Ask most plants whether their robots are precise and the answer is yes. They repeat the same move within 0.1 mm, often well below it. But repeatability only means the robot returns to the same spot. It does not mean that spot is correct relative to the robot base or the part.
An off-the-shelf industrial robot typically has an absolute accuracy of several millimeters or worse. Causes include manufacturing tolerances in every robot unit, incorrect joint zero-mastering, mechanical flexibility, and gear backlash. Automated calibration addresses this by measuring the robot and compensating for the errors.
The problem goes beyond the robot
A complete cell adds more sources of error. The end-effector may flex, and its working point (such as the tip of a drill) probably differs from the design intent. The fixture on the floor is rarely exactly where it was drawn. External axes and positioners make it worse.
Closing the metrological chain
To reach the highest absolute accuracy, you need to close the entire chain linking the tool center point (TCP) on one end to the point on the part on the other. This is where automated calibration helps. The robot moves to a variety of positions, a measurement is taken at each one, and software identifies the discrepancies and compensates for them. With the DynaCal robot calibration system, the robot, TCP, and fixture are calibrated in one step, and external axes or a positioner can be included.
Where it is used
Robot calibration for absolute accuracy is used in virtually every kind of robotic application except very low-accuracy ones such as painting. Typical uses include arc welding, waterjet cutting, drilling and routing, assembly, spot welding, and gauging.
What you gain
Less manual programming time
Better process quality
Standard robots reaching accuracies associated with more expensive equipment such as CNC machines and CMMs
Support for offline programming, cloning of robot cells, mirroring across lines, and robot swapping
Keeping it accurate
Calibration is generally done once at installation, though it can be repeated for maintenance or robot replacement. During production, in-line recovery tools such as AutoCal and AccuBeam can detect and correct accuracy issues promptly so the cell keeps the accuracy achieved at installation.
About Dynalog: Founded in Detroit in 1990, Dynalog provides robot calibration, measurement, and performance analysis systems used in more than 20 countries.