Repeatability is a robot's ability to return to the same position repeatedly. Command the robot to go to a point. It goes there. Command it again. It goes to the same point again. The distance between the two positions is the repeatability error. It is usually measured in millimeters or microns.

Repeatability is different from accuracy. Accuracy is closeness to the commanded target. Repeatability is closeness of repeated movements to each other. A robot can be very repeatable but not accurate. It might consistently miss the target by the same offset. That is fine for many tasks. If the robot always misses by 2 millimeters to the left, you can adjust the target 2 millimeters to the right. Repeatability is what matters in manufacturing.

Factors affecting repeatability

• Backlash in gears and joints.
• Thermal expansion of links and components.
• Mechanical wear over time.
• Encoder resolution and calibration.
• Control loop tuning and settling time.

Industrial robots are extremely repeatable. A typical six-axis robot can repeat to within 0.05 millimeters. That is why they are used for welding and assembly. The robot does not need to know exactly where the part is. It just needs to go to the same place every time. Collaborative robots are slightly less repeatable because they use compliant joints for safety. That is a trade-off. Repeatability is measured by running the robot through a cycle hundreds of times and recording the positions. The spread of those positions is the repeatability. A robot with poor repeatability cannot do precise work. A robot with good repeatability can do the same task forever without variation.