A Coordinate-Free Approach to the Design of Generalized Griffis-Duffy Platforms

Architectural singularity belongs to the Type II singularity,in which a parallel manipulator(PM)gains one or more degrees of freedom and becomes uncontrollable.PMs remaining permanently in a singularity are beneficial for linearto-rotary motion conversion.Griffis-Duffy(GD)platform is a mobile structure admitting a Bricard motion.In this paper,we present a coordinate-free approach to the design of generalized GD platforms,which consists in determining the shape and attachment of both the moving platform and the fixed base.The generalized GD platform is treated as a combination of six coaxial single-loop mechanisms under the same constraints.Owing to the inversion,hidden in the geometric structure of these single-loop mechanisms,the mapping from a line to a circle establishes the geometric transformation between the fixed base and the moving platform based on the center of inversion,and describes the shape and attachment of the generalized GD platform.Moreover,the center of inversion not only identifies the location of rotation axis,but also affects the shape of the platform mechanism.A graphical construction of generalized GD platforms using inversion,proposed in the paper,provides geometrically feasible solutions of the manipulator design for the requirement of the location of rotation axis.

Porphyromonas gingivalis bacteremia increases the permeability of the blood-brain barrier via the Mfsd2a/Caveolin-1 mediated transcytosis pathway

Bacteremia induced by periodontal infection is an important factor for periodontitis to threaten general health. P. gingivalis DNA/virulence factors have been found in the brain tissues from patients with Alzheimer’s disease(AD). The blood-brain barrier(BBB) is essential for keeping toxic substances from entering brain tissues. However, the effect of P. gingivalis bacteremia on BBB permeability and its underlying mechanism remains unclear. In the present study, rats were injected by tail vein with P. gingivalis three times a week for eight weeks to induce bacteremia. An in vitro BBB model infected with P. gingivalis was also established. We found that the infiltration of Evans blue dye and Albumin protein deposition in the rat brain tissues were increased in the rat brain tissues with P. gingivalis bacteremia and P. gingivalis could pass through the in vitro BBB model. Caveolae were detected after P. gingivalis infection in BMECs both in vivo and in vitro. Caveolin-1(Cav-1) expression was enhanced after P. gingivalis infection.Downregulation of Cav-1 rescued P. gingivalis-enhanced BMECs permeability. We further found P. gingivalis-gingipain could be colocalized with Cav-1 and the strong hydrogen bonding between Cav-1 and arg-specific-gingipain(RgpA) were detected.Moreover, P. gingivalis significantly inhibited the major facilitator superfamily domain containing 2a(Mfsd2a) expression. Mfsd2a overexpression reversed P. gingivalis-increased BMECs permeability and Cav-1 expression. These results revealed that Mfsd2a/Cav-1 mediated transcytosis is a key pathway governing BBB BMECs permeability induced by P. gingivalis, which may contribute to P. gingivalis/virulence factors entrance and the subsequent neurological impairments.

Actuation Spaces Synthesis of Lower-Mobility Parallel Mechanisms Based on Screw Theory

The lower-mobility parallel mechanism has been widely used in the engineering field due to its numerous excellent characteristics.However,little work has been devoted to the actuator selection and placement that best satisfy the system's functional requirements during concept design.In this study,a unified approach for synthesizing the actuation spaces of both rigid and flexure parallel mechanisms has been presented,and all possible combinations of inputs could be obtained,laying a theoretical foundation for the subsequent optimization of inputs.According to the linear independence of actuation space and constraint space of the lower-mobility parallel mechanism,a general expression of actuation spaces in the format of screw systems is deduced,a unified synthesis process for the lower-mobility parallel mechanism is derived,and the efficiency of the method is validated with two selective examples based on screw theory.This study presents a theoretical framework for the input selection problems of parallel mechanisms,aiming to help designers select and place actuators in a correct and even optimal way after the configuration design.

