Haibo Qu

Type synthesis of 4-dof nonoverconstrained parallel mechanisms based on screw theory

A systematic method is developed for the type synthesis of 4-DOF nonoverconstrained parallel mechanisms with three translations and one rotation inspired by the design of H-4. First, the motion requirements of primary platform and secondary platform of the 4-DOF nonoverconstrained parallel mechanisms are analyzed, and the conflict between the number of actuators and the constraint system for nonoverconstrained parallel mechanism is solved. Then, the research topic of type synthesis of 4-DOF nonoverconstrained parallel mechanisms is transformed into the type synthesis of the secondary platform with three translational DOF linked by two chains. On the basis of the screw and reciprocal theory, all possible secondary limbs with 3-DOF, 4-DOF, and 5-DOF are synthesized, respectively. Finally, the configurations and spatial assembly conditions of all possible secondary limbs are provided and some typical mechanisms are sketched as examples.

Type Synthesis of 2R2T Parallel Mechanisms Based on Motion Equivalent Chain Method

In this paper, the motion equivalent chain method is proposed and then applied to the type synthesis of a class of 2R2T parallel mechanism. The equivalent serial chains are synthesized for a specific 2R2T motion pattern based on screw theory. Feasible limb structures that provide a constraint couple and a constraint force are enumerated according to the reciprocity of the twist and wrench systems. Several motion equivalent single loop chains are constructed with the equivalent serial chains. Using motion equivalent single loop chains to replace the equivalent serial chains, a class of 2R2T parallel mechanisms is obtained based on the foundation of motion equivalent single loop chain structures.

Theory of degrees of freedom for parallel mechanisms with three spherical joints and its applications

The analysis of degrees of freedom(DOF) of a moving platform is the fundamental problem in kinematics of parallel mechanism. However, many problems should be considered to correctly perform the DOF calculation by using the traditional DOF criterion, and it is difficult to find a DOF criterion suitable for all kinds of mechanisms. A rule that can be used to determine the position and orientation of the moving platform is presented. Based on the proposed rule, a new form of DOF criterion is proposed, which is suitable for a class of parallel mechanisms with three spherical joints attached to the moving platform. The basic types of generalized limb structures are given based on the possible dimension of achieving the center of spherical joint attached to the moving platform, and the general steps of analyzing the DOF are presented. This proposed formula simplifies the DOF analysis of parallel mechanisms with spherical joints attached to the moving platform, and plays an important role in structural synthesis of such parallel mechanisms.

A new method for isotropic analysis of limited dof parallel manipulators with terminal constraints

Based on the terminal constraints system (TCS) and reciprocal screw theory, a novel method is presented to determine the isotropic configurations of limited degree-of-freedom (DOF) parallel manipulators. From the available physical meaning of isotropy, the criteria to determine the isotropic configurations can be transformed to investigate whether the TCS acting on the moving platform works equally well in all directions. From the TCS study, the simplest form of constraints system matrix can be obtained. Then the constraint condition number is defined to measure the isotropy of spatial parallel manipulator based on the TCS. This method not only avoids solving the Jacobian matrix for some complex structural parallel manipulators but also points out the physical meaning of isotropy, which indicates that the TCS acting on the moving platform works equally well in all directions. Three examples are employed to illustrate this method.

A class of reconfigurable parallel mechanisms with five-bar metamorphic linkage

This paper presents a planar five-bar metamorphic linkage which has five phases resulting from locking of motors. Reconfigurable limbs are constructed by integrating the five-bar metamorphic linage as sub-chains. The branch transition of metamorphic linkage is analyzed. By adding appropriate joints to the planer five-bar metamorphic linkage, reconfigurable limbs whose constraint can switch among no constraint, a constrained force and a constrained couple are obtained. Serial limb structures that can provide a constraint force and a constraint couple are synthesized based on screw theory. Reconfigurable limbs that have five configurations associated with the five phases of the five-bar metamorphic linkage are assembled with 4-DOF (degrees-of-freedom) serial chains. A class of reconfigurable parallel mechanisms is derived by connecting the moving platform to the base with three identical kinematic limbs. These parallel mechanisms can perform various output motion modes such as 3T, 3R, 2T1R, 1T2R, 3T1R, 2T2R, 1T3R, 2T3R, 3T2R and 3T3R. Finally, the potential application of the proposed mechanisms is analyzed and conclusions are drawn.

A serial of novel four degrees of freedom parallel mechanisms with large rotational workspace

SUMMARY In this paper, a class of novel four Degrees of Freedom (DOF) non-overconstrained parallel mechanisms with large rotational workspace is presented based on screw theory. First, the conflict between the number of independent constraints applied on the moving platform and the number of kinematic limbs for 4-DOF non-overconstrained parallel mechanism is identified. To solve this conflict, the platform partition method is introduced, and two secondary platforms are employed in each of the parallel mechanisms. Then, the motion requirements of the secondary platforms are analyzed and all the possible kinematic chains are enumerated. The geometrical assembly conditions of all possible secondary limbs are analyzed and some typical non-overconstrained parallel mechanisms are generated. In each of the parallel mechanisms, a planetary gear train is used to connect both of the secondary platforms. The large rotational workspace of the moving platform is obtained due to the relative motion of the two secondary platforms. Finally, the kinematics analysis of a typical parallel mechanism is conducted.

A novel 4-UPU translational parallel mechanism with fault-tolerant configurations

This paper aims at designing a pure translational parallel mechanism constructed by UPU (universal-prismatic-universal joint) kinematic limbs. First, the typical problem of unexpected rotations is pointed out from analyzing the typical 3-UPU parallel mechanism, and the reason for unexpected rotations of parallel mechanism constructed by UPU kinematic limbs is analyzed. Then, in order to design a pure translational parallel mechanism constructed by UPU without the unexpected rotations, the 2-UPU single loop is chosen as the basic structure to construct the 4-UPU translational parallel mechanism. Each 2-UPU single loop can be used to constrain a rotation about an axis of the linear complexes, which defined the unexpected rotations. Therefore, a novel type of 4-UPU pure translational parallel mechanism with redundant actuations is proposed. Since the existence of redundantly actuated kinematic limb, this proposed parallel mechanism possesses analytical forward kinematics, and its singularity can be avoided completely. Finally, the workspace and fault-tolerant performance are analyzed. When the proposed 4-UPU parallel mechanism is located in a fault-tolerant configuration, the moving platform can still possess movable ability to realize the given task even if one kinematic limb is in locked-joint failure mode, and the fault-tolerant workspace is obtained.

Structural synthesis of a class of 3-DOF wrist mechanisms with redundantly-actuated closed-loop units

In this paper, a new method is proposed for the structural synthesis of a class of redundantly-actuated parallel wrists (RaPWs) with three rotational degrees of freedom of the moving platform and symmetrical structures based on screw theory. First, the new procedure for structural synthesis of RaPWs with closed-loop actuated unit is proposed and the constraint system of the moving platform of RaPWs is analyzed. Then, considering the inclusion relation between the primary constraint system and the limb constraint system, the type of kinematic limb is determined. The synthesis of type-1 and type-2 kinematic limbs is dealt with based on the obtained closed-loop actuated unit and open-loop sub-limb. Next, the RaPWs are synthesized and a number of new RaPWs have been identified. Finally, the condition for proper actuator selections of RaPWs is revealed, and one example is used to perform the validation.

Structure synthesis of reconfigurable parallel mechanisms with closed-loop metamorphic linkages

This paper proposes a class of closed-loop metamorphic linkages, which has different phases resulting from links annexing or locking of motors. Reconfigurable limbs are obtained by assembling these metamorphic linkages with kinematic chains. The potential metamorphic linkages are presented and the working phase transformation of the metamorphic linkages is analyzed. After adding suitable kinematic joints to the metamorphic linkage, the reconfigurable limbs whose constraint can be switched among different constraint forces and couples are synthesized. The serial limbs that can provide u (u = 0, 1, 2) constraint forces and v (v = 0, 1, 2) constraint couples are constructed by using screw theory method. The reconfigurable limbs which possess different configurations are combined with serial kinematic chains. By connecting the end moving platform to the fixed base with three identical kinematic limbs, a family of reconfigurable mechanisms with closed-loop metamorphic linkages is derived. These mechanisms have various output motion modes, such as 3R, 1T2R, 2T1R, and 3T.

Design and Optimization of a Novel Three-Dimensional Force Sensor with Parallel Structure

To measure large external forces exerted on a loading platform, a novel three-dimensional force sensor is developed in this paper. The proposed sensor was designed with a parallel mechanism with three degrees of freedom. Kinematic analysis of this sensor was performed. Due to its structural characteristics, the working principle of the sensor was analyzed using a Jacobian matrix. The sensitivity diversity index and measuring capability were both calculated. The analysis showed that the proposed sensor is more suitable for measuring large forces than existing strain sensors. In addition, compared with existing strain sensors, this sensor is more suitable for measuring forces along the x and y axes. By changing the stiffness coefficients of the springs, the proposed sensor has reconfigurability. This sensor can change its measuring capability to meet different requirements. Next, the mode shapes and natural frequencies of the proposed sensor were performed. Finally, based on these performance indices, the design variables were optimized using a Multi-Objective Genetic Algorithm.