Automated friction brake assisted vehicle stop

A method of assisting deceleration during a stop of a motor vehicle having a drivetrain including a traction motor, a road wheel operatively connected to the drivetrain, a friction brake configured to decelerate the road wheel, and an electronic controller includes detecting, via the electronic controller, a request to stop the vehicle. The method also includes commanding the traction motor to provide regenerative braking in response to the request to stop the vehicle. The method additionally includes determining a current vehicle operating state. The method also includes determining an amount of brake drag torque to be generated by the friction brake based on the current vehicle operating state. The method further includes commanding an application of the determined amount of the brake drag torque in parallel with the regenerative braking, thereby operating the friction brake as a mechanical drivetrain damper while assisting the regenerative braking to stop the motor vehicle.

INTRODUCTION

The disclosure relates to an automated friction brake assisted stop in a motor vehicle employing a drivetrain with an electric motor.

Motor vehicles generally employ a powerplant to generate propulsion and braking systems to inhibit the vehicle's motion. A traditional brake is typically a mechanical friction device designed to inhibit motion by converting kinetic energy into heat. Such mechanical braking systems apply a retarding force, typically via specifically adapted frictional elements at the vehicle's rotating axles or wheels, to slow the vehicle.

Friction brakes often include stationary shoes or pads that are lined with friction material and configured to be engaged with a rotating wear surface, such as a rotor or a drum. Common configurations include shoes that contact to rub on the outside of a rotating drum, commonly called a “band brake”, a rotating drum with shoes that expand to rub the inside of a drum, commonly called a “drum brake”, and pads that pinch a rotating disc, commonly called a “disc brake”. Generally, vehicle friction brakes absorb thermal energy and store the energy mainly in the brake disc or brake drum while the brakes are being applied, and then gradually transfer stored heat to the ambient.

Other methods of energy conversion may also be employed. For example, electric or hybrid-electric vehicles using traction motors for propulsion frequently employ regenerative braking, where the traction motor is operated in energy generation mode to retard vehicle motion. Generally, regenerative braking converts much of the vehicle's kinetic energy to electric energy, which may then be stored for later use in onboard batteries. Many modern electric and hybrid-electric vehicles employ braking systems that include a combination of mechanical friction and regenerative braking.

Occasionally, operation of braking systems may be accompanied by noise, vibration, and harshness (NVH) concerns in the host vehicle. In some instances, such NVH concerns may be due to performance characteristics of the mechanical braking system's friction elements. In other situations, NVH concerns may be experienced during transitions between vehicle drive, coast, and braking modes, for example uncovering or exacerbating effects of mechanical lash in the underlying propulsion and/or suspension systems.

SUMMARY

A method of assisting deceleration during a stop of a motor vehicle having a drivetrain including a traction motor, a road wheel operatively connected to the drivetrain, a friction brake configured to decelerate the road wheel, and an electronic controller includes detecting, via the electronic controller, a request to stop the vehicle. The method also includes commanding, via the electronic controller, the traction motor to provide regenerative braking in response to the request to stop the vehicle. The method additionally includes determining, via the electronic controller, a current vehicle operating state. The method also includes determining, via the electronic controller, an amount of brake drag torque to be generated by the friction brake based on the current vehicle operating state. The method further includes commanding, via the electronic controller, an application of the determined amount of the brake drag torque in parallel with the regenerative braking, thereby operating the friction brake as a mechanical drivetrain damper while assisting the regenerative braking to stop the motor vehicle.

According to the method, determining the current vehicle operating state may include determining a current grade of the vehicle.

Additionally, determining the amount of brake drag torque may be accomplished via a first look-up table.

According to the method, determining the current vehicle operating state may include determining a current road speed of the vehicle.

Additionally, commanding the application of the determined amount of the brake drag torque may be accomplished when the current road speed of the vehicle is below a vehicle road speed threshold, such as 3 kph.

The method may also include determining a rate of ramp-up of the brake drag torque based on the current vehicle operating state and the determined amount of the brake drag torque.

