Abstract:
A vehicle is provided having a brake pedal with a detectable travel position and apply force, an electronic braking system component, and a controller having a stored threshold braking force and an algorithm. The algorithm determines a first braking torque request corresponding to the apply force, and a second braking torque request corresponding to the travel position. The first request applies when the apply force is greater than the threshold, and the second request applies when it is not. A calculated third request transitions linearly to the second request when apply pressure drops below the threshold upon pedal release. A method is also provided that includes recording the apply force, travel position, and force and travel-based tables, comparing the apply force to the threshold, applying the braking system component using the force-based table when the apply force exceeds the threshold, and otherwise using a calculated braking torque and travel-based table.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/908,319, filed Mar. 27, 2007, and which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a brake pedal sensor transition control apparatus and method of use with a vehicle having an electronic braking system, and in particular to a vehicle having a controller with brake pedal transition logic for interpolating or transitioning between separate force-based and travel-based braking torque sensor requests during brake pedal release, thereby optimizing the performance and feel of the brake pedal. 
     BACKGROUND OF THE INVENTION 
     Conventional automotive vehicles typically include a mechanical brake pedal that is continuously connected to a brake lever or arm. The motion of the arm in turn actuates a braking mechanism, such as a disc brake and/or drum brake, to thereby slow or stop the vehicle. The rate of deceleration imparted to the vehicle depends largely on the amount of force applied to depress the brake pedal, as well as the travel position of the brake pedal within or along its fixed range of motion. Conventional hydraulic braking systems are powered by a supply of pressurized brake fluid from a master cylinder. Such a mechanical, fluid-driven braking system generally responds relatively quickly and accurately to the force applied to the brake pedal through its entire range of motion, providing what could be described as a “normal” or conventional brake pedal feel. 
     By contrast, an electronic or by-wire braking system is often used in electric vehicles, as well as in hybrid vehicles which are alternately and selectively powered by an internal combustion engine and one or more electric motor/generators. In an electronic braking system (EBS), the braking command or input applied as a force to a brake pedal by an operator is converted by an encoder device into an electrical signal. This electrical braking signal, also known as a braking torque request, is then transmitted or communicated to the point of application, wherein one or more actuators operate in response to the signal to slow or stop the vehicle. Braking torque may be applied directly using a conventional braking mechanism, or more commonly by applying an individual electronic braking unit positioned in proximity to each wheel, and/or to the transmission output member, thereby slowing the vehicle in a precisely controlled manner. 
     In an electric or hybrid vehicle, the brake pedal is isolated from the point of braking torque application, and therefore is attached to one or more pedal sensors which detect or measure the pressure on the brake pedal and convert the pressure into the transmittable electrical signal. The controller has preprogrammed braking system logic for translating the detected brake pedal measurements into corresponding braking torque requests. Typically, such braking logic includes an accessible braking torque lookup table containing specific braking torque requests corresponding to the various detected brake pedal forces. 
     However, certain pressure sensors may have less than optimal resolution, particularly under low force conditions. Using a brake pedal force sensor alone under these conditions may result in an error or variance in the braking torque request communicated to the electronic braking system when compared to the vehicle operator&#39;s intended braking force. While sensors used to measure a brake pedal&#39;s relative position within or along its range of motion, i.e., the brake pedal travel position, generally have better resolution at these low pressure ranges, hysteresis within the braking system may also potentially lead to errors or variances in the resulting applied braking torque request in the event of an attempted direct or immediate switch to such a sensor during low pressure applications. 
     SUMMARY OF THE INVENTION 
     Accordingly, a vehicle is provided having a brake pedal and an electronic braking system component that is configured to slow or stop the vehicle in response to a detected apply force applied to a brake pedal, and a detected travel position of the brake pedal. The vehicle also includes a controller having a braking system control algorithm which determines a first braking torque request corresponding to the detected apply force and a second braking torque request corresponding to the detected travel position. The braking system is applied using the first braking torque request when the detected apply force is greater than a stored threshold braking force, and using the second braking torque request when the detected apply force is less than the stored threshold braking force. 
