Accelerometer data logger

ProfJP

Hi,
I am a physics teacher and I would like to (try to) build some accelerometer data logger, to study... a water rocket! After having searched a little, I think it would be convenient to use some Analog Device accelerometer circuit (ADXL series) and, may be, a Basic Stamp. But, although I am quite familiar with most things in an analog undergraduate physics laboratory, I am less than a beginner in digital circuits and especially in microcontrollers and especially in Basic Stamps . Could somebody give me some indications or advices? First of all, I have a few questions :
- I have read somewhere that the acquisitions could be made by a Basic Stamp with at least 10 s between points. If so, my project will not be possible like this. Is it true?
- Output of most ADXL are analogic ; so I would need to use some A/D converter to generate input signal to Basic Stamp. Which one would be convenient?
- Some ADXL have PWM Output. Would it be convenient to make Basic Stamp input?
Thanks.

alinious

Hello ProfJP,

Parallax sells the Memsic 2125 Dual-axis Accelerometer (www.parallax.com/detail.asp?product_id=28017), which is easily interfacable to the Basic Stamp 2 series of microcontrollers. It is mounted on a small printed circuit board which can easily be plugged into a breadboard for prototyping. The Memsic 2125 uses pulse width modulation for the output of both the X and Y axis. The BS2 series has a pulse width modulation command which allows for direct (no ADC needed) interfacing of the Memsic 2125. Also, on the webpage for the link I posted above, there is source code examples that show how to interface the Memsic 2125 to the BS2 along with calculating GForce, acceleration, and tilt (rotation).

I have spent the last two days working with the Memsic 2125 and found it to be a great device of which I am very satisfied with. I to am also working on building an acceleration datalogger for my Estes model rockets as part of my hobby interest. If you would like, I could also post some of my code which shows how I calculated acceleration and tilt.

Hope this helps,

Alan Balich

Post Edited (alinious) : 11/19/2005 4:36:53 PM GMT

Bruce Bates

Professor JP -

Welcome to the world of Basic Stamps. Basic Stamps have been used in many physics applications, in the field, and in academia and are well respected by those who use them for their ease of use, and rapid develpment characteristics. Both are exceptionally important for classroom use, and for prototyping.

Just in passing, there is an entire section of the Parallax web site dedicated to "Stamps in Class". Stamps in Class is a wide breadth progam which extends the presentation of the Parallax Basic Stamp from grammar schools to post-graduate work. Free course work is readily available along with instructor's manuals as well. Take a gander when you get a chance:
http://www.parallax.com/html_pages/edu/index.asp

Please find attached to this message, Analog Devices Application Note number AN-596 which describes in some detail the use of the Parallax Basic Stamp with the Analog ADXL202/215. If you were to limit your choices of Analog Devices MEMs accelerometers to just those with PWM output, these same techniques would apply. That's NOT to say that the Basic Stamp can't be used to interface with other Analog Devices (or other) accelerometers, just that the interfacing technique would need to be different than that shown in this applications note. So too, the Basic Stamp programming methods.

Regards,

Bruce Bates

Post Edited (Bruce Bates) : 11/19/2005 4:47:15 PM GMT

achilles03

Professor,
I've built a couple of accelerometer dataloggers, and you can achieve sample rates around 75 samples/sec (with the standard BS2), and even 150Hz or more with the BS2p, BS2sx, etc. The components I've used to build an accelerometer data logger include an accelerometer (of course), a analog to digital converter (ADC), and an eeprom for extra storage (although you might not need the eeprom, depending on how many samples you'll need).

Analog devices has a few good accelerometers, but make sure you know what the sensor output is. The ones I've used (like the ADXL150, which is now, unfortunately, out of production) have an analog output 0-5VDC, and I use a 12-bit ADC to convert it into a number that the BS2 can store. You can either store this number on the (limited) eeprom on the BS2, or add an extra eeprom and store 1 or 2 minutes of data (at a sample rate of 75Hz).

Freescale Semiconductor also has some good analog (0-5VDC) accelerometers with ranges up to 100+g's, and you can probably get one or two free samples of their accelerometers. MMA3201 is one of the ones I used, but you can hunt around and take you're pick. But make sure to get one with about twice the range of what you expect to see from the water rocket.

Some other sensors have PWM outputs, but I don't use too many of those. Just make sure you're getting the output you want before ordering.

Hope that helps,
Dave

ProfJP

Thank you very much, Alan, Bruce and Dave. All these advices are very precious and it seems me quite clearer. Now I have... to do it!