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Author Topic: Help Choosing Accelerometer  (Read 799 times)
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Hello everyone, I am a member of a rocket design team at the University of Washington and I need a little help. We are building a sounding rocket for an ESRA competition at the end of the month. The competition is accuracy based, the rocket with apogee closest to 10,000ft wins. So, to increase the accuracy of our apogee we have been designing an actively controlled drag brake system. We had planned to use our altimeter to determine the rockets velocity on the go to control the air brakes but an altimeter is just not accurate enough. What I believe we need is an accelerometer, but I'm having trouble finding one that will work. We are using an Arduino Duemilanove with a motor shield that controls a screw the deploys and retracts the air brakes. Our max acceleration will be around 12g's but the acceleration during air brake deployment is a maximum of 2.5g's. I've tried finding an accelerometer but can't seem to find anything that will work with the Arduino board and survive our high g loading. If anyone could point me to an accelerometer that will work with our set up to determine velocity I'll love you forever.
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How about one of these...

http://www.signalquest.com/sq-xld.htm

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Thanks for the response but it looks like the SQ-XLD is self contained. We need an accelerometer that can pass acceleration to the Arduino on accent to determine velocity on the fly.
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Freescale makes a large range of accelerometers, with a large range of ranges. For example, their MMA7330 has two ranges, 4g and 12g. You can use 12g when the acceleration is high and switch down to 4g for more resolution under software control.

Accelerometer outputs are inherently very noisy. If you're going to integrate to velocity and position you're going to need as much accuracy as you can muster, and the 10-bit+/-3LSB precision of the built-in A/D's on the Arduino are probably not going to cut it. I suggest connecting the accelerometer output directly to a buffer op-amp and high-quality A/D converter as close to the accelerometer as possible, and send the data (e.g., over SPI) to the Arduino.
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If you search DigiKey or Mouser for Accelerometer with SPI / I2C interface you will find quite a few of them that can return measurements in digital form or even have ability to preset alarms, etc...

Few accelerometers listed there will survive 18g or more. SparkFun has breakout boards for some digital accelerometers that have 2g and 8g ranges.

However, as RuggedCircuits suggeste you will need all the precision and a lot of math to determine height from integrating velocity and acceleration.

If competition rules allow GPS and / or you already have one then you could use GPS to find your altitude and use accelerometer only in 2g range for feedback on your braking. Then any smaller accelerometer will work as long as it can survive 12g and doesn't need to measure them.

Alternatively you can bypass accelerometer and determine your acceleration from GPS position / velocity delta readings.

Search for ArduPilot as an example of GPS driven controls.
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or grab a cheap wii nunchuck off ebay smiley-wink u get a 3 axis acc a couple of buttons and analog stick smiley-wink it also outputs i2c.

if thats any help. if not u can harvest it for its acc and parts.
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You can also get free samples from http://freescale.com

Direct link:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=0112691118

Watch the packages though, most of those are surface mount reflow chips :-)
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