Force Analysis of the Overconstrained Mechanisms Based on Equivalent Stiffness Considering Limb Axial Deformation

Overconstrained mechanism has the advantages of large bearing capacity and high motion reliability,but its force analysis is complex and difficult because the mechanism system contains overconstraints.Considering the limb axial deformation,taking typical 2SS+P and 7-SS passive overconstrained mechanisms,2SPS+P and 7-SPS active overconstrained mechanisms,and 2SPS+P and 7-SPS passive-input overconstrained mechanisms as examples,a new force analysis method based on the idea of equivalent stiffness is proposed.The equivalent stiffness matrix of passive overconstrained mechanism is derived by combining the force balance and deformation compatibility equations with consideration of axial elastic limb deformations.The relationship between the constraint wrench magnitudes and the external force,limb stiffness is established.The equivalent stiffness matrix of active overconstrained mechanism is derived by combining the force balance and displacement compatibility equations.Here,the relationship between the magnitudes of the actuated wrenches and the external force,limb stiffness is investigated.Combining with the equivalent stiffness of the passive overconstrained mechanism,an analytical relationship between the actuated forces of passive-input overconstrained mechanism and the output displacement,limb stiffness is explored.Finally,adaptability of the equivalent stiffness to overconstrained mechanisms is discussed,and the effect of the limb stiffness on overconstrained mechanisms force distribution is revealed.The research results provide a theoretical reference for the design,research and practical application of overconstrained mechanism.

Configuration Synthesis and Lightweight Networking of Deployable Mechanism Based on a Novel Pyramid Module

Deployable mechanism with preferable deployable performance,strong expansibility,and lightweight has attracted much attention because of their potential in aerospace.A basic deployable pyramid unit with good deployability and expandability is proposed to construct a sizeable deployable mechanism.Firstly,the basic unit folding principle and expansion method is proposed.The configuration synthesis method of adding constraint chains of spatial closed-loop mechanism is used to synthesize the basic unit.Then,the degree of freedom of the basic unit is analyzed using the screw theory and the link dismantling method.Next,the three-dimensional models of the pyramid unit,expansion unit,and array unit are established,and the folding motion simulation analysis is carried out.Based on the number of components,weight reduction rate,and deployable rate,the performance characteristics of the three types of mechanisms are described in detail.Finally,prototypes of the pyramid unit,combination unit,and expansion unit are developed to verify further the correctness of the configuration synthesis based on the pyramid.The proposed deployable mechanism provides aference for the design and application of antennas with a large aperture,high deployable rate,and lightweight.It has a good application prospect in the aerospace field.

Design and analysis of truss deployable antenna mechanism based on a novel symmetric hexagonal profile division method

As the deployment,supporting,and stability mechanisms of satellite antennas,space-deployable mechanisms play a key role in the field of aerospace.In order to design truss deployable antenna supporting mechanisms with large folding rate,high accuracy,easy deployment and strong stability,aiming at the geometric division of the parabolic reflector,a novel method based on symmetric hexagonal division and its corresponding modular truss deployable antenna mechanism is proposed,and the original method based on asymmetric triangular division and its corresponding mechanisms are presented for comparative analysis.Then,the screw theory is employed to analyze the mobility of different mechanisms.Furthermore,the improved three-dimensional mesh method is used to divide the reflector surface of a large parabolic antenna designed by the two different methods,and the profile accuracy and the type of links are taken as the evaluation indexes to quantitatively analyze the division results.Finally,a three-dimensional model of the modular deployable mechanism based on the symmetric hexagonal design is developed,and the deployable mechanisms with different configurations based on the two design methods are compared and analyzed from the mechanical perspective.The research results provide a good theoretical reference for the design of deployable truss antenna mechanisms and their application in the aerospace field.

Design and analysis of a truss deployable antenna mechanism based on a 3UU-3URU unit

Space deployable structures with large calibers, high accuracy, and large folding ratios are indispensable equipment in the aerospace field. Given that the single-DOF 3 RR-3 RRR deployable unit cannot be fully folded, this study proposes a 3 UU-3 URU deployable unit with two kinds of DOF: folding movement and orientation adjustment. First, based on the G-K formula, the DOF of the 3 UU-3 URU unit is analyzed. Then, the 3 UU-3 URU unit is used to construct a deployable truss antenna with a curved surface, and the DOF of the whole deployable antenna containing multiple 3 UU-3 URU units is calculated. The structural design of a deployable antenna with two loops is carried out with specific parameters and geometric relations. Next, a DOF simulation of a basic combination unit composed of three 3 UU-3 URU units is performed. Finally, a prototype of the basic combination unit is manufactured, and the DOF of the mechanism is experimentally verified.

Accurate and efficient follower log repair for Raft-replicated database systems

State machine replication has been widely used in modern cluster-based database systems.Most commonly deployed configurations adopt the Raft-like consensus protocol,which has a single strong leader which replicates the log to other followers.Since the followers can handle read requests and many real workloads are usually read-intensive,the recovery speed of a crashed follower may significantly impact on the throughput.Different from traditional database recovery,the recovering follower needs to repair its local log first.Original Raft protocol takes many network round trips to do log comparison between leader and the crashed follower.To reduce network round trips,an optimization method is to truncate the follower’s uncertain log entries behind the latest local commit point,and then to directly fetch all committed log entries from the leader in one round trip.However,if the commit point is not persisted,the recovering follower has to get the whole log from the leader.In this paper,we propose an accurate and efficient log repair(AELR)algorithm for follower recovery.AELR is more robust and resilient to follower failure,and it only needs one network round trip to fetch the least number of log entries for follower recovery.This approach is implemented in the open source database system OceanBase.We experimentally show that the system adopting AELR has a good performance in terms of recovery time.

High-availability in-memory key-value store using RDMA and Optane DCPMM

1 Introduction and main contributiions Emerging hardwares like remote Direct Memory Access(RDMA)capable networks and persistent memory(PM)are promising to build fast high availability in-memory key-value stores.The recent advent of Intel Optane DC Persistent Memory Modules(Optane DCPMM)brings the future closer.However,existing studies to combine the two devices cannot deliver the desired performance due to their two-phase protocols for log shipping and most of them were based on emulation that perform sub-optimally on real PM hardware.

Plover:parallel logging for replication systems

Logging and replication are commonly used recovery approaches in database systems.To guarantee that the database state is not corrupted due to system crash,database systems rely on a centralized logging method to persist log entries into a stable storage device;to prevent data loss due to device failure,a primary server in the database system periodically replicates its state to backup servers by copying log entries over networks.As the transaction execution in a modern database system is highly parallelized,the centralized logging with a single I/O channel tends to inhibit the scalability of the system.Meanwhile,log entries generated at high speed make a network with limited bandwidth a potential bottleneck for replication.In this paper,we propose an in-memory transaction engine named Plover with parallel logging and speedy replication for primary-backup replication systems.The parallel logging enables concurrent execution of logging by utilizing multiple log buffers associated with multiple stable storages.All log entries in the log buffers maintain a global sequence number(GSN),which ensures a partial order among transactions.The kernel of the speedy replication is an adaptive shipping method,which allows to transfer data increments instead of log entries to backups under heavy workloads.Experimental results using the YCSB and TPC-C benchmarks show that Plover scales well with the increasing number of worker threads and stable storage devices.And our adaptive shipping requires only one fifth network bandwidth of the conventional log shipping.