     A pair of braking torque lookup tables are accessible by the controller to determine a corresponding one of the first and second braking torque requests, with the lookup tables including a force-based table providing the first braking torque request, and a travel-based table providing the second braking torque request. The braking system component is then applied using only a calculated third braking torque request when the detected apply force drops below the stored threshold braking force upon release of the brake pedal, with the braking system switching to the second braking torque request only when the third braking torque request equals the second braking torque request, thereby smoothly transitioning between the force-based and travel position-based braking torque requests. 
     The third braking torque request is selected from a point along a calculated linearized curve between the first and second braking torque requests, and neither of the first or second braking torque requests is executed when a separate panic braking condition is detected. 
     A brake pedal sensor transition method is also provided for use with a hybrid vehicle having an electronic braking system component and a brake pedal. The method includes recording an applied braking force and travel position of the brake pedal, accessing a force-based braking torque lookup table corresponding to a first braking torque request, and accessing a separate travel position-based braking torque lookup table corresponding to a second braking torque request. The method further includes comparing the recorded braking force to a threshold value, and applying the braking system component using the force-based table when the braking force exceeds the threshold value, and using the travel-based table when the braking force is less than the threshold value. 
     The braking system component is applied according to a calculated transition braking torque, until the transition braking torque is equal to a corresponding braking torque value from the travel position-based braking torque lookup table. Calculating the transition braking torque includes performing a least squares linearization method to generate a linearized torque transition curve between a torque value point selected from the force-based braking torque lookup table and a torque value point selected from the travel position-based braking torque lookup table. 
     A method of interpolating a driver-requested braking torque request includes calculating a first braking torque request using a detected pedal force, determining a second braking torque request using a detected pedal travel position, and saving the first braking torque request as a linearization entry point for transitioning to the second brake torque request. The method further includes setting a linearization exit point equal to the corresponding second braking torque request, interpolating a third braking torque request between the entry and exit points, and applying the braking system component using the third braking torque request until the third braking torque request is equal to the exit point. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a vehicle chassis having a controller with brake pedal transition logic according to the invention; 
         FIG. 2  is a flow chart describing the method or algorithm of the invention for interpolating or transitioning between brake pedal pressure and travel position lookup tables; and 
         FIG. 3  is a schematic curve showing the brake pedal transition of the invention during a brake pedal release. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in  FIG. 1  a vehicle  10  having a plurality of wheels  28 , an electronic braking system control unit or controller  18 , and an engine  25  that is selectively connectable to a transmission  17  having a rotatable output member  24 . The vehicle  10  is preferably a hybrid vehicle having an alternate power supply  14  including an energy storage device (ESD)  47 , such as a rechargeable battery or battery pack, and at least one electric motor/generator  42  operable for alternately powering or propelling the vehicle  10  and/or one or more of its various subsystems. However, the vehicle  10  may be any vehicle utilizing an electronic braking system (EBS) controller  18  as described herein, such as an electric vehicle or a fuel-cell powered vehicle. 
     The EBS controller  18 , referred to hereinafter for simplicity as the controller  18 , is operable for detecting a braking force (arrow A) applied to a brake pedal  27  using a pair of brake pedal sensors  40  and  41  connected thereto. The controller  18  calculates or otherwise determines a corresponding braking torque request (B C ) in response to the detected and recorded measurements or readings. The controller  18  then communicates a braking torque request (B C ) to an individual electronic braking unit  30  positioned in proximity to each of the wheels  28  and/or to the output member  24  by-wire and/or via datalink. The braking torque request (B C ) is transmitted across one or more command signal transmission channels or lines  50  to the electronic braking unit  30  and/or the output member  24 , where the braking torque request (B C ) can act to oppose the torque of the electronic braking unit  30  and/or the output member  24  to slow or stop the vehicle  10  as needed. 
     Depending on the specific drive configuration of the vehicle  10 , the output member  24  may be driveably connected to a rear differential  31 , which is configured to distribute rotational force or torque from a rotatable output member  24 , such as a driveshaft, to rear drive axle  26  for powering or driving a plurality of wheels  28  at the rear of the vehicle  10 . Although not shown in  FIG. 1 , the vehicle  10  may also have a substantially similar front differential suitable for distributing torque to the front drive axle  11  for powering or driving a plurality of wheels  28 , such as in a four-wheel or all-wheel drive configuration. The transmission  17  is configured to deliver a variable transmission output speed N to the output member  24 , with the transmission output speed N being variably opposable by the braking request (B C ), as determined by the controller  18 . 
