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Version 1.0.7
License Apache-2.0
Build type AMENT_CMAKE
Use RECOMMENDED

Repository Summary

Checkout URI https://github.com/ros-planning/navigation2.git
VCS Type git
VCS Version galactic
Last Updated 2021-09-15
Dev Status DEVELOPED
CI status No Continuous Integration
Released RELEASED
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Package Description

Smac global planning plugin: A*, Hybrid-A*, State Lattice

Additional Links

No additional links.

Maintainers

  • Steve Macenski

Authors

No additional authors.

Smac Planner

The SmacPlanner is a plugin for the Nav2 Planner server. It includes currently 2 distinct plugins: - SmacPlannerHybrid: a highly optimized fully reconfigurable Hybrid-A* implementation supporting Dubin and Reeds-Shepp models (ackermann and car models). - SmacPlanner2D: a highly optimized fully reconfigurable grid-based A* implementation supporting Moore and Von Neumann models.

It also introduces the following basic building blocks: - CostmapDownsampler: A library to take in a costmap object and downsample it to another resolution. - AStar: A generic and highly optimized A* template library used by the planning plugins to search. Template implementations are provided for grid-A*, SE2 Hybrid-A* planning. Additional template for 3D planning also could be made available. - CollisionChecker: Collision check based on a robot's radius or footprint. - Smoother: A path smoother to smooth out 2D, Hybrid-A* paths.

We have users reporting using this on: - Delivery robots - Industrial robots

Introduction

The nav2_smac_planner package contains an optimized templated A* search algorithm used to create multiple A*-based planners for multiple types of robot platforms. It was built by Steve Macenski while at Samsung Research. We support circular differential-drive and omni-directional drive robots using the SmacPlanner2D planner which implements a cost-aware A* planner. We support cars, car-like, and ackermann vehicles using the SmacPlannerHybrid plugin which implements a Hybrid-A* planner. These plugins are also useful for curvature constrained planning, like when planning robot at high speeds to make sure they don't flip over or otherwise skid out of control. It is also applicable to non-round robots (such as large rectangular or arbitrary shaped robots of differential/omnidirectional drivetrains) that need pose-based collision checking.

The SmacPlannerHybrid implements the Hybrid-A* planner as proposed in Practical Search Techniques in Path Planning for Autonomous Driving, including hybrid searching, gradient descent smoothing, analytic expansions and hueristic functions.

In summary...

The SmacPlannerHybrid is designed to work with: - Ackermann, car, and car-like robots - High speed or curvature constrained robots (as to not flip over, skid, or dump load at high speeds) - Arbitrary shaped, non-circular differential or omnidirectional robots requiring SE2 collision checking with constrained curvatures

The SmacPlanner2D is designed to work with: - Circular, differential or omnidirectional robots - Relatively small robots with respect to environment size (e.g. RC car in a hallway or large robot in a convention center) that can be approximated by circular footprint.

Features

We further improve in the following ways: - Remove need for upsampling by searching with 10x smaller motion primitives (same as their upsampling ratio). - Multi-resolution search allowing planning to occur at a coarser resolution for wider spaces (O(N^2) faster). - Cost-aware penalty functions in search resulting in far smoother plans (further reducing requirement to smooth). - Gradient descent smoother - Faster planning than original paper by highly optimizing the template A* algorithm. - Faster planning via precomputed heuristic, motion primitive, and other values. - Automatically adjusted search motion model sizes by motion model, costmap resolution, and bin sizing. - Closest path on approach within tolerance if exact path cannot be found or in invalid space. - Multi-model hybrid searching including Dubin and Reeds-Shepp models. More models may be trivially added. - High unit and integration test coverage, doxygen documentation. - Uses modern C++14 language features and individual components are easily reusable. - Speed optimizations: no data structure graph lookups in main loop, near-zero copy main loop, dynamically generated graph and dynamic programming-based obstacle heuristic, optional recomputation of heuristics for subsiquent planning requests of the same goal, etc. - Templated Nodes and A* implementation to support additional robot extensions. - Selective re-evaluation of the obstacle heuristic per goal/map or each iteration, which can speed up subsiquent replanning 20x or more.

All of these features (multi-resolution, models, smoother, etc) are also available in the SmacPlanner2D plugin.

