Patent Publication Number: US-2021182632-A1

Title: Method and apparatus for estimation road surface type using ultrasonic signal

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and an apparatus for estimating a road surface type using an ultrasonic signal, more specifically, to a method for estimating the type of a road surface using a machine-learning artificial neural network model for a reflected ultrasonic signal, and an apparatus for performing the same. 
     2. Description of the Related Art 
     Unlike ordinary ground, it becomes difficult for a person to increase or decrease walking speed when walking on slippery ice. As with ground vehicles, it is very important to know the maximum frictional force of the road surface in motion control. Since the road surface friction coefficient determines how much acceleration/deceleration can be performed in the longitudinal and transverse motion, it becomes a very important factor in both the stability control aspect and the maximum motion performance control aspect of the ground vehicle. 
     In a vehicle, if the maximum coefficient of friction between the tire and the road surface is known, the maximum vertical and lateral force that the vehicle can exert can be determined in advance. Therefore, it is possible to prevent the tire from being locked, to prevent accidents caused by inability to steer, or to bring out the maximum moving performance of the vehicle. Therefore, one can drive the vehicle more safely and sporty. 
     In order to estimate the coefficient of friction, the following method has been conventionally adopted: 
     1) Estimation of maximum friction coefficient between tire and road surface through vehicle kinematic measurement 
       FIG. 1A  is a graph showing the relationship between the tire slip amount according to the road surface and the longitudinal and lateral forces of the tire, and  FIG. 1B  is a graph showing the experimental data of the relationship between the tire slip amount according to the road surface and the normalized tire longitudinal load. 
     The maximum coefficient of friction between tire and road surface can be estimated through vehicle kinematic measurement. In this method, the longitudinal and lateral force applied to the tire and the amount of tire slip are calculated using the wheel angular velocity sensor, the vehicle acceleration sensor, and the vehicle angular velocity change rate sensor. And the tire slip amount, that is, the tire longitudinal and lateral force relationship data, is obtained as shown in  FIGS. 1A and 1B . After that, in the section where the slip amount is small, the slip amount-tire longitudinal and lateral force slope is calculated, and the maximum point of the tire longitudinal and lateral force in the total tire slip amount is predicted. If one can find the maximum point of the tire longitudinal and lateral force and divide it by the tire load, this is the maximum coefficient of friction between the tire and the road surface. However, in this method, the longitudinal and lateral force of the tire are not accurate in the section where the tire slip amount is small, due to noise included in sensors such as wheel speed sensor, vehicle acceleration sensor, vehicle angular velocity change rate sensor, and also due to change in tire vertical load because of vehicle load change and change in center of gravity because of vehicle movement. Accordingly, the slope between the amount of slip and the longitudinal and lateral forces of the tires obtained using inaccurate data becomes also inaccurate, and the maximum friction coefficient between tire and road surfaces using the maximum point of the longitudinal and lateral forces of the tire on the extension line is also inaccurate. That is, it is used only limited case because it requires estimation for estimation. 
     2) Check the road surface condition using camera and estimate the maximum friction coefficient between tire and road surface: 
     This is a method of estimating the road surface friction coefficient by checking road conditions such as dry asphalt, wet asphalt, and snowy roads using image information from the camera. This allows the maximum friction coefficient between tire road surfaces to be estimated by mapping the maximum road surface friction coefficient to the corresponding road surface conditions. However, this method not only requires an expensive camera sensor module, but also requires a high-performance signal processing device. In addition, if there is another object on the road surface, the probability of misrecognition increases. And the probability that the road surface is incorrectly estimated is very high according to the surrounding illumination. 
     SUMMARY OF THE INVENTION 
     The present invention was invented to solve the above-mentioned problem and has a purpose to provide a method and apparatus for providing highly accurate road surface information at low cost by establishing a model between the characteristics of the ultrasonic signal reflected from the road surface and the road surface condition through machine learning and estimating the type of the road surface using the same. 
