Vehicle control device

Abstract

A vehicle control device (110) comprises: a lateral acceleration calculation unit (113) that calculates, on the basis of the curvature of a curve and the vehicle speed of a vehicle (10), a future lateral acceleration to be generated in the vehicle; a notification unit (114) that makes notification of, through a notification device (150), a hands-on request for requesting switching from a hands-off state to a hands-on state; and a control determination unit (115) that determines the details of control of the vehicle in accordance with a calculation result of the lateral acceleration calculation unit. When the vehicle travels on a curve in the hands-off state, in a case where the lateral acceleration is greater than a preset threshold value, the control determination unit determines control for making notification of the hands-on request through the notification unit, and in a case where the lateral acceleration is not greater than the threshold value, the control determination unit determines control for continuing the hands-off state.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Application No. 2021-193738 filed on November 30, 2021, the contents of which are incorporated herein by reference. [Technical Field]

[0002] The present disclosure relates to a vehicle control device. [Background technology]

[0003] For example, as described in Patent Document 1, an information processing device for a vehicle capable of both automatic and manual driving is known. In this information processing device, a point where automatic driving has been switched to manual driving is pre-stored in map information as a malfunction point where automatic driving malfunctions. The device then notifies the driver that the malfunction point is approaching. When such a notification is received, a hands-on request is issued to request a switch from a hands-off state in which the driver is not holding the steering wheel to a hands-on state in which the driver is holding the steering wheel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-32333 Summary of the Invention

[0005] However, in the above configuration, since a hands-on request is issued uniformly based on pre-stored map information, unnecessary hands-on requests may be issued depending on the actual driving conditions of the vehicle. For example, when traveling around a curve that has been registered as a problem area, a hands-on request may be issued even when the lateral acceleration of the vehicle is small due to low-speed driving in a traffic jam, and the driver's safety and comfort would not be compromised if the hands were off. In this case, the driver is forced to perform unnecessary operations. Therefore, a vehicle control device is desired that can issue a hands-on request when necessary while not issuing a hands-on request when traveling around a curve.

[0006] The present disclosure can be realized in the following forms.

[0007] According to one aspect of the present disclosure, there is provided a vehicle control device for a vehicle capable of both automatic and manual driving, the vehicle control device including: a driving path setting unit that sets a target driving path for the vehicle; a lateral acceleration calculation unit that calculates a future lateral acceleration of the vehicle based on the curvature of a curve in the direction of travel of the vehicle and the vehicle speed; a notification unit that issues a hands-on request by an alarm device during execution of the automatic driving, requesting a switch from a hands-off state in which a driver of the vehicle is not holding the steering wheel to a hands-on state in which the driver holds the steering wheel; and a control decision unit that determines a control content for the vehicle based on a calculation result of the lateral acceleration calculation unit. When the vehicle is traveling around the curve in the hands-off state, if the lateral acceleration calculated by the lateral acceleration calculation unit is greater than a predetermined threshold, the control decision unit determines, as the control content, to control the notification unit to notify the hands-on request, and if the lateral acceleration is equal to or less than the threshold, to control the vehicle to continue the hands-off state.

[0008] According to this configuration, when the vehicle is traveling around a curve in a hands-off state, the control decision unit determines control to issue a hands-on request if the lateral acceleration calculated by the lateral acceleration calculation unit is greater than a predetermined threshold, and determines control to maintain the hands-off state if the lateral acceleration is equal to or less than the threshold. For example, when the lateral acceleration of the vehicle is small, such as when traveling at low speed in a traffic jam, maintaining the hands-off state does not impair the driver's safety or comfort, and there is no problem with continuing the hands-off state. In such cases, the above configuration allows the hands-off state to be maintained without issuing a hands-on request. In other words, when traveling around a curve, it is possible to not issue a hands-on request when it is unnecessary, but to issue a hands-on request when it is necessary. [Brief explanation of the drawings]

[0009] The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: [Figure 1] FIG. 1 is a block diagram showing a schematic configuration of a vehicle control device according to a first embodiment of the present disclosure; [Figure 2] FIG. 2 is a flowchart showing a process of notifying a hands-on request executed by a vehicle control device; [Figure 3] FIG. 3 is a schematic diagram for explaining the calculation of the lateral acceleration. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, several embodiments of the present disclosure will be described with reference to FIGS.

