Sensor (application to measurement) Lecture 3 (Chapter 2)

Sensor is a Transducer: What is a transducer ? An electromechanical device that converts a mechanical change into a change in an electrical signal Sensors Physical parameter Electrical Output Electrical Input Physical Output e.g. Piezoelectric: Force -> voltage Actuators 4/21/2017 SME3252: Mechatronics Lecture 3

Essential Elements Input Instrumentation System Output Sensing Element True value of variables Instrumentation System Output Measured value of variables Sensing Element Conditioning Element Processing Element Displaying Element Chapter 2 Chapter 3 Chapter 4,5,6 4/21/2017 SME3252: Mechatronics Lecture 3

Example of Instrumentation System 4/21/2017 SME3252: Mechatronics Lecture 3

2.1: Sensor Measurement in mechanical engineering: Displacement, position and proximity Velocity and motion Force Pressure Fluid flow Liquid level Temperature 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 Application : Bottle sensing. Type           : Photoelectric Sensor

Rolling of steel plate

Application : Liquid level indicator. Type           : Capacitive Sensor

Milling process

Translational and Rotational Potentiometers Translational or angular displacement is proportional to resistance. Taken from www.fyslab.hut.fi/kurssit/Tfy-3.441/ luennot/Luento3.pdf

high alternating stresses that cause damage to blade 2.3.2: Strain-gage high alternating stresses that cause damage to blade Strain gages allow monitoring of dynamic torsional motions

2.3.3: Capacitive sensor

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 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. LVDT Linear Variable Differential Transformer Taken from http://www.pages.drexel.edu/~pyo22/mem351-2004/lecture04/pp062-073lvdt.pdf

2.3.6: Inductive sensor

2.3.7: Optical encoder Provide digital output from linear and angular displacement

2.3.9: Proximity switch simplest 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 - Load Cell Performance 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: Infinite Environmental Temp. operating: 0 to 130 °F Temp. compensated: 30 to 130 °F Mechanical Static overload: 50% over capacity Full Scale P eT eL R3 , R4 , R1 R2

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

What is piezoelectricity ? 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