     According to an aspect of the present invention, a method for estimation road surface type using an ultrasonic signal, comprising: (a) extracting a 1D feature vector through one or more 1D convolutional layers using an input signal derived from a reflection signal in the time domain of the ultrasonic signal reflected from the road surface after being emitted to the road surface; (b) receiving the 1D feature vector as an input and estimating a probability value for each road surface type in an artificial neural network having one or more layers; and, (c) determining the road surface type from the estimated probability value for each road surface type is provided. 
     According to other aspect of the present invention, an apparatus for estimation road surface type using an ultrasonic signal, comprising: at least one processor; and, at least one memory storing computer-executable instructions, wherein the computer-executable instructions stored in said at least one memory, when executed by the at least one processor, causes the at least one processor to perform operations comprising: (a) extracting a 1D feature vector through one or more 1D convolutional layers using an input signal derived from a reflection signal in the time domain of the ultrasonic signal reflected from the road surface after being emitted to the road surface; (b) receiving the 1D feature vector as an input and estimating a probability value for each road surface type in an artificial neural network having one or more layers; and, (c) determining the road surface type from the estimated probability value for each road surface type is provided. 
     According to another aspect of the present invention, an apparatus for estimation road surface type using an ultrasonic signal, comprising: at least one processor; a memory for storing a convolution filter for convolution operation; a feature extractor for extracting a 1D feature vector through one or more 1D convolutional layers using an input signal derived from a reflection signal in the time domain of the ultrasonic signal reflected from the road surface after being emitted to the road surface; and a classifier that receives the 1D feature vector as an input and estimates a probability value for each road surface type in an artificial neural network having one or more layers is provided. 
     According to the present invention, a method and apparatus for providing highly accurate road surface information at low cost by establishing a model between the characteristics of the ultrasonic signal reflected from the road surface and the road surface condition through machine learning and estimating the type of the road surface using the same are provided. In particular, the present invention contributes more safe driving by accurately detecting even road surface with thin ice (i.e., black ice), which was not possible in the conventional road surface friction coefficient estimation method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A and 1B  are graphs showing the relationship between the amount of tire slip and the longitudinal and lateral tire force according to the road surface. 
         FIG. 2  shows the configuration of a computer device equipped with an application for estimating a road surface using an ultrasonic signal according to the present invention. 
         FIG. 3  shows a flow chart of a first embodiment of the present invention for estimating a road surface type using an ultrasonic signal. 
         FIG. 4  is a schematic diagram showing each process of the first embodiment of  FIG. 3 . 
         FIG. 5A  shows an embodiment of the present invention in which the convolutional layer includes five layers (Conv1 to Conv5), and the classifier includes three layers (FC6 to FC8). 
         FIG. 5B  shows an exemplary convolution operation performed on 6×6 data. 
         FIG. 5C  is an exemplary convolution filter for being used in the convolution operation in  FIG. 5B . 
         FIG. 6  shows a test result of the first embodiment of  FIG. 3 . 
         FIG. 7  is a flow chart a second embodiment of the present invention for estimating a road surface type using an ultrasonic signal. 
         FIG. 8  is a schematic diagram showing each process of the second embodiment of  FIG. 7 . 
         FIG. 9  shows a test result of the second embodiment of  FIG. 7 . 
         FIG. 10  shows another configuration of a road surface type estimation apparatus for estimating a road surface using an ultrasonic signal according to the present invention. 
         FIG. 11  shows a road surface type estimation test result at each different sampling frequencies of an ultrasonic sensor system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings and, based on the principle that the inventor can appropriately define the concept of terms in order to describe his own invention in the best way, it should be interpreted as a meaning and concept consistent with the technical idea of the present invention. Accordingly, the embodiments described in the present specification and the configurations shown in the drawings are only the most preferred embodiment of the present invention, and do not represent all the technical spirit of the present invention. It should be understood that there may be various equivalents and modifications that may be substituted for them at the time of application. 
       FIG. 2  shows the configuration of a computer device in which an application for estimating a road surface type using an ultrasonic signal according to the present invention is installed. 
