Obligatory First Featherweight Build Log (FeatherDozer)

[Sam_Gad]

Design Philosophy & CAD

As was mentioned my in previous post, I wanted to build a reliable first build but apply the engineering knowledge gained at both university and in my day-to-day working environment. So, in its most basic form, FeatherDozer in a compact 2WD robot, armed with an electronic, rear hinged lifter.

Before this final design, there were many concept that ended up being shelved due to an aspect that I wanted to keep to: manufacturability. Due to my limited shed and tool capacity as well as my desire to make as much of the robot in house, I wanted to be sure I can achieve the design before starting the build.

Drive
Due to the compact design, it became clear that I did not have the space or the weight for conventional brushed motors. Therefore, the drive comes from 2 brushless outrunners connected to a live driveshaft via a single gear reduction. This was to limit the number of moving parts for each side of the drive as well as the inherent lack of space (maybe I should have just made it a bit bigger). The chosen wheels come from an industrial belt sander as they were an off the shelf part with have good grip qualities and a solid aluminium inner hub.

Front Scoop and Weapon Mechanism
The front “scoop” consists of 4 main sections:

• 1 main upper Hardox section
• 1 HDPE Front sacrificial panel
• 2 Lower HDPE “forks”/ side scoops

The “forks” are designed to not only get underneath opponents, but due to being made from HDPE, they can flex with both impacts and the floor to always be as low as possible. I decided to make the whole scoop lift as one as it seemed simpler to me at the time (not sure why now I think of it) as well as be able to act as a raised shield when required. This causes a number of issues which we will get to in later posts. When fully lifted, the front is protected by an inner HDPE impact panel. This is shown below.

The lifting mechanism is powered by a linear actuator, however after initial testing of off the shelf systems, I found a number of limitations such as weight, size and quality of materials used (due to being a mass-produced part).

Therefore, I chose build my own system using parts from a car scissor jack, a brushed motor with a planetary gearbox and a decent amount of hope. The leadscrew from the jack is retained in the centre of the design by the weapon motor and gearbox housing and the front plate that separates the fuse module from the rest of the machine. This was the first aspect of the design modelled which gives the robot its final structure characteristics. For now, this system will remain brushed as the electrical system can be basic (2 high power relays to create a H-bridge motor controller). The image below is in the early build phase (hence the mock up of the arm in wood) which shows the full final actuator mechanism in the robot.

IMG_20190915_203435 - Reduced.jpgDF MK1.0 CAD 5 Lifted.PNG

Chassis
With the weapon mechanism determined, the main chassis can be split into a left and right module. Each module consists of a main longitudinal bulkhead, inner bulkhead to support the weapon mechanism and a transverse bulkhead that acts as the battery separation from the wheels, driveshaft outer bearing carrier and retains the rear armour/ wheel guards. The two modules are then connected together by the rear bulkhead as well as a number of internal spacers (as shown in red in the picture below).

DF MK1.0 CAD 2 Inner Structure.PNG

Electronics and battery positioning
As the central part of the inner structure is taken up by the weapon system, the main electronics had to go either side of the module to keep the design balanced as well as be accessible between fights. Each side of the main inner structure houses a battery and drive speed controller. The electronics are protected by a HDPE curved cover that is shock mounted to dissipate the energy of blows from an overhead weapon (e.g. an axe).

DF MK1.0 CAD 4 Top View.PNG

Other aspects
From the images it might be apparent that the self-righting mechanism was a bit of an afterthought. This is because it was! From initial testing it became clear that without an additional structure at the rear of the lifting arm, the robot did not have the articulation to self-right. This system shown was limited due to time constraints and was added just before my first event. As a result it is a poor design. This has been changed in the latest version (Mk1.5) but is untested and seems to be a major limitation to this design which will be fixed in the next robot.


In the next entry I will go over some of the build process leading up to my first event in late July 2020.