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Sensor (application to measurement)
Lecture 3
(Chapter 2)
04/21/23 SME3252: Mechatronics Lecture 3
2
Sensor is a Transducer:What is a transducer ?
An electromechanical device that converts a mechanical change into a change in an electrical
signal
ActuatorsSensors
Physical paramete
r
Electrical Output
Electrical Input
Physical Output
e.g. Piezoelectric:
Force -> voltage
04/21/23 SME3252: Mechatronics Lecture 3
3
Essential Elements
InstrumentationSystem
Input
True valueof variables
Output
Measured valueof variables
ConditioningElement
SensingElement
DisplayingElement
ProcessingElement
Chapter 2 Chapter 3 Chapter 4,5,6
04/21/23 SME3252: Mechatronics Lecture 3
4
Example of Instrumentation SystemExample of Instrumentation System
2.1: Sensor
• Measurement in mechanical engineering:1. Displacement, position and proximity2. Velocity and motion3. Force4. Pressure5. Fluid flow6. Liquid level7. Temperature8. Light intensity
2.2 Performance terminology
• Range and span• Error• Accuracy• Sensitivity• Hysteresis error• Nonlinearity error• Repeatability /
reproducibility
• Stability – zero drift• Resolution• Output impedence
Examples - • Ranges: 70 to 1000kPa, 2000 to 70000 kPa• Supply voltage: 10 V d.c or a.c. rms• Full range output: 40 mV• Nonlinearity and hysteresis:±0.5% full range
output• Temperature range: -540C to +1200C when
operating• Thermal zero shift: 0.030% full range output /0C
2.3: Displacement, position and proximity
• Displacement – how much the object has been moved
• Position – position of an object with a reference point
• Proximity – position sensor to detect when an object has moved
• 2 basic types of displacement / position sensor – contact and noncontact
Displacement measurement
Examples of application:
• Location & position of object on a conveyor
• Orientation of steel plate in a rolling mill
• Liquid/solid level measurement
• Location or position of work piece in milling operation
Application : Bottle sensing. Type : Photoelectric Sensor
Location & position of object on a conveyor
Rolling of steel plate
Application : Liquid level indicator. Type : Capacitive Sensor
Milling process
2.3.1: PotentiometerTranslational and Rotational
Potentiometers
Translational or angular displacement is proportional to resistance.
Taken from www.fyslab.hut.fi/kurssit/Tfy-3.441/ luennot/Luento3.pdf
2.3.2: Strain-gage
Strain gages allow monitoring of dynamic torsional motions
high alternating stresses that cause
damage to blade
2.3.3: Capacitive sensor
17
How does it works: Capacitive
A capacitive touchscreen consists of a glass
panel with a capacitive (charge storing)
material coating its surface. Circuits located at
the corners of the screen measure the
capacitance of a person touching the overlay.
Frequency changes are measured to
determine the X and Y coordinates of the
touch command. The touchscreen controller
sends data via serial port to the host
computer and emulates a mouse.
2.3.4: LVDT
Inductive Sensors - LVDT
LVDT
Linear Variable Differential Transformer
Taken from
http://www.pages.drexel.edu/~pyo22/mem351-2004/lecture04/pp062-073lvdt.pdf
An LVDT is used as a sensitive displacement sensor: for example, in a cardiac assist device or a basic research project to study displacement produced by a contracting muscle.
2.3.6: Inductive sensor
2.3.7: Optical encoder
• Provide digital output from linear and angular displacement
2.3.9: Proximity switchsimplest form of digital displacement sensor
• many forms: lever or push-rod operated microswitches; float switches; pressure switches; etc.
A limit switch A float switch
Lim Kim Fung, PSM 2006/2007
2.4: Velocity sensor
A) Linear velocity measurement
• Linear velocity transducer or LVT
• Based on inductive transducer principle
• Magnetic field associated with velocity to be measured moves w.r.t fixed conductor
B) Angular velocity measurement
• Many common machine have rotating shafts – angular velocity or shaft speed
• Stroboscopic – flashing light, mark on pulley, rpm
• Photoelectric – used photodetector
2.5: Force measurement - 2.5: Force measurement - Load CellLoad Cell
PerformancePerformance Load range: 5 to 250 lbs Non-Linearity: 0.05% F.S. Hysteresis: 0.03% F.S. Non-Repeatability: 0.03% F.S. Output: 3 mV/V Resolution: InfiniteEnvironmentalEnvironmental Temp. operating: 0 to 130 °F Temp. compensated: 30 to 130 °FMechanicalMechanical Static overload: 50% over capacity
FullScale
P
eT
eL
R3
, R4
, R1
R2
P
2.6: Fluid pressure
• hydraulic pressure is used to measure force applied to diaphragm
• when force F applied, pressure is developed in fluid (normally oil),device to measure normally Bourdon gage
2.6.1: Piezoelectric Sensors
What is piezoelectricity ?
Strain causes a redistribution of charges and results in a net electric dipole (a dipole is kind of a battery!)
A piezoelectric material produces voltage by distributing charge (under mechanical strain/stress)
Discovered in 1880 by Pierre Curie in quartz crystals. The greek word “piezein”, which means “to press”Examples --- Quartz, Barium titanate, tourmaline
2.7: Liquid flow
The most common principals for fluid flow metering are:
• Differential pressure flowmeter
• Velocity flowmeter
• Positive displacement flowmeter
• Open channel flowmeter
• The most common types of differential pressure flowmeters are:
• Orifice Plates
• Flow Nozzles
• Venturi Tubes
• Variable Area - Rotameters
2.9: Temperature
• RTD
• Thermistor
• Thermocouple
2.10: Photoelectric sensor
• Photodiode
• Phototransistor
• Photocell
• Solar cell
Presence of object
Samsul Tongaji, PSM 2006/2007
Exercises (Textbook)
Identify / suggest suitable sensors for the following applications:
• Control system for a furnace to monitor rate of heating oil flows along a pipe line
• Control system to determine difference of liquid levels in two container
• Control system to control thickness of rolled sheet that emerges from rollers
End of Lecture 3
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