Patent Publication Number: US-2021178904-A1

Title: Apparatus for controlling regenerative braking torque of an electric vehicle and a method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to Korean Patent Application No. 10-2019-0167762, filed in the Korean Intellectual Property Office on Dec. 16, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a technique of controlling regenerative braking torque of a driving motor to prevent an anti-lock brake system (ABS) provided in an electric vehicle from entering an operating range. 
     BACKGROUND 
     An anti-lock brake system (ABS) provided in a vehicle does not continuously apply a braking force to wheels when braking. An ABS performs a pumping operation periodically (e.g., 10 times or more per second) to prevent the wheels from being locked. In this case, the ABS compares the speed of a vehicle body and the speed of a wheel, and when the difference exceeds a threshold value, determines that the wheel is locked and starts operation. 
     The electric vehicle is equipped with a regenerative braking system that obtains energy by operating a drive motor as a generator when braking. The electric vehicle equipped with ABS cannot recover sufficient energy through regenerative braking because the regenerative braking must be stopped when ABS operates during regenerative braking. 
     A conventional technique of improving the energy recovery rate through regenerative braking determines a target slip amount and controls the motor to follow the target slip amount. Accordingly, it is necessary to estimate the speed of a vehicle body in order to determine the target slip amount. In addition, an additional sensor is required to estimate the speed of the vehicle body. 
     The matters described in this background section are intended to promote an understanding of the background of the disclosure and may include matters that are not already known to those of ordinary skill in in the art. 
     SUMMARY 
     The present disclosure has been made to solve the above-mentioned problems occurring in the prior art while advantages achieved by the prior art are maintained intact. 
     An aspect of the present disclosure provides an apparatus for controlling regenerative braking torque of an electric vehicle and another aspect is to provide a method thereof. The apparatus and method can compensate the regenerative braking torque of a driving motor based on a behavior model of the electric vehicle, such that an anti-lock brake system (ABS) is prevented from entering the operating range to the maximum limit to maximize the energy recovery rate through regenerative braking. 
     The technical problems to be solved by the present inventive concept are not limited to the aforementioned problems. Any other technical problems not mentioned herein should be clearly understood from the following description by those having ordinary skill in the art to which the present disclosure pertains. 
     According to an aspect of the present disclosure, an apparatus is disclosed for controlling regenerative braking torque of an electric vehicle on which an ABS is mounted. The apparatus includes a disturbance extractor that extracts a disturbance in a specific frequency band from a difference between a behavior model and an actual behavior of the electric vehicle. The apparatus also includes a torque compensator that compensates for the regenerative braking torque based on the disturbance extracted by the disturbance extractor. 
     The torque compensator may calculate compensation torque to offset the disturbance extracted by the disturbance extractor and may subtract the compensation torque from the regenerative braking torque. 
     The torque compensator may prevent a hysteresis phenomenon from occurring based on the calculated compensation torque. 
     The torque compensator may set a changing rate of the compensation torque. In this case, the torque compensator may dividedly apply the compensation torque when the regenerative braking torque is increased and may collectively apply the compensation torque when the regenerative braking torque is decreased. 
     The disturbance extractor may include: an inverse nominal model in a form of a transfer function, which may output torque when a wheel speed is input; a first subtractor that subtracts the compensated regenerative braking torque through proportional integral derivative (PID) control from the torque output from the inverse nominal model to extract a primary disturbance; and a filter that filters the primary disturbance extracted by the first subtractor to extract a final disturbance. 
     The filter may include: a first low-pass filter (LPF) that passes a first frequency component in a low frequency band; a second LPF that passes a second frequency component in the low frequency band; a second subtractor that subtracts the second frequency component from the first frequency component to extract the final disturbance; and a third LPF that removes a noise component of the final disturbance. 
     The torque compensator may include a compensation torque calculator that calculates a compensation torque that offsets the final disturbance extracted by the filter and a hysteresis comparator that prevents hysteresis caused by the compensation torque calculated by the compensation torque calculator The torque compensator may also include a rate limiter that equally divides the compensation torque received from the hysteresis comparator and inputs the equally divided compensation torque to a third subtractor when the regenerative braking torque is increased, and that collectively inputs the compensation torque received from the hysteresis comparator within a reference time when the regenerative braking torque is reduced. The third subtractor may be configured to subtract the compensation torque input from the rate limiter from the regenerative braking torque to compensate for the regenerative braking torque. 
     The torque compensator may delay an operation of the ABS until a time point at which the compensation for the regenerative braking torque is possible based on the inverse nominal model. 
     According to another aspect of the present disclosure, a method is disclosed of controlling regenerative braking torque of an electric vehicle on which an ABS is mounted. The method includes extracting, by a disturbance extractor, a disturbance in a specific frequency band from a difference between a behavior model and an actual behavior of the electric vehicle. The method also includes compensating, by a torque compensator, for the regenerative braking torque based on the disturbance extracted by the disturbance extractor. 
     The compensating for the regenerative braking torque may include calculating compensation torque to offset the disturbance extracted by the disturbance extractor. The method may also include subtracting the compensation torque from the regenerative braking torque. 
     The compensating for the regenerative braking torque may further include preventing a hysteresis phenomenon from occurring based on the calculated compensation torque. 
