Patent Publication Number: US-11378967-B2

Title: Enumeration based failure prevention QP smoother for autonomous vehicles

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure relate generally to operating autonomous driving vehicles. More particularly, embodiments of the disclosure relate to an enumeration based failure prevention quadratic programming (QP) smoother for autonomous driving vehicles. 
     BACKGROUND 
     Vehicles operating in an autonomous mode (e.g., driverless) can relieve occupants, especially the driver, from some driving-related responsibilities. When operating in an autonomous mode, the vehicle can navigate to various locations using onboard sensors, allowing the vehicle to travel with minimal human interaction or in some cases without any passengers. Motion planning and control are critical operations in autonomous driving. Particularly, trajectory planning is a critical component in an autonomous driving system. Conventional trajectory planning techniques rely heavily on high-quality reference lines, which are guidance paths, e.g., a center line of a road, for autonomous driving vehicles, to generate stable trajectories. 
     Reference lines can be generated from map data points, typically a sequence of two-dimensional (2D) points in the world coordinate. Reference lines directly generated from the map data points are raw reference lines, which may lack the required smoothness and therefore can lead to unstable and oscillating trajectories between planning cycles. As such, raw reference lines typically need to be smoothed using various smoothing techniques for better driving experiences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
         FIG. 1  is a block diagram illustrating a networked system according to one embodiment. 
         FIG. 2  is a block diagram illustrating an example of an autonomous vehicle according to one embodiment. 
         FIGS. 3A-3B  are block diagrams illustrating an example of a perception and planning system used with an autonomous vehicle according to one embodiment. 
         FIG. 4  illustrates an example smoothing module according to one embodiment. 
         FIG. 5  illustrates an example reference line to be smoothed according to one embodiment. 
         FIGS. 6-7  illustrate an example use of an enumeration based failure prevention QP spline smoother according to one embodiment. 
         FIG. 8  is a flow diagram illustrating an example of a process for smoothing a reference line according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments and aspects of the disclosures will be described with reference to details discussed below, and the accompanying drawings will illustrate the various embodiments. The following description and drawings are illustrative of the disclosure and are not to be construed as limiting the disclosure. Numerous specific details are described to provide a thorough understanding of various embodiments of the present disclosure. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments of the present disclosures. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in conjunction with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     Various reference line smoothing techniques can be used to smooth a raw reference line for more comfortable driving. For example, a reference line can be smoothed using a QP spline smoothing algorithm (e.g., smoother). However, it is often unknown why QP fails to converge when the smoothing algorithm fails to generate a solution. In this case, an enumeration-based failure prevention QP spline smoother can enumerate and iteratively execute a QP spline smoother varying (or enumerating) one or more input variables for the QP spline smoother until convergence or until some predetermined criteria are satisfied to prevent (or minimizes) failures in convergence. 
     In a first aspect, the present disclosure provides a computer-implemented method for operating an autonomous driving vehicle (ADV). According to one embodiment, a system receives an initial reference line representing a route from a first location to a second location associated with an autonomous driving vehicle (ADV), where the initial reference line includes a number of reference line segments. The system smooths the line segments by applying a quadratic programming (QP) spline smoothing algorithm to the line segments. The system determines that the QP spline smoothing algorithm for the line segments fails to converge. The system adjusts (or enumerates) a location of an end point of a line segment. The system smooths the line segments by applying a QP spline smoothing algorithm to the adjusted (or enumerated) line segments. The system generates a smoothed reference line based on the smoothed line segments to be used as a reference line of the route to control the ADV. 
       FIG. 1  is a block diagram illustrating an autonomous vehicle network configuration according to one embodiment of the disclosure. Referring to  FIG. 1 , network configuration  100  includes autonomous vehicle  101  that may be communicatively coupled to one or more servers  103 - 104  over a network  102 . Although there is one autonomous vehicle shown, multiple autonomous vehicles can be coupled to each other and/or coupled to servers  103 - 104  over network  102 . Network  102  may be any type of networks such as a local area network (LAN), a wide area network (WAN) such as the Internet, a cellular network, a satellite network, or a combination thereof, wired or wireless. Server(s)  103 - 104  may be any kind of servers or a cluster of servers, such as Web or cloud servers, application servers, backend servers, or a combination thereof. Servers  103 - 104  may be data analytics servers, content servers, traffic information servers, map and point of interest (MPOI) severs, or location servers, etc. 
     An autonomous vehicle refers to a vehicle that can be configured to in an autonomous mode in which the vehicle navigates through an environment with little or no input from a driver. Such an autonomous vehicle can include a sensor system having one or more sensors that are configured to detect information about the environment in which the vehicle operates. The vehicle and its associated controller(s) use the detected information to navigate through the environment. Autonomous vehicle  101  can operate in a manual mode, a full autonomous mode, or a partial autonomous mode. 