[0011] A. First embodiment: A1. Configuration of vehicle control device 110: As shown in FIG. 1, a vehicle 10 is equipped with an automatic driving control system 100. The vehicle 10 is capable of automatic driving and manual driving. In automatic driving, the vehicle 10 is steered and driven automatically even if the driver does not operate a steering wheel for operating the vehicle 10. In some automatic driving modes, the driver performs steering and the acceleration and deceleration are automatically controlled. In manual driving, the driver operates the steering wheel to steer and drive the vehicle 10.

[0012] In this embodiment, the autonomous driving control system 100 includes a vehicle control device 110, a surrounding sensor 120, an internal sensor 130, a road information storage unit 140, an autonomous driving control unit 210, a driving force control ECU (Electronic Control Unit) 220, a braking force control ECU 230, and a steering control ECU 240. The vehicle control device 110, the autonomous driving control unit 210, the driving force control ECU 220, the braking force control ECU 230, and the steering control ECU 240 are connected via an in-vehicle network 250.

[0013] The surrounding sensor 120 acquires surrounding information outside the vehicle that is necessary for autonomous driving. The surrounding sensor 120 includes a camera 121 and an object sensor 122. The camera 121 captures images of the surroundings of the vehicle 10. The object sensor 122 detects the situation around the vehicle 10. Examples of the object sensor 122 include object sensors that use reflected waves such as laser radar, millimeter wave radar, and ultrasonic sensors.

[0014] The internal sensor 130 includes a vehicle position sensor 131, an acceleration sensor 132, a vehicle speed sensor 133, and a yaw rate sensor 134. The vehicle position sensor 131 detects the current position of the vehicle 10. Examples of the vehicle position sensor 131 include a Global Navigation Satellite System(s) (GNSS) and a gyro sensor.

[0015] The acceleration sensor 132 is a detector that detects the acceleration of the vehicle 10. The acceleration sensor 132 includes, for example, a longitudinal acceleration sensor that detects longitudinal acceleration in the longitudinal direction of the vehicle 10, and a lateral acceleration sensor that detects lateral acceleration of the vehicle 10. The vehicle speed sensor 133 measures the current traveling speed of the vehicle 10. The yaw rate sensor 134 is a detector that detects the yaw rate (rotational angular velocity) around the vertical axis of the center of gravity of the vehicle 10. A gyro sensor, for example, can be used as the yaw rate sensor 134. The peripheral sensor 120 and the internal sensor 130 transmit the various acquired data to the vehicle control device 110.

[0016] The road information storage unit 140 stores detailed road information and the like relating to the roads on which the vehicle 10 is scheduled to travel. The road information includes, for example, the number of lanes, lane width, center coordinates of each lane, positions of stop lines, positions of traffic lights, positions of guardrails, road gradient, road types of curves and straight sections, radius of curvature of curves, length of curve sections, and the like.

[0017] The notification device 150 is a device that notifies various pieces of information to the occupants (mainly the driver) of the vehicle 10 using images and sounds. The notification device 150 includes a display device and a speaker. The display device may be, for example, a HUD (Head-Up Display) or a display device provided on an instrument panel. Note that the term "image" also includes moving images and character strings.

[0018] The vehicle control device 110 includes a driving route setting unit 111, a surrounding information recognition unit 112, a lateral acceleration calculation unit 113, a notification unit 114, a control decision unit 115, and a communication unit 116. The vehicle control device 110 is composed of a microcomputer configured with a central processing unit (CPU), RAM, and ROM, and realizes the functions of each of these units by the microcomputer executing a pre-installed program. However, some or all of the functions of each of these units may be realized by hardware circuits.

[0019] The driving route setting unit 111 sets a route along which the vehicle 10 will travel. More specifically, the driving route setting unit 111 sets a target driving route to a predetermined destination using road information stored in the road information storage unit 140. In this embodiment, the "target driving route" does not simply refer to a route to the destination, but refers to a detailed route including the driving lanes, driving positions on the road, etc.