According to the method, determining the rate of ramp-up of the brake drag torque may be accomplished via a second look-up table based on the current road speed of the vehicle and the current grade of the vehicle.

Additionally, commanding the application of the determined amount of the brake drag torque may include commanding the rate of ramp-up of the brake drag torque.

The method may also include determining a desired incremental amount of the brake drag torque. Additionally, the method may include determining whether a difference between the desired incremental amount of the brake drag torque and the determined amount of the brake drag torque is greater than a predetermined incremental brake drag torque limit. Furthermore, the method may include commanding the application of the determined amount of the brake drag torque when the difference between the desired incremental amount of the brake drag torque and the determined amount of the brake drag torque is greater than the predetermined incremental brake drag torque limit.

The motor vehicle may include an accelerator switch in communication with the electronic controller. The method may further include monitoring, via the electronic controller, the accelerator switch for a vehicle acceleration request. According to the method, in such an embodiment, commanding the application of the determined amount of the brake drag torque may be accomplished when the vehicle acceleration request has not been detected.

A vehicle having an electronic controller configured or programmed to execute such a method is also disclosed.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers refer to like components,FIG.1shows a schematic view of a motor vehicle10positioned relative to a road surface12. The vehicle10may be a mobile platform, such as a passenger vehicle, an all-terrain vehicle (ATV), an airplane, etc., used for personal, commercial, or industrial purpose. As shown, the vehicle10includes a vehicle body14disposed along a longitudinal axis16and having respective left, right, front, and back sides. The vehicle body14also defines a vehicle interior18configured to accommodate a vehicle operator, passengers, and cargo.

With continued reference toFIG.1, the vehicle10includes a plurality of road wheels, specifically shown as front wheels22A and rear wheels22B. The vehicle10also includes a drivetrain24configured to provide propulsion thereof. The drivetrain24includes one or more traction motors or electric motor-generators26operatively connected to at least some of the road wheels22A and22B and configured to generate motor drive torque Tm. As shown, the drivetrain24may additionally include an internal combustion engine28configured to generate engine drive torque Teand a transmission30operatively connecting the engine to at least some of the road wheels22A,22B for transmitting engine torque thereto. The drivetrain24may additionally include a fuel cell (not shown) operatively connected to at least some of the road wheels22A and22B.

As shown inFIG.1, a vehicle suspension system32operatively connects the body14to the respective road wheels22A and22B for maintaining contact between the wheels and the road surface12, and for maintaining handling of the vehicle10. As also shown inFIG.1, a vehicle steering system34is operatively connected to the front wheels22A for steering the vehicle10. The steering system34includes a steering wheel36that is operatively connected to the front wheels22A via a steering rack38. The steering wheel36is arranged inside the passenger compartment of the vehicle10, such that an operator of the vehicle may command the vehicle to assume a particular direction with respect to the road surface12. Additionally, an accelerator switch or pedal40is positioned inside the passenger compartment of the vehicle10, wherein the accelerator switch is operatively connected to the drivetrain24for commanding propulsion of the vehicle10.

A vehicle braking system42is operatively connected to the respective front and rear wheels22A,22B for retarding rotation of the wheels and decelerating the vehicle10. The braking system42includes a friction brake subassembly, or friction brake,44arranged at each of the respective front and rear wheels22A,22B and operatively connected to the vehicle suspension system32. In other words, the braking system42may include a plurality of friction brake subassemblies44. Each brake subassembly44may be configured as either a disc brake (shown inFIG.2) or a drum brake (shown inFIG.3). Each brake subassembly44includes a rotor46configured for synchronous rotation with the respective wheel22A or22B about a wheel axis48. Each brake subassembly44additionally includes an actuator50arranged in a brake caliper50-1of a disc brake (shown inFIG.2) or in a foundation50-2of a drum brake (shown inFIG.3), and configured to generate an actuator or brake force F. The actuator50may be configured as a hydraulically actuated piston, e.g., operated via hydraulic brake pressure P generated at a master brake cylinder52, or an electrically actuated servo-motor (not shown).

