Patent Publication Number: US-2021171044-A1

Title: Apparatus for Estimating Friction Coefficient of Road Surface and Method Thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Korean Patent Application No. 10-2019-0159980, filed in the Korean Intellectual Property Office on Dec. 4, 2019, which application is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a technique for estimating a friction coefficient of a road surface and a method thereof. 
     BACKGROUND 
     As an engine performance of a vehicle increases, a driving speed of the vehicle also increases. Accordingly, various electronic control systems are mounted on the vehicle to improve driving stability and secure braking stability. 
     The electronic control system of the above-described vehicle includes an anti-lock brake system (ABS) that secures braking stability by repeating braking and braking release depending on slip of a wheel during braking of the vehicle, a tracking control system (TCS) preventing sudden acceleration of the vehicle and slippage of the driving wheel during rapid acceleration, and an electronic stability control (ESC) which controls brake pressure by combining the ABS and the TCS to improve driving stability of the vehicle. 
     In particular, the ESC determines whether a state of the driving vehicle is understeer or oversteer through a plurality of sensors such as a wheel speed sensor, a brake pressure sensor, a steering angle sensor, a yaw rate sensor, a lateral acceleration sensor, and the like and then individually controls a braking operation of an inner wheel and an outer wheel responding to the determined state, to stably maintain a vehicle posture. 
     It is very important to estimate a friction coefficient of a road surface with high accuracy because the electronic control system determines the state of the vehicle based on the friction coefficient of the road surface on which the vehicle is running and performs control responding thereto. 
     In a conventional technique for estimating a friction coefficient of a road surface, which is a technique for estimating the friction coefficient of the road surface based on longitudinal acceleration, lateral acceleration, yaw rate and steering angle of the vehicle, a longitudinal force acting on the front and rear wheels is estimated using the longitudinal acceleration and the lateral acceleration, a lateral longitudinal force acting on the front and rear wheels is estimated using the lateral acceleration, the yaw rate, and the steering angle, a longitudinal force acting on the front and rear wheels respectively using the longitudinal acceleration of the vehicle, and the friction coefficient of the road surface is estimated using the estimated longitudinal force and lateral force. 
     The above-described conventional friction coefficient of the road surface estimation technique is capable of estimating the friction coefficient of the road surface only in a sudden braking or turning situation, but there is a problem in that the friction coefficient of the road surface is not capable of being estimated in a gentle turning situation. 
     The above information disclosed in this section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. 
     SUMMARY 
     The present disclosure relates to a technique for estimating a friction coefficient of a road surface in a vehicle and a method thereof. Particular embodiments relate to a technique for estimating a friction coefficient of a road surface in a vehicle to which a rear wheel steering (RWS) system is applied and a method thereof. The embodiments of the present disclosure have been made to solve problems occurring in the prior art while advantages achieved by the prior art are maintained intact. 
     An embodiment of the present disclosure provides an apparatus for estimating a friction coefficient of a road surface capable of estimating the friction coefficient even in a moderate turning situation as well as a sudden braking or turning situation of a vehicle equipped with a rear wheel steering (RWS) system, in which the friction coefficient of the road surface is estimated based on a control current value of a RWS motor and a movement amount (stroke value) of a rear wheel steering link responding to the control current value of the RWS motor, and a method thereof. 
     The technical problems to be solved by the present inventive concept are not limited to the aforementioned problems, and any other technical problems not mentioned herein will be clearly understood from the following description by those skilled in the art to which the present disclosure pertains. 
     According to an embodiment of the present disclosure, an apparatus may include a current sensor that measures a control current value of a rear wheel steering (RWS) motor, a stroke sensor that measures a stroke value indicating a movement amount of a rear wheel steering link, and a controller that estimates the friction coefficient of the road surface based on the control current value measured by the current sensor and the stroke value measured by the stroke sensor. 
     The apparatus may further include a storage device that stores a map in which the friction coefficient of the road surface responding to the control current value and the stroke value is recorded. 
     Here, the controller may estimate the friction coefficient of the road surface based on the map stored in the storage device. 
     The apparatus may further include a vehicle speed sensor that measures a speed of a vehicle, and a storage device that stores a map in which a friction coefficient of a road surface responding to the control current value and the stroke value is recorded in a first speed section of the vehicle and a map in which a friction coefficient of a road surface responding to the control current value and the stroke value is recorded in a second speed section of the vehicle. 
