model rocket accelerometer

Have questions about a SparkFun product or board? This is the place to be.

Moderator: phalanx

model rocket accelerometer

Postby nikolaj » Sat Mar 01, 2008 6:45 pm

Hi, I am trying to figure out what accelerometer packages to get to setup a triple axis accelerometer on a model rocket. From what I have read, i think i need +/- 50g, and I can't find any tri-axis all in one packages that have that range, so i have been moving towards the ADXL150/250 sensors. I am confused by the "packages" .. e.g. JQC and AQC, and I am not sure how i would best get the axis perpendicular to the circuit board where I mount the chips. I plan on reading the data at first with an AT Tiny and eventually maybe an arduino stamp for more advanced stuff. Anybody have any good resources or experience with model rocket accelerometers? Is there any way I can get one through sparkfun that can handle the Gs I expect (on a nice tidy breakout board of course!)? Thanks!
nikolaj
 
Posts: 20
Joined: Fri Feb 29, 2008 11:30 am

Postby ttabbal » Sat Mar 01, 2008 9:20 pm

I don't think SFE has any high-G accels available. I'm designing a flight computer for rocketry as well. I've decided to go with a single axis unit for the vertical axis (along the thrust line). I used a ADXL193. I'm thinking of trying a low-G triple axis unit to try to catch the other two axis for attempting to log deployment events and such. I don't know if it will handle the Gs. I'll probably build one and try it as I have the parts available anyway. It's a little ways off, I got distracted with a RF tracking unit. :)

I believe the high-G units have the same footprint as the lower ones, so the SFE breakout boards should work. You would have to solder it yourself though. :)
ttabbal
 
Posts: 231
Joined: Wed Aug 15, 2007 2:28 pm

Postby Shifted » Sun Mar 02, 2008 11:00 am

What data are you hoping to get out of the accelerometers? You want attitude, or are you hoping to integrate and get a flight path or altitude? If you just want altitude, I would suggest using a barometer and sampling the pressure as you go up, the lowest pressure could then be converted to your altitude.
Shifted
 
Posts: 250
Joined: Sat Nov 24, 2007 10:46 pm
Location: Texas

Postby nikolaj » Sun Mar 02, 2008 11:05 am

using a tri axis for the other directions is interesting, although I would be sad if my rocket had a catastrophic failure and I couldn't graph it in 3d and forever admire its trajectory.

Regarding the high G accelerometer, looking at octopart/digikey I see there is a "cerpak" and soic package. Neither seem suited for the SFE breakout, but maybe I am missing something. I am a novice solderer with primitive tools.
nikolaj
 
Posts: 20
Joined: Fri Feb 29, 2008 11:30 am

Postby ttabbal » Mon Mar 03, 2008 7:58 am

nikolaj wrote:using a tri axis for the other directions is interesting, although I would be sad if my rocket had a catastrophic failure and I couldn't graph it in 3d and forever admire its trajectory.

Regarding the high G accelerometer, looking at octopart/digikey I see there is a "cerpak" and soic package. Neither seem suited for the SFE breakout, but maybe I am missing something. I am a novice solderer with primitive tools.


The ADXL193 is in an 8 lead CLCC package. That might be the "cerpack" you saw. That's the only package in the datasheet I have. I don't see a SOIC in there. I thought it was the same as other single axis accel units from Analog. I haven't checked that out though. The CLCC package is a little tricky to solder, so practice some SMD soldering and get a good iron. SOIC is a little easier to solder, but you will still want to practice on an old PCB or something.

I have been wanting to use the accel to detect things like motor burnout for staging and such. A flight path is an interesting idea, but you do need 3 axis data to even get close. And the errors will add up. I have wondered if I could use a gyro sensor to get the path data. It seems better suited to that use. I'm not sure if they can provide accurate readings under thrust though. The datasheets I looked at only mention G forces in the absolute maximums section. I'm using barometer readings for altitude.

Of course, I can get a GPS sensor for the cost of the gyro unit. Not super accurate, but close enough to get decent path data.
ttabbal
 
Posts: 231
Joined: Wed Aug 15, 2007 2:28 pm

What are you using for your uP

Postby uwfalcon » Mon Mar 03, 2008 10:09 am

I thought about building a dead simple altimeter out of an avr butterfly.
If you aren't breaking things you aren't working hard enough.
uwfalcon
 
Posts: 4
Joined: Tue Oct 02, 2007 9:25 pm

Postby Philba » Mon Mar 03, 2008 1:21 pm

if you can't find 3-axis units that meet your needs, just use 2 2-axis ones. You can use the redundant axis to correlate the readings.

Soldering SOIC SM parts is very easy. very very easy. I think it's actually easier than DIP parts but then that's just an opinion...
Philba
 
Posts: 2503
Joined: Sun Feb 13, 2005 11:33 pm
Location: Seattle

Postby ttabbal » Mon Mar 03, 2008 3:23 pm

I found some parts by Freescale that handle pretty high G levels (250G+). They make single axis units both in X and Z axis. You can mount an X axis unit 90 degrees from the other X unit and get X+Y. The Z unit keeps you from having to mount one vertically to the board. They are in an SOIC-16 package for easy soldering.

