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[Nav2] Request for Comment: Path Smoothing

Hi all,

It’s your friendly neighborhood navigator here. In a cross collaboration with RoboTech Vision, we’re looking at abstracting out the smoother in the Smac Planner into its own server and adding in a few different path smoothing options. This will enable users to take any path from any planner and request it to be smoothed in accordance with a set of smoothing algorithm plugins (analogous to the design of the controller and planner servers). This will mean we have now 4 primary task servers: Recovery, Smoothing, Planning, and Control.

We wanted your comments / questions / ideas / suggestions on the topic of path smoothing so we make sure to take into account all of the different criteria users might have on the subject. Below is a brief description of our current thoughts as a basis but we’d appreciate anything on this subject you care to share or suggest, including algorithm ideas, use-cases, or more.

We are currently looking at making an abstract nav2_smoother package containing an action server lifecycle node. This action server will expose a new SmoothPath action (defined roughly below) that can be called to smooth a path. This server will contain a map of named plugins that can be configured independently and utilized using the smoother_id parameter. The server will contain a costmap and footprint subscribers to the global costmap to use if cost information is valuable to your smoother plugin, given in the configuration stage of the lifecycle node.

#goal definition
nav_msgs/Path path #input path
string smoother_id #plugin to use
builtin_interfaces/Duration max_smoothing_duration #max time for smoothing
---
#result definition
nav_msgs/Path path #output smoothed path
builtin_interfaces/Duration smoothing_duration #how much time was spent smoothing
bool was_completed #did smoothing complete successfully in the time allotted
---
#feedback

While we haven’t discussed at length the API for the plugins, it would likely be something like:

  • void configure(costmap subscriber, footprint subscriber, tf, node, name): get parameters, setup the plugin
  • path_and_time smooth(path_and_time): do the major path smoothing action on the path within the time window provided. Returns smoothed path in used time struct.
  • activate / deactivate / etc: filling in lifecycle requirements

In terms of techniques, our current thinking is to support the current Smac Planner smoother which is a more basic optimization-based solution with collision checking to stop smoothing if it hits something. We will also add in an improved version of the old Smac Planner smoother which is a much more intelligent Ceres based cost-aware smoother that is curvature constained. While this definitely works better, also takes much longer to compute. So its up to a designer what they prefer but they each have their niche.

Other thoughts of methods would be spline-based solutions such as b-splines or bezier curves, as well as elastic-band based techniques. We do not have implementations of those at hand to plugin-ize but if there was serious desire for them over the optimization-based solutions, that would be something we would consider.

Thanks for your time, please let me know if you have any questions / comments / suggestions / requests so we can incorporate them in our design and review process for this work.

Happy smoothing,

Steve

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This separation would be very helpful for scenarios where we want to mix and match planners and different smoothers, or if we have an application for smoothing paths that may not come from a planner (e.g. teach-and-repeat).

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