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Spatial compliance measurement of a clamping table with integrated force sensors

Friedrich Christian, Ihlenfeldt Steffen

Journal of Machine Engineering

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2022

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Vol. 22, No. 1

70--83

EN

Force sensor integration into machine components is a promising approach to measure spatial process forces, especially, when regarding hexapod structures and kinematics. Rigid still-standing hexapod frameworks, such as clamping tables, are particular suitable for this approach, as no dynamic influences need to be taken into account within the measurement model and they allow a measurement in 6 degrees of freedom. On the other hand, the stiffness of rigid frameworks is reduced by force sensor integration significantly. In addition, many approaches apply joints or flexure hinges to reduced lateral forces and improve the measuring quality, which reduce the stiffness even more. In this contribution, the compliance of a clamping table with integrated force sensors and flexure hinges is determined by experimental measurements using a multiline laser interferometer, by analytic calculation, and by finite element simulation. In conclusion, the amount of stiffness reduction by force sensors and flexure hinges is quantified and different methods for compliance determination are compared.

2

Accuracy assessment of articulated industrial robots using the Extended- and the Loaded-Double-Ball-Bar

Marwitz Johann August, Theissen Nikolas Alexander, Gonzalez Monica Katherine, Friedrich Christian, Hellmich Arvid, Archenti Andreas, Ihlenfeldt Steffen

Journal of Machine Engineering

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2022

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Vol. 22, No. 2

80--98

EN

This research paper outlines the methodology and application of geometric and static accuracy assessment of articulated industrial robots using the Extended Double Ball Bar (EDBB) as well as the Loaded Double Ball Bar (LDBB). In a first experiment, the EDBB is used to assess the geometric accuracy of a Comau NJ-130 robot. Advanced measuring trajectories are investigated that regard poses or axes configurations, which maximize the error influences of individual robot components, and, in this manner, increase the sensitivity for a large number of individual error parameters. The developed error-sensitive trajectories are validated in experimental studies and compared to the circular trajectories according to ISO 203-4. Next, the LDBB is used to assess an ABB IRB6700 manipulator under quasi-static loads of up to 600 Newton using circular testing according to ISO 230-4. The stiffness is identified from the loaded circular trajectories. Then, the stiffness is used to perform a reverse calculation to identify the kinematic errors on the path deviations. The concept is validated in a case study of quasi-static loaded circular testing using the LDBB compared to a Leica AT960 laser tracker (LT).

3

Stiffness evaluation of a hexapod machine tool with integrated force sensors

Friedrich Christian, Kauschinger Bernd, Ihlenfeldt Steffen

Journal of Machine Engineering

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2020

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Vol. 20, No. 1

58--69

EN

Today, in-process force measurement is required by many manufacturing applications, such as process monitoring, quality assurance, or adaptive process control. A very promising force measurement approach bases on sensor-integration into the machine structure and is particularly suitable for hexapod structures and kinematics, where it allows a measurement in 6 degrees of freedom. On the other hand, a sensor integration also affects the machine. Especially for strain-gauge-based force sensors, a stiffness reduction is predicted, as their measuring principle requires a deformation. The practical consequences of these influences are investigated in this contribution. In particular, this work presents extensive experimental studies of the stiffness change caused by sensor integration for a single hexapod strut as well as for the complete hexapod machine tool. The results are evaluated in comparison to compliances of other components, such as the kinematic joints, and to stiffness changes resulting from sensor-integration into the end-effector or the application of a commercial force/torque sensor at the end-effector. In conclusion, the studies support the approach of structure-integrated force measurement for parallel kinematics, as the stiffness loss is rather small in many cases.

4

Fast evaluation of the volumetric motion accuracy of multi-axis machine tools using a Double-Ballbar

Kauschinger Bernd, Friedrich Christian, Zhou Ruiqing, Ihlenfeldt Steffen

Journal of Machine Engineering

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2020

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Vol. 20, No. 3

44--62

EN

The proof of manufacturing accuracy requires continuous verification and crosscheck of the motion accuracy of machine tools. Machining in 5 to 6 axes intensifies the problem of measurement and evaluation of volumetric motion accuracy in up to 6 degrees of freedom (DOF) in the whole workspace. Although, there are many known, even standardized, measuring methods, they are either expensive, time-consuming, not applicable in an operational state of the machine under shop floor conditions, or their significance is limited to only 1 or 2 feed-axes. Appropriate methods to be run regularly, fast and cost-efficient by the machine operator as a performance test are still desired. The article presents a new approach that meets these requirements. It is based on a Double-Ballbar (DBB) with enlarged measuring range and volumetric measuring paths of up to 6 DOF with all feed-axes moving simultaneously during continuous measurement, instead of plane circular paths according to ISO 230-4. After an explanation of the proposed method, the developed DBB device is introduced, including its mechanical and sensor design, the data interface, and results of experimental investigations on the measuring accuracy. Furthermore, relevant problems regarding the design, optimization, and programming of appropriate 6 DOF measuring paths are discussed and experimental results are presented that show the advantage compared to other measuring paths.