     The road surface type estimation apparatus  100  includes a processor  110 , a nonvolatile storage unit  120  storing programs and data, a volatile memory  130  storing running programs, a communication unit  140  for performing communication with other devices, and a bus that is an internal communication path between these units. As a running program, there may be a device driver, an operating system, and various applications. Although not shown, a power supply unit such as a battery may also be included. 
     The road surface type estimation system using ultrasonic signals further includes the ultrasonic transmitter  10  and the ultrasonic receiver  20  together with the road surface type estimation apparatus  100  using ultrasonic signals. 
     In a road surface type estimation system using an ultrasonic signal mounted on a mobile object moving on the ground, for example, a vehicle, the ultrasonic transmitter  10  emits ultrasonic waves toward the road surface, and the ultrasonic waves reflected from the road surface is received by the ultrasonic receiver  20 . The ultrasonic receiver  20  transmits the reflection signal to the road surface type estimation application  220 . In this way, the road surface type estimation application  220  receives the reflection signal regarding the ultrasonic wave reflected from the road surface from the ultrasonic receiver  20 . When the ultrasonic receiver  20  continuously transmits the reflection signal, the road surface type estimation device  100  properly transmits the reflection signal to the road surface type estimation application  220  via a data interface device (not shown). The data interface device may serve as a kind of buffer in the process of transmitting the reflection signal to the road surface type estimation application  220 . In addition, an ultrasonic transmission/reception control driver  210  may be installed in the road surface type estimation apparatus  100 , and may serve to control the operation of the ultrasonic transmitter  10  and the ultrasonic receiver  20  as described above. Furthermore, the ultrasonic transmission/reception control driver  210  is connected to the vehicle electronic control device  300  through the communication unit  140 . The vehicle electronic control device  300  may control the operation of the ultrasonic transmitter  10  and the ultrasonic receiver  20 . 
     When the ultrasonic sensor system including the ultrasonic transmitter  10  and the ultrasonic receiver  20  is mounted on a vehicle and used for detecting the condition of the road surface, the ultrasonic sensor system has a wide field view in order to detect the condition of the entire road surface. 
     However, if necessary, an ultrasonic sensor system having a small field view may be also used. In the vehicle, as an embodiment, each ultrasonic sensor system having a small field view may be separately mounted on the left and right sides, and each of the ultrasonic sensor systems may be used to detect the conditions of the road surfaces on the left side and the right side based on the center line of the vehicle. 
     In addition, when such ultrasonic sensor system having a small field view is installed on the left and right sides of the vehicle, it can be used especially for detecting lanes on the road. For example, when a driving vehicle happens to cross a lane to the left or right due to the driver&#39;s drowsy driving etc., a warning may be given to the driver by detecting that the left or right lane has entered in a certain range. 
     The road surface type estimation application  220  performs a role of estimating the road surface type by using the ultrasonic signal as described above. Such a road surface type estimation application  220  is a program installed and operated in the road surface type estimation apparatus  100 . It will be described hereinafter in detail with reference to  FIGS. 3 to 9 . 
       FIG. 3  is a flow chart illustrating the first embodiment of the present invention for estimating a road surface type using an ultrasonic signal and  FIG. 4  is a schematic diagram showing each process of the first embodiment. 
       FIG. 3  illustrates a process performed by the operation of the road surface type estimation application  220  of  FIG. 2 . First, a reflection signal is received from the ultrasonic receiver  20  (S 310 ). The received reflection signal  410  (refer to  FIG. 4 ) is a signal representing the intensity of the reflected ultrasonic wave over time. 
     From the signal extracted from the reflection signal in the time domain, a 1D feature vector per time is extracted through an 1D convolution layer, and an input to be entered into a classifier is determined (S 340 ). By moving a window having a predetermined time interval on the received (S 310 ) reflection signal  410  and performing a Fourier Transform (Short-Time Fourier Transform), a signal in the time-frequency domain, i.e., 2D spectrogram  420  (refer to  FIG. 4 ) is calculated (S 320 ). The calculated 2D spectrogram  420  is converted into a 1D signal  430  for each time (refer to  FIG. 4 ) (S 330 ), and then convolution is performed (S 340 ). If necessary, before converting the calculated 2D spectrogram  420  into a time-based 1D signal  430 , convolution may be first performed by the convolution filter  51  (S 321 , see  FIG. 4 ). 
