Anjali V. Kulkarni Ravdeep Chawla anjalik@iitk.ac.in
ravdeepchawla@gmail.com Indian Institute of Technology Punjab Engg. College,
Kanpur-208016, INDIA Chandigarh-160012, INDIA Abstract- A snake is an extremely capable organism that can
conquer harsh terrains like rock and sand with apparent ease.
The present work highlights the design, development and testing of a snake-like robot prototype. So called ‘SnakeBOT’, is a modular, wheeled snake-like robot. It simulates the sinusoid motion of a snake and is controlled by human voice such that it could actively respond to its milieu. The motion commands to SnakeBOT are delivered via an Infra red (IR) link. The speech recognition software converts the chosen set of commands (forward, left, right and stop) into motion commands. These are transmitted via the IR transmitter interfaced to parallel port of the control PC. On-Off keying technique is used for
transmission. IR receiver residing on the tail module of the robot receives these commands and the snakeBOT motion is
performed accordingly. The details of the structure of the SnakeBOT and its analysis of motion while on level terrain are presented. The applications of these kinds of robots are mainly in space exploration, disaster management, bomb disarmament, etc.
Keywords: SnakeBOT, slithering motion, navigation, motion control, uneven terrain
I. INTRODUCTION
In an effort to relieve the burden of time-consuming activities, a versatile robot is required to follow in man's footsteps. The pioneering work in developing biologically inspired robots was carried out by S. Hirose [1]. He developed cord mechanism [2] [3], oblique swivel mechanism [4] [5], and heavy articulated mobile robot [6] [7]
[8]. Chirikjian and Burdick developed ‘Snakey’ [9] [10] [11]
[12], and snake robot locomotion theory. Most robotic vehicles use a wheel and axle based propulsion system [13]
[14] simply because it gives lots of flexibility and high speeds even on rugged terrain. Moreover, if the diameter of the wheel is appropriate, it can easily climb small steps. A wheeled system provides greater traction while navigating variable terrain. Use of passive wheels has been experimented in [15] for achieving the smooth motion. Autonomous gating has been implemented in [16] [17] [18]. Use of specially designed joints has been discussed in [19] [20] [21] to achieve variety of motions. Another novel coupled-drive-based joint mechanism has been designed [22] and studied
the dynamics of snake robots and their motion [23]. A highly flexible robot prototype named the GMD-Snake [24]
describes the design and implementation of dynamic distributed real-time control applications. Saito and colleagues have established a mathematical framework for the modeling, analysis, and synthesis of serpentine locomotion with a multilink robotic snake [25]. The research will continue in all the directions to explore the maximal use of snake-like robots.
In this paper the design and development of a free moving wheeled snake-like robot prototype named as ‘SnakeBOT’ is discussed. It is a modular robot comprising four body modules and one tail module. At a time it uses four of eight actuated wheels for propulsion, creating a larger surface area of contact giving greater traction. The low center of gravity creates stability, and the small size helps in passing through small crevices.
SnakeBOT is nimble on its wheels with its extremely flexible joints accounting for the ease with which it can climb up obstacles and move around them with one or more of the modules standing almost vertical and the others pushing the snake along the obstacle. During practical tests, SnakeBOT easily climbed over the keyboard of the computer and a small step of 2 cm height with ease. Such a serpent is very cost effective, compact in size and dexterous enough to replace expensive rovers used in space exploration and can be useful in disaster management.
The present paper is organized as follows, part I give the introduction; in part II the structure of the SnakeBOT is described. Section III gives the details of the voice and IR control. Section IV describes the slithering motion of the snake with mathematical analysis including the simulation of the actual snake. Finally section V presents the important conclusions and discusses the future work that can be carried out.
T. Sobh et al. (eds.), Innovative Algorithms and Techniques in Automation, Industrial Electronics and Telecommunications, 43–47.
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II. STRUCTURE OF THE SNAKEBOT
The structure of the SnakeBOT is modular. It is mainly divided in three parts: Body, Tail, and Skin. Body and tail modules run in a chain. Body comprises of 4 similar body modules. Total length of the snakeBOT is around 41 cm. It is 7.3 cm wide. The skin is the top cover, which covers the robot in a specially designed way so as to provide the flexibility and ease while moving. This design of the snakeBOT provides following important features:
• The combined torque provided by the modules is responsible for navigation on an uneven elevation.
• Even if one of the modules is twisted and rises above the ground at an angle around 70° to the horizontal, the other modules are not affected and continue to follow their path.
• The S-shaped curve formed by the snake during motion aids moving around obstacles.
A. Body
Coupling similar body modules forms the body. In the present design, four body modules are connected to form the main body. A body module consists of three major parts:
1. Motor holding assembly 2. Two Motors
3. Two Wheels
Motor holding assembly has three sub parts as shown in Fig. 1. The bottom and top covering lids and the inner box, which holds the motors. It is the basic building block of the body module. The slots inside the inner box hold the motors.
The slots are placed side by side to minimize the dimensions of a single module. The system is both dynamically and statically stable as the weight is uniformly distributed.
Placement of motors side by side as shown gives the effective sinusoidal motion. DC motors running at 80 rpm with a gear reduction of 175:1 are used. These give the necessary torque to sustain motion in the adverse conditions. The bottom and top covering lids cap the two motors. This ensures the proper positioning of the motors. Wheels made up of nylon 66 material are directly connected to the motor shafts. . The wheel design is shown in Fig. 2. The large diameter (43 mm) helps in scaling obstacles that are comparable in size to the modules. The tire inserts on the wheels are designed to give maximum traction and reduce the chances of toppling. Tires are made up of molded rubber. Fig. 3 shows the front and rear view of the assembled single body module. Specially designed joints as discussed connect body modules to each other later.
