Abstract:
A vehicle has a controller with a threshold brake pedal apply force and an algorithm for determining a first braking torque request corresponding to a pedal apply force, a second braking torque request corresponding to a pedal travel position, and a calculated third braking torque request. The third torque request is calculated by multiplying the first torque request by an average percentage variance of the second to the first torque request. An adaptive electronic brake system (EBS) includes force and travel sensors connected to a brake pedal for determining a force-based and travel position braking torque request, and an algorithm for adapting one of the force-based and travel position-based braking torque requests to the other despite variances over time in a force/travel relationship therebetween.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an adaptive control method and apparatus usable with an electronic braking system (EBS) having a brake pedal travel position sensor and a brake pedal apply force sensor, the method and apparatus being operable for continuously applying a variable multiplier to a travel position-based braking torque request to thereby more closely approximate a force-based braking torque request. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional automotive vehicles typically include a mechanical brake pedal that is connected to a brake lever or arm. The motion of the brake arm in turn actuates a braking mechanism, such as a disc brake or a drum brake, to thereby slow and/or stop the vehicle. The rate of deceleration imparted to the vehicle depends on the amount of force applied to actuate or depress the brake pedal and on the travel position of the brake pedal within or along the range of motion of the brake pedal. Conventional hydraulic braking systems in particular are powered by a supply of pressurized brake fluid delivered from a master cylinder. Such a mechanical, fluid-powered braking system responds relatively quickly and accurately to a force applied to the brake pedal through the pedal&#39;s entire range of motion, providing what could be described as a “normal” or conventional brake pedal “feel”. 
         [0003]    By way of contrast, a by-wire or electronic braking system (EBS) is often used in electric vehicles, as well as in hybrid vehicles which are alternately and selectively powered by an internal combustion engine or fuel cell and one or more electric motor/generators. Using an EBS, the braking command or input applied as a force to a brake pedal by an operator of the vehicle is converted by an encoder device into an electrical braking signal. This electrical braking signal, also known as a braking torque request, is then rapidly transmitted or communicated to the point of application, where one or more actuators operate in response to the signal to slow or stop the vehicle. Braking torque in a vehicle having an EBS may be applied directly using an electro-hydraulic and/or an electromechanical braking mechanism to apply pressure to brake calipers to slow the vehicle, or more commonly by applying an opposing torque to an individual electronic braking unit positioned in proximity to each wheel, and/or to a transmission output shaft, thereby slowing the vehicle in a precisely controlled manner. 
         [0004]    In an electric or a hybrid vehicle, the brake pedal is isolated from the actual point of application of the braking torque, and therefore is attached to one or more pedal sensors which detect or measure the pressure on the brake pedal and/or the position of the brake pedal and convert the measurements into the aforementioned transmittable electrical signal. A controller has preprogrammed braking system logic for translating the electrical signal into a corresponding braking torque request. Typically, such braking logic includes one or more accessible braking torque look-up tables containing specific braking torque requests corresponding to the detected brake pedal forces for a particular sensor type. 
         [0005]    Certain pressure or force sensors may have less than optimal resolution, particularly under low force conditions, such as might occur when very light pressure is applied to a brake pedal. Using a brake pedal force sensor alone under these conditions may result in an error or variance in the braking torque request that is communicated to the EBS 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, generally have better resolution at lower pressure ranges, mechanical 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 between sensors during low pressure applications. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, a vehicle is provided having a brake pedal for delivering a braking request, a first sensor for detecting a brake pedal travel position and direction of travel of the brake pedal, a second sensor for detecting a brake pedal apply force, and an electronic braking system (EBS) component configured to slow or stop the vehicle in response to the braking request. The vehicle includes a controller having a stored threshold brake apply force window and an adaptive algorithm for determining a travel position-based braking torque request, a force-based braking torque request, and an adapted braking torque request. 
         [0007]    In one aspect of the invention, the adapted braking torque request is determined by multiplying the travel position-based braking torque request by an adjustable multiplier factor, and the controller applies the EBS component according to the adapted braking torque request when the braking request is based on the detectable brake pedal travel position. 
