Patent Publication Number: US-2018031065-A1

Title: Electromechanical brake system and method

Description:
FIELD OF THE INVENTION 
     The subject invention relates to an electromechanical brake system, and more particularly, to a method of determining brake pad lining wear as part of the electromechanical brake system. 
     BACKGROUND 
     Traditional service braking systems of a vehicle are typically hydraulic fluid based systems actuated by a driver depressing a brake pedal that generally actuates a master cylinder. In-turn, the master cylinder pressurizes hydraulic fluid in a series of hydraulic fluid lines routed to respective actuators at brakes located adjacent to each wheel of the vehicle. Such hydraulic braking may be supplemented by a hydraulic modulator assembly that facilitates anti-lock braking, traction control, and vehicle stability augmentation features. The wheel brakes may be primarily operated by the manually actuated master cylinder with supplemental actuation pressure gradients supplied by the hydraulic modulator assembly during anti-lock, traction control, and stability enhancement modes of operation. Hydraulic brake systems are known to include dedicated brake wear sensors (e.g., clip-on sensors) generally placed upon or secured to the brake pad itself for detecting brake wear. Unfortunately, such clip-on sensors may not be robust, are costly, and may require replacement with worn brake pads. 
     More recent brake designs may include brake assemblies with an electromechanical park brake feature as part of the actuator. With such a feature, the driver merely presses a button to electrically actuate the brakes into a park brake mode. Yet further, recent brake system designs may not include any hydraulics. Such systems may be known as brake-by-wire (BBW) systems that electrically actuate the brakes during both service and park brake modes of operation. Such systems with electro-mechanical attributes may still include the clip-on sensors applicable for hydraulic systems to determine brake pad wear. 
     Accordingly, it is desirable to provide a brake pad wear check process and related hardware that may better utilize attributes of electromechanical brake components to reduce system costs, simplify complexity, and improve robustness. 
     SUMMARY OF THE INVENTION 
     In one exemplary embodiment of the present disclosure, an electromechanical brake system includes a structure defining a chamber, a member adapted to rotate with a vehicle wheel, an electric motor mounted to the structure, a piston, a brake pad, and a controller. The piston is coupled to the electric motor for reciprocation within the chamber between a variable actuated position and a retracted position. The brake pad is movably supported by the structure, operatively coupled to the piston, and adapted to be in braking contact with the member when the piston is in the actuated position, and spaced from the member when the piston is in the retracted position. The controller is configured to control reciprocation of the piston between the actuated and retracted positions via energization of the motor, perform a brake pad wear check by calculating a change in a parameter associated with a change in distance between the retracted position and the variable actuated position, and comparing the change in parameter to a preprogrammed threshold value. 
     In another exemplary embodiment, a method of performing a brake pad wear check includes the step of driving a piston from a retracted position to a clamped position via an electric motor. A current spike induced by the electric motor when the piston is in the clamped position is then sensed by a controller. A parameter associated with the movement from the retracted position and to the clamped position is then measured. The parameter is then compared to a predetermined threshold to determine brake pad wear. 
     The above features and advantages and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which: 
         FIG. 1  is a schematic plan view of a vehicle having an electromechanical brake system as one, non-limiting, example in accordance with the present disclosure; 
         FIG. 2  is a side view of a brake assembly of the electromechanical brake system with sections removed to show internal detail; and 
         FIG. 3  is a flow chart of a method of performing a brake pad wear check. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the terms module and controller refer to processing circuitry that may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     In accordance with an exemplary embodiment of the invention,  FIG. 1  is a schematic of a vehicle  20  that may include a powertrain  22  (i.e., an engine, transmission and differential), a plurality of rotating wheels  24  (i.e., four illustrated), and an electromechanical brake system  26  that may include a brake assembly  28  for each respective wheel  24 , a brake pedal assembly  30 , and a controller  32 . The powertrain  22  is adapted to drive at least one of the wheels  24  thereby propelling the vehicle  20  upon a surface (e.g., road). The electromechanical brake system  26  is configured to generally slow the speed and/or stop motion of the vehicle  20 . The vehicle  20  may be an automobile, truck, van, sport utility vehicle, or any other self-propelled or towed conveyance suitable for transporting a burden. The electromechanical brake system  26  may be a brake-by-wire (BBW) system, or may be any variety of more traditional hydraulic braking systems having an electromechanical brake assembly  28 . In one example, the brake assembly  28  may be constructed and arranged to operate hydraulically during normal braking of the vehicle  20 , and operate electromechanically when actuating a parking brake. 
