Mini-Wheg

January / March 2017

Individual Project

A mini-wheg is a small motorised vehicle that features 'whegs' (wheel - legs). 

What is a mini-wheg?

Specifications

Purpose

My mini-wheg served the purpose of exploring and capturing on camera locations too remote or difficult for humans to get to, such as caves.

Find the full report here.

Ideation

I began by sketching out initial ideas and than using paper cut outs to find an arrangement of components that worked.

Component Analysis

Motor

Required tractive force:

Required Torque:

Required Motor Power:

Maximum achievable tractive force:

D

F

Fm

Fmax

FN

g

h0

P

Γ

ω

μ

Wheg diameter

Tractive force

Weight

Maximum achievable tractive force

Normal force

Acceleration due to gravity

Height of object

Motor power

Motor torque

Angular velocity

Coefficient of friction

Conclusion: Optimal wheg diameter was 65 mm with a motor of  1.79 W (after applying a 1.75 safety factor). 

Gearbox

Iteration 1 took up too much space

Motor speed = 8870 RPM at nominal voltage of 3V

Desired velocity = 5 m/s

Ideal gear ratio = 60.38

Actual gear ratio = 58.4

In hindsight, a worm gear would have been much more suitable than this crown gear

Iteration 2 was more compact

Drivetrain

A suitable belt was selected using the ContiTech manual.

A tensioner arm kept the longer belt in tension. A gear ratio of 1:1 was used for the pulleys, to minimise the space taken up by the belts.

Steering

The steering mechanism involved a motor actuating a rack and pinion attached to a linkage and eventually the whegs.

A servo motor was used to actuate the steering mechanism, providing the required accuracy and range of motion.

Minimum required servo torque:

Ackerman angles were calculated to provide the necessary turning circle radius.

A cup and ball style, simplified universal joint allowing the whegs to steer whilst still rotating about the axle

Compliant Mechanism

A torsion spring allows the whegs, usually at 60 degrees out of phase with one another to come into phase when an obstacle is encountered, doubling the available torque to overcome it.

Ingress Protection

A flexible polymer barrier would complete the casing around the whegs at the attachment points shown in red.

Whegs

Iterative redesign of the whegs utilising FEA to identify failure load and stress raisers.

Shaft

The stress experienced by the shaft was calculated to select appropriate material and diameter 

Free body force diagrams

Torsional shear stress:

Final values:

Centre of Mass

The centre of mass was successfully kept central, giving the MiniWheg optimal balance both right way up and upside down.

Final Design

© Anna Bernbaum