SCALAR: Simultaneous Calibration of 2-D Laser and Robot Kinematic Parameters Using Planarity and Distance Constraints

In this paper, we propose SCALAR, a calibration method to simultaneously calibrate the kinematic parameters of a 6-DoF robot and the extrinsic parameters of a 2-D laser range finder (LRF) attached to the robot's flange. The calibration setup requires only a flat plate with two small holes carved on it at a known distance from each other and a sharp tool-tip attached to the robot's flange. The calibration is formulated as a nonlinear optimization problem where the laser and the tool-tip are used to provide the planar and distance constraints, and the optimization problem is solved using the Levenberg-Marquardt algorithm. We demonstrate through experiments that SCALAR can reduce the mean and the maximum tool position error from 0.44 to 0.19 mm and from 1.41 to 0.50 mm, respectively.

Improving the accuracy of parallel robots

The main objective of this thesis is the accuracy improvement of parallel robots. Accuracy can be improved either by precise manufacturing and assembly or by calibration of each individual robot using a kinematic model which takes geometric deviations into account. The latter has the advantage of leading to low cost solutions but requires sophisticated modeling of the robot's structure which is usually considerably more complex than the derivation of its nominal model. To substantiate the theoretical tools proposed in this thesis two examples of parallel structures are chosen. One of them is the Delta robot with three translational degrees of freedom whereas the second example is a novel structure called Argos having three rotational degrees of freedom. For experimental verification a mock-up was built for each of the two structures. Four calibration steps, modeling, measurement, identification, and implementation are investigated. Investigations were restricted to static errors due to geometric deviations assuming rigid bodies. First a formula is proposed which allows to calculate the number of independent kinematic parameters required for a complete model of a parallel structure. Then a systematic parameterization is introduced and applied to derive four calibration models, two for each example. Two measurement devices are described which were built to determine the position and orientation (pose) of the end-effectors of the two robots. For the Delta robot two additional set-ups using no external (additional) measurement device are proposed. For parameter identification different methods were tested by simulation. Calibration based on the implicit model is proposed as a standard method to calibrate parallel robots. Another calibration method is introduced, referred to as semiparametric calibration, which leads to low computational effort. Fast solutions of the direct and inverse problems had to be found. For the first time al1 the solutions of the direct problem for the Delta robot were found by means of an algorithm introduced by Husty. In addition a fast numeric algorithm for the Delta's direct problem is proposed. The main contribution of this thesis is the experimental verification of calibration methods to improve the accuracy of parallel robots. Using these calibration methods for the two robots, ARGOS and DELTA, between a three- to a twelve-fold improvement of accuracy was achieved and experimentally verified.

EPFL1996

Optimisation de la conception du robot parallèle Delta cube de très haute précision

Tiavina Niaritsiry

In the high-precision industry, most operations require the use of robots able to accomplish highly accurate and repeatable motions. In order to meet the desired level of absolute accuracy, one has to limit or even suppress the effects of different sources of inaccuracy by means of an appropriate calibration. However, calibration techniques cannot be applied to compensate inaccuracies occuring along passive (non-actuated) degrees of freedom (e.g orientation errors on the end-effector of a 3 degrees-of-freedom (DOF) in translation robot). The main goal of this thesis is to evaluate and benchmark the effects of these different sources of inaccuracy. Moreover, a set of design rules are proposed in order to optimize flexure parallel robots regarding their absolute accuracy. The robots studied in this work are of "Delta Cube" type, having their kinematic chains composed of translational stages and space parallelograms (acting as universal joints). These robots have the following features: their 3 DOF are the (x, y, z) translations on the Euclidean space, the stroke of each DOF goes from ±1 mm to ± 4 mm, their repeatabilities are at the nanometre range (thanks to the absence of dry friction and mechanical play), their resolutions are within a few nanometers (only limited by the captors used), their small sizes go from 1,5 dm3 to 13 dm3. The analysis of the effects of the different sources of inaccuracy has been focused on the following points: the basic elements composing the robot structure (translational stage, space parallelogram). Studying the influence on the absolute accuracy caused by a variation of a given geometric dimension provides knowledge on how to optimize the overall robot dimensions regarding absolute accuracy; the assembly of the different basic elements on the kinematic chain (in particular their relative orientation) is also critical for the absolute accuracy of the robot since it may cause parasitic effects such as orientation errors in the end-effector; the type and location of the different captors, motors and the frame can also influence accuracy. The analysis of the influence of each source of inaccuracy (manufacturing tolerances, temperature variations, assembly defaults, effect of external loads) and their coupling has been mainly performed numerically by means of Finite-Element Models. Theoretical results have been verified by experimental data. Considering the simulation results for each basic structure as well as the comparison of different assembly variants, general design rules have been proposed in order to optimize a given flexure parallel robot regarding absolute accuracy and considering also the application the robot is made for. This work is a contribution to the design of high-precision flexure parallel robots regarding absolute accuracy. It is also an important tool for the engineer in order to make the calibration work easier.

