This is bighak
A number of members attended Maker Faire in 2013, primarily as visitors, with some helping with the Hackspace organised solder area. For 2014 a last minute application to be an exhibitor was submitted, and to everyone’s delight, was accepted.
Between the group there was a good number of personal projects to fill the expectant trestle tables, but what better than a group project to really make an impact? Whilst enjoying a post-build-night pint, inspiration struck…
“Does anyone remember bigtrak? You know, that programmable space tank from the 70s. How about a ride-on version? We could call it ‘bighak'”
“bigtrak? That was the first platform I ever programmed on!”
“Me too! BLART BLART BLART”
“I’ve got some wheelchair motors & golf kart wheels in the shed I was planning to use on a project, but this sounds like more fun!
‘You know what, I think we could do it’
And so the fun began.
Thankfully the Hackspace’s local has a blackboard for darts scoring.
An ambitious project, especially with the exhibition deadline only 3 months away, there was no time to spare! Core tasks were quickly assigned and the group got to work. Within a couple of weeks we were driving a crude prototype around the paths of Hitchin – emphasis on the ‘crude’, it being mainly constructed of old decking timber.
Proof of principle – skid turning in action
Following the success of the initial trial the team began to source the other materials. With some wheelchair motors and golf kart wheels already donated by one of the group, other members were also keen to help; “What are you making the chassis out of? I’ve got some aluminium honeycomb that would be perfect”, “I’ve got some 12V batteries I salvaged from somewhere, any use?”… And so the wombling continued: an old bed frame, a storm damaged greenhouse, a past-its-best child’s ride-on jeep…
Group Build Day
The first build day was devoted to the chassis. Having been modelled in CAD software, 1:1 templates of all the body panels were printed off in advance and affixed to the aluminium honeycomb on a sunny March afternoon. Cutting the sheets was surprisingly easy work and in no time ‘the tub’ started to take shape’
Attention to detail for the bodywork was paramount. From an early stage it was decided that it had to look as close to the original as possible. Having never built a giant version of a 70s space tank before the team considered several different methods to create the complex body shape, settling on plywood sheets + angled batons + lots of car body filler, with a bit of salvaged solid insulation material thrown in for good measure.
Once the basic shape was created, on went the car body filler and out came the sander. Many hours of devoted sculpting followed, with the detail of every curve and contour agonised over for longer than was strictly healthy.
“You know what, I think it would have been easier if we just CNC’d the whole nose cone”
[through dust covered goggles, having just coughed lungs inside-out] “Now you say that!”
There were a few bodywork details that were beyond even our master craftsmen, so the team resorted to 3D modelling & CNC routing for the more intricate features. Due to time constraints solid foam insulation was used, with the team acknowledging that it wouldn’t be a robust long-term solution.
Finally several layers of Plastikote were applied to give the oh-so-important glossy/plastic toy look
Chassis and Axles
If the bodywork had to look good, the chassis had to be perfect! Honeycomb aluminium is very strong and light, but as with anything, there is a compromise. It is very susceptible to point loads and de-lamination. Several different styles of joints were tested to make sure we got it right. The biggest fear was structural failure. We had just enough aluminium for one unit.
If anything broke, there were no replacement parts available without heavy cost and delay. Being unfamiliar with the material, Luke Clampitt’s advice and assistance was invaluable in getting the project finished on time and to a high enough quality.
Sanding and Painting
With time at a premium, a lot of elbow grease went into the filling, sanding and painting of the tub and bodywork. As with all the best OCD workers, the sanding team still want to fill and smooth down the surface at least one more time! There were many complex curves on the nose cone as well as the side pods. After the first coat of spray paint the bighak really started to look the part, so much so that everyone was extremely proud of the work done.
It’s always interesting to see where we got some of the components for this build. Here you can see we bought the jeep for it’s wheels. The jeep was dismantled to see if any part of it could be re-used. bighak’s power comes from two paired wheel chair motors. They came with a motor controller and joystick which is going to be ‘hacked’ to allow computer control to mimic the keypad control of the original bigtrak.
When thinking about building a programming interface for bighak, we knew that we wanted it to be as close to the original toy as possible, but be something that people could do while waiting fortheir turn as a pilot. So we opted for a mobile web based solution, using people’s phones as a programming device
Once they have entered their program, they press ‘go’ on the keypad, and their program gets sent to the web service, and stored in our database. They are then returned a QR code to be displayed on their device, which contains a JSON encoded version of their set of commands.
When it’s their turn to take bighak for a spin, they can show the code (which is on their own specific url) to a pi camera on the dashboard, and the python control code grabs the QR code and parses it. When the pilot hits the start button, python code running on the raspberry pi takes care of all the timing and steering logic, and communicates over serial to an attached arduino, which acts as an adaptor emulating the proprietary joystick for the wheelchair controller.
reproduced, in part, with kind permission of Dave Booth from boothing.com