Tuesday, 27 September 2011

Cancelled Test

We had been planing to conduct another test last weekend but unfortunately  it had to be cancelled the night before.

The source of the problem was that the new H-Bridge motor controller dent have the power to drive the valve motors. I hadn't tried it before then because I thought it would be a trivial task and a waste of time, but I learnt yet again that few things are simple. When buying the motor controller I noted at the motor's specs and noted that it retired 3A while moving. The motor controller could provide this, but I didn't take into consideration that it required allot more current to get moving. So If I hand started the motor it would work, but it couldn't start on its own.

I had been working really really hard last week to get everything finished and it is quite disappointing to have to cancel the test. We probably won't get another shot to test before the Christmas break, because I have exams coming up which I really need to concentrate on.

In the last 6 months I have learnt a really important lesson - things are usually harder than you think they are going to be - The entire reason I used actuated valves was so that I can vary the amount of fuel or oxidizer going into the engine. I think that to keep things simple I will replace the actuated valves with solenoids. Once I get the engine going they can be switched back.

Another example of how things were not as simple as I thought they were going to be is the relay back. Apparently (didn't  say it on their ebay page)  they wired active high. This means they are closed when they are open and open when they are off. If I used them like that it would mean that if the relays lost power everything would open (solenoids etc.) at the same time as the igniters ignited. Not the ideal situation. Ariel suggested I modify them, by removing a resistor and attaching the signal line to one side of the removed resistor which worked quite well.

So I guess I really need to make less assumptions and check everything!

On the valve front I would like to continue development of actuated valves as they will be extremely useful, but the current implementation is a bit haphazard. It will work for just opening/closing but I am not sure I will ever get the accuracy I want out of them without some major modifications. In the past I had thought that all the play had to do with the method used to attach the motor shaft to bearing plate. I have now realized that there is also quite allot of play in the reduction gearbox itself.

Originally I had considered using a large RC servo to actuate valves, but wasn't sure if they would have enough torque. After talking to someone the other day who told me about a 1/4 scale servo he had that nearly took his finger off and i thought... thats exactly what I need! (not the finger bit). I bought  cheap 1/4 scale servo, which I will test soon (once I get a torque wrench). It reckon it has 3.2Nm of torque, which might be enough to open a valve. If not there are two options. Get a better servo (the one I got was relatively inexpensive), or get a valve with less resistance. The valve I am using at the moment is good because its cheap and rated to high pressures but it has a few downsides. The torque required seems to vary greatly depending on the pretension on the ball and the pressure, so i might be able to reduce the torque required at a particular pressure. Alternatively I could get a valve with bearings. I would like to research the different types of valve available.

On another note I now have a engineering job which is keeping me busy. In someways having a job is good because I now have more money to put into rocketry, but the downside is I have less time to use the stuff I buy. I have so far avoided buying stuff I don't need and turning into one of those old men... but it is a struggle! At some stage I would like to purchase a CNC mill, but I really don't have any need for it at the moment (not to mention any place to put it). I have been considering moving into a dedicated workshop space for a while now and now that I have a income I am able to. Ariel and I are looking at something about 100m^2 which should provide plenty of room. We have inspected a few places in the inner suburbs, but they are all a bit pricey, so we will likely go a bit out of the city. Over the holidays I will be working full time so I just hope that I will have enough time to work on rocket stuff. I would also like to start building up a group of rocket enthusiasts to work on projects. There have been manny discussions about the best way to organize such a group.

Thursday, 15 September 2011

I have been quite busy with uni and work over the last two weeks but have tried to find the time to finnish off the things that need completing before the next test.

I had planned on conducting a test this weekend, but for a number of reasons it was postponed to next saturday. Although I am keen for another test it was probably the right decision to postpose the test, because although I could have probably got the electronics don't in time one of the things that was done poorly in the last test was rushing, and hopefully I can have everything done this weekend so I will have one week to double check everything is working (100% !)

In the way of electronics I have:
  1. Installed the new relay bank - Much much neater now. (diagnosing was a problem at the last site and I have a spare it can be swapped out with.
  2. Installed the speed controllers
  3. Made 12v and 5v regulator boards for the router and usb server
  4. Tested the trimpots and started making their breakout board
  5. Tested the pressure transducers and started making their board
  6. Allot of wiring in-between the new components
I still have to finish the sensor break out boards. Annoyingly the only precision trimpots I could for less than a bizillion dollars find were 10 turn. It didn't say that on the ebay listing but I should have known from how long they were. This means that I only have 1/4 of a volt between the half turn from full open to full close. If the trimpot is +- %5 this means there is 0.25v error (same as the resolution they have)! Although thats the absolute error over all the windings in the pot, and i suspect the actual error would be less (as it will stay roughly in the same position) its still not good. I might pick up a cheap one turn pot (10-20% error that will have less error and more range (1v error of its range @ 20%) which all be better than the 10 turn precision one. I am also having troube with the vregs overheating. I am not exactly sure what current I am drawing but they are rated to 3A and they are getting quite hot. I will have to fix a heat sink, and a big one. Heat could be an issue in the un-ventilated boxes. i also have to wire up the transducers so that the max output is 5v, where the default output is about 6.5.

I also bought two needle valves I would like to connect up after each main actuated valve. The reason for that is I am paranoid that too much propellant will go into the engine during start up with bad consequences. A needle valve will allow me to precisely calibrate the maximum flow.

Saturday, 3 September 2011

yesterday Buren and I worked for most of yesterday and finished the improved actuated valves (minus two shaft couplers I have to lathe). I also tested the pressure transducers and found them to be simple to use. We then started on a few modifications to the plumbing including adding a manual vent to the ox side plumbing. I was hopping to have the electrical components I ordered by now but they should arrive next week and won't take long to install. I also made a start on rewiring the relay box by removing all the old relays and mounting and wiring the new motor controller.

Hopefully  the new motor controllers combined with the feedback will mean good control over valve position, but I suspect that it won't be good enough for precise throttle control (like the kind that would be needed on a hovering vehicle). A ball valve is needed because its quick to actuate, but I was thinking why not use a actuated needle valve and a ball valve in series. A needle valve is not quick enough to be used as the main valve (would take a few seconds to open) but they are much more accurate at setting flow rates, and once the engine is going thrust won't need to change any after than this anyway. Just one possible mod.

Friday, 2 September 2011

I am still waiting for some of the electronics parts to arrive so haven't been able to do much work on electronics.

I also ordered new Nitrogen and Compressed air bottles from supagas (as they were much cheaper than BOC ) for the next test. Looking through the price list I was trying to decide what size to get, and decided to go for the 9.5m^3 over the 3.5m^ as it was only $10. I didn't consider the size of the bottles, and the G bottle is a bit larger than I was expecting. I will probably have to lie to down in the trailer (horizontally) but having lots of extra gas means I will be able to throughly pressure test without worrying about running out of gas.

We also did a wireless test of the new WIFI control system and found it worked excellently at 200m with the routers default antenna.

Thought I would post a few photos of Buren has taken of the last month.