SCALAR - Simultaneous Calibration of 2D Laser And Robot's Kinematic Parameters Using Three Planar Constraints

Industrial robots are increasingly used in various applications where the robot accuracy becomes very important, hence calibrations of the robot's kinematic parameters and the measurement system's extrinsic parameters are required. However, the existing calibration approaches are either too cumbersome or require another expensive external measure- ment system such as laser tracker or measurement spinarm. In this paper, we propose SCALAR, a calibration method to simultaneously improve the kinematic parameters of a 6-DoF robot and the extrinsic parameters of a 2D Laser Range Finder (LRF) that is attached to the robot. Three flat planes are placed around the robot, and for each plane the robot moves to several poses such that the LRF's ray intersect the respective plane. Geometric planar constraints are then used to optimize the calibration parameters using Levenberg-Marquardt nonlinear optimization algorithm. We demonstrate through simulations that SCALAR can reduce the average position and orientation errors of the robot system from 14.6 mm and 4.05 degrees to 0.09 mm and 0.02 degrees.

Concept of Modular Kinematics to Design Ultra-high Precision Parallel Robots

Murielle Richard

Miniaturisation will be the key challenge for the next decade in numerous industrial fields, such as microelectronics, optics and biomedical engineering. Although most of their products already achieve footprints of some square millimeters, the trend towards the integration of a maximum number of elements in a minimal volume requires even more compact components. This tendency creates a growing need for industrial robots able to perform micromanipulation and microassembly tasks with a submicrometric precision. Nonetheless, the design of such machines is nowadays costly, both in time and money, mostly because of the twofold complexity of their development: first, from a kinematic standpoint, the use of a parallel structure consists in a particularly interesting approach to build ultra-high precision robots. However, the synthesis of such a kinematics proves especially challenging for machines presenting more than 3 degrees of freedom. Moreover, the resulting robots are scarcely flexible: if the industrial specifications are modified, which for example necessitates to add a degree of freedom or to change the position of a rotation centre, the design process has to be restarted, often from the very beginning. The second challenge consists in the mechanical design of flexure-based mechanisms: flexure hinges are joints which are based on the elasticity of the matter. They allow to perform motions which are without friction, backlash and wear; their use is thus mandatory to achieve the aimed submicrometric precision. Albeit the synthesis of planar and low-degree of freedom structures is now widely investigated, the development of a whole tridimensional flexure-based robot is still infrequent, especially in the industrial context. This thesis thus introduces a modular design methodology which significantly reduces the time-to-market of ultra-high precision robots. This procedure can be compared to a robotic Lego, where a finite number of conceptual building bricks allows to easily design and modify parallel robots. Furthermore, this work shows that the machines resulting from this approach present similar or even improved performances compared to robots developed more traditionally. The key aspect of this thesis consists in the concept of modular kinematics, which aims at facilitating the synthesis of parallel kinematics thanks to solution catalogues. At this step of the methodology, the conceptual building bricks and the kinematics are totally independent from any mechanical design: they can thus be used to synthesise a large variety of robots, from machine-tools to microscale robots. An exhaustive conceptual solution catalogue groups all kinematics generated by the combination of the building bricks. Then, a reduced solution catalogue for ultra-high precision is proposed: based on selection criteria linked with the design and machining of flexure-based mechanisms, it allows to reduce the total number of solutions and thus facilitates the practical use of the concept. The second part of this work details the mechanical design of the building bricks, whose main challenge consists in increasing the ratio between the working ranges of the mechanisms and their overall size. One or more flexure-based solutions have been developed for each motorised brick: a special emphasis is given to the original use of a Remote Centre of Motion, which allows to achieve high rotation angles while drastically reducing parasitic translations. The development of a standardised actuation sub-brick, common to all motorised bricks, introduces a new level of modularity, thus increasing even more the flexibility of the methodology. As for non-actuated bricks, original designs and uncommon uses of well-known mechanisms are proposed. A case study on a 5-degree of freedom robot, Legolas 5, finally illustrates the practical use of the methodology: first, the selection in the solution catalogue of a kinematics adapted to the specifications of the robot is detailed. Then, the development of the Legolas 5 prototype highlights the mechanical design of the necessary building bricks, as well as assembly subtleties, such as force alignment and gravity compensation, which allow to shrewdly design a high-performance robot. The measurements of this machine have shown motion resolution and repeatability of 50 nm in translation and 1.9 µrad in rotation (limited by the sensor resolution). This case study has generated the Legolas family, a new family of ultra-high precision parallel robots, which notably includes the orthogonal version of the Delta kinematics: using only 6 of the conceptual building bricks, one solution can be built for each of the 19 possible robot mobilities. The promising characterisation of the Legolas 5 tends to suggest that the robots from this family will be interesting candidates to fulfill the upcoming need for quickly designed and high-performance industrial ultra-high precision machines.