The 2D A* implementation also does not have any of the weird artifacts introduced by the gradient wavefront-based 2D A* implementation in the NavFn Planner. While this 2D A* planner is slightly slower, I believe it's well worth the increased quality in paths. Though the SmacPlanner2D is grid-based, any reasonable local trajectory planner - including those supported by Nav2 - will not have any issue with grid-based plans.

Note: In prior releases, a CG smoother largely implementing the original Hybrid-A* paper's. However, this smoother failed to consistently provide useful results, took too much compute time, and was deprecated. While smoothing a path 95% of the time seems like a "good" solution, we need something far more reliable for practical use. Since we are working with mobile robots and not autonomous cars at 60 mph, we can take some different liberties in smoothing knowing that our local trajectory planners are pretty smart. If you are looking for it, it now lives in the test/ directory in a depreciated folder. This smoother has been replaced by a B-Spline inspired solution which is faster, far more consistent, and simpler to understand. While this smoother is not cost-aware, we have added cost-aware penalty functions in the planners themselves to push the plans away from high-cost spaces and we do check for validity of smoothed sections to ensure feasibility.

Metrics

The original Hybrid-A* implementation boasted planning times of 50-300ms for planning across 102,400 cell maps with 72 angular bins. We see much faster results in our evaluations:

  • 2-20ms for planning across 147,456 (1.4x larger) cell maps with 72 angular bins.
  • 30-120ms for planning across 344,128 (3.3x larger) cell map with 72 angular bins.

For example, the following path (roughly 85 meters) path took 33ms to compute.

alt text

Design

The basic design centralizes a templated A* implementation that handles the search of a graph of nodes. The implementation is templated by the nodes, NodeT, which contain the methods needed to compute the hueristics, travel costs, and search neighborhoods. The outcome of this design is then a standard A* implementation that can be used to traverse any type of graph as long as a node template can be created for it.

We provide by default a 2D node template (Node2D) which does 2D grid-search with either 4 or 8-connected neighborhoods, but the smoother smooths it out on turns. We also provide a Hybrid A* node template (NodeHybrid) which does SE2 (X, Y, theta) search and collision checking on Dubin or Reeds-Shepp motion models. Additional templates could be easily made and included for 3D grid search and non-grid base searching like routing.

In the ROS2 facing plugin, we take in the global goals and pre-process the data to feed into the templated A* used. This includes processing any requests to downsample the costmap to another resolution to speed up search and smoothing the resulting A* path. For the SmacPlannerHybrid plugin, the path is promised to be kinematically feasible due to the kinematically valid models used in branching search. The 2D A* is also promised to be feasible for differential and omni-directional robots.

We isolated the A*, costmap downsampler, smoother, and Node template objects from ROS2 to allow them to be easily testable independently of ROS or the planner. The only place ROS is used is in the planner plugins themselves.

Parameters

See inline description of parameters in the SmacPlanner. This includes comments as specific parameters apply to SmacPlanner2D and SmacPlanner in place.

planner_server:
  ros__parameters:
    planner_plugins: ["GridBased"]
    use_sim_time: True

