Monday, 22 August 2011

The last week I have been busy catching up with uni work so I haven't had time to do any rocket stuff, but I have ordered all the parts I need with long lead times.

I decided to replace all the relays in the relay box with a nice neat relay bank as the transistors on the old ones are dead (not too difficult to replace, but i still haven't identified the cause) and don't want to rick the same problem. The new ones are optically isolated from the micro controller which is a nice feature. Also the igniter will be on a separate battery.

I have also decided (reluctantly) to ditch the wireless control scheme and go for wired. I really liked the wireless control system and haven't had any major issues with it, but it is not %100 reliable (does loose connection from time to time, although doesn't affect operation) and after the last test I have come to realize everything has to work %100 of the time (not %99) because if something can go wrong it will. Wireless does have its uses, but I think a cable is better for this application.

The new control scheme will use a 300m length of cat6 cable, through which I understand I should be able to maintain at least 100M/bit speeds. There will be a usb-network server on the test stand which the micro controller is plugged into and shared over the network to a computer. The other advantage of a cable is that I will have enough bandwidth to run streaming video from the stand. The usb server supports 4 devices so I plan on having one usb tilt-pan camera and another fixed webcam. This should give me a good view of whats going on with the stand. One concern I have is that the usb server won't work well with the Arduino, bit I have ordered one and will know soon. I can always use a computer on the test stand sharing usb, but would prefer not to. I had considered a more direct approach of using a arduino with a lan port, but I really didn't want to go into programing for that and although the hub adds one layer of abstraction (and source of failure) it will act as a serial link (same as the xbee) so I won't need to modify the control program at all. Also if the cable is struggling (which it shouldn't) I can uses booster.

Valve feedback is the other major thing that need implementing. After some discussion with ariel about the different options (trimpot, 12-way encoder and optical encoder) I deeded to go with a simple trimpot. I did find a nice 100 count encoder which was relatively cheap and would be more accurate but the problem is its not absolute and could need calibrating. I thought of implementing an auto calibration with two limit switches but why complicate something if you don't have to, and with a %5 trimpot i will have all the accuracy I can handle.

Also I need to add a manual vent valve to the nitrous side as after I lost control of the stand we had no way of venting other than manually turning the throttle valve. I was thinking of attaching a rope to the two manual vent valves so that in the event of a total loss of test stand control I can still manually vent from a safe distance. Again I could go for a complex redundant electronic backup system (that vents electronically) but why complicate things! Also I want to remake the valve hollers out of thicker aluminum C section, as I noticed some flex in the 3mm one.

Much work also needs to be done on the operations manual. I really want it to be something that is useful, not something that is done because it needs to be so I plan on compiling it over a few weeks so I can give it good thought.

I plan on working with Buren on sunday on the mechanical things, and we should be able to get most of them done. We are hopping to be ready for anther test in the next 2-3 weeks or so. I also need to buy a markee, table and chairs before the next test. I was tempted the other day to get a new generator (11KVA petrol for $1000!) of ebay, something I have wanted for a while but don't really have the need for at the moment Using all my power (pun intended) I managed to avoid becoming one of those people who buy tools not because they need them but because they want them (which is something that annoys me) With a surge protector (so it doesn't ruin my charger again) my 500W  one will be enough to power all our devices. Also  at the last test Marco suggested I have something fun I do before each test (the Russians used to pee on their rockets for luck). I have thought of a good pre-test ritual, although I am not sure everyone else will enjoy it as much as I will - hint - we have a somewhat different taste in music......

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