Vincenzo Parenti-Castelli

Mobility Analysis of the 3-UPU Parallel Mechanism Assembled for a Pure Translational Motion

The occurrence of singular configurations in parallel mechanisms must be avoided during motion since the actuators cannot control motion eyen in the neighborhood of these configurations. As a consequence, the knowledge of the singular configurations of the mechanism is important for control purposes, for singularity-free path planning, and also represents basic information for the synthesis of a desired mechanism workspace free from singularities. In this paper the mobility analysis of the 3-UPU parallel mechanism assembled for obtaining a pure translation motion of the output platform is performed and both translation and rotation singularity loci are presented in analytic form and their geometric interpretation is given.

A Family of 3-DOF Translational Parallel Manipulators

This article addresses parallel manipulators with fewer than six degrees of freedom, whose use may prove valuable in those applications in which a higher mobility is uncalled for. In particular, a family of 3-dof manipulators containing only revolute joints or at the most revolute and prismatic ones is studied. Design and assembly conditions sufficient to provide the travelling platform with a pure translational motion are determined and two sub-families that fulfill the imposed constraint are found: one is already known in the literature, while the other is original. The new architecture does not exhibit rotation singularities, i.e., configurations in which the platform gains rotational degrees of freedom. A geometric interpretation of the translation singularities is provided.

A Translational 3-dof Parallel Manipulator

This paper presents a new architecture of a three degree-of-freedom parallel manipulator that, if properly assembled, exhibits pure translational motion of the output link (platform) with respect to the frame (base). To prove the existence of a pure translational motion a mobility analysis of the manipulator has been carried out. Moreover, a singularity analysis has also been conducted that shows the occurrence of configurations of the manipulator where the pure translational feature is no longer controllable.

A Novel Fully Decoupled Two-Degrees-of-Freedom Parallel Wrist:

In this paper we present a novel pointing parallel mechanism with fully decoupled degrees of freedom. The mechanism consists of two interconnected slider-crank linkages, each one of which independently actuates one of the Euler angles of the output link. Motors can be mounted on the frame. The parallel architecture provides the mechanism with compactness and robustness, while decoupling of motion guarantees a wide workspace, an easy mathematical treatment, and a simple control. Such a mechanism may be suitable for applications that require aiming bodies such as tools, telescopes, antennas, cameras, solar panels, human or humanoid artificial limbs, particularly if requisites on dimensions and stoutness on the one hand and mobility on the other are stringent.

Recent advances in robot kinematics

1. Plenary. 2. Control and Optimisation. 3. Performance. 4. Workspace and Trajectory Analysis. 5. Modelling and Computation. 6. Analysis and Simulation. 7. Performance of Parallel Mechanisms. 8. Analysis of Parallel Mechanisms. Author Index.

A new technique for clearance influence analysis in spatial mechanisms

This paper presents a technique for assessing the influence that clearance in the kinematic pairs of a mechanism has on accuracy. The technique works for both planar and spatial, open-, and closed-chain mechanisms, but not for overconstrained mechanisms. It can be defined deterministic, because it precisely determines the actual pose of the link of interest, and does not rely on probability density functions. The most innovative aspect is that, unlike other existing techniques, knowledge of the forces acting on the mechanism is not needed; on the contrary, given a mechanism configuration, the maximum pose error of the link of interest-that is, the maximum displacement of the link of interest caused by clearance take-up-is directly determined. In this way, a problem which is intrinsically kinematic is solved by a kinematic method, rather than by a kinetostatic one as usually done in the literature. This allows for a more general approach to the clearance problem, as it removes one of the inputs (the external load) usually required for a kinetostatic analysis. The numerical effectiveness of the method is shown on a pure translational parallel manipulator.

Mobility analysis of the 3-UPU parallel mechanism assembled for a pure translational motion

The occurrence of singular configurations in parallel mechanisms must be avoided during motion since the actuators cannot control motion even in the neighbourhood of these configurations. As a consequence, the knowledge of the singular configurations of the mechanism is important for control purposes, for singularity-tree path planning and also represents basic information for the synthesis of a desired mechanism workspace free front singularities. In this paper the mobility analysis of the 3-UPU parallel mechanism assembled for obtaining a pure translation motion of the output platform is performed and both translation and rotation singularity loci are presented in an analytic form and their geometric interpretation is given.

Dynamics of a Class of Parallel Wrists

This paper presents a dynamic model of parallel wrists with all links constrained to have a spherical motion with the same center. The model can also be applied to serial wrists. The model, based on Lagrangian formulation of dynamics, exploits the feature that all the links have the same fixed point. Three parameters defining the platform orientation are used as generalized coordinates. This choice allows the use of the generalized inertia matrix (GIM) appearing in the model to calculate effective dynamic performance indices proposed in a previous paper. The model can solve both the direct and the inverse dynamic problems. It also contains the Jacobian matrix useful to characterize the kinematic behavior of parallel manipulators. By the model it is shown that the best performances are reached in the workspace regions where the manipulator has a good kinematic and dynamic isotropy, whereas the incidence of nonlinear forces on performances is relevant at high end-effector speed. A numerical example is provided.

Mathematical models of passive motion at the human ankle joint by equivalent spatial parallel mechanisms.

The paper presents a theoretical model of the ankle joint, i.e. tibio-talar articulation, which shows how the articular surfaces and the ligaments, acting together as a mechanism, can control the passive kinematics of the joint. The authors had previously shown that, in virtually unloaded conditions, the ankle behaves as a single degree-of-freedom system, and that two ligament fibres remain nearly isometric throughout the flexion arc. Two different equivalent spatial parallel mechanisms together with corresponding kinematic models were formulated. These assumed isometricity of fibres within the calcaneal-fibular and tibio-calcaneal ligaments and rigidity of the articulating surfaces, taken as three sphere-plane contacts in one model, and as a single spherical pair in the other. Geometry parameters for the models were obtained from three specimens. Motion predictions compare quite well with the measured motion of the specimens. The differences are accounted for by the simplifications adopted to represent the complex anatomical structures, and might be reduced by future more realistic representations of the natural articular surfaces.

A New Algorithm Based on Two Extra-Sensors for Real-Time Computation of the Actual Configuration of the Generalized Stewart-Gough Manipulator

It is well known that the direct position analysis of fully-parallel manipulators provides more than one solution, i.e., more than one configuration of the mechanism is possible for a given set of the actuated variables of motion. Extra information is, thus, necessary to find the actual configuration of the manipulator. This paper presents a new algorithm for the real-time computation of the actual configuration of the generalized Stewart-Gough manipulator, also known as 6-6 fully-parallel manipulator with general geometry. The proposed algorithm makes use of two extra rotary sensors in addition to five out of the six sensors normally implemented in the servosystems of the manipulator. A one-to-one correspondence between the sensor measurements and the manipulator configuration is provided. With respect to other algorithms recently presented in the literature, the proposed method greatly reduces the computational burden. Finally a case study shows the effectiveness of the proposed procedure.