Modular Model of Snake Robot

Transcription

1 INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI EE304 DESIGN LABORATORY PROJECT REPORT Modular Model of Snake Robot Guide: Dr. Prithwijit Guha, Assistant Professor, IIT Guwahati Name Swapnil Gupta Somitra Baldua Isht Dwivedi Ankur Kunder Ayush Pathania Pawan Dixit Roll No.

2 Introduction Biological snakes occupy a wide variety of ecological niches, ranging from arid desert to tropical forests as well as swimming in rivers and oceans. Their body construction and locomotion technique has proved to be an extremely effective and efficient strategy. By attempting to build robots that emulate and perhaps match the capabilities of their biological counterparts, it is possible that we will create useful tools capable of carrying sensors, or surveillance operations by operating in challenging environments where physical human presence is unfeasible or impossible. Snake robots are robots inspired from biological snakes in structure, path trajectory and mechanism of movement. Biological snakes have inspired a variety of robotic designs since 1920s. One of the earliest Biomechanical studies of snakes was done by Shigeo Hirose, in 1970 who modelled a snake body as a continuous curve that could not move sideways. Serpenoid curve was first introduced by Hirose. This curve shows a path along which a continuous snake has an optimal motion. The motor torques and friction forces are minimum and smooth, so the power consumption is optimal. According to his studies, the tangential angles of this path must be a sinusoidal function. The continuous curve is defined such that any (x,y) point on the curve satisfies: where and are serpenoid parameters; different types of serpenoid curves can be defined by varying them. represents the position on the curve and the speed of motion can be defined by the speed of changes in specifies undulation, periods and the angular speed.

3 Different types of Serpenoid curves * Based on similar mathematical formulations large varieties of snake models are proposed. Innovative mechanical designs have flourished with one of the most sophisticated being GMD- SNAKE 2 which has actuated joints between each segment, along with powered wheels all around the circumference. This enables an approximation of rectilinear progression, but such wheels may not be effective on fibrous obstacles. In recent times to make the models more adaptive, environment sensing tools are also used to make the design more advance and robust. (* Source: Paper named A Modified Serpenoid equation for snake robots by Dehghani, Mohammad, Center for Mechatronics and Automation, School of Mechanical Engineering, Univ. of Tehran, Iran)

4 Challenges The major challenge in this project was hardware construction. As per the requirement of applications, in which this snake bot may be used, it was decided to make this robot self-contained in terms of power and computation, eliminate the drag effect of external factors and use transmitterreceiver pair to interact with robot wirelessly. Apart from that, mathematical analysis of 6 different reference frames alltogether (each attached to a module), software portion of the project that involved controlling 6 servo motors to generate precise trajectory, and making the snake move forward with all passive wheels it was all together an extremely challenging task. Hardware Design To keep the modules lightweight and torque requirement low -aluminum sheets were used for its construction. Laser cutting of these aluminum sheets were done in order to get the precise shape of modules. AutoCAD drawing of each module: Each module consist of 2 parts which were attached with each other via clips made manually, using tin. The dimensions of the above module were all decided by keeping in mind the size of servo motors, wheels and batteries to be used.

5 Metal gear standard servo motors of torque capacity 13.5kg/cm were used to make the joints active. Wheels, made from nylon, of 3.3cm diameter were used. (Lego wheels of 3cm diameter were the most ideal choice, but due to unavailability we selected the next best possible) Batteries were added in order to make snake self-contained in terms of power. Connection wires were soldered to batteries to avoid loose connections. Every connecting wire either had an insulating covering or was covered properly to avoid short circuiting. All drilling work, construction of hinges (via clips), cutting of wheels and Aluminum in proper and precise shape was done manually. Top view of a single module

6 Side views of a single module

7 (Side view of two modules connected together) Physical Parameters of the design: Length of each module 14.5cm width of each module 4.5cm Height of each module 7.5cm Radius of the wheel 3.3cm Max. torque of the motor 13.5kg/cm at 6V Arduino Mega is used as microcontroller. It is mounted on a separately designed front module which is made three wheeled, in order to give stability to the whole body.

