In robotics, an end effector is the device at the end of a robotic arm, designed to interact with the environment. The exact nature of this device depends on the application of the robot. In the strict definition, which originates from serial robotic manipulators, the end effector means the last link (or end) of the robot. At this endpoint, the tools are attached. In a wider sense, an end effector can be seen as the part of a robot that interacts with the work environment. This does not refer to the wheels of a mobile robot or the feet of a humanoid robot, which are not end effectors but rather part of a robot's mobility. End effectors may consist of a gripper or a tool. When referring to robotic prehension there are four general categories of robot grippers:

Impactive: jaws or claws which physically grasp by direct impact upon the object.

Ingressive: pins, needles or hackles which physically penetrate the surface of the object (used in textile, carbon, and gl

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

Optimization-based motion planning for legged robots

Alexander Winkler

Legged machines have the potential to traverse terrain that wheeled robots cannot. These capabilities are useful in scenarios such as stairs in homes or debris-filled disaster scenes, such as earthquake areas. This thesis develops one of the algorithms necessary to achieve such a task, a motion-planner, which translates a high-level task into a desired motion-plan for a legged robot to execute. A core difficulty in legged locomotion is that a forward body movement cannot be directly generated, but results from contact forces between the end-effectors and the environment. Furthermore, there are various physical restrictions on the contact forces that can be generated. These can make hand-crafting valid motions for all the interdependent quantities such as body motion, end-effector motion and contact forces tedious and even infeasible for complex tasks. Alternatively, Trajectory Optimization can be utilized to generate motions in a more general, automated way. Only a high-level task is specified, while the algorithm determines the motions and forces taking into account the physical restrictions of legged locomotion. This approach is attractive because once the problem has been properly modeled, the algorithm would, in an ideal case, produce motions for any high-level task, solving legged locomotion planning on a general level. This thesis presents approaches to transcribe, using e.g. collocation, the legged locomotion problem into a mathematical optimization problem, then solvable by off-the-shelf software. The three developed motion-planning algorithms generate motion-plans for increasingly complex terrains and tasks. Simulation and experiments on the quadruped robots HyQ and ANYmal verify the physical correctness of the motion-plans. The final formulation and main contribution of this thesis holistically determines the gait- sequence, step-timings, footholds, contact forces, swing-leg motions and 6-dimensional body motion, given a desired goal state and 2.5D height map of the non-flat terrain. We model the robot as a single rigid body (SRBD) controlled by the contact forces at the end- effectors. Our novel phase-based parameterization of end-effector motion and forces allows optimizing over the discrete gait sequence using only continuous decision variables. The algorithm efficiently generates highly dynamic motion-plans with flight-phases for legged systems, such as monopeds, bipeds, and quadrupeds.

ETH Zurich2018

Improving the accuracy of parallel robots

EPFL1996

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

Tiavina Niaritsiry

EPFL2006

Optimization-based motion planning for legged robots

Alexander Winkler

ETH Zurich2018

Robot end effector

Impactive: jaws or claws which physically grasp by direct impact upon the object.

Ingressive: pins, needles or hackles which physically penetrate the surface of the object (used in textile, carbon, and gl

Improving the accuracy of parallel robots

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

Optimization-based motion planning for legged robots

Improving the accuracy of parallel robots

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

Optimization-based motion planning for legged robots