Virginia Tech Fall 2019 - Present
A truss structure in an ill-supported state (ie by damage or during assembly) will cause abnormal stress and strain concentrations throughout the structure, leading to deformation and increasing the likelihood of further damage. By correcting for this improper support through external forcing agents, the structure will be returned to an operational state. Combining state estimation, sensor feedback, and structural calculations provides strategic forces to mimic a healthy strut. This will result in a stabilized truss where misalignments will be resolved and further efforts to repair damages can occur.
As seen throughout the history of space truss analysis, starting with the ISS, the deformation of large damaged space structures is a critical concern. Moving into the ISAM paradigm allows for an existing framework to rectify these damages. Although the ISAM state of play and many individual ISAM projects cite repair as a critical technology, it is vastly underdeveloped in favor of flashier assembly methodologies. By actively pursuing repair efforts, space structures can be built to last beyond their project lifespan, replacing damaged elements with new healthy members. A cornerstone seen in terrestrial damage cases focuses on the support cases for stabilizing preexisting damaged structures for reconstruction. These active applied force stabilization methods have yet to be fully explored within the ISAM sphere and by bridging this gap, the reality of in-space repair efforts is one step closer to implementation.
Truss deflection due to a damaged member
This work aims to fill a gap in research targeting the repair sector of the ISAM field
Structures run the risk of being damaged leading to deflections, stress concentrations, and cascading damage
This damage can lead to mission failure and early termination of structure lifetime
Working in an assembly scope, presents an opportunity for socketable element repair in structures
Before this takes place though, the structure must be realigned to prevent further damage
This research creates a methodology and application for properly realigning and stabilizing structures
In addition, this research opens up assembly avenues for procedure steps in which elements are cantilevered or otherwise improperly supported
A series of six trials was run to prove the adaptability of the system. For each trial, one parameter of the truss architecture or loading was changed to allow for a more direct comparison of damage behavior.
Correction Methodology
For a determined set of timesteps defined by the system:
Define a number of timesteps
Divide the total axial force needed by the number of time steps
Apply each discretized iteration of the force at the damaged node site
Transform that iterated force into the local node frame
Repeat until aligned
This is continued along the Primary Force Control section which describes the force-informed motion path generated by the Ansys solver and the inverse kinematics code. Once the system has achieved the desired location, it is anticipated that there will be some small misalignment still due to the fact that simulated models never completely represent real-world systems. This small misalignment can then be corrected through positioning alone. This is due to the fact that the system is much more stable after correcting the majority of the deflection.
Trial Results
Trial 3 Results
10 Bay Structure, 7.5m, 1 in hollow cylindrical strut, -z loading at the tip (4 kN)
Critical model values:
Healthy tip deflection: 1.388 m
Damaged tip deflection: 1.998 m
Healthy member axial force: 6.750 kN
Upper Right: Healthy Truss Deflection, Lower Left: Healthy Truss Axial Force, Lower Right: Damaged Truss Deflection
When correcting for this motion, JACS must follow the prescribed motion path assigned to it. This motion path can be calculated through a series of inverse kinemaics waypoints to create a trajectory. Here in 20 time steps the path taken can be seen in green on the right figure to provide that force into the truss. The corresponding angles for the 3 pairs of linear actuators are also plotted to shoe that the motion taken in generally 1 dimension dominant but requires the full degrees the stweart platform provides for an optimal path.
Robot Actuator Angles and Plotted Realignment Path
Trial 3 Applied Robotic Force and Corresponding Tip Motion for Each Timestep
Trial 3 Truss Correction [10x scale]