So we spent some of the session on Saturday playing around with the servos for Hummingbird, we started by taken one of them apart so we could solder a new wire in there and get some feedback on the current position:
We have been soldering… Upgrading the servos to give us some position feedback.
We then spend the rest of the day wiring up the two servos, one for the x-axis and one for the y-axis, creating a circuit and attaching it to the Arduino so we could control them independently:
Starting to wire up the servos, one for x-axis, one for y-axis. Now to program them to work with the 2-axis joystick (which will eventually be replaced by some gyroscopic sensor).
As you can see, the servos are currently being controlled by a 2D joystick; this is just a concept idea, eventually that control will be replaced by a gyroscopic and magnetic sensor which will tell the servos which direction the motor needs to point to stay on course.
See them in action below:
Now we just need to order those gyroscopic and magnetic sensors!
Over the holidays, Joe (seen in the first picture here with the excitingly stripy rocket) put his model-making skills to use by assembling a foamcore model of Hummingbird. The gimbal even moves! I’ve just been told it wasn’t actually supposed to move, so I should probably stop playing with it…
Having something physical gives a better idea of how everything fits together.
The model pointed out something that’d be useful to change: the original concept had a single platform where the electronics would sit, but we now reckon it’d work better to have a second platform just for the flight computer. Of course, unlike the version in the model, the roll cage needs to be extended over this too.
Here’s a quick concept layout of Hummingbird. It’s inspired by David Wyatt’s Kestrel design, but uses a gimballed rocket motor instead of cold gas thrusters. (if you’re wondering what that’s about, there’ll be more about different attitude control systems in a later blog post!)
It’ll be a very modular vehicle, so it’s easy to swap out parts for improved parts once it’s been test-flown a few times and we’ve spotted things that need improving (or, erm, after we’ve broken things and need to replace them…). Features you can see above include:
A platform to make batteries and the flight computer easily accessible and swappable.
Landing legs made of carbon fibre kite spars. Not shown is some sort of shock absorber – this is definitely needed, becase a solid-fuelled motor can’t be throttled, so we’ll always land with a bit of residual vertical speed. (If you’re really on the ball you’ll have spotted that even with a single motor it’s possible to set the thrust-to-weight ratio slightly above 1 and then move towards the ground by “tacking” or “fluttering” from side to side… but that’s a bit of a complicated flight profile for the first batch of flights!)
A roll cage, just in case Hummingbird lands on its head.
The red cylinder in the middle is a Klima D3-P motor. For scale, it’s 70mm long.
The grey boxes are placeholders for small servos as used in RC models, something like this:
CAD model found on GrabCAD of one possible micro servo.