Today, let’s talk about what accelerometers and gyroscopes are, how they work, and what they look
like in action. Accelerometers and gyroscopes both detect motion but accelerometers detect
linear motion, that is, the acceleration along an axis. Gyroscopes, on the other hand, use the
Coriolis Effect to detect angular velocity, or how quickly an object is turning. It should be noted
that accelerometers do NOT report the current speed, only the acceleration in a direction.
Equally, gyroscopes do not report current angle, only the rate at which a device is turning.
To get either of these, you would need to integrate it over time
We’re talking about both accelerometers and gyroscopes because they usually come inside
one package called an Inertial Measurement Unit or IMU. An IMU can include more than just these
two devices but we’re only going to worry about accelerometers and gyroscopes at the moment.
The amazing thing about an IMU, though, is how it uses microelectromechanical systems, or MEMS, that
combine mechanical and electrical components into structures that are only a few microns across.
The use of this incredible technology allows many
different sensors to be included in small, highly portable packages.
Accelerometers that use capacitive sensing are made up of microscopic structures similar to this,
with several key components. First, there is the proof mass, or seismic mass,
which is an H-shaped structure with sense fingers extending from it. This proof mass is
tethered to the substrate at both ends and can move back and forth between the tethered ends.
Electrodes are structures fixed in the substrate and remain stationary,
unlike the proof mass which can move about when there are acceleration forces on it.
It’s important to emphasize that proof mass and electrodes do not touch together but form
a comb-like structure. If you recall from our introduction to capacitors tutorial, having
two objects close to each other but not touching like this produces a capacitance. As a reminder,
capacitance is based on a few factors but, for what’s important here, one of those factors is
the distance between the different plates. The closer the plates get without touching,
the greater the capacitance. If they move farther apart, the capacitance goes down.
Keeping this in mind, let’s take a look at a circuit made up of two stationary and one
free-to-move metal plate. Together they form what is called a “differential capacitor”. In this
differential capacitor, we measure the difference in the charges that form in the bottom capacitor,
the bottom and middle plate, and the top capacitor, the middle and top plate. If we apply
a voltage to both the proof mass and the electrode while the sense fingers are perfectly centered,
they’ll have an equal build-up of capacitance. Now if we move the middle plate closer to the
bottom plate, the capacitance of the bottom capacitor increases while the top one decreases.
This is exactly how an accelerometer works. As the accelerometer moves back and forth,
the charge stored between the fingers changes. The fingers get closer on one side,
increasing the capacitance on that side while the fingers get farther apart on the other side,
decreasing the capacitance between those two plates. The change in differential capacitance
is then recorded and passed through charge amplification, signal conditioning, low-pass
filtering before it gets converted to a digital signal using an ADC. In this way, these changes
in capacitances are measured and used to quantify the amount of acceleration they’re experiencing.
While there are other technologies, capacitive sensing is widely used due to its high accuracy,
stability, low power dissipation, noise immunity, and generally simple physical structure.
But even with their benefits, sometimes, the motion can be very hard to detect
using a single differential capacitor. This makes it necessary to use multiple
movable and fixed electrodes, all connected in a parallel configuration.
This allows the system to become more accurate and sensitive to changes. But an exclusively parallel
configuration would only let us sense motion in one direction. To detect motion in multiple
directions, we need to mount the sensors within the accelerometer in multiple directions as well,
at 90 degree angles to each other. Now we can detect motion in all three dimensions.
Now that we understand how these work in theory, let’s see what the output looks like from a real
IMU. I have a low-cost MPU6050 attached to an Arduino Uno that is outputting the
acceleration and rotation to my laptop. As you can see it is still measuring an acceleration
downward in the form of gravity. But, now, as I move it up and down in a single direction,
you can see the numbers increase both positively and negatively in that axis.
And then if I go this direction, we can see the movement in that axis. One of the interesting
things is when I take the accelerometer and I change its direction and orientation
so that you can see the force of gravity in the different axes.
Now, we can get a general idea of what is going on by looking at this data but, to do real analysis,
you’ll need some sort of logic to process the information into something useful.
These, literally microscopic, sensors have become so small, low-power, inexpensive,
and accurate that they have been integrated into nearly everything. While initially only used
in industrial robotics or automobiles, these are now small enough to be found in nearly every smart
device, interpreting our conscious and unconscious movement to provide more data about what we do
and what we want to do. I hope that you’ve now gained a greater understanding of what they do,
the underlying physics principles on which they’re based, and the output you can get from
one in real life. If you’ve liked this video or found it useful, please like this video
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