         [0008]    In another aspect of the invention, the travel position-based braking torque request is continuously multiplied by the multiplier factor to determine the adapted braking torque request. 
         [0009]    In another aspect of the invention, the adjustable multiplier factor is adjusted only when the controller determined the presence of a predetermined vehicle condition. 
         [0010]    In another aspect of the invention, the controller has a stored window of acceptable error, and is operable for calculating an average percentage variance of the travel position-based braking torque request over the force-based braking torque request. 
         [0011]    In another aspect of the invention, the average percentage variance is calculated only when the detectable brake pedal travel direction is not decreasing. 
         [0012]    In another aspect of the invention, a method is provided for adapting a travel position-based braking torque request to approximate a force-based braking torque request in a hybrid vehicle having an electronic braking system (EBS) actuated by a brake pedal. The method includes comparing a travel position-based braking torque request to a force-based braking torque request to determine an average percentage variance therebetween, continuously multiplying the travel position-based braking torque request by an adjustable multiplier factor to calculate an adapted braking torque request, and adjusting the value of the adjustable multiplier factor upon determination of a predetermined vehicle condition. The average percentage variance is calculated when the detected apply force on the brake pedal has a value falling within a stored threshold brake apply force range and the detected brake pedal travel direction is not decreasing. 
         [0013]    In another aspect of the invention, the predetermined condition is determined when the average percentage variance falls outside of a stored window of acceptable error. 
         [0014]    In another aspect of the invention, the adjustable multiplier factor is initialized to 1, and then adjusted by a stored percentage upon determination of the predetermined vehicle condition. 
         [0015]    In another aspect of the invention, an adaptive EBS is provided for a vehicle having a depressible brake pedal with a detectable apply force and a detectable travel position, including a force sensor operatively connected to the brake pedal for determining a force-based braking torque request and a travel sensor operatively connected to the brake pedal for determining a travel position-based braking torque request. The EBS controller has an algorithm configured to adapt the travel position-based braking torque request to the force-based braking torque request, despite physical changes in a relationship between the force-based and travel position-based braking torque requests. 
         [0016]    In another aspect of the invention, the controller is configured to continuously multiply the travel position-based braking torque request by an adjustable multiplier factor during operation of the vehicle. 
         [0017]    In another aspect of the invention, the controller is operable for calculating an average percentage variance between the travel position-based and force-based torque requests based on a predetermined number of samples, and the adjustable multiplier is adjusted by a predetermined amount when the calculated average percentage variance falls outside of a stored window of acceptable error. 
         [0018]    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 
         [0019]      FIG. 1  is a schematic representation of a vehicle chassis having a controller with an adaptive brake pedal algorithm according to the invention; 
           [0020]      FIG. 2  is a flow chart describing the adaptive brake pedal algorithm of the invention; and 
           [0021]      FIG. 3  is a schematic curve or graphic illustration showing a representative brake pedal sensor transition according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    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 (EBS) control unit or controller  18 , and an engine  25  that is selectively connectable to a transmission  17  having a rotatable output member  24 . 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, vehicle  10  may be any vehicle utilizing an EBS controller  18  as described herein, such as an electric vehicle or a fuel-cell powered vehicle. 
         [0023]    EBS controller  18 , referred to hereinafter for simplicity as controller  18 , is operable for detecting a braking or apply force (arrow A) applied to a brake pedal  27  using a pair of brake pedal sensors  40  and  41  connected thereto. Controller  18  is configured to calculate, select, or otherwise determine a corresponding braking request (B C ) in response to the detected and recorded measurements or readings. Controller  18  then communicates the braking request (B C ) to a braking mechanism to slow or stop the vehicle  10 , preferably to an individual electronic braking unit  30  positioned in proximity to each of the wheels  28  and/or to output member  24  by-wire and/or via datalink. The braking request (B C ) is transmitted across one or more command signal transmission channels or lines  50  to the electronic braking units  30  and/or output member  24 , where the braking request (B C ) can act to oppose the torque of each electronic braking unit  30  and/or output member  24  to slow or stop vehicle  10  as needed. 
