Series Of Joints Research Articles

Polyhedral-based Modeling and Algorithms for Tolerancing Analysis

The cumulative stack-up of geometric variations in mechanical systems can be modeled summing and intersecting sets of constraints. These constraints derive from tolerance zones or from contact restrictions between parts. The advantage of this approach is its robustness for treating any kind of mechanisms, including the over-constrained ones. However, the sum of constraints, which must be computed when simulating the accumulation of defects in serial joints, is a very time-consuming operation. In previous papers, we proposed to virtually limit the degrees of freedom of the toleranced features and joints turning the polyhedra into polytopes to avoid manipulating unbounded objects. Even though this approach enables to process the whole mechanism, it also introduces bounding or cap facets which increase the complexity of the operand sets after each operation until becoming far too significant. In this work, we introduce algorithms summing, intersecting and testing inclusions. As they operate on sets of constraints using unbounded polyhedral objects, we identify the smaller sub-space in which the projection of these operands are bounded sets. Calculating the sum in this sub-space allows reducing the operands complexity significantly and consequently the computational time. Then, checking the final inclusion informs us not only about the compliance of the mechanism tolerances with respect to the functional specification but also to quantify how far we are from this target. Finally prismatic polyhedra integrate ISO and contacts specifications in a very natural way and are able to perform a full kinematic analysis of the mechanism. After presenting the geometric properties on which this approach rely, we demonstrate it on an industrial case. Then we compare the computation times, prove the robustness of the new method and show how to quantify the functional condition compliance with respect to a given set of tolerances.

Pure Condition of Both 6R Serial and 3-PRS Parallel Robots Using Grassmann-Cayley Algebra

This research presents Pure Condition approach, which has used in analyzing simultaneously the singularity configuration and the rigidity of mechanism. The study cases analysis is implemented on variable joints orientation of 6R (Revolute) Serial Manipulators (SMs) and variable actuated joints position of 3-PRS (Prismatic-Revolute-Spherical) Parallel Manipulators (PMs) using Grassmann-Cayley Algebra (GCA). In this work we require in Projective Space both Twist System (TS) and Global Wrench System (GWS) respectively for serial and parallel manipulators which represent the Jacobian Matrix (J) in symbolic approach to Plucker coordinate vector of lines and unify framework on static and kinematics respectively. This paper, works, is designed to determine geometrically at symbolic level the vanished points of inverse form of this Jacobian Matrix (J) which called superbracket in GCA. The investigation vary to those reported early by introducing GCA approach on the singularity of serial robot, variable joints orientation and actuated positions on robot manipulators (RMs) to analyze rigidity frame work and singularity configuration which involve simultaneously Pure Condition. And the results also revealed a single singularity condition which contains all particulars cases and three general cases such as the shoulder, elbow and wrist singularity for SMs while double, single and undermined singularities respectively for 3-PRS, 3-PRS and 3-PRS PMs which contain all generals and particulars cases.

Underactuated miniature bending joint composed of serial pulleyless rolling joints

This paper proposes a miniature bending joint composed of serial rolling joints; it refers to the classification of small joints of other products and studies on surgical applications according to the structures and transmissions. To achieve this, a pulleyless rolling joint is introduced as a unit component and the basic design principle is established for the joint. A simple mechanism guarantees a smooth rolling contact using elastic fixtures. Underactuation, which prevents S-shaped curvature similar to buckling, is applied to drive the bending joint with a limited number of wire cables. Using a 1-DOF real-sized prototype, experiments were performed to measure the motion accuracy and payload capacity from various angles to evaluate the efficacy of an improved design that enhances the payload on the distal end. This novel joint is applicable to a wider variety of surgical tools and catheters relative to other candidates owing to its small constant curvature, reasonable payload capacity, and miniature size.

Analysis and optimization of a wire actuated, single effect n-R robotic structure

SUMMARYThis paper investigates the kinematics and the optimization of a generic robotic structure composed by N serial rotary joints and actuated with a mono-directional tendon system. In the first part of the paper, the specific case that brought us to develop this study is introduced; the main motivations and the scenario with its specific constraints and design choices have been described.Since a complete and detailed analysis of an n-R serial structure with this kind of characteristics could not be found in the literature, the study of the kinematics and the parameter optimization of such a structure is treated as generally as possible, in order to make the procedure and the results applicable for any similar structure. Finally, in the last part, through the introduction of specific constraints and the definition of the parameters, the general analysis has been applied to the specific case of study: the preliminary study of a finger exoskeleton for an astronaut suit.

