In this paper, we study dynamic isotropy using natural frequency analysis for a class of symmetric spatial parallel mechanisms (SSPMs) with 2 p ( p≥3) struts. This kind of dynamic isotropy has been defined as the square roots of the eigenvalues of the equivalent mass-spring systems formed via the interactions between rigid-body mechanical systems and their driving systems. Analytic expressions for these eigenvalues are then derived in the task space, which is linearly dependent on p. Furthermore, a general compliance center was found for all the SSPMs in which the parallel mechanisms are fully decoupled. Based on the dynamically decoupling, then, dynamic isotropy for the SSPM is discussed which shows that though taking the inertial parameters into consideration, the SSPM can not attain complete dynamic isotropy and the optimal dynamic isotropy index is the quartic root of two. At the end of the paper, to demonstrate these results, an example is given.

A shaking table is an important facility for earthquake damage testing of large scale structures like high buildings,large bridges and infrastructural systems.To improve the payload capacity in the test for large-scale structures or real-scale structures,hydraulically redundant actuated parallel mechanisms are usually adopted.By exploring the interactions between the mechanical parts and the hydraulially actuated systems,an integrated model of hydraulically redundant actuated shaking table was provided.Then,a framework of degree-of-freedom(DoF) control for this type of shaking table was put forward.To regulate forces among the redundant actuated systems and to reduce force couplings,an inner force balancing control method based on pseudo-inverse of Jacobian matrix was also employed.Simulations validated the proposed DoF control and inner force balancing method.

Geometry parameters and lengths of legs often have close relations with the singularity of a Gough-Stewart platform. When these parameters are improperly selected,the platform may have hunt singularities. The relations among the geometrical parameters and the leg lengths when in hunt singularity for 3-3、6-3、 6-6 Gough-Stewart are derived according to their geometrical constrains. The obtained relations can be used as a geometrical criterions,for a Gough-Stewart platform with given geometrical parameters,to determine if the mechanism may have or close to Hunt singularity in its workspace. Two experiments are performed,which validates the hunt singularity criterions.

The hydraulically driven 6-DOF parallel Gough-Stewart mechanism was used as a motion simulator to simulate the real motions of carrier for testing firing control systems in laboratory.The bidirectional force/torque versus velocity interactions between mechanical dynamics system and its hydraulic driving system was explored to establish an integrated dynamic model.Then the couplings of pose outputs were presented and experimental results show that the couplings between these outputs will become more intense with the increase of motion frequency within natural frequency of the Gough-Stewart mechanism.Moreover,for the safety of the motion simulator,a simple criterion which only uses the lengths of legs instead of complicated and time-consuming method of condition number of Jacobian is proposed to distinguish singular area from the workspace.Amplitude and Phase Control(APC) and random wave duplication techniques were employed to greatly improve the simulation fidelity not only for sine waveform but for random waveform as well.

Parallel mechanisms are widely adopted in simulation system as motion simulator in virtue of high load/weight,high stiffness etc.If improperly designed,there will exist singularities in its workspace which constitutes great threat to the safety of motion simulator.Hunt singularity is a kind of frequently occurred singularity.Based on Ma and Gosselin's classification for parallel mechanism singularity,several kinds of singularities like architecture singularity,typical configuration singularities(Hunt singularity and Fitcher singularity)were analyzed.Kinematic and dynamic performance were presented for the typical Gough-Stewart platform when it was approaching to Hunt singularity.