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2006 - 2007: NASA Tetrahedral Robotics Research




The tetrahedral system that we explored is classified as ART (Autonomous Reconfigurable Technology), which means it uses modular/reconfigurable components to construct complex robotic structures. The individual components that comprise this system are designed to be as simple as possible. TET robotics employ linear actuators in tetrahedral configurations. The coordinated extension and contraction of these linear actuators allows for a myriad of complex motions. In addition, the ability to replace broken parts easily and configure for different tasks makes such technology desirable for space robotics. NASA is presently exploring this technology for rovers, but has a large array of other applications in mind.



The autonomy of these structures comes from the controller hierarchy. NASA wants to limit the amount of commands it needs to send to the rover, and the lag-time between transmission and reception is appreciable even at the speed of light. For this reason it is ideal to be able to send a basic set of commands like move to this location and perform test. To do this, 3 control levels are required. The first involves the command sent by NASA. Then the rover's central brain must translate this command into the necessary length configurations for the structure. This includes both walking and avoiding obstacles. These length command then proceed to each strut to be controlled independently by a PID or some type of decentralized adaptive control for precise movement. Coordination on the second control level is being explored in two main ways. The simplest approach involves constructing a library of basic movement commands (walking gaits) and executing them in a desired order. After a base library has been complete a neural based control is sought to learn new motions through interacting with the environment.

Summer 2006

NASA / ESMD Faculty Student Fellowship GSFC outside of D.C.
Dynamic modeling and control implementation for a 12TET

Using inertial tensors and motion constraints, we were able to construct dynamic kinematics models from the Euler-Lagrange equations constructed within SimMechanics. This software made the assembly of modular components quick and easy. It operates within MATLAB using a graphical block interface just like Simulink. In fact it can interact with Simulink and other packages including custom MATLAB code. If you are not familiar with Simulink, it is very similar to LabView, but in my opinion it offers considerably more modeling power. Using SimMechanic, a single strut was constructed with the desired frictional and motor behaviors, and then it was simply cut and paste into nodal configurations. Once the model was obtained Simulink and MATLAB code was used to implement the strut control and high level walking gait commands.

During that summer, We were employed on site at Goddard Space Flight Center in Maryland for modeling and control work. The other two members of the Hope College team were Aaron Silver and our mentor Dr. Miguel Abrahantes. We teamed up with another controls team from Georgia. All of our work was coordinated. We worked alongside NASA employees and other student teams exploring the mechanical and electrical engineering aspects of the project. Each week we reported progress and discussed new ideas. Our models became invaluable to understanding the dynamics of the 12TET and optimal mechanical designs such as minimizing the diameter and spread of the struts meeting at the node.



12TET Kinematics model constructed in MATLAB w/ Simulink & SimMechanics:




Below is a concept video from the prototype that was built in parallel to our model.




Spring 2007

NASA/MSGC Hope College: 
Exploration of potential strut controllers Application of Decentralized Adaptive Controller in 1TET Model

The adaptive Seraji controller was determined to be unstable over long executions and discrete inputs.
The struts responded well to PI control, which was ultimately chosen for application in 4TET.

Summer 2007

NASA/ESMD Hope College:
Design, Construction & Control of a 4TET Prototype

The 4TET followed the construction of a 1TET assembled as an Engineering Design project. This was the first closed loop controlled walking gait of an over-constrained tetrahedral robot ever! The summer team consisted of Dr. Miguel Abrahantes, myself, Aaron Silver and Dan Lithio. Collectively we developed the control scheme and hardware for the 4TET 's autonomous control. The control system took 5 weeks to implement and cost $1k in hardware.

Control: MATLAB, USB DAQ, IFI controller Board, Custom String POTs, ESCs and Relays

We used an onboard processor from IFI that read in 2 analog signals from a reference passed through the laptop. The analog signals were multiplexed to correspond to each strut. This let the processor know what length the struts should be. The struts reported their lengths via spring loaded string potentiometers I constructed from retractable key chains. The struts themselves were made from power car antennas. The on board control was a simple PI control where the weighted addition of error and integral error defined how much power to send to the motor. The non-responsive dead-zone of the motor was corrected for. The power was controlled with 7 PWM signals out of the processor. Alternate struts were controlled via relays because we never need to control the 3 floor struts. Each PWM went to a Victor 883 electronic speed controller which was capable of reversible polarity and was provided with a 12V power supply.




Our implementation that summer required a wired tether, but Miguel implemented a wireless communication system the following summer.


Closing thoughts on the Project

This project was of particular interest to me as it extended on the concept of emergence within a complex system. Consider a system composed of simple fundamental units with simple interactions between each unit. In the presence of an energy flow through an open system, entropy within the system can actually decrease as long as entropy external to the system is increasing. This phenomenon is capable of capturing all the order we see around us, be it man made or natural. What I find most fascinating about this is even the simplest systems are capable of producing chaos while the most complex systems are capable of producing awe inspiring order . Mathematics is the key. It provides a window into the most fascinating realms of reality thought to be forever unintelligible.