BIPED TRANSFORMER Group No. 9 Name Roll Number Aditya Vikram Singh 11010305 Dhiraj Gandhi 11010321 Jagjeet Singh 11010326 Mayank Sanghani 11010336 Sriram Kumar 11010365 Vikas Singh 11010371 Abstract: This paper proposes the design and control of a hybrid legged wheel robot. Biped robots have better mobility than conventional wheeled robots in rough terrain, but they tend to tip over easily. To be able to move efficiently in various environments, such as on plane surface, rough terrain, up and down slopes, or in regions containing obstacles, a wheeled motion added to a walking motion would be a much better option. When the ground conditions and stability constraint are satisfied, it is desirable to switch between the two mechanisms in minimal time with static stability maintained at each stage. To incorporate the walking of a biped robot for uneven surfaces along with high speed mobility of a wheeled robot, it is state of the art research topic in design for mobility. A walking gait pattern is presented whose parameter estimation required extra attention because of the extra weight due to wheels and motors on each foot of the biped robot. A special setup was designed and a unique transformation strategy was developed to lift the foot from the ground and bring the wheels in contact to the ground simultaneously. Wheeled motion consisted of 2 wheels powered by dc motors and castor wheels to provide stability. In the final stages of the project, the efficiency of the design and control strategy was analyzed by testing the mobility of the prototype on
object obstructed surface for biped locomotion and on plane surface for wheeled motion. Introduction The human body is a wonder. It performs more tasks simultaneously than any known man made device. We as humans have been trying to model our body s behaviour via robotics since the 1960s, when we first modelled a leg. As part of the semester design project, we were asked to design something of utility value. We decided to model and prototype a robot that both walks on two feet, and rolls on wheels. The inspiration for the idea came from the idea of how useful it would be to have legs to walk on uneven surfaces and climb stairs, while having wheels to increase our transverse speed when the conditions were favorable, for example, on a road. The utility of this design is that it opens up the possibilities of having multiple modes of motion in a single body, be it a robot or exoskeleton, while developing on the work being done in gait analysis. We went about this in 3 parts: 1. Designing the leg. 2. Analyzing the walking pattern/gait. 3. Modelling the transformation from biped to wheeled robot. As there were constraints of both time and money, it was decided to keep the design of the leg fairly simple. The model essentially included having 3 actuating joints in each leg, similar to having a hip, a knee, and an ankle. This was accomplished by having 3 servo-encoder motors in each leg, linked together with the leg chassis. The entire structure was modelled in CAD for better clarity. However, the length of the individual links could only be decided after knowing the range of angles each motor would be subject to in the walking motion, which led to the next part: Gait analysis.
The main question in the construction of the biped was figuring out how to make the legs actuate with the given number of motors. This required simulating the walking motion in MATLAB, and optimizing the angles of links to derive a stable condition. It is easily understood that when one leg is in the air, the entire weight of the body is on one foot. This required designing the foot as well, to ensure that the ZMP, or Zero Moment Point, fell within its convex hull. This was accomplished by modelling the forces acting on the body while walking, as well as considering various foot-shapes, ranging from C-shaped to squares. This was added to the CAD design. After having decided on the walking model and the link lengths and actuation angles, the next step was modelling the transformation. Wishing to have a speedy and quickly reversible change in the movement method, it was decided to have the wheels attached to the feet themselves. This enabled the wheels to have contact with the ground simply by actuating the ankle motor, and 2 castor wheels were attached for support, which would make contact with the ground simply by bending the topmost motor forward. This was all done keeping in mind the standing stability and balance of the bot. The transformation was again added to the CAD model, after which work on building the prototype began. Prototype: The prototype was built exactly to conform to the CAD model. It used the following parts: 1. 6 Servo encoders motors for the legs. 2. 2 DC motors for the wheels 3. Arduino microcontroller 4. Motor casings of sheet metal 5. Leg chassis made of steel. 6. Acrylic sheets for platform and feet. 7. 2 Castor wheels
The parts were assembled together, and the code for the movement was written in ARDUINO. The balance was adjusted and re-adjusted to minimize vibrations. The motors were connected to a DC source.the final result was as expected, and a step length of 4 mm was achieved with a bot height of 30 cm. The time taken for transformation to wheeled form was 4 seconds. Overall, a highly engaging and enjoyable project with hopefully real world significance. Micro-controller Controller Interface Mechanical assembly Arduino Duemilanove Servo shield Foot Joint 1 actuating Servo motor Knee Joint 1 Actuating Servo Motor Hip Joint 1 Actuating Servo Motor Foot Joint 2 actuating Servo motor Knee Joint 2 Actuating Servo Motor Hip Joint 2 Actuating Servo Motor (motor Driver) DC Motor 1 for left DC Motor wheel DC Motor 2 for Right Wheel
Mechanical Design and Prototype Details: Soild Works Model of the Walking part: Robot with wheels (links excluded ) Parts added for transformation: Wheels with dc- motors mounted on both foot.
