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Increasing accuracy through optimized robot calibration

Precision applications, 3D printing, or the assembly of miniature electronics sometimes push robots to the limits of their accuracy. Mechanical improvements of high-precision robots are not enough – what is needed is additional optimization of the control and calibration systems. This article describes how a robot's accuracy of less than 0.1 millimeters can be achieved.

Robots: changing requirements

Since industrial robots made their debut in the 1950s, both the demands and the implementation have changed significantly. Originally, their tasks were heavily focused on repetition, with only low requirements regarding accuracy. Lightweight construction, electric servo drives, gearbox design, controllers, and improved path planning have had a positive impact on accuracy so that today, path deviations of less than 1 millimeter are no longer unusual.

However, some applications today demand path accuracies of less than 0.1 millimeters, such as:

  • Laser welding and cutting
  • Precision coating and inkjet technology
  • 3D printing
  • Assembly of miniature electronics or aircraft.

When looking to increase robot accuracy, engineers for the longest time focused on the mechanical systems in order to get as close as possible to the ideal, design model. Today, the focus has shifted to increased computational capacity and more precise measurement technology for improving control: The actual properties of the mechanical systems need to be detected so that the model can be expanded to include the deviations from the ideal. When it comes to series production, in particular, it pays off to keep the focus on robot calibration and compensation functions.

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Robot accuracy: causes and factors

The creation of a reliable compensation model initially requires engineers to identify the dominant factors precisely. The more effects that need to be taken into consideration, the greater the effort required for the calibration and validation of the compensation functions.

The most important factors are:

  • Robot geometry
  • Effects of the gearbox, such as transmission, elasticity, backlash on reversal, hysteresis, or gear friction
  • Limited stiffness due to joint elasticity and bearing errors
  • Servo errors
  • Vibration
  • Higher-order errors.

The drawback of a complex compensation model with many parameters is that it may become unreliable. Each additional parameter entails a new dimension of possibilities and dependencies. For this reason, the measurements must include independent variations of all these dimensions in order to reliably identify the effect of each parameter.

White Paper: Improving Robot Accuracy

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Nabtesco & KEBA: the most precise industrial robot in the world

KEBA’s KeMotion Robot Control software has long included functionality for the calibration of robots and takes into account not just the geometric parameters but also factors such as gearbox elasticity.

The cooperation between Nabtesco and KEBA now contributes gear-specific data and characteristics to the robot controller. Cycloid gearbox specialist Nabtesco is the first gearbox manufacturer in the robotics sector to cooperate with a maker of control systems.

Their common goal: industrial robots with the greatest accuracy available on the market. Because gearbox effects have a significant influence on the path accuracy of robots.

Robot calibration with KEBA software

The software calibration is supported by the creation of robot programs needed for the measurement process. The results can be imported into the robot configuration, avoiding problems with parameter conventions and conversions.

The KeMotion dynamic model for serial and parallel robots contains precise friction models that permit a high accuracy of the robot motion even at low speeds and with directional changes.

KEBA software also supports robot validation. This requires knowing the causes of the deviations. For example, the use of a laser tracker with a real-time interface makes it possible to perform dynamic measurements and accuracy analyses.

The analysis can then form the basis for further improvement steps, be it enhanced calibration or the compensation of mechanical properties, improved servo control or even pointers to where the mechanical systems need to be refined.

For more details on the important aspects of “robot accuracy and robot calibration”, check out our white paper. Download it here:

White Paper: Improving Robot Accuracy

Download now

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