     In one embodiment, autonomous vehicle  101  includes, but is not limited to, perception and planning system  110 , vehicle control system  111 , wireless communication system  112 , user interface system  113 , and sensor system  115 . Autonomous vehicle  101  may further include certain common components included in ordinary vehicles, such as, an engine, wheels, steering wheel, transmission, etc., which may be controlled by vehicle control system  111  and/or perception and planning system  110  using a variety of communication signals and/or commands, such as, for example, acceleration signals or commands, deceleration signals or commands, steering signals or commands, braking signals or commands, etc. 
     Components  110 - 115  may be communicatively coupled to each other via an interconnect, a bus, a network, or a combination thereof. For example, components  110 - 115  may be communicatively coupled to each other via a controller area network (CAN) bus. A CAN bus is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other in applications without a host computer. It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles, but is also used in many other contexts. 
     Referring now to  FIG. 2 , in one embodiment, sensor system  115  includes, but it is not limited to, one or more cameras  211 , global positioning system (GPS) unit  212 , inertial measurement unit (IMU)  213 , radar unit  214 , and a light detection and range (LIDAR) unit  215 . GPS system  212  may include a transceiver operable to provide information regarding the position of the autonomous vehicle. IMU unit  213  may sense position and orientation changes of the autonomous vehicle based on inertial acceleration. Radar unit  214  may represent a system that utilizes radio signals to sense objects within the local environment of the autonomous vehicle. In some embodiments, in addition to sensing objects, radar unit  214  may additionally sense the speed and/or heading of the objects. LIDAR unit  215  may sense objects in the environment in which the autonomous vehicle is located using lasers. LIDAR unit  215  could include one or more laser sources, a laser scanner, and one or more detectors, among other system components. Cameras  211  may include one or more devices to capture images of the environment surrounding the autonomous vehicle. Cameras  211  may be still cameras and/or video cameras. A camera may be mechanically movable, for example, by mounting the camera on a rotating and/or tilting a platform. 
     Sensor system  115  may further include other sensors, such as, a sonar sensor, an infrared sensor, a steering sensor, a throttle sensor, a braking sensor, and an audio sensor (e.g., microphone). An audio sensor may be configured to capture sound from the environment surrounding the autonomous vehicle. A steering sensor may be configured to sense the steering angle of a steering wheel, wheels of the vehicle, or a combination thereof. A throttle sensor and a braking sensor sense the throttle position and braking position of the vehicle, respectively. In some situations, a throttle sensor and a braking sensor may be integrated as an integrated throttle/braking sensor. 
     In one embodiment, vehicle control system  111  includes, but is not limited to, steering unit  201 , throttle unit  202  (also referred to as an acceleration unit), and braking unit  203 . Steering unit  201  is to adjust the direction or heading of the vehicle. Throttle unit  202  is to control the speed of the motor or engine that in turn controls the speed and acceleration of the vehicle. Braking unit  203  is to decelerate the vehicle by providing friction to slow the wheels or tires of the vehicle. Note that the components as shown in  FIG. 2  may be implemented in hardware, software, or a combination thereof. 
     Referring back to  FIG. 1 , wireless communication system  112  is to allow communication between autonomous vehicle  101  and external systems, such as devices, sensors, other vehicles, etc. For example, wireless communication system  112  can wirelessly communicate with one or more devices directly or via a communication network, such as servers  103 - 104  over network  102 . Wireless communication system  112  can use any cellular communication network or a wireless local area network (WLAN), e.g., using WiFi to communicate with another component or system. Wireless communication system  112  could communicate directly with a device (e.g., a mobile device of a passenger, a display device, a speaker within vehicle  101 ), for example, using an infrared link, Bluetooth, etc. User interface system  113  may be part of peripheral devices implemented within vehicle  101  including, for example, a keyword, a touch screen display device, a microphone, and a speaker, etc. 
     Some or all of the functions of autonomous vehicle  101  may be controlled or managed by perception and planning system  110 , especially when operating in an autonomous driving mode. Perception and planning system  110  includes the necessary hardware (e.g., processor(s), memory, storage) and software (e.g., operating system, planning and routing programs) to receive information from sensor system  115 , control system  111 , wireless communication system  112 , and/or user interface system  113 , process the received information, plan a route or path from a starting point to a destination point, and then drive vehicle  101  based on the planning and control information. Alternatively, perception and planning system  110  may be integrated with vehicle control system  111 . 
     For example, a user as a passenger may specify a starting location and a destination of a trip, for example, via a user interface. Perception and planning system  110  obtains the trip related data. For example, perception and planning system  110  may obtain location and route information from an MPOI server, which may be a part of servers  103 - 104 . The location server provides location services and the MPOI server provides map services and the POIs of certain locations. Alternatively, such location and MPOI information may be cached locally in a persistent storage device of perception and planning system  110 . 