Specifications: Motors: Speed Supply Voltage shaft diameter Weight 200 RPM 12V 6mm 117.00 grams Wheel Diameter: 9 cm Angle of inclination from foot (ground): 30 degree Two casters with links on main body.
Links: Length Material Angle connected from main body 21 cm Aluminium 30 degree Wheeled Mode: Walking Mode: Locomotion In a biped robot, during its motion, it is necessary to be able to balance the robot on one foot. This requires to shift the center of mass of the whole body on one foot for the duration of an entire step. To achieve this, we had two possible options: To place a mass on the top of the robot which could be actuated. To actuate the angle between the knee and the foot.
The first option required us to add an extra mass to the bot, which would make it bulky, while there was no such requirement for the second method. So we opted for the second option. The first step in this method was to lift one foot off the ground. Now for the bot to be stable its ZMP (Zero Moment Point) must lie within the convex hull of its contact points. But when one foot is lifted off the ground, the ZMP moves out of this convex hull, so to bring it back within the convex contact area of one foot, we need actuate motor 1. We calculated the range of angle δ for which the ZMP would lie within the convex hull of contact area. The range of angle δ was found to be from 14.7 o to 45.5 o. We took the mean of this range and decided to actuate motor 1 by 30 o. We also needed to actuate the motor fast enough so that the time period for which the ZMP would lie outside the convex hull would be small enough so that the bot does not fall. By hit and trial we found 0.5 seconds to be appropriate. Keeping in consideration the initial and final values of angles and smooth motion of links, we get the following conditions for δ(t): 1. δ(0) = 0 o Initial δ 2. δ(0.5) = 30 o Final δ 3. δ (0) = 0 Starts with velocity zero 4. δ (0.5) = 0 Ends with velocity zero Considering these conditions, the most suitable equation for δ(t) is δ(t) = at 3 + bt 2 + ct +d Solving the above equation, we get the values of coefficients as: a = -480 b = 360 c = 0 d = 0
The graph shows the variation of angle δ with time Till now, we have just lifted one foot off the ground and then stabilized the bot. The next step is to actuate the motors at the knee and hip joints such that the bot moves forward. To give a better idea of actuation of each motor, we consider that we actuate only one motor at a time. We do this only to give a less complex explanation but in reality all motors are actuated simultaneously. FIGURE
MOTOR 2 3 4 5 ACTUATED -θ +θ -θ +θ Viewing from the plane tilted by angle δ The sequence in which motors are actuated and their direction of rotation are also shown in the figure. Upon analysis, we found that each motor is to be actuated by the same angle θ. In this case also we have to ensure that the ZMP stays within the convex hull of contact area. This limits the maximum step size of the bot, which we calculated and found that the maximum possible value of y is 5.82 cm. To be on the safer side we chose y = 4 cm. The corresponding value of θ is found to be 10 o. After the first step the side view of the bot is as shown: From the second step onwards each motor in the two hip joints and two knee joints have to be actuated by 2θ. Also time period of 2 seconds was found to be appropriate for completing this step. This time will decide the speed of walking. We assume that each motor is actuated simultaneously and identically. Now we have the following conditions for θ(t):
1. θ(0) = 0 o 2. θ(2) = 20 o 3. θ (0) = 0 4. θ (2) = 0 Considering these conditions, the most suitable equation for θ(t) is: θ(t) = At 3 + Bt 2 + Ct + D Solving the above equation for the coefficients we get: A = -320 B = 240 C = 0 D = 0 Transformation As described earlier in the design of the robot, to transform the bot into a wheeled robot, two castor wheels were attached on two horizontal (when the bot is upright) arms on the top and two motors were attached on the foot at some angle. The transformation process starts with the actuation of knee joints. Both the knee joints are actuated simultaneously to an angle of 30 o with the vertical. Then the hip joints are rotated by 60 o each such that arms on which the castor wheels are attached are now vertical. After this both the joints at the foot are actuated in opposite directions such that the wheels come in contact to the ground and become vertical. It is also ensured that the foot is no longer in contact with the ground.