     The method may further include setting a changing rate of the calculated compensation torque. In this case, the setting of the changing rate of the calculated compensation torque may include dividedly applying the compensation torque when the regenerative braking torque is increased and may include collectively applying the compensation torque when the regenerative braking torque is decreased. 
     The extracting of the disturbance may include extracting a primary disturbance by subtracting the compensated regenerative braking torque through PID control from the torque output from an inverse nominal model and may include extracting a final disturbance by filtering the extracted primary disturbance. The extracting of the final disturbance may include: passing a first frequency component in a low frequency band; passing a second frequency component in the low frequency band; extracting the final disturbance by subtracting the second frequency component from the first frequency component; and removing a noise component of the final disturbance. 
     The compensating for the regenerative braking torque may include: calculating, by compensation torque calculator, a compensation torque that offsets the final disturbance; preventing, by a hysteresis comparator, hysteresis caused by the calculated compensation torque; equally dividing, by a rate limiter, the compensation torque received from the hysteresis comparator and inputting equally divided compensation torque to a subtractor when the regenerative braking torque is increased; collectively inputting, by the rate limiter, the compensation torque received from the hysteresis comparator within a reference time to the subtractor when the regenerative braking torque is reduced; and subtracting, by the subtractor, the compensation torque input from the rate limiter from the regenerative braking torque to compensate for the regenerative braking torque. 
     The compensating for the regenerative braking torque may include delaying an operation of the ABS until a time point at which the compensation for the regenerative braking torque is possible based on the inverse nominal model. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings: 
         FIG. 1  is a view illustrating a configuration of an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure; 
         FIG. 2  is a view illustrating a relationship between a slip ratio and a braking force used to derive an inverse nominal model provided in the apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure; 
         FIG. 3  is a view illustrating the structure of a filter provided in an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure; 
         FIG. 4  is a view illustrating the output of each low-pass filter (LPF) in the filter provided in an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure; 
         FIG. 5  is a view illustrating the performance of an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure; 
         FIG. 6  is a view illustrating a configuration of an apparatus for controlling regenerative braking torque of an electric vehicle according to another embodiment of the present disclosure; 
         FIG. 7  is a flowchart illustrating a method of controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure; and 
         FIG. 8  is a block diagram illustrating a computing system for executing a method of controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some embodiments of the present disclosure are described in detail with reference to the drawings. In adding the reference numerals to the components of each drawing, it should be noted that the identical or equivalent component is designated by the identical numeral even when they are displayed on other drawings. Further, in describing the embodiments of the present disclosure, a detailed description of well-known features or functions has been omitted in order not to unnecessarily obscure the gist of the present disclosure. 
     In describing the components of the embodiment according to the present disclosure, terms such as first, second, “A”, “B”, (a), (b), and the like may be used. These terms are merely intended to distinguish one component from another component. Such terms do not limit the nature, sequence, or order of the constituent components. Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as those generally understood by those having ordinary skill in the art to which the present disclosure pertains. Such terms as those defined in a generally used dictionary are to be interpreted as having meanings equal to the contextual meanings in the relevant field of art. Such terms are not to be interpreted as having ideal or excessively formal meanings unless clearly defined as having such in the present application. 
       FIG. 1  is a view illustrating a configuration of an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     As shown in  FIG. 1 , an apparatus  100  for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure may include a disturbance extractor  10  and a torque compensator  20 . In this case, according to a scheme of implementing the apparatus  100  for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure, each component may be combined with each other to be implemented as one, and some components may be omitted. In particular, the functions of the disturbance extractor  10  and the torque compensator  20  may be implemented to be performed by a controller. In this case, the controller may be implemented in the form of hardware or software, or in a combination of hardware and software. In one example, the controller may be implemented with a microprocessor, but the controller is not limited thereto. 
     Referring to each component, first, the disturbance extractor  10  extracts the disturbance of a specific frequency band from the difference between a behavior model and the actual behavior of the electric vehicle. 
     The disturbance extractor  10  may include an inverse nominal model  11 , a subtractor  12 , and a filter  13 . 
     The inverse nominal model  11  may be implemented as a behavior model of an electric vehicle in the form of a transfer function (G n   −1 ) that outputs torque when a wheel speed is input. 
     Hereinafter, the inverse nominal model  11  is described in detail with reference to  FIG. 2 . 
       FIG. 2  is a view illustrating a relationship between a slip ratio and a braking force used to derive the inverse nominal model provided in the apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     As shown in  FIG. 2 , reference numeral ‘ 210 ’ indicates the relationship between the slip ratio and the braking force of the electric vehicle corresponding to the frictional forces of different road surfaces. Although there is a difference in the maximum braking force for each road surface, the maximum braking force is stably maintained in a specific slip region. 
     The inertia J whl  of a wheel and the inertia J eq  of the electric vehicle are summarized in relation to the slip ratio as in the following Equation 1. 
         J   eq   =J   whl   =±mR   eff   2 (1−λ)  [Equation 1]
 