[0020] The surrounding information recognition unit 112 recognizes surrounding information of the vehicle 10 using a detection signal from the surrounding sensor 120. More specifically, based on an image captured by the camera 121 and an output signal from the object sensor 122, the surrounding information recognition unit 112 recognizes, as surrounding information, the presence and positions of lane markings on the left and right sides of the road on which the vehicle is traveling (hereinafter referred to as "lane markers"), the presence and positions and instructions of traffic lights, the presence, positions, sizes, distances, and traveling directions of other vehicles, the presence and actions of drivers of other vehicles, the presence and positions of people around other vehicles, etc. Note that the surrounding information recognition unit 112 may acquire and recognize some or all of this information via wireless communication with traffic lights, an external server, etc.

[0021] The lateral acceleration calculation unit 113 calculates the future lateral acceleration that will occur in the vehicle 10 when the vehicle 10 travels around a curve in the traveling direction of the vehicle 10. Specifically, the lateral acceleration calculation unit 113 calculates the future lateral acceleration when traveling around a curve based on the curvature of the curve and the vehicle speed of the vehicle 10. Details of the calculation of the lateral acceleration will be described later.

[0022] The notification unit 114 notifies the occupants of various information such as the driving route and vehicle position information using the notification device 150, which is capable of image display and audio output. The notification unit 114 notifies the occupants of hands-on request information in accordance with the driving conditions of the vehicle 10, in accordance with the processing of the control decision unit 115. A hands-on request is a request to switch from a hands-off state, in which the driver is not holding the steering wheel during autonomous driving, to a hands-on state, in which the driver is holding the steering wheel.

[0023] The control decision unit 115 decides the control content of the vehicle 10 according to the calculation result of the lateral acceleration calculation unit 113, and outputs the control content to the autonomous driving control unit 210 via the in-vehicle network 250 to control the vehicle 10. The communication unit 116 acquires, for example, traffic information, weather information, accident information, obstacle information, traffic regulation information, and the like from an information center (not shown) via an antenna (not shown). The communication unit 116 may acquire various information from other vehicles through vehicle-to-vehicle communication. The communication unit 116 may also acquire various information from roadside units installed at various points on the road through road-to-vehicle communication.

[0024] The autonomous driving control unit 210 includes a microcomputer configured with a central processing unit (CPU), RAM, and ROM, and realizes an autonomous driving function by having the microcomputer execute a pre-installed program. The autonomous driving control unit 210 controls, for example, the driving force control ECU 220, the braking force control ECU 230, and the steering control ECU 240 so that the vehicle 10 travels along a route determined by the travel route setting unit 111. For example, when the vehicle 10 changes lanes to an adjacent lane, the autonomous driving control unit 210 may perform merging assistance so that the vehicle 10 travels from the reference line of the lane in which the vehicle 10 is traveling to the reference line of the adjacent lane.

[0025] The driving force control ECU 220 is an electronic control device that controls an actuator, such as an engine, that generates driving force for the vehicle 10. When the driver drives manually, the driving force control ECU 220 controls the power source, such as the engine or electric motor, according to the amount of accelerator pedal operation. On the other hand, when autonomous driving is performed, the driving force control ECU 220 controls the power source according to the required driving force calculated by the autonomous driving control unit 210.

[0026] The braking force control ECU 230 is an electronic control device that controls a brake actuator that generates a braking force for the vehicle 10. When the driver drives manually, the braking force control ECU 230 controls the brake actuator in accordance with the amount of operation of the brake pedal. On the other hand, when autonomous driving is performed, the braking force control ECU 230 controls the brake actuator in accordance with the required braking force calculated by the autonomous driving control unit 210.

[0027] The steering control ECU 240 is an electronic control device that controls a motor that generates steering torque for the vehicle 10. When the driver drives manually, the steering control ECU 240 controls the motor in accordance with the operation of the steering wheel to generate an assist torque for the steering operation. This allows the driver to operate the steering wheel with a small amount of force, thereby realizing steering of the vehicle 10. On the other hand, when autonomous driving is performed, the steering control ECU 240 performs steering by controlling the motor in accordance with the required steering angle calculated by the autonomous driving control unit 210.

[0028] A2. Processing by vehicle control device 110: In autonomous driving, the driving route setting unit 111 creates a driving plan for the vehicle 10 for the next few seconds based on the current position detected by the vehicle position sensor 131 and the positions, speeds, etc. of other vehicles around the vehicle 10. This driving plan includes a steering plan and an acceleration / deceleration plan for the vehicle 10 for the next few seconds.