The wheeled robot can again be transformed back to its upright position. For this all the steps done previously are done in reverse order, that is, first the foot joints are actuated so that the bot now rests on the castor wheels and the two feet, then the hip joints are rotated by 60 o and then the knee joints are rotated by 30 o. The bot is now in the upright position as it was before the initial transformation, so that it can walk.
Conclusion and Future Plan of Action: In this paper, we analyzed the pros and cons of both biped walking and wheeled motion of a mobile robot. Based on this we came up with the idea of having both these modes in the same robot. First the walking gaits were decided and parameter estimation was done by making the model in Solid Works and carrying on simulations in Matlab and Working Model Software. This was followed by fabricating the biped walking mode part and confirming the validity of the results. After being satisfied with the walking motion, a transformation strategy was designed to transform this into a wheeled robot. The design was modified to incorporate this strategy. There on dc motors, wheels, castors and arm links were added to use the wheeled motion for plane surfaces. A code was written in Arduino environment to execute walking mode, transformation, wheeled mode and reverse transformation. The results obtained after analyzing the mobility of prototype in varied platforms and surfaces were very satisfactory and the project received lot of appreciation from the observers. 1. Currently there were vibrations observed during walking mode. Our first aim would be to modify the design and walking gait pattern to minimize the vibrations during walking and incorporate dynamic stability as well. 2. Using camera to apply image processing so as to get a feedback of the environment. Using this feedback, the robot should decide whether to go with biped walking mode or wheeled mode. 3. Object detecting sensors to avoid collision with wall and prevent falling down during obstacles like stairs. 4. Using force measurement sensors at the foot surface to analyze the stress exerted on the foot and come up with a more accurate foot dimension and material. 5. Try the inverted Pendulum approach and transform the biped walking mode into a 2 wheeled self-balancing robot by using Inertia Measurement Units (IMU s) and accelerometers.
Acknowledgement: 1. Mechanical Department of IIT Guwahati for providing financial support to come up with a working prototype. 2. We are very thankful to our guide Prof Narayana Reddy who provided the required technical, motivational and financial support throughout the project References: 1. C. Hernández-Santos1, E. Rodriguez-Leal1,*, R. Soto1 and J.L. Gordillo1, Kinematics and Dynamics of a New 16 DOF Humanoid Biped Robot with Active Toe Joint, Center for Robotics and Intelligent Systems, Tecnológico de Monterrey, Campus Monterrey, México. 2. Masaaki Kumagai and Kaoru Tamada Wheel Locomotion of a Biped Robot Using Passive Rollers, Biped Robot Roller, Walking Using a Variable- Curvature Truck. 3. Meiqiang Zhu,Jun Wang, Ming Li,Yajing Lin Static Gait Analysis and Planning of Biped Robot,China University of Mining and Technology. 4. C. Hernández-Santos, R. Soto, E. Rodríguez Design and Dynamic Modeling of Humanoid Biped Robot e-robot Centro de Robótica y Sistemas Inteligentes. 5. Marlon Fernando Velásquez-Lobo, Juan Manuel, José Luis Vázquez- González Modeling a Biped Robot on Matlab/SimMechanics. 6. Hashimoto, Yusuke Sugahara, Hun-ok Lim and Atsuo Takanishi, Swizzle Movement for Biped Walking Robot Having Passive Wheels. 7. Masaaki Kumagai and Kaoru Tamada, Wheel Locomotion of a Biped Robot Using Passive Rollers Biped Robot Roller Walking Using a Variable- Curvature Truck, Tohoku Gakuin University. 8. Qiang Huang, Member, Kazuhito Yokoi, Shuuji Kajita, Kenji Kaneko, Hirohiko Arai, Member, Noriho Koyachi and Kazuo Tanie, Planning Walking Patterns for a Biped Robot. 9. Marcin Szarek and Gözde Özcan, Biped Robot development of an autonomous walking robot. 10. Pranav Audhut Bhounsule, A controller design framework for bipedal robots: trajectory optimization and event-based stabilization.
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