Detection and removal of excess materials in aircraft wings using continuum robot end-effectors

Excess materials are left inside aircraft wings due to manual operation errors,and the removal of excess materials is very crucial.To increase removal efficiency,a continuum robot(CR)with a removal end-effector and a stereo camera is used to remove excess objects.The size and weight characteristics of excess materials in aircraft wings are analyzed.A novel negative pressure end-effector and a two-finger gripper are designed based on the CR.The negative pressure end-effector aims to remove nuts,small rivets,and small volumes of aluminum shavings.A two-finger gripper is designed to remove large volumes of aluminum shavings.A stereo camera is used to achieve automatic detection and localization of excess materials.Due to poor lighting conditions in the aircraft wing compartment,supplementary lighting devices are used to improve environmental lighting.Then,You Only Look Once(YOLO)v5 is used to classify and detect excess objects,and two training data sets of excess objects in two wings are constructed.Due to the limited texture features inside the aircraft wings,this paper adopts an image-matching method based on the results of YOLO v5 detection.This matching method avoids the performance instability problem based on Oriented Fast and Rotated BRIEF feature point matching.Experimental verification reveals that the detection accuracy of each type of excess exceeds 90%,and the visual localization error is less than 2 mm for four types of excess objects.Results show the two end-effectors can work well for the task of removing excess material from the aircraft wings using a CR.

Cusp points and assembly changing motions in the PRR-PR-PRR planar parallel manipulator

Most parallel manipulators have multiple solutions to the direct kinematic problem.The ability to perform assembly changing motions has received the attention of a few researchers.Cusp points play an important role in the kinematic behavior.This study investigates the cusp points and assembly changing motions in a two degrees of freedom planar parallel manipulator.The direct kinematic problem of the manipulator yields a quartic polynomial equation.Each root in the equation determines the assembly configuration,and four solutions are obtained for a given set of actuated joint coordinates.By regarding the discriminant of the repeated roots of the quartic equation as an implicit function of two actuated joint coordinates,the direct kinematic singularity loci in the joint space are determined by the implicit function.Cusp points are then obtained by the intersection of a quadratic curve and a cubic curve.Two assembly changing motions by encircling different cusp points are highlighted,for each pair of solutions with the same sign of the determinants of the direct Jacobian matrices.

Design and modeling of continuum robot based on virtual-center of motion mechanism

Continuum robot has attracted extensive attention since its emergence.It has multi-degree of freedom and high compliance,which give it significant advantages when traveling and operating in narrow spaces.The flexural virtual-center of motion(VCM)mechanism can be machined integrally,and this way eliminates the assembly between joints.Thus,it is well suited for use as a continuum robot joint.Therefore,a design method for continuum robots based on the VCM mechanism is proposed in this study.First,a novel VCM mechanism is formed using a double leaf-type isosceles-trapezoidal flexural pivot(D-LITFP),which is composed of a series of superimposed LITFPs,to enlarge its stroke.Then,the pseudo-rigid body(PRB)model of the leaf is extended to the VCM mechanism,and the stiffness and stroke of the D-LITFP are modeled.Second,the VCM mechanism is combined to form a flexural joint suitable for the continuum robot.Finally,experiments and simulations are used to validate the accuracy and validity of the PRB model by analyzing the performance(stiffness and stroke)of the VCM mechanism.Furthermore,the motion performance of the designed continuum robot is evaluated.Results show that the maximum stroke of the VCM mechanism is approximately 14.2°,the axial compressive strength is approximately 1915 N/mm,and the repeatable positioning accuracies of the continuum robot is approximately±1.47°(bending angle)and±2.46°(bending direction).

;ast forward kinematics algorithm for real-time and ligh-precision control of the 3-RPS parallel mechanism

A new forward kinematics algorithm for the mechanism of 3-RPS （R： Revolute; P： Prismatic; S： Spherical） parallel manipulators is proposed in this study. This algorithm is primarily based on the special geometric conditions of the 3-RPS parallel mechanism, and it eliminates the errors produced by parasitic motions to improve and ensure accuracy. Specifically, the errors can be less than 10 6. In this method, only the group of solutions that is consistent with the actual situation of the platform is obtained rapidly. This algorithm substantially improves calculation efficiency because the selected initial values are reasonable, and all the formulas in the calculation are analytical. This novel forward kinematics algorithm is well suited for real-time and high-precision control of the 3-RPS parallel mechanism.

A brief review on nonlinear modeling methods and applications of compliant mechanisms

Compliant mechanisms （CMs） have become one of the most popular research themes in mechanisms and robotics because of their merits. This paper aims to provide a brief systematic review on the advances of nonlinear static modeling approaches and the applications of CMs to promote interdisciplinaly/multidisciplinary development for associated theories and other new applications. It also predicts likely fh^tre directions of applications and theory development.

Conceptual design,modeling and compliance characterization of a novel 2-DOF rotational pointing mechanism for fast steering mirror

In this paper,a novel 2-DOF rotational pointing mechanism(RPM)is designed inspired by the guidelines of the graphical approach.The mechanism integrates with a fast steering mirror(FSM)for compensating pointing errors of a laser beam.The design intends to achieve an angular travel of±10 mrad and steers a 25 mm mirror aperture.A planar flexure with beam flexures accompanied in parallel with an axial flexure build-up mechanism configuration.Compliant mechanismbased RPM ensures high precision and compactness.Compliance characteristics are established based on the stiffness matrix method for four different planar flexure layouts.One layout with best in-plane rotational compliance is then assessed for performance sensitivity to mechanism dimension parameters and parasitic error,thus informing the design space.Rotational stiffness in both the inplane rotational axes and stress is determined based on finite element analysis(FEA).The wire electrical discharge machining(EDM)is employed for developing the proof of concept for the RPM and is then assembled in FSM.Experiments are conducted to determine the rotational stiffness and angular travel about both in-plane rotational axes.Comparison among theoretical,numerical and experiments reveal excellent linearity of rotational stiffness along the rotational travel range.The maximum theoretical error is less than 5.5%compared with FEA while,the experimental error has a mean of 5%and 3%for both rotational axes thus satisfying the intended design requirement.

A novel shape memory alloy actuated soft gripper imitated hand behavior

The limited length shrinkage of shape memory alloy(SMA)wire seriously limits the motion range of SMA-based gripper.In this paper,a new soft finger without silicone gel was designed based on pre bent SMA wire,and the finger was back to its original shape by heating SMA wire,rather than relying only on heat exchange with the environment.Through imitating palm movement,a structure with adjustable spacing between fingers was made using SMA spring and rigid spring.The hook structure design at the fingertip can form self-locking to further improve the load capacity of gripper.Through the long thin rod model,the relationship of the initial pre bent angle on the bending angle and output force of the finger was analyzed.The stress-strain model of SMA spring was established for the selection of rigid spring.Three grasping modes were proposed to adapt to the weight of the objects.Through the test of the gripper,it was proved that the gripper had large bending amplitude,bending force,and response rate.The design provides a new idea for the lightweight design and convenient design of soft gripper based on SMA.

Understanding coupled factors that affect the modelling accuracy of typical planar compliant mechanisms

In order to accurately model compliant mechanism utilizing plate flexures, qualitative planar stress （Young＇s modulus） and planar strain （plate modulus） assumptions are not feasible. This paper investigates a quantitative equivalent modulus using nonlinear finite element analysis （FEA） to reflect coupled factors in affecting the modelling accuracy of two typical distrib- uted-compliance mechanisms. It has been shown that all parameters have influences on the equivalent modulus with different degrees; that the presence of large load-stiffening effect makes the equivalent modulus significantly deviate from the planar assumptions in two ideal scenarios; and that a plate modulus assumption is more reasonable for a very large out-of-plane thickness if the beam length is large.

Special issue： Mechanisms and robotics

This special issue contains ten papers from researchers in seven countries on three continents and seeks to capture the current interest topics of and late.~;t research results from the Mechanisms and Robotics （M＆R） community. The topics include fundamental synthesis and analysis theory, algorithms, design, control, and experimental validation for mechanisms and robots, as well as their validation in a variety of applications. The papers are organized according to their contributions to the core theoretical methodologies of M＆R, with five papers appearing first. The application areas that follow are biornimetic mechanical devices （two oaoers~ and industrial robots （three papers）.