Difference among motion planning, path planning, and trajectory generation?

原创已于 2025-04-17 11:36:17 修改 · 672 阅读

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于 2025-04-17 11:31:18 首次发布

tl;dr

Generally, motion planning and trajectory generation are kind of interchangeable.
Sometimes, trajectory generation can be viewed as a subclass of motion planning.
If motion planning is viewed as path planning, then trajectory generation is a bigger thing.

Sometimes the two words （motion planning and trajectory generation）are used interchangeably. However, there are some subtle nuances between motion planning and trajectory generation.

When talking about motion planning, it is planning motions for robots to move from point A to point B (such as for mobile robots, etc.) or pose A to pose B (such as for manipulators, etc.). In order to do so, a number of constraints need to be taken into account: collision avoidance, joint limits, velocity/acceleration limits, jerk limits, dynamic balance, torque bounds, and many more. In this sense, not only the the robots is considered but also its environment (e.g., to avoid collision, to how remain balanced). Considering this, motion planning is kind of trajectory generation with lots of constraints. They may be considered the same thing. So, yes, you can use them interchangeably.

When talking about trajectory generation, the scope can be narrower than that of motion planning. Often time, in trajectory generation, people really focus on generating a trajectories—with joint limits, velocity and acceleration constraints—and just that. One case I can think of which is called trajectory generation but not motion planning (or at least, that’s what I perceive) is as follows. Think of when we want to execute a trajectory on a manipulator. We send commands to the controller as a set of waypoints, i.e., discrete points (supposedly closed to one another) spread across the trajectory, often at a fixed interval equal to the controller’s sampling time. The controller then has to regenerate trajectory segments between two consecutive waypoints, such that manipulator reaches the next waypoint within the fixed time interval while staying within joint limits, velocity limits, and acceleration limits. However, the controller does not really consider even collision avoidance or anything else. That is called trajectory generation.

Sometimes, people may think of motion planning as path planning. That’s another thing since, strictly speaking, a path is not equal to a trajectory. A trajectory is a path and information of how to traverse the path with respect to time, a.k.a a velocity profile. Considering this, trajectory generation is kind of a bigger thing.