     Referring to  FIG. 2 , each brake assembly  28  of the electromechanical brake system  26  may include a structure  34 , a member  36 , a drive unit  38 , a piston  40 , and at least one brake pad  42 . The structure  34  may be, or may include, a caliper, and may define the boundaries of a piston chamber  44 . The member  36  is constructed and arranged to generally rotate with the wheel  24 , and may be a brake disc or rotor. The piston  40  is disposed for reciprocation in the piston chamber  44 , and is generally driven by the drive unit  38 . The brake pad  42  may be two brake pads located on opposite sides of the brake disc  36  and generally supported by the caliper  34  for movement toward and away from the respective sides of the brake disc  36 . 
     The drive unit  38  may include an electric motor  46 , a geared drive  48 , a screw  50 , and a nut  52 . In operation, the electric motor  46  generally powers or drives the geared drive  48  which rotates the screw  50  that may extend along an axis  54  extending in the direction of piston and brake pad travel. Because the nut  52  is threaded to the screw  50 , the nut  52  bears upon the piston  40 , which may contact the brake pad  42  and press the brake pad against the brake disc  36  as the screw  50  rotates. Reverse rotation of the screw  50  causes the brake pad to release from the brake disc  36  and moves the piston  40  away from the brake disc  36 . It is contemplated and understood that the nut  52  may be an integral part (e.g., one unitary part) of the piston  40  or may be two separate components. It is further contemplated and understood that the drive unit  38  may be part of a BBW system, or may be dedicated toward a park brake feature. 
     In other, non-limiting, embodiments, the brake assembly  28  may be a drum brake assembly, or other types. The drive unit  38  may be an electro-hydraulic brake actuator (EHBA) or other actuator capable of moving the brake pad  42  against the rotating member  36  in response to an electrical command signal from the controller  32 . More specifically, the drive unit  38  may be, or may include, any type of motor capable of acting upon a received electric signal and, as a consequence, converting energy into motion that controls movement of the piston  40 . Thus, the motor  46  may be a direct current motor configured to generate electro-hydraulic pressure delivered to, for example, the piston  40 . 
     Referring to  FIG. 1 , and with respect to the embodiment of a BBW system  26 , the controller  32  may include a computer-based processor (e.g., microprocessor) and a computer readable and writeable storage medium. In operation, the controller  32  may receive one or more electrical signals from the brake pedal assembly  30  over a pathway (see arrow  55 ) indicative of driver braking intent. In-turn, the controller  32  may process such signals, and based at least in-part on those signals, output an electrical command signal to the drive unit  38  over a pathway (see arrow  57 ). Based on any variety of vehicle conditions, the command signals directed to each wheel  24  may be the same or may be distinct signals for each wheel  24 . The pathways  55 ,  57  may be wired pathways, wireless pathways, or a combination of both. Non-limiting examples of the controller  32  may include an arithmetic logic unit that performs arithmetic and logical operations; an electronic control unit that extracts, decodes, and executes instructions from a memory; and, an array unit that utilizes multiple parallel computing elements. 
     Other examples of the controller  32  may include an engine control module, and an application specific integrated circuit. It is further contemplated and understood that the controller  32  may include redundant controllers, and/or the system may include other redundancies, to improve reliability of the BBW system  26 . 
     In another embodiment, where the brake system  26  may not be a BBW system, and instead may include more traditional hydraulic components for normal service or operation of the brake system  26 , the brake assembly  28  may include an electromechanical park brake feature. That is, the system  26  may use hydraulic components for decelerating a vehicle down via driver actuation of the brake pedal assembly  30 , but when initializing a parking brake feature, the brake assembly  28  operates electromechanically utilizing the drive unit  38  (see  FIG. 2 ). 
     The brake system  26  and/or brake assembly  28  may include a subsystem and/or the ability to determine brake pad lining wear. To facilitate this ability, the brake system  26  may further include a vehicle level sensor  58  (e.g., multi-axis accelerometer) and an ignition position module  60  that may be hardware and/or software based. The vehicle level sensor  58  and the ignition position module  60  are configured to both input data (e.g., signal) to the controller  32  used as part of a process to determine brake pad wear. 
     In order to check for brake pad wear, the controller  32  may be configured to measure or detect an increase in piston  40  travel along axis  54 , and between a clamped or actuated position and a retracted position. The actuated position generally signifies firm contact of the brake pad  42  to the brake disc  36 , and varies in placement depending upon the degree of wear of the brake pad  42 . The retracted position is generally a reference point positioned along the axis  54 , and generally denoting a pre-established and constant distance of, for example, the piston  40  from the brake disc  36 . When the piston  40  is in the retracted position, the brake pad  42  is not in contact with the brake disc  36  (i.e., is spaced from the brake disc). 
