Why does my weld path look fine at slow speed but drift when I run production speed?

Path accuracy degrades with speed. Slower motion gives the servos time to track the setpoints and keeps corner rounding small, so the TCP hugs the commanded line. Push the speed up and dynamic tracking error and corner blending both grow, so the path bows off where it was fine before.

If my robot's repeatability spec is tight, doesn't that guarantee good path accuracy?

No. Repeatability is measured at stopped taught points, not along a moving trajectory. A robot can return to its endpoints within a very tight scatter and still deviate from the commanded line between those points. They are separate properties.

Is path accuracy a single number I can look up in the datasheet?

Generally not the way repeatability is. Path deviation depends on speed, payload, reach into the workspace, and the shape of the path itself, so it isn't a single fixed figure. It's a behavior you characterize under your actual conditions.

Performance

Path Accuracy

Path accuracy is how closely a robot's actual TCP trajectory follows the commanded line or curve between programmed points, measured during continuous motion rather than at stops.

What it is

Path accuracy describes how faithfully the tool center point tracks the programmed geometry while it is moving. If you command a straight line from A to B, path accuracy is the deviation between the real path the TCP sweeps and that ideal line, sampled all the way along, not just at the endpoints.

It's a continuous-motion property. Point-to-point positioning accuracy asks whether the robot lands on the right spot when it stops; path accuracy asks whether it stays on the right route the whole way there. A robot can hit its endpoints well and still bow off the commanded line in between.

How it actually works

The controller interpolates the commanded path into a stream of setpoints and each axis servo chases them. During continuous motion the joints trade off dynamic loads, servo lag, and the geometry of the arm, so the real TCP tends to fall inside or off the commanded curve. Higher speed makes it worse: at a corner between two segments the robot rounds through rather than snapping the vertex, and the amount of rounding grows with speed and with how aggressively the blend is tuned.

This is why a seam that looks perfect at jog speed can drift at production speed. The interpolation is the same; the tracking error under dynamic load is not. Path accuracy has no single fixed millimeter value the way a robot's repeatability spec does, because it depends on speed, payload, reach, and the path itself.

How it differs

Repeatability · Repeatability measures how tightly a robot returns to the same taught point over many cycles, a scatter around a stopped position. Path accuracy is about the continuous route between points while moving. A robot can be extremely repeatable at its endpoints and still track a curved path loosely at speed.
Positioning accuracy · Positioning accuracy is the endpoint question: does the TCP reach the commanded coordinate when it stops. Path accuracy is the in-between question: does the TCP stay on the commanded line while it travels. They are measured differently and can fail independently.

Where you meet it in the field

Arc welding · Bead quality depends on the torch tracking the programmed weld path at travel speed; path deviation shows up as an inconsistent bead or missed seam.
Laser welding · The beam has to stay on the seam while the robot moves, so path deviation at speed pulls the focal spot off the joint.

Common questions

Why does my weld path look fine at slow speed but drift when I run production speed?: Path accuracy degrades with speed. Slower motion gives the servos time to track the setpoints and keeps corner rounding small, so the TCP hugs the commanded line. Push the speed up and dynamic tracking error and corner blending both grow, so the path bows off where it was fine before.
If my robot's repeatability spec is tight, doesn't that guarantee good path accuracy?: No. Repeatability is measured at stopped taught points, not along a moving trajectory. A robot can return to its endpoints within a very tight scatter and still deviate from the commanded line between those points. They are separate properties.
Is path accuracy a single number I can look up in the datasheet?: Generally not the way repeatability is. Path deviation depends on speed, payload, reach into the workspace, and the shape of the path itself, so it isn't a single fixed figure. It's a behavior you characterize under your actual conditions.