     In Equation 1, ‘m’ is the mass of the electric vehicle, ‘R eff ’ is a tire dynamic radius, and ‘λ’ is a slip ratio, respectively. In this case, ‘λ’ may be expressed as in the following Equation 2. 
     
       
         
           
             
               
                 
                   
                     
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                         ( 
                         
                           slip 
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                            
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                             R 
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                            
                           ω 
                         
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                         v 
                       
                       v 
                     
                   
                   , 
                   
                     
                       
                         R 
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                       ω 
                     
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                     v 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 2, ‘ω’ represents the number of wheel revolutions and ‘v’ represents a vehicle speed, respectively. 
     Assuming that the slip ratio is 0 in Equation 1, the inertia J n  of the nominal model is expressed as in the following Equation 3. 
         J   n   =J   whl   +mR   eff   2   [Equation 3]
 
     Finally, the nominal model G n (s) is expressed as in the following Equation 4. 
     
       
         
           
             
               
                 
                   
                     
                       G 
                       n 
                     
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                       ( 
                       s 
                       ) 
                     
                   
                   = 
                   
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                         J 
                         n 
                       
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                       s 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Therefore, the inverse nominal model G n (d) −1  is expressed as in the following Equation 5. 
         G   n ( d ) −1   =J   n   s   [Equation 5]
 
