Electrical Box Design Overview (2020-2021)

In 2021, the electrical box (ebox) was completely overhauled to address a number of issues with the previous design.

KEY PROJECT CONTRIBUTORS

Progress To Date: E-Box Progress Tracker (2020-2021)

TL;DR

The new e-box is comprised of a thin 0.06" single piece weldment to maintain a light weight while providing a stable site for connector mounting on the front panel. The front panel has mounting spots for multiple PP15 connectors, a pair of PP75 connectors, two USB connectors, two ethernet connectors and two general purpose cable tie racks for strain reliefs. The frame will also include punched in PEM fasteners to reduce required hardware, and will be powder coated to provide electrical insulation. It features a clear acrylic lid with a nice handle, quick-lock latch mechanism and detachable hinges to allow the easy access to the interior of the ebox. Four high CFM fans are placed on the left and right of the ebox (two intake and two exhaust) to prevent the Jetson from overheating and throttling in Utah. These fans feature fine grain filters on the intake and course grain filters on the exhaust to prevent dust from entering the ebox. The ebox will also have interior hardware to allow for a second level comprised of a 1/8" thick acrylic sheet on elevated standoffs. The exact shape of this second level is still to be determined. 

ISSUES WITH PREVIOUS DESIGN

The 2020 ebox was constructed using corrugated plastic sheets and aluminum angle. This design was very light weight, but was faced with the issues described below:

• Outer walls would easily bow with any applied force.
• Front panel was one large rectangular cutout (circled in red) that offered no stability for connector mount points or cable strain relief, so you could easily accidently yank and damage a wire.
• Lid was fastened to the top of the box using four thumb screws (circled in blue), so it was pretty inconvenient to open the box and make adjustments as you had constantly screw and unscrew the lid.
• Second level was added to this ebox hastily using a piece of plywood, rather than having a properly dedicated second level. 
• Ebox was not insulated from the rest of the frame; during wiring for 2021 SAR video submission there was an incident where a live wire in the ebox touched one of the metal brackets. This charged the bracket, and led to one of the electronic components shorting and breaking. 

2021 DESIGN FEATURES

Outer Frame

For the 2021 ebox, we wanted to make sure that the box frame was very strong and stable, while maintaining a light weight. We designed a single piece weldment from 0.06" thick 5052 aluminum (using material and thickness recommendations from our fabrication sponsor, demtool). This piece will be laser cut, folded and welded on all edges, so the frame should be very stable. Various cutouts are included on different parts of the ebox to allow for the mounting of a number of different components. The frame was designed using guidelines provided by our machining sponsor, and careful attention was directed towards ensuring that interior bend radius was equal to material thickness, minimum distance between cutouts and bend lines were met and small gaps were present at folded edges. 

As part of the fabrication process, PEM nuts will also be punched into the frame where possible. PEM nuts require a minimum distance between the centerline of the punch hole and the nearest edge, so it unfortunately it wasn't possible to replace all of the ebox nuts with PEM nuts without drastically spacing out some of the fastener holes.  Stainless steel pem nuts (S-440-2ZI and SSs-032-2ZI) were used to replace 4-40 and 10-32 nuts in a couple of spots to reduce the amount of hardware required to assemble the ebox. This was also a good exercise to experiment with different types of fabrication processes . After PEM nuts are installed, the entire electrical box will be powder coated with a light gray smooth medium gloss finish to act as an insulating medium between the ebox and metal rover frame. Light gray was selected as the powder coating colour as an effort to have the box reflect more light from the sun in hot environments like Utah. 

Front/Rear Panel

The front panel was designed to provide strain reliefs for the wires coming in and out of the box. It contains cutouts for PP15, PP75, USB, Ethernet and general cable access. We designed 3D printed mounting jaws for all the power pole connectors (PP15 and PP75) that lock the cable connectors on the front panel of the ebox, so if it's associated wires or yanked or pulled force will be transmitted to the connector and front panel of the ebox rather than the cross section of the wire. Connectors for USB and Ethernet extensions were spec'd out on Amazon, and are fastened to the front of the ebox using plastic nuts.  We also designed 3D cable tie racks for two cutouts on the front panel to provide a strain relief for any other general wires passing through the box via zip ties. 

Fans

During the design process of our 2021 electrical box, additional concerns were raised that the Jetson would likely overheat and throttle in the hot environment of Utah. We wanted to ensure that the electrical box would remain at ambient temperatures throughout the entirety of comp, so we selected high CFM fans. The CFM of fans in Melvin's PC were used as a reference to select CFM total for the fans in the ebox and the idea is to turn on fans in stages at comp as the Jetson's temperature reaches certain thresholds (rather than leaving them on all the time). We spec'd out fine grain filters for the intake fans, and course grain filters for the exhaust fans to prevent the fans from sucking dust into the box. One thing to remember is that the addition of these filters will reduce the effective CFM of the cooling fans. These particular filters were chosen as they were in stock at the time this design was formulated, but they should perform well to prevent dust from being sucked into the box. Once we get our hands on these filters, we will test them out and if necessary design and 3D print some dust filter covers for additional protection. 3D printed finger guards were designed to be placed on interior facing side of the fans as a means of protection, and cutouts were added on the left and right walls of the electrical box to provide mounting spots for fans. 

Lid

The lid was completely overhauled compared to our previous design. The new lid is comprised mainly of 1/4" clear acrylic sheet and features a sturdy handle and 3D printed detachable hinges as a mounting point with the outer frame. Now, the lid should be easy to open and remove if necessary, provided quick access to all interior components in the ebox. The lid also uses 3D printed stress relief place around all fasteners on the lid to prevent the acrylic lid from cracking from over torqued fasteners. The lid also features a low force spring quick lock mechanism to tightly secure the lid to the outer frame as the rover drives around. We will also be laser engraving our team logo to the top of the electrical box to add some personal flare to our box

Second Level

The 2021 ebox will also have dedicated infrastructure for a second level. A 1/8" thick sheet of acrylic will be laser cut to a suitable shape and placed on 3" long female standoffs using a combination of screws and washers to provide an elevated second level for interior components that are commonly accessed by electrical members (such as the Jetson, or the PDB). The shape of this second level isn't exactly finalized, as we are a still a but unsure of what the final ebox layout will be. Currently, the plan is to wait for the outer frame to finish it's fabrication process, then have electrical take all the interior components, figure out a suitable layout, then laser cut the acrylic sheet for the second level. In the mean time, additional custom 3D printed "standoffs" were designed for corners of the second level that may not be able to use female threaded standoff mounting holes. These 3D printed "standoffs" feature a nut pocket, small holes on the bottom face to ask as wells for applied adhesives, and are slightly oversized compared to the height clearance of the second level to ensure that they stay in place. 

Outer Frame "Standoffs"

Lastly, to ensure that the electrical box was fully electrically isolated from the rest of the rover, we also designed rectangular 3D printed parts that act similarly standoffs to fasten the box to the chassis using electrically isolated fasteners. These parts are large in size to increase the surface area between the standoffs and the chassis to ensure a stable fastening connection. Furthermore, they elevate the box by about an inch to ensure provide better clearance between the front panel of the ebox and the roboteq mount on the rover.