CAD Model Completed

After several late nights working on my CAD model I think I have everything done.

Learning from past mistakes, I included a sketch of the maximum allowed robot size. I also used this to label which motor sockets on the Motor2040 board each of my wheels is connected to, so that I reassemble it correctly when I disassemble my test chassis and rebuild it with the 3D printed parts from this new design. I reused the Pi 5 case design, but reprinted it in better quality in ABS. I added mounting space for a USB-C battery pack which after testing I was happy to confirm is capable of powering the Raspberry Pi 5 with no issue. The twin cell Li-Ion battery pack between the wheels powers the motors (via a 6V UBEC) so they get a constant voltage supply whatever the level of charge of the battery. Using separate batteries also ensures the motors don't cause brown outs for the Pi power supply when the going gets tough.

The whole CAD design is highly modular. This enables each part to be printed separately, reducing the time lost if a print fails or a part needs modifying and reprinting. It also allowed me to get parts printing while I was still working on the rest of the model, saving valuable time.

I really need to get working on my own robot!

Just one month to go until competition day, and I have not started building my own robot yet! Panic has started to set in and I have done some initial work on the CAD for a small chassis to hold the motors and a battery. Based on the design of my small tracked robot, but needing 4 motors instead of 2. I have a design which uses commercial camcorder batteries still commonly used for LED video panel lights. So they are easy to swap out and drop onto a charger.

I spent more time than I probably should on designing a model of the mechanum wheels. But now I have them to add to my common robotics parts CAD model library. I will share them on GrabCAD when I have time to think about things after the competition, so that other members of the robotics community can use them in their own CAD models. I'll do the same with the battery model. I have learned it really pays to model every component and put in all the machine screws and hex nuts in the CAD assemblies. This allows interference detection to be calculated and avoids costly mistakes which require re-printing parts when you discover a servo collides with a nut of some such problem when you try to assemble your design for real.

Working on Attachments with my Young Persons Team

Since my holiday coding, all my Pi Wars time has again been spent mentoring my young person's team. Together we designed a barrel grabber to go on the front of their robot, and started 3D printing the parts for their proper robot. By the time we get to competition day, I will have built 4 robots in total! 2 test platforms and 2 actual competition robots. I am teaching my young persons CAD using SOLIDWORKS. We based the grabber off designs we saw when searching for robot claws on eBay and other direct from China tech selling websites. This is what we came up with.

But I have a cunning plan. Having 3D printed all the parts to fit onto their robot chassis, I am going to design the front of my own robot to have exactly the same mounting points. So I can print a second grabber, camera mount and sensor mounting block and put them onto my own robot. Hopefully this will not over balance my much smaller robot. I plan to put my battery on the back to act as a counter balance for the grabber.

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.

Designing the Main Chassis and Complete Rocker Bogie Assembly

Having built a pair of working prototype rocker bogie wheel assemblies, I turned my attention to the main body of the rover. Initially I just used a simple box shape, keeping the front of the chassis well back from the front steerable wheels to avoid collisions. The rear of the chassis had to overlap the area where the rear steerable wheels servo links came under the body in order to fit the top pivot for the linking bar assembly. But I now had a basic chassis design which enabled me to complete my CAD model and see a working mechanical model of the full rocker bogie suspension.

A rendered animation of the rocker bogie suspension

To avoid the rear wheel steering links colliding with the underside of the body, I modified the box so that the bottom was shorter than the total length of the body. This gave the servo steering links more clearance where they passed under the body. I made the front section of the body just long enough to fit in the Raspberry Pi 3 plus two additional support pillars to provide a second pair of holes for the main pivot bolts to pass through. This gave a more rigid main pivot axis assembly than if the bolts just passed through one piece of 3mm sheet material on each side.

I mounted the Raspberry Pi 3 board in the box, with the USB and Ethernet ports poking through a cut out. This was technically the first Raspberry Pi case I had ever designed. The rear of the box still needed to extend far enough back to support the linking bar assembly on the top. To allow this, the base of the box stepped up to a lesser depth at the rear.

The body design with a step up on the underside at the rear 

to allow the steering links to pass underneath the chassis

At this stage I was still pondering over how to build the linking bar assembly. I had modelled a bracket to hold the horizontal linking bar, which I was going to have to 3D print. I also needed to solve how to attach fittings to the ends of the cylindrical bar to link to the rocker arms. At about this time, somebody posted a picture of the NASA Mars Curiosity rover in a lab in reply to one of my progress posts on Twitter.

 Image Credit: NASA/JPL-Caltech

Up to this point I had not realised how large the real Mars Curiosity rover was! But I also noticed the linking bar was a flat bar, not a metal rod as I had modelled based on the JPL educational scale model project which inspired me to design a Pi Wars sized version of my own. That looked a lot easier to build. The picture also gave me another revelation. I did not need to mount the bar on the back of my rover. It could go in front of the main pivot axis of the body. This gave me more room as the body already extended further out in that direction to accommodate the Raspberry Pi. It also better distributed the weight of the body between the horizontal pivot axis and the top mounting point for the linking bar.

I now had a complete design worth building a prototype of. I laser cut it all from 3mm plywood because this was cheaper than plastic, easier to modify with small tools after cutting the pieces and I could re-purpose all the off-cuts and waste pieces as kindling to light the fire at home.

My first prototype, almost fully assembled (one rocker bogie arm is attached here)

This first mechanical prototype enabled me to verify that the design worked in practice. It also allowed me to test how much motion was possible in the rocker bogie arms before the rear steering legs collided with the underside of the chassis. I was not sure there was enough range of movement, and it also made me realise the weight was not well balanced around the main pivot axis of the chassis. I needed to think about how to solve these problems, and also work out how to fit all the electronics and a power source into the body.

Refining My Rocker Bogie Design

Using my newly learned skills in CAD and laser cutting I started to build the rocker arms for my robot. I was pleased with the way my servos fitted into the arm, but concerned that using the servo itself as the only mounting point for the steering leg was not going to be strong enough. That tiny screw attaching the horn to the servo just did not feel like it would handle all the strain put on the robot leg.

My first prototype steering leg assembly

I had only attached one leg to the first part of an arm and already I was needing to redesign. Some browsing of possible bolt styles I might use online led me to shoulder bolts. These have a solid cylindrical section (the shoulder) with a short threaded section on the end. I redesigned my leg and arm ends to attach the leg using a long shoulder bolt as the axis. A nylon spacer provided the required separation of the leg from the arm. Moving the servo along enabled me to attach the servo horn to the leg via a hinged link. The shoulder bolts were also perfect for the other pivots in the rocker bogie arms. So with the entire arm remodelled in my CAD software I was ready to assemble a complete version.

My first attempt to put it all together in laser cut 3mm plywood proved that the design would work, but I could not fit the nuts onto the small bolts which held the servo mounting plates in place. I had not left enough room for the nuts to fit alongside the spacers on the pivot bolts. It was at this point that I discovered the collision detection feature of the CAD software and sure enough it highlighted the problem in my CAD model. Lesson learned I made some more adjustments and was finally able to build and assemble a complete pair of rocker bogie arms.