    GridBased:
      plugin: "nav2_smac_planner/SmacPlanner"
      tolerance: 0.5                      # tolerance for planning if unable to reach exact pose, in meters, for 2D node
      downsample_costmap: false           # whether or not to downsample the map
      downsampling_factor: 1              # multiplier for the resolution of the costmap layer (e.g. 2 on a 5cm costmap would be 10cm)
      allow_unknown: false                # allow traveling in unknown space
      max_iterations: 1000000             # maximum total iterations to search for before failing (in case unreachable), set to -1 to disable
      max_on_approach_iterations: 1000    # maximum number of iterations to attempt to reach goal once in tolerance, 2D only
      max_planning_time: 3.5              # max time in s for planner to plan, smooth, and upsample. Will scale maximum smoothing and upsampling times based on remaining time after planning.
      motion_model_for_search: "DUBIN"    # 2D Moore, Von Neumann; Hybrid Dubin, Redds-Shepp; State Lattice set internally
      cost_travel_multiplier: 2.0         # For 2D: Cost multiplier to apply to search to steer away from high cost areas. Larger values will place in the center of aisles more exactly (if non-`FREE` cost potential field exists) but take slightly longer to compute. To optimize for speed, a value of 1.0 is reasonable. A reasonable tradeoff value is 2.0. A value of 0.0 effective disables steering away from obstacles and acts like a naive binary search A*.
      angle_quantization_bins: 64         # For Hybrid nodes: Number of angle bins for search, must be 1 for 2D node (no angle search)
      minimum_turning_radius: 0.40        # For Hybrid nodes: minimum turning radius in m of path / vehicle
      angle_quantization_bins: 64         # For Hybrid/Lattice nodes: Number of angle bins for search, must be 1 for 2D node (no angle search)
      analytic_expansion_ratio: 3.5       # For Hybrid/Lattice nodes: The ratio to attempt analytic expansions during search for final approach.
      minimum_turning_radius: 0.40        # For Hybrid/Lattice nodes: minimum turning radius in m of path / vehicle
      reverse_penalty: 2.1                # For Reeds-Shepp model: penalty to apply if motion is reversing, must be => 1
      change_penalty: 0.15                # For Hybrid nodes: penalty to apply if motion is changing directions, must be >= 0
      non_straight_penalty: 1.50          # For Hybrid nodes: penalty to apply if motion is non-straight, must be => 1
      cost_penalty: 1.7                   # For Hybrid nodes: penalty to apply to higher cost areas when adding into the obstacle map dynamic programming distance expansion heuristic. This drives the robot more towards the center of passages. A value between 1.3 - 3.5 is reasonable.
      lookup_table_size: 20               # For Hybrid nodes: Size of the dubin/reeds-sheep distance window to cache, in meters.
      cache_obstacle_heuristic: True      # For Hybrid nodes: Cache the obstacle map dynamic programming distance expansion heuristic between subsiquent replannings of the same goal location. Dramatically speeds up replanning performance (40x) if costmap is largely static.     
      smoother:
        max_iterations: 1000
        w_smooth: 0.3
        w_data: 0.2
        tolerance: 1e-10

Topics

Topic Type
unsmoothed_path nav_msgs/Path

Install

sudo apt-get install ros-<ros2-distro>-smac-planner

Etc (Important Side Notes)

Potential Fields

Many users and default navigation configuration files I find are really missing the point of the inflation layer. While it's true that you can simply inflate a small radius around the walls, the true value of the inflation layer is creating a consistent potential field around the entire map.

Some of the most popular tuning guides for Navigation / Nav2 even call this out specifically that there's substantial benefit to creating a gentle potential field across the width of the map - after inscribed costs are applied - yet very few users do this.

This habit actually results in paths produced by NavFn, Global Planner, and now SmacPlanner to be somewhat suboptimal. They really want to look for a smooth potential field rather than wide open 0-cost spaces in order to stay in the middle of spaces and deal with close-by moving obstacles better.

So it is my recommendation in using this package, as well as all other cost-aware search planners available in ROS, to increase your inflation layer cost scale in order to adequately produce a smooth potential across the entire map. For very large open spaces, its fine to have 0-cost areas in the middle, but for halls, aisles, and similar; please create a smooth potential to provide the best performance.

Hybrid-A* Turning Radius'

A very reasonable and logical assumption would be to set the minimum_turning_radius to the kinematic limits of your vehicle. For an ackermann car, that's a physical quantity; while for differential or omni robots, its a bit of a dance around what kind of turns you'd like your robot to be able to make. Obviously setting this to something insanely small (like 20 cm) means you have alot of options, but also probably means the raw output plans won't be very straight and smooth when you have 2+ meter wide aisles to work in.

I assert that you should also consider the environment you operate within when setting this. While you should absolutely not set this to be any smaller than the actual limits of your vehicle, there are some useful side effects of increasing this value in practical use. If you work in an area wider than the turning circle of your robot, you have some options that will ultimately improve the performance of the planner (in terms of CPU and compute time) as well as generate paths that are more "smooth" directly out of the planner -- not requiring any explicit path smoothing.

By default, 0.4m is the setting which I think is "reasonable" for the smaller scale industrial grade robots (think Simbe, the small Fetch, or Locus robots) resulting in faster plans and less "wobbly" motions that do not require post-smoothing -- further improving CPU performance. I selected 0.4m as a trade off between practical robots mentioned above and hobbyist users with a tiny-little-turtlebot-3 which might still need to navigate around some smaller cavities.

