So one of the cool things about a smartphone is that what's displayed on
the screen of the phone changes in response to how I orient the phone.
So for example, if I take phone in the portrait mode and
I rotate it so that it comes in the landscape mode,
the display changes accordingly, or what's on the screen changes accordingly.
And this is enabled by this fascinating piece of
technology which is the MEMS accelerometer.
So if I open up this phone and look under the hood on the printed circuit board
there are a plethora of sensors on this PCB for this particular iPhone.
And I want to call out three of them which are responsible for
what is called as motion sensing on this phone.
The first one is a accelerometer, which essentially what it does,
it it measures acceleration, which includes the force of,
which includes the acceleration due to force of gravity.
Along these three axes, and hence it's a three axis accelerometer.
The other one is this IC over here, which is a gyroscope.
And what it does, it measures the angle of rotation along these three axes,
which is often referred by these terms, pitch, roll, and yaw,
which we'll talk more about when we talk about gyroscopes in the subsequent videos.
And the third one is this magnetometer or the compass.
Okay?
Many times you might see these terms called as six axis
motion sensor, or six axis IMU, where IMU stand, stands for
inertia motion or inertial measurement unit.
Okay? So don't get daunted.
You know what, they're essentially saying by the six axis IMU is that
it's combining these three axis data from the accelerometer.
Along with these three axes of data from the gyroscope, and they're using
it in conjunction, or using both of these data sets to enable best, better motion
sensing that what you would get from using any of these sensors individually.
And again sometime you might even read terms like nine axis IMUs.
Which again means that they're combining with another three axle beta from
this magnetic compass and this magnetic compass might be useful,
very useful in the case where you have application which involves navigation.
Similarly the data from the gyroscope and accelerometer would be very useful for
apps which involve gaming and what it means is that you're
using data from multiple of these sensors which are involved with motion sensing.
Okay, so that was an application of MEMS accelerometer in a consumer electronics.
A much older application where MEMS sensors have
been prevalent are in our cars.
And as shown over here is a, a MEMS accelerometer.
Which is especially very helpful regarding the deployment of airbags.
So let's say you were driving along in this direction, and
suddenly you had an accident or you hit something.
And that would result in a very large negative acceleration allowing this
direction, and again, the MEMS accelerometer would come to the rescue and
detect this very large acceleration they get of acceleration.
And accordingly apply an airbag, which would prevent you from,
in this case, hitting the wheel of the car and injuring yourself.
Similarly, if there was another, if you were, if your car was going in this
direction, and another automobile comes and hits you in the, from the side.
Okay? So again, this accelerometer will
detect that that acceleration in another direction.
And hopefully you have a car which has side airbags.
So again, it will deploy it, these side airbags which would prevent you
from hitting the door or the side of the car.
Okay. So I'm very fascinated by
how these things work, okay.
So let's talk a little bit about the basics of MEMS accelerometer.
And these accelerometer either detect the change in capacitance or
they detect the change in resistance.
Majority of the market is dominated by
these accelerometer which measure the change in capacitance.
So the operating principle is very similar to what you
have in mass attached to a spring.
Okay?
So we are all familiar with this problem from high school.
So whenever you have acceleration or a force acting on
this spring, the length of the spring changes, or
expands, or it contracts depending upon this force.
So the very same concept is in play over here.
What you have in this MEMS system is that you have miniature version of this spring.
And you have a mass and it's also referred to as the proof mass.
Or sometimes it's also referred to as the seismic mass.
And it's usually confined to move along a certain direction.
In this case it's confined to move let's say along the X-direction, okay.
So how do you convert that into capacitance, okay?
So what you have typically are this proof mass or
this seismic mass, it would have a fin structured like this.
And then you would have these other plates which are fixed while this mass is moving.
And then you measure the capacitance, 'kay?
Between this mass and this fixed outer plate, okay?
So let's say the distance between these two plates when
this mass was addressed are when there are no acceleration.
Was denoted by this distance D.
And then this mass move by this distance X in response to
the acceleration that was acting upon this proof mass.
Kay? So we can you know,
figure out that one of these capacitance will increase.
And the other one will decrease.
