SmacPlannerHybrid & Identical Poses: A Nav2 Navigation Bug?

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SmacPlannerHybrid & Identical Poses: A Nav2 Navigation Bug?

Hey guys! Ever run into a weird issue while navigating your robot with ROS2 Navigation2? I recently stumbled upon a head-scratcher: the SmacPlannerHybrid planner seems to choke when given a NavigateThroughPoses goal with two identical consecutive poses. Let's dive into this, shall we?

The Problem: SmacPlannerHybrid and Identical Poses

So, here's the deal. I was setting up my navigation stack using the SmacPlannerHybrid, a popular choice, and everything was humming along nicely. Then, I needed my robot to hit a series of waypoints, which meant using the NavigateThroughPoses action. This action takes a list of poses, and the robot should navigate through them. Simple enough, right?

Well, not quite. If, for some reason, two consecutive poses in my list happened to be identical (same x, y, orientation), the SmacPlannerHybrid would fail. It wouldn't generate a valid path, and I'd get a warning in the logs: "GridBased: failed to create plan, no valid path found." Yikes! That's not what we want, especially if these identical poses are legitimate, maybe to ensure the robot stops in the exact spot. This whole thing makes you wonder, is this a bug, a corner case, or something else entirely?

Let's break down the issue and what I've found, including steps to reproduce, what I expected, and what actually happened.

Reproducing the Issue

To make sure we're all on the same page, here's how you can replicate this issue:

  1. Set up your environment: Make sure you're running ROS2 Humble on Ubuntu 22.04. This is where I saw the problem, and based on the package versions, it seems pretty specific to this setup.
  2. Use the correct Hardware: The issue was found on a Computer with an Intel Core i9-14900K.
  3. Ensure you have the right ROS2 package versions: The tests were done on version 1.1.18-1jammy.20250915.225626. Double-check those versions to make sure we're comparing apples to apples.
  4. Launch Navigation2: Start your Navigation2 stack. Make sure you're using the SmacPlannerHybrid planner. I'm using Fast-RTPS for DDS implementation, but I don't think that's the root cause.
  5. Send the Goal: Now, here's the crucial part. Send a NavigateThroughPoses goal with two identical poses in a row. An example of what the goal message looks like is shown in the bug report above.
  6. Observe the behavior: You should see the planner failing, and that "GridBased: failed to create plan" warning in your terminal.

So, if you follow these steps, you should see the same behavior I did. And if you're like me, you probably expected something different. Let's see what that is.

Expected vs. Actual Behavior

What I Expected

Honestly, I expected the robot to navigate through all the poses, including the identical ones. The logic, to me, seemed straightforward: If the robot is already at the first pose, and then the next pose is the same as the current one, the robot should simply stay put or perhaps barely adjust its position. It's like saying "go to this spot, and then...stay at the spot." It is not a difficult request.

What Actually Happened

Instead, the SmacPlannerHybrid planner throws a fit and fails to create a valid path. I think it would be great if the planner would accept it. It makes you wonder if there's an implicit assumption in the planner's code that prevents it from handling consecutive identical poses gracefully. In this scenario, the expected behavior would be a successful navigation, perhaps with a slight pause at the identical pose. But the reality is the planner gets stuck.

Digging Deeper: Possible Causes and Workarounds

Why does this happen? Well, I can only guess because I haven't seen the code. But here are some potential reasons for this behavior:

  • Path Planning Algorithm: The SmacPlannerHybrid likely relies on a path planning algorithm that might struggle with zero-distance movements. Some algorithms are designed to find the shortest path between distinct points and don't explicitly handle the case where the points are the same.
  • Collision Checking: Another possibility is that the collision-checking process gets confused when two identical poses are provided. Perhaps the collision checker thinks that the robot is overlapping with itself, causing the planner to reject the path.
  • Simplification Logic: It's also possible that the planner includes some logic to simplify the path by removing redundant poses. If it's too aggressive about simplifying, it could be unintentionally discarding identical consecutive poses.

Workarounds (for now)

Okay, so what can you do to work around this issue? Here are a few ideas:

  • Avoid Consecutive Identical Poses: The easiest solution is to avoid sending consecutive identical poses to the NavigateThroughPoses action. You can preprocess your pose list to remove duplicates before sending it.
  • Introduce Small Offsets: If you need the robot to stay at a specific location, you can insert a very small offset between the poses. Just a tiny nudge in position or a slight rotation might be enough to satisfy the planner.
  • Consider a Different Planner (Navfn): While investigating this, I tested the same scenario with the Navfn planner, and it handled the identical poses without a hitch. If this is a blocker for your application, switching to Navfn might be a viable solution.
  • Custom Action: If you need to stay at a certain spot, and can't use these workarounds, you might need to create a custom action that does what you want. It might use some basic ROS commands to tell the robot to pause at a certain location. Then, you can put this logic between the other poses to get the behavior you are looking for.

Is This a Bug? Or a Feature? (or at least a corner case)

Is this a bug in SmacPlannerHybrid, or is this just a corner case that it wasn't designed to handle? I'm not entirely sure, but the behavior is inconsistent with the expected behavior of the action. It's probably worth opening an issue on the Navigation2 GitHub repository so the developers can take a look. If the planner can be improved to handle this scenario, that would make it more robust. But, it might also not be possible for some design reason. So let's see!

Conclusion: Navigating the Navigation Maze

So, there you have it, folks! The SmacPlannerHybrid planner and identical poses: a potential gotcha to watch out for when using NavigateThroughPoses. Understanding this behavior will help you debug navigation issues and design more robust robotic applications. Keep experimenting, keep learning, and don't be afraid to dig into the details. That is how you will solve this and other difficult issues.