     Convolution may be performed through one or more convolutional layers (S 340 ). As described above, the 1D feature vector  440  (refer to  FIG. 4 ) calculated through the one or more convolutional layers is determined as an input of the classifier (S 340 ). 
     The classifier  450  (refer to  FIG. 4 ) is an artificial neural network having multi-layers. The classifier  450  using such an artificial neural network is determined as a final classifier model by learning a weight value using training data in an artificial neural network having the same layer configuration. Here, the weight means the weight of the connection value of one or more layers of the artificial neural network. 
     The one or more convolutional layers (S 340 ) for calculating the 1D feature vector for the classifier  450  and the classifier  450  having a multi-layer artificial neural network forms together a convolutional neural network (CNN). The aforementioned weight may mean a weight of a connection value of one or more layers of the artificial neural network  450  or values of a convolution filter  52  in the one or more convolutional layers. The weight of the connection value of one or more layers of the artificial neural network can be determined through machine learning and also the value of the convolution filter  52  in the one or more convolutional layers can be determined through machine learning. The training data is input to the artificial neural network, and the gradient descent method is used for optimization in the direction of minimizing the loss between the output value and the actual correct answer. 
     The convolutional artificial neural network (CNN) model is determined by the weight or convolutional layer filter value which is determined by training and estimates a probability value  460  (refer to  FIG. 4 ) for each type of road surface from which the ultrasonic wave is reflected (S 350 ). The type of the road surface can be determined as the type having the largest probability value (S 360 ). Furthermore, the friction coefficient of the corresponding road surface may be estimated from the determined road surface type (S 370 ). Such a convolutional artificial neural network (CNN) model is included in the road surface type estimation application  220 . 
     As described above with reference to  FIG. 2 , when the ultrasonic sensor system including the ultrasonic transmitter  10  and the ultrasonic receiver  20  is mounted on a vehicle and used for detecting the condition of the road surface, such an ultrasonic sensor system may be used to detect the state of the entire road surface by having a wide field view. An ultrasonic sensor system having a small field view that detects the condition of a part of the road surface may also be used. 
     When an ultrasonic sensor system having a wide field view is mounted on a vehicle, the type of the road surface currently on which the vehicle is moving may be determined by using the state of the road surface detected by the ultrasonic sensor system (S 360 ). 
     When an ultrasonic sensor system having a small field view is mounted, each ultrasonic sensor system may be separately mounted on the left and right sides of the vehicle. Using the road surface type detected by each ultrasonic sensor system, the road surface type of the left side and the right side may be classified and determined (S 360 ). 
     In addition, when the ultrasonic sensor system having such a small field view is installed on the left and right sides of the vehicle, it can be particularly used for detecting a lane applied on a road. That is, the type of the road surface detected by the ultrasonic sensor system may be determined as the lane (S 360 ). As shown in  FIG. 2 , for example, when a driving vehicle is likely to cross a lane to the left or right due to a driver&#39;s drowsy driving etc., it may be used to give a warning to the driver by detecting that the left or right lane has entered in a certain range. 
       FIG. 5A  shows an embodiment in which a convolutional layer includes five layers (Conv1 to Conv5), and a classifier includes three layers (FC6 to FC8). For example, in Conv1, ‘201’ means the size of the convolutional layer, and the stride value means the size at which the convolutional filter jumps. 
       FIGS. 5B and 5C  are examples for explaining the execution of convolution. In  FIG. 5B , convolution is performed on 6×6 data  5 B using the convolution filter of  FIG. 5C . First, the first data b-1 of  FIG. 5B  is dot-producted by the convolution filter ( FIG. 5C ) to obtain a value of three (3). The window moves 2 spaces (stride=2) to obtain data b-2. Data b-2 is dot-producted with the convolution filter ( FIG. 5C ) and a value of −3 is obtained. When the convolution operation is performed for all data, the number of data is significantly reduced and a new data representing the characteristics of the original data is calculated. 