     Controller  18  includes programmable memory  19  and a microprocessor  80  configured to rapidly execute the necessary control logic for implementing and controlling the electronic braking units  30  and/or the output member  24  as needed, using a brake pedal transition logic method or algorithm  100  (see  FIG. 2 ) which is programmed or stored in memory  19 . The controller  18  is electrically connected, whether directly by-wire or indirectly via datalink signal as described hereinabove, to a brake pedal travel sensor  41  and a brake pedal pressure or force sensor  40 , each of which are in electric communication with brake pedal  27 . 
     Travel sensor  41  (also labeled B T  in  FIG. 1 ) is preferably a pedal range sensor configured or adapted to precisely detect, measure, or otherwise determine the relative position of the brake pedal  27  along a fixed range of motion when the brake pedal  27  is depressed or actuated. Pressure or force sensor  40  (also labeled B P  in  FIG. 1 ) is preferably a pressure transducer or other suitable pressure sensor configured or adapted to precisely detect, measure, or otherwise determine an apply pressure or force (arrow A) imparted to the brake pedal  27  by an operator of the vehicle  10 . The measurements or readings taken by travel sensor  41  and force sensor  40  are transmittable or communicable to the controller  18  or are otherwise determinable thereby as needed for use with the algorithm  100  of the invention, which is described in further detail later hereinbelow. 
     Memory  19  of the controller  18  is preloaded or preprogrammed with a pair of lookup tables  90  and  91 , which are braking torque data tables readily accessible by controller  18  in implementing or executing algorithm  100 . Lookup table  91 , referred to hereinafter for clarity as travel table  91 , corresponds to the measurements or readings of travel sensor  41  and contains a commanded braking torque request (B C ) appropriate for the detected position of travel sensor  41 . Likewise, the lookup table  90 , referred to hereinafter as the force table  90 , corresponds to the recorded measurements or readings of force sensor  40  and contains a commanded braking torque request (B C ) appropriate for the detected force measurement as determined by the force sensor  40 . 
     Controller  18  preferably normally operates according to the force table  90 , and force table  90  therefore acts as the preferred or default table. However, as described previously hereinabove, pressure or force sensors such as the force sensor  40  tend to have relatively low resolution under low apply pressure conditions, while travel sensors such as travel sensor  41  do not typically share these particular limitations. Therefore, travel table  91  is preferably selectively used in place of force table  90  during periods of low force resolution in order to avoid a perceptible discontinuity or variance between the braking torque request (B C ) and the operator&#39;s intended braking force. Discontinuity may result from hysteresis or lag in response time in the various components comprising the electronic braking system, and therefore skipping or transitioning instantaneously between the travel table  91  and the force table  90  may result in a less than optimal braking performance. 
     Accordingly, turning to  FIG. 2 , the algorithm  100  is provided to interpolate and smoothly transition from force table  90  to the travel table  91  during these periods of low force resolution, such as would occur upon release of brake pedal  27  (see  FIG. 1 ) after a stop. In this manner, braking performance is optimized, and the “feel” of the brake pedal  27  closely approximates the motion and feel of a conventional, mechanical braking pedal. The algorithm  100  is preferably executed on a continuous cycle or control loop of approximately 5 to 10 milliseconds, but which may be performed more or less frequently depending on the available speed or power of microprocessor  80  (see  FIG. 1 ). 
     Beginning with step  102 , the algorithm  100  determines whether one or more predetermined braking conditions have occurred within the immediately prior completed control loop. For example, step  102  may determine whether the brake pedal  27  is releasing, and/or whether certain other linearization entrance criteria have been met, as will be described later hereinbelow. Because the algorithm  100  effectively performs linear data fitting or another linearization process to interpolate between the separate force and travel tables  90  and  91 , respectively, the flag set at step  102  is referred to in  FIG. 2  as a “linear flag”. If at step  102  it is determined that the predetermined braking conditions have been met, algorithm  100  proceeds directly to step  114 . Otherwise, the algorithm  100  proceeds to step  104 . 
     At step  104 , it having been determined in step  102  that predetermined braking conditions have not occurred during the previous control loop, the algorithm  100  uses measurements taken by the travel sensor  41  to determine whether the travel of the brake pedal  27  is decreasing, i.e., is moving in a direction opposite that used to apply the brakes, such as when a driver releases brake pedal  27  after a stop. If brake pedal travel is decreasing, algorithm  100  proceeds to step  106 . If not, algorithm  100  proceeds to step  108 . 
