Tech Front - Getting a Better Grip with Robots
Industrial robots have made much progress in recent years, becoming more nimble and adding advanced technologies such as force sensing and 3D vision systems. But one area that has lagged in development is the ability of robots to pick up and grasp objects as easily as a dexterous human hand.
Tackling the difficult challenges of dexterous manipulation by robots was the aim of a workshop recently hosted by the National Institute of Standards and Technology (NIST; Gaithersburg, MD). Featuring speakers from manufacturing companies and developers of robot arms and advanced robot hands, or grippers, the NIST workshop attendees included representatives from Procter & Gamble, Boeing, Southwest Research Institute, Schunk, Robotiq, SynTouch, Kuka Robotics, Rethink Robotics and Yaskawa Motoman.
The proceedings of the workshop are summarized in a new NIST report entitled, "Dexterous Manipulation for Manufacturing Applications Workshop," by NIST authors Joseph A. Falco, Jeremy A. Marvel and Elena R. Messina. The workshop is being used as a basis for NIST developing a technology roadmap to guide measurement science research in the field.
Several factors currently driving the quest for improved robot arms and grippers are quality control, cost, throughput and worker safety. According to one manufacturer at the workshop, roughly two-thirds of his company’s worker compensation cases are ergonomic disorders, mostly due to repetitive strain, overextension and overexertion.
New robotic arms are starting to come in pairs that are mounted to either a fixed or rotary torso with each arm having seven joints instead of the conventional six, according to the NIST report. This option boosts the dexterity of a robot and allows it to move its elbow around obstacles while maintaining precise placement at its working point. Robots also have incorporated force sensing, a technology that introduces new assembly capabilities and can improve safety, so robots can operate close to workers.
Robotic grippers, manipulators and end effectors currently are largely single-purpose tools, designed to grasp parts with a specific shape. More capable, multiple-function grippers, often called robotic hands, are edging into the market, but they are a far cry from the universal gripper that is a holy grail of robotics research, according to NIST.
Both commercial and university researchers see promise for developing more highly dexterous robotic arms, the NIST report noted, but it remains to be seen whether these newly developed robotic hands can withstand harsh factory environments.
"Manufacturers face many challenges in implementing next-generation automation, especially when their production mix involves low volumes of parts with high mixtures of part types," according to the NIST report. "Small batches, which might include even batches of one, require quick turnover rates and the ability to reprogram a multiplicity of involved processes." These types of factory operations will require much more advanced, adaptive grippers that are, as one workshop attendee noted, "blind to shape."
The workshop’s discussions focused on the need for metrics and tests for judging the dexterity and overall performance of emerging robotic arm and hand technologies. Key areas of need include metrics for assessing reliability, ease of programming, reachability, energy consumption per payload, and coordination if multiple arms or hands are used, as well as testing methods. Workshop participants also recommended a framework to evaluate tasks and parts as a means to evaluate the effectiveness of candidate technologies, with competitions as an incentive to drive research and development.
For more information or to download a copy of the report, see http://tinyurl.com/RoboticGrippers. ME
Novel additive processes and developments in process planning for additive manufacturing were reported during several sessions at the annual NAMRC41-MSEC 2013 advanced manufacturing conference hosted June 10–14 by the University of Wisconsin-Madison.
In their NAMRC paper, L. Tse and K. Barton of the University of Michigan (Ann Arbor, MI) describe a flexible deposition approach for micro/nanomanufacturing using integrated electrohydrodynamic jet printing. E-jet printing shows superior resolution over traditional inkjet printing while being cost competitive with such techniques as microscale lithography. The transition to e-jet is hampered by limitations such as process throughput and substrate flexibility. With the design of a novel printhead, the authors address a key limitation associated with conductivity and flatness requirements. Challenges related to nozzle alignment and process control are discussed, and the functionality of the design is demonstrated through the fabrication and implementation of a working prototype.