[0029] The process shown in Fig. 2 is repeatedly executed at predetermined time intervals while the vehicle 10 is traveling. As shown in Fig. 2, in step 11 (hereinafter, step will be abbreviated as "S"), it is determined whether the current driving state of the vehicle 10 is a hands-off state. For example, based on a control signal from the automatic driving control unit 210, it is determined whether the vehicle is in a hands-off state during automatic driving.

[0030] If it is determined that the hands are off (S11: YES), the process proceeds to S12, where it is determined whether the vehicle 10 will be turning a curve in the future. "Turning a curve in the future" includes the vehicle 10 just before the curve and while the vehicle 10 is turning a curve. Similarly, "when turning a curve" hereinafter also includes the vehicle 10 just before the curve and while the vehicle 10 is turning a curve, and hereinafter will also be referred to simply as "when turning a curve."

[0031] If it is determined that the vehicle 10 is traveling around a curve (S12: YES), the process proceeds to S13, where the lateral acceleration calculation unit 113 calculates the lateral acceleration An. Then, in S14, it is determined whether the lateral acceleration An is greater than a predetermined threshold value A0. The threshold value A0 is set in advance through experiments or the like as an upper limit value of the lateral acceleration that is assumed to be able to maintain a level of safety and comfort that gives the driver a sense of security when traveling around a curve.

[0032] If it is determined that the lateral acceleration An is greater than the predetermined threshold A0 (S14: YES), the process proceeds to S15, where a hands-on request is issued by the notification device 150. Specifically, an image representing the hands-on request may be displayed on the display, or a sound informing the driver of the hands-on request may be emitted from the speaker.

[0033] On the other hand, if the lateral acceleration An is equal to or less than the predetermined threshold Ao (S14: NO), the process proceeds to S16, the hands-off state continues, and no hands-on request is made. After the processes of S15 and S16, this processing routine ends. Note that if it is determined in S11 that the hands-off state is not in effect (S11: NO), or if it is determined in S12 that the vehicle is not traveling around a curve (S12: NO), the subsequent control is not performed, and the process of FIG. 2 ends.

[0034] A3. Details of lateral acceleration calculation (S13) and hands-on requirement determination (S14): Next, the processing of S13 and S14 will be described in detail. As shown in Fig. 3, the lateral acceleration calculation unit 113 calculates the lateral acceleration An (n is a positive integer) for each section obtained by dividing the curve recognition distance D into a plurality of sections. The curve recognition distance D is the total length of the portion ahead of the vehicle 10 that is recognized as a curve. The sections are obtained by dividing the curve recognition distance D at equal intervals of several meters (for example, 5 meters), and are referred to as section 1, section 2, ..., section n in order from closest to the vehicle 10. The future lateral acceleration An when traveling around a curve is calculated based on the curvature Rn of the curve and the current vehicle speed V of the vehicle 10 using the following equation (1): Lateral acceleration An=(vehicle speed V) 2×curvature Rn (1)

[0035] For example, when the vehicle 10 is at a first point P1 which is the start point of Section 1 shown in FIG. 3, the lateral acceleration A1 of Section 1 is calculated by the following equation (2). Lateral acceleration A1=(vehicle speed V1) 2 ×curvature R1 (2) In equation (2), vehicle speed V1 is the vehicle speed at the first point P1, and curvature R1 is the curvature at the first point P1. The current vehicle speed V of the vehicle 10 can be obtained from the vehicle speed sensor 133. The curvature R1 at the first point P1 can be obtained from the road information storage unit 140. The lateral accelerations A2 to An for sections 2 to n can be calculated in a similar manner, and the vehicle speed V is calculated using the vehicle speed V1 at the first point P1, and the curvatures at the start points P2 to Pn of each section can be used for the curvature Rn. The curvature of the curve in the map information stored in the road information storage unit 140 is used for the curvature Rn.