Costmap Resolutions

We provide for the Hybrid-A* and 2D A* implementations a costmap downsampler option. This can be incredible beneficial when planning very long paths in larger spaces. The motion models for SE2 planning and neighborhood search in 2D planning is proportional to the costmap resolution. By downsampling it, you can N^2 reduce the number of expansions required to achieve a particular goal. However, the lower the resolution, the larger small obstacles appear and you won't be able to get super close to obstacles. This is a trade-off to make and test. Some numbers I've seen are 2-4x drops in planning CPU time for a 2-3x downsample rate. For long and complex paths, I was able to get it << 100ms at only a 2x downsample rate from a plan that otherwise took upward of 400ms.

I recommend users using a 5cm resolution costmap and playing with the different values of downsampling rate until they achieve what they think is optimal performance (lowest number of expansions vs. necessity to achieve fine goal poses). Then, I would recommend to change the global costmap resolution to this new value. That way you don't own the compute of downsampling and maintaining a higher-resolution costmap that isn't used.

Remember, the global costmap is only there to provide an environment for the planner to work in. It is not there for human-viewing even if a more fine resolution costmap is more human "pleasing". If you use multiple planners in the planner server, then you will want to use the highest resolution for the most needed planner and then use the downsampler to downsample to the Hybrid-A* resolution.

No path found for clearly valid goals or long compute times

Before addressing the section below, make sure you have an appropriately set max iterations parameter. If you have a 1 km2 sized warehouse, clearly 5000 expansions will be insufficient. Try increasing this value if you're unable to achieve goals or disable it with the -1 value to see if you are now able to plan within a reasonable time period. If you still have issues, there is a secondary effect which could be happening that is good to be aware of.

In maps with small gaps or holes, you may see an issue planning to certain regions. If there are gaps small enough to be untraversible yet large enough that inflation doesn't close it up with inflated costs, then it is recommended to lightly touch up the map or increase your inflation to remove those spaces from non-lethal space.

Seeing the figures below, you'll see an attempt to plan into a "U" shaped region across the map. The first figure shows the small gap in the map (from an imperfect SLAM session) which is nearly traversible, but not quite. From the starting location, that gap yeilds the shortest path to the goal, so the heuristics will attempt to route the paths in that direction. However, it is not possible to actually pass with a kinematically valid path with the footprint set. As a result, the planner expands all of its maximum 1,000,000 iterations attempting to fit through it (visualized in red). If an infinite number of iterations were allowed, eventually a valid path would be found, but might take significant time.

By simply increasing the footprint (a bit hackier, the best solution is to edit the map to make this area impassible), then that gap is now properly blocked as un-navigable. In the second figure, you can see that the heuristics influence the expansion down a navigable route and is able to find a path in less than 10,000 iterations (or about 110ms). It is easy now!

As such, it is recommended if you have sparse SLAM maps, gaps or holes in your map, that you lightly post-process them to fill those gaps or increasing your footprint's padding or radius to make these areas invalid. Without it, it might waste expansions on this small corridor that: A) you dont want your robot actually using B) probably isnt actually valid and a SLAM artifact and C) if there's a more open space, you'd rather it use that.

One interesting thing to note from the second figure is that you see a number of expansions in open space. This is due to travel / heuristic values being so similar, tuning values of the penalty weights can have a decent impact there. The defaults are set as a good middle ground between large open spaces and confined aisles (environment specific tuning could be done to reduce the number of expansions for a specific map, speeding up the planner). The planner actually runs substantially faster the more confined the areas of search / environments are -- but still plenty fast for even wide open areas!

Sometimes visualizing the expansions is very useful to debug potential concerns (why does this goal take longer to compute, why can't I find a path, etc), should you on rare occasion run into an issue. The following snippet is what I used to visualize the expansion in the images above which may help you in future endevours.

// In createPath()
static auto node = std::make_shared<rclcpp::Node>("test");
static auto pub = node->create_publisher<geometry_msgs::msg::PoseArray>("expansions", 1);
geometry_msgs::msg::PoseArray msg;
geometry_msgs::msg::Pose msg_pose;
msg.header.stamp = node->now();
msg.header.frame_id = "map";

...

// Each time we expand a new node 
msg_pose.position.x = _costmap->getOriginX() + (current_node->pose.x * _costmap->getResolution());
msg_pose.position.y = _costmap->getOriginY() + (current_node->pose.y * _costmap->getResolution());
msg.poses.push_back(msg_pose);

... 

// On backtrace or failure
pub->publish(msg);

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