mechatronics sensors transducers

1. Professor Charlton S. Inao
Professor Mechatronics System Design
Defence Engineering College
Bishoftu, Ethiopia
PE-4030
Chapter 2/b Part two

2. Instructional Objectives
 To understand the working principle and applications of the following sensors:
1. Liquid Flow Sensor
1.1 Orifice
1.2 Turbine Meter
2. Level Sensor
2.1 Floats
2.2 Differential Pressure
3. Temperature Sensor
3.1 Liquid in Glass
3.2 Bimetallic Strip
3.3 Thermistors
3.4 Electrical Résistance Thermometers
3.5 Thermocouples
4. Light Sensor
• To practice how to select sensor based on industrial requirements.

3. Flow Sensors

4. 1.1 Orifice
The orifice plate is simply a disc ,
with a central hole, which is placed in
the tube through which the fluid is
flowing
The pressure difference is measured
between a point equal to the diameter
of the tube upstream and a point
equal to half of the diameter
downstream. It does not work well
with the slurries. The accuracy is
typically about + 1.5% of full range
and is non-linear.

5. • The turbine flowmeter consists of
a multi-bladed motor that is
supported centrally in the pipe
along which the flow occurs.The
fluid rotates the motor , the
angualr velocity being
approximately proportional to the
flow rate. The rate of the
revolution of the rotor can be
determined using a magnetic pick
up which produces an induced
emf pulse every time the rotor
blade passes it as th e blades are
made from magnetic material or
have small magnets mounted at
their tips.
1.2 Turbine Meter
The pulses are counted and so the
number of revolutions of the rotor
can be determined. The meter is
expensive with a n accuracy of
typically about + 0.3%

6. Level Sensors
A direct method of monitoring the
level of liquid in a vessel is by
monitoring the movement of the
float. . The displacement of the float
causes a lever arm to rotate and so
move a slider across the
potentiometer. The result is an
output of voltage related to the
height of the liquid.
2.1 Floats
2.0 Indirect Method
1. Monitoring of the weight of the
vessel by load cell
Weight= Ahρg
Note: hρg = P
2. Measurement of pressure at some
point in the liquid P= hρg

7. Float Swtich

8. 2.2Differential Pressure
The differential pressure cell
determines the pressure difference
between the liquid at the base of the
vessel and atmospheric pressure,
the vessel being open to atmospheric
pressure.
The differential pressure cell
monitors the difference in pressure
between the vase of the vessel and
the air or gas above the surface of
the liquid.

9. Temperature Scales
• Celsius(º C)- common SI unit of relative temp
• K=C +273
• Kelvin(K)-Standard SI unit of absolute
thermodynamic temperature
• Fahrenheit-(º F)English unit of relative
temperature. T= 9/5C +32
• Rankine(ºR) English system unit of absolute
thermodynamic temperature. R=F +460
Temperature Measurements

10. Temperature Sensors
• 3.1 Liquid in Glass
-A simple non electrical temperature measuring
device which typically uses alcohol or mercury
as the working fluid, which expands and
contracts relative to the glass container.
When making measurements in a liquid, the
depth of immersion is important

11. Temperature Sensors
• 3.2 Bi–Metallic Strip
Another nonelectrical temperature
measuring device. I tis composed of
two or more metal layers having
different coefficient of thermal
expansion. Since these layers are
permanently bonded together, the
structure will deform when
temperature changes, due t to the
difference in the thermal expansions
of the two metal layers. The
deflection can be related to the
temperature of the strip.
The mechanical motion of the
strip makes or breaks an
electrical contact to turn a
heating or cooling system On or
OFF.

12. Temperature Sensors
• 3.3Resistance Temperature Detector(RTDs)
RTD is constructed of metal wire wound around
a ceramic or glass core and hermetically
sealed. The resistance of the metallic wire
increases with temperature. The resistance
Temperature relationship is approximated by
the following linear expression:
R=Ro[1 +α(T-To)]

13. Where To=reference temperature
Ro= resistance at the reference
temperature
α=calibration constant
The reference temperature is usually the ice point of the
water(0º C).
The most commonly used metal in RTD is platinum, because of
its high melting point, resistance to oxidation, predictable
tem characteristics, and stable calibration values.
The operating range of typical platinum RTD is –220 deg
centigrade to 750 deg centigrade.

14. 3.4 Thermistor-is a semiconductor device whose
resistance changes exponentially with temperature.
Thermistors have much narrower operating ranges
than RTDs.
Its resistance –temperature relationship is usually
expressed in the form
R= Roe[β(1/T-1/To)]
Where To= reference temperature
β =a calibration constant called the characteristic temperature of the

15. Temperature Sensors
• 3.5 Thermocouples
Two dissimilar metals in contact
form a thermoelectric
junction occur in pairs,
resulting in what is called
thermocouple.This is known
as Seebeck effect.
The thermocouple voltage is
directly proportional to the
junction temperature
difference
V= α(T1-T2)
Where α is called the Seebeck
coefficient; T1 and T2 is the junction
temperature of metals A and B.

16. Thermocouple Circuit

17. The Five Basic Law of
Thermocouple
1. Law of leadwire temperatures. The
thermoelectric voltage due to two junctions in a circuit
consisting of two different conducting metals depends only
on the junction temperature s T1 and T2. The temperature
environment of the leads away from the junctions(T3,T4,T5)
does not influence the measured voltage.

18. 2. Law of intermediate leadwire metals. A third
metal C introduced in the circuit constituting
thermcocouple has no influence on the resulting voltage
as long as the temperature of the two new junctions(A-
C and C-A) are the same(T3=T4). A voltage measurement
device that creates two new junctions can be inserted
into the thermocouple circuit without altering the
resulting voltage.

19. 3.Law of Intermediate
Junction Metals. If a
third metal is introduced
within the junction
creating two new
junctions(A-C and C-B) ,
the measured voltage will
not be affected as long as
the two new junctions are
at the same
temperature(T1=T3).If
T1=/T3, the effective
temperature at C is the
average of the
temperatures(T1 +T3)/2.

20. 4.Law of Intermediate Temperatures. Junction pairs at
T1 and T3 produce the same voltage as two sets of junction
pairs spanning the same temperature range(T1 to T2 and T2 to
T3), therefore
V1/3=V1/2+ V2/3
The voltage resulting from measuring temperature T1 relative to
T3 , is the same as the sum of the voltages resulting from T1
relative to T2 and T2 relative to T3.

21. 5. Law of the intermediate metals. The voltage
produced by two metals A and B is the same
as the sum of the voltages produced by each
metal A and B relative to the third metal C.
VA/B=VA/C +VB/C
The results supports the use of standard reference
metal(e.g . Platinum) be used a s a basis to calibrate all
other metals.

22. Thermocouple
Configuration
Thermocouple
Data

24. Thermocouple

25. Thermocouple Type, Materials,
Range, Sensitivity

26. Thermocouple Junction
Temperature and Output voltage
Junction Temperature (C) Output Voltage (mV)
0 0
10 0.507
20 1.019
30 1.536
40 2.058
50 2.585
60 3.115
70 3.649
80 4.186
90 4.725
100 5.268

27. Light Sensors

28. Since distance is velocity multiplied by time, wavelength can be expressed
as the velocity of electromagnetic waves multiplied by the time of one cycle
of frequency f. Since the accepted speed of light is 186,000 miles per
second or 300,000,000 meters per second, this is: ë(in meters) =
300,000,000 meters/sec × 1/f(in seconds) or, ë(in meters) = 300/f(in MHz)
If visible light (white light) is passed through a prism, , the visible light
separates into its color components.
The electromagnetic spectrum is divided into radio waves and light waves by
frequency. Light waves are further divided by into infrared, visible, ultraviolet
and X-rays. The spectrum is either expressed in frequency or wavelength.
Wavelength is the distance that an electromagnetic wave travels through
space in one cycle of its frequency.
The Electromagnetic Spectrum

29. The frequency of visible light is from 400 million megahertz to 750 million
megahertz. The wavelength is from 750 nanometers (10−9) to 400
nanometers. Light sensors extend into the infrared frequency range below
visible light and into the ultraviolet light frequency range above visible light.
Cadmium sulfide sensors are most sensitive in the green light region of visible
light, while solar cells and phototransistor sensors are most sensitive in the
infrared region.

30. Light Sensors
Light sensor diodes make
the resistance of the circuit
decreases and the current
increases as the
light/illuminance increases,
at constant voltage.
•Used in control of
street lamp
•Used in the
automatic /digital
camera
•Used in the
automotive and
military industry

31. Selection of Sensors
1. Identify the nature of the measurement
required
• Variable to be measured
• Nominal value
• Range of Value
• Accuracy required
• The required speed of measurement
• Reliability required
• Environmental conditions

32. 2. Identify the nature of the output required from the
sensor, this determining the signal conditioning
requirements in order to give suitable output signals
from the measurement.
3. Identify the possible sensors, taking into account
such factors as range, accuracy, linearity, speed of
response, reliability, maintainability, life, power
supply requirements, ruggedness, availability and
cost.
4.Identify the signal conditioning requirements. Eg.
Measurement of level of a corrosive acid in a vessel.
Using a load cell, which gives an electrical output,
calibrated to the level, ie. When empty and when
full.

33. The End