Indian Institute of Technology Kanpur
National Programme on Technology Enhanced
Learning (NPTEL)
Introduction to Experiments in Flight
Prof.A.K.Ghosh
Department of Aerospace Engineering
IIT-Kanpur
Lecture-09
Sensors Part-I
Hello friends, So far in this course you have seen how to measure different deflection of con-
trol surface, how to calculate centre of gravity and all these experiments require different sort
of sensors to be calibrated and recorded. The software which we used for recording is lab
view. Now for all this sensor data collection, we use the module NI DAC. Now today we will
be concentrating on, how this sensors work and how do we collect data using this sensor.
Before this lecture we use NI DAC, but for illustration purpose or demonstration purpose we
will be using a processor board that is arduino uno and MEMS sensor that is micro electro
mechanical systems and the code will be using a arduino IDE or you can use languages such
as C++ or Matlab to write the code. So today’s lecture will be basically on various types of
sensors and their working principles and how do we collect data from them.
So different sensors which we will be discussing will be first pressure sensor now these pres-
sure sensors usually are used to calculate the altitude and air speed.
(Refer Slide Time: 01:58)
The second sensor which we will be concentrating on Accelerometer, third is rate gyro. We
will see the importance of magneto meter and then we will see how angle of sensor attack
works and why it is needed. These were the sensors which we showed you during the data
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collection of flight data recorder. We saw different total pressure and differential pressure,
what is the acceleration in all three axis, what will be the roll, yaw and pitch rate for different
axis.
(Refer Slide Time: 01:58)
We did not mention this magnetometer, but instead of that we mention what was your roll
angle, your pitch angle and the yaw angle. And we mention angle of attack as well as side
slip angle in FDR, we also saw the result of control surface deflection that is deflection of
elevator, deflection of aileron, deflection of rudder.
(Refer Slide Time: 03:47)
These were the data we saw in flight data recorder, excluding your magnetometer, data and
your GPS data, which will be your sixth sensor, which we will be concentrating on. So lets
start with your first sensor that is your pressure sensor. As I already told you that pressure
sensors are used to calculate altitude as well as airspeed.
So first altitude, now the general equation used to calculate altitude is a simple hydrostatic
equation given by,
P2 − P1 = ρg(Z2 − Z1 ).
Here ρ is density, (Z2 - Z1 ) represent the depth. g is acceleration due to gravity. P1 , P2 are
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(Refer Slide Time: 03:47)
the pressure at position 1 and 2 respectively. Now in case of height just you have to substitute
instead of the this depth we have to substitute it with altitude so the equation will become,
Palt − Pground = −ρg(halt − hground ).
The values halt and hground are heights above sea level. The minus sign because, in case of
fluids the depth that is positive Z direction. That is depth is downwards this is known as posi-
tive Z, whereas, in case of height, we calculate that above sea level, this is negative Z. Hence,
we get a negative Z. Also you can see that as altitude increases this value will decreases. So
the overall equation will be positive. One thing we have approximated is that density remains
constant for all height.
(Refer Slide Time: 06:41)
In case of water or any fluid, you can approximate that the density remains constant as you
go beyond a certain depth. As height increases your density decreases. So if we want a more
accurate result, you have to incorporate that changing density as the height increases. So
density with height is given as change of density that is,
−(n+1)
Th
ρh = ρ0
T0
g0
where n = aR . Now g you know this is acceleration due to gravity, a is your lapse rate that is
-0.0065 and gas constant represented by R which is equal to 287 and the temperature T0 at
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sea level is 288.16 K.
(Refer Slide Time: 06:41)
(Refer Slide Time: 08:27)
Now using this approximation you will get exact value of density at that particular height.
That you can substitute in this particular equation the output which we get from digital sys-
tems are always in terms of voltages or in terms of current that is why the output which you
will be getting will be something in terms of voltage that we have already seen in case of
previous experiments where we calibrated your deflection of control surface and the output,
which we were getting was in terms of voltage.
So we have to calibrate that and output for instance for this something pressure absolute will
be in terms of say,
γPabs = Pground + Palt + noise
(Refer Slide Time: 09:41)
Now as a second application of pressure sensor is to calculate airspeed. Now the principle for
calculating airspeed and altitude is same. The difference between them are for altitude block
diagram is as follows.