As shown inFIGS.2and3, each brake subassembly44also includes one or more brake components or pads54, each having a wearable friction lining or element56. The friction lining56is configured to be pressed into contact with the rotor46by the actuator force F for retarding rotation of the respective wheel22A or22B to decelerate the vehicle10. The actuator force F may be controlled via a signal generated by a brake switch or pedal58and communicated electronically to the master brake cylinder52(shown inFIG.1). The brake switch58is generally positioned inside the passenger compartment of the interior18, and is adapted to be controlled by the operator of the vehicle10.

With reference toFIG.1, the vehicle10also includes a first sensor60configured to detect a deceleration request to stop the vehicle10, such as via an application of the brake switch58or a vehicle-based request for a stop without a driver-initiated deceleration request. The vehicle10further includes one or more second sensors62configured to detect an operating state of the vehicle10, such as the vehicle road speed (V) and a grade or inclination (G) of the vehicle. Additionally, the vehicle10includes a third sensor64configured to detect a vehicle acceleration request, such as via an application of the accelerator switch40. The actuator force F may be controlled via the brake switch58to provide sufficient torque at the respective wheel22A or22B to bring the vehicle10to a stop, some drag torque Tdvia light contact between the friction lining(s)56and the rotor46to generate nominal or trace retardation of wheel rotation, and various magnitudes of the actuator force in between to decelerate the vehicle at a desired rate.

Alternatively, the actuator force F may be similarly controlled via an on-board vehicle electronic controller66as part of a system for assisting vehicle deceleration to a stop using the friction brake(s)44. As shown inFIG.1, the electronic controller66is in communication with the sensors60and62. The electronic controller66may alternatively be referred to as a control module, a control unit, a controller, a vehicle10controller, a computer, etc. The electronic controller66may include a computer and/or processor68, and include software, hardware, memory, algorithms, connections (such as to sensors60and62), etc., for managing and controlling the operation of the vehicle10. As such, a method, described in detail below and generally represented inFIG.4, may be embodied as a program or algorithm operable on the electronic controller66. It should be appreciated that the electronic controller66may include a device capable of analyzing data from the sensors60and62, comparing data, making the decisions required to control the operation of the vehicle10, and executing the required tasks to control the operation of the subject vehicle.

The electronic controller66may be embodied as one or multiple digital computers or host machines each having one or more processors68, read only memory (ROM), random access memory (RAM), electrically-programmable read only memory (EPROM), optical drives, magnetic drives, etc., a high-speed clock, analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, and input/output (I/O) circuitry, I/O devices, and communication interfaces, as well as signal conditioning and buffer electronics. The computer-readable memory may include non-transitory/tangible medium which participates in providing data or computer-readable instructions. Memory may be non-volatile or volatile. Non-volatile media may include, for example, optical or magnetic disks and other persistent memory. Example volatile media may include dynamic random-access memory (DRAM), which may constitute a main memory. Other examples of embodiments for memory include a floppy, flexible disk, or hard disk, magnetic tape or other magnetic medium, a CD-ROM, DVD, and/or other optical medium, as well as other possible memory devices such as flash memory.

The electronic controller66also includes a tangible, non-transitory memory70on which are recorded computer-executable instructions, including one or more algorithms, for regulating operation of the motor vehicle10. Algorithms required by the controller66or accessible thereby may be stored in the memory and automatically executed to provide the required functionality. The subject algorithm(s) may specifically include an algorithm72for assisting a stop of the motor vehicle10to be described in detail below. The processor68of the electronic controller66is configured to execute the algorithm72. The electronic controller66is further configured to command an application of friction brake44drag torque (Td) to thereby operate as a mechanical vehicle drivetrain damper to address noise, vibration, and harshness (NVH) concerns during a regenerative braking vehicle stop.

