Conceptual design of a lateral undulation and side winding motion mechanism in a snake robot V.S.Rajashekhar 1, and R.Senthil 2 1 Student, Adhiparasakthi Engineering College/Department of Mechanical Engineering, Chennai, India Email: vsrajashekhar@gmail.com 2 Professor, Adhiparasakthi Engineering College/Department of Mechanical Engineering, Chennai, India Email: senradarjun@yahoo.co.in Abstract A snake robot s high degree of freedom allows the structure to move in various directions on its path. The flapping cum forward push action by the links attached to the servo motors and stepper motor makes the snake robot exhibit linear and lateral undulation movement. The side winding movement is made possible by expanding the elastic band comprising of ball bearings in such a way that it lifts off from the ground and exhibits the movement through axial rotation. B. Side winding Only two points of contact are maintained with the ground. The segments are not in contact with the surfaces. They are lifted and moved to the side. They then become the new contact points. The previous contact point is then lifted and moved. Repeating this pattern, the snake moves in a sidewise direction. This is shown in Fig. 2. Index terms Lateral undulation mechanism, Friction pads, Piston rod connector, Side winding mechanism, Snake robot. I. INTRODUCTION Linear, lateral undulation movement and side winding movement play an important role in the snake robot locomotion. The snake robot moves forward due to the various mechanisms exhibited by the various sections of it. By these movements, the snake robot finds applications in industries to search for small parts that are lost while fixing and repairing. It can be used in the maintenance of thermal power plants where they can easily crawl throughout the boiler surface. It can also be used for security purposes and to help the fire rescue team to travel in the smoke and find people or identify the source of fire. In this paper, mechanisms for the linear, lateral undulation and side winding locomotion are first designed and explained about their working. Then their arrangement in a snake robot is shown. II. BIOLOGICAL SNAKE MOVEMENTS The lateral undulation and the side winding motion of a snake are as follows. A. Lateral undulation The lateral undulation gait produces propulsion, by simultaneously moving the body sections. The sections continuously move from side to side perpendicular to the direction of forward motion. This is shown in Fig. 1. Figure 2. Side winding motion [1] III. LINEAR AND LATERAL UNDULATION MECHANISM A. Introduction The linear and lateral undulation motion is made possible by a periodic braking mechanism. It has a suitable friction pad in order to make a particular segment stable when the motors push forward the segment that is in front of it. Due to this, alternate compression and expansion occurs in the compression sections that are attached in between the segments and the snake robot moves forward. In order to reduce the friction between the base of the snake robot and the moving surface, friction reducing bearings are used. B. Friction reducing bearings In this type of movement, there are four ball bearings in the lower side of each segment to provide a smooth motion on a flat surface. Figure 3. The screw provides adjustable holding [2] 1 Author, Tel.: +91 9840468079; 2 Co-Author, Tel.: +91 9840468079; Received on May 30, 2011; Accepted on July 26, 2011 Figure 1. Lateral undulation motion [1] 17 Figure 4. Ball bearing at the base
C. Friction pads The friction pads are usually a flat plate with a suitable friction material on its base. If it has to move on a smooth surface like glass, rubber projections are kept. Figure 5. Friction pads There are two friction pads, one on each side attached to the ends of the rod for all the segments up to a required length based on the segments of the snake robot. D. Braking mechanism (Periodic braking) The segment 1 has to be stable when the segment 2 carries itself forward. So stability of the segment 1 is made possible by using the braking mechanism. The connecting rod which is connected to a circular disk attached to a servo motor is used to produce reciprocating motion of the slider. Two cylindrical sticks, one on each side are connected to the slider at one end and the other end is passed through a fixed point in the space between the segment wall and the reciprocating axis. The friction pads are attached to the other end which fixes itself to the ground periodically and provides the braking mechanism. Due to the reciprocating motion, the friction pads press themselves on the floor and the braking action occurs when the slider is on the top. When the slider is at the bottom, brakes are off. When the segment 1 has to be stable, the friction pads press themselves to the ground. When the segment 1 has to move forward, the friction pads lift themselves and the segment 2 becomes stable due to the braking mechanism. Figure 7. Position of friction pads when the slider is at 2 different points E. Linear movement of the segments A spring is attached to the end of the connecting rod so that stress free movement is made possible. The spring used here is a hard spring (i.e.) slacking does not occur. By the alternating compression and expansion of the spring, the forward movement of various segments is made possible. The motor used to provide the forward motion is a stepper motor. Figure 8. Arrangements of motor and springs The movement of the segment 1 is due to the force given by the segment 2 which is held stationary. Here the segment 1 moves forward as shown in Fig. 9. The movement of the segment 2 (which is now free) in the forward direction is due to the stable nature of the segment 1 due to the braking mechanism and due to the rotation of the connecting rod in segment 2 and also due to the forward force given by the segment 3 as shown in Fig. 10. This action continues and hence the forward motion of the snake robot in the segments 2 to n is made possible. Figure 6. Reciprocating motion of the slider [2] Figure 9. The segment 1 moves forward. 18
