Yundou Xu

Measurement Theory and Experimental Study of Fault-Tolerant Fully Pre-Stressed Parallel Six-Component Force Sensor

To meet the high reliability requirement under complicated environment and overcome the disadvantages of parallel structure six-component force sensor with traditional spherical pairs, the design concepts of fault-tolerance and fully prestressing are proposed to the design of parallel six-component force sensor. A kind of fault-tolerant and fully prestressed parallel structure force sensor using modified spherical pairs is proposed. The structural characteristics of the sensors are analyzed in comparison with traditional Stewart platform-based sensor. The complete mathematic model including the models with and without signal fault is established using screw theory and considering the relations of stiffness and force distribution. The mathematical expression to obtain the lower bound of prestressed force is determined. The static calibration experimental study of a sensor prototype is carried out, and the experimental results prove the superiority of the structure and the correctness of theoretical analysis. Besides, a fault diagnosis method is proposed and verified with a tracking task, which provides a basis for the applied research of the fault-tolerant sensor.

Type synthesis of spatial mechanisms for forging manipulators

In this study, a systematic method is proposed to synthesize the parallel and hybrid serial–parallel mechanisms for the forging manipulators based on the screw theory. First, several typical configurations of five-degrees-of-freedom parallel mechanism for the forging manipulators are synthesized, and they are all non-overconstrained mechanisms. Then, two kinds of hybrid serial–parallel mechanisms for the forging manipulators with the advantages of motion decoupling are constructed, which are also not overconstrained. The configurations obtained in this study would provide more type selection for the heavy-duty forging manipulators in engineering.

Stiffness modelling and comparison of the 5-UPS/PRPU parallel machine tool with its non-redundant counterpart:

This article presents a derivation of the stiffness matrix of a general redundantly actuated parallel mechanism based on the overall Jacobian. The Jacobian of the constraints and actuations is derived using reciprocal screw theory. Based on the mapping relationship between constraint, actuated and external forces combined with the principle of virtual work, a compatibility equation for the deformation of all of the limbs is achieved, and the stiffness model of the general redundantly actuated parallel mechanism is derived. The 5-UPS/PRPU redundantly actuated parallel machine tool is used to illustrate this method. The parallel machine tool comprehensively reveals the effect of the elastic deformation of active–passive joints and some basic transmission parts. The stiffness model is further validated by experimental data. Moreover, the global stiffness matrix of the general redundantly actuated parallel mechanism can be separated into two parts via matrix decomposition. The first part is the stiffness matrix of the corresponding non-redundant parallel mechanism, and the second part is the stiffness matrix of the redundantly actuated limbs (actuators). The redundantly actuated 5-UPS/PRPU parallel machine tool is also investigated for further analysis. The different stiffness characteristics of the machine tool and its corresponding non-redundant 5-UPS/PRPU parallel machine tool are compared. Actuation redundancy is found to improve the stiffness performance of the machine tool efficiently.

Task-oriented Design Method and Experimental Research of Six-component Force Sensor

As the isotropy performance widely used for evaluating parallel six-component force sensor is not suitable for a certain task, the task-oriented design method of the sensor is proposed. The mathematic model of sensor is built with the screw theory; based on the task requirement, the task ellipsoid is established. Then, with ellipsoid method, the design method for the sensor structure is proposed and the structure constraints are deduced. In order to perform peg-in-hole assembly task with the task-oriented sensor, the sensor prototype is designed and manufactured. Based on the assembly platform, the experimental research is carried out. The research results are useful for the design and application of six-component force sensor.

Rotational Axes Analysis of the 2-RPU/SPR 2R1T Parallel Mechanism

This paper presents a novel 2-RPU/SPR 2R1T parallel mechanism, which has one translational degree of freedom (DOF) and two rotational DOFs. The properties of the rotational axes of the mechanism are analyzed by using screw theory, and the results show that the 2-RPU/SPR mechanism has two rotational continuous axes. The position analysis of the 2-RPU/SPR mechanism is also carried out. The proposed 2R1T mechanism has very simple kinematic model, which reduces difficulties in calibrating kinematic parameters.

A Contrastive Study on the Force Analysis of the Active and Passive Overconstrained Parallel Mechanisms

This paper reveals that the distinction between the statically indeterminate force problem of the active overconstrained parallel mechanisms (PMs), i.e., the redundantly actuated PMs, and that of the passive overconstrained PMs is the displacement coordination condition. For the redundantly actuated PMs, the displacement consists of the input displacements of the actuators and the elastic deformations of the system. While for the passive overconstrained PMs, it just refers to the elastic deformations of the system. Besides, taking the typical 2RPR+P redundantly actuated mechanism as an example, the relationship between the input displacement and the driving force/torque of an actuator under the position and torque control modes are discussed. The results will provide an important theoretical guidance for the control of the redundantly actuated mechanisms.

Motion Decoupling Analysis of a Kind of 2R Parallel Mechanism with Two Continuous Rotational Axes

The two-rotational-degrees-of-freedom (2R) parallel mechanism (PM) with two continuous rotational axes (CRAs) has a simple kinematic model. Therefore, it is easy to implement trajectory planning, parameter calibration, and motion control, which allows for a very variety of application prospects. Aiming at three different structural forms of a kind of 2R-PM with two CRAs (2UPS-RR mechanism), comparative analyses on motion decoupling were carried out in this paper, the results showed that two rotational degrees of freedom are completely decoupled for a structural form of the studied mechanism, and the regular features that make it be completely decoupled were also summarized.

Type synthesis of two-degrees-of-freedom rotational parallel mechanism with two continuous rotational axes

The two-rotational-degrees-of-freedom(2R) parallel mechanism(PM) with two continuous rotational axes(CRAs) has a simple kinematic model. It is therefore easy to implement trajectory planning, parameter calibration, and motion control, which allows for a variety of application prospects. However, no systematic analysis on structural constraints of the 2R-PM with two CRAs has been performed, and there are only a few types of 2R-PM with two CRAs. Thus, a theory regarding the type synthesis of the 2R-PM with two CRAs is systematically established. First, combining the theories of reciprocal screw and space geometry, the spatial arrangement relationships of the constraint forces applied to the moving platform by the branches are explored, which give the 2R-PM two CRAs. The different distributions of the constraint forces in each branch are also studied. On the basis of the obtained structural constraints of branches, and considering the geometric relationships of constraint forces in each branch, the appropriate kinematic chains are constructed. Through the reasonable configuration of branch kinematic chains corresponding to every structural constraint, a series of new 2R-PMs with two CRAs are finally obtained.

A Five-Degree-of-Freedom Hybrid Manipulator for Machining of Complex Curved Surface

In this paper, a novel five-axis hybrid manipulator is constructed based on the 2-RPU&UPR parallel mechanism, which has less joints and all the axes of rotation are continuous. Then, the structure of the key components of the new five-axis hybrid manipulator is designed. Next, the analytic expression of the inverse position of the hybrid manipulator is established, based on which, the workpiece of the spherical surface is machined. All rotational degrees of freedom of this five-axis hybrid manipulator are of continuous axes, the analytical expression of the inverse position model can be obtained, so the trajectory planning and motion control can be realized easily. Moreover, the hybrid manipulator has less joints, which can guarantee high rigidity and high precision operation, so it has great application prospects.

Isotropy Analysis of a Stiffness Decoupling 8/4-4 Parallel Force Sensing Mechanism

A stiffness decoupling 8/4-4 parallel force sensing mechanism (PFSM) is presented. Its mathematic model is established with screw theory. The force mapping relation is studied and the stiffness matrix is found to be a diagonal matrix, which proves the stiffness decoupling characteristics of the mechanism. According to the concept of fully isotropy, the isotropy conditions are analyzed, the parameters which meet fully isotropy are given. The 8/4-4 PFSM’s configuration under isotropy parameters is analyzed. Based on this configuration, an 8/4-4 mechanism cluster which meets the fully isotropy is presented. The cluster’s configuration is classified and induced into four main configurations according to the different parameter conditions.