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CarND Path Planning Project

Introduction

This project implements a path planner, that creates smooth safe paths
for autonomous vehicles along a highway with mild or heavy traffic. The highway and the real time data are provided to the planner by Udacity's simulator.

The simulator communicates with the planner via WebSocket interface and provides real time information including

  • Vehicle's current position in Cartesian coordinates
  • Vehicle's current position in Frenet coordinates
  • Vehicle's speed and yaw
  • A list of points from the previous path provided by the planner that have not been "consumed" yet.
  • Sensor fusion data for other vehicles on the same side of the road.

On the planner side, it uses those information to create a set of discrete points in Cartesian coordinates that will be consumed by the simulator. Consumption is done by "moving" the vehicle to the point we provide, using a point every 0.02 seconds (50 Frames per second). The simulator does not drive the car from point A to point B but "teleports" the vehicle from position A to position B.

The car should move respecting certain limits of Speed, Acceleration and Jerk.

Navigator Architecture

The architecture is described in the following diagram:

uml

The Websocket interface enables communication layer between the Simulator and path planner. The communication is in main.cpp. The link between the Navigator and WebSockets interface is kept as simple as one line of code

// line:64
auto msg = "42[\"control\"," + navigator(j) + "]";

Simply overloading the () operator on the Navigator struct and passing the json object containing the new sensor fusion data.

The Navigator Struct

The whole path planning cycle is very self explanatory in the code itself and is documented well enough. Pasting here for reference

 // 1. Update or pass
  car.readPreviousPath(json); // reads previous path, end_path data
  if (!plan.shouldUpdate()) { return car.previousCurve.toJson(); }

  //  2. Update data and calculate local data
  car.updateData(json, 1, true);
  car.updateLocalData(road.orientation(car.s));
  plan.reportedLag = car.lag();
  road.updateData(json);

  // 3. Create new goal
  auto newMode = plan.select_mode(car, road);
  car.mode = newMode;
  auto newGoal = plan.realizePlan(newMode, car, road);

  // 4. Generate new trajectory
  auto trajectory = tr_generator.generateTrajectory(newGoal, car, road);

  // 5. Keep the final plan for next starting state
  car.currentGoal = trajectory.goal;

  // 6. Create new discrete curve
  auto numberOfPoints = (int) (road.rcfg.frames * newGoal.duration);
  auto newCurve = std::move(curveHandler.createCurve(trajectory, road, numberOfPoints));

  // 7. Merge with previous curve
  auto mergedCurve = curveHandler.appendCurve(newCurve, car.previousCurve);

  return mergedCurve.toJson();

The Behavior Planner

The behavior planner is a simple finite state machine. For each state, we define possible next states. Each of them is evaluated and "quantized". The state with the least cost is selected as the next state. The next state is to realize the plan, by creating an end state as a goal, based on the current position (or expected position) of the vehicle and it sensor fusion data. The goal, encapsulated in a StateGoal struct is passed to the trajectory generator to create the actual continuous path from start state to end state.

The states of the FSM are described in the following simple map object.

map<string, vector<string>> Planner::next_modes = {
    {"KL", {"EB", "MF", "KL", "LCL", "LCR"}},
    {"MF", {"EB", "MF", "KL", "LCL", "LCR"}},
    {"EB", {"EB", "MF", "LCL", "LCR"}},
    {"LCL", {"KL"}},
    {"LCR", {"KL"}},
    {"OFF", {"ON"}},
    {"ON", {"KL", "MF", "LCL", "LCR"}}
};
// KL : KEEL LANE
// MF : MATCH FRONT VEHICLE SPEED
// EB : EMERGENCY BREAK (for cars suddenly getting in my way)
// LCL & LCR : CHANGING LANES 
// OFF: THE INITIAL STATE
// ON: STARTING THE ENGINE. INITIALIZING

Trajectory Generator

Trajectory Generator takes the goal generated by the behavior planner, creates alternate goals, by perturbing the initial plan to create similar to the original plan, and then generate JMT (jerk minimized trajectories) polynomials that are evaluated against certain parameters, such as collision with other vehicles (in every point of the path), exceeding limits for velocity, acceleration, jerk and positions (s and d) and finally selecting the best one to be digitized and send back to the simulator

Discrete curves generator

Receives the polynomials from the Trajectory generator, creates a discrete set of points equidistant (as the simulator prescribes) and merges the curve with the curve send us back by the simulator, which are just the points that have not yet been consumed by simulator.