Typically, mechanical systems, such as the drivetrain24and suspension32of the vehicle10, have clearance or lost motion, a.k.a., backlash or lash, caused by clearance between system components. Such lash may be defined as the maximum distance or angle through which some part of a mechanical system may be moved in one direction without applying appreciable force or motion to the next part in mechanical sequence. An example of lash in the context of gears and geartrains is the amount of clearance between mated gear teeth. Gear lash may be seen when the direction of geartrain movement is reversed and the slack or lost motion is taken up before the reversal of motion is complete. Variations in lash in mechanical linkages may be due to allowances for lubrication, manufacturing tolerances, deflection under load, and thermal contraction/expansion. Additionally, in many mechanical systems some backlash is allowed specifically to prevent jamming. Backlash in drivetrain24and suspension32of the vehicle10may be exposed under operation of the braking system42, and generate NVH concerns, such as bumping and clunking. Such, NVH concerns may be experienced during transitions between vehicle drive, coast, and braking modes. Additional NVH concerns, such as creaking, may be experienced due to performance characteristics of the friction element(s)56.

Specifically, the electronic controller66is configured to detect, using the first sensor60, a request74to stop the vehicle10. The electronic controller66is also configured to command the traction motor26to provide regenerative braking in response to the request74to stop the vehicle10. The electronic controller66is additionally configured to determine, using the second vehicle sensor(s)62, a current vehicle operating state, such as current vehicle road speed (V) and current vehicle grade (G). The electronic controller66is also configured to determine an amount of brake drag torque (Td) to be generated by the friction brake44based on the current vehicle operating state. The electronic controller66is further configured to command an application of the determined amount of the brake drag torque (Td) in parallel with the regenerative braking, to thereby generate friction damping via the friction brake44, while assisting the regenerative braking to stop the vehicle10. Specifically, the algorithm72may compare the current road speed (V) to a vehicle road speed threshold78, such as 3 kph.

The electronic controller66may be configured to then command application of the determined amount of the brake drag torque (Td) when the current road speed (V) of the vehicle10is below the road speed threshold78. The electronic controller66may be additionally configured to monitor the accelerator switch40for a vehicle acceleration request80and command application of the determined amount of the brake drag torque (Td) when the vehicle acceleration request has not been detected. The electronic controller66may be also configured to determine the amount of brake drag torque (Td) via a first look-up table82saved into the controller's memory. The first look-up table82may include data of brake drag torque (Td) versus the vehicle grade (G) empirically developed on a representative vehicle under controlled test conditions.

The electronic controller66may be additionally configured to determine a rate of ramp-up of the brake drag torque (Td′) based on the current vehicle operating state, such as the current road speed (V) and grade (G) of the vehicle10, and the determined amount of the brake drag torque (Td). Accordingly, the electronic controller66may command the determined rate of ramp-up of the brake drag torque (Td′) during application of the determined amount of the brake drag torque (Td). The electronic controller66may be specifically configured to determine the rate of ramp-up of the brake drag torque (Td′) via a second look-up table84saved into the controller's memory. The second look-up table84may include data of ramp-up of the brake drag torque (Td′) versus the empirically developed vehicle road speed (V) and grade (G). Data from the first look-up table82may be specifically employed as an input to the second look-up table84.

In a particular embodiment, the electronic controller66may be configured to determine a desired incremental amount of the brake drag torque (ΔTd) based on the determined amount of the brake drag torque (Td) and the determined rate of ramp-up of the brake drag torque (Td′). In such an embodiment, the electronic controller66may be additionally configured to determine whether a difference between the desired incremental amount of the brake drag torque (ΔTd) and the determined amount of the brake drag torque (Td) is greater than a predetermined incremental brake drag torque limit86. The electronic controller66may be configured to then command the application of the determined amount of the brake drag torque (Td) when the difference between the desired incremental amount of the brake drag torque (ΔTd) and the determined amount of the brake drag torque (Td) is greater than the predetermined incremental brake drag torque limit86.

The electronic controller66may be further programmed to fully engage the friction brake(s)44once the vehicle10has come to a complete stop and the regenerative braking is no longer active. Accordingly, the electronic controller66is intended to regulate application of the friction brake(s)44to apply the drag torque (Td) and thereby generate vehicle driveline hysteresis during vehicle deceleration while coming to a stop. The hysteresis provided by the drag torque (Td) is thereby intended to operate as a mechanical or friction damper to minimize NVH issues due to driveline lash during a regenerative braking vehicle stop. Additionally, the vehicle driveline hysteresis provided by the drag torque (Td) during the latter stages of vehicle deceleration permits a smooth and uninterrupted transition to a brake-held stationary vehicle10.