     The controller  32  may determine when the piston  40  along with the brake pad  42  is in the actuated position by receiving a signal indicative of a first energy peak, or current spike, that results when the electric motor  46  strains to further move the brake pad  42  against the brake disc  36 . The retracted position is similarly determined except that the energy peak may be established once during a setup or initialization process by the system  26  to determine the reference point location along axis  54 . More specifically, a stop  62  may be carried between a surface  64  carried by the caliper  34  and defining in-part a boundary of the piston chamber  44 , and a face  66  that may be carried by a trailing skirt  68  of the piston  40 . In one example, the surface  64  of the caliper  34 , and the face  66  of the trailing skirt  68  may be generally annular in shape and opposed to one-another. 
     During an initialization process, the controller  32  may energize the motor  46  causing the piston  40  to axially move toward the surface  64  of the caliper  34 . This movement continues until the surface  64  of the piston  40  contacts the face  66  (i.e., stop  62  is engaged). Immediately after contact, the continued exertion of the motor  46  causes a second energy peak (e.g., current spike) that is received by the controller  32 . The actual reference point utilized during prescheduled brake pad wear checks, and determined by the controller  32 , may be at some distance before the stop  62  engages. It is further contemplated and understood that the reference point may be alternatively established utilizing a motor position sensor with a designed ‘home’ retraction position. One example of a motor position sensor may be an encoder. 
     In one embodiment, an internal clock of the controller  32  measures a time duration between first and second energy peak occurrences. Because the rate of axial travel of the piston  40  may be known, an axial distance of piston travel between actuated and retracted positions, associated with the reference point, may be calculated by the controller  32 . The piston travel distance between states will increase as the brake pad wears. The controller  32  may be configured to monitor the change in distance until a pre-programmed threshold is reached. When the threshold is reached, the controller  32  may be programmed to react. As one example, the controller  32  may notify the vehicle driver in the form of a visual indicator. It is further contemplated and understood that the distance measured may not be a function of time. Instead, the controller  32  may recognize a change in motor position from the reference point indicated by a motor encoder. It is contemplated and understand that the controller  32  may generally monitor any parameter (e.g., travel distance and/or time) associated with piston travel between the actuated and retracted positions, and compare this parameter to a predetermined threshold value thus establishing, for example, a remaining life estimate of the brake pad. 
     Referring to  FIG. 3 , the controller  32  may be programmed to perform periodic brake pad wear checks contingent upon certain vehicle conditions being met. That is, in one embodiment, a diagnostic test to collect pad wear information may be run in accordance with the flow chart of  FIG. 3 . In block  100  of a method of performing brake pad wear checks, the controller  32  may determine if a pre-programmed check interval has lapsed. If yes, and in block  102 , the controller may confirm the vehicle  20  is in park (e.g., via a transmission position), the ignition or key is off via the ignition position module  60 , and the vehicle  20  is parked on substantially level ground via the level sensor  58 . If yes to all three self-verifications and in block  104 , the controller  32  may initiate a brake pad wear check by self-energizing the park brake feature via the motor  46 . Also, and if yes to all three self-verifications, and in block  106 , if a driver electrically activates the park brake feature, the controller  32  may initiate the park brake check. 
     In block  108 , the initialization process is generally conducted by retracting the piston  40 . In block  110 , after the controller  32  initiates a brake pad wear check, the controller  32  may confirm that a reference point was pre-established. If “no” to a reference point being pre-established, and in block  112 , a brake system diagnosis may be in order, and/or the initialization process may be performed as previously described (see arrow  114 ). If “yes” to a reference point being pre-established, and in block  116 , the controller actuates the piston  40  via the drive unit  38 , and moves the piston from the retracted position and into the actuated or clamped state. In block  118 , the resulting travel data is stored within, for example, the memory of the controller  32 . In block  120 , the controller  32  may calculate and update remaining brake lining thickness within an algorithm while re-initializing the clock for the next check (see arrow  122 ). In block  124 , information relative to the remaining brake pad life may be communicated to the driver. 
     Advantages and benefits of the present disclosure include the ability to provide a driver with actual continuous measurement of remaining brake pad life without the use of dedicated, consumable, sensors, thus avoiding the expense of electrical sensor(s) and wiring harnesses. Another advantage is the avoidance of brake pad geometry changes and caliper design and tooling changes that may be driven by known clip-in electrical sensor and wire routing. Yet further, a brake system is provided that may include lower costs, improved accuracy in determining brake pad wear, and reduced complexity. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the application.