Optimal Design for Biped Robot Parallel Ankle Joint

After analyzing the motion mechanism of human ankle, a new type of parallel ankle joint mechanism for a biped robot is designed. The parameterized model of the mechanism is established and its kinematic and dynamic analyses are carried out, with the conclusions of the first-order influence matrix of the parallel mechanism and the relationship between the driving force in the concertina motion and the zero moment point trajectory. Considering the great difference between the actual angular velocity and the driving moment in the biped robot’s walking ankle joint, the structure parameters of the parallel mechanism are optimized through iterative calculation according to the dynamic characteristics of the ankle joint during walking, which make the driving peak power and speed of the two driving elements of the ankle joint tend to coincide and reduce the dynamic unbalance in the biped robot’s walking ankle joint. According to the analysis of the experimental results of the biped robot’s ankle joint motion, the peak-power used to drive the optimized parallel ankle joints is only about half of that used to drive serial joints, which lowers the demand of the design power for the driving components. On the premise that the dynamic characteristic of biped robot ankle joints during walking are satisfied, choosing driving components with lower power is beneficial to the reduction of bulk, mass and the energy consumption of the joints.

A Hybrid Robotic System for Harvesting Heavy Produce

Certain agricultural work requires a very stiff robot arm to reduce vibration and damage to products. However, the structure of conventional articulated robots tends to be too weak to manipulate heavy objects, such as melons and watermelons. The Stewart platform is a typical parallel robotic mechanism with very high stiffness, but this advantage is balanced by a small workspace and large installation space. This research proposes a new type of agricultural robot arm constructed by combining a parallel and a serial mechanism. This robot consists of a hybrid manipulator, a robot controller and an end-effector. The hybrid manipulator has serial joints with two extra degrees of freedom, making the workspace wider. It was also designed to harvest heavy fruits such as melons, even from behind obstacles such as leaves or tree branches. Position control results of the developed hybrid robot showed that it could be controlled within a ±1 mm and ±0.2° range of error.

멜론 재배작업용 하이브리드 매니플레이터 개발

Various robots were developed for harvesting fruits and vegetables. However, each robot was designed for a specific task such as harvesting apples or vegetables. This has been a big hurdle in application of robots to agriculture. A new type of hybrid manipulator with both parallel and serial joints was developed and designed to apply to various kinds of field operations. The hybrid manipulator had 2 extra degree of freedom in serial joints which made it flexible in switching one to the other type of hybrid manipulator, for example, PUMA to SCARA. And it was designed to harvest heavy fruits such as musky melons or water melons even behind leaves or branches of tree. This hybrid manipulator showed less than <TEX>$\pm1mm$</TEX> position error. It was concluded that the hybrid manipulator was an effective and feasible tool to perform various works and to increase working performance.

Analysis and Design of a Direct-Drive Arm With a Five-Bar-Link Parallel Drive Mechanism

The direct-drive arm that has no gears between motors and their loads have several important advantages including no backlash, small friction, and high mechanical stiffness. The arm mechanism, however, becomes extremely massive, when each motor is directly attached to its joint along a serial linkage mechanism. The complicated dynamics resulting from varying inertia, interactions, and nonlinearities, is also more prominent than that of a robot with gears. This paper describes a lightweight arm mechanism with invariant and decoupled inertia characteristics. Instead of having motors at serial joints, a parallel drive mechanism with a closed-loop five bar linkage is utilized. The dynamic behavior of this mechanism is analyzed and the condition for the elimination of the interactions and nonlinearities in the mass properties is derived. The decoupled and invariant arm dynamics significantly reduces the complexity of controlling the direct-drive arm. In the latter half of the paper, a prototype robot developed on this basis is described. By using high torque brushless motors which were specially designed for the direct-drive robot, top speed and maximum acceleration were increased by an order-of-magnitude to about 10 m/s and 5 G, respectively.