     Although not shown, among the above-described convolutional layers, a max pooling layer may be included. Max pooling does not reduce data in the way shown in  FIG. 5B , but reduces data by selecting the largest value of 3×3 data b-1. One or more convolutional layers and one or more max pooling layers may be included. In the specification of the present application, the “convolutional layer” includes not only one or more convolutional layers but also one or more max pooling layers. 
       FIG. 6  is a test result of the first embodiment of the present invention shown in  FIG. 3 . 
     In this test, training and test were performed with a 4624 DB set for 20 minutes, with the convolutional layer and the classifier layer configured as shown in  FIG. 5A . Of the 4624 DB sets, training was performed for 3233 DB sets to determine weights and filter values to form a convolutional neural network model, and test were performed with the remaining 1391 DB sets. 
     The number on the diagonal indicates cases where the actual road surface type (Ground Truth) and the road surface type estimated through the convolutional neural network (CNN) of the present invention (Estimation) coincide, that is, the number of times accurate estimation has been made. According to the results shown in  FIG. 6 , the number of incorrect estimates is only two times when the painted road surface is incorrectly estimated as asphalt. 
     As described above, when the ultrasonic sensor system having a small field view is installed on the left and right sides of the vehicle, in particular, it can be used for detecting the lane applied on the road. In this case, although not shown in  FIG. 6 , ‘Lane’ may be further included among the road surface type. 
       FIG. 7  is a flowchart illustrating a second embodiment of the present invention, and  FIG. 8  is a schematic diagram illustrating each process of the second embodiment shown in  FIG. 7 . 
       FIG. 7  is a diagram showing a process performed by the operation of the road surface type estimation application  220  of  FIG. 2  as the second embodiment. First, a reflection signal is transmitted from the ultrasonic receiver  20  (S 710 ). The received ultrasonic reflection signal  810  (refer to  FIG. 8 ) is a signal representing the intensity of the reflected ultrasonic wave with respect to time. 
     Unlike the first embodiment of  FIG. 3 , Short-Time Fourier Transform (STFT) is not performed on the reflection signal in the time domain. The road surface type is estimated by using the reflection signal in the time domain as an input to the convolutional neural network. 
     As in the case of  FIG. 3 , convolution may be performed through one or more convolutional layers (S 720 ). The 1D feature vector  820  (refer to  FIG. 8 ), which is calculated through the one or more convolutional layers, is determined as an input of the classifier (S 721 , see  FIG. 8 ). 
     The classifier  830  (refer to  FIG. 8 ) is also formed as an artificial neural network includes multi-layers, as described above with reference to  FIGS. 3 and 4 . The classifier  830  using such an artificial neural network is determined as a final classifier model by learning a weight value using training data in an artificial neural network having the same model architecture. 
     As described above with reference to  FIGS. 3 and 4 , the convolutional layer (S 720 ) that calculates the 1D feature vector and the classifier  830  are combined to be a convolutional neural network (CNN). The above-described weight may mean a connection value of one or more layers of the artificial neural network  830  or a value of a convolution filter in a convolutional layer. This means that not only the connection value of one or more layers of the artificial neural network can be determined through machine learning, but also the convolutional filter value in the convolutional layer can be determined through machine learning. The training data is input to the artificial neural network, and optimization is performed in a gradient descent method in the direction of minimizing the loss between the output value and the actual correct answer. 
     A convolutional artificial neural network (CNN) model is determined based on the weight of the artificial neural network and the convolutional layer filter value which were determined by learning. The convolutional artificial neural network (CNN) model estimates a probability value  840  (refer to  FIG. 8 ) for each road surface type (S 730 ). From the road surface type having the largest probability value, the type of the corresponding road surface can be determined (S 740 ). The friction coefficient of the corresponding road surface may also be estimated from the determined road surface type (S 750 ). Such a convolutional neural network (CNN) model is included in the road surface type estimation application  220 . 