Our task here is to create a smooth curve preserving continuity not only in the graph, but also in the first order (velocity) and second order derivatives.

The final result (set of points) is sent back to the simulator

Thoughts on the project

The project had many small and big problems to solve. Some of the early decisions made when programming the project affect the way you can handle things later.

  • How to handle the Frenet to Cartesian conversion is one problem. One may choose to completely work on Cartesian coordinates, using the cars local coordinate system. The big benefit of this decision is that although you work on on linear paths, you are dealing with real lengths and real velocities, which the Frenet system hides from you

  • I choose to work with a pre generated approximation (by splines) of the track, but also a "on the fly" generation of a local coordinate system is possible (and actually implemented). I choose to keep in the final code the one that is calculated once before the start of the simulation, off loading the planner from the extra repeated calculation of splines.

  • My initial selection was to work on the goal state space consisting of (position, velocity, acceleration). In hindsight, I would choose to include the jerk component there too. The problem of leaving it out, is that although i can enforce continuity in acceleration values , there is no way to enforce continuity in derivatives of accelerations, so have no control on the magnitude of jerk.

  • The above choice forced me to require acceleration to be zero at goal endpoints, thus minimizing the danger of jerk, but loosing performance since the car was to start from zero acceleration, accelerate and move back to zero acceleration every cycle.

  • How you combine previous curve with next curve, was a very big problem for me. In the beginning I wanted to merge the two curves at a point of my choice, but enforcing continuity in derivatives is very hard. You can try smoothing the resulting curve, and nugging points to create a smooth result, but its very fragile and car easily be broken.

  • A possible solution to this was to create another JMT curve, joining the start and end state, but the big problem here is that there is no guarantee that this is actually a function (meaning, that at the edge of the road we can have points with the same X's and different Y's. Maybe its very rare, but I thought it was dangerous to spend much time there.

  • A viable solution would be to treat the problem as a Ipopt (interior position optimizer problem) and use a solver to do the work. It was a completely viable solution but I thought It was a bit like "cheating"

Limitation and possible enhancements

The project is far from perfect and can be improved in many ways. Some limitations are

  • Although it recognizes possible collisions (other cars running onto it) it does nothing about it. The main reason about that is , as mentioned above, the decision not to include the jerk in the goal state. As of that the perturbations cannot go wildly of the initial goal without breaking the jerk (and acceleration) restrictions.
  • There are some rare incidents of accidents happening in the opposite lane and cars ending on the other side of the road. Those cannot be avoided since there are no sensor fusion data for that side of the road.
  • The vehicle is very conservative in overtaking requiring the vehicle in the adjacent lane to be far enough to make the change. To improve that it needs another state to match or better the trailing vehicle's speed and of course let accelerations be non zero and end states for the car to have enough acceleration to overtake.

Dependencies

  • cmake >= 3.5 click here for installation instructions
  • make >= 4.1
  • gcc/g++ >= 5.4 (Linux & Mac)
  • Visual C++ (Windows)
  • uWebSockets v.14+
    • Run either install-mac.sh or install-ubuntu.sh. (Linux & Mac)
    • Install vcpkg and then install uWebsockets using 'vcpkg install uWebsockets`
    • If you need to run on a earlier webSockets version you need to change in the main file the pointer *ws to simply ws and change the three send commands from ws->send to ws.send
  • Udacity's Simulator. Latest version here.

Build Instructions

  • clone repository
  • Linux
    • mkdir build & cd build
    • cmake .. && make
  • Windows
    • Use Visual C++ compiler community edition (MinGW has some problems with uWebSockets)

simulator