Conventional Parallel Mechanisms Research Articles

Mobility of overconstrained parallel mechanisms with reconfigurable end-effectors

Reconfigurable parallel mechanisms have legs or end-effectors with additional degrees of freedom or with the ability to change their mobility. This allows them to reconfigure into a variety of conventional parallel mechanisms. One type of reconfigurable parallel mechanism can be obtained by replacing the end-effector with a closed-loop linkage. The parts of this linkage that are connected to the legs may be considered as the end-effectors, having their own mobility relative to the base and a coupled mobility with each other. Classical methods for mobility analysis cannot be directly applied to this type of mechanism. This article presents a general method to calculate and represent the coupled mobility of the multiple end-effectors using nullspace operations on a matrix of mobility. The nullspace approach provides an efficient way to calculate the mobility in a closed-loop chain while the matrix of mobility allows for calculating the multiple end-effectors mobility interaction. The general method can be performed analytically to obtain insights on the mechanism in any configuration, or algorithmically to obtain numerical values of the mobility in a particular configuration. Two illustrative examples showcasing the method are presented.

Reconfigurable fully constrained cable driven parallel mechanism for avoiding interference between cables

Cable driven parallel mechanisms (CDPMs) have attracted much attention due to their many advantages over conventional parallel mechanisms, such as the significantly large workspace and the dynamics capacity. One of the main issues involved in designing CDPMs is avoiding cable-cable collision, especially when an operator is sharing the same workspace with the moving parts of the mechanism. This paper aims to model and simulate a reconfigurable fully constrained CDPM and solve the forward and inverse kinematics given that the attachment points on the rails move up and down in real time, unlike conventional CDPMs where the attachment points are firmly fixed on specific positions on the rails. The new idea of reconfiguration is then used to avoid interference between two cables in real time by moving one cable's attachment point on the frame to increase the shortest distance between them while keeping the trajectory of the end effector unchanged. This new approach was tested by creating a simulated intended cable interference trajectory, hence detecting and avoiding cable collision using the proposed real time reconfiguration while maintaining the end effector trajectory.

A Robotic System With Multichannel Flexible Parallel Manipulators for Single Port Access Surgery

Robot-assisted single port access surgery (SPAS) brings significant benefits to the patients. However, it is challenging due to the limited incision size and requirements in precision, load capacity, and dexterity. To address these challenges, this paper presents a multichannel SPAS robotic system consisting of two channels of 5-degree-of-freedom (DOF) flexible manipulators and an endoscope channel. Each channel of the manipulators is designed based on a 3-DOF parallel mechanism with three sets of super-elastic Ni–Ti rods and universal joints. This design optimizes the structure for flexibility and safety considerations with a simplified structure compared with conventional parallel mechanisms. Flexible shafts are used for torque transmission from the actuation unit, allowing the motors to be farther away from the patient-side robotic system. The kinematics of the manipulator is derived, and then the reachable workspace and dexterity are analyzed. Furthermore, a prototype of the proposed robotic system is presented and evaluated through adequate experiments. The flexibility of the manipulator is verified via stiffness characterization test. The results of the accuracy tests confirmed that the robotic system can implement manipulation arm with acceptable accuracy. The feasibility and effectiveness of applying the robotic system to the practical operation are also demonstrated through experimentations.

Structural synthesis of a class of two-loop generalized parallel mechanisms

The conventional parallel mechanism (CPM) is characterized by connecting serial kinematic limbs or chains to the moving platform or linkage. The mobility and motion pattern of the CPM may be further constrained by adding a coupling sub-chain to serial kinematic chains. That will result in a new class of mechanisms called generalized parallel mechanisms or GPMs for short. While the systematic mobility analysis and structural synthesis of GPMs are more challenging and have rarely been studied, the application of the coupling sub-chain possesses the potential for enhanced functionality and performance. This paper proposes a novel approach for synthesizing two-loop mechanisms with coupling sub-chains that form the basic building blocks for GPMs. The screw theory based constraint synthesis method is advocated. Examples are enumerated to demonstrate the feasibility of the proposed approach.

Kinematic and static analysis of a planar compliant tensegrity-like parallel mechanism

In this work, a novel planar compliant tensegrity-like parallel mechanism composed of a set of rods and strings is introduced. The mechanism has low mass and high strength to weight ratio compared with conventional parallel mechanisms. An energy based method is employed to find the analytical solutions to the forward and inverse position analysis. Afterwards, the singular configurations are detailed. The actuator and output workspaces are then mapped. Moreover, an analytical approach based on an energy formulation is used to evaluate the stiffness of the mechanism. Finally the characteristics of the mechanism are reported.

Haptic Device Using a Newly Developed Redundant Parallel Mechanism

A number of haptic devices have recently become available on the commercial market, and these devices are becoming common not only in research but in consumer use as well. In this paper, a new parallel mechanism, referred to herein as DELTA-R (meaning DELTA-Redundant, formerly referred to as DELTA-4) is proposed for a new haptic device having high-quality force display capability and operability. DELTA-R allows three-degree-of-freedom (DOF) translational motions. The key features of DELTA-R, as compared with conventional parallel mechanisms, are redundant actuation, a smaller footprint, a larger working area, and improved access to the end effector. The prototype is equipped with a 3-DOF rotation mechanism, the center of motion of which is located on the wrist position of the operator. An evaluation test of the force display was conducted using a prototype of the proposed mechanism. This paper describes the kinematic design, kinematic modeling, kinematic analysis, prototype implementation, and evaluations.

パラレルリンク機構を用いた力覚提示装置ＤＥＬＴＡ‐４の開発

In this paper, a new haptic device “DELTA-4” within a high force feedback capability and operability is proposed. DELTA-4 realizes 3 degrees of freedom of translational motions by a novel redundant parallel link mechanism. These benefits can be obtained by DELTA-4 comparing with conventional parallel mechanisms: wider working area, smaller footprint and easier access to the wide range of working area from operators. In this paper, kinematics and its analysis of DELTA-4 are introduced. In addition, an evaluation experiment of force display is presented.

1A1-A12 力覚提示装置のための3自由度冗長型パラレルメカニズムDELTA-4の開発

DELTA-4 realizes 3 degrees of freedom of translational motions by a novel redundant parallel link mechanism. These benefits can be obtained by DELTA-4 comparing with conventional parallel mechanisms: wider working area, smaller footprint and easier access to the wide range of working area from operators. In this paper, evaluation experiments of DELTA-4 are described about an accuracy of force display, a mechanical stiffness and a back drivability. In addition, an evaluation experiment of force display is presented.

Synthesis of the internal parallel cam mechanism

Parallel cam mechanisms have been studied in different ways. In this paper, a parametric formulation which can cover every configuration of the parallel cam mechanism is derived. As a result of parametric analysis, a new, last mechanism was found. This cam is essentially an internal cam mechanism. Based on the assumption of an equally distributed roller follower, the basic configurations of the mechanism are derived from both the pressure angle and the undercutting constraints. As a result, the possible number of rollers per spider plate is two. Thus the feasible area of the design parameters of the mechanisms are obtained. The advantages of the mechanism over the conventional parallel mechanism are the saving of space and a larger angular stroke of output.