The last couple of weekends, I’ve been working on the least sexy part of the robot – the mounting frame. As has been mentioned, the space inside the robot is VERY tight this year, so making everything fit is a real challenge.
We need to fit in:
The Pi and its (not quite) Hat
2 x Motor controllers
Servo controller board
2 x Battery monitors
2 x PSUs
All in a space 94 x 83 x 89mm. And we need to think about thermal management. Looks like we’ll have to mount the batteries externally!
Our solution is this 3D printed frame. It holds the circuit boards vertically (for good convection cooling), puts the Pi and its connections at the back (the back is removable for easy access).
All the other little boards are mounted on the reverse of the Pi mounting plate (hidden in the photo). The whole thing lifts out of the robot if we need access to one of the boards buried near the bottom.
As mentioned in my previous post, this year we needed (an excuse) to learn KiCad and build a custom PCB. Thankfully, we did succeed in soldering it up , despite the tiny pitch on some of the components.
The PCB dives into a few parts. I expect you’ll all recognise the Pi header in the top left. Above that, in yellow on the annotated image, we have the SPI peripherals: the screen and the IMU (which we use mainly for the gyroscope).
Below the header, in pink, we have the Parallax propeller chip, a fast microcontroller that we use to decode the signals from the motors. Each motor can put out 200k pulses per second, which isn’t really possible to handle from the GPIO pins because Linux can’t really handle that many interrupts per second.
To the right, in yellow, we have connectors for the “noisy” off-board components. These sit over their own ground plane, so that, if we want to, we can drive them from a completely isolated power supply. From top to bottom:
“noisy” 5v power
motor driver control 1
motor encoder 1
motor driver control 2
motor encoder 2
2 x power monitors
To bridge the gap between the microcontroller and the noisy world of the motors, (in red) we have a pair of ISO7742 chips. These provide two input and two output signals, which are level shifted from 3.3v to 5v and are isolated through an internal capacitor. Unlike an optoisolator, they were super-simple to use, requiring 3.3v and 5v power and grounds, a couple of decoupling capacitors and some pull-ups on their enable pins.
Similarly, below that, we have an isolated i2c line for driving the servo board (which runs from the “noisy” 5v power supply.
In the bottom left (in green) we have 6 connectors for optical time-of-flight sensors.
The time of flight sensors, Propeller, servo controller and voltage monitors are all i2c controlled, which poses a couple of problems:
i2c busses tend to become unstable with more than a handful of devices (because each device adds capacitance to the bus, making it harder for any device to drive the bus)
we have no control over the addresses of many of the devices; for example, all the time-of-flight sensors use the same address.
To address those problems, we included an i2c multiplexer in the design (to the left of the Propeller), allowing us to switch any combination of devices on and off the bus.
Despite having very little space to play with, we were able to squeeze in a bit of prototyping area, which we’ve used to address errata. For example, I found that I’d missed a couple of pull-ups on the i2c port that the propeller was attached to. A bit of thin kynar wire to the rescue:
Having seen various tracked robots on Thingiverse and especially this amazing one, I thought we should try and implement Wall-E’s tracks ourselves.
We could have gone with a simple rubber band or timing belt (and in retrospect that would have been MUCH easier), but I really fancied seeing how far I could push 3D printed parts.
So I had a long browse through thingiverse looking at lots of track designs and started to draw up my own. The FPV rover design had an interesting idea for fine adjustment – they used 2 different sizes of 3D printed pins to join the tracks together to make the whole thing slightly tighter or looser as needed.
In the end I settled on a design which had sprocket wheels mounted on either side of a supporting frame (to avoid nasty torques on the frame). Obviously the layout of the sprocket wheels on the frame had to match the ‘real’ Wall-E, but I decided to make the sprocket teeth larger (and therefore stronger).
Then the track elements needed designing. I went with a design which used the links between the tracks as the raised sections and the sprocket teeth sat in a deep well, but did not protrude from the other side. Like this:
A matching pin is shown too. After a few trail and error prints to fine tune the pin diameter, well depth, we got something that worked. And then we needed to print about 36 of them per set of tracks (3 x 4 hour sessions of printing).
The final problem was how to connect these to the motor. We wanted a fair bit of speed, so I’d ended up buying pololu motors with a 4:1 gearbox. Having seen these run, I was a bit worried about the high speed, so wanted to gear them down a touch. I found a bevel gear generator plugin in Onshape and ended up with this:
And that worked!
In fact running these is slightly terrifying – fairly sure if you got your finger in there it’d get a nasty nip…