I also like SOIC better than DIP.
ttabbal
 
Posts: 231
Joined: Wed Aug 15, 2007 2:28 pm

Postby Shifted » Mon Mar 03, 2008 4:17 pm

You really haven't said what it is that you are trying to measure... do you want to know the accelerations, or is there some other acceleration based data that you are after, like altitude? Maybe there is an easier way to get what you are looking for besides very expensive high g accelerometers....
Shifted
 
Posts: 250
Joined: Sat Nov 24, 2007 10:46 pm
Location: Texas

Postby nikolaj » Mon Mar 03, 2008 5:03 pm

ttabal: interesting. For little model rockets i think 250G might be a bit much, but I wonder if there are similar Z-axis chips in the 50G range. I will check out freescale's site.

Shifted: i would like to gather the accelerations, and then after the flight integrate to find the flightpath. I would like to keep the project costs "reasonable", but I don't mind paying 100 bucks for the right accelerometers and pcbs.

Really, though, this is an arbitrary project serving as a glorified excuse to launch model rockets and learn more about embedded electronics. :)
nikolaj
 
Posts: 20
Joined: Fri Feb 29, 2008 11:30 am

Postby Shifted » Tue Mar 04, 2008 9:17 am

I do a lot of work with accelerometers, so let me give you a little insight...

A GOOD (not great, not bad) 30+g accelerometer will run you $500 for a 3 axis. Now that's just accelerometer's alone, which will not give you enough data to get a flight path (since accelerometers are affected by rotation, you need to take that into account, along with the gravity vector). You will need a 6 DOF unit. I have one from MemSense sitting on my desk that cost $2500, and even it doesn't produce data "good" enough to get a 3-D position from.

The ONLY 3-D "grade" accelerometers out there are made by companies like Honeywell ($20,000), Northrup Grumman ($50k+), Sperry Marine ($50k), L3 Communications ($50k+), L3 Communications, etc.

I'm not saying its not possible to get 3D position with low grade accelerometers, but don't expect a lot of accuracy, and expect to work for MONTHS on the multi-dimensional scented extended Kalman Filter to track all 6 degrees of freedom. If you do get one working, start a company and sell it, you can see from above, its a small market and is very expensive.

BTW, the reason that those are so expensive is that they use fiber optic or motorized gyroscopes along with very sophisticated (and large) lever-arm accelerometers and huge filtering algorithms.

I'm working with an ADIS16350 right now thats a +/-10g, +/-300 degree/sec, 6 DOF accelerometer/gyroscope, its shelf cost is $550 and is a 12 week lead time (although I think Digikey has some in stock right now). I'm hoping to make a 1-D position sensor out of it (only care about up/down motion but needs to be accurate to 2cm), on a ship that also experiences yaw/pitch/roll/surge/sway/heave. I'm 2 months into development and I've yet to have anything working, my Kalman filter seems to diverge very quickly, and its only accurate for a few seconds.
Shifted
 
Posts: 250
Joined: Sat Nov 24, 2007 10:46 pm
Location: Texas

Postby ttabbal » Wed Mar 05, 2008 3:53 pm

nikolaj wrote:ttabal: interesting. For little model rockets i think 250G might be a bit much, but I wonder if there are similar Z-axis chips in the 50G range. I will check out freescale's site.


heh.... Yeah, I'm a high-power guy, so 50G goes quickly. :)

Really, though, this is an arbitrary project serving as a glorified excuse to launch model rockets and learn more about embedded electronics. :)


That's me as well. My rockets are just bigger and have more powerful motors. :)

Honestly, I'm not liking accelerometers for this stuff for the reasons posted by Shifted. I still want a single channel high-G unit for detecting motor burnout, but I don't think I want to integrate all the errors to try to get a flight path. I want motor burnout for staging high power motors. It also makes a good sanity check for deployment events. :)

I also have RF telemetry and GPS tracking on my to-do list for the first spin of the PCB. Though I might make a simpler unit first to validate some of the design.

Maybe we could do flight path by interpolating based on barometric altitude, GPS positions and the accelerometer to calculate thrust? That new GPS unit SFE is selling with the helix antenna is calling me. :) It's not going to fit in a BT-20 though. :)
ttabbal
 
Posts: 231
Joined: Wed Aug 15, 2007 2:28 pm

thanks

Postby nikolaj » Fri Mar 07, 2008 5:39 pm

Wow everyone, thanks for all the info.

Shifted: very interesting post. I knew I would be fighting a losing battle with regards to accuracy on flight path data but I didn't realize it would be that grim! Your insight will definitely help me be satisified with a single-axis accelerometer to just sense significant changes of acceleration along the thrust axis and compute actions from them, and maybe stick to barometric sensor for rudimentory altitude measurements. As ttabal noted, I am very much in the low power rocket realm still, and am hunting more for something to get started, and am nowhere near refinement.

Thanks to everyone for the thoughts/advice. I realized this was a significantly more involved project.. maybe that sunlight tracking rocket might be a bit easer :)

either way I will post more if (pun Intended) this project ever gets off the ground.

Thanks again!
nikolaj
 
Posts: 20
Joined: Fri Feb 29, 2008 11:30 am


Return to SparkFun Product Questions

Who is online

Users browsing this forum: Bing [Bot] and 5 guests