Calibration of parallel robots

kalib abb.1

The possible applications of a parallel robot depend on several characteristic features, including in particular accuracy. In this context, a distinction is made between two parameters: repeatability and absolute accuracy. Repeatability refers to the position and orientation deviations that occur when repeating the approach of learned layers. Absolute accuracy, on the other hand, is the distance between the target and actual positions. The latter is usually by an order of magnitude worse than the repeatability. The reasons for this are the inadequate internal control modeling of the robot. Since this modeling is a rigid-body model, the inaccuracies in the geometric parameters due to manufacturing tolerances and assembly errors are the main source of error. Accordingly, a procedure is necessary to correct the kinematic model stored in the controller and to identify the correct kinematic parameters. In this way, a significant reduction of the position and orientation deviation is achieved.

kalib abb.2

The Institute of Mechanism Theory, Machine Dynamics and Robotic has an optical measuring system from Metris with a measuring volume of 17 m3, a measuring accuracy of 60 μm and a measuring frequency of 1000 Hz, see figure 1, which allows a fast and reliable identification of the kinematic parameters, see figure 2.

In addition to measuring data from incremental encoders, the position and orientation of the working platform was measured. This redundant information is used to determine the deviations between the measured values and the calculated values, see Figure 2 If the non-linear correlation between the kinematic parameters and the deviations is formulated, an over-determined system of equations can be established by a sufficiently large number of measuring points. The solution of this equation system provides the actual parameters of the parallel robot.



Institute of Mechanism Theory, Machine Dynamics and Robotics

RWTH Aachen University

Eilfschornsteinstraße 18

52062 Aachen



Phone: +49 241 80 95546

Fax:      +49 241 80 92263


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