The Characteristic Analysis Of Pentahedral Deployable Truss Unit

With the continuous development and progress of space technology,the domestic demand for large-space structures is increasing rapidly.In order to meet the needs of large-scale space truss assembly;in space orbit,this paper designed a pentahedral truss unit based on the characteristic that pentahedral truss unit can be combined into various types of truss structure.The structure can be placed in a retracted state on the ground to facilitate launching.When it is launched to the designated space location,the constraint of the pentahedral expandable truss unit is released and the unit is unfolded.Then the space truss structure is obtained by space assembly based on this unit.In this paper,two folding methods are designed for pentahedral deployable truss unit,and the degree of freedom,kinematics and dynamics of the unit are analyzed.Based on the necessary stability criterion of Maxwell geometry system,the fully expanded pentahedral deployable truss unit model is analyzed.The analysis result shows the stability of the basic unit structure can be guaranteed by the expanded pentahedral truss unit.This is the premise of the overall structural stability.Through the pre-set connecting rod on the basic unit,multiple pentahedral deployable truss units can be connected into a variety of truss structures.In this paper,the constraint helix theory and anti-helix theory are applied to analyze the degrees of freedom of the single closed-loop structure and the whole structure of the pentahedral deployable truss unit.The results show that when the bottom of the unit is always parallel to the set coordinate axis plane,the single closed-loop degree of freedom and the overall degree of freedom of the unit are 1;when the bottom of the unit is not parallel to the set coordinate axis plane,the single closed-loop degree of freedom and the overall degree of freedom of the unit are not 1.By using the D-H coordinate transformation method,the kinematic analysis of the pentahedral deployable truss unit with the unfolding degree of freedom is 1.Select the representative points of the two motions on the unit.Two special points of motion on the unit are selected,and the position parameter equation of the special point is obtained by coordinate transformation.The derivation of the position parameter equation leads to the velocity parameter equation of the special point;the derivation of the velocity equation leads to the acceleration parameter equation of the special point.Based on the designed pentahedral deployable truss unit model,the position,velocity and acceleration curves of special points can be calculated by derived position,velocity and acceleration parameter equations.Lagrangian method is used to analyze the dynamics of the pentahedral deployable truss unit.The dynamic characteristics of the pentahedral deployable truss unit driven by torsion spring are obtained,and the energy parameter equation of each part of the pentahedral truss unit is obtained.The relevant parameters of the designed pentahedral deployable truss unit model are introduced into the obtained energy parameter equation,and the acceleration curves of two points under different torsion spring stiffness coefficients are calculated.Using ADAMS to simulate the unfolding process of pentahedral deployable truss unit,and the simulation results are compared with kinematic parameter equation.The comparison results show that the kinematic equations derived from the theory fit the simulation results basically,which indicates the theoretical analysis.The derived position,velocity and acceleration parameter equations can well describe the expansion process of the pentahedral deployable truss unit.