Initial Project Proposal

Problem Statement/Background:

Historically, exploration of astronomical objects without atmosphere (such as the moon, or asteroids) have encountered issues with abrasive dust damaging bearing surfaces, seizing up motors and rotary joints in short periods of time. As astronomical mining and space colonization are becoming increasingly relevant, effective locomotion on the surface of astronomical bodies will be increasingly important. Current solutions to locomotion on astronomical bodies often involve wheels, which face significant challenges with uneven ground, small obstacles, and dust-accelerated degradation of joints and motors. 

Problem details and complexities:

Effective movement on astronomical objects needs to be done in a manner that is capable of handling the obstacles and uneven terrain present. Craters, rocks, and sand are common features that pose challenges to typical movement patterns on earth, which are often designed for paved roads and other developed infrastructure. 

In addition to the challenge of obstacles, bearing surfaces should be minimized, if not eliminated entirely due to the aforementioned issue with abrasive dust leading to accelerated wear. Many of the conventional engineering solutions used on earth for joints will face issues in space due to dust, large temperature differences, and vacuums.

This problem is difficult to solve with simple joints as a rotating or sliding element possesses bearing surfaces and is unable to avoid the obstacles mentioned previously. Wheels or sliding tracks for movement work best in clean, flat, environments with supporting transportation infrastructure, three things which are missing in space. As such, more complex movement patterns are needed to navigate the uneven terrain found outside of earth.

Proposed mechanism:

In order to address the previously mentioned issues, we propose a walking mechanism with a high foot-lift to best navigate the harsh terrains that are expected on asteroids and other planets. This greater foot lift will allow the robot to step over and on top of obstacles, rather than being caught and getting stuck. Flexural joints will be used in place of conventional rotary joints, as compliant mechanisms are agnostic to dust ingression, and feature no bearing surfaces. Lastly, in order to fully eliminate bearing surfaces, a conventional rotary motor could be replaced by a nitinol linear motor. Such a motor would be temperature actuated to contract, similar to a muscle, and would not face the same wear issues that rotary motors and their bearings would encounter in a dusty environment. A nitinol motor could also be more cost effective as it saves weight, which comes at a premium in space missions, as it does not require the rigid housing, windings, or permanent magnets that conventional motors would require, but instead uses nitinol wire, a heating element, and springs. 

Proposed scope of work for the final project:

Given the ambitious nature of this project, it was decided that the scope of the project would center around making a compliant mechanism that takes a linear motion input and converts it into a walking motion. Some interesting, yet non-critical goals would include using nitinol wire to actuate the linear motion, as well as incorporating multiple legs to create a walking robot. 

Analysis will be conducted on the kinematics of the joints and the linages, with attention paid to the effects of flexible joints in comparison to conventional joint construction methods. We expect to make assumptions that allow us to model the flexural mechanism in a way similar to conventional mechanisms with rigid linkages and joints. We will then analyze whether the simplifications used for modelling flexural mechanisms are appropriate based on the actual mechanism. 

Currently, the team is most excited about exploring the various fabrication techniques that are possible for compliant mechanisms, including but not limited to laser cutting, FDM printing, and SLA printing. We hope to produce compliant joints with enough flexibility to function without compromising the structural integrity of the walking mechanism. 

The team is most worried about finding a mechanism that can effectively convert linear motion to a walking motion, specifically one with the high foot raise that we specified (most walking mechanisms move with a low foot raise, where obstacles would pose a significant challenge). 

Preliminary Design Ideas:

Initial ideas for the compliant mechanisms involve either single materials, such as laser cutting the entire mechanism out of polyethylene, or multiple materials, such as using low-durometer rubber for the flexural joints, with rigid plastics serving as the links. In terms of the nitinol motors, we hope to explore using nitinol wire springs with resistive heating elements to create the main contraction force, with an opposing spring serving to relax and release the muscle once the power to the heating element is removed. Other linear actuators are also being considered at the moment.

2D fully compliant mechanisms can typically only have rockers, and not cranks, as a crank would destroy the flexural element, twisting it until it is broken. This proves a major challenge with almost all conventional mechanisms. One way of addressing this could be to use a material that only bends in one direction to allow the foot to drag in one direction and grip in the other, to use a redirecting element to convert linear motion to circular (note that this does introduce a bearing element, however it would be low speed and possibly self clearing), or to implement a crank by having a compliant linkage that is into the plane (essentially spins around like a noodle). 

Next Section: Project Analysis and Prototype