Sensor (application to measurement)

Lecture 3

(Chapter 2)

04/19/23 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

04/19/23 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

04/19/23 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