Robot calibration

Robot calibration is the process of identifying the real geometrical parameters in the kinematic structure of an industrial robot, i.e., the relative position and orientation of links and joints in the robot.

A calibrated robot has a higher absolute positioning accuracy than an uncalibrated one, i.e., the real position of the robot end effector corresponds better to the position calculated from the mathematical model of the robot. Absolute positioning accuracy is particularly releveant in connection with robot exchangability and off-line programming of precision applications. Besides the calibration of the robot, the calibration of its tools and the workpieces it works with (the so-called cell calibration) can minimize occurring inaccuracies and improve process security.

Measurement systems

There are several options for calibrating and monitoring industrial robots:

• touching reference parts
• theodolites
• callipers
• ultrasonic distance sensors
• laser triangulation

ISO standards for accuracy

The international standard ISO 9283 sets different performance criteria for industrial robotics and suggests test procedures in order to achieve the correct parameters. The most important criteria are accuracy of pose (AP) and repeatability of pose (RP).

Repeatability is particularly important when the robot is moved towards the command positions manually. If the robot program is generated by a 3D simulation, absolute accuracy is vital here as well. It can be negatively influenced by kinematic factors, as well as joint offsets and deviations in the lengths and angles of each robot link.

Kinematic modeling of robots

The robot errors detected by position measurement can be minimized by numerical optimization. A complete kinematic model of the geometric structure can be developed, whose parameters then can be calculated by mathematical optimization. The common system behaviour can be described with the vector model function as well as input and output vectors.

Sample applications

In industry there is a general trend towards substitution of machine tools and special machines by industrial robots for certain manufacturing tasks whose accuracy demands can be fulfilled by calibrated robots. In-line measurement in automotive manufacturing is an example, where industrial robots carry sensors to measure the car's unibody shell. Such as measuring station can be used for multiple models, simply by changing the parameters loaded into the software.

Further Examples for precision applications are robot-guided welding in car body manufacturing; assembly of mobile phones and other electronic devices; drilling, riveting and milling in aerospace industry; and an increasing number of medical applications.