Modeling and Simulation of the Seismic TETwalker Concept

Transcription

1 Modeling and Simulation of the Seismic TETwalker Concept Bryce L. Carmichael and Christopher M. Gifford University of Kansas 2335 Irving Hill Road Lawrence, KS Technical Report CReSIS TR 134 November 8, 2007 This work was supported by a grant from the National Science Foundation (#ANT ).

2 1 Abstract The objective of this project was to adapt the design of a robot that was originally created at NASA Goddard Space Flight Center called the TETwalker, in a computer simulation software program to demonstrate the collection of seismic data of ice sheets in Antarctica and Greenland. We will take their design and adapt it for seismic data collection by placing seismic sensors (geophones) in each ground node of the tetrahedral structure, or in the center node for deployment. Seismic methods are analyzed in order to determine which design could possibly be more efficient and reliable in polar environments in terms of geophone deployment and environmental characteristics. D I. INTRODUCTION ue to the reality of climate change of today s society, people have been making attempts to determine how we can detect if the disaster will actually happen and the effects on the world. The Center for Remote Sensing of Ice Sheets (CReSIS) creates innovative technologies in the pursuit of understanding global warming and its effects on ice sheets [7]. The integration of radar and seismic mapping has played a major role in obtaining accurate estimates of ice sheet involvement to sea level rise. The TETwalker robot was designed at NASA Goddard Space Flight Center (GSFC). The TETwalker is a pyramid-shaped robot which changes it center of gravity to topple over obstacles. Initially, the purpose of the TETwalker was for space exploration. One of the primary objectives of the robot was to adapt the technology to record seismic data in polar regions. By including geophones in the TETwalker design, data could be collected faster and more efficiently in extremely rugged and/or previously-inaccessible terrain. The TETwalker has been modeled in MSC.visualNastran 4D [5], a computer modeling and simulation software that allows the user to design and animate their creation without programming. We also attempted to simulate the deployment and retrieval based on the new seismic-based design. A miniature, replica of the TETwalker has been constructed in order to show the actual perspective of what the robot would look like and to demonstrate how deployment and retrieval would take place. In the future, NASA plans to send numerous TETwalkers to the moon, other planets, and the asteroid belt as an attempt to gain more knowledge about those areas [8]. This paper focuses on seismic sensor integration with the TETwalker robot for polar seismic surveying, robotic deployment, and seismic sensor retrieval in polar regions. Collecting data from numerous robots will enhance seismic surveying that currently limits human involvement due to the harsh weather in Antarctica and Greenland. II. BACKGROUND In 2005, the Center for Remote Sensing of Ice Sheets (CReSIS) was established by the National Science Foundation (NSF). CReSIS is a science and technology center that helps analyze climate change and response of ice sheets to sea level rise using the latest computer-orientated technologies and models [7]. In order for CReSIS to obtain accurate estimates and measurements of ice sheets and their internal state, radars and seismic sensors, or geophones, are used [3][4]. A seismic source sends vibration energy into the ground. Depending on how fast the vibrations reflect back to the surface, seismic sensors can be used to create images to help us better determine whether water or till lies beneath the glacier s bottom surface as well as subsurface geology and the ice sheet s major internal layers. The Tetrahedral Walker, or TETwalker, is a robot, originally developed at NASA Goddard Space Flight Center (GSFC) as an attempt to further explore the planets [1][2][8]. This shape-shifting robot is a prototype for extending machines that will one day explore our neighboring planets by altering its shape to maneuver smoothly over rocky, difficult terrain. The main structure of the TETwalker is a tetrahedron consisting of telescoping struts and nodes. The nodes act as electric motors that are connected to the struts, and which form the sides of the robot. The TETwalker travels by moving its center of gravity to one side until it topples over due to the unbalanced weight in that direction. As the robot continues to repeat this motion, it will result in a side-to-side moving transition. Rather than the small design NASA had originally planned the TETwalker to have, the robot is

