11.1 Project Proposal
Introduction
Legged robots are an interesting area of robotics because they can move in ways that wheeled robots cannot, especially on uneven or unpredictable terrain. Quadruped robots, which move like animals with four limbs, are particularly interesting because they provide a good balance between stability and mobility. Researchers often study these robots for applications such as exploration, inspection, and assistive technologies. In addition to their practical uses, they also provide a fascinating platform for studying how mechanical systems can replicate complex biological movements.
The purpose of this project is to create a robotic leg that can be used in a quadruped robot capable of standing, walking, and jumping. Designing a robotic system that can transition between these behaviors is both technically challenging and interesting from a design perspective. Beyond basic locomotion, the ability to perform dynamic motions such as hopping introduces additional considerations like energy storage, force generation, and landing stability. By studying these behaviors, engineers can better understand how mechanical systems can mimic dynamic biological motion while maintaining control and repeatability.
Problem Statement
Designing a robotic leg capable of controlled locomotion and repeated jumping introduces several mechanical challenges. The leg must generate and coordinate multiple types of motion while supporting external loads and maintaining stability. During walking-type movements, the mechanism must support the robot’s weight while controlling foot placement in both the vertical and horizontal directions to maintain balance. Jumping introduces a more dynamic requirement, where the leg must rapidly generate a large upward force to achieve lift-off and then safely absorb impact forces during landing.
These motions require a complex relationship between joint movement, force transmission, and timing throughout the mechanism. The leg must transition between slow, controlled positioning during standing or walking and high-power extension during a jump. This creates competing mechanical requirements, such as the need for both high torque and fast actuation while maintaining stability throughout the structure. Simple single-axis joints are not sufficient to achieve this behavior, since the system must coordinate motion across multiple links while controlling the direction and magnitude of forces. Therefore, the main challenge is designing a mechanism capable of producing the required motion profiles and force outputs while maintaining mechanical stability and repeatability.
Proposed Mechanism
To achieve the desired motion requirements in both walking and jumping conditions, we plan to implement two vertically stacked servo motors that will slide vertically along two rails. The motor on top will control the hip joint rotation (A-drive), and the motor on the bottom will control a capstan drive (B-drive) that uses tensioned rope to control the knee joint rotation. A small drum is attached to the shaft of the bottom motor and is linked to a larger drum through a tensioned rope attached to the ends of the larger drum. With this configuration, the drums become meshed together in the same way as a set of gears.
The large drum in our proposed mechanism is represented by the arc at the bottom of a larger triangular link. This triangular linkage rotates about the top motor as the servos slide vertically along the rails. Attached to the top of the triangular link is our B-drive, which exerts a force on the long leg based on the input rotation of the triangular link (relative to A-drive controlled by top motor). The top motor lies at the triangular link center of rotation and is attached to one end of our A-drive, controlling its angular velocity. The other end of the A-drive is attached to the longer leg, serving as the long leg center of rotation.
Our team is still debating the usage of a spring between the middle of the A-drive and the top of the long leg. While implementing a spring here would enhance jumping capabilities by increasing tension, it would add additional complications to our already-complex kinematic analysis. Omitting the spring for simplicity, we are currently looking at a four bar sub-assembly in parallelogram configuration with the ground at the top motor. Because we are still looking at potential alternatives to the spring, we are unsure of our complete assembly linkage analysis.
Proposed Scope
The ultimate goal of this problem is the creation of a quadrupedal walking dog with the ability to maneuver around in a 2D plane and have the ability to jump. However, for the project scope of this class, the primary goal is to create a singular grounded robotic dog leg with the ability to walk around in the X/Y direction and have the ability to jump upwards. Its main functions then being standing, single locomotive walking, basic vertical jumping, and actuating legs allowing for turning along with the ability to support its own weight.
In addition to being able to do these basic functions, we have set a number of goals as a bonus to determine if the robotic dog functions within and beyond our expectations. These benchmarks being the ability to continuously jump 1000 times, jumping with a payload, having a perfectly damped system, and a compliant mechanism allowing for a hip flexor motion without an addition of extra motors.
For the completion of our goals, we must accomplish certain phases of our design which we have listed. First is the system architecture planning where we decide what approach we will be using. These were options such as Direct Drive vs geared, 2 vs 3 DOF, 4 bar vs other bar mechanisms, and choosing a Capstan drive or other drive system. Following this we work on our other phases doing our Modeling/Sizing phase where we do our jumping physics, actuator sizing, and stability kinematics along with our Mechanical Design phase where we do our CAD design and figure out the optimal structural layout. The next steps would be our Electrical Integrations phase where we create our wiring diagram and work on our code then we move to getting our Jump/Gait Behavior phase ready through repeated testing and trials searching for the optimized positioning allowing for stabilization, walking, and jumping goals to be accomplished. Most of these phases will be worked concurrently with each other to be ready for our preliminary design demo.
Preliminary Design
The initial design inspiration was CARA; the capstan drives based dog-analog robot by Aaed Musa and DINGO; the small low-cost dog by Nathan Ferguson. In his build process, there is an initial testing phase where a single leg is tested on a stand; this is the origin of this projects design.
From the beginning a number of important design trades needed to be done to limit the projects' scope.
Servo Drives VS. BLDC Pancake drives
Servos Chosen due to inbuilt gearboxes, lower cost, easier driver requirements, and form factors.
Capstan Drive VS. Linkage drive
There is still research needed to be done here, provisionally a capstan drive will be used to simplify the mechanism and maintain high torque transparency throughout the legs stroke as opposed to a linkage system.
4 Bar linkage system VS. 7 bar system
4 bar linkage was chosen for the sake of project siplicity despite loss of torque and ROM.
In addition to these trades, a basic design layout was conceived to begin working on
Leg will be non-grashof
Grueber Equation (basic 4 bar above vs. 4bar w two actuators)
M = 3(4-1) - 2(4) -0 = 1 DOF
M = 3(5-1) -2(4) -0 = 2 DOF
Basic Kinematic systems level diagram: See mechanism section