Wednesday, October 30, 2019

Wall-E project (body CAD second step)

Since my last post I've been CAD-ing my way through the body of our Wall-E project. I am quite pleased with the amount of progress I've been able to make in the after-hours of the evening.. despite work, side-work, kid-work, and my role as full-time musician and maid.

In any event.. There are several things I 'fixed' in my model - including separating the parts to be printed by color, so that I have to do less painting. And I also thought through the screw mounts a bit more in detail. I feel my design will be stronger.

Also, the Neck mount has room for three 626zz bearings, with M6 screws as axels. I will also be hollowing the inside of the Neck mount for better cable management. I was not happy with the way this was handled in the inspirational model.. and also, some portions of that model just were not as they are in the movie - so, naturally I had to fix those pieces.

Rear Right Upper View:

 Front Right Upper View:
Front View with door and electronics panel hidden:
The only thing I'm really not pleased with are the proportions of the electronics panel in the front view. I had to mount the 1.8" LCD sideways, because it would not fit?! Due to the PCB mounts being larger than the actual panel itself.. so, I scaled the recessed vent holes on the right side to about 35% while the LCD is 65%. Of course, if you look at screen shots from the movie, it's actually 50/50.. but oh well. We can't make everything perfect..

I suppose I could scale the entire body by 20-30mm.. but there are a ton of extrusion operations to fix if I do that change.. and I'm just not THAT motivated. Maybe in version 2 I keep thinking.. (?) - when I have to scale the body anyway to fit retractable wheels, anyway.

Or who knows.. I may get the motivation pretty soon.. even thought there are lot of sketches to change to propagate +20mm into the height of that panel. It's a pain to think about.. but I probably could knock it out in a few hours if I really wanted to, as most of the extrusions are referencing other faces or vertices rather than hard measurements.

So, more to come..

Sunday, October 13, 2019

Wall-E project (chassis, first step)

It is all but redundant to say that's I've been busy on embedded projects. Alas, I somehow find time to build a robot?!

My son and I are really excited about our next robot project. We are building a mini Wall-E. I started with an engineering student from Ireland's Wall-E on Thingiverse. However, there were several issues with this design, that make it more of a re-design project.

  1. I did not have the motors v1 author (Simon B. @chillibasket) used.. both because the ones he used were sourced in UK and because I have other ones I can source which are similar size.
  2. Since the robot is a smaller sized robot (unlike Beast), I have to use reduced size 60 RPM motors.. even though I have some 200 RPM motors - but there is only 5mm of play between the motors, if I use those. Also, these cheaper motors do not have an encoder, and that's an issue for me! Because, obviously I need an autonomous Wall-E eventually..
  3. @chillibasket printed his axels, which are not very strong and make a LOT of noise.. so, I wanted to add 626zz bearings and use an M6 screw for my axel for strength and something that does not sound like a mill wheel.
  4. Later, I will need to redesign the head exclusively to add a StereoPi for the eyes. Nevermind the variable baseline between the cameras.. I'll address that later.
  5. The gripper in the v1 design does not actuate, again.. later.
  6. The wheels cannot retract into the body so that he folds into a box. I have a plan for this, but it will likely get addressed in my re-redesign.. but I'm thinking about it.

Further thoughts about retracting wheels: I already purchased some worm gear motors with encoders for this. However, I just want to get something working first! Then I can think about more complicated design features like navigation, mapping and localization and retractable wheels. I already did some rough measurements and I would need to think through a lot of other things like a 2-axis gantry mount, and where the heck the electronics would fit in the re-redesign.. possibly and false wall? It's much more complicated and the body would likely need to bigger to accommodate the mechanics and electronics.

So.. I gave up on my Luzbot 3D printer.. 3mm filament is temperamental. It breaks a lot, much more fragile over time than 1.75mm filament.. and I also got supoer spoiled while contracting for a company with a lot of Makerbot printers for several months. They are the most user friendly printer I have used yet. I also acquired a prefect condition (30 print-hours old) Makerbot Rep2 for free.. and a second one with a rebuilt gantry a similarly low print-hours for a few hundred dollars. So, I moved the Luzbot to a storage/practice space, and I have started running my new printers 24hrs a day!

So, 320 parts later..

The initial track prints had issues. I mistakenly printed with support for the overhang, which left a blemish in the underside. Over time the tracks started to curl up as the pins where inserted. After I revisited the print, I found the mistake, and reprinted 50 parts.. The final track is flat now, and it worked great. The 626zz bearing are a huge improvement. I also added wheel covers, and used M6 1.5mm washers as spacers (rather than printing them). However, I did not account for this in the CAD and this forced me to go back and reduce the tread gear teeth by 1mm so that I did not get binding as the track moves (which as also stopping to motor from turning). Now we have functionally wheels.

I chose some cheap Nextron 60 RPM motors from Amazon. But since they did not have encoders, I also order some shaft mount hall-effect encoders as well. Alas, they also needed a mount.. so after disassembling one of the motors to make sure it would work and that I had enough room.. and a couple caliper measurements later.. I had to print the encoder mount for the motor I used. I have not had to revisit this part. After final assembly I noticed different speeds for each tread, but this seems to because of different torques on the main shaft axel screw. So, loosing one side or tightening the other side works for now. Obviously I might have to revisit this part of the design later, if I extremely worried about drift. In my experience the wheel slip is either huge or non-existent depending on the chassis design.. in this case, I it is going to a lot of slip, I'm sure.

Lastly, I had to redesign the motor mount for a couple reasons. The first is that I soldered 90° angled 10mil pins into the encoder breakout, and I needed relief for this on the motor enclosure side. I also made the enclosure fit a lot better. I added width, so it straps together with M3x30mm screws. And in the previous design there was only one M3x6mm into the motor mount.. which I feel is way too few obviously, so I added four more for a total of five mount screws. Basically, it is not going anywhere. Additionally, I added an M3x4mm set screw into the shaft gear, because the pressure fit in the previous design rips right through with enough motor torque. Theres also an additional 4mm of pressure fit with the set screw, and the new design works a little better.

Here is the final assembly, with the wheel caps added. I purchased some M3 2mm, 2.5mm and 5mm bits for my Dewalt. There are about 200 screws in the chassis along with the 300 parts. Phew, this is not even the complicated part yet..

Then, I started the body. And, Wall-e hit a wall. I reached the build volume limit of my Makerbot with the bottom plate. It does not fit vertically or horizontally. I have access to larger printers, via friends.. but really I do want to try to build it entirely on the Rep2's. So, I had to split the part into two prints, which I think will be okay. I put the split on an off-center portion of the motor mounts, and I think there will be enough rigidity from the remaining screws the side mount screws.


Here are the resulting split bottom plate assembly after super glued and bolted. It is actually much stronger than I thought it would be. It also helps that I did not line up the spit seam with the seam on the motor mounts. It seems rigid enough for the robots goals.