Wednesday, October 13, 2010

The Work Continues...

Well, yesterday saw my first step into a larger world - I etched my first PCB(s). Normally, I'd just use experimenter/vero/whatever-board - when you're trying experimental stuff, it's easy enough to use. The problem with that stuff, though, is it's only for relatively low amperage. For driving motors, you need to be able to control reasonable currents... Assuming around 4-6 amps, I decided it was easier to make a small board to mount the MOSFETs which, for the uninitiated, will be the electronic switches for controlling speed and direction.

So what is called for is a couple of small boards, with big tracks - so I decided to make a couple of small boards, using Copper clad board, a special pen, and the required etching compound. Of course, manually drawing onto the copper (protecting the bit that I don't want eaten away) means that it won't be consistent, nor the optimal size... But I'm making this for myself, and just wanted something quick to do the job. Plus it's the first board I've etched - clearly a milestone.

Easy enough to do, but not an optimum result. What aren't readily visible are the bits that haven't been protected enough - but it's enough to do the job, especially when I give a coat of solder over the tracks to ensure good power transfer. It will be enough to control the power, give the MOSFETs something to be attached easily enough...

For those not technically minded, MOSFETs are a type of transistor made for switching high power - and hence good for running motors. For those technically minded, I've been reading up on Complementary Pairs - and how easy they seem to use in an H-bridge.

The end goal being that although controlling motors is simple enough, I want to not only have a good control system, but plenty of feedback to any control system - ie proprioception - available. So, experimental circuitry is being made and will be tested

I could probably buy what I want, but cost is a killer, and I'm learning plenty.

Wednesday, October 6, 2010

Droid with a good motivator...

My new motors arrived from Oatley Electronics on Friday... They're a bit heavier than they look - but hardly surprising when you think about it, really. Brushed motors, which means that they're not as efficient and won't last as long as would be ideal - but so much easier to drive than any other type, such as brushless.

They're 100W at 24V, so they'll put out a bit of power, while not needing too much amperage. Of course, it means playing around, and plenty of batteries in order to get the full power... On the other hand, because they are brushed motors, it will be possible to run them at lower voltages - and therefore lower power.

I've just checked from the standardised astromech plans (available online), and it looks like there will be plenty of room in the feet to fit them - the whole frame is about 5 inches wide, 3.5" at the top, while feet are 7 wide at the bottom but 3" or so at the top... What I will do is do up a paper foot to double check... If I have to fudge... I suppose it will have to be... I doubt any hobby builder would call me out about it - once you know how much work it is to do, you don't denigrate others... If someone does, I reckon that a couple of hundred watts propelling quite a few kilograms bearing down on them will change their minds.

The other step is to build a couple of motor driver boards - a few high power MOSFETs, a current shunt (to test for motor stall, etc., rotation encoders...