3 2 currently much larger in size than its future successors. The TETwalker will be made smaller by replacing the nodes with micro- and nano-electro-mechanical systems [8]. Struts will be replaced with metal tape or carbon nanotubes that will also greatly increase the number that can be packed into a rocket because the new struts would be fully flexible, allowing the pyramid to shrink to the point where all its nodes touch. An effort was made in order to figure out how to integrate both the TETwalker and CReSIS seismic sensors in to one complete design. Since the struts on the TETwalker lengthen and shorten when necessary due to unbalanced weight and gravity, it was relevant that the TETwalker and seismic data could work the same way together. The idea consists of the TETwalker moving on the surface, using its center strut to raise and press down in order to plant the sensor into the ground. When it is finished collecting data at that location, the robot will raise its center strut, along with the seismic sensor, and continue moving along the surface while repeating the same pattern as before. III. MODELING AND SIMULATION SOFTWARE To simulate the TETwalker, MSC.visualNastran 4D [5] was used in order to provide a more accurate depiction of what the robot would look and act like in a real environment. MSC.visualNastran 4D is software that integrates motion and animation in a modeling application that does not require any programming by the user. The designed model is simulated by the user to determine whether or not the product will function under real-life circumstances which include physics and gravity. It also allows you to determine component stress and the physics of motion and gravity with the model. Fig. 1 shows images of the modeling software during use. Fig. 1: Using MSC.visualNastran 4D [5] to model the TETwalker s overall structure of nodes and struts. IV. APPROACH The purpose of our research was to figure out a way for the TETwalker to plant its seismic sensor while simulating its movement on the surface. The tetrahedral shape of the TETwalker was a concept originally created by NASA in order for the robot to maneuver on the ground, but the foundation of the shape proved difficult to coordinate when planting the seismic sensors. The design of the robot was altered into a cube rather than a tetrahedron. Rather than the struts extending and toppling over, a potential design could have all the sides of the cube unfold and plant the seismic sensors into the ground. However, the forces required to unfold and refold the robot structure proved too difficult a task. It was impossible for all the struts to lengthen and shorten while folding together in place. The decision was made to use the original triangular pyramid-like, tetrahedron shape rather than the cubed robot design due to the difficulties performing the detaching and attaching actions of the struts from the nodes. Since the seismic sensor had to be placed on the ground, the robot would have to lower and retrieve it using the same mechanism. This lead to the design of the TETwalker having the center strut raise and lower the seismic sensor into the ground while having other struts hold it in place to help uphold the geophone orientation and placement precision during deployment.

4 3 The robot s nodes were initially modeled using cubes. Although cubes weren t exactly the same shape as actual nodes, they were very similar compared to the other shapes in the program. Three cubes representing ground nodes were placed on the surface of the ground plane to provide the foundation for the tetrahedron shape, while one representing the center sensor node was placed in the center of the tetrahedral structure. Linear actuators were used as struts and placed diagonally between the cubes in order to connect to the center cube above the ground plane. In MSC.visualNastran, linear actuators are multi-purpose constraints that exert enough necessary force to maintain the specified length, velocity, or acceleration between its endpoints. The linear actuator also applies force to endpoints that are equal in magnitude and opposite in direction (meaning the actuator grows in both directions). From the center node, a linear actuator was placed vertically to act as the strut placing the seismic sensor in to the ground. The seismic sensor was represented by a small cube, as shown in Fig. 4. Three linear actuators were then connected to the seismic sensor to provide support and more actuation for placement and mobility. Fig. 3 shows models of the Seismic TETwalker in deployment position and with the seismic sensor deployed with cubes for nodes and linear actuators for struts. Fig. 3: TETwalker design showing the deployment of a seismic sensor using a linear-actuator-like strut. V. SEISMIC SENSOR DEPLOYMENT SIMULATION When it came time to animate the TETwalker, the newly designed model would not work properly. The linear actuators would lengthen to extreme distances and push different parts of the model off the ground. This is an inherent challenge and difficulty with the robot architecture and design in that for one strut to move, multiple other struts must also move. The second model of the TETwalker was created using rigid cylinders rather than using linear actuators for the base design for the tetrahedron. We did not want the base of the TETwalker to push in and out but rather stand still to demonstrate how the robot topples for mobility. The next objective was to simulate the actual deployment of the seismic sensor. Also, spheres replaced the usage of cubes as the nodes. The TETwalker could maneuver easier while using spheres rather than cubes since they would roll rather than move piece by piece as the TETwalker would topple over. Unlike the previous model, an object was needed in order to hold all the parts together. Shorter cylinders were created and the struts and nodes were connected with revolute joints. All spheres and cylinders were arranged in the same areas as nodes and struts in the previous model. Fig. 4 shows the updated model design.