EPFL2012

Distributed Scalable Multi-Robot Learning using Particle Swarm Optimization

Alcherio Martinoli, James Pugh

Designing effective behavioral controllers for mobile robots can be difficult and tedious; this process can be circumvented by using online learning techniques which allow robots to generate their own controllers online in an automated fash- ion. In multi-robot systems, robots operating in parallel can potentially learn at a much faster rate by sharing information amongst themselves. In this work, we use an adapted version of the Particle Swarm Optimization algorithm in order to accomplish distributed online robotic learning in groups of robots with access to only local infor- mation. The effectiveness of the learning technique on a benchmark task (generating high-performance obstacle avoidance behavior) is evaluated for robot groups of various sizes, with the maximum group size allowing each robot to individually contain and manage a single PSO particle. To increase the realism of the technique, different PSO neighborhoods based on limitations of real robotic communication are tested and com- pared in this scenario. We explore the effect of varying communication power for one of these communication-based PSO neighborhoods. To validate the effectiveness of these learning techniques, fully distributed online learning experiments are run using a group of 10 real robots, generating results which support the findings from our simulations.

2009

Active origami platforms for human-robot interactions

Frédéric Henri Vaskin Giraud

Robots are employed to assist humans in lengthy, challenging, and repetitive tasks. However, the fields of rehabilitation, haptics, and assistive robotics have shown a significant need to support and interact with people in their everyday life. To facilitate these further needs, robots must overcome constraints in compactness, degrees of freedom (DoF), and mechanical performance. However, developing a robot with these characteristics is challenging for conventional mechanics. First, creating a meso-scale device requires the assembly of small components, which is laborious and increases manufacturing cost, time, and complexity. In addition, devices with several DoF and functionalities also require more parts to create numerous links and joints, actuators, and thus space, affecting the compactness of systems. Finally, aiming for mechanical performance in terms of force, speed, and motion range requires transmissions or bulky actuators, which also influence the size of the system. This thesis attempts to overcome the limitations of conventional mechanics by using origami robots, an alternative design approach for creating meso-scale structures with numerous folding joints. It consists in stacking layers of functional material to achieve low-profile and scalable structures that fold into 3D. Using this method, we developed three hybrid robots combining the decades of experience of conventional mechanics with the advantages of origami robotics. First, to enhance the performances of robots at the meso-scale, the creation of compact low-profile compliant transmissions benefiting from origami's minimal assembly of functional layers was explored. The transmissions obtained can reproduce conventional kinematics or generate unconventional motions using materials properties. Second, the development of multi-DoF compact robots was studied using the aforementioned low-profile compliant transmissions to create a multi-functional structure. The resulting robot is a fingertip haptic device called Haptigami that can render vibrotactile and three-DoF force feedback without compromising bulkiness. Then, this study aimed at creating a meso-scale, multi-DoF, and mechanically efficient robot. We combined the previous device contributions and developed Flexure Variable Stiffness Actuators (F-VSA), a novel compliant actuator that uses the origami flexure hinges' inherent stiffness to avoid hindering mechanical simplicity. Finally, we developed a software architecture that enables high-level control of the human-robot hardware and is compatible with multiple simulation engines. We used this architecture to control our robots and provide feedback to the user, based on interactions in a virtual environment. These novel robots rely on non-conventional structures, actuators, and kinematics that require exploring and testing suitable control strategies. Hence, kinematic and stiffness models were developed and used to tune the position, force, and stiffness of the end-effector. Several characterization platforms and experimental protocols suitable for testing our unique robotic systems were also developed to assess their precision and efficiency. Consequently, the results of this study pave the way for the development of robots that adapt to multiple environments, interactions, and application scenarios, compatible with the human bandwidth without hindering the user's motion and comfort in everyday life.