     Here, the controller may select the map among a plurality of maps stored in the storage device based on the vehicle speed measured by the vehicle speed sensor and estimate the friction coefficient of the road surface based on the selected map. 
     According to an embodiment of the present disclosure, a method may include measuring a control current value of a rear wheel steering (RWS) motor in a current sensor, measuring a stroke value indicating a movement amount of a rear wheel steering link in a stroke sensor, and estimating the friction coefficient of the road surface based on the control current value measured by the current sensor and the stroke value measured by the stroke sensor in a controller. 
     The method may further include storing a map in which the friction coefficient of the road surface responding to the control current value and the stroke value is recorded in a storage device. 
     Here, the estimating of the friction coefficient of the road surface may include estimating the friction coefficient of the road surface based on the map stored in the storage device. 
     The method may further include measuring a speed of the vehicle in a vehicle speed sensor, and storing a map in which a friction coefficient of a road surface responding to the control current value and the stroke value is recorded in a first speed section of the vehicle and a map in which a friction coefficient of a road surface responding to the control current value and the stroke value is recorded in a second speed section of the vehicle, in a storage device. 
     Here, the estimating of the friction coefficient of the road surface may include selecting the map among a plurality of maps stored in the storage device based on the vehicle speed measured by the vehicle speed sensor, and estimating the friction coefficient of the road surface based on the selected map. 
     According to an embodiment of the present disclosure, an apparatus may include a first current sensor that measures a control current value of a left rear wheel steering (RWS) motor, a first stroke sensor that measures a stroke value indicating a movement amount of a left rear wheel steering link, a second current sensor that measures a control current value of a right RWS motor, a second stroke sensor that measures a stroke value indicating a movement amount of a right rear wheel steering link, and a controller that estimates a friction coefficient of a first road surface based on the control current value measured by the first current sensor and the stroke value measured by the first stroke sensor, estimates a friction coefficient of a second road surface based on the control current value measured by the second current sensor and the stroke value measured by the second stroke sensor, and estimates a friction coefficient of a final road surface using the friction coefficient of the first road surface and the friction coefficient of the second road surface. 
     Here, the controller may estimate an average value of the friction coefficient of the first road surface and the friction coefficient of the second road surface as the friction coefficient of the final road surface. 
     In addition, the controller may determine the road surface on which the vehicle is traveling as a split road surface when a difference between the friction coefficient of the first road surface and the friction coefficient of the second road surface exceeds a threshold. 
     The apparatus may further include a storage device that stores a map in which the friction coefficient of the road surface responding to the control current value and the stroke value is recorded. 
     Here, the controller may estimate the friction coefficient of the first road surface and the friction coefficient of the second road surface based on the map stored in the storage device. 
     The apparatus may further include a vehicle speed sensor that measures a speed of a vehicle and a storage device that stores a map in which the friction coefficient of the road surface responding to the control current value and the stroke value is recorded in a first speed section of the vehicle and a map in which the friction coefficient of the road surface responding to the control current value and the stroke value is recorded in a second speed section of the vehicle. 
     Here, the controller may select a map of a plurality of maps stored in the storage device based on the vehicle speed measured by the vehicle speed sensor and estimate the friction coefficient of the first road surface and the friction coefficient of the second road surface based on the selected map. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of embodiments of the present disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an apparatus for estimating a friction coefficient of a road surface according to an embodiment of the present disclosure; 
         FIG. 2  is an example of a first map and a second map provided in an apparatus for estimating a friction coefficient of a road surface according to an embodiment of the present disclosure; 
         FIG. 3  is a flowchart for a method of estimating a friction coefficient of a road surface according to an embodiment of the present disclosure; 
         FIG. 4  is a block diagram of an apparatus for estimating a friction coefficient of a road surface according to another embodiment of the present disclosure; 
         FIG. 5  is a flowchart for a method of estimating a friction coefficient of a road surface according to another embodiment of the present disclosure; and 
         FIG. 6  is a block diagram illustrating a computing system for implementing a method of estimating a friction coefficient of a road surface according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, some embodiments of the present disclosure will be described in detail with reference to the exemplary 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 will be omitted in order not to unnecessarily obscure the gist of the embodiments of the present disclosure. 