     As described above with reference to  FIG. 2 , when an ultrasonic sensor system including an ultrasonic transmitter  10  and an ultrasonic receiver  20  is mounted on a vehicle and used for detecting the condition of the road surface, an ultrasonic sensor system having a wide field view can be used to detect the condition of the entire surface. If necessary, an ultrasonic sensor system with a small field view may be used to detect the condition of a part of the road surface. In each cases, the embodiment of estimating the road surface type (S 740 ) is the same as the embodiment in  FIG. 3  described with respect to the road surface type estimation (S 360 ). 
       FIG. 9  is a test result of the second embodiment shown in  FIG. 7 . 
     Training and test were performed using reflection signal in the time domain as an input to the convolutional neural network without performing a Short-Time Fourier Transform (STFT). The convolutional layer and the classifier layer are configured as shown in  FIG. 5A . Training and test were performed on the 4977 DB set. Among 4977 DB sets, 3484 DB sets were used to determine weights, filter values etc., by performing machine learning to form a convolutional neural network (CNN) model. Test was performed with the remaining 1493 DB sets. 
     The number on the diagonal indicates cases where the actual road surface type (Ground Truth) and the road surface type estimated through the convolutional neural network (CNN) of the present invention (Estimation) coincide, that is, the number of times accurate estimation has been made. According to the results of  FIG. 9 , the painted road surface was incorrectly estimated as asphalt or cement road surface, and the cement road surface was incorrectly estimated as a dirt road surface. From these results, it can be seen that there is no significant differences in road surface estimation accuracy from the case of applying CNN after performing Short-Time Fourier Transform (STFT) on the reflection signal in the time domain. 
     Meanwhile, when an ultrasonic sensor system having a small field view is installed on the left and right sides of the vehicle, respectively, it can be particularly used for detecting lanes applied on the road. In this case, although not shown in  FIG. 9 , the detection ‘Lane’ may be further included in the road surface type to be detected. 
       FIG. 10  shows the configuration of a road surface type estimation apparatus  500  using an ultrasonic signal according to the present invention. 
     In the road surface type estimation apparatus  100  according to the present invention of  FIG. 2 , the estimation of the road surface type is performed by the operation of the road surface type estimation application  220 . In  FIG. 10 , each step is operated in hardware. 
     In  FIG. 2 , operations of the STFT converter  530 , feature extractor  540 , and classifier  550  of  FIG. 10  are performed by the operation of the road surface type estimation application  220 . In the case of  FIG. 10 , each of these modules is configured in hardware by using an electronic circuit on a chip. In such hardware configuration, a road surface estimation process by a convolutional neural network (CNN) is performed faster. The feature extractor  540  and the classifier  550  may be configured as separate chips. Or both the feature extractor  540  and the classifier  550  may be included in one chip. In addition, the reflection signal in the time domain may be directly used as an input of the feature extractor  540  without performing Short-Time Fourier Transform (STFT). In this case, the STFT converter  530  is not required. 
     The road surface type estimation apparatus  500  of  FIG. 10  includes at least one processor  130 , a memory  520  for storing weights of an artificial neural network and of a convolution filter, an STFT converter  530 , a feature extractor  540  and a classifier  550 . 
     The STFT converter  530 , the feature extractor  540 , and the classifier  550  may operate under the control of the processor  510 . In addition, although not shown in  FIG. 10 , when the ultrasonic receiver  20  continuously transmits a reflection signal, a data interface device (not shown) may be provided as described in  FIG. 2 . By doing so, the reflection signal may be received and properly transmitted to the STFT converter  530  or the feature extractor  540 . That is, the data interface device may serve as a buffer in the process of transmitting the reflection signal to the STFT converter  530  or the feature extractor  540 . The data interface device may also be composed of a chip or the like. 
