Joint move or linear move
Two ways between the same two points, and only one of them is a straight line.
§ 1Two ways between the same two points, and only one of them is a straight line
When you command a robot arm to move from one recorded pose to the next, you must also choose how it gets there. The choice is not cosmetic — it determines whether the tool follows a predictable path or a mathematically convenient one.
A joint move (often labelled J or MoveJ in controller syntax) tells each axis to rotate at a scaled rate so that all six joints start and finish together. The controller does not care about the shape the tool traces through space. The path curves wherever the kinematics dictate, and that shape changes if you alter the start pose. Joint moves are fast, smooth, and computationally cheap. They are the right call for travelling between poses where nothing in between matters — picking up from a fixed nest, retreating to a home position, or repositioning over a clear workspace.
A linear move (L or MoveL) commands the tool centre point to travel in a straight Cartesian line between the two poses, while simultaneously interpolating the tool orientation. The controller works backwards from that constraint, solving inverse kinematics at every interpolation step to find the joint angles that put the TCP exactly on that line. The path is predictable and repeatable in space, not just at the endpoints.
That predictability has a cost. Because the controller must keep the TCP on a line, it can be forced into joint configurations that approach awkward territory — most acutely near singular poses, where the maths becomes unstable and the arm may decelerate sharply or fault out. A joint move navigates around those configurations naturally; a linear move can drive straight into them.
The practical rule is straightforward. Use a linear move whenever the tool must travel a defined path: welding a seam, dispensing adhesive, cutting, or deburring. The line is the process. Use a joint move for everything else, because it is faster, more robust, and puts less stress on the joints.
Where this matters most is near fixtures and clamps. A joint move that looked clear in simulation can arc unexpectedly close to tooling if the start pose shifts. Dry-run at reduced speed before committing — the path you see then is the path you get in production.
Joint moves are fast, smooth, and computationally cheap.