     In describing the components of the embodiments 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, and the 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 skilled 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 aft, and 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 block diagram of an apparatus for estimating a friction coefficient of a road surface according to an embodiment of the present disclosure and illustrates a configuration which is capable of being applied to a vehicle equipped with a right and left integrated RWS system. 
     As illustrated in  FIG. 1 , an apparatus  100  for estimating a friction coefficient of a road surface according to an embodiment of the present disclosure may include a storage device  11 , a vehicle speed sensor  12 , a current sensor  13 , a stroke sensor  14 , and a controller  15 . Here, each component may be combined with each other and implemented as one, or some components may be omitted based on a method of implementing the friction coefficient estimating apparatus  100  of the road surface according to an embodiment of the present disclosure. 
     The above-described components will be described, respectively. The storage device  11  may store various logics, algorithms and programs required in a process, where, in the vehicle equipped with the rear wheel steering (RWS) system, the friction coefficient of the road surface is estimated based on a control current value of a RWS motor and a movement amount (stroke value) of a rear wheel steering link responding to the control current value of the RWS motor. 
     For reference, the RWS system means a chassis control system which allows a rear wheel steering angle to operate in a reverse phase of a front wheel steering angle in a low-speed driving state of the vehicle to reduce a turning radius when turning and allows the rear wheel steering angle to operate in the same phase of the front wheel steering angle to improve stability when turning. The above-described RWS system steers rear wheels in the reverse phase to the steering of front wheels in a low-speed section and steers the rear wheels in the same phase as the steering of the front wheels in a high-speed section. 
     The storage device  11  may store a map in which the friction coefficient of the road surface responding to the control current value of the RWS motor and the movement amount (stroke value) of the rear wheel steering link is recorded. 
     The storage device  11  may store a first map in which the friction coefficient of the road surface responding to the control current value of the RWS motor and the movement amount (stroke value) of the rear wheel steering link in the low-speed section of the vehicle is recorded and a second map in which the friction coefficient of the road surface responding to the control current value of the RWS motor and the movement amount (stroke value) of the rear wheel steering link in the high-speed section of the vehicle is recorded. Here, it is possible to distinguish between low-speed and high-speed based on a value between 40 kph and 80 kph. For example, less than 60 kph may be set at low-speed and more than 60 kph may be set at high-speed. 
       FIG. 2  is an example of a first map and a second map provided in an apparatus for estimating a friction coefficient of a road surface according to an embodiment of the present disclosure. 
     As illustrated in  FIG. 2 , in a low-speed section (a first speed section) and a high-speed section (a second speed section) of the vehicle, a horizontal axis represents a control current value (mA) of a RWS motor, and a vertical axis represents a stroke value (mm). 
     In the low-speed section of the vehicle, a graph having a friction coefficient of 0.3, a graph having a friction coefficient of 0.6, and a graph having a friction coefficient of to are illustrated and in the high-speed section of the vehicle, a graph having a friction coefficient of 0.3, a graph having a friction coefficient of 0.6, and a graph having a friction coefficient of to are illustrated. However, this is to help understanding and the number of graphs representing the friction coefficient may be arbitrarily changed according to a designer&#39;s intention. 
     Through  FIG. 2 , it may be seen that, in the case of a graph having the same coefficient of friction, a slope of the graph illustrated in the high-speed section is greater than that of the graph of the friction coefficient illustrated in the low-speed section. 
     The storage device  11  may include a at least one type of storage medium of a memory having a flash memory type, hard disk type, micro type, and card type (e.g., secure digital (SD) card or extreme digital (XD) card), and a random access memory (RAM), a static RAM (SRAM), a read-only memory (ROM), a programmable ROM (PROM), an electrically erasable PROM (EEPROM), a magnetic RAM (MRAM), a magnetic disk, and an optical disk. 
     The vehicle speed sensor  12  measures the speed of the vehicle. Of course, the controller  15  may estimate the speed of the vehicle without the vehicle speed sensor  12 . 
     The current sensor  13  may measure the control current value (mA) input from an RWS motor controller  21  to an RWS motor  22 . Here, the RWS motor  22  steers a rear wheel  23  by moving the steering link of the rear wheel  23  based on the control current value. 
     The stroke sensor  14  may measure the movement amount (mm) of the steering link of the rear wheel  23 . 
