How does the accelerometer in DE0-Nano work (in words)?

[Altera_Forum]

I was just wondering, how would you describe the function of the accelerometer in the DE0-Nano card? No code, just description in words please :)

[Altera_Forum]

--- Quote Start ---  I was just wondering, how would you describe the function of the accelerometer in the DE0-Nano card? No code, just description in words please :)  --- Quote End ---    Sindredit - the datasheet of the accelerometer from the product page is particularly useful in this regard:  http://www.analog.com/static/imported-files/data_sheets/adxl345.pdf  Page 13 of the datasheet has the information you want. To paraphrase : the accelerometer is a Micro ElectroMechanical System (MEMS) type. It has capacitors whose one end is fixed, the other is attached to a moving mass. Since q = C*V (assuming capacitor is linear), differentiating both sides we get i = CdV/dt + VdC/dt. Note that we CANNOT ignore dC/dt because the moving mass causes one of the plates to move and this in turn causes a change in capacitance. If I remember my basic MEMS class, most MEMS capacitors work this way.  Have fun.  Bharath

[Altera_Forum]

Thank you for your reply mbharat. Can you please check if this statement is correct then? 

 The accelerometer is a Micro ElectroMechanical System (MEMS) type.  It has capacitors whose one end is fixed, the other is attached to a moving mass. The capacitor is linear, resulting in a charge that is subject to the equation  (1) q = C v where q is charge, C is the capacitance and v is the potential difference.  For a parallel plate capacitance is given by  (2) C = ε_0 A / d  where A is the plate area and d is the distance between the plates.  The potential difference V between the plates can be expressed as  (3) v = Ed  where E is the electric field strength between the plates.  By substitution of these to the original formula for q, we get  (4) q = C v = ε (A/d) (Ed) = εEA = εɸ where ɸ = EA is the electric flux through the surface.  As the capacitor charges to the change of q be power, i_c = dq / dt. By deriving the equation above with respect to time, we get  (5) i_c = (dq)/(dt)  If we put together the equations we have now we can get the displacement current in  the area between the plates of the capacitor:  (6) i = C (dv/dt) + v (dc/dt)  This displacement current is what is reflected and ADXL345 accelerometer used for the measurement of acceleration.

[Altera_Forum]

--- Quote Start ---  Thank you for your reply mbharat. Can you please check if this statement is correct then?   --- Quote End ---    Yup, your mathematical description pretty much nailed it down. To close this thread, here is a link to the MEMS course from my alma matter (UC Berkeley):  http://www-inst.eecs.berkeley.edu/~n245/sp08/  The lectures cover material relevant to MEMS variable capacitors (varactors). There are lecture videos online as well. Although it is designated as a graduate course, it is understandable by someone with basic engineering or science knowledge (IMO).  Good luck.  Bharath

[Altera_Forum]

A thorough datasheet literature reveals, that your description doesn't meet the ADXL operation principle exactly: 

   --- Quote Start ---  Deflection of the structure is measured using differential capacitors that consist of independent fixed plates and plates attached to the moving mass. Acceleration deflects the proof mass and unbalances the differential capacitor, resulting in a sensor output whose amplitude is proportional to acceleration. Phase-sensitive demodulation is used to determine the magnitude and polarity of the acceleration.  --- Quote End ---    Measuring displacement current won't be able to detect static acceleration, but ADXL does. As the datasheet explains, it's achieved by an AC capacitance measurement.

[Altera_Forum]

FvM: So what you are saying is that all of my physics/math is based on something wrong, or that I am just missing a sentence?

[Altera_Forum]

The equations are correct as such (seriously speaking, I didn't check them in detail), but they don't describe the operation principle of the sensor.  

 In simple words, you're assuming that a movement is sensed by a generated displacement current. But that's not true, instead the position of the mass is sensed by a differential capacitance sensor. Both quantities are different by one d/dt operation, very important for the sensor's capability of sensing a static acceleration.

[Altera_Forum]

FvM, thank you for explaining it to me.  

 I feel a bit smarter today than I did previously ;)  I have just written a new page where I explained how it works. I will meet up with my mentor tomorrow and get him to go trough the papers with me.  

[Altera_Forum]

I found a description of ADXL MEMS sensor geometry and detection principle in this ADI literature, at page 3-15 f http://www.analog.com/library/analogdialogue/archives/43-09/edch%203%20sensors.pdf

[Altera_Forum]

--- Quote Start ---  I found a description of ADXL MEMS sensor geometry and detection principle in this ADI literature, at page 3-15 f http://www.analog.com/library/analogdialogue/archives/43-09/edch%203%20sensors.pdf  --- Quote End ---    FvM : thanks for clarifying. Couple of questions :   1. The essential principle of operation for ADXL is Figure 3.14 on p. 18 in the document above, correct? 2. Going through the first paragraph under figure 3.14, the deflection of the beam produces a change in capacitance AND this change in capacitance is sensed using a mismatch in phase between the 1 MHz square waves (hence, the concept of "AC capacitance")?  Thanks again.  Bart

[Altera_Forum]

FvM, thank you for the link to the sensor pdf. 

I found page 3.15 to 3.17 particular interesting.