5 4 Fig. 4: Adjusted TETwalker model used for toppling and geophone deployment (red square) studies. By the time all the parts were in place, it was time to figure out how to get the center strut to plant and retrieve the seismic sensor. Alteration of the linear actuator properties was necessary in order to approximate the time in which the actuator would need to move up and down. Timing of strut movement was critical in the design as the need to extend/retract a single strut requires many other struts to also change in length or pivot. A table of lengths and times to achieve those lengths were created to control the behavior of each strut using a cubic spline interpolation function. This controlled how fast each strut extended and retracted to perform the toppling motion. The final product of the actuators resulted in the times of 0, 0.5, 1.0, 1.5, 2.5, 3.0, 3.5, 4.5, and 5.0 seconds. The actuators would correspondingly move to lengths of 0.1, 0.035, 0.1, 0.1, 0.035, and meters at those times. Each actuator rises for a short period of time before it then lowers itself to the ground, simulating the robot s motion of actually deploying the sensor onto/into the surface. The actuator continues to rise to retrieve the sensor, and the above steps are repeated for each deployment location. Fig. 5 shows images of the deployment and retrieval simulation. Fig. 5: TETwalker in ready position (left), deployed position (middle), and retrieved position (right). VI. MOBILITY SIMULATION MSC.visualNastran was used to simulate mobility as it integrates lifelike physics. The TETwalker travels by moving its center node to one side until the unbalanced weight topples in the direction due to the gravity feature in the program. Originally, we removed all of the center pieces of the TETwalker and made all the struts into linear actuators. We attempted to lengthen two of the three struts connected to the center node. This resulted in the two struts pushing the node over the shortened one. In order to shorten and lengthen all actuators, we created a table for each in the software which consisted of the distance in which the strut would lengthen or shorten and the amount of time to perform the action. For the two nodes that would be lengthening, we set the lengths as 0.288, 0.7, and.07 meters and respectively set their times to 0, 1, and 3 seconds. For the shortened strut, we set the lengths as 0.277, 0.377, and meters at the times of 0, 1, and 3 seconds.

6 5 After the two struts push over the shortened one, the newly placed struts in the middle push up together to raise the center node again. Two out of the three nodes would lengthen to 0.361, 0.261, 0.361, 0.561, and meters at the times of 0, 0.75, 1.25, 1.5, and 2.25 seconds while the other one stayed the same length. When viewing the simulation, the robot s movement was very choppy and slow since it was having difficulty raising all the nodes on its own. The linear actuators must be in a stable enough position so that the robot can bring itself back into the upright and stable position to insure the center of gravity can properly be shifted using the available linear actuator lengths. Fig. 6 shows the extension of the top node to alter the center of gravity in order to topple. Fig. 6: TETwalker movement simulation, starting from the upright position (left), extending the top node to the ground (middle), and raising one of the back nodes back to the top in upright position (right). We decided to focus more on modeling a single toppling motion to better understand system dynamics. This design consisted of three linear actuators, just enough to extend a node to alter the center of gravity. Out of the three linear actuators, only one actuator was required to lengthen while the others to remain the same length. This actuator was set on the times of 0, 0.25, and 0.36 seconds that correspond with the lengths of 0.173, 0.25, and 0.36 meters. To assist in the TETwalker s toppling motion, the top node s weight had to be altered in order for the transition to work effectively. Originally, the weight of the center node was set at 0.1 kilograms, but needed to be changed to 4 kilograms to successfully topple. When the simulation was finalized, the actuator would extend itself enough to actually turn the TETwalker over while pulling another node in its direction, allowing the robot to continue to topple. Fig. 7 shows the toppling effect observed in the simulation. Fig. 7: TETwalker toppling over by extending and retracting a single node to push and pull itself back up. Using this style of mobility, larger and more articulated versions of this robot are envisioned to scale extremely rough terrain. Fig. 8 shows a NASA simulation image depicting these robots and their abilities.

7 6 Fig. 8: More articulated TETwalker scaling a large obstacle [6]. VII. CONCLUSION Resulting simulations of the Seismic TETwalker prove the possibility of combining the robot with seismic sensors for polar seismic surveying. There is also explanation of how certain parts of the robot will function together to topple, meet its final destination, and deploy and retrieve a geophone. The various struts moving together in unison resulted in a more dynamic system of travel and the ability to collect seismic data more efficiently. In order for the toppling effect to function efficiently, the extension speed and strut movement had to be precise to properly alter its center of gravity. In contrast, the center node and strut that deploy and retrieve the seismic sensors had to be precisely timed in terms of extension and retraction. The shape and size of these robots can change dynamically and even be potentially used for communications and other platforms needed for future missions. Modeling of these more advanced TETwalker versions is left as future work. REFERENCES [1] Clark, P E, et al. SMART Power Systems for ANT Missions. Chantally, VA: [2] Clark, P E, and M L Rilee. BEES for ANTS: Space Mission Applications for the Autonomous Nano Technology Swarm. Greenbelt, MD: [3] Gifford, Christopher M, Robotic Seismic Sensors for Polar Environments. Lawrence, KS: [4] Gifford, Christopher M, and Arvin Agah. Precise Formation of Multi-Robot Systems, Lawrence, KS: [5] "MSC.visualNastran 4D." MSC Software. 6 June 2007 < [6] Christensen, Bill. "TETWalker: Shape-Shifting Robot Swarm." Technovelgy. 13 June 2007 < [7] CReSIS (Center for Remote Sensing of Ice Sheets). Available [Online], June URL: [8] "Shape-Shifting Robot Nanotech Swarms on Mars." Available [Online], March URL: