DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER
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DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER

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Walking on uneven terrain is always a benchmark problem for autonomous guided vehicles. In the present work, the same issue is dealt with the help of a legged mobile robot. Various comparisons are ...

Walking on uneven terrain is always a benchmark problem for autonomous guided vehicles. In the present work, the same issue is dealt with the help of a legged mobile robot. Various comparisons are made among two, four, and sixlegged walking machine and a four-legged walking machine is selected based on the suitability criterion. In this paper, the emphasis is given for minimization of the design and controlling complexities for the four-legged walking machine. A prototype devised to test various gaits. For the walking and turning, an improved gait is presented. The legs are designed with one degree of freedom each. The actuation is tested on normal DC geared motors as well as DC servo motors. A comparison is made between the two actuators. For proper walking, a control scheme is prepared and real time tests are performed by implementing it on the Arduino microcontroller. The present work is helpful to analyze the performance of a legged autonomous walking machine on unstructured environment.

Keywords: Walking Machining, Legged AGV, Mobile Robotics, Servo Motor Control

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DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER Document Transcript

  • 1. International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 3-6, 2013) DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER Tanmai Bhargava, Shivam Aggarwala, Sangram K. Dasb, Arshad Javedc* a Graduate Student, Department of Mechanical Engineering, Birla Institute of technology and Science, Pilani, Rajasthan, India b Research Scholar, Center for Robotics and Intelligent System, Birla Institute of technology and Science, Pilani, Rajasthan, India c Lecturer, Department of Mechanical Engineering, Birla Institute of technology and Science, Pilani, Rajasthan, India * corresponding author e-mail: arshadjaved2@gmail.com ABSTRACT Walking on uneven terrain is always a benchmark problem for autonomous guided vehicles. In the present work, the same issue is dealt with the help of a legged mobile robot. Various comparisons are made among two, four, and sixlegged walking machine and a four-legged walking machine is selected based on the suitability criterion. In this paper, the emphasis is given for minimization of the design and controlling complexities for the four-legged walking machine. A prototype devised to test various gaits. For the walking and turning, an improved gait is presented. The legs are designed with one degree of freedom each. The actuation is tested on normal DC geared motors as well as DC servo motors. A comparison is made between the two actuators. For proper walking, a control scheme is prepared and real time tests are performed by implementing it on the Arduino microcontroller. The present work is helpful to analyze the performance of a legged autonomous walking machine on unstructured environment. Keywords: Walking Machining, Legged AGV, Mobile Robotics, Servo Motor Control leg to reduce the controlling to its minimum level. Keeping this condition, the present work is focused upon the design of a four legged walking robot with 1DOF in each leg. The major task to design this walking machine is to analyze the walking pattern. To serve this purpose a prototype of four legged walking robot is made using DC geared motors with manual switch control. By doing different experiments, the correct walking patterns are identified. Based on the obtained walking patter the final control is made through micro-controller on RC servo motors. 1. Introduction A smooth and controlled navigation in an unstructured environment is a benchmark problem for an Autonomous guided vehicles (AGVs) [1]. The modes of navigation are primarily provided by either wheels, or legs. Wheels are important to provide smooth motion in an even terrain. On the other hand, the legged motion provides flexibility to walk on the uneven terrain efficiently. In legged motion-based AGV, there are several designs, which provided different levels of smoothness and stability. A twolegged walker similar to the human legs, provides high speed of walking and quick turn capabilities [2]. However, its stability and dynamic control are highly complex. A four-legged walking machine provides smoother motion than the two legged and less complicated control [3, 4]. In the similar way the smoothness of walking and precision in motion can be achieved by the six-or higher number of legged machines. In order to provide a controlled motion the controlling complexities again increased in six or higher numbers of legged machines. In addition to the number of legs, the degree of freedom (DOF) associated with each leg also plays a major role in the motion of an AGV. By increasing the DOF, the walking precision can be improved and greater capability of obstacle avoidance and uneven terrain navigation can be achieved. However, the controlling becomes more complex. Considering these issues, it is evident to employ four legs with single DOF in each The presented manuscript is organized in following way. In section 2, the design of the prototype and gait experiment analysis is presented. In section 3, the construction, working and controlling of the RC servomotor based walker is explained. Finally, the conclusion is made. 2. Gait Experiment and Analysis for Four Legged Walker The motion of a four legged walker is the most vital aspect of this paper. A lot of experimentation is done on the walking sequence of a four legged walker, which is required to be controlled by servo motors. The aim of these experiments are to find out the exact sequence of the movements of the four legs relative to each other. For conducting experiments, a DC geared based model is devised as shown in Fig. 1. It is controlled using manual switching. For each motor, Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 48
  • 2. International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 3-6, 2013) forward and reverse motions are generated through the switch as shown in Fig. 2. The gait sequence, which is obtained after this experimentation, is given here. Assuming the four legs are initially still at their respective positions (Fig.3). The steps of walking are described below: Step 1: Let the left front leg moves ahead by a small distance. The front right leg will be at its same position but it will be having a tendency to go backward. The left and right back leg will also remain in its initial position but it will also be a tendency to move forward. Step 2: Now the left front leg is at its same new position. The right back leg steps forward. When the right back leg steps forward, the left back leg remains in its same position but it tends to move backward. At the same time, the right front leg lifts up in a backward direction in order to move forward. Step 3: Now the right front leg starts moving forward. At this time, the left front leg is at its same position but it tends to move backward. The left back leg is inclined backward and is on the verge of moving forward. The right back leg is at its same position and tends slightly backward. Step 4: Now the right front leg is at its new position. Now the left back leg also lifts from the ground to move forward while front left leg will be at its same position but it will be having a tendency to go backward. The right back leg will also remain in its initial position but it will also be a tendency to move forward. Fig. 1 DC geared motor-based four-legged walker Fig. 2 Switch control of four legged walker Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 49
  • 3. International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 3-6, 2013) (a) (d) (b) (e) compact form of close loop controlled actuator. It has four modules packed in a single system. These modules are, Controller circuit- It is the main controlling part, which implements the controlling scheme by taking the signals coming from feedback element. Feedback Potentiometer- The feedback of the rotation of the motor shaft is taken by a potentiometer. It is connected through a gear train to the motor shaft. The Motor- This is a small DC motor, which is controlled by an H-bridge inbuilt in the main controller. The Gearbox- Normally a small DC motor comes with a very high RPM. A small gearbox is provided to reduce the RMP of the motor. (c) (f) Fig.3 Gait for four-legged walker 3.1 Operation of RC servomotor The above cycle (Fig. 3) continues to cover the specified distance. Taking the maximum angle that can be moved by the leg-link to be 100 and link length to be 5 cm the distance moved by the walker in each cycle is 1.74 cm (17.4 mm). The operation of RC servomotor is based upon the feedback signal. When there is no input signal from the controller, the servo does not move. By giving an input signal motions are made. These signals are in terms of a pulse of a specified pulse-width and frequency, for a required angle of motion (Fig. 6). The controller will de-code this signal into a reference voltage. Thus, each voltage corresponds to different positions of the drive shaft. The controller then reads the actual drive shaft position by reading the voltage of the feedback potentiometer connected to the drive shaft. By making the comparison between the required voltage to the actual voltage, the rotations are performed. If the reference voltage is lesser than the potentiometer- voltage, the motor rotates in one way. If it is greater than the potentiometer-voltage, the motor rotates the in the other way. Then the comparison of voltages made again. After the comparison, if the two voltages are found to be equal, it means that the required position has been achieved. The servomotor works continuously. It is never idle. It always checks if the voltage differences of the potentiometer and required voltage and try to correct the position. Hence, the input signal is always required. Fig.4 turning motion by walker The above cycle (Fig.4) shoes turning moment for the walker. Taking the maximum angle that can be moved by the left link to be 100 and right link to be 50, the length of the arc subtended by the walker in each cycle is 1.305 cm (13.05 mm) and an angle of 780. 3.2 Constructional details The main parts of four-leg walker are, chassis, servomotors, leg links, controller and battery. In the present work, the main emphasis is given to the reduction of controller task. Hence, the torque computations for the drive motor etc. are neglected by selecting an adequate amount of motor-torque. In this regard, the chassis design is also simplified by selecting a hollow rectangular box in which all components will be accommodated. The over 3. Design of Four Legged Walker Based on the walking pattern analyzed in the prototype experiment an ARDUINO microcontroller based control is developed [5]. In this four-legged walking machine, actuation is achieved by radio controlled (RC) servomotor. RC servomotor is a Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 50