     The subtractor  12  subtracts the regenerative braking torque (a regenerative braking torque value) compensated through proportional integral derivative (PID) control from the output (torque value) of the inverse nominal model. The subtraction result represents a primary disturbance. 
     The filter  13  extracts a final disturbance of a specific frequency band from the primary disturbance. 
     The filter  13  may be implemented with a low-pass filter (LPF) to extract the final disturbance from which high frequency noise is removed. 
     The filter  13  may be implemented with a high-pass filter (HPF) to extract the final disturbance above a specific frequency. 
     The filter  13  may be implemented with a band-pass filter (BPF) to extract the final disturbance of a specific frequency band. 
     As shown in  FIG. 3 , the filter  13  may be implemented with a plurality of LPFs as shown in  FIG. 3 . 
       FIG. 3  is a view illustrating the structure of a filter provided in an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     As shown in  FIG. 3 , an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure may include a plurality of LPF&#39;s. The apparatus may include: a first LPF  131  having a first time constant to pass a high-frequency component in a low frequency band; a second LPF  132  having a second time constant to pass a low-frequency component in the low frequency band; a subtractor  133  for subtracting the frequency component passing through the second LPF  132  from the frequency component passing through the first LPF  131 ; and a third LPF  134  for removing a noise component from the subtraction result of the subtractor  133 . 
     The first LPF  131  filters the primary disturbance d raw  to pass the first frequency component  d   hf . In this case, the first frequency component represents the disturbance detected when a wheel slip occurs. 
     The second LPF  132  filters the primary disturbance draw to pass the second frequency component  d   lf  In this case, the second frequency component represents the disturbance with respect to the gradient, load change, or change in running load. 
     The subtractor  133  extracts the final disturbance {circumflex over (d)} add  by subtracting the second frequency component from the first frequency component. 
     The third LPF  134  removes the noise component of the final disturbance. 
       FIG. 4  is a view illustrating the output of each LPF in the filter provided in an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     In  FIG. 4 , reference numeral  411  denotes a first frequency component as an output of the first LPF  131 , reference numeral  412  denotes a second frequency component as an output of the second LPF  132 , and reference numeral  413  denotes the final disturbance as an output of the third LPF  134 . 
     Next, the torque compensator  20  compensates for the regenerative braking torque based on the disturbance extracted by the disturbance extractor  10 . That is, the torque compensator calculates compensation torque (compensation torque to offset the disturbance) by which the disturbance extracted by the disturbance extractor  10  becomes 0 (zero). The torque compensator also subtracts the calculated compensation torque (compensation) from the regenerative braking torque (regenerative braking request torque). 
     To prevent a hysteresis phenomenon based on the calculated compensation torque, the torque compensator  20  may perform the compensation for the regenerative braking torque when the calculated compensation torque is less than a first reference value. The torque compensator  20  may not perform the compensation when the calculated compensation torque is greater than or equal to a second reference value. In this case, the first reference value is set to a value greater than the second reference value. 
     The torque compensator  20  may set a change ratio of the compensation torque. For example, when the compensation torque is −10 (in case of increasing the regenerative braking torque) and it is required to add 10 to the regenerative braking torque within 100 ms, it may be added by 1 in 10 ms increments instead of 10 at a time. This is to prevent shock. 
     As another example, when the compensation torque is 10 (in case of reducing the regenerative braking torque) and it is required to subtract 10 from the regenerative braking torque within 100 ms, the torque compensator  20  is allowed to have a fast response characteristic by subtracting 10 from the regenerative braking torque at a time. 
     The torque compensator  20  may include a compensation torque calculator  21 , a hysteresis comparator  22 , a rate limiter  23 , and a subtractor  24 . 
     The compensation torque calculator  21  calculates the compensation torque (compensation torque to offset the disturbance), which allows the final disturbance extracted by the disturbance extractor  10  to become 0 (zero). 
     To prevent the hysteresis phenomenon from being caused by the compensation torque calculated by the compensation torque calculator  21 , the hysteresis comparator  22  transmits the calculated compensation torque to the rate limiter  23  when the compensation torque calculated by the compensation torque calculator  21  is less than the first reference value. The hysteresis compensator  22  does not transmit the calculated compensation torque to the rate limiter  23  when the compensation torque calculated by the compensation torque calculator  21  is greater than or equal to the second reference value. 
     In the case of increasing the regenerative braking torque, the rate limiter  23  equally divides the compensation torque received from the hysteresis comparator  22  within a reference time and inputs the equally divided compensation torque to the subtractor  24 . In the case of reducing the regenerative braking torque, the rate limiter  23  collectively inputs the compensation torque received from the hysteresis comparator  22  to the subtractor  24  within the reference time. 
