Linear axis for industrial robots

Industrial robots (IR) with a serial kinematic structure enable high flexibility in modern production processes while taking up little space. The combination of these serial kinematics with external linear axes results in versatile and adaptable machine concepts for manufacturing, even for the production of large components.

• high flexibility of the system thanks to the modular configurable linear axis
• less space required due to the smaller footprint of the robot and linear axis carriage
• increased productivity thanks to coordinated station changes and task sharing using several industrial robots on one linear axis
• fail-safe, as individual robots can be maintained or replaced independently without affecting the functionality of the other robots
• lower investment volume

At the Fraunhofer IFAM in Stade, these system concepts are used in various process steps in the production of large components. They include, for example, machining, assembly, joining, application as well as metrology processes. In order to meet the high accuracy requirements of these processes, our expertise includes hardware and software developments to improve the properties of the “linear axis with industrial robot” system concept.

Calibration of geometric errors

For the calibration of the linear axis, methods have been developed to determine and compensate for deviations from linear models. Conventional control systems usually offer the option of describing the external additional axis with a 3D vector. For example, cubic B-splines can be used to capture further deviations in order to approximate the interpolation of the measured deviations to the linear model. Using these mathematical functions, different curves of data points can be described continuously and differentiable. In addition to the translational deviations, the rotational errors must also be taken into account, which can lead to large deviations due to the kinematically induced lever arms at the robot tool center point (TCP). With the use of 3D coordinate measuring technology, the Fraunhofer IFAM in Stade offers the possibility of measuring the error variables.

The calibration methods developed are not only limited to external robot axes, but are also suitable for increasing the accuracy of linear drive axes in machine tools and large special machines.

Hardware and software optimization of the linear axis for application 

As part of further optimization options for linear feed axes, various approaches were pursued to increase the performance and accuracy of the linear axis.

• By using two preloaded rack and pinion drives, reversal effects can be effectively compensated and this contributes to increasing the drive stiffness of the linear axis carriage. This is crucial for ensuring accurate robot processes.
• The integration of sensors on the output side significantly improves the absolute accuracy of the overall system. Recording the actual position on the load side enables the compensation of errors caused by the gear ratio.
• Another focus is on the structural rigidity of the linear axis. The serial design of industrial robots results in large lever arms between the linear axis carriage and the load application point. This geometry not only leads to increased susceptibility to inaccuracies, but also to changing torque loads caused by the effects of gravity. A suitably rigid design of the linear axis is able to minimize the influence of these loads to such an extent that they can be considered negligible.
• As an alternative to mechanical optimization, it is possible to analyse and compensate for the errors that occur by means of measurement or model-based compensation. In this case, error influences in the direction of travel of the linear axis – such as reversal effects, transmission errors or static friction influences – can be compensated directly by the linear feed axis. Error types that have an effect in several spatial directions and lead to changes in the orientation of the linear axis carriage require compensation by the mounted industrial robot and are part of the “CaliRob” software developed at Fraunhofer IFAM.

Path-accurate industrial robots for large components using external linear axis

Fraunhofer IFAM has extensive expertise in this area, not only in the development of high-precision linear axes but also in their individual calibration in order to maximize the performance and accuracy of the linear feed axis and thus the overall system with industrial robots. In addition, calibration of the linear axis can be combined with robot calibration to ensure a holistic understanding of the system and optimum functionality. The aim is to significantly increase the efficiency and reliability of the systems through customized solutions and well-founded analyses. The industrially usable software “CaliRob” was developed at the Fraunhofer IFAM in Stade for this purpose. With the help of model-based error compensation, it is possible to optimize existing systems cost-effectively and increase their accuracy.

The combination of mechanical and software improvements creates a solid basis for the reliable and precise machining of large components in industrial applications, for example in the production of aircraft, rail vehicles, ships, commercial vehicles and wind turbines.

These methods enable an additional increase in the accuracy and robustness of the system, which further enhances efficiency in the machining of large components. In the RoMaNi 2 R&D project, path accuracies of 0.15 millimeters were achieved for large components of up to 8 meters.