  • 4. International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 3-6, 2013) construction is shown in Fig. 5. Four V3003 servomotors are used in the present work. The specifications of the servomotor are given in Table 1. The required position signal is given in terms of a pulse. For the selected motor the values of these pluses corresponding to the different position is shown in Fig. 6. Fig. 6 Pulse signal ranging 3.3 Controlling For the controlling ARDUINO microcontroller is used. It has the capability to provide multiple signals simultaneously. In the present work, it sends the separate controlling signal to four servomotors. As stated earlier, these controlling signals are the pulses corresponding to the required position of the motor shaft. Based on the gait analysis the motion sequences of the legs are already identified. For a straight motion, left, and right turning, the modules of corresponding signals can be made. In the microcontroller, these modules are coded by the means of programming [6, 7]. For illustration the codes of the straight-line motion is given in Appendix. In the present work the provision of obstacle avoidance etc. are not provided. However, by making simple changes in the controlling scheme and by incorporating an Infrared or Ultrasonic proximity sensor it can be easily done. Full View Fig. 5 Servo motor-based four-legged walker 4. Conclusion A four-legged walker has been an area of interest due to its varied applications and better stability. The gait or the manner of walking is one of the vital parts of the analysis of the four-legged walker. In the present work, a gait analysis is performed be the means of a various experiments on a prototype four-legged walker. The relative positions and motions of the legs are figured out which is explained. In addition, the walking pattern for forward and turning is found out. The four-legged walker can be controlled by any of the varied number of types of electric motors but RC servomotor has better properties as compared to other motors like DC motors. A RC servomotor based walker is developed for the autonomous walking. In this, the control is implemented by using microcontroller. The presented work proposes a design of a walker with the minimum controlling complexities. It is helpful to analyze the walking of legged AGVs. Table 1: Specification of the servomotor Operating Voltage 4.8-6.0 V Stall Torque 3 Kg-cm at 4.8V 3.2 Kg-cm at 6V No load Operating Speed 0.2 sec/ 60° at 4.8V 0.18 sec/ 60° at 6V Weight 38gm Size 41.3mm×20.3mm×38.7mm Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 51
  • 5. International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 3-6, 2013) } for(pos4 = 90; pos4>=80; pos4-=1) { myservo4.write(pos4); delay(15); } if(i = 2) for(pos2 = 80; pos2 < 111; pos2 += 1) { myservo2.write(pos2); delay(15); } for(pos1 = 111; pos1>=80; pos1-=1) { myservo1.write(pos1); delay(15); } if(i = 3) for(pos4 = 80; pos4 < 111; pos4 += 1) { myservo4.write(pos4); delay(15); } for(pos3 = 111; pos3>=80; pos3-=1) { myservo3.write(pos3); delay(15); } if(i = 4) for(pos2 = 111; pos2 < 89; pos2-= 1) { myservo2.write(pos2); delay(15); } for(pos1 = 80; pos1>=90; pos1+=1) { myservo1.write(pos1); delay(15); } for(pos4 = 111; pos4 < 89; pos4-= 1) { myservo4.write(pos4); delay(15); } for(pos3 = 80; pos3>=90; pos3+=1) { myservo3.write(pos3); delay(15); } } } REFERENCES 1. Siegwart R., Nourbakhsh I. (2004), 'Introduction to Autonomous Mobile Robots', The MIT Press, Cambridge, Massachusetts, London, England, pp 5-152. 2. Tlalolini D., Chevallereau C., Aoustin Y. (2011), 'HumanLike Walking: Optimal Motion of a Bipedal Robot With ToeRotation Motion', IEEE/ASME Transactions on Mechatronics, Vol. 16, pp 310 - 320. 3. Gurfinkel V. S., Gurfinkel E. V., Shneider A. Yu., Devjanin E. A., Lensky A. V., Shtilman L. G. (1981), 'Walking Robot with Supervisory Control', Mechanism and Machine Theory, Vol. 16, pp 31–36. 4. Peng S., Lam C. P., Cole G. R. (2003), 'A Biologically Inspired Four Legged Walking Robot', Proceedings of IEEE International Conference on Robotics and Automation, Taipei, Taiwan, Vol. 2, pp 2024 – 2030 5. Getting Started with http://www.arduino.cc/ 6. Servo Control tutorial on, http://playground.arduino.cc/Learning/SingleServoExample 7. Servo library, available at, http://arduino.cc/en/reference/servo Arduino, available on, Appendix ARDUINO program to run servo motor #include <Servo.h> Servo myservo1; Servo myservo2; Servo myservo3; Servo myservo4; int pos1 = 90, pos2 = 90, pos3 = 90, pos4 = 90, i = 0; void setup() { myservo1.attach(9); myservo2.attach(10); myservo3.attach(11); myservo4.attach(12); } void loop() { for(i = 0; i < 5; i+=1) { if(i = 0) for(pos1 = 90; pos1 < 111; pos1 += 1) { myservo1.write(pos1); delay(15); } for(pos2 = 90; pos2>=80; pos2-=1) { myservo2.write(pos2); delay(15); } if(i = 1) for(pos3 = 90; pos3 < 111; pos3 += 1) { myservo3.write(pos3); delay(15); Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 52