Motion Planning Algorithms

When you need this

Most users do not need to understand the planning algorithm. You define frames, obstacles, and constraints, then call Move. The planner handles the rest.

This page is useful when:

• You need to tune planning performance (timeout, resolution).
• You are debugging why the planner fails to find a path.
• You want to understand what the planner is doing under the hood.

The cBiRRT algorithm

Viam’s motion service uses cBiRRT (Constrained Bidirectional Rapidly-Exploring Random Tree), based on Berenson et al. 2009. This is the only planning algorithm in the builtin motion service. There is no configuration option to select a different algorithm.

How it works

cBiRRT grows two search trees simultaneously: one from the start configuration and one from the goal configuration. At each iteration, the algorithm:

1. Samples a random configuration in joint space.
2. Extends the nearest node in one tree toward the sample, checking constraints and collisions at each step.
3. Attempts to connect the two trees. If a connection is found, the path is complete.
4. Projects each new configuration onto the constraint manifold, ensuring constraints are satisfied throughout the path.

The bidirectional approach is significantly faster than growing a single tree, because both trees converge toward the middle simultaneously.

Why it works well for robot arms

• High-dimensional spaces: A 6-axis arm has 6 degrees of freedom. cBiRRT handles this efficiently through random sampling.
• Constraint satisfaction: The projection step ensures constraints (linear, orientation) are satisfied, not just checked.
• Bidirectional search: Finding a path in 6D space from both ends is much faster than searching from one end only.

What it does not do

• No optimal paths: cBiRRT finds a valid path, not the shortest or smoothest path. The builtin service applies path smoothing (default 30 iterations) after planning to improve the result.
• No dynamic replanning: The planner computes a full path before execution begins. It does not adjust the path during execution.

Tuning parameters

The planner accepts configuration through the motion service config and through per-request options. Most users do not need to change these.

Service-level config

These are set in the motion service configuration:

Planning defaults

These defaults are compiled into the builtin motion service:

When planning fails

If the planner returns an error (“no path found” or similar), consider:

1. Loosen constraints. Tight linear or orientation tolerances can make the constrained space too small to navigate.
2. Reduce obstacle sizes. Oversized obstacles leave no room for the arm to pass.
3. Check joint limits. If the kinematics file has joint limits tighter than the physical arm, the planner may not explore valid configurations.
4. Check the destination. The destination pose must be reachable and not inside an obstacle.
5. Increase timeout. For complex environments, the default 300-second timeout may not be enough, though this is rare.

What’s Next

• Configure Motion Constraints: the constraint types that cBiRRT enforces during planning.
• Define Obstacles: collision geometry that the planner routes around.
• Motion Service Configuration: full configuration reference.