[0036] 2 is executed when the vehicle 10 travels beyond the second point P2, the lateral acceleration is calculated for each section obtained by dividing the curve recognition distance D in the direction of travel of the vehicle 10 at that time into equal intervals. When determining whether the calculated lateral acceleration An is greater than the threshold Ao (S14), a hands-on request is issued if any of the calculated lateral accelerations An for each section is greater than the threshold Ao (S15). The timing of the hands-on request notification can be set appropriately as long as it is several seconds before the vehicle approaches a curve section exceeding the threshold Ao.

[0037] [effect] (1) According to the vehicle control device 110 of the first embodiment, when the vehicle 10 is traveling around a curve in a hands-off state, if the lateral acceleration An that will occur in the vehicle 10 when traveling the curve recognition distance in the future exceeds the threshold Ao, a hands-on request is issued (S15). If the lateral acceleration An is equal to or less than the threshold Ao, the hands-off state is continued (S16). In other words, the decision on whether to accept the hands-on request is made taking into account the driving situation, namely the vehicle speed when traveling around a curve, so unnecessary hands-on requests can be reduced compared to a configuration in which a hands-on request is issued uniformly based on pre-stored map information.

[0038] For example, even when a hands-on request should be issued when traveling at normal speeds around a curve, if the lateral acceleration An occurring in the vehicle 10 is small due to low-speed travel in traffic congestion, the driver's safety and comfort will not be compromised even if the hands are off, and there is no problem with continuing the hands-off state. In such cases, the hands-off state can be continued without issuing a hands-on request, which improves the accuracy of hands-on requests when traveling around a curve.

[0039] (2) According to the vehicle control device 110 of the first embodiment, the lateral acceleration calculation unit 113 calculates the lateral acceleration An for each section obtained by dividing the curve recognition distance D into equal intervals. Therefore, compared to a configuration in which the lateral acceleration is calculated using the curvature that is a representative value of the entire curve recognition distance D, more precise control can be implemented, and the vehicle 10 can travel safely around an upcoming curve.

[0040] (3) According to the vehicle control device 110 of the first embodiment, the timing for notifying the hands-on request is set several seconds before the curve section exceeding the threshold Ao is approached, so that the notification can be made before the driver becomes anxious, thereby maintaining safety and comfort.

[0041] B. Second embodiment: Next, a second embodiment will be described. The overall configuration (FIG. 2) and processing (FIG. 3) of the vehicle control device in the second embodiment and a third embodiment described later are substantially the same as those in the first embodiment, and therefore description thereof will be omitted. In the first embodiment, the curvature of the curve in the map information stored in the road information storage unit 140 was used as the curvature Rn of the curve when calculating the lateral acceleration An. Instead, in the second embodiment, a curvature estimated from actual road information in the traveling direction read from the periphery sensor 120 is used as the curvature Rn.

[0042] The actual road information is information obtained while the vehicle 10 is actually traveling. Specifically, the actual road information is information about lane markers 21 (see FIG. 3) estimated by the surrounding information recognition unit 112 based on a forward image captured by the camera 121. The curvature is estimated from the shape of the estimated lane markers 21. As in the first embodiment, the lateral acceleration calculation unit 113 calculates the lateral acceleration An for each of sections 1, 2, ... n, which are equal intervals of the curve recognition distance D.

[0043] According to the second embodiment, it is possible to achieve the same effects as those of the first embodiment. Furthermore, since the curvature estimated from actual road information is used, it is possible to calculate the lateral acceleration An even when map information is not available, and it is possible to improve the calculation accuracy of the lateral acceleration An.

[0044] C. Third embodiment: Next, a third embodiment will be described. In the third embodiment, the curvature Rn is the curvature of the target driving route T set by the driving route setting unit 111. As shown in FIG. 3, the target driving route T on a curve is drawn such that, for example, the vehicle enters the curve from the outside of the road when approaching the curve and travels on the inside of the road at the end of the curve. The target driving route T is drawn to draw a line for smoothly traveling around the curve, and therefore does not completely match the map information or actual driving route information. For this reason, the curvature of the target driving route T is information that is more in line with actual traveling than the curvature based on the map information or actual driving route information.

[0045] According to the third embodiment, it is possible to achieve the same effects as the second embodiment, and further, by using the curvature of the target driving path T, which is more accurate information that is more in line with actual driving, it is possible to improve the calculation accuracy of the lateral acceleration An.