     At step  106 , it having been determined in step  104  that travel of the brake pedal  27  is decreasing, the algorithm  100  compares the detected force applied to the brake pedal  27 , as measured or determined by the force sensor  40 , to a calibrated or threshold force value stored in memory  19 . This threshold force value is predetermined based on the design criteria of a given vehicle  10 , and may be set according to design parameters depending on the available resolution and performance of specific EBS components (such as braking units  30  and/or output member  24 ) and/or the force sensor  40 . If at step  106  it is determined that detected pedal force exceeds the stored threshold force value, algorithm  100  proceeds to step  108 . Otherwise algorithm  100  proceeds to step  110 . 
     At step  108 , the algorithm  100  proceeds according to a standard or default driver braking request algorithm. Preferably, this standard algorithm entails accessing force table  90  to determine the correct braking torque request (B C ) to apply, and then applying the request (B C ) to the braking units  30  and/or the output member  24  (see  FIG. 1 ) as needed to slow of stop the vehicle  10 . The algorithm  100  will effectively remain in step  108  unless the next control loop detects a different result at step  106 . 
     At step  110 , it having been determined in step  106  that the detected braking force is less than the stored threshold braking force, algorithm  100  compares the detected travel position of the brake pedal  27  to a threshold travel value stored in memory  19 . If the detected travel position is less than the stored threshold travel position value, algorithm  100  proceeds to step  108  and executes the force table  90  as explained hereinabove. By so doing, the algorithm  100  optimizes the feel of the brake pedal  27 , preventing a sudden or abrupt transition to the travel table  91 , and any consequent sudden or abrupt application of the braking units  30  and/or braking of the output member  24  (see  FIG. 1 ). If however at step  110  it is determined that detected pedal travel exceeds the stored threshold travel position value, algorithm  100  proceeds to step  112 . 
     At step  112 , the algorithm  100  performs a final status check to determine whether the brake request measured at the brake pedal  27  exceeds the corresponding braking torque of the force table  90 . Step  112  will normally result in a determination that the two values are indeed equal, as the algorithm  100  at that instant should be operating according to the force table  90 , and not according to the travel table  91 . If the values are different, however, the algorithm  100  proceeds to step  108  and there executes a standard driver brake request algorithm, which in this instance would be determined by travel table  91 . However, if at step  112  it is determined that the values are indeed equal, algorithm  100  proceeds to step  114 . 
     At step  114 , the algorithm  100  determines whether the combination of braking force and travel, as determined by the force and travel sensors  40  and  41 , respectively, are indicative of an emergency or “panic” braking condition. Measurements indicative of such a condition, and the commanded braking torque request (B C ) responsive to the emergency braking condition, are preferably preprogrammed in memory  19  where they are readily accessible by the algorithm  100  in making this determination. If an emergency braking condition is detected, the algorithm  100  proceeds to step  116 . Otherwise, the algorithm  100  proceeds to step  118 . 
     At step  116 , the algorithm  100  immediately enters or activates the stored emergency or “panic” braking algorithm (not shown) mentioned at step  114 . The algorithm  100  is then reinitiated at step  102  when the controller  18  determines that such a condition has ceased, or the vehicle  10  has stopped. 
     At step  118 , it having been determined at step  114  that an emergency or “panic” braking condition does not exist, a force-based request flag is set to zero, thus signaling that the controller  18  will no longer operate according to the force table  90 , but will instead begin a transition to the travel table  91 . The remaining portion of the algorithm  100  subsequent to step  118  describes the linearization or interpolation between the force table  90  and the travel table  91 , with the setting of the force-based request flag to zero in the current step signaling the transition, after which the algorithm  100  proceeds to step  120 . 
     At step  120 , a “snapshot” is taken of the current detected force and travel position levels, as measured by the force sensor  40  and the travel sensor  41 , respectively. These values are stored or recorded in memory  19 . Once complete, the algorithm  100  proceeds to step  122 . 
     At step  122 , the algorithm  100  determines whether the stored pedal force (see step  120 ) corresponds to a braking torque request (B C ) that is less than the linearization “exit point” E, as determined at step  120  and shown in  FIG. 3 . Turning briefly to  FIG. 3 , this figure is an exemplary line graph describing the interrelation between a force-based brake request curve A, a travel-based brake request curve C, and a separate calculated brake request curve B. Each of the points of curves A and C are taken directly from the previously described force-based and travel position-based lookup tables  90  and  91 , respectively, while the points comprising calculated curve B are calculated or interpolated according to the algorithm  100 , as described hereinbelow. 
     Point D is referred to hereinafter as the “entry point”, referring to the “entry” onto transition curve B and departure from force-based brake request curve A. Likewise, point E is referred to hereinafter as the “exit point”, referring to the “exit” from calculated curve B and onto travel-based brake request curve C. If at step  122  the algorithm  100  determines that the stored pedal force (see step  120 ) corresponds to a braking torque request (B C ) that is less than a linearization “exit point” E, the algorithm  100  proceeds to step  126 . Otherwise, the algorithm  100  proceeds to step  124 . 
     At step  124 , and turning back to  FIG. 2 , the algorithm  100  launches a calculated brake request curve B (see  FIG. 3 ), which is an interpolated and linearized transition curve that is fit to and interconnects force curve A and travel curve C. Curve B may be generated using known linear data fitting methods, such as the least squares method or the method of least absolute deviations, with the slope of curve B dependent on entry point D determined by the force captured in step  120 , and on the exit point E (see  FIG. 3 ), with exit point E being the corresponding torque point along travel-based brake request curve C, as determined from travel table  91 . After completing the linearization process of step  124 , the algorithm  100  proceeds to step  128 . 
     In step  126 , which is reached upon a determination that the detected pedal force corresponds to a braking torque request (B C ) that is less than that corresponding to exit point E (see  FIG. 3 ), the algorithm  100  brakes the vehicle  10  according to travel-based brake request curve C, as determined by the travel table  91 . Algorithm  100  remains at step  126  until the following control loop determines a status change, beginning with step  104 . 
     In the remaining steps  128 - 136 , a final portion of the algorithm  100  is used to determine if an operator of the vehicle  10  (see  FIG. 1 ) is reapplying the brake pedal  27 . If the brake pedal  27  is being reapplied, the algorithm  100  determines how to proceed. 
     At step  128 , the algorithm  100  performs a simple calculation by subtracting the linearization torque request, i.e., the braking torque request (B C ) as determined by the transition curve B (see  FIG. 3 ) during the linearization process performed in step  124 , from the corresponding force-based torque request, as determined by the force-based brake request curve A and the force table  90  (see  FIG. 1 ). This difference is compared to a stored threshold difference. If the stored threshold difference is greater than the calculated difference, the algorithm  100  proceeds to step  130 , otherwise the algorithm  100  proceeds to step  132 . 
     At step  130 , the algorithm  100  “passes” the linearization request, i.e., commands or controls braking of the vehicle  10  according to the linearized or calculated brake request curve B (see  FIG. 3 ). In this manner, the braking torque request (B C ) applied to stop or slow the vehicle  10  is exclusively the calculated or interpolated torque request according to calculated brake request curve B. Algorithm  100  remains at step  130  until the following or subsequent control loop detects a status change, beginning with step  104 , as discussed previously hereinabove. 
     In step  132 , the algorithm  100  determines whether the travel of brake pedal  27  (see  FIG. 1 ) is increasing, i.e., the brake pedal  27  is being reapplied, as detected by the travel sensor  41 . If increasing travel is detected, the algorithm  100  proceeds to step  134 . Otherwise, the algorithm  100  proceeds to step  130 . 
     At step  134 , the algorithm  100  determines whether the amount of force (arrow A of  FIG. 1 ) applied to the brake pedal  27  exceeds a stored threshold force value. If so, the algorithm  100  determines that the brake pedal  27  is being reapplied rather than released, and proceeds to step  136 . Otherwise, the algorithm  100  proceeds to step  130 . 
     At step  136 , the algorithm  100  passes the force-based torque request, i.e., actuates the braking units  30  and/or brakes the output member  24  according to force curve A of  FIG. 3 , as determined by the force table  90  and described previously hereinabove, and remains on force-based brake request curve A until the following or subsequent control loop detects a braking status change, beginning with step  104 , as discussed previously hereinabove. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.