MOSSBot

2018. Individual Project.

Mechanical Engineering  •  Machine Design  •  Failure Analysis  •  Component Selection  •  Technical Drawing  •  Report writing  •  SolidWorks  •  MatLab

MOSSBot is a small, passively self-righting jumping robot designed to meet a target specification. 

MOSSBot's development was guided by calculation starting with numerically-solved projectile motion with drag in MatLab, through detailed industry-standard equations for motor and battery selection as well as failure analysis of various components.

A CAD model of the final design and set of technical drawings of select parts were produced in SolidWorks. These drawings were included in a 20 page technical report of the design and engineering features, process and outcome written at the end of the project.

Outcome

Simplicity was key throughout the design to eliminate unnecessary components, reduce the overall mass and ensure that whatever went into the design could be properly validated.

3

4 (i) & 2 (inside compartment)

1 (i)

4 (ii) & 5 (inside compartment)

1 (ii)

1. Rack and sector pinion with spring-actuated piston:

A simple energy storage and release mechanism whereby the piston compresses a spring as it is displaced by the sector pinion, and then releases to launch the robot without the need of any other parts as the sector pinion disengages with the rack.

2. Continuous high-torque servo motor:

Actuates the sector pinion and, as a suitable off-the-shelf component, was the cheapest and most effective way of achieving a high torque output in a compact space. Its housing provides additional ingress protection of the motor and gear train.


3. Aluminium Cage structure:

In combination with the appropriate positioning of the robot’s centre of mass, the spherical cage structure allows the robot to passively self-right after a jump. As the robot prepares for its next jump and the piston moves into the structure, the spherical structure lets the robot roll back into its jumping position, with the piston inclined at the right launch angle. 

4. Sealed Compartments:

These compartments provide ingress protection only to the necessary parts (the battery, electronics and servo motor), thus saving on material, cost and overall mass. They achieve IP54 rating using geometric features and two O-rings (one static, one dynamic) specified to BS 4581.

 

5. Li-Po Battery:

A small, lightweight battery capable of storing energy for 156 jumps.

Process

Initial Research, Ideation & Early Calculations

A literature review of existing robots and jumping methods, feeding into ideation using morphological analysis. Initial calculation including determination of launch velocity (3 m/s) and angle (60°) through discretised projectile motion with drag in MatLab, from which I estimated the energy required with losses.

Design Development

First proposed macro structure consisting of orientation agnostic diamond-shaped cage and inner cylindrical shell housing components. Simple hand-calculations to specify initial components (spring, motor, battery) and identification of possible failure modes (bending of cage members and gear teeth).

Revised macro-architecture to spherical cage with mounting platform for a smaller, lighter and more efficient design with a simplified actuation and different self-righting. Determination of overall geometry and dimension based on length of piston (dependent on required spring size).

Further calculation to select a spring and from this determine compression force, required motor torque and power, and gear train ratio. Modelling of spring release using simple harmonic motion.

Detailed selection process to specify motor and battery. Specification of a standard dynamic (rotary) and static O-ring using BS 4518 to seal in motor, battery and electronics. Integration of all these components into macro-structure.

Engineering Validation

Failure analysis and mitigation through material and geometric adjustments of various key components: selection of macro-architecture (cage and base plate) materials using Ashby plot of density against Young's modulus with performance index of stiffness plotted, and detailed calculation of failure of pinion gear and shaft.

CAD & Technical Drawing

CAD development in SolidWorks and technical drawing following BS EN ISO 8888 standard and using tolerances and fits following standards BS 4500, BS EN ISO 2768-1, BS EN ISO 6410-1 and BS EN ISO 2203.