Figure 10. The segment 2 moves forward F. Rightward and leftward movements (Lateral Undulation) The movement of the snake robot towards the right and the left is controlled by stopping the right and the left stepper motors respectively (i.e.) if the snake has to move towards its right, the stepper motor(2) is stopped whereas the stepper motor(1) is operated. This makes the segments move towards the right. If the snake has to move towards its left, the stepper motor (1) is stopped whereas the stepper motor (2) is operated. This makes the segments move towards the left. These movements are shown in Fig. 11. B. Elastic band It consists of ball bearings at places which have surface contact i.e. in the space between the two hollow cylinders. The bearings are free to move across their axis of rotation. These ball bearings reduce the friction during rotation. It is shown in Fig. 12. The ends of the hollow steel cylinders have a ball bearing. These bearings attach themselves to the hole made on the elastic band. The holes on the elastic band are surrounded by metallic structure in order to prevent the wear of the material. It is as shown in Fig. 13. Figure 12. Ball bearing arrangement Figure 13. The hole where the ball bearing is inserted Figure 11. The leftward and rightward turning of the snake robot A. Introduction IV. SIDE WINDING MECHANISM The side winding mechanism can be used when the snake robot has to move sidewise or when it is stuck up on its path. Here there is a rotating pressure cylinder which has 8 hollow cylinders. The 8 hollow cylinders have an elastic band around them with 8 ball bearing arrangement and 8 holes where the hollow cylinder s ball bearings are attached. A piston with a piston rod connector is controlled by a servo motor. It is used to pressurize the fluid in the rotating pressure cylinder. Due to this fluid pressure, the 8 hollow cylinders expand with the elastic band thereby lifting the snake robot s segment from the ground. Then a stepper motor is used to provide an axial rotation to the rotating pressure cylinder which then moves sidewise. C. Piston rod connector It is a modified passive capture joint which is present in the middle of the piston rod. It has a cylinder in the middle which connects the two ends of the piston rods. The end which is connected to the piston of the pressure cylinder also acts as a shaft when the pressure cylinder rotates. Therefore the three degrees of freedom of the passive capture joint is reduced to two degrees of freedom as shown in Fig. 15. Figure 14. The passive capture joint [2] 19
an octagonal structure. When the alternating segments rotate, the side winding takes place. The expansion of the elastic band is as shown in Fig. 17. Figure 15. The modified passive capture joint D. Rotating pressure cylinder The movement of the piston is guided by the reciprocating motion due to the circular disk attached to the servo motor. The piston rod contains a special connector (modified passive capture joint) in the middle of it. The rotating section has 8 hollow cylinders, each displaced by an angle of 45 degrees. These cylinders have a rubber envelope around it. The pressure cylinder is used to exert pressure on the 8 hollow steel cylinders which later expands. There are two valves used in it, one opens inward and the other opens outward. It is as shown in Fig.16. E. Working of rotating pressure cylinder When the piston rod is pushed forward, one of the valves opens inward and provides pressure and hence the 8 cylinders of the rotating segment expand and thus the snake robot lifts off from the surface. The piston at this stage stays stable. When the axial rotation is done to the pressure cylinder using the motors, the part of the se gment rotates. As the motor s shaft rotates, the rotating segment also rotates and the snake robot moves sidewise. This motor is prevented from self-rotation with the help of compressible envelope that joins the two sections. When it reaches the destination spot, the hollow steel cylinders contract and the normal motion takes place. It is due to the inward pull of the piston rod. The air from the rotating section flows into the piston cylinder through the second valve that opens outward. Figure 17. Expanded steel cylinders V. ARRANGEMENT OF MOTORS IN THE SEGMENT A. Position of motors There are 5 motors that have to be positioned in each segment. All the 5 motors work in such a way so that mechanisms for various motions are created. In the segment 1 there is no requirement of servo motor to provide forward motion. In addition, a rotating pressure cylinder is attached to it and hence two totally. The arrangement is as shown in Fig. 18. Figure 18. Position of motors in segment 1(Top view) In segments 2 to n, the forward motion is required and hence it has 2 motors are in the front. But there is only one rotating segment. The arrangement is as shown in Fig. 19. Figure 16. Pressure cylinder (cross sectional view) F. Expansion of elastic band The hollow steel cylinders expand due to the fluid pressure exerted on it. This in turn makes the elastic band to expand. On the whole the segments lift off from the ground forming 20 Figure 19. Block diagram of position of motors in segments 2 to n (Top view) B. Position of the rotating pressure cylinder The pressure cylinder is placed in such a way that its axis of rotation coincides with the axis of rotation of motor (5). The arrangement is as shown in Fig. 20.
CONCLUSIONS Thus in this paper we have created two locomotion mechanism i.e. Linear/lateral undulation mechanism and side winding mechanism which makes it move on a rough surface with high degrees of freedom. It therefore gives a new face to the locomotion mechanism design that is employed in the snake robot movement. Figure 20. Block diagram of position of pressure cylinder (side view) C. Final view of the snake robot With all the mechanisms set in its position, the snake robot has the following appearance. Figure 21. A 3 segment snake robot without the envelope REFERENCES [1] Limbless Locomotion: Learning to crawl with a Snake Robot by Kevin J. Dowling (Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Robotics) [2] Mechanisms and Mechanical Devices by Sclater & Chironis [3] Design of a Modular Snake Robot by Cornell Wright, Aaron Johnson, Aaron Peck, Zachary McCord, Allison Naaktgeboren, Philip Gianfortoni, Manuel Gonzalez-Rivero, Ross Hatton and Howie Choset. [4] www.snakerobots.com [5] Simulation and Realization of Combined Snake Robot by V. Racek, J. Sitar, D. Maga [6] Design of Combined Snake Robot by V. Racek, J. Sitar, D. Maga [7] History of Robots from Wikipedia, the free encyclopedia [8] Robot from Wikipedia, the free encyclopedia [9] Snake-arm robot from Wikipedia, the free encyclopedia Electric Drives by Vedam Subrahmanyam Figure 22. Final view of a 3 segment snake robot 21