Air Pressure Sensor

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joeman
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Air Pressure Sensor

Postby joeman » Sat Sep 10, 2016 9:22 pm

Hello,
I've been creating a small air pressure sensor that will fit inside the Nose Cone of the Junior rocket.
It only weighs 5 grams, uses a "Trinket Mini", a BMP180 and a really small 50mAH battery. A few pics below are for those curious.

So far, it is able to detect a pressure reduction (currently set to 50Pa) which is will be sufficient to detect a launch. It will then write "selected" values to the EEPROM of the AtTiny85 chip...which can be retrieved using a serial cable.
Major issue has been getting program to fit in side the 5k of program space.

One of the last little issues is how to keep it inside the nose cone without gluing the cap on. I've been thinking of an interference fit...some stickytape....but have a feeling it might not be good enough... Though It does seems to be quite hard to remove and suspect it might be fine because of the very light mass. (low momentum)

Any other ideas/methods?


Cheers

Joe
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SpaceManMat
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Re: Air Pressure Sensor

Postby SpaceManMat » Sat Sep 10, 2016 10:13 pm

Looks like a great project.

How about countersunk radial screws for retension?
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joeman
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Re: Air Pressure Sensor

Postby joeman » Sun Sep 11, 2016 10:32 am

Thanks Mat.
Yes, I thought about that and then thought it might be a bit hard, with the thickness/strength of the plastic.

What I'm going to look at is dropping some spare shock-cord down to the tip of the nose cone with a bit of glue to hold it in place and then bring it back through the hole. This will be in addition to the interference fit.

Photo attached.

One thing I've discovered during testing is the need to have the sensor inside the nosecone during testing, to shield it from fluctuation in wind which tend to throw the sensor off completely!

All look good.

Cheers

Joe
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OverTheTop
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Re: Air Pressure Sensor

Postby OverTheTop » Sun Sep 11, 2016 11:37 am

Hi Joe,
Project is looking good :)

One thing I've discovered during testing is the need to have the sensor inside the nosecone during testing, to shield it from fluctuation in wind which tend to throw the sensor off completely!


Correct. What you want on the rocket, providing air pressure to the altimeter, are static ports. These are holes located where there is no direct airflow component normal to the surface and minimal turbulence in the airflow across them. That gives a direct indication of static pressure outside the airframe, and is not confounded by any dynamic pressure effects

There is a rule of thumb which says that you keep static ports more than three calibers away from any changes in diameter or fins to keep the pressure fluctuations (due to additional dynamic pressures) down to a reasonable level.

I can't see where the static ports are on your NC. I assume there is a breather into the area below the NC inside the airframe, and then static ports in that airframe to the outside? One thing to watch with venting an altimeter to the inside of an airframe is that if that part of the airframe has a parachute ejection gases can make their way into the altimeter. This can be bad as they are corrosive. This needs to be managed somehow (filtering, segregation, conformal coating, planned obsolescence...).

There is a discussion on static port sizing on this page FYI:
viewtopic.php?t=1415

Another thing to watch with these baro sensors is that they are modulated by light. Make sure there is no sunlight shining onto the board as it causes changes in the measured pressure.

A general comment: Remember, everyone has recommendations guidlines or rules. Sometimes however we have other constraints we need to work with so need to be clever to solve the problems or find a way to tolerate them. You get to make those decisions as the builder and flyer. Learn about the problem, think it through, and go with the solution that suits you.
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joeman
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Re: Air Pressure Sensor

Postby joeman » Sun Sep 11, 2016 2:12 pm

That is awesome!

Yes. I was trying to visualize the air going through the hole to equalise the pressure inside...not wanting air to be forced INTO the cavity or to have air sucked out; and how big to drill the hole, etc. And there we have it, some equations to figure it out! I'm certainly learning heaps.

FYI. I did drill a very small hole into the Nose Cone, just wasn't visible in the picture. I did it on the section that is parallel with the air-frame, but alas, not far enough down as I'd require. I'm a little concerned with potential weakening of the air-frame if I drill a hole in it; Though on reflection, I guess I could drill a small hole and put a little bit of glue around it to provide some additional strength. Something for me to think about.

Thanks for your words of wisdom. I sometimes ask questions on the forums, even if I "think" I know the best approach. Sometimes I just like to dip into the collective wisdom of others. Often I find that forum posts lead on to something else very useful, interesting, that then leads on to some creativeness to deal with a particular engineering problem I am having. Hopefully, I can make a positive and meaningful contribution back to the forums.

Thanks OTT and Mat.
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SpaceManMat
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Re: Air Pressure Sensor

Postby SpaceManMat » Sun Sep 11, 2016 4:26 pm

One other thing Joe. If you drill a hole through the shoulder of the nose cone and then through the body tube, then you might want to consider adding a key so that the nose cannot spin on the body tube and block the hole. Not too big a problem if your just logging data, but if you are using the altimeter to deploy your chute then it is probably not going to end well.

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ogivemeahome
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Re: Air Pressure Sensor

Postby ogivemeahome » Mon Sep 12, 2016 4:50 pm

joeman wrote:I'm a little concerned with potential weakening of the air-frame if I drill a hole in it; Though on reflection, I guess I could drill a small hole and put a little bit of glue around it to provide some additional strength.


When you're drilling the holes, put something solid underneath the hole to drill into. This helps prevent 'breakout' (material tearing away as opposed to being cut cleanly). It helps if you use a sharp (brand new) drill bit.

You'll want to keep the inside of the airframe free of any 'burrs' - put some coarse sandpaper on a stick and sand carefully, apply super glue and sand again, repeat until smooth.

(All these tips apply mostly to cardboard tube - hope it helps.)
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OverTheTop
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Re: Air Pressure Sensor

Postby OverTheTop » Mon Sep 12, 2016 6:39 pm

apply super glue and sand again, repeat until smooth.


The same treatment for 3D printed parts works very well also.

Holes can be funny things. Counter intuitive sometimes...
https://en.wikipedia.org/wiki/Stress_concentration
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joeman
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Re: Air Pressure Sensor

Postby joeman » Tue Sep 13, 2016 9:29 pm

Mat: Nice ideal with the notch. Yes, I could have put the static hole back a bit further with that procedure.

I ended up doing a flight with the sensor in a Junior rocket today and to my amazement it (the sensor) actually worked. More amazement from a programming side
(not the rocket launch side of things).

Of course the numbers might be not truly representative of the air pressure, but definitely in the ball-park!

I'll post a graph some time.

If anyone interested in the program/methodology/logic, happy to share.

Cheers

Joe
L1 - Callisto (H133) - 11-Jan-2016
L2 - March Fly (1633K940) - 18-Mar-2018 - RSO


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