17-2 Project Prototype

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Section I: Inspiration

What if we could design a walking robot similar in function to quadrupedal systems like Boston Dynamics’ Spot, but driven by simple linkages instead of multiple actuators?

The idea is to explore whether complex, navigable walking behavior can be accomplished using four-bar mechanisms and minimal motor input. This concept draws inspiration from underactuated robotic hands, which are capable of producing adaptive and complex grasping motions using a single servo or actuator. By using passive mechanics and carefully designed linkages, these systems achieve functionality that would otherwise require significantly more control inputs.

Applying this philosophy to locomotion, the goal is to develop a walking robot that can generate stable and repeatable gait patterns using only one or two motors. This presents an interesting engineering challenge: translating simple rotational input into coordinated, multi-phase leg motion through purely mechanical design.

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Section II: Iteration Documentation and Conceptualization

Iteration 1: Initial Kinematic Proof of Concept

As with any design process, developing a proof of concept is a very important, critical first step. Also equally as important is making sure that this proof of concept is both adaptaive, and simple, capable of being modified and refined as the design evolves to better meet project goals and constraints.

For a linkage-based walking mechanism, early-stage concepts can be really easily developed and tested using digital simulation tools such as MotionGen. These sofwares make it easily experiment and test different linkage configurations, geometries, and motion behaviors, making it quicker to iterate toward an effective design.

Our first iteration focuses on the digital construction of a standard Jansen eight-bar mechanism. This mechanism transforms constant rotational input into a cyclical walking motion that mimics a natural gait. Specifically, the motion can be broken into three distinct phases:

• Lift phase, where the foot rises off the ground

• Support phase, where the foot maintains contact with the ground to provide stability to the body/chassis

• Stride phase, where the foot moves forward in preparation for the next step.

As shown in the position analysis generated by MotionGen, the kinematic proof of concept demonstrates that the Jansen linkage operates as a system of interconnected four-bar linkages arranged in parallel, with two fixed (grounded) pivot points. This configuration lets the mechanism produce a smooth, continuous walking trajectory from a single rotational input.

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Iteration 2: Prototype CAD and Design

For this iteration, we focused on building a complete CAD assembly of the 8-bar Jansen linkage so we could simulate and visualize the motion in a 3D environment. At this stage, the model is simplified and focused mainly on the linkage geometry and movement. It does not include hardware components like bolts, joints, or fasteners.

Some links are double-reinforced to better represent structural support and to prevent unrealistic deformation in the simulation. The main goal here was to assemble the full leg mechanism and verify that the motion behaves as expected when driven by a rotational input. This allows us to check for issues like interference, range of motion, and overall walking behavior before moving into a more detailed and physically accurate design.

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Iteration 3: Physical Prototype of Jansen Mechanism

The third iteration produced the first fully functional physical prototype. This stage showed the practical challenges that were not apparent in CAD, particularly in determining proper spacing for bolt heads, washers, and overall clearance between moving parts. A critical aspect of the process was establishing the correct assembly sequence to ensure the linkages moved freely without interference. Even minor spacing issues could cause binding or restrict motion, so careful adjustments were necessary to achieve smooth operation. The designer for our team focused on the Jansen mechanism, Harrison, used his prior experience from developing a fully 3D-printed RC car.

Similar to assembling suspension linkages in the RC car, he used thru holes to insert M4 bolts at each joint, separated by washers to reduce friction. Lock nuts were tightened to secure the linkages, then backed off by a quarter turn to reduce the vertical play and still allow smooth motion. His experience with RC car linkages and managing hardware allowed the assembly to come together relatively quickly in terms of figuring out spacing, layering, and overall assembly, since there are multiple moving links in a compact arrangement.

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Section III: Bill of Materials

Section IV: Kinematic Analysis Plots

In this section is the complete kinematic analysis of a Jansen linkage’s end plot at the endpoint which makes contact with the ground. An animation of the Jansen linkage has been attached to the assignment submission rather than this document.

To calculate mobility of a Jansen Linkage, one can make use of the planar Grubler-Kutzbach mobility formula:

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For a Jansen linkage: L = 8 (including ground)

H = 0 (no higher pairs)

Joint counting: There are 7 physical pin joints, but 3 of them are ternary joints, and each ternary joint counts as 2 binary joints, so making use of equation 4.1,

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Final answer: M = 1

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