Performance
Repeatability
Repeatability is how consistently a robot returns its tool to the same point when commanded to the same pose again and again, expressed as a tight ± band in millimeters.
What it is
Repeatability describes consistency of returns. Send a robot to the same taught position a hundred times and measure where the tool actually lands each time. The spread of those landings is the repeatability figure, usually quoted as a ± value like ±0.02 mm.
It captures how tightly the arm clusters around wherever it decided that taught point is, regardless of whether that is the point you actually meant. A small, tight cluster is good repeatability, even if the whole cluster sits a few millimeters off the true target.
How it actually works
The number comes from returning to a single taught pose repeatedly and measuring the scatter of the tool center point across those returns. The FANUC LR Mate 200iD, for example, is published at ±0.01 mm. That means returns land inside a sphere roughly 0.02 mm across, smaller than the width of a few sheets of paper.
Repeatability holds up because the robot uses the same encoder counts, the same joint angles, and the same mechanical path to reach a stored point. It reproduces a recorded joint state rather than solving fresh geometry on each move. Backlash, thermal drift, and payload flex are what widen that scatter band, which is why specs are quoted at a rated load and stable temperature.
How it differs
- Accuracy · Repeatability is how tightly the robot clusters its returns to the same taught point. Accuracy is how close the robot gets to the commanded or true position in space. A robot can be extremely repeatable and still inaccurate, consistently landing on the same wrong spot, which is why arms are specced primarily on repeatability.
Where you meet it in the field
- FANUC LR Mate 200iD · A compact arm published at ±0.01 mm repeatability, the figure worked through above.
- Arc welding · Bead consistency along a taught seam leans on the arm returning to the same path pass after pass.
Common questions
- Does better repeatability mean the robot is more accurate?
- No. They are separate specs. Repeatability measures how consistently the arm returns to the same taught point; accuracy measures how close it gets to the true point in space. An arm can repeat to ±0.02 mm and still be several millimeters off the coordinate you actually asked for.
- Why does my repeatable robot start drifting during a shift?
- The published figure assumes stable conditions. As joints warm up, dimensions shift slightly with temperature, and payload flex or wear in the drivetrain widens the scatter band. This is why repeatability is quoted at a rated load and why some cells run a warm-up cycle before precision work.
- Is repeatability the same as the position error the controller reports?
- No. Repeatability is a manufacturing spec measured by repeated physical returns to one pose. The position error a controller tracks in real time is a servo following figure, related in spirit but measured and reported differently.