Working principle of Triple-axis Accelerometer

Slide 1

Welcome to webinar series on Intel Higher Education Challenge 2017. In this webinar, we are going to learn about I2C protocol and it implementation for Gesture recognition using ADXL345 three axis accelerometer

Slide 2

In this webinar, we are going to learn about following topics:

Lets understand the physics of acceleration measurement in Accelerometer using spring and mass systems. Springs generally obey Hooke's law. If this system undergoes an acceleration, then by using Newton's 2^nd law and force conserving equation, we get F = ma = kx
                          
 F=kx, where k is the constant of proportionality (spring coefficient) between displacement (x) and force (F).
Hence an acceleration a will cause the mass to be displaced by
                                          x=ma/k    -------------- (i) 
alternatively, if we observe a displacement of x, we know that the mass has undergone an acceleration of
                                         a=kx/m      --------------- (ii)

Thus, if we can somehow measure displacement x, we can measure the actual acceleration a on the body.
For accelerometers that use the MEMS technology, two approaches for measuring dominate the market: (i) capacitive sensing, (ii) piezoelectric sensing.

These approaches convert mechanical information into electrical signals by methods that are particular to the silicon technology.
The case of measuring using the capacitive mechanism is considered next

                                                                                                                                                                                                 

A change in causes a change in the capacitance defined by                            

                                   C=ε_0  A/x_a                ---------- (iii)

where ε_0 is the permittivity of the air gap, and A is the surface area on either side of the proof mass where the springs are located, x_a is the distance between capacitor plate and mass m.

By varying the distance x_a , we can vary capacitance ‘C’. Combining Equation  (ii) & (iii), we can find acceleration  ‘a’ in mass ‘m’.

               a=(k.A.ε_0)/mC         --------------- (iv)

This way, you can calculate acceleration in all three axis x, y and z coordinates.

Hope this explination helped many of you to understand the basic physics behind working of accelerometers.

Happy Learning! Please post your comments below :-)