     While autonomous vehicle  101  is moving along the route, perception and planning system  110  may also obtain real-time traffic information from a traffic information system or server (TIS). Note that servers  103 - 104  may be operated by a third party entity. Alternatively, the functionalities of servers  103 - 104  may be integrated with perception and planning system  110 . Based on the real-time traffic information, MPOI information, and location information, as well as real-time local environment data detected or sensed by sensor system  115  (e.g., obstacles, objects, nearby vehicles), perception and planning system  110  can plan an optimal route and drive vehicle  101 , for example, via control system  111 , according to the planned route to reach the specified destination safely and efficiently. 
     Server  103  may be a data analytics system to perform data analytics services for a variety of clients. In one embodiment, data analytics system  103  includes data collector  121  and machine learning engine  122 . Data collector  121  collects driving statistics  123  from a variety of vehicles, either autonomous vehicles or regular vehicles driven by human drivers. Driving statistics  123  include information indicating the driving commands (e.g., throttle, brake, steering commands) issued and responses of the vehicles (e.g., speeds, accelerations, decelerations, directions) captured by sensors of the vehicles at different points in time. Driving statistics  123  may further include information describing the driving environments at different points in time, such as, for example, routes (including starting and destination locations), MPOIs, road conditions, weather conditions, etc. 
     Based on driving statistics  123 , machine learning engine  122  generates or trains a set of rules, algorithms, and/or predictive models  124  for a variety of purposes. In one embodiment, for example, algorithms  124  may include an algorithm for QP smoothing of reference lines. These functions can be uploaded onto the autonomous driving vehicle to be used to generate a smooth path at real time. 
     In one embodiment, algorithms  124  may include a QP spline smoothing algorithm (e.g., QP spline smoother) to perform smoothing of a reference line for path and speed planning. The QP spline smoother can be an enumeration-based QP spline smoother that can iteratively retry the QP spline smoothing with varying input parameters when the QP spline smoothing algorithm does not converge. That way the QP spline smoothing algorithm convergence failure (e.g., non-convergence) rate is reduced for the QP spline smoothing algorithm has a high chance of convergence with a particular set of input parameters. Some of the varying input parameters include enumerated segment lengths, a number of splines, and the order of spline for the QP spline smoothing algorithm. 
       FIGS. 3A and 3B  are block diagrams illustrating an example of a perception and planning system used with an autonomous vehicle according to one embodiment. System  300  may be implemented as a part of autonomous vehicle  101  of  FIG. 1  including, but is not limited to, perception and planning system  110 , control system  111 , and sensor system  115 . Referring to  FIGS. 3A-3B , perception and planning system  110  includes, but is not limited to, localization module  301 , perception module  302 , prediction module  303 , decision module  304 , planning module  305 , control module  306 , routing module  307 , and smoothing module  308 . 
     Some or all of modules  301 - 308  may be implemented in software, hardware, or a combination thereof. For example, these modules may be installed in persistent storage device  352 , loaded into memory  351 , and executed by one or more processors (not shown). Note that some or all of these modules may be communicatively coupled to or integrated with some or all modules of vehicle control system  111  of  FIG. 2 . Some of modules  301 - 308  may be integrated together as an integrated module. For example, decision module  304  and planning module  305  may be integrated as a single module; and routing module  307  and smoothing module  308  may be integrated as a single module. 
     Localization module  301  determines a current location of autonomous vehicle  300  (e.g., leveraging GPS unit  212 ) and manages any data related to a trip or route of a user. Localization module  301  (also referred to as a map and route module) manages any data related to a trip or route of a user. A user may log in and specify a starting location and a destination of a trip, for example, via a user interface. Localization module  301  communicates with other components of autonomous vehicle  300 , such as map and route information  311 , to obtain the trip related data. For example, localization module  301  may obtain location and route information from a location server and a map and POI (MPOI) server. A location server provides location services and an MPOI server provides map services and the POIs of certain locations, which may be cached as part of map and route information  311 . While autonomous vehicle  300  is moving along the route, localization module  301  may also obtain real-time traffic information from a traffic information system or server. 
     Based on the sensor data provided by sensor system  115  and localization information obtained by localization module  301 , a perception of the surrounding environment is determined by perception module  302 . The perception information may represent what an ordinary driver would perceive surrounding a vehicle in which the driver is driving. The perception can include the lane configuration (e.g., straight or curve lanes), traffic light signals, a relative position of another vehicle, a pedestrian, a building, crosswalk, or other traffic related signs (e.g., stop signs, yield signs), etc., for example, in a form of an object. 
     Perception module  302  may include a computer vision system or functionalities of a computer vision system to process and analyze images captured by one or more cameras in order to identify objects and/or features in the environment of autonomous vehicle. The objects can include traffic signals, road way boundaries, other vehicles, pedestrians, and/or obstacles, etc. The computer vision system may use an object recognition algorithm, video tracking, and other computer vision techniques. In some embodiments, the computer vision system can map an environment, track objects, and estimate the speed of objects, etc. Perception module  302  can also detect objects based on other sensors data provided by other sensors such as a radar and/or LIDAR. 
     For each of the objects, prediction module  303  predicts what the object will behave under the circumstances. The prediction is performed based on the perception data perceiving the driving environment at the point in time in view of a set of map/rout information  311  and traffic rules  312 . For example, if the object is a vehicle at an opposing direction and the current driving environment includes an intersection, prediction module  303  will predict whether the vehicle will likely move straight forward or make a turn. If the perception data indicates that the intersection has no traffic light, prediction module  303  may predict that the vehicle may have to fully stop prior to enter the intersection. If the perception data indicates that the vehicle is currently at a left-turn only lane or a right-turn only lane, prediction module  303  may predict that the vehicle will more likely make a left turn or right turn respectively. 
     For each of the objects, decision module  304  makes a decision regarding how to handle the object. For example, for a particular object (e.g., another vehicle in a crossing route) as well as its metadata describing the object (e.g., a speed, direction, turning angle), decision module  304  decides how to encounter the object (e.g., overtake, yield, stop, pass). Decision module  304  may make such decisions according to a set of rules such as traffic rules or driving rules  312 , which may be stored in persistent storage device  352 . 
     Based on a decision for each of the objects perceived, planning module  305  plans a path or route for the autonomous vehicle, as well as driving parameters (e.g., distance, speed, and/or turning angle). That is, for a given object, decision module  304  decides what to do with the object, while planning module  305  determines how to do it. For example, for a given object, decision module  304  may decide to pass the object, while planning module  305  may determine whether to pass on the left side or right side of the object. Planning and control data is generated by planning module  305  including information describing how vehicle  300  would move in a next moving cycle (e.g., next route/path segment). For example, the planning and control data may instruct vehicle  300  to move 10 meters at a speed of 30 miles per hour (mph), then change to a right lane at the speed of 25 mph. 
     Based on the planning and control data, control module  306  controls and drives the autonomous vehicle, by sending proper commands or signals to vehicle control system  111 , according to a route or path defined by the planning and control data. The planning and control data include sufficient information to drive the vehicle from a first point to a second point of a route or path using appropriate vehicle settings or driving parameters (e.g., throttle, braking, and turning commands) at different points in time along the path or route. 
     In one embodiment, the planning phase is performed in a number of planning cycles, also referred to as command cycles, such as, for example, in every time interval of 100 milliseconds (ms). For each of the planning cycles or command cycles, one or more control commands will be issued based on the planning and control data. That is, for every 100 ms, planning module  305  plans a next route segment or path segment, for example, including a target position and the time required for the ADV to reach the target position. Alternatively, planning module  305  may further specify the specific speed, direction, and/or steering angle, etc. In one embodiment, planning module  305  plans a route segment or path segment for the next predetermined period of time such as 5 seconds. For each planning cycle, planning module  305  plans a target position for the current cycle (e.g., next 5 seconds) based on a target position planned in a previous cycle. Control module  306  then generates one or more control commands (e.g., throttle, brake, steering control commands) based on the planning and control data of the current cycle. 
     Note that decision module  304  and planning module  305  may be integrated as an integrated module. Decision module  304 /planning module  305  may include a navigation system or functionalities of a navigation system to determine a driving path for the autonomous vehicle. For example, the navigation system may determine a series of speeds and directional headings to effect movement of the autonomous vehicle along a path that substantially avoids perceived obstacles while generally advancing the autonomous vehicle along a roadway-based path leading to an ultimate destination. The destination may be set according to user inputs via user interface system  113 . The navigation system may update the driving path dynamically while the autonomous vehicle is in operation. The navigation system can incorporate data from a GPS system and one or more maps so as to determine the driving path for the autonomous vehicle. 
     Decision module  304 /planning module  305  may further include a collision avoidance system or functionalities of a collision avoidance system to identify, evaluate, and avoid or otherwise negotiate potential obstacles in the environment of the autonomous vehicle. For example, the collision avoidance system may effect changes in the navigation of the autonomous vehicle by operating one or more subsystems in control system  111  to undertake swerving maneuvers, turning maneuvers, braking maneuvers, etc. The collision avoidance system may automatically determine feasible obstacle avoidance maneuvers on the basis of surrounding traffic patterns, road conditions, etc. The collision avoidance system may be configured such that a swerving maneuver is not undertaken when other sensor systems detect vehicles, construction barriers, etc. in the region adjacent the autonomous vehicle that would be swerved into. The collision avoidance system may automatically select the maneuver that is both available and maximizes safety of occupants of the autonomous vehicle. The collision avoidance system may select an avoidance maneuver predicted to cause the least amount of acceleration in a passenger cabin of the autonomous vehicle. 
     Routing module  307  can generate reference routes, for example, from map information such as information of road segments, vehicular lanes of road segments, and distances from lanes to curb. For example, a road can be divided into sections or segments {A, B, and C} to denote three road segments. Three lanes of road segment A can be enumerated {A 1 , A 2 , and A 3 }. A reference route is generated by generating reference points along the reference route. For example, for a vehicular lane, routing module  307  can connect midpoints of two opposing curbs or extremities of the vehicular lane provided by a map data. Based on the midpoints and machine learning data representing collected data points of vehicles previously driven on the vehicular lane at different points in time, routing module  307  can calculate the reference points by selecting a subset of the collected data points within a predetermined proximity of the vehicular lane and applying a smoothing function to the midpoints in view of the subset of collected data points. 
     Based on reference points or lane reference points, routing module  307  may generate a reference line by interpolating the reference points such that the generated reference line is used as a reference line for controlling ADVs on the vehicular lane. In some embodiments, a reference points table and a road segments table representing the reference lines are downloaded in real-time to ADVs such that the ADVs can generate reference lines based on the ADVs&#39; geographical location and driving direction. For example, in one embodiment, an ADV can generate a reference line by requesting routing service for a path segment by a path segment identifier representing an upcoming road section ahead and/or based on the ADV&#39;s GPS location. Based on a path segment identifier, a routing service can return to the ADV reference points table containing reference points for all lanes of road segments of interest. ADV can look up reference points for a lane for a path segment to generate a reference line for controlling the ADV on the vehicular lane. 
     Smoothing module  308  can generate a smooth road reference line based on a reference line provided by routing module  307 . For example, smoothing module  308  selects a number of control points along a reference line. In one embodiment, the control points can be reference points of the reference line provided by routing module  307  or some interpolated points along the reference line which are approximately equally distant to their adjacent points. Smoothing module  308  can use a quadratic programming spline smoother to generate the smooth road reference line. 
     As described above, route or routing module  307  manages any data related to a trip or route of a user. The user of the ADV specifies a starting and a destination location to obtain trip related data. Trip related data includes route segments and a reference line or reference points of the route segment. For example, based on route map info  311 , route module  307  generates a route or road segments table and a reference points table. The reference points are in relations to road segments and/or lanes in the road segments table. The reference points can be interpolated to form one or more reference lines to control the ADV. The reference points can be specific to road segments and/or specific lanes of road segments. 
     For example, a road segments table can be a name-value pair to include previous and next road lanes for road segments A-D. E.g., a road segments table may be: {(A 1 , B 1 ), (B 1 , C 1 ), (C 1 , D 1 )} for road segments A-D for lane 1. A reference points table may include reference points in x-y coordinates for road segments lanes, e.g., {(A 1 , (x 1 , y 1 )), (B 1 , (x 2 , y 2 )), (C 1 , (x 3 , y 3 )), (D 1 , (x 4 , y 4 ))}, where A 1  . . . D 1  refers to lane 1 of road segments A-D, and (x 1 , y 1 ) . . . (x 4 , y 4 ) are corresponding real world coordinates. In one embodiment, road segments and/or lanes are divided into a predetermined length such as approximately 200 meter segments/lanes. In another embodiment, road segments and/or lanes are divided into variable length segments/lanes depending on road conditions such as road curvatures. In some embodiments, each road segment and/or lane can include several reference points. In some embodiments, reference points can be converted to other coordinate systems, e.g., latitude-longitude. 
     In some embodiments, reference points can be converted into a relative coordinates system, such as station-lateral (SL) coordinates. A station-lateral coordinate system is a coordinate system that references a fixed reference point to follow a reference line. For example, a (S, L)=(1, 0) coordinate can denote one meter ahead of a stationary point (i.e., the reference point) on the reference line with zero meter lateral offset. A (S, L)=(2, 1) reference point can denote two meters ahead of the stationary reference point along the reference line and one meter lateral offset from the reference line, e.g., offset to the left by one meter. 
     In one embodiment, smoothing module  308  generates a smooth reference line based on reference points representing a reference line provided by routing module  307 . The smooth reference line can be converted into a relative coordinate system such as a SL coordinate system before a decision module and/or a planning module such as decision module  304  and/and planning module  305  incorporates the smooth reference line with perceived obstacles and/or traffic information. 
     In one embodiment, decision module  304  generates a rough path profile based on a reference line (the reference line having been smoothed by smoothing module  308  as described above) provided by routing module  307  and based on obstacles and/or traffic information perceived by the ADV, surrounding the ADV. The rough path profile can be a part of path/speed profiles  313  which may be stored in persistent storage device  352 . The rough path profile is generated by selecting points along the reference line. For each of the points, decision module  304  moves the point to the left or right (e.g., candidate movements) of the reference line based on one or more obstacle decisions on how to encounter the object, while the rest of points remain steady. The candidate movements can be performed iteratively using dynamic programming to path candidates in search of a path candidate with a lowest path cost using cost functions, as part of costs functions  315  of  FIG. 3A , thereby generating a rough path profile. Examples of cost functions include costs based on: a curvature of a route path, a distance from the ADV to perceived obstacles, and a distance of the ADV to the reference line. In one embodiment, the generated rough path profile includes a station-lateral map, as part of SL maps/ST graphs  314  which may be stored in persistent storage devices  352 . 
