Wednesday, 13 March 2019

Another Redesign: Electronics Cartridge

One problem I identified with my robot chassis design was that I could not access any of the electronics without taking the whole thing apart. It was not possible to remove the SD Card from the Raspberry Pi without unscrewing the Pi either. One option for the SD Card access which Brian Corteil suggested was to boot the Pi from a USB thumb drive instead. I had not realised this was possible, but some testing with various tiny sized USB memory sticks showed this worked very well (at least with the Pi 3b). But I was still concerned that if I had any wiring problems on competition day it would be difficult to access the internals to fix things.

The idea I had to solve this was to redesign my chassis so that all the electronics were mounted on a plug-in cartridge which could be pulled out of the chassis to easily access everything. This also allowed me to design a second tier to mount breakout boards on above the Raspberry Pi.

CAD model of my electronics cartridge

Having a 2 tier electronics tray posed some problems with how to route the ribbon cable which connects the touch screen display to the Pi. I went through a few iterations before building a prototype cartridge. If I routed the ribbon cable straight up through the upper tier then it limited the space to arrange the breakout boards on that tier. I ended up with a design where the ribbon cable was folded to change the orientation through 90 degrees from the Pi to the display. The fold was contained on the lower tier, which required the height of the lower tier to be higher than originally planned. Space was tight for the upper tier, and I did not take into account the height of the Du Pont connectors used to attach all the wires to the breakout boards. Another redesign of my entire chassis was required to allow another 5mm of head room. I modelled just one Du Pont connector with a wire bending over coming from it (you can see it in the CAD model above) to check I had the required headroom for the wiring.

Redesigned chassis with the electronics cartridge inserted

It all looked great in CAD, but when I came to assemble it I realised I needed a lot longer cables to provide enough slack to actually slide out the cartridge from the chassis. My ribbon cable was not long enough and 40 way ribbon cables seemed very expensive when I looked online at the usual electronics suppliers. Then I remembered somebody had pointed out that IDE hard disk cables in older PCs were just this type of cable. I checked my box of parts for modding PCs and found I had several. They all had 3 connectors, but I simply cut one off close to the middle connector to give a suitable length ribbon cable. All my other Du Pont connector leads I was making myself from a reel of ribbon cable, so I made these long enough to provide some slack.

The reality was that all this extra cable took up a lot of room, and the cartridge was very tightly packed when assembled. It was very hard to slide out due to the cables all catching on bolts, and in hindsight I could have done with another 10mm of head room in the chassis. But I can just about slide it out with a lot of cable wrangling if required to plug in a new lead. I later extended the i2c breakout to a second bus board near the front of the chassis so that I could easily plug and unplug additional ic2 devices mounted on the front of the robot without needing to remove the cartridge.

The assembled electronics cartridge with the multiple folds in the long ribbon cable. Most of the i2c device cables are missing here, apart from one connecting to the PWM breakout used to drive the servos and motor power. The 4 servo cables are also missing here.

Shown with the chassis front and front-side panels removed. The cables completely pack out the space in the cartridge.

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