Table of Contents
Problem
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Mimicking a human-like gait motion in bipedal mechanisms is a problem that has been solved analytically; however, these gait simulations often encounter issues in translation to physical devices. This challenge arises from the number of manufacturing concerns that are difficult to model in simulation such as friction and alignment of the system. These issues, can easily hamper the finely tuned motion preventing movement from being achieved. As a team, we set out to build a bipedal walking mechanism capable of replicating this illusive motion using our knowledge of kinematics and mechanical design.
Project Description
While this problem has been solved and attempted countless time by large companies and researchers alike, success is often achieved using mechatronics controlling a very complex model that achieves balance and consistent motion through programmed weight redistribution and machine learning. Our project took a different approach - we aimed to create a completely mechanical solution to this problem - our goal was to create a machine that would walk using nothing more than mechanisms, a motor, and a power supply.
To create a motion similar to a typical human walking gait, we began with a basic four bar mechanism. We proposed to to create a linkage that would produce an endpoint trajectory similar to the analytically established walking gait. We will discuss the kinematics of this gait in further detail in this writeup. After exploring the capabilities of a four bar linkage, we used Chebyshev's Lamba Mechanism to create the desired D-shaped motion of a bipedal step. Still, there was work to be done creating prototypes that were robust enough to maintain this motion without much departure from Chebyshev's analytical model. Through several iterations, we created a well calibrated Chebyshev mechanism minimizing any play in the joints of the linkage.
Finally, staying true to our goal of solving our problem completely mechanically, we were tasked with developing a counterweight system that would allow us to shift the weight of our robot from side-to-side corresponding with whichever foot was currently on the ground. In this process we developed several iterations to shift a large counterweight from leg to leg. we maintained the following goals: our final design was to be balanced completely through the use of mechanisms, our mechanism was to be powered using one motor, our mechanism would only have one degree of freedom, and finally our mechanism would not be capable of statically balancing without a weight. Many people build bipedal mechanisms that balance using large feet that tesselate as the robot moves forward - these systems balance with ease. we considered this option, but concluded that it was important to our team that we solve and calibrate the balance problem using a mechanical system rather than a change in geometry.
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Our assignment is to produce a novel and interesting mechanism. It can be almost anything but we need to demonstrate that we are able to take the tools we have learned in this class and use them to solve a realistic problem. We need to take our analytical solutions and use them to inform decisions that solve larger problems outside of the textbook.
Background
Bipedal locomotion is an exciting area of robotics research. Many different solutions have been developed for the problem of how to make a two-legged robot walk reliably. Most of these solutions are extremely complex, utilizing high-degree-of-freedom bodies and sophisticated electronics and software for motion planning and joint control. The most difficult problem in bipedal locomotion tends to be balancing during motion. Powering the robot is easy, making the robot move through the commanded trajectories is easy, but making it balance while walking is very difficult. We wanted to see if we could find a solution to the problem of producing balanced bipedal locomotion using the materials covered in this class.
Solution
We propose to create a walking bipedal robot using the information covered in this class. It is fairly simple to produce a simulacrum of "walking motion", but much more difficult to actually make a robot walk and carry its own weight. More importantly, it will be difficult to make that robot balance when it has to shift its center of gravity from one foot to the other during each step. Inexpensive toys can demonstrate bipedal locomotion, but only for the special case that the feet overlap and the body's center of gravity is motionless. This is not an accurate representation of the locomotion of any living animal or human-sized bipedal robot.
So our robot must overcome three main obstacles: it must walk to move itself, it must be able to carry its own weight, and it must use its own weight to maintain balance while walking. We cannot imagine a single mechanism that accomplishes all of these goals, but a combination of several complementary mechanisms should suffice.