     The feature extractor  540  extracts a 1D feature vector through a 1D convolution layer by inputting a signal derived from the reflection signal in the time domain. In this case, as an input signal for the convolutional layer of the feature extractor  540 , a reflection signal in the time domain is used. 
     Alternatively, by further providing an STFT converter  530 , the STFT converter  530  may divide the reflection signal in the time domain at predetermined time intervals, and perform Short-Time Fourier Transform on each the divided reflection signal. The time-frequency domain signal generated from such operation may be provided as an input signal to the convolutional layer of the feature extractor. When performing such convolution, a convolution filter is stored in the memory  520 , and the feature extractor  540  reads the convolution filter of the memory  520  to perform convolution in the convolution layer. The memory  520  may be a volatile memory such as DRAM or SRAM. It may be stored in a nonvolatile memory (not shown) and loaded into the memory  520  when convolution is performed. 
     The classifier  550  receives such 1D feature vector as an input and estimates a probability value for each road surface type in a neural network having one or more layers. 
     The process of determining the classifier input signal through convolution of the feature extractor  540  and the process of estimating the road surface through the artificial neural network in the classifier  550  go through the same process as the flowchart of  FIG. 3 or 7 . 
     The classifier  550  configured as an electronic circuit chip may also determine a CNN model through machine learning for a neural network of one or more layers. The final classifier model is determined by learning the weight value using the training data in the artificial neural network having the same architecture. 
     As described above with reference to  FIGS. 3 to 5 , the feature extractor  540  and the classifier  550  are combined to form a convolutional neural network (CNN). The above-described weight may mean a weight of a connection value of one or more layers of the artificial neural network of the classifier  550  or a convolution filter value in a convolutional layer. As already described, the weight of the connection value of one or more layers of the artificial neural network of the classifier  550  can be determined through machine learning, and also the convolutional filter value in the convolutional layer of the feature extractor  540  can be determined through machine learning. In addition to the convolutional filter, the weight of the connection value of one or more layers of the artificial neural network may be uploaded from the non-volatile memory (not shown) to the volatile memory  520 . The classifier  550  reads the weight from the memory  520  and uses them for estimation of road surface type. 
     In this way, the model determined through machine learning is configured as hardware, that is, as an electronic circuit chip to form a feature extractor  540  and a classifier  550 . The feature extractor  540  is formed of a chip including an electronic circuit implementing the function of the feature extractor  540  as described above, and the classifier  550  is formed of a chip including an electronic circuit implementing the function of the classifier  550  as described above. The feature extractor  540  and the classifier  550  may be configured as separate chips, respectively, or may be formed as a single chip including the feature extractor  540  and the classifier  550 . 
       FIG. 11  shows a road surface type estimation test result at different sampling frequencies that can be adopted in the ultrasonic sensor system used to estimate the road surface type using an ultrasonic signal according to the present invention. 
     As described above, the ultrasonic sensor system includes an ultrasonic transmitter  10  and an ultrasonic receiver  20 . Since the cost of the ultrasonic sensor system with a lower sampling frequency is cheaper, it is preferable to lower the sampling frequency within the limit of maintaining the performance required. 
     As shown in  FIG. 11 , at the sampling frequency of 1M Samples/sec, the accuracy of the road surface type estimation was 99.7% on average, at the sampling frequency of 40K Samples/sec, 99.6% on average, and at the sampling frequency of 20K Samples/sec, 99.4% on average. From these results, it can be determined that the sampling frequency that can be adopted in the ultrasonic sensor system used for estimating the road surface type using the ultrasonic signal according to the present invention is in a range of 20K Samples/sec-1M Samples/sec. 
     The road surface type estimation system based on ultrasonic signals employing the road surface type estimation apparatus  100  and  500  including an ultrasonic transmitter  10  and an ultrasonic receiver  20 , described with reference to  FIGS. 2 to 11 , may be mounted on the a moving object such as a vehicle and road surface type information on which the moving object is running can be provided in real time, so that it can be used for operation control of the moving object. This can be of great help in smooth driving, especially when it is installed and used in an autonomous vehicle in the future.