Fig. 1. Motor holding assembly
Fig. 2. Wheels
B. Tail
The tail is the inactive part of the SnakeBOT. It houses the battery pack; IR control and motors drive circuitry. Four idle wheels (castors) are attached to the bottom of this structure to make it mobile. Tail module is connected to the last body module by means of signal and power wires. Theses wires run all along the length of the snakeBOT
C. Skin
The skin is the outer cover provided on the snakeBOT. The skin consists of two parts: the joints and the covering of the modules. The joints in the skin are made of tire tube used in trucks. Cuts are provided at appropriate places (Fig. 4) to make turning easier. The joint made of tier tube provide rotational degrees of freedom. The elasticity of tire tube is advantageous as it enables smooth motion and helps in returning back to the same orientation. Fig. 5 shows the actual joint while the snake tries to turn. The covering of the module is made up of hosepipe. Figure 6 shows the completed snakeBOT with 4 body modules and the tail module.
KULKARNI AND CHAWLA 44
Fig. 3. Front and rear view of the body module
Fig. 4. Cuts in the tire tube for Turning Fig. 5. Joint
Fig. 6. The snakeBOT
The control of the snakeBOT is described in the following section.
III. VOICE AND IR CONTROL
The snakeBOT simulates the sinusoid motion of a snake by design of its structure. Moreover actuation of only one wheel in one module and the alternate wheels in the body structure are also responsible for achieving the sinusoidal motion. It is controlled by human voice such that it could actively respond to its milieu. It is trained to respond to the voice commands from the user. A trained Microsoft’s speech recognition engine recognizes these voice commands. The speech
recognition software converts the chosen set of commands (forward, left, right and stop) into text. Corresponding to this text, a bit stream is sent to one of the signal lines of the parallel port. The bit stream is then transmitted over an IR
Fig. 7. Block diagram of the control strategy transmitter (working at 38 KHz) using On-Off Keying (OOF) technique. The IR receiver residing on the tail module of the robot receives these commands and the snakeBOT motion is performed accordingly.
Fig. 7 gives the block diagram of the control strategy.
The bit stream decides the On-Off timing of the motors to determine the direction of motion of the robot. Thus, for moving in the forward direction, the left motor is made ‘On’
for 200 ms and right motor is made ‘Off’ for the same time.
During the next 200ms time interval the right motor is made
‘On’ and left motor is ‘Off’. This results in forward sinusoidal motion. For turning sideways, the ‘On’ and “Off’
times are unequal and it is 200ms-400ms or 400ms-200ms accordingly. Thus for turning left, the right motor is ‘On’ for 400ms while left motor is ‘Off’ and during the next time period the left motor is ‘On’ for 200ms and right motor is
‘Off’ for that time interval. The resulting wavy turn is as shown in the following section describing the simulation.
L298 IC is used as the amplifier to driver the motors. The control and drive electronics resides on the tail module.
IV. MOTION ANALYSIS AND SIMULATION SnakeBOT has 8 actuated wheels i.e. two in each of the four modules as described in section II. Periodic switching on alternate wheel in the consecutive body module of the serpent such that only four of the eight wheels are moving at a specific moment generates the slithering motion. We have generated a simulation for analyzing the motion of this serpent on ground level by neglecting the inertia of the motors, noise due to the IR transmitter and receiver circuit.
The analysis is carried out for one of the body modules. The rest of the modules follow the first one with a specific delay.
Hence the analysis of one module is an apt description of motion of the whole serpent. The motion of the center of the mass of the first body module is analyzed in the simulation.
Fig. 8 gives all the dimensions of an individual body module. The radius of curvature of the arc is the distance between the contact points of wheels and is around 73 mm.
STRUCTURE AND ANALYSIS OF A SNAKE-LIKE ROBOT 45
Fig. 8. Dimensional details of a body module
Fig. 9 Simulated ‘forward’ motion
The DC motors rotate at 80 rpm at 12 V and thus, the linear velocity of the tires with diameter 43 mm is around 360 mm/s and the angular velocity comes out to be around 5.14 rad/s.
With this data the motion of the snakeBOT is simulated using Visual Basic software.
The simulated ‘forward’ motion is as shown in Fig. 9. It gives the trace of the centre of mass.
In Fig. 10 complete ‘left’ turn is described with the center point of the module tracing a wavy circle. The turning radius is calculated as half of the maximum displacement of the center point. This distance is shown in the bottom right corner of Fig 9 comes out to be 92 mm.
Fig. 11 gives the simulation result for turning ‘right’
condition. While turning right the turning radius comes out to be 94 mm. The simulation results are the true replica of the
nature of motion of snakeBOT. The turning radius are observed more than in the practical motion due to the inertia of the motors which is neglected in the simulation results.
V. CONCLUSION
In this paper, the structure of a free moving wheeled serpentine robot has been presented. Its wavy motion on level terrain is analyzed and simulated and tested with the real robot. It gives the feasibility of such kind of robots and their practical implementation. These kinds of robots can negotiate the clumsy workspace due to their wavy motion.
Fig. 10 Simulated ‘left’ turn
Fig. 11 Simulated ‘right’ turn KULKARNI AND CHAWLA
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ACKNOWLEDGMENT
Thanks are due to Dr. Bhaskar Dasgupta of ME Depatment for giving the idea of developing snake-like robot. Mr.
Rajendra, Mr. Sanjay helped in fabricating the modules. Their help is gratefully acknowledged.
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