     In one embodiment, decision module  304  generates a rough speed profile (as part of path/speed profiles  313 ) based on the generated rough path profile. The rough speed profile indicates the best speed at a particular point in time controlling the ADV. Similar to the rough path profile, candidate speeds at different points in time can be iterated using dynamic programming to find speed candidates (e.g., speed up or slow down) with a lowest speed cost based on cost functions, as part of costs functions  315  of  FIG. 3A , in view of obstacles perceived by the ADV. The rough speed profile can be used to decide whether the ADV should overtake or avoid an obstacle, and to the left or right of the obstacle. In one embodiment, the rough speed profile includes a station-time (ST) graph (as part of SL maps/ST graphs  314 ). Station-time graph indicates a distance travelled with respect to time. 
     In one embodiment, planning module  305  recalculates the rough path profile in view of obstacle decisions and/or artificial barriers to forbid the planning module  305  to search the geometric spaces of the barriers. For example, if the rough speed profile determined to nudge an obstacle from the left, planning module  305  can set a barrier (in the form of an obstacle) to the right of the obstacle to prevent a calculation for the ADV to nudge an obstacle from the right. In one embodiment, the rough path profile is recalculated by optimizing a path cost function (as part of cost functions  315 ) using quadratic programming (QP) and/or other smoothing algorithms. In one embodiment, the recalculated rough path profile includes a station-lateral map (as part of SL maps/ST graphs  314 ). 
       FIG. 4  illustrates an example smoothing module in accordance with an embodiment. As shown in  FIG. 4 , the smoothing module  308  can include a failure prevention QP spline smoother  401 . The smoothing module  308  can generate a smooth reference line based on a reference line provided by routing module  307  by the use of the QP spline smoother  401 . By applying the QP smoother to a raw reference line, the ADV can generate a smoothed reference line with the desired continuity and smoothness of curvature. 
     According to one embodiment, failure prevention QP spline smoother  401  includes reference line receiver  402 , reference line segmenter  403 , QP solver  404 , QP convergence determiner  405 , segment adjuster  406 , and smooth reference line generator  407 . Reference line receiver  402  can receive a raw reference line (e.g., raw reference line points) from map and route information for ADV. Reference line segmenter  403  can segment the reference line (e.g., raw reference line points or the interpolated points) into reference line segments. QP solver  404  can apply a QP smoothing algorithm to the reference line segments. QP convergence determiner  405  can determine if the QP smoothing algorithm converged (e.g., provides a solution). Segment adjust  406  can adjust a parameter for a segment (e.g., length, number of spline, order of spline, etc.). Smooth reference line generator  407  can generate a smooth reference line. 
     As an illustrative example, given a raw reference line to be smoothed, the smoothing module  308  can select a number of control points along the reference line. The control points can be reference points of the reference line provided by routing module  307  or some interpolated points along the reference line which are approximately equally distant to their adjacent points. 
     The reference points can divide the reference line into a number of reference line segments. For example, the reference line segments can include a first reference line segment, a second reference segment, and a third reference line segment in a sequential order. The smoothing module  308  can apply the failure prevention QP spline smoother  401  to each of the reference line segments. The smoothed reference line segment can subsequently be optimized in view of a set of constraints, such that an output of the objective function reaches a minimum value while the set of constraints are satisfied. The above operations can be repeatedly performed until each reference segment is smoothly connected. The smoothed reference line segments can be connected to generate the smoothed reference line, which can be used as a reference line of the route to control the ADV. 
       FIG. 5  illustrates an example reference line to be smoothed according to one embodiment. Referring to  FIG. 5 , line segments A-F can be a raw reference line received from routing module  307 . The raw reference line can be a center line of a lane of a road segment obtained from a high definition map, and can include a number of reference points that divide the raw reference line into a number of reference line segments. The reference points can be (x 1 , y 1 ), (x 2 , y 2 ), (x 3 , y 3 ), (x 4 , y 4 ), (x 5 , y 5 ), (x 6 , y 6 ) and (x n , y n ) for the reference line segments A, B, C, D, E, and F, respectively. 
       FIGS. 6-7  illustrate an example use of an enumeration based failure prevention QP spline smoother according to one embodiment. As shown in  FIG. 6 , a road segment (i.e., the first reference line segment A shown in  FIG. 5 ) for a lane (e.g., lane 1) can include a number of control points such as control points  601 . The control points can be approximately equally spaced apart, for example, approximately five to ten meters apart. 
     Based on control points  601 , the failure prevention QP spline smoother  401  can apply a 2D spline optimization to generate a smooth reference line, such as spline  605 . A spline is a curve represented by one or more (e.g., piecewise) polynomials (e.g., A 1   a , A 1   b , A 1   c , etc.) for the lane joined together to form the curve. For example, a polynomial or a polynomial function can represent a segment between adjacent control points. In one embodiment, each polynomial function within the spline can be a two dimensional polynomial(s), e.g.,
 