A multinozzle array (MNA) printhead for e-jet printing is described by M. Takagi and P. Ferreira of the University of Illinois at Urbana-Champaign in a NAMRC paper. The printhead has the capability to use each nozzle individually or simultaneously print with the whole array. The concept four-nozzle printhead has the potential to be increased for batch pattern/image printing. Also studied is the use of a pulse-width DC voltage printing setup so printed image quality can be controlled depending on the desired application. Achieved droplet size standard deviation across the printhead is approximately 0.5–0.6 µm. A simple cost analysis is performed to determine feasibility of a one-time-use nozzle printhead. This paper is submitted to the Journal of Manufacturing Processes.
Electron beam additive fabrication (EBAF) is attracting attention for its usability to fabricate intricately designed parts as a whole. The technique not only increases production rates but also reduces costs by minimizing waste material. Ti-6Al-4V, used in the aerospace industry for its excellent mechanical properties, low density, great resistance to corrosion and nonmagnetism, is the most common material currently being fabricated with EBAF. This MSEC paper by L. Roy and L. Jannesari Ladani of the University of Alabama at Tuscaloosa investigates the anisotropy effect caused by build layer direction. Different mechanical properties such as yield strength, ultimate tensile strength and modulus of elasticity are determined using tensile tests. The tests show that yield strength and ultimate tensile strength for flat-build samples have distinguishably higher values than those of the side-build and top-build samples.
Another MSEC paper from the University of Alabama, by researchers X. Gong and Y.K. Chou, focuses on characterization of sintered Ti-6Al-4V powders in electron beam additive manufacturing (EBAM). Preheating the powders prior to the melting stage, resulting in sintering, reduces the residual stresses in the build parts. Analysis of specimens of sintered powders by optical microscopy and scanning electron microscopy shows that some powders are partially melted and necks are formed. Necking—evidence of sintering—is noted on both the build plane and the side surface (normal to the build plane).
A NAMRC paper by Y.K. Chou describes numerical evaluations of thermal effects in EBAM. Different thermal properties will result in distinct thermal responses, including the melt pool size. A finite element thermal model previously developed by the author to investigate thermal behavior of Ti-6Al-4V is used for numerical simulations with various materials. The melting temperature of work materials is intuitively the most dominant factor to the melt pool size. However, if a material has a very high thermal conductivity, such as copper, the role of the thermal conductivity may outweigh the melting point.
Process Planning for Additive
The NAMRC paper by S. Allavarapu, R. Paul and S. Anand of the University of Cincinnati presents a new file format using curved bi-quadratic Bezier patches, which can approximate a CAD model with higher accuracy as compared with an STL file. The first format uses curved Bezier patches with linear edges, and the second format uses curved Bezier patches with curved edges. Also from NAMRC and U-Cincinnati, N. Panhalkar, R. Paul and S. Anand describe a k-d tree based adaptive slicing methodology. A user-defined cusp height error criteria determines the depth of division for the k-d tree representation of the part. The k-d tree based decomposition of part volume determines the layer thickness at each slice level. The adaptive slice thicknesses generated from the k-d tree are then used to virtually manufacture sample parts, and the volumetric errors for the manufactured parts are calculated. The errors are compared with those of the virtual parts manufactured using uniform slice thicknesses.
A reactionary process planning algorithm for an unconstrained hybrid process integrating additive, subtractive and inspection processes, called iAtractive, is proposed by Z. Zhu, V. Dhokia and S. Newman of the University of Bath (UK) in a NAMRC paper. The iAtractive process aims to accurately manufacture complex geometries without being constrained by the capability of individual additive and subtractive processes. This process planning algorithm enables a part to be manufactured taking into account process capabilities, production time and material consumption. This approach is also adapted for the remanufacture of existing parts. This paper is in press for the Journal of Manufacturing Processes (http://tinyurl.com/NAMRC-Zhu).
All NAMRC41 papers are available on the Proceedings of NAMRI/SME CD ($150; $100 for SME members), http://tinyurl.com/NAMRC41. ME
TechFront is edited by Senior Editors Patrick Waurzyniak, firstname.lastname@example.org, and Ellen Kehoe, email@example.com.
This article was first published in the September 2013 edition of Manufacturing Engineering magazine. Click here for PDF.