High-precision robots: What to do when absolute accuracy is poor?

Robots are virtually all the time extremely repeatable. Absolute accuracy, alternatively, usually leaves one thing to be desired. A staff on the Fraunhofer Institute for Manufacturing Engineering and Automation IPA is uncovering the explanations for this and serving to firms to realize extra environment friendly robotic techniques.

Repeatability and absolute accuracy are established parameters for assessing the standard and precision with which a robotic performs a selected process. If a robotic performs a selected process many occasions with virtually equivalent outcomes, this demonstrates superb repeatability. Many industrial robots obtain values within the vary of 0.1 mm or higher.

In distinction, absolute accuracy describes how precisely a robotic’s spatial coordinates are achieved in relation to its base system, no matter earlier actions. And right here, there may be usually nonetheless room for enchancment. Sometimes, values between 0.5 and 1.5 mm are achieved. This tendency towards inaccuracy isn’t an issue for some purposes, however it’s for others.

Disadvantages of inaccurate robots

It’s troublesome to make use of robots with low absolute accuracy in processes that aren’t repeated identically, resembling within the manufacturing of small batch sizes. When welding an meeting, for instance, it’s significantly necessary that the wire tip is positioned exactly within the heart of the weld seam. Actual positioning leads to high-quality weld seams that meet trade necessities.

Nevertheless, deviations of only a few tenths of a millimeter can considerably impair high quality and even result in rejections. Absolute accuracy isn’t solely necessary for robot-based welding, but in addition for purposes resembling choose and place, overspray-free portray, or robot-based drilling and deburring.

One other drawback of inaccurate robots turns into obvious in offline programming (OLP), which can be ceaselessly used within the processes talked about above. Right here, the robotic program is created in a simulation after which transferred to the true robotic. The benefit lies in the potential of the robotic sustaining operation whereas programming is carried out. Due to this fact, whereas the robotic is processing one part, the following part will be programmed in parallel.

But when absolute accuracy is inadequate, the created program can’t be used straight. As a substitute, time-consuming corrections are mandatory, resulting in undesirable machine downtime, which in flip impairs effectivity and productiveness.

The usually-achieved absolute accuracy of 0.5 to 1.5 mm is subsequently inadequate for advanced purposes and may restrict and even forestall the usage of robots in sure purposes. It additionally ceaselessly results in further work steps, such because the re-teaching, which reduces the effectivity of the whole manufacturing chain. In lots of industrial situations, an absolute accuracy that’s much like the repeatability is fascinating and even required.

Causes for a scarcity of absolute accuracy

Along with the accuracy measurements supplied in accordance with ISO 9283, firms repeatedly flip to Fraunhofer IPA with the query of how they’ll enhance the efficiency of their robots by way of precision. The causes of inaccuracies are manifold and will be divided into hardware-related, software-related, and externally associated causes.

{Hardware}-related causes could embrace the design of the robotic, resembling inferior encoders or gears, or inadequate rigidity, whether or not it’s an articulated arm, a SCARA robotic, or a gantry system. Poor calibration of the robotic is likely one of the principal causes for inaccuracy on the software program aspect, together with inadequate controller efficiency.

As well as, an growing variety of robotic techniques are moreover geared up with “cognitive” capabilities, significantly these associated to notion. These so-called closed-loop techniques work with sensors that detect the robotic’s setting, and their software program processes the measured information to regulate the robotic’s path accordingly. In such a system, poor sensor information, a low-performing controller, or suboptimal interplay between the elements within the robotic cell often trigger issues.

Lastly, exterior components resembling temperature and air strain, but in addition course of forces that happen, can have a destructive affect on absolute accuracy.

The measurements made by these units have an accuracy of +- 40 µm wherever within the working house. Supply: Fraunhofer IPA

Complete evaluation with new benchmark

If firms that use robots, in addition to the robotic producers themselves, are dissatisfied with absolutely the accuracy of their techniques, the whole utility ought to be examined and analyzed. As described above, a person investigation of the causes is important.

The automation staff at Fraunhofer IPA is a impartial mission companion that may conduct this root-cause evaluation in a methodical method primarily based on in depth skilled data and expertise.

In a typical mission, the staff first information the present scenario utilizing sensor expertise such because the Leica Absolute Tracker AT960 to create a strong database. The laser tracker information the positions of the robotic with submillimeter accuracy. Particular efficiency indicators are calculated from the recorded information, which can be utilized to find out the efficiency of the robotic system.

As well as, the traits of the robotic’s actions are examined, which additionally present beneficial insights. In an additional step, the specialists examine the reason for the undesirable habits. To do that, they look at each the {hardware} and software program of the robotic system.

As well as, the staff has developed a closed-loop benchmark particularly for the closed-loop robotic techniques talked about above. The benchmark is appropriate for robots from many producers. Within the benchmark, a reference-giving gadget, a “metronome,” is used to impress the robotic into sure behaviors, which permits conclusions to be drawn about its efficiency.

Whereas ISO 9283 solely focuses on the robotic itself to measure absolute accuracy, the benchmark developed can look at every thing that makes up a closed-loop managed course of or process. As an illustration, this consists of the efficiency of the whole robotic cell, together with the sensor expertise and the supposed course of.

The benchmark process was introduced on the 56th Worldwide Robotics Symposium (ISR 2023) and on the twenty first Worldwide Convention on Automation Expertise and Mechanical Engineering (CASE 2025).

With an correct robotic, the deliberate path will be executed with out handbook interplay on the true robotic. Supply: Fraunhofer IPA

Added worth of excessive absolute accuracy

A robotic with enough absolute accuracy cannot solely exchange dearer gear but in addition enhance flexibility and decrease rework. In lots of purposes, it subsequently is sensible to research anomalies and enhance absolute accuracy.

A robotic that works exactly and reliably provides financial benefits, in addition to will increase an organization’s competitiveness. Investing on this space is subsequently a strategic determination that pays off in the long run.

Concerning the writer

Martin Finkbeiner is a mission supervisor on the Fraunhofer Institute for Manufacturing Engineering and Automation IPA. He will be reached at martin.finkbeiner@ipa.fraunhofer.de and +49 711 9701046.

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