 x ( t )= p   0   +p   1   t+p   2   t   2   + . . . +p   n   t   n , and  y ( t )= q   0   +q   1   t+q   2   t   2   + . . . +q   n   t   n ,
 
where x, y represents a two dimensional (x, y) geometric coordinate for a polynomial to the nth order, and p 0 . . . n  and q 0 . . . n  are coefficients of the two dimensional polynomial to be solved.
 
     In another embodiment, the polynomial function can be one dimensional. E.g.,
 
 l ( s )= p   0   +p   1   s+p   2   s   2   + . . . +p   n   s   n ,
 
where s, l represents a station-lateral one dimensional (s, l) geometric coordinate for a polynomial to the nth order, and p 0 . . . n  are coefficients of the one dimensional polynomials to be solved.
 
     In one embodiment, failure prevention QP spline smoother  401  can configure an order of polynomial for the spline or piecewise polynomials to ensure a desired threshold of spline smoothness. In one embodiment, the piecewise polynomials can be preconfigured to a fifth order polynomial. Based on the control point  601 , failure prevention QP spline smoother  401  can define a boundary area, such as boundary box  603  with a predefined dimension, such as, approximately 0.2 meters by 0.2 meters, to surround each of the control points  601 . The boundary areas can represent an inequality constraint that the smooth reference line (e.g., spline or piecewise polynomials)  605  must touch or pass through. 
     In one embodiment, failure prevention QP spline smoother  401  can add a set of initial constraints to the piecewise polynomials. The set of initial constraint can correspond to a current geographical location and/or a current directional heading of the ADV, for example,
 
 x (0)= x   0  and  y (0)= y   0 ,
 
 x ′(0)= dx   0  and  y ′(0)= dy   0 ,
 
where (x 0 , y 0 ) is the current x-y coordinate of the ADV geographical location, (dx 0 , dy 0 ) is a current direction of the ADV, and x(0), y(0) corresponds to the initial values of the first x-y polynomial. In some embodiments, smoother  401  can add a set of end constraints corresponding to a location and a direction of the ADV when the ADV reaches a destination point.
 
     In some embodiments, failure prevention QP spline smoother  401  can select a target function with various kernels or costs functions which the spline will target on. Example target functions can include smoothing kernels and/or guidance kernels such as:
 
 w   1 ∫( x ′) 2 ( t ) dt+w   2 ∫( y ′) 2 ( t ) dt+w   3 ∫( x ″) 2 ( t ) dt+w   4 ∫( y ″) 2 ( t ) dt+w   5 ∫( x ″′) 2 ( t ) dt+w   6 ∫( y ″′) 2 ( t ) dt+w   7 ∫[ x ( t )− x   ref ( t )] 2   dt+w   8 ∫[ y ( t )− y   ref ( t )] 2   dt  
 
where x(t), y(t) are x-y two dimensional piecewise polynomials, w 1 , . . . , w 8  are weight factors, (x′) 2  (t), (y′) 2 (t) are the first derivative squares of the piecewise polynomials, (x″) 2 (t), (y″) 2 (t) are the second derivative squares of the piecewise polynomials, (x″′) 2 (t), (y″′) 2  (t) are the third derivative squares of the piecewise polynomials, and x ref (t), y ref (t) are x-y reference route values of average human driving routes from previously collected data.
 
     In one embodiment, failure prevention QP spline smoother  401  can solve the target function to generate a smooth reference line. In one embodiment, a QP optimization can be performed on the target function such that the target function reaches a predetermined threshold (e.g., minimum), while the set of constraints are satisfied. Once the target function has been optimized in view of the constraints, the coefficients of the polynomial functions can be determined. Then the location of the path points (e.g., control points) along the path can be determined using the polynomial function with the optimized coefficients, which represents a smooth reference line. As described above, the smoothing function is incorporated into the target function to be solved, i.e., the smoothing is not a post processing step ensuring the optimized reference line after applying a smoothing function would still be bound to the set of defined constraints. 
     In one embodiment, each smoothed reference line segments can be optimized in view of one or more constraints such as road boundary constraints, perceived obstacles, and some limitations of the ADV, e.g., turning radius. Once all the reference line segments are smoothed and/or optimized, the smoothing module  308  can connect the smoothed reference line segments together to create a smoothed reference line. 
     In one embodiment, if it is determined that the QP optimization fails to provide a solution, e.g., the QP optimization does not reach a predetermined threshold (e.g., minimum), given a set of constraints, different parameters of the spline segment can be varied and the QP optimization can be re-executed. 
     Referring to  FIG. 7 , in one embodiment, one parameter of the spline segment to be varied (enumerated) is the end points of one or more of the spline segments, end points of a road/line segment, and/or an end point of the raw reference line. For example, referring to  FIG. 7 , in one embodiment, spline segment Ala initially can stretch from point (x 1 , x 2 ) to end point (control point  601 ) and the end point can be enumerated through a predetermined number of points (in this case, four end points,  702 - 705 ). The QP optimization can then be executed for each of the enumerations or until the QP optimization reaches a convergence. 
     In another embodiment, an end point of the raw reference line (e.g., (xn, yn)) can be enumerated. The raw reference line can then be segmented into a predetermined number of road segments. In another embodiment, an end point of individual road segments (e.g., end points of A-F) can be enumerated. For each enumeration, road segments can be segmented into a predetermined number of spline segments (e.g., segments between control points) and QP optimization can be applied to the spline segments, until the QP optimization reaches a convergence. 
     In one embodiment, the number of spline segments for a road segment can be varied (e.g., enumerated) over a predetermined range. For example, if a road segment is segmented into 10 spline segments, the number of spline segments can be enumerated between 8-12. In one embodiment, an order of spline for a subset or all of the spline segments can be varied over a predetermined range. For example, if the default order of spline is 5, the order of spline can be enumerated between 3-7. For each of the enumeration, the QP smoothing algorithm can be applied to the spline segments until convergence. 
     Here, varying the spline segment parameters allows QP optimization to reach convergence through a number of iterations (chance to converge) for each planning cycle and the enumerations would not alter a solution for the smoothing. In one embodiment, for end point enumerations, end points are enumerated over a small fraction (e.g., 50 centimeters) of the overall length of a road/line segment (e.g., 100 meters). the small deviation in the enumerations is not critical in planning because there is no impact for smoothing a 100 meters, versus the enumerated, e.g., 101, 100.5, 99.5, or 99 meters, as the ADV would recalculate a subsequent route in a next planning cycle (100 ms per planning cycle). Note that enumerating an order of spline and a number of spline segments may alter a smoothness for the smoothed reference line but the additional parameters for enumeration can increase a likelihood of convergence. 
       FIG. 8  is a flow diagram illustrating an example of a process for failure prevention smoothing of a reference line according to one embodiment. Process  800  may be performed by processing logic which may include software, hardware, or a combination thereof. For example, process  800  may be performed by failure prevention QP spline smoother  401  of  FIG. 4 . Referring to  FIG. 8 , in operation  801 , the processing logic receives an initial reference line representing a route from a first location to a second location associated with an autonomous driving vehicle (ADV), where the initial reference line includes a number of line segments. In operation  802 , processing logic smooths the line segments by applying a quadratic programming (QP) spline smoothing algorithm to the line segments. In operation  803 , processing logic determines that the QP spline smoothing algorithm for the line segments fails to converge. In operation  804 , processing logic adjusts (or enumerates) a location of an end point of a line segment. In operation  805 , processing logic smooths the line segments by applying a QP spline smoothing algorithm to the adjusted (or enumerated) line segment. In operation  806 , processing logic generates a smoothed reference line based on the smoothed line segments to be used as a reference line of the route to control the ADV. 
     In one embodiment, processing logic further adjusts a number of spline for the QP spline smoothing algorithm and smooths the line segments by applying the adjusted QP spline smoothing algorithm to the line segments. In one embodiment, processing logic further adjusting an order of spline for the QP spline smoothing algorithm and smooths the line segments by applying the adjusted QP spline smoothing algorithm to the line segment. 
     In one embodiment, the location of the end point is adjusted so a length of the line segment lengthens or shortens by a predetermined length. In one embodiment, the length of each of the plurality of line segments is dynamically determined based on road conditions of the route and a speed of the ADV. 
     In one embodiment, at least one of the line segments is different in length from at least one other line segment. In one embodiment, the end point is adjusted a predetermined number of iterations before adjusting a number of spline for the QP spline smoothing algorithm. 
     Note that some or all of the components as shown and described above may be implemented in software, hardware, or a combination thereof. For example, such components can be implemented as software installed and stored in a persistent storage device, which can be loaded and executed in a memory by a processor (not shown) to carry out the processes or operations described throughout this application. Alternatively, such components can be implemented as executable code programmed or embedded into dedicated hardware such as an integrated circuit (e.g., an application specific IC or ASIC), a digital signal processor (DSP), or a field programmable gate array (FPGA), which can be accessed via a corresponding driver and/or operating system from an application. Furthermore, such components can be implemented as specific hardware logic in a processor or processor core as part of an instruction set accessible by a software component via one or more specific instructions. 
     Some portions of the preceding detailed descriptions have been presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the ways used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of operations leading to a desired result. The operations are those requiring physical manipulations of physical quantities. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the above discussion, it is appreciated that throughout the description, discussions utilizing terms such as those set forth in the claims below, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Embodiments of the disclosure also relate to an apparatus for performing the operations herein. Such a computer program is stored in a non-transitory computer readable medium. A machine-readable medium includes any mechanism for storing information in a form readable by a machine (e.g., a computer). For example, a machine-readable (e.g., computer-readable) medium includes a machine (e.g., a computer) readable storage medium (e.g., read only memory (“ROM”), random access memory (“RAM”), magnetic disk storage media, optical storage media, flash memory devices). 
     The processes or methods depicted in the preceding figures may be performed by processing logic that comprises hardware (e.g. circuitry, dedicated logic, etc.), software (e.g., embodied on a non-transitory computer readable medium), or a combination of both. Although the processes or methods are described above in terms of some sequential operations, it should be appreciated that some of the operations described may be performed in a different order. Moreover, some operations may be performed in parallel rather than sequentially. 
     Embodiments of the present disclosure are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of embodiments of the disclosure as described herein. 
     In the foregoing specification, embodiments of the disclosure have been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.