8 MATHEMATICAL MODELLING OF SNAKE ROBOT The snake robot is modelled as ball stick model which consists of n links connected by n-1 joints. The link is of mass m i, length 2*l i and moment of inertia J i (=m i * l i 3 /3). Symbols D and A stands for difference and addition operators respectively. The vector e is the basis of kernel of D. Consider the free body diagram of each link

9 f i is the friction force on the module, g i is the contact forces due to the adjacent modules, u i is the torque due to joint forces and t i torque due to frictional forces. Position of each link in the Cartesian plane The velocity components of the modules in the normal and tangential direction Keeping c t and c n as the friction coefficients and dm i mass of the infinitely small segment

10 Hence the torque and force due to friction is given by Now these values we will use in the equation of motion Equations of Motion Translational motion Rotational motion Now x,y and angle are constrained by Hence the translational velocity is given by

11 Decomposing the translational motion into two parts. Serpenoid Curve Angle between each segment with x-axis measured counter clockwise is Hence Relative angles that determine the shape of the discrete serpenoid curve

12 Undulatory motion of the snake can be imitated by changing the relative angles of the snake robot in the following way Software Design The Arduino Mega2560 is a microcontroller board based on the ATmega1280. It has 54 digital input/output pins (of which 14 can be used as PWM outputs), 16 analog inputs, 4 UARTs (hardware serial ports), a 16 MHz crystal oscillator and a USB connection. The board can operate on an external supply of 6 to 20 volts. Zigbee is an Arduino compatible trans-receiver. It is long range high speed serial wireless communication module which can give range of 30 meters indoor or 100 meters outdoor. It is generally used for robot to robots or robots to PC communication. It supports data rates of up to 115kbps. One Zigbee module has been connected with Arduino Mega board to play the role of signal receiver and mounted on the robot itself, to make the snake wireless. One more Zigbee has been connected to laptop via interfacing board to play the role of transmitter i.e. sending control signals from laptop to robot.

13 Assuming all the pre calculations have been done, Code for Arduino can be written in a simple C language. #include <Servo.h> #include <math.h> Servo myservo[8]; int pos = 0; int i = 0; int theta = 0; void setup() { for(i = 0; i<8; i+=1) { myservo[i].attach(i+2);// put your setup code here, to run once: } } void loop() { for(pos = 0; pos< 3.14;pos +=0.1) { for(i = 0; i<8; i+=1) { theta = 30*sin(pos*180/ *i); //f(y<0) // { y = myservo[i].write(theta); delay(15); } } }

14 Conclusions: As targeted in challenges initially, the actual design try to address all the desirable requirements of an ideal wireless snake robot upto a certain extent. The final design can be used as a base for testing and further researching in path optimization, obstacle detection or even wireless power transfer over a short range (by removing batteries and finding alternate method to power servo motors) Further improvements: Hardware of the robot can be improved, if better facilities are available for its construction. Its shape can be made circular to provide a more robust and overall covered body to the robot hence making its appearance closer to biological snakes. Its height can also be reduced, if smaller wheels are available. Aluminum Body can be completely covered with insulation to avoid any chance of short circuit. There is an immense scope of improvement in Path planning, methods of turning, speed of locomotion, energy efficiency and terrain adaptability. References: Modelling, Analysis and Synthesis of Serpentine Locomotion with Multi-Link Robotic Snake By M.Saito, M.Fukaya and T.Iwasaki Introduction To Robotics : J.J Craig ReBiS Reconfigurable Bipedal Snake Robot By Rohan Thakker, Ajinkya Kamat, Sachin Bharambe, Shital Chiddarwar, and K. M. Bhurchandi A Modular and Waterproof Snake Robot Joint Mechanism with a Novel Force/Torque Sensor -By P. Liljebäck, K.Y. Pettersen, Stavdahl, J.T.Gravdahl Sine-Wave Locomotion in a Robotic Snake Model Form and Programming - By Mark W Sherman 'A review on modelling, implementation, and control of snake robots'-by P. Liljebäck, K.Y. Pettersen, Stavdahl, J.T.Gravdahla