike occurs when a hard pedal type failure occurs. When the results of the diagnostic routine finds a pressure at the detection curve  100  that is within a predetermined tolerance range of an expected pressure value P 1 , this indicates a good check or satisfactory condition. When the results of the diagnostic routine finds a pressure at the detection curve  100  that is at least a predetermined amount higher than the expected pressure value P 1  (example P 2  in  FIG. 2 ), this indicates a “hard pedal” condition. In other words, the pressure rises significantly faster than expected for a normally-functioning system as the piston  62  is stroked, and the controller predicts or determines the occurrence of hard pedal condition. Hard pedal refers to the circumstance when brake pedal travel is far less than a design amount for a given target circuit pressure. The controller may also be programmed to identify a fluid leak if during the stroking of the piston  62 , the fluid pressure does not increase within a predetermined tolerance range of the expected pressure value P 1  at the detection curve  100 . 
     The controller can be programmed to conduct the diagnostic routine in response to detecting an abnormal value from the primary pressure sensor  88 , or the controller may be programmed to conduct the diagnostic routine upon each identification of a predetermined vehicle condition (e.g., acceleration), or the controller may be programmed to conduct the diagnostic routine on a less frequent basis, such as a single time during a period of continuous vehicle operation (e.g., first acceleration). Vehicle acceleration can be identified by the controller on the basis of a sensor output, such as a wheel speed sensor, an accelerometer, or a throttle position sensor, for example. The controller may be triggered to perform the diagnostic routine only when the vehicle is accelerating beyond a predetermined threshold or only when the throttle is open more than a predetermined threshold to avoid conditions in which the driver is likely to depress on the brake pedal  28  while the diagnosis is being performed. 
     The diagnostic routine serves as a hardware check that allows the controller to determine whether there is any mechanical failure of the simulator circuit (e.g., simulator valve  84  not opening, pedal feel simulator  80  stuck and not receiving fluid). In the case of a mechanical failure in the simulator circuit, the braking system  20  may not be enabled to perform brake-by-wire braking, since the simulator circuit is required to accept the fluid from the master cylinder  24  when the braking pressure comes from a source other than the master cylinder  24 . However, when the controller can determine based on the diagnostic routine that there is nothing mechanically wrong with the simulator circuit, the system  20  can be operated in brake-by-wire operation. If the controller has determined that the primary pressure sensor  88  is reporting abnormal values and cannot be relied upon to generate the braking request signal for brake-by-wire operation, the braking system  20  transitions to a secondary brake-by-wire mode in which the braking request signal is generated by the pedal travel sensor  36 , assuming that the diagnostic routine has indicated no physical failure in the simulator circuit. This method of operation enables the braking system  20  to perform in a more sophisticated manner and achieve better performance, by retaining brake-by-wire operation when the primary pressure sensor  88  fails, as long as the operability of the simulator circuit hardware is confirmed in the diagnostic routine. 
     The process carried out by the program of the controller as described above is visually represented in the flow diagram of  FIG. 3 . At step  200 , the controller operates the braking system  20  in a first mode, which is the normal or primary brake-by-wire mode in which the braking force (i.e., hydraulic fluid pressure) to the wheel cylinders WC is produced by the brake pressure generator  60 , not the master cylinder  24 , in proportion to the driver&#39;s demand as manifested by the fluid pressure sensed by the primary pressure sensor  88 . The apply pressure control valves  72   1 ,  72   2  may open and close as required to modulate the pressure to the two braking circuits. During step  200 , the simulator valve  84  is open and the two isolation valves  52   1 ,  52   2  are closed. At step  204 , the controller identifies the diagnostic time as described above. This may be part of a normal repetitive routine, or only triggered by the detection of an abnormality in the output of the primary pressure sensor  88 . Examples of the manner in which the controller detects the abnormality have been described above. At step  208 , the controller puts the braking system  20  into the diagnostic configuration. This includes controlling the system valves as shown in  FIG. 1  to establish a diagnostic circuit (not for braking) that includes the brake pressure generator, the simulator circuit, and the secondary pressure sensor  76 . In the diagnostic configuration, the simulator valve  84  is actuated to open. The braking system  20 , in particular the program of the controller, then carries out the diagnostic routine to determine whether there is any fault with the mechanical components, such as those of the simulator circuit that normally enable fluid to be received from the master cylinder  24  during brake-by-wire operation. 
     At step  212 , the controller (e.g., by control of the motor  64 ) drives the piston  62  to stoke or advance to push fluid toward the simulator circuit. At step  216 , the controller is programmed to compare the resulting pressure increase, as measured by the secondary pressure sensor  76 , to the piston displacement. Thus, the controller can observe the relationship between actuation amount of the brake pressure generator  60  and the resulting fluid pressure increase and can compare this data to data or values stored in a memory of the controller (e.g., the data corresponding to the detection curve  100  of  FIG. 2 ) to determine if the simulator hardware is in proper working condition at step  220 . As described above, this can include determining whether a “hard pedal” condition occurs. When the simulator hardware is found to be in non-working condition resulting in the hard pedal condition, the pedal feel simulator  80  is prevented from receiving fluid in the designed manner, such as the pedal feel simulator  80  having an internal component (e.g., a spring) being stuck or the simulator valve  84  being stuck closed. As such, the controller de-activates brake-by-wire operation at step  224 . For this mode of operation, the system valves default to their normally-biased positions. The braking system  20  is then operable in a “coupled” or “direct” braking mode in which fluid pressure at the master cylinder  24  is propagated to the wheel cylinders WC, and the brake pressure generator  60  is left idle. In addition to transitioning out of the primary brake-by-wire mode, the controller can also trigger and store an error code at step  236  when a malfunction of the simulator hardware is determined at step  220 . 
     When the controller determines at step  220  that the simulator hardware is working properly, the process continues to step  228  whereby the state of the primary pressure sensor  88  is considered. It is noted that the state of the primary pressure sensor  88  can be identified (e.g., identification of an abnormal value) in advance of the diagnostic time, and in some cases may trigger the diagnostic routine. If the simulator hardware is checked to be OK and the primary pressure sensor  88  has not reported abnormal values, the process returns to normal primary mode brake-by-wire operation at step  200 . On the other hand, when the simulator hardware is checked to be OK and the primary pressure sensor  88  has reported abnormal values, the controller at step  232  is programmed to activate a secondary brake-by-wire mode. The controller can also trigger and store an error code at step  238  when the primary pressure sensor output is deemed abnormal. Rather than generic “system fault” errors, the error codes of steps  236  and  238  can include information identifying whether the simulator hardware, or the primary pressure sensor  88 , was confirmed to be in working order or not. Thus, a service technician can more readily identify the source of the problem and more conveniently provide an appropriate repair or replacement. Optionally, the error code may be displayed to the driver in an instrument panel of the vehicle, either in a generic or specific format. 
     When the diagnostic routine proceeds to step  232  after confirming proper operation of the simulator hardware, the system  20  commences brake-by-wire operation in the secondary mode. In this mode, brake pedal actuation is detected by the pedal travel sensor  36 , and the driver isolation valves  52   1 ,  52   2  are actuated to close and the simulator isolation valve  84  is actuated to open. As pedal feedback is provided by the pedal feel simulator  80 , a braking request of the driver is sensed and reported to the controller by a sensor (e.g., the pedal travel sensor  36 ) other than the primary or secondary pressure sensors  88 ,  76 . Brake force (i.e., hydraulic fluid pressure) corresponding to the braking request is generated by the brake pressure generator  60  and applied to the corresponding wheel cylinders WC through the respective apply pressure control valves  72   1 ,  72   2 .