Monday, 31 October 2011

Error in accelerometer

A Repost of something I did while working on a project a while ago.... Throughout it might be of use to someone.
Today i wrote a fairly simple program to plot position from accelerometer values. It wasn’t too complicated, just a simple python program that reads accelerometer values from the 9DOF accelerometer package (relayed from the ardwino). The hardest part was stripping out the data i wanted from everything else the accelerometer outputs. I had to restart my computer manny manny times initially because i hadn’t figured out how to close the serial stream and for some reason if you close idle with the stream open the keyboard stops working…. I used two of the equations of motion, v2 = v1 + at and s2 = s1 + vt to convert from acceleration to velocity, then to time. There were other methods but this was the easiest to implement.
And what were the results? Not good…….
Here is a plot of acceleration, velocity and position (green, blue and red consecutively) in the x axis Vs. samples (25Hz)  for the acceleration sitting still. As can be seen the error in velocity isnt too bad, but error in position is an order of magnitude more (about 0.25m per second).
I had been hopping for something in the order of a few centimetres per second. This pretty much kills dead reckoning.

Saturday, 29 October 2011

Uni has been hectic lately but all my assignments/thiesis are done so all thats left now is two exams.

I have been learning allot about peroxide over the last month as its the direction I would like to go in. I have managed to find  good supply of bulk %50 food grade peroxide, but it is heavily stabilized.

The first picture is the %50 food grade peroxide. As can be seen it has quite allot of floaties which I believe to be either colloidal stannate or silicate. The bottom picture is %35 reagent grade peroxide as can be seen there are very few particles suspended in it. The few that are visible are a few visible i believe are from not washing the container properly. The reagent grade is probably lightly stabilized. 

I wondered how much the stabilization ada an affect on decomposition, so I did a test comparing decomposition of the two grades. I found out that despite being lower in concentration the %35 it was significantly more reactive when a exposed potassium permanganate catalyst. The first video is of food grade and the second video is of lab grade. As can be seen the %35 reacted faster than the %50

I have ordered a hydrometer so I can measure the exact concentration of the two grades. Using a TDS (total dissolved solids) I measured 0.33g/l for the food grade and 0.26g/l for the lab grade. This was interesting and made me think that the lab grade was more stabalsied than I had initially thought. It also means that the lack of reactivity of the %50 is dependent on the colloid, although without knowing what dissolved stabilizers are present a conclusion can't really be drawn on how much.

For removing the coloid I am going to try a fine carbon filter. For the dissolved stabilizers I will use an ion exchange resign. I have some resin and will give filtering it a go soon, and see what affect ti has on dissolved solids. Ii should be easy to see if the carbon filter has any affect.

I am still looking in to methods for contentrating it. Ideally I would like to use %90 for fuel but initially I all probably concentrate it to %80. A method which seems to be popular in the rocket belt community is distillation under a vacuum. It has a high yield with little loss but also results in the occasional explosion. Normally peroxide vapor at atmospheric pressure will explode, so distillation is done under vacuum at which the vapor is stable. The problem is if you loose your vacuum the peroxide in your distillation column will probably decompose (violently). The articles I have read written by people who produce HTP for their rocket belts all say: design your apparatus for an explosion and stand behind a shield. Everyone who distills peroxide seems to have had at least one explosion. It seems you would have to be crazy to try distillation but then again these are the people that ride rocket packs! 

The other method which is most popular among the amature rocket community is sparing. This involves bubbling a dry gas through the peroxide. Since peroxide has a vapor pressure 10 times less than water the water will evaporate 9 times faster (in theory).

Fractional crystallization (freezing) is another method that doesn't seem to be widely used. It takes advantage of that fact that the freezing point of water is different to peroxide. The difference depends on the concentration of peroxide/water. Its interesting that for concentrations of bellow %62 the water will freeze whereas for higher concentrations the peroxide will freeze first. What is sometimes done is to sparge to around %65 then separate the peroxide by fractional freezing. 

I read a very interesting article on a patent that NASA own that uses a membrane filter with sparing.  A plan of this can be seen bellow.

The membrane (similar to a reverse ozmosis membrane) has a selectivity towards water (it prefers to only let water through) although because peroxide and water are similarly sized so it does let quite a bit thorough  (the articles says 2:1 water :peroxide is good)  The sparge gas then takes away the liquid that gets through the membrane. I do see a flaw in this method, or possibly I am missing something. Assuming that the membrane filter tubes have air in them (the sparge gas), some peroxide and water (more water than peroxide) but all liquid that gets through will be evaporated if the membrane tube is to not get filled up with liquid. If thats the case then I can't see what the point of the sparge gas is. You may as we'll just  use it like a typical reverse osmosis setup. Doing that you can turn %50 to %75 (2:1 selectivity). You can either throw away the waste of save it then repeat the process with it (25 to 37.5). 

I will probably just concentrate on sparging for the moment as it seems to be the easiest to get going. I have started designing a regenerative desiccant dryer, and I plan on using a aquarium air pump to pump the air for sparging. I should be able to get it decently concentrated just using normal air on a reasonable dry day.

I am becoming concerned about the safety aspects of using using peroxide. Sparging is quite a safe method but there is still the issue of safely handling it. Also it can become quite dangerous if contaminated with things that will burn. I don't plan on concentrating any significant amount until I get the new workshop sorted out, which should be over the next few weeks. I have ordered a full face shield (at the moment I am wearing enclosed protective goggles) and will probably order a safety shower for the new workshop. I am considering buying a protective PVC suit to wear but that may be overkill. Armadillo and lots of others regular handled %90 just take basic precautions of eye wear and pure cotton clothing. I would like to develop some strict guidelines which we will all use to handle the peroxide (no matter what concentration it is).

We are still planing to get the nitrous engine up and running, but we are just waiting for another opportunity to test. This will be in the first few weeks of the holidays.