Accelerometers & Gyros for N00bs

[EricVanWyk]

I think the best way to understand a MEMs accelerometer is to look at one - http://archives.sensorsmag.com/artic...03/20/fig1.jpg is a 3axis
http://www.sensorsmag.com/sensors/da...34974/fig6.jpg is an early prototype that is easier to understand.

Essentially, you have a mass (the middle bit) attached by springs (the bits that wind back and forth) to the frame (the outer bit). When you accelerate the frame, the mass lags behind it. The distance it lags is proportional to the acceleration.

This gives us an acceleration-to-relative-position transducer. By measuring the relative position of the mass compared to the frame, we know acceleration. This is usually done capacitively. Capacitance is proportional to the distance between the plates, so now we have an acceleration-to-capacitance transducer.

Often you will see fingers like this ( http://www.sensorsmag.com/sensors/da...34974/fig2.jpg ). These form the capacitors. Note that the actual capacitance is somewhere in the range of rediculosmallfarads.

To make the final leap, a circuit must convert capacitance to voltage. This is done in many ways. The easiest to conceptualize is putting an RC oscillator that varies its frequency with C. Measuring frequency is easily accomplished with digital logic.

In reality, companies will use more interesting / complicated methods to make the final capacitance to voltage leap. However, the basic mass spring -> capacitance is reasonably standard. The innovation there is how many axis one can fit on the same silicon.

And that creates a MEMs accelerometer. By changing the geometry of the springs and fingers you can create a gyro, but I have a harder time clearly showing this geometry.