So what is often measured is the difference between these two capacitance,
c1 and c2.
So c1 could be denoted by, it would be proportional, of course,
to the area that this capacitor has.
And it would be proportional, inversely proportional to this distance.
So, in this case, c1 would essentially respond as this.
While c2, which is the capacitance with respect to
other other fin would behave something like this.
And you can prove that the difference in the capacitance it's proportional to,
of course, the area, and it's proportional to the amount that this proof mass moves.
And it has this relationship with d, okay?
So if we want to sense essentially a larger capacitance delta,
it's advisable that you'll amount that this this
proof mass or this seismic mass moves is large as that.
'Kay?
The other equation which is in play over here, is related to this spring.
And this is again equation that I'm sure you're all familiar from your
high school physics, that the spring equation or the Hooke's Law.
So, the displacement that this proof mass experiences is proportional to,
again, the force which is proportional to the mass times the acceleration,
okay, given by this Hooke's Law this spring equation.
So again, you can express this displacement as a function
of a the acceleration, the mass of this proof mass and
the stiffness constant of the spring which is denoted by K over here, okay.
So we can express this by using this resonant frequency as well.
Where this resonant frequency would be given by this formula.
That again you might be familiar from you high school physics.
So a few things to point out over here.
The first is that let's say I want to measure a very
small amount of acceleration.
So again if I want to sense a certain amount of
capacitance difference I want this X to be large.
So I want essentially this proof math to be large in value or
I want the mass of this proof math to be large.
Especially if I want to measure accelerations which are small in nature.
The other thing which is related to this resonant frequency is that it's
proportional to the stiffness constant inversely proportional to the mass, okay.
So usually if you have a high resonant frequency or your measure this
sample the data from this accelerometer at a high frequency, would that would usually
mean that the displacement would be less because the other infrequency was high.
So you're capacitance you know it has to
be very sensitive to a small amount of change in this capacitance.
One of the ways that you can actually increase this capacitance is by increasing
this area, so which is shown in this picture over here, right.
So many times what you would do to maximize this change in capacitance
is design your system in such a way that you have a lot of area,
which increases your capacitance.
So you would have this comb-like structure, okay?
And then you would have multitude of plates inside it, okay?
So that way you maximize the area that you have with those fixed plates.
'Kay?
So all these blue plates would be connected to each other.
Similarly all these pink plates would be connected to each other.
And one of them would be responsible for measuring c1 and
the other one would be responsible for measure this c2.
'Kay?
So shown in action in this picture is this
proof mass over here right, any of the same color.
So it's shown here as this proof mast.
And, again, you can see it has multiple of these fins,
'kay, to trying to maximize this data c.
And it has these six plates.
Okay?
Which are all connected these blue plates are all connected with each other.
Similarly you have these pink plates over here.
Okay?
And these would be all also connected to each other.
So you're maximizing the capacitance that you're measuring.
And you're maximizing the delta between them.
'Kay? So then one question that you may ask is
okay, so this is for measuring the capacitance along, let's say, x axis.
How do I measure the acceleration along the y axis?
So that might be simple, you know.
With not stretching your imagination too much you can think of another
proof math which is oriented along this [UNKNOWN] direction.
Okay?
And again associated with this you would have a set of plates.
Six plates which again measure these capacitances, and
that would give you acceleration largest along the x axis but
also along the, also along the y axis.
Another way you can achieve that by another
way you can achieve that two x's accelerometer is shown over here.
So, in this design you only have one proof mass.
Okay, so this is your proof mass, but
it's anchored in such a way that it can move in both X and Y directions.
Kay?
And it would have a set of plates over here, see?
And a site of fins over here.
And when this x,
when this proof [INAUDIBLE] sends that acceleration along x direction.
These sets of fins and
these sets of face plates shown over here would detect that capacitance change.
And it also has another set of plates, which are perpendicular.
And another set which are perpendicular over here, which would measure
the change in capacitance and acceleration in the y direction.
So, this is a design that accomplishes a two axis
sensing of acceleration using just one proof mass.
So now, how will I make a three axis accelerometer?
Let's talk about that in the next video.