EPFL2006

Orthèses fonctionnelles à cinématique parallèle et sérielle pour la rééducation des membres inférieurs

Carl Schmitt

Robotics applied to rehabilitation requires specific manipulators: Powered Orthoses. They are orthopedic devices equipped with motors and captors that enable locomotor assistance. These powered orthoses must be capable of reproducing physiological articular trajectories and taking over or simulating the segmentary charges of a movement, mainly walking. One needs to obtain rather high dynamic performances with the help of small activators enabling mechanical integration bearable for its user. This doctoral thesis deals with the conception of such POs as well as it proposes original parallel kinematics that are compared with serial kinematics, i.e. ordinary exoskeletons. We have chosen to limit our study to motor re-education of lower limbs associated to neurological disorders: paratetraplegia, hemiplegia, cerebral palsy, etc. This project was initiated by the Fondation Suisse pour les Cyberthèses (Swiss Foundation for Cyberthoses) in 1999, in collaboration with the Laboratoire de Systèmes Robotiques (Laboratory of Robotic Systems) of the Ecole Polytechnique Fédérale de Lausanne (EPFL). It aims at developing systems of motor re-education and walking assistance, associating powered orthosis with trans-cutaneous and closed-loop electrical muscle stimulation: the MotionMaker™ and the WalkTrainer™. Its goal is to create active muscular participation that respects body dynamics of movements and to quicken re-education, whenever possible. In cases of total paralysis, its purpose is to activate lower limbs in order to reduce side-effects and complications resulting from immobilization. The FSC also makes it its ambition to conceive powered orthoses for autonomous walking with functional electric stimulation in its research programme: the WalkMaker™. To begin with, this doctoral thesis defines the biomechanical bases relative to lower limbs and pelvis. Anthropometrical, kinematic and dynamic data of body segments, as well as space-time parameters of walking have been specified. These data have been used in the theoretical models of the conception of the Powered Orthoses, and applied to the numerical simulations carried out in this study. Then we have presented the state of research on orthoses. The number of projects and the diversity of technologies offer a good illustration of the challenge posed when conceiving Powered Orthoses. So far there are no autonomous Powered Orthoses for re-education of ground walking subsequent to neurological trauma. If it is possible to find treadmills on the market, they are however deficient for medical purposes because of the subject's passivity and lack of pelvis mobility. We have then conceived two Powered Orthoses for a stationary training device: firstly, a serial orthosis of the exoskeleton type has been conceived with three articulations: hip, knee and ankle. Its activators consist in connectingrod and crank systems. Secondly, we have devised a device of leg manipulation with a parallel structure (in the shape of the Greek letter lambda λ). We have modelled these two powered orthoses and carried out a numerical simulation of two movements in order to compare their performances: leg press and cycling. The λ parallel powered orthosis gives better results. Before these findings, we built the prototype of the serial powered orthosis for clinical tests for feasibility of closed-loop controlled electric stimulation. The thesis then offers the design of an unprecedented powered orthosis to be integrated into a walker. We have analyzed and modelled a parallel structure with orthogonal connections and another one with λ connections. The comparative results of the numerical simulations of normal speed walking show that the orthogonal powered orthosis is optimal for an autonomous walker. We have built a prototype, and walking tests with healthy subjects prove the feasibility of such a concept. Finally we carried out two studies for a leg-powered orthosis, compatible with a pelvic PO and the walker. An exoskeleton is compared with a parallel structure. For these two systems, the numerical simulations and models give all the kinematic and dynamic features of the activators. Following these results, we chose the parallel PO so as to design an experimental prototype. It is currently being built as we are writing these lines. To complete the procedures, a chapter deals with the integration of the POs into a walker, with the motorization of the mobile frame and with a system of active body relieving. We also present a system of optical measurements of pelvic movements for diagnosis or for biomechanical studies. The last chapter offer guidelines for development in powered orthosis.

EPFL2007