     The controller  15  performs overall control so that each of the components may perform its functions normally. The controller  15  may be implemented in a form of hardware, software, or a combination of hardware and software. Preferably, the controller  15  may be implemented as a microprocessor, but is not limited thereto. 
     In particular, the controller  15  may be applied to a vehicle equipped with a right and left integrated rear wheel steering (RWS) system to perform various controls in a process which estimates the friction coefficient of the road surface based on the control current value of the RWS motor  22  and the movement amount (stroke value) of the steering link of the rear wheel  23  responding to the control current value of the RWS motor  22 . 
     The controller  15  may estimate the friction coefficient of the road surface based on the control current value of the RWS motor  22  measured by the current sensor  13  and the movement amount of the steering link of the rear wheel  23  measured by the stroke sensor  14 . 
     The controller  15  may estimate the friction coefficient of the road surface responding to the control current value of the RWS motor  22  measured by the current sensor  13  and the movement amount of the steering link of the rear wheel  23  measured by the stroke sensor  14 , based on the maps stored in the storage device  11 . 
     The controller  15  may select one map among a plurality of maps stored in the storage device  11  based on the speed of the vehicle measured by the vehicle speed sensor  12  and estimate the friction coefficient of the road surface responding to the control current value of the RWS motor  22  measured by the current sensor  13  and the movement amount of the steering link of the rear wheel  23  measured by the stroke sensor  14 . 
     The friction coefficient of the road surface estimated by an embodiment of the present disclosure may be used to limit a target yaw rate for a chassis integration control. 
       FIG. 3  is a flowchart for a method of estimating a friction coefficient of a road surface according to an embodiment of the present disclosure. 
     First, the current sensor  13  measures the control current value of the RWS motor  22  in  301 . 
     Thereafter, the stroke sensor  14  measures the stroke value representing the movement amount of the rear wheel steering link in  302 . 
     Then, the controller  15  estimates the friction coefficient of the road surface based on the current value measured by the current sensor  13  and the stroke value measured by the stroke sensor  14  in  303 . 
       FIG. 4  is a block diagram of an apparatus for estimating a friction coefficient of a road surface according to another embodiment of the present disclosure and illustrates a configuration which is capable of being applied to a vehicle equipped with a left and right independent RWS system. 
     As illustrated in  FIG. 4 , an apparatus  400  for estimating a friction coefficient of a road surface according to another embodiment of the present disclosure may include a storage device  41 , a vehicle speed sensor  42 , a first current sensor  43 , a first stroke sensor  44 , a second current sensor  45 , a second stroke sensor  46 , and a controller  47 . Here, the storage device  41  stores the same data as the storage device  11  described above, and the vehicle speed sensor  42  also performs the same function as the vehicle speed sensor  12  described above, and will not be described any further. 
     The above-described components will be described, respectively. The first current sensor  43  may measure a control current value (mA) input from a first RWS motor controller  51  to a first RWS motor  52 . Here, the first RWS motor  52  steers a left rear wheel  53  by moving the steering link of the left rear wheel  53  based on the control current value. 
     The first stroke sensor  44  may measure the stroke value representing a movement amount (mm) of the steering link of the left rear wheel  53 . 
     The second current sensor  45  may measure a control current value (mA) input from a second RWS motor controller  61  to a second RWS motor  62 . Here, the second RWS motor  62  steers a right rear wheel  63  by moving the steering link of the right rear wheel  63  based on the control current value. 
     The second stroke sensor  46  may measure a stroke value representing a movement amount (mm) of the steering link of the right rear wheel  63 . 
     The controller  47  performs overall control so that each of the components may perform its functions normally. The controller  47  may be implemented in a form of hardware, software, or a combination of hardware and software. Preferably, the controller  47  may be implemented as a microprocessor, but is not limited thereto. 
     In the vehicle equipped with the left and right independent RWS system, the controller  47  may estimate a friction coefficient of a first road surface based on the control current value measured by the first current sensor  43  and the stroke value measured by the first stroke sensor  44 , estimate a friction coefficient of a second road surface based on the control current value measured by the second current sensor  45  and the stroke value measured by the second stroke sensor  46 , and a friction coefficient of a final road surface using the friction coefficient of the first road surface and the friction coefficient of the second road surface. 