[0046] Furthermore, even if the curve curvature cannot be read because the distant lane marker 21 is obscured by the preceding vehicle 22, the lateral acceleration An can be calculated and a hands-on request can be issued accurately.

[0047] D. Other Embodiments: (D1) In the above embodiments, as shown in the above formula (1), the current vehicle speed V of the vehicle 10 is used for all sections when calculating the future lateral acceleration An, but the vehicle speed when traveling in the future section may be read ahead and used. In this case, the lateral acceleration An is calculated using the following formula (3). Lateral acceleration An = (predicted vehicle speed Vn) 2 ×curvature Rn (3)

[0048] Here, the predicted vehicle speed Vn is calculated by the following equation (4). Look-ahead vehicle speed Vn = current vehicle speed V + target longitudinal acceleration × look-ahead time (4) The target longitudinal acceleration can be obtained, for example, from an adaptive cruise control (ACC) system that measures the distance between the preceding vehicle 22 and the vehicle 10 using a sensor and automatically adjusts the speed so that the distance does not become smaller than a set distance.

[0049] (D2) In each of the above embodiments, a hands-on request is notified at the turn of a curve section where the lateral acceleration An exceeds the threshold value Ao, but if it is possible to turn the curve and a minimum level of driver safety is ensured, the notification may be made after the curve section in question is reached.

[0050] (D3) In each of the above embodiments, if only one section of the curve recognition distance D exceeds the threshold value Ao, the hands-on request may be reset after traveling through that section, and a notification may be given that a hands-off state is now possible.

[0051] (D4) In each of the above embodiments, the lateral acceleration calculation unit 113 calculates the lateral acceleration An for each section obtained by dividing the curve recognition distance D at equal intervals, but the sections do not have to be divided at equal intervals. For example, sections with a relatively constant curvature do not need to be divided. The vehicle control device 110 and the methods described herein may be implemented by a special-purpose computer configured with a processor and memory programmed to execute one or more functions embodied in a computer program. Alternatively, the vehicle control device 110 and the methods described herein may be implemented by a special-purpose computer configured with a processor implemented by one or more dedicated hardware logic circuits. Alternatively, the vehicle control device 110 and the methods described herein may be implemented by one or more special-purpose computers configured with a processor and memory programmed to execute one or more functions and a processor implemented by one or more hardware logic circuits. Furthermore, the computer program may be stored as instructions executed by a computer on a computer-readable non-transitory storage medium.

Claims

1. A vehicle control device provided in a vehicle (10) capable of automatic driving and manual driving, a travel route setting unit (111) that sets a target travel route for the vehicle; a lateral acceleration calculation unit (113) that calculates a future lateral acceleration that will occur in the vehicle based on the curvature of a curve in the traveling direction of the vehicle and the vehicle speed of the vehicle; a notification unit (114) that notifies, by a notification device (150), a hands-on request to switch from a hands-off state in which the driver of the vehicle is not holding the steering wheel to a hands-on state in which the driver is holding the steering wheel during execution of the automatic driving; a control decision unit (115) that decides the control content of the vehicle in accordance with the calculation result of the lateral acceleration calculation unit; Equipped with The control decision unit When the vehicle is traveling around the curve in the hands-off state, A vehicle control device that, when the lateral acceleration calculated by the lateral acceleration calculation unit is greater than a predetermined threshold, determines control to notify the hands-on request by the notification unit as the control content, and, when the lateral acceleration is equal to or less than the threshold, determines control to continue the hands-off state as the control content.

2. the lateral acceleration calculation unit calculates the lateral acceleration for each of a plurality of sections obtained by dividing the curve, The vehicle control device according to claim 1 , wherein the control decision unit decides, as the control content, to perform control to notify the hands-on request before the vehicle enters the section where the lateral acceleration is greater than the threshold value.

3. 3. The vehicle control device according to claim 1, wherein the lateral acceleration calculation unit uses a curvature of map information as the curvature of the curve.

4. 3. The vehicle control device according to claim 1, wherein the lateral acceleration calculation unit uses, as the curvature of the curve, a curvature estimated from actual road information in the traveling direction.

5. 3. The vehicle control device according to claim 1, wherein the lateral acceleration calculation unit uses a curvature of the target driving route set by the driving route setting unit as the curvature of the curve.

Images