Since my last post I've done some research on how to design and build a multi-copter. I couldn't think of any interesting pictures to put aside the text so its all just writing unfortunately.
So first up, build materials. Last post I was determined to use carbon fibre, being the top material for strength and mass. I was talking to some mates at uni about this and they both reckon I shouldn't bother and should just use fibre glass instead for the time being. The reason for this mainly being price. Secondary to this, as I'm new to this type of fabrication, I most likely will make mistakes and so would be better just to go with fibre glass due to costs in stuff ups or bad production causing the same strength as fibre glass. The other question I asked them was how to join/bond two pieces together and they said I could use the same epoxy that is used to do the layers of fibre glass for the bonding. If I wanted it even stronger, there is a fibre glass tape that could be put round the joint.
Next, the safety guards. Using safety guards basically turn the air flow from open-axial-flow into ducted-axial-flow. This has effects on efficiency and power. I'm still unsure about these effects so I think I might talk to a lecturer at uni this coming week. If it was ducted (and this might help open-axial as well) I would put stators behind the blades. This increases efficiency and reduces noise and turbulence by taking the swirl out of the air (Ref: Fluid Mechanics - Fundamentals and Applications by Yunus A. Cengel and John M. Cimbala).
In the same section of the text book, it has how to calculate the pitch throughout the span of the propellers. So I now partly understand this calculation except that the book gets one variable from a requirement but doesn't say where this comes from. It may be that I haven't read other parts of the book and so don't know but it seemed like a variable that was only for propeller design. I'm also thinking with the propeller design that once I become better at using fibre glass (or carbon fibre) that I slightly change the propeller design and put a wingtip/winglet at the end of the propellers to improve efficiency and noise. It has been done on a few military helicopters and other rotary wings/propellers. I'm aiming for 12 inch or larger diameter propellers which I'll buy rather than make.
Now to the electronics and control systems. I've had a look at two control systems, ArduCopter and OpenPilot. Open Pilot I think is the better option for my uses, as being ARM based, its much more extendible to an autonomous system than ArduCopter which is Audrino based. I had a look at using the OpenPilot CopterControl unit but this only has outputs for 6 servos and as I'm designing for 8 servos at least (octocopter), So I think I'll make my own unit. I'll borrow some of their code though and try and make a plug-in framework or something so I can add artificial intelligence to the unit.
Talking of servos, one small rant. I don't understand why the hobby websites don't give proper datasheets for their servos. It would be very useful if they had current vs torque curves so you could design battery and propeller requirements earlier and before buying.
As I said, I want to start using ARM based processors. I found a blog post that says how to get JTAG and debugging integrated with the eclipse programming environment under Mac OS X. I'll then use this to help with the 3D printer and multicopter control systems. My only problem at the moment is for prototyping, I would use a breakout board for the chip and I'm unsure how I'm going to get from the breakout board to the breadboard as the pins are double rows. I'm thinking I'll have to just use female connectors on the breakout so I individually wire them to the breadboard rather than plugging in the whole lot which will be quite messy with the amount of cables but I think I'll have to suffice. The other option is to make my own breakout board but at the moment I don't have the tank to make my own boards. Talking of which, I'm thinking of changing EDA to DipTrace from KiCAD as KiCAD is very basic and doesn't have all the features I would like. I would use Altium but its Windows only and the company seems to be having some problems at the moment (they fired their CEO recently for example).
Last week one of the things I was very interested in looking at was fuel cells. I've since had quite a good look at the technology and there are many different types, but the PEM type looks the best suited, its also the most readily available. I understand the theory behind PEM cells but I can't work out how to build one. Due to this and the cost, even if I could make one, I've decided I'll just use batteries for the time being. I may, once I have everything working, switch over still but it will be a few thousand dollars to buy a large enough cell, the storage containers and the electrolysis device for creating the hydrogen. The prices of these may come down in the next few years anyway as its a fast moving area of development due to the use of this technology being explored for use in vehicles.
Two more things I've researched this week. The first being vibrations. I realised that if I was going to have a look at damped vibrations that I would need to first be able to measure the vibrations (well obviously!). I had a look round and found a Piezo electric sensor from Sparkfun. The product page has a very interesting technical document on how piezo electric sensors work and the design considerations. I only read the first 20 or so pages as I had a lecture but seemed to be a very good and well written document. This week I'll be doing basics of vibrations in class but I don't think we'll be covering damping. We've done more damping and simple harmonic motion in numerical methods maths than engineering classes so far. This morning I was wondering how to work out the coefficients for the spring and damping. The spring one was obvious, as you simply swap round the force function so it turns out to be
\[ k=\frac{F_{s}}{x} \]
and the damping coefficient is made up to be
\[ c=-\frac{F_{c}}{\frac{dx}{dt}} \]
I don't quite understand how once you have solved the equation, how this helps but I'll do some more research while I'm designing and building.
The final thing I checked out this week was cameras. I'm thinking of getting a camera module and using that. One mate from uni suggested a GoPro for taking shots which I do agree would work better for taking videos etc but this won't work for the autonomous systems as they need a live stream and the only streamed video that can be accessed from GoPros has a few seconds delay. Other cameras like Canon EOS series I think have live output but are hideously expensive and I wouldn't want to risk damaging one until I knew it was safe.
So that is what I researched this week. I think I did a pretty good job considering I still had uni lectures and assignments to do. I'll try and get cracking with the design soon and then post the updates. As I'm not trying to push the boundaries for the hardware of this project, the design shouldn't be too strenuous.