     The subtractor  24  compensates for the regenerative braking torque by subtracting the compensation torque input from the rate limiter  23  from the regenerative braking torque. 
       FIG. 5  is a view illustrating the performance of an apparatus for controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     As shown in  FIG. 5 , according to the related art, it may be understood that regenerative braking is stopped by operating the ABS at a specific time point  510  because the regenerative braking torque is not controlled. In this case, the specific time point  510  is a time point at which the difference between a vehicle speed  511  and a wheel speed  512  exceeds a threshold. 
     To the contrary, according to a scheme of the present disclosure, because the regenerative braking torque is controlled, the operation of the ABS may be prevented or delayed as much as possible to extend the regenerative braking time. That is, when it is possible to continuously compensate for the regenerative braking torque based on the inverse nominal model or the nominal model, the operation of the ABS may be completely prevented. In addition, it is possible to delay the operation of the ABS until the time point at which it is possible to compensate for the regenerative braking torque based on the inverse nominal model or the nominal model. 
       FIG. 6  is a view illustrating a configuration of an apparatus for controlling regenerative braking torque of an electric vehicle according to another embodiment of the present disclosure. In this embodiment, the structure of the torque compensator  20  is the same as that of  FIG. 1 . 
     As shown in  FIG. 6 , a disturbance extractor  30  extracts the disturbance of a specific frequency band from the difference between the behavior model and the actual behavior of the electric vehicle. 
     The disturbance extractor  30  may include a nominal model  31 , a subtractor  32 , and a filter  33 . 
     The nominal model  31  may be implemented in the form of a transfer function G n  that outputs a wheel speed when torque is input as a behavior model of an electric vehicle. 
     The subtractor  32  performs an operation of subtracting the wheel speed of the vehicle from the output (wheel speed) of the nominal model. The calculation result represents the primary disturbance. 
     The filter  33  extracts the final disturbance of a specific frequency band from the primary disturbance. 
       FIG. 7  is a flowchart illustrating a method of controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     First, in operation  701 , the disturbance extractor  10  extracts the disturbance (torque) of a specific frequency band from the difference between the behavior model and the actual behavior of the electric vehicle. 
     Thereafter, in operation  702 , the torque compensator  20  compensates for the regenerative braking torque based on the disturbance extracted by the disturbance extractor  10 . 
       FIG. 8  is a block diagram illustrating a computing system for executing a method of controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure. 
     Referring to  FIG. 8 , as described above, a method of controlling regenerative braking torque of an electric vehicle according to an embodiment of the present disclosure may be implemented through a computing system. A computing system  1000  may include at least one processor  1100 , a memory  1300 , a user interface input device  1400 , a user interface output device  1500 , storage  1600 , and a network interface  1700  connected through a system bus  1200 . 
     The processor  1100  may be a central processing unit (CPU) or a semiconductor device that processes instructions stored in the memory  1300  and/or the storage  1600 . The memory  1300  and the storage  1600  may include various types of volatile or non-volatile storage media. For example, the memory  1300  may include a read only memory (ROM) and a random access memory (RAM). 
     Accordingly, the processes of the method or algorithm described in relation to the embodiments of the present disclosure may be implemented directly by hardware executed by the processor  1100 , a software module, or a combination thereof. The software module may reside in a storage medium (i.e., the memory  1300  and/or the storage  1600 ), such as a RAM, a flash memory, a ROM, an EPROM, an EEPROM, a register, a hard disk, solid state drive (SSD), a detachable disk, or a CD-ROM. The disclosed storage medium in this example is coupled to the processor  1100 . The processor  1100  may read information from the storage medium and may write information in the storage medium. In another method, the storage medium may be integrated with the processor  1100 . The processor and the storage medium may reside in an application specific integrated circuit (ASIC). The ASIC may reside in a user terminal. In another method, the processor and the storage medium may reside in the user terminal as an individual component. 
     According to the embodiments of the present disclosure, the apparatus and method for controlling regenerative braking torque of an electric vehicle can compensate the regenerative braking torque of the driving motor based on the behavior model of the electric vehicle. Thus, an ABS is prevented from entering the operating range to the maximum limit to maximize the energy recovery rate through regenerative braking. 
     The above description is an exemplification of the technical spirit of the present disclosure. The present disclosure may be variously altered and modified by those having ordinary skill in the art to which the present disclosure pertains without departing from the essential features of the present disclosure. 
     Therefore, the disclosed embodiments of the present disclosure do not limit the technical spirit of the present disclosure but instead are illustrative. The scope of the technical spirit of the present disclosure is not limited by the disclosed embodiments. The scope of the present disclosure should be construed by the claims, and it should be understood that all the technical spirits within the equivalent range fall within the scope of the present disclosure.