4. Sensors and Transducers Overview

8/10/2019 4. Sensors and Transducers Overview

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D R . T A R E K A . T U T U N J I

P H I L A D E L P H I A U N I V E R S I T Y , J O R D A N

2 0 1 3

Sensors and Transducers:

Overview


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Introduction

A sensoris defined as a device that produces anoutput signal for the purpose of sensing of a physicalphenomenon.

Sensors are also referred to as transducers.

A transduceris defined as a device that converts asignal from one physical form to a correspondingsignal, which has a different physical form.


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Mechatronics Measurement System

[Ref.] Shetty


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General Instrumentation System

[Ref.] Shetty


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Sensor Classification

[Ref.] Shetty


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Transducers Classification

Potentiometric Potentiometric transducers apply the principle of change in resistance of

material in the sensor.

Capacitance Capacitance transducers apply the principle of capacitance variation

between a set of plate assemblies. Inductance Inductance transducers are based on the principle of variation of

inductanceby the insertion of core material into an inductor. Inductancevariations serve as a measure of displacement.

Piezoelectric Piezoelectric transducers are based on the principle of charge

generation.Whenever certain piezoelectric crystals are subjected tomechanical motion, an electric voltage is induced. This effect can bereversed by applying an electric voltage and deforming the crystal.


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Elements of Instrumentation System

[Ref.] Shetty


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Home Heating System Example

[Ref.] Shetty


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Quality Parameters

Sensitivity

Resolution

Accuracy

Precision Backlash

Repeatability

Linearity


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Linear and Rotational Sensors

Linear and rotational position sensors are two of themost fundamental of all measurements used in atypical mechatronics system.

Position sensors produce an electrical output that isproportional to the displacement. Contact: Strain gage, LVDT, RVDT, and tachometer.

Noncontact : encoders, hall effect, capacitance, andinductance.


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Linear and Rotational Sensors

Hall effect, fiber, optic inductance, capacitance, andstrain gage are considered high resolution sensors, butsuitable for only very small range (typically from 0.1 mmto 5 mm).

The differential transformers on the other hand, have amuch larger range with good resolution.

Among many linear displacement sensors, strain gageprovides high resolution at low noise level and is leastexpensive.


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Potentiometer

[Ref.] Kilian + Shetty


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Potentiometer as Position Sensor

[Ref.] Kilian


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Potentiometers Features and Applications

Features Linear potentiometers are often considered when an electrical

signal proportional to displacement is required, but also wherecost should be kept low and high accuracy is not critical.

Typical rotary potentiometers have a range of +-170. Theirlinearity varies from 0.01 to 1.5%.

Applications Used for position monitoring of products on assembly lines

and checking dimensions of the product in quality control

systems. Rotary potentiometers are used in applications involving

rotational measurement for applications ranging frommachine tools to aircraft.


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Linear Variable Displacement Transformer (LVDT)

[Ref.] Kilian


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Linear Variable Displacement Transformer (LVDT)

[Ref.] Kilian


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LVDT Interface Circuit

[Ref.] Kilian


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LVDT Features and Applications

Features: High resolution, high accuracy, and good stability make them an ideal

for applications involving short displacement measurements. Sensitive transducers provide resolution down to about 0.05 mm. They

have operating ranges from about 0.1 to 300 mm. Accuracy is 0.5 mm of full-scale reading. Less sensitive to wide ranges in temperature than potentiometers.

Applications Measurement of precision gap between weld torch and work surface in

welding applications. Measurement of the thickness of plates in rolling mills.

Detection of surface irregularity of parts after they are machined. Angular speed measurement of a rotating device. Precise detection of specimen size. Liquid level applications.


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Rotary Encoders Applications

Encoders are used for measurement of linear or angularposition, velocity, and direction of movement.

Used in computerized manufacturing machines, motion-control applications, and quality assurance of equipment.

Used in tensile-test instruments to precisely measure theball screw position.

Used in automated test stands used when angularpositions of windshield wiper drives and switch positions

are tested. Incremental encoders commonly are used for counting

applications.


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Optical Encoding: Absolute

[Ref.] Kilian


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Optical Encoding: Incremental

[Ref.] Kilian


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Encoder Interface Circuit

[Ref.] Kilian


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Tachometer

[Ref.] Kilian


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Tachometer Example


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Vibration Sensors: Seismic Mass


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Vibration Sensors: Piezoelectric

[Ref.] Shetty


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Force, Torque, and Pressure Sensors

Common force/torque sensors are: strain gauge andpiezoelectric.

Both are available to measure force and/or torque either inone axis or multiple axes.

The strain gauge make use of mechanical members thatexperiences elastic deflection when loaded. These types ofsensors are limited by their natural frequency.

The piezoelectric sensors are particularly suitable for dynamicloadings in a wide range of frequencies. They provide highstiffness, high resolution over a wide measurement range, andare compact.


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Strain Gauges

[Ref.] Shetty


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Strain Gauges

[Ref.] Kilian


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Load Cells

[Ref.] Kilian


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Strain Gauges

Features A high gauge factor increases its sensitivity and causes a larger change in

resistance for a particular strain. High resistance of the strain gauge minimizes the effect of resistance

variation in the signal processing circuitry. Choose gauge characteristicssuch that resistance is a linear function of strain.

For dynamic measurements, the linearity should be maintained overthe desired frequency range.

Low temperature coefficient and absence of the hysteresis effect addto the precision.

Applications Strain-gauge transducers are used for measuring strain, force, torque,

pressure, and vibration. In some applications, strain gauges are used as a primary or secondary

sensor in combination with other sensors.


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Tactical Force Sensor

[Ref.] Kilian


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Conductive Foam Tactical Sensor

[Ref.] Kilian


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Pressure Sensors: Bourdon and Bellows

[Ref.] Kilian


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Semiconductor Pressure Sensor

[Ref.] Kilian


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Flow Sensors

The fluid medium can be liquid, gas, or a mixture of the two. The flow could be laminar or turbulent and can be a time-

varying phenomenon.

The venturi meter and orifice plate restrict the flow and use

the pressure difference to determine the flow rate.

The rotameter and the turbine meterswhen placed in theflow path, rotate at a speed proportional to the flowrate.

The electromagnetic flow meters use noncontact method.Magnetic field is applied in the transverse direction of the flowand the fluid acts as the conductor to induce voltageproportional to the flow rate.


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Flow Sensors

Ultrasonic flow meters measure fluid velocity by passinghigh-frequency sound waves through fluid. Transmitters (T) provide the sound signal source. As the wave travels

towards the receivers (R), its velocity is influenced by the velocity ofthe fluid flow due to the doppler effect.

The control circuit compares the time to interpret theflow rate. This can be used for very high flow rates andcan also be used for both upstream and downstream

flow. The other advantage is that it can be used forcorrosