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Simulation-Driven Design of a 3D Printed, Pneumatically Actuated, Lightweight Robot

This is a guest post by Gabriel Dämmer, research associate at Festo and the Institute of Polymer Product Engineering at Johannes Kepler University.
Combining additive manufacturing (AM) with structural optimization can result in functionally integrated and stiff but lightweight structures. These attributes are especially desirable for lightweight robots because space is limited, and structural compliance and weight strongly affect system performance. The creation of printed robots requires suitable design strategies based on material sciences, additive manufacturing technologies, and structural optimization. However, not many examples of printed robots and consistent design strategies are available today.

With this in mind, the prototypic robot arm AM SCARA (Selective Compliance Assembly Robot Arm) was developed as a multidisciplinary research platform. The development of the robot covered the necessary steps from material characterization to the manufacturing and testing of the final parts. The aim of the project was not only to gain knowledge in distinct disciplines, but to improve additive manufacturing related development processes for upcoming generations of products in the automation industry. The additive manufacturing technologies applied in this project were PolyJet™ printing of elastomeric and thermosetting acrylates, selective laser sintering of thermoplastic polyamide, and selective laser melting of aluminum powder.
Exterior view of the AM SCARA. Source: Festo

Research and development was completed in a Ph.D. project based on cooperation between Festo AG & Co. KG, and the Institute of Polymer Product Engineering (IPPE) at Johannes Kepler University. Preliminary studies and PolyJet-printed parts resulted from the DIMAP[1] research project.
The AM SCARA is a four degree of freedom (4DOF) manipulator, entirely driven by pneumatic actuators. For control of the actuator torques, printed air channels in combination with piezo valves were integrated in the robot links. The links were manufactured by selective laser sintering and designed following a simulation-driven design approach with Altair software solutions. One of the technological highlights is the multi-material gripper with an integrated elastomeric rotary drive that was printed as one whole part using PolyJet™ technology by cirp[2]. A vacuum nozzle was integrated into the gripper and connected to four suction cups through printed air channels. The robot has a reach of 400 millimeters and payloads of more than one kilogram can be lifted.
Internal view of the AM SCARA. Source: Festo

Structural Optimization at the Center of Mechanics Development
The AM SCARA was designed from scratch because no comparable system existed at that time. Conceptual sketches considered the kinematics, sequence of assembly steps, and dimensions of the mechatronic sub systems like the piezo valve modules and electronics. The concept was translated into a computer-aided design (CAD) model and all design features were supplemented that would affect the structural behavior and definition of design space.
After importing the robot arm geometry into Altair Inspire™ and the gripper into Altair HyperMesh™ a finite element analysis (FEA) was run as a baseline for the optimization. Multiple load cases were defined, representing loads in a typical pick and place application. Extensive testing of the structural materials, SLS and PolyJet™, were performed at IPPE. This covered monotonic tensile tests with variations in temperature and strain rate, as well as creep and relaxation tests at different levels of stress and displacement.
Development process of the AM SCARA. Source: Festo

Based on the results, material models were calibrated and the application limits of the materials were determined. The formulation of the topology optimization problem contained the objective of minimum weighted compliance of the structures, constraints for symmetry, and the permissible volume fraction in the design space. Due to the layer-wise buildup of structures and easily removable support materials in the PolyJet™ and SLS Process, no manufacturing related constraints were formulated. Links were completely optimized in Altair Inspire and the gripper was optimized using Altair HyperMesh, Altair OptiStruct™ and Altair HyperView™. Each tool played a vital role in the process including pre-processing, solving/optimization and post-processing the results. A fully isotropic and linear elastic material model was used for the underlying FEA load cases which was in accordance with the results from material characterization for small strains.
The optimized structures were re-modeled using the PolyNurbs functionality in Altair Inspire. A re-analysis of the PolyNurbs geometry was performed and the structure was inspected for strain peaks. For design finalization, the geometry was re-imported into the CAD software and the final details, including printed threads, were added before the parts were built into real-world structures.
Current Applications and Future Research
The challenge of this project was to achieve a fully functional pneumatically actuated robot that was assembled mostly of additively manufactured parts. With this goal reached, a clear picture had been shown of which components of a pneumatic lightweight robot should be considered for additive manufacturing and which require further research. Experiences and data gained in this project enables to build functional prototypes for early stage testing of pneumatic-mechatronic systems.
An upcoming research project will cover comparisons of various additive and conventional technologies for the manufacturing of structural lightweight parts, and geometrically complex valve modules. In the future, the simulation-driven design framework could allow for the development of “tailored robots.” In this new class of additively manufactured robots, lightweight structures and actuation systems would be designed specifically for each customer application.
Experience the vision of tailored robots at formnext 2019.
Gabriel Dämmer is a research associate at Festo and the Institute of Polymer Product Engineering at Johannes Kepler University and specializes in simulation-driven design of printed polymeric structures. During his studies, Gabriel focused on polymer technology and product development and holds Mechanical Engineering degrees from the Esslingen University of Applied Sciences and University of Stuttgart, both located in Germany. During the EU-funded DIMAP project (2015-2018), Gabriel was responsible for the mechanical design and development of a PolyJet-printed lightweight robot. Currently he is working together with his colleagues on novel types of printed pneumatic actuators and functionally integrated lightweight structures.

[1] The DIMAP project was funded as part of the Horizon 2020 framework program for research and innovation under the grant agreement 685937. Project work was carried out between 2015 and 2018.
[2]cirp GmbH Römerstraße 8, 71296 Heimsheim, Germany.