FIG.4depicts a method100of assisting deceleration of a motor vehicle, such as the vehicle10described above with respect toFIGS.1-3, during the vehicle's stop. The method100commences in frame102with detecting movement of the vehicle10relative to the road surface12. In frame102the method may also include detecting via the controller66, such as using the first sensor60, the request74to stop the vehicle10. The method100then proceeds from frame102to frame104. In frame104, the method includes commanding, via the electronic controller66, the traction motor26to provide regenerative braking in response to the request74to stop the vehicle10. The method100then proceeds from frame104to frame106. In frame106, the method includes determining via the electronic controller66the current vehicle operating state, such as by using the second vehicle sensor(s)62to detect the current vehicle road speed (V) and current vehicle grade (G). Following frame106, the method100advances to frame108.

In frame108the method100includes determining, via the electronic controller66, the amount of brake drag torque (Td) to be generated by the friction brake(s)44based on the current vehicle operating state. Determining the amount of brake drag torque (Td) may be accomplished via the first look-up table82having data of brake drag torque (Td) versus the vehicle grade (G). In frame108the method100may also include determining, via the electronic controller66, a particular rate of ramp-up of the brake drag torque (Td′) based on the current vehicle operating state and the determined amount of the brake drag torque (Td). Determining the rate of ramp-up of the brake drag torque (Td) may be accomplished via the second look-up table84having data of ramp-up of the brake drag torque (Td′) versus the vehicle road speed (V) and grade (G).

In frame108the method100may additionally include determining, via the electronic controller66, a particular desired incremental amount of the brake drag torque (ΔTd) based on the determined amount of the brake drag torque (Td) and the determined rate of ramp-up of the brake drag torque (Td′). As described above with respect toFIGS.1-3, in such an embodiment the method may further include determining, via the electronic controller66, whether the difference between the desired incremental amount of the brake drag torque (ΔTd) and the determined amount of the brake drag torque (Td) is greater than the predetermined incremental brake drag torque limit86. After frame108, the method100proceeds to frame110.

In frame110, the method100includes commanding via the electronic controller66the application of the determined amount of the brake drag torque (Td) in parallel with the regenerative braking. Specifically, commanding the application of the determined amount of the brake drag torque (Td) may be accomplished when the current road speed (V) of the vehicle10is below a particular vehicle road speed threshold78. Also, commanding the application of the determined amount of the brake drag torque (Td) may include commanding the rate of ramp-up of the brake drag torque (Td′). Additionally, commanding the application of the determined amount of the brake drag torque (Td) may be accomplished when the difference between the desired incremental amount of the brake drag torque (ΔTd) and the determined amount of the brake drag torque (Td) is greater than the predetermined incremental brake drag torque limit86. Furthermore, commanding the application of the determined amount of the brake drag torque (Td) in frame110may be accomplished when the vehicle acceleration request80has not been detected. Otherwise, the method may loop back to frame102.

As described above with respect toFIGS.1-3, the commanded brake drag torque (Td) operates the friction brake(s)44as a mechanical drivetrain damper to minimize NVH concerns, while assisting the regenerative braking to stop the motor vehicle10. The method may proceed from frame110to frame112where the electronic controller66includes commanding the friction brake(s)44to fully engage once the vehicle10has come to a complete stop and the regenerative braking is no longer active. Following a complete vehicle stop, the method may restart in frame102with detecting resumed movement of the vehicle10relative to the road surface12, thus enabling repetition of the method algorithm in frames102through112for assisting regenerative braking to stop the motor vehicle10while minimizing NVH concerns. The method100may also terminate at frame114. Accordingly, with respect to the method100, regenerative braking and brake drag torque (Td) are employed to achieve different goals. Regenerative braking is primarily used to bring the vehicle10to a stop, while brake drag torque (Td) is used to target NVH/driveline issues.