Tilt sensing using accelerometer

simon_say

Hi, im new here and im stuck with accelerometers... i have been reading this...

http://www.freescale.com/files/sensors/doc/app_note/AN3107.pdf

on page 3 i get confused. An accelerometer is most sensitive when it is perpendicular to gravity (i.e. 0 degree) and least sensitive at 90 degree. It explains that the resolution at 0 degree when using an 8bit ADC is 0.92 degree. I understand how to calcuate this but then i get stuck with the resolution at 90degree. Im guessing that the sensitivity in the equation to calculate 90 degree resolution is not 800mv/g as it was for the 0 degree calculation?

Please can somebody help me calculate the 6.51 degree they have calculated as ive spent over two days trying several things!! Also im using adxl203 and it has 17.5mg sensitivity upto 45 then 12.2mg sensitivity above 45 to 90. What does this mean in terms of voltage/degree? i get 2.5V as offset and 3.5V at 90 degrees. How do i calculate the max resolution at 90 degree?
I would appreciate any response.
Many thanks in advance!!

Craig Eid

Simon,

You can use a single axis accelerometer to accurately measure tilt when the angle is than +/-60 degrees. The best results occur at an angle below +/-30 degrees, resulting in an error of less than 1%. Since the tilt calculation is derived from a sin (angle) calculation, there is minimal change for an angle greater than +/-60 degrees, resulting in an inability to measure tilt at +/-90 degrees with a single axis accelerometer.

Parallax provides documentation here for the Memsic 2125 accelerometer that describes a technique to measure tilt and acceleration. Read the datasheet for the Analog Devices part and the two application notes and you should have the information required measure tilt at +/-90 degrees with a dual axis accelerometer.

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··········· Triad Research and Development
Electical Engineering Design and Consulting Services

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Post Edited (Craig Eid) : 4/10/2009 7:05:01 AM GMT

Jessica Uelmen

Your duplicate post from the Stamps in Class forum has been removed. Please refrain from duplicate posting.

Thank you!

Jessica

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Jessica Uelmen
Education Department
Parallax, Inc.

simon_say

Thank you very much for the reply but im already using the adxl203. The output is a DC voltage where for angles 0-45 the output is proportional to the voltage. However above 45 the sensitivity decreases. i have found new link below, which is good but i still do not understand how the sensitivity of the accelerometer is calculated on page 3. Does anybody know how the column from the table on page 3 for "dg/dDeg AX TSX(mg/°)" was calculated?

http://www.freescale.com/files/sensors/doc/app_note/AN3461.pdf

MAny thanks in advance again and thank you a]gain for your response!!!

Craig Eid

Simon,
·
You are correct that the sensitivity at 90 degrees in the referenced equation is not 800mv/g; the sensitivity·is 0mv/g. The data you are referencing is a plot of Sin(X) vs X [noparse][[/noparse]degrees].· As noted previously, there is minimal change for an angle greater than 60 degrees which significantly reduces the sensitivity.·Please review the tabular results in the attachment for·an understanding of the variation of Sin(X) vs X·over the range of +/- 90 degrees.

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························ Craig Eid


··········· Triad Research and Development
Electical Engineering Design and Consulting Services

··················· www.TriadRD.com

Ole Man Earl

The Wii nunchuck knockoff called the 'EDGE' is only $14.95 at most Best Buy's. It has a 3 axis Freescale accelerometer and Freescale 8 bit uP chip in it and it talks via i2c bus. I am using it in my quadrocopter UAV project.