     Based on the maps stored in the storage device  41 , the controller  47  may estimate the friction coefficient of the first road surface based on the control current value measured by the first current sensor  43  and the stroke value measured by the first stroke sensor  44 , estimate the friction coefficient of the second road surface based on the control current value measured by the second current sensor  45  and the stroke value measured by the second stroke sensor  46 , and the friction coefficient of the final road surface using the friction coefficient of the first road surface and the friction coefficient of the second road surface. 
     The controller  47  may select one of a plurality of maps stored in the storage device  41  based on the speed of the vehicle measured by the vehicle speed sensor  42 , and based on the selected map, may estimate the friction coefficient of the first road surface based on the control current value measured by the first current sensor  43  and the stroke value measured by the first stroke sensor  44 , estimate the friction coefficient of the second road surface based on the control current value measured by the second current sensor  45  and the stroke value measured by the second stroke sensor  46 , and the friction coefficient of the final road surface using the friction coefficient of the first road surface and the friction coefficient of the second road surface. 
     When a difference between the friction coefficient of the first road surface and the friction coefficient of the second road surface exceeds a threshold, the controller  47  may determine the road surface of the road on which the vehicle is traveling as a split road surface. 
     The controller  47  may estimate an average of the friction coefficient of the first road surface and the friction coefficient of the second road surface as the friction coefficient of the final road surface. 
     The friction coefficient of the road surface estimated by another embodiment of the present disclosure may be used to limit the target yaw rate for chassis integration control. 
       FIG. 5  is a flowchart for a method of estimating a friction coefficient of a road surface according to another embodiment of the present disclosure. 
     First, the first current sensor  43  measures the control current value of the left rear wheel steering (RWS) motor in  501 . Here, the left RWS motor means the first RWS motor  52 . 
     Thereafter, the first stroke sensor  44  measures a stroke value representing the movement amount of the left rear wheel steering link in  502 . 
     Then, the controller  47  estimates the friction coefficient of the first road surface based on the control current value measured by the first current sensor  43  and the stroke value measured by the first stroke sensor  44  in  503 . 
     Then, the second current sensor  45  measures the control current value of the right RWS motor in  504 . Here, the right RWS motor means the second RWS motor  62 . 
     Thereafter, the second stroke sensor  46  measures the stroke value representing the movement amount of the right rear wheel steering link in  505 . 
     Then, the controller  47  estimates the friction coefficient of the second road surface based on the control current value measured by the second current sensor  45  and the stroke value measured by the second stroke sensor  46  in  506 . 
     Thereafter, the controller  47  finally estimates the friction coefficient of the road surface using the friction coefficient of the first road surface and the friction coefficient of the second road surface in  507 . 
       FIG. 6  is a block diagram illustrating a computing system for implementing a method of estimating a friction coefficient of a road surface according to an embodiment of the present disclosure. 
     Referring to  FIG. 6 , 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 , which are connected with each other via a 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 ROM (Read Only Memory) and a RAM (Random Access Memory). 
     Thus, the operations of the method or the algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware or a software module executed by the processor  1100 , or in a combination thereof. The software module may reside on a storage medium (that is, 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, a removable disk, or a CD-ROM. The exemplary storage medium may be coupled to the processor  1100 , and the processor  1100  may read information out of the storage medium and may record information in the storage medium. Alternatively, 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 within a user terminal. In another case, the processor and the storage medium may reside in the user terminal as separate components. 
     In the apparatus for estimating the friction coefficient of the road surface according to an embodiment of the present disclosure and the method thereof, in the vehicle equipped with the rear wheel steering (RWS), the friction coefficient of the road surface may be estimated based on the control current value of the RWS motor and the movement amount (stroke value) of the rear wheel steering link responding to the control current value of the RWS motor to estimate the friction coefficient of the road surface in the moderated turning situation as well as the sudden braking or turning situation of the vehicle. 
     Hereinabove, although the present disclosure has been described with reference to exemplary embodiments and the accompanying drawings, the present disclosure is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present disclosure pertains without departing from the spirit and scope of the present disclosure claimed in the following claims. 
     Therefore, the exemplary embodiments of the present disclosure are provided to explain the spirit and scope of the present disclosure, but not to limit them, so that the spirit and scope of the present disclosure is not limited by the embodiments. The scope of the present disclosure should be construed on the basis of the accompanying claims, and all the technical ideas within the scope equivalent to the claims should be included in the scope of the present disclosure.