Marius Stücheli

Dynamic Monitoring Reveals Motor Task Characteristics in Prehistoric Technical Gestures.

Reconstructing ancient technical gestures associated with simple tool actions is crucial for understanding the co-evolution of the human forelimb and its associated control-related cognitive functions on the one hand, and of the human technological arsenal on the other hand. Although the topic of gesture is an old one in Paleolithic archaeology and in anthropology in general, very few studies have taken advantage of the new technologies from the science of kinematics in order to improve replicative experimental protocols. Recent work in paleoanthropology has shown the potential of monitored replicative experiments to reconstruct tool-use-related motions through the study of fossil bones, but so far comparatively little has been done to examine the dynamics of the tool itself. In this paper, we demonstrate that we can statistically differentiate gestures used in a simple scraping task through dynamic monitoring. Dynamics combines kinematics (position, orientation, and speed) with contact mechanical parameters (force and torque). Taken together, these parameters are important because they play a role in the formation of a visible archaeological signature, use-wear. We present our new affordable, yet precise methodology for measuring the dynamics of a simple hide-scraping task, carried out using a pull-to (PT) and a push-away (PA) gesture. A strain gage force sensor combined with a visual tag tracking system records force, torque, as well as position and orientation of hafted flint stone tools. The set-up allows switching between two tool configurations, one with distal and the other one with perpendicular hafting of the scrapers, to allow for ethnographically plausible reconstructions. The data show statistically significant differences between the two gestures: scraping away from the body (PA) generates higher shearing forces, but requires greater hand torque. Moreover, most benchmarks associated with the PA gesture are more highly variable than in the PT gesture. These results demonstrate that different gestures used in ‘common’ prehistoric tasks can be distinguished quantitatively based on their dynamic parameters. Future research needs to assess our ability to reconstruct these parameters from observed use-wear patterns.

A robust manipulation strategy based on impedance control parameters changes and smooth trajectories

This paper proposes a new manipulation strategy that combines smooth trajectories with an impedance controller which parameters are adapted in time of execution. The proposal uses a single control law during all manipulation phases, i.e. when the robot is in contact with the object, and during regrasping when the robot moves freely in space. The manipulator impedance changes during the different manipulation phases and this is achieved modifying mass, stiffness and damping parameters of the controller. Controller stability is further enhanced by the adoption of smooth trajectories that are generated using a compact and computationally light Squeezed Screws Planner. Additionally, the approach shows good response to the occurrence of unplanned contact or collisions. Experimental results with an industrial manipulator unscrewing a jar cap, provides interesting data about performance and show the benefits of the adopted manipulation strategy.

Mechatronic Machine Elements: On Their Relevance in Cyber-Physical Systems

This paper discusses the appearance of mechatronic machine elements (MME) in the greater context of cyber-physical systems (CPS). For this purpose it establishes classifications for CPS. Three groups of MME are identified, characterized and illustrated with examples. Regarding the advantages of MME, the text explains how they allow for new functions in mechanical systems and how they help to reduce the development effort for complex products. As desirable characteristics for MME in general are identified independent communication abilities, “plug-and-play” integration in networks and mechanics and the preprocessing of sensor data on the element itself. CPS with specifically designed MME can further become a valuable tool in product development for information collection.

Anthropomorphic and Linear Arm Models for Mechanical Power Tool Testing

Vibrational dynamics of hand-held power tools are relevant for ergonomics and system performance. In application of these tools, the human user is in the force flow, thus has a relevant influence on the vibrations occurring in the human body. For development purpose and valid comparison between tools, reproducible testing is needed. The testing is most useful if it simulates different user types, working poses and muscle activation states, as observed in the tools’ application. The admittance of an anthropomorphic two link arm system strongly depends on the arm pose angles. This work examines how well this angle dependent behaviour can be approximated by a linear mass-spring-damper system, which is easier to build for test rigs. Parameter optimization under the assumption of fixed masses in the linear replacement system showed unsatisfactory fitting of the system dynamics for some arm poses. Therefore the authors recommend to consider building future test rigs in an anthropomorphic arm setup. The work further reveals the importance of implementing adjustable stiffness and damping in test rigs.Copyright

Development of VariLeg, an exoskeleton with variable stiffness actuation: first results and user evaluation from the CYBATHLON 2016

BackgroundPowered exoskeletons are a promising approach to restore the ability to walk after spinal cord injury (SCI). However, current exoskeletons remain limited in their walking speed and ability to support tasks of daily living, such as stair climbing or overcoming ramps. Moreover, training progress for such advanced mobility tasks is rarely reported in literature. The work presented here aims to demonstrate the basic functionality of the VariLeg exoskeleton and its ability to enable people with motor complete SCI to perform mobility tasks of daily life.MethodsVariLeg is a novel powered lower limb exoskeleton that enables adjustments to the compliance in the leg, with the objective of improving the robustness of walking on uneven terrain. This is achieved by an actuation system with variable mechanical stiffness in the knee joint, which was validated through test bench experiments. The feasibility and usability of the exoskeleton was tested with two paraplegic users with motor complete thoracic lesions at Th4 and Th12. The users trained three times a week, in 60 min sessions over four months with the aim of participating in the CYBATHLON 2016 competition, which served as a field test for the usability of the exoskeleton. The progress on basic walking skills and on advanced mobility tasks such as incline walking and stair climbing is reported. Within this first study, the exoskeleton was used with a constant knee stiffness.ResultsTest bench evaluation of the variable stiffness actuation system demonstrate that the stiffness could be rendered with an error lower than 30 Nm/rad. During training with the exoskeleton, both users acquired proficient skills in basic balancing, walking and slalom walking. In advanced mobility tasks, such as climbing ramps and stairs, only basic (needing support) to intermediate (able to perform task independently in 25% of the attempts) skill levels were achieved. After 4 months of training, one user competed at the CYBATHLON 2016 and was able to perform 3 (stand-sit-stand, slalom and tilted path) out of 6 obstacles of the track. No adverse events occurred during the training or the competition.ConclusionDemonstration of the applicability to restore ambulation for people with motor complete SCI was achieved. The CYBATHLON highlighted the importance of training and gaining experience in piloting an exoskeleton, which were just as important as the technical realization of the robot.

Work density analysis of adjustable stiffness mechanisms

Mechanical compliance is important for a robust and safe physical interaction of robots with humans and unstructured environments. Using adjustable stiffness, the advantages of compliant and stiff systems can be combined and thus the versatility of a robot increased. The realisation of adjustable stiffness in robot joints through compliant mechanisms shows several advantages over active control approaches, especially in terms of robustness. The compactness of adjustable stiffness mechanisms (ASMs) is important for their integration in robotic systems. An important aspect of compactness in ASMs is the storable work per volume, i.e. the work density. Therefore we propose a set of benchmarks to analyse the work density of elastic mechanisms on different design levels. The application of these benchmarks is demonstrated on a novel ASM, which is part of the adjustable impedance element AIE Uno, and on DLRs FSJ. The analysis of these ASMs demonstrates the application and the benefit of the proposed benchmarks. The benchmarks support the choice between alternative solutions and the identification of improvement potential in an existing design.