         [0024]    Depending on the specific drive configuration of vehicle  10 , 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 vehicle  10 . Although not shown in  FIG. 1 , vehicle  10  may also have a substantially similar front differential suitable for distributing torque to front drive axle  11  for powering or driving a plurality of wheels  28 , such as in a four-wheel or all-wheel drive configuration. Transmission  17  is configured to deliver a variable transmission output speed N to output member  24 , with transmission output speed N being variably opposable by braking request (B C ), as determined by controller  18 . 
         [0025]    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 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 . 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 . 
         [0026]    Travel sensor  41  (also labeled B T  in  FIG. 1 ) is preferably a pedal position and range sensor configured or adapted to precisely detect, measure, or otherwise determine the relative position and direction of travel of 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 brake pedal  27  by an operator of vehicle  10 . The measurements or readings taken by travel sensor  41  and force sensor  40  are transmittable or communicable to controller  18  or are otherwise determinable thereby as needed for use with algorithm  100  of the invention, which is described in further detail later hereinbelow. 
         [0027]    Memory  19  of controller  18  is preloaded or preprogrammed with a pair of braking torque look-up tables  90  and  91 , i.e. braking torque data tables readily accessible by controller  18  in implementing or executing algorithm  100 . Lookup table  90 , referred to hereinafter as force table  90 , corresponds to the recorded measurements or readings of force sensor  40  and contains an associated commanded braking request (B C ) appropriate for each of the detected force measurements as determined by force sensor  40 . Likewise, look-up 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 request (B C ) appropriate for the detected position of travel sensor  41 . 
         [0028]    Controller  18  preferably normally operates according to force table  90 , and force table  90  therefore acts as a preferred or default table. However, as described previously hereinabove, pressure or force sensors such as 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  may be used in place of force table  90  during periods of low force resolution of force sensor  40  in order to avoid a perceptible discontinuity or variance between the braking request (B C ) and the operator&#39;s intended braking force. However, discontinuity may result from mechanical hysteresis or lag in response time in the various components comprising the electronic braking system, and therefore transitioning instantaneously between force table  90  and travel table  91  and may result in a less than optimal braking performance. Accordingly, a separate variable and adaptive braking torque request is provided, which is initially a travel position-based braking torque request (TBR) determined by travel table  91 , and which is modified as needed by a multiplier M (see  FIG. 2 ) to more closely model or approximate a force-based braking torque request (FBR) as determined by force table  90 . 
         [0029]    Turning to  FIG. 2 , algorithm  100  is provided to actively adapt a travel-position based braking torque request (TBR), i.e. the braking request (B C ) determined based on or using travel sensor  41  (see  FIG. 1 ), to more closely approximate or match a force-based braking torque request (FBR), i.e. the braking request (B C ) based on or determined using force sensor  40  (see  FIG. 1 ), thereby facilitating a smooth and efficient switch or transition between travel and force sensors  41  and  40 , respectively. As explained hereinabove, an electronic braking system (EBS) may have a slightly different force/travel relationship relative to the actual position of brake pedal  27  (see  FIG. 1 ). Additionally, this force/travel relationship may vary over time, for example due to the introduction of air into a vehicle hydraulic system through service and/or absorption of water into a supply of brake fluid. Some measurable difference in the force/travel relationship may also result from different driver apply rates, as no two people will apply brake pedal  27  in exactly the same manner over time. Accordingly, using algorithm  100 , the variance over time of a particular electronic braking system (EBS), build variation, and/or component tolerances may be properly accounted for. 
         [0030]    Using algorithm  100  of the invention, electronic braking performance is optimized, and the “feel” of brake pedal  27  (see  FIG. 1 ) should more closely approximate the motion and feel of a conventional mechanical braking pedal as described hereinabove. To ensure the continuous adaptation or virtual “learning” of algorithm  100 , algorithm  100  is preferably executed per a sufficiently rapid and continuous cycle or control loop, preferably 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 ). Finally, within algorithm  100  a force-based braking torque request (FBR) remains constant while travel position-based braking torque request (TBR) is permitted to adapt or vary to thereby model or approximate the force-based braking torque request (FBR). 
         [0031]    Beginning with step  101 , an integer counter (C) is zeroed or initialized in memory  19  of controller  18  (see  FIG. 1 ). Integer counter (C) is any digital or analog device capable of counting upward by positive whole numbers in response to a predetermined event, and of retaining the current count for access by algorithm  100  as needed and described later hereinbelow. After counter (C) has been properly zeroed or initialized, algorithm  100  proceeds to step  102 . 
         [0032]    At step  102 , algorithm  100  detects or otherwise determines that a braking event has occurred. A “braking event” as used herein defines an application of a detectable or measurable apply force (arrow A) to brake pedal  27  (see  FIG. 1 ). Such an apply force (arrow A) likewise imparts pedal motion or “travel”, which in turn is measurable or otherwise detectable by travel sensor  41  as described hereinabove. Algorithm  100  repeats step  102  according to the predetermined control loop, preferably every 5 to 10 milliseconds, until such a braking event is detected. Once a braking event has been properly detected, algorithm  100  proceeds to step  104 . 
         [0033]    At step  104 , algorithm  100  determines whether the brake pedal apply force (arrow A, see  FIG. 1 ) detected at step  102  falls within a stored, predetermined threshold brake apply force range, abbreviated in  FIG. 2  simply as “range”. For example, the lower threshold value of such a range may be set to an apply force value above which the resolution of force sensor  40  (see  FIG. 1 ) is sufficient to enable execution of braking request (B C ) according to force table  90  (see  FIG. 1 ), while the upper threshold value may be set to an appropriate value so as to avoid saturation with travel sensor  41  (see  FIG. 1 ). Step  104  permits execution of the remaining steps of algorithm  100  to be limited or restricted to a specific portion or range of detected apply force (arrow A). If algorithm  100  determines that the detected apply force (arrow A) falls within the stored threshold brake apply force range, it proceeds to step  106 . Otherwise, algorithm  100  repeats step  102 . 
         [0034]    At step  106 , algorithm  100  determines whether the detected motion or travel of brake pedal  27  (see  FIG. 1 ) is not decreasing, i.e. is traveling in a proper direction for application of brake pedal  27  (see  FIG. 1 ). If at step  106  algorithm  100  determines that pedal travel is not decreasing, algorithm  100  proceeds to step  108 . Otherwise, algorithm  100  returns to step  102 . The inclusion of step  106  in algorithm  100  is preferred in order to minimize or eliminate the influence of mechanical hysteresis on the subsequent error-adjustment or adaptive method steps described hereinbelow. 
         [0035]    At step  108 , upon detection of a braking event corresponding to travel within the predetermined calibration window described hereinabove at steps  104  and  106 , respectively, algorithm  100  next calculates an instantaneous travel position-based error or variance percentage, abbreviated in  FIG. 2  as “ITB % Error”. Step  108  determines, on a percentage basis, the variance between a force-based request (FBR) and travel position-based request (TBR). Accordingly, the value “ITB % Error” is calculated according to the equation [[TBR−FBR]/FBR]×100%. Once the value “ITB % Error” has been properly calculated, it is temporarily stored in memory  19  of EBS controller  18  (see  FIG. 1 ). Algorithm  100  then proceeds to step  110 . 
         [0036]    At step  110 , algorithm  100  sums the calculated ITB % Error from a predetermined number of samples to thereby arrive at an intermediate or nominal error total (Y). Nominal error total (Y) is temporarily stored in memory  19  of controller  18  (see  FIG. 1 ). Algorithm  100  then proceeds to step  112 . 
         [0037]    At step  112 , algorithm  100  increments the integer counter (C) by one count, then proceeds to step  114 . 
         [0038]    At step  114 , algorithm  100  compares the value of integer counter (C) to a predetermined value or multiple (n) corresponding the number of samples (N) used to calculate nominal error total (Y) (see step  110 ). If counter (C) equals the predetermined multiple (n), algorithm  100  proceeds to step  116 . Multiple (n) is selected to provide a sufficient number of braking events to occur before modifying the adapted braking torque request (ABR). Preferably, multiple (n) is set to at least 10, however any multiple (n) may be selected as appropriate within the scope of the invention as needed for a particular vehicle or electronic braking system. If the value of counter (C) does not equal multiple (n), algorithm  100  returns to step  102  without making any adjustments to the travel position-based request (TBR). 
         [0039]    At step  116 , algorithm  100  retrieves the stored nominal error total (Y) and, using the predetermined number of data samples (N), performs a simple averaging function, i.e. average=Y/N, to arrive at an average ITB % Error value, abbreviated “ITB % ave ” in  FIG. 2  for simplicity. Once algorithm  100  has calculated the value “ITB % ave ” it proceeds to step  118 . 
         [0040]    At step  118 , algorithm  100  compares the value “ITB % ave ” calculated in step  116  to a stored set, range, or acceptable window of error, which describes a band or window of permissible variance. Within this window, for example, an instantaneous transition between travel position-based request (TBR) and force-based request (FBR) may be considered permissible as falling within an acceptable performance tolerance. If the value “ITB % ave ” falls within this window, algorithm  100  returns to step  102  without making any adjustment to the travel position-based request (TBR). Otherwise, algorithm  100  proceeds to step  120 . 
         [0041]    At step  120 , an error-adjustment factor or adjustable multiplier factor (M) is modified or adjusted by a predetermined value, percentage, or quantity. That is, the adjustable multiplier factor (M) is adjusted by an appropriate preset value, percentage, or quantity, preferably approximately 0.2 to 0.8%, although other values or percentages may be used as needed within the scope of the invention. Upon initial launch of a vehicle  10  (see  FIG. 1 ), i.e. when a vehicle  10  is released to the consumer from production, multiplier factor (M) is initialized to a value of 1, i.e., preprogrammed or recorded in memory  19  of controller  18  (see  FIG. 1 ) to equal 1, so that the adapted braking torque request (ABR) is initially equal to the travel position-based request (TBR). Multiplier factor (M) is then adjusted by a preset value, percentage, or quantity periodically based on the detected presence of a predetermined vehicle condition, such as explained in step  118 . This occurs in order to allow the travel position-based request (TBR) to be continuously adjusted to approximate over time the force-based request (FBR). To ensure that the travel position-based request (TBR) remains within a reasonable range, multiplier factor (M) is preferably bounded by predetermined upper and lower limits. 
         [0042]    Turning to  FIG. 3 , a representative set of braking torque curves  52  is shown describing the effect of algorithm  100  of  FIG. 2  on an exemplary travel position-based request (TBR). Line A describes a representative travel position-based request (TBR), while line B describes a representative force-based request (FBR), as described previously hereinabove. The area or band between arrows D represents a variance or difference in a requested braking torque (Y-axis) at a particular moment in time (X-axis). Transitioning instantaneously between a travel position-based request (TBR) and force-based request (FBR) when such a variance is present may, depending on the magnitude of the variance, result in a perceptible discontinuity in applied braking torque. 
         [0043]    For example, if at point E an instantaneous transition where to occur between force-based request (FBR) line B and travel position-based request (TBR) line A, or between line A and line B, such a transition will result in a negligible perceptible effect, as at point E lines A and B are approximately equal or coextensive. However, at point F of force-based request (FBR) line B, an instantaneous transition from point F to a corresponding point F′ of travel position-based request (TBR) line A would result in a braking torque variance of approximately 1000 Nm, which would likely be readily perceptible to a user of vehicle  10  (see  FIG. 1 ). 
         [0044]    Therefore, using algorithm  100  as described hereinabove with reference to  FIG. 2 , an alternate or adapted braking torque request (ABR) is provided, as represented by line C. Using algorithm  100 , for example, the adapted braking torque request (ABR) represented by line C more closely approximates the force-based request (FBR) of line B by adjusting the travel position-based request (TBR) on an adaptive basis. As shown in  FIG. 3 , transitioning instantaneously between the adapted braking torque request (ABR) of line C and the force-based request (FBR) of line B provides a more robust, fluid transition than would otherwise be possible. 
         [0045]    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.