(Refer Slide Time: 10:59)
You have a chamber which is divided into two volumes by a small diaphragm here you have a
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(Refer Slide Time: 08:27)
(Refer Slide Time: 09:41)
standard pressure some known pressure you have here and this vent is opened for ambient air
so the pressure difference here P, pressure difference between this two volumes will be will
cause a deflection in this diaphragm which will be again given to you in terms of voltages
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which can be calibrated and you get what will be your present altitude based on the pressure
which you have got.
(Refer Slide Time: 10:59)
We have already shown you this will be your output which is the difference between your
pressure at ground and particular altitude plus some noise part will be there. You can if in-
struments are quite accurate then noise can be neglected but we consider some noise part here
and the difference between these, this was your altitude for air speed. You have a pitot P1 and
P2 are the volumes chambers this is same diaphragm now it has two outlet one is at the front
this will be open to your air flow which will give you total pressure.
(Refer Slide Time: 14:20)
And this is a small vent here which will give you static pressure Pstatic , now this will with
respect to the your total pressure you will get a pressure P1 on left volume and due to static
pressure you will get pressure on right volume. The difference between these two pressure
will be result in giving a deflection to diaphragm will again which again will give you a volt-
age compared to that deflection which can be again calibrated. And the difference between
these two pressures can be noted in terms of voltage and the relation which goes with for
calculation of airspeed is,
1
PT = Pstatic + ρVa2
2
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(Refer Slide Time: 14:20)
Va is a velocity here. So this a basic difference between your pressure sensor for airspeed cal-
culation and your altitude calculations. This is basic difference between the block diagrams.
(Refer Slide Time: 17:38)
The second sensor which we talk about Accelerometer remember when we showed you the
flight recorded data flight data recorder. You saw there were three times named as ax , ay ,
az these represent acceleration in X, Y and Z direction. This is MEMS sensors, this is your
proof mass.
These are your springs, these and these are your springs, these all are fixed plate. The working
principle of the accelerometer is when the body is in acceleration this proof mass oscillates
for suppose this the acceleration experienced in this particular direction your proof mass will
oscillate in this direction. These are spring with constant K. So the capacitance between these
plates will change and corresponding to that capacitance you will get a voltage. This voltages
when calibrated will represent your acceleration in particular direction.
Now since this proof mass can only oscillate in one particular direction. So for three axis
you have to have three different accelerometer for three different axis that is X, Y and Z. This
proof mass will move in only in X direction, for Y it will move only in Y direction, and for
Z it will move only in Z direction. And based on that change in capacitance c1 and c2 you
can calibrate that to get what will be required the voltage and what will be the acceleration of
that particular accelerometer.
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(Refer Slide Time: 17:38)
Next sensor which we will discuss will be rate gyro. In rate gyro the force which you experi-
ence is due to Coriolis acceleration. That is suppose you have a body which is moving with
velocity V and when it is subjected to an external rotation are you can say angular velocity.
It will experience a force which will be perpendicular to your axis of rotation and velocity,
that is, it will experience force in this particular direction. Suppose this is X direction then
this will be acceleration or force experienced in X direction.
(Refer Slide Time: 19:31)
Now once this force is experienced, the phenomena or the principle behind calculating what
will be the voltage corresponding to this angular motion what will be the force that you can
calibrate, the principle will be same, acceleration experienced will move the proof mass and
proof mass will vary the capacitance and according to the capacitance we will get a voltage.
(Refer Slide Time: 22:26)
The block diagram for this is a little bit complex corresponding to acceleration, here acceler-
ation is different because this is due to Coriolis force. The block diagram for rate gyro can
be seen in the figure ??. There are fixed plates which are mounted inside a casing.
Now when the body is not rotating or it does not experience any angular velocity. Suppose
your body is moving in its particular direction so the outer casing will be moving in that par-
ticular direction that is oscillation will be in this particular direction either this or outer case
will be moving. Once it experience our angular velocity say about this axis it experience a
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(Refer Slide Time: 19:31)
(Refer Slide Time: 22:26)
angular velocity.
Now this particular inner casing will be moving, but these are fixed plate as you know so this
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will be moving in this particular direction or oscillating in this particular direction. This will
result in change of capacitance between these plates and as I already told due to change of
capacitance you will get a respective voltage and that you can calibrate to get what are the
rates in XYZ that is roll, pitch and yaw rates.
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