Accelerometer - analog vs digital & recommendation ?

Using Arduino Sensors
1

Hi everyone!
I just started playing with the Arduino, got it last pay day and it's quite fun! I'm looking at accelerometers for my own project, and I'd like to find out the difference between using accelerometers that have analog vs digital outputs. The only place where I found some info was in this data sheet: http://www.freescale.com/files/sensors/doc/app_note/AN3468.pdf

From my basic understanding for accelerometers with analog outputs

  • Can be read in by arduino's analog pins using analogRead()
  • From http://www.arduino.cc/en/Reference/AnalogRead - it seems that with an analog accelerometer, I can sample it several thousand times a second.
  • ADC of the arduino is 10bit; if I want a finer resolution, I may need to look into an external ADC?

Digital outputs:

  • Using i2C or other protocols, so there is a library to use
  • There is a limit on how fast you can sample the data
  • Some I've seen have 13 bit adc resolution

A little on my project, it's very simple. I want to attach LEDs to my arm, if I'm say, squatting down, the number of LEDs lit up will depend on how fast I move. Conversely, if I'm jumping up, another set of LEDs will indicate upward movement, with the number of them indicating how fast the upward movement is.

Question 1: In such a case, what kind of accelerometer would make more sense?

Question 2: Recommendations! I'm looking for a 3-axis accelerometer, my budget is £30-50. It's not solely for this project it's going to be recycled for other purposes, so I don't mind purchasing something more expensive, say with gyro on board.

Thank you! I hope the info provided is sufficient.

Cheers!

2

ADC of the arduino is 10bit; if I want a finer resolution, I may need to look into an external ADC?

external ADC, or much more easy a digital accelerometer (some got 14 bit).

it seems that with an analog accelerometer, I can sample it several thousand times a second.

Yes, you can sample in standard mode about 9Khz. With some tricks until 70Khz. accelerometer will handel max 1-2Khz. But for your project it seems total overkill. I think a < 100Hz should be sufficient (and you get non fluctuating readings).

For myself I used a BMA180, and I am very happy with it (14bit, from 10 to 2400Hz, low noise, set range of +-1G to +-16G). I wrote a GPL library for it: jayduino - Arduino stuff written by me that would help you to setup quickly the sensor. Sparkfun has a nice breakout board Triple Axis Accelerometer Breakout - BMA180 - SEN-09723 - SparkFun Electronics. You will need a 3.3V level converter like this one: http://www.sparkfun.com/products/8745. Total costs 30$ US.

3

Hi there! Thanks for the info! That's a very nice recommendation you got there, I may get it :slight_smile: I also came across this page http://www.sparkfun.com/tutorials/167 after looking at your links, gave me a good idea what's the differences between the various accelerometers. Thanks!

4

I also came across this page http://www.sparkfun.com/tutorials/167 after looking at your links, gave me a good idea what's the differences between the various accelerometers. Thanks!

Oh, thank you, that is a cool collection I wasn't aware of! :grin:

5

I've decided to do a few projects with accelerometers and gyros too. I just ordered this accelerometer http://shop.moderndevice.com/products/mma7260qt-3-axis-accelerometer and this gyro http://shop.moderndevice.com/products/idg-500-dual-axis-gyro.

Since they have not arrived yet, I can't give you any recommendation now. But I did do quite a bit of shopping around.

Good luck on your project. Sounds fun and interesting.

6

scjurgen:

I also came across this page http://www.sparkfun.com/tutorials/167 after looking at your links, gave me a good idea what's the differences between the various accelerometers. Thanks!

Oh, thank you, that is a cool collection I wasn't aware of! :grin:

You're welcome :slight_smile:

Daanii: Those two look quite interesting. I'd love to explore gyros one day soon :slight_smile:

7

That's a very common path I'm seeing lot of people here are following.. accelerometer.. then gyro.. then magnetometer..

You might wanna have a look at my FreeIMU project as well as my orientation sensing library.

8

Fabio:

That looks good. No immediate need but I got a copy because I expect to look at it soon...

I am trying the BMA180 Accelerometers. Maybe they have less noise. I will try to comment ASAP.

9

I am trying the BMA180 Accelerometers. Maybe they have less noise. I will try to comment ASAP.

I ear this often .. but I still didn't see a good test with "scientific" proof of that.

Enjoy my code.

10

Hi Fabio, your project looks very interesting! I'll be sure to look at the libraries in more detail when I get my accelerometer :slight_smile:

Cheers!

11

Fabio:

I have run tests on the BMA 180. I have not tested the ADXL245. Maybe I should. :slight_smile:

I will try to post the results as I stated in another thread. If I can I will combine all the graphs and tables into a PDF.

The results are interesting...

12

That would be great. Please write down the configuration of the sensor you are using.

Also you may wanna try check out this: http://www.analog.com/static/imported-files/application_notes/AN-1063.pdf
(Oversampling technique to improve the ADXL345 output resolution)

13

Fabio:

I will see if I can post a table ...

The following table was developed with the following settings.

The G-Force range was set to 1G
The Band Pass was set to 150 Hz.
The Mode was set to Mode Zero (0) through One (1) as indicated.

I also took off 2.31V from a voltage divider with a .01 uf cap across the 2.31Volts.

I was using an ISR and sampling at about 145-150 SPS

Accelerometer Noise-- The BOSCH BMA180

Mode 0 Axis Axis Axis
Measurement X Y Z
Low Value -147 -511 8029
High Value 167 -192 8168
Sample Count 14938 14938 14938
Average -2 -327 -8160
Std Dev. 38.52 32.87 6.77

Mode 1 Axis Axis Axis
Measurement X Y Z
Low Value 3064 -968 8150
High Value 3364 -677 8165
Sample Count 16174 16174 16174
Average 3230 -818 8165
Std Dev. 29.34 30.08 1.16

Mode 2 Axis Axis Axis
Measurement X Y Z
Low Value 3091 -977 8155
High Value 3300 -771 8165
Sample Count 12390 12390 12390
Average 3196 -870 8162
Std Dev. 22.52 22.84 0.6478

Mode 3 Axis Axis Axis
Measurement X Y Z
Low Value -53 -408 8125
High Value 93 -214 8164
Sample Count 11083 11083 11083
Average 19 -312 8159
Std Dev. 19.27 21.39 1.71

[/table]

Maybe you can throw that into your own spreadsheet and have a look...

Hmm does not look hopeful.

In the next day or two I should do some trials on Low Frequency noise. I need to move everything off a breadboard onto a mounting system.

I have a PDF I could send you -- maybe you could duplicate the trials at the most sensitive settings...

Hope that helps.

14

please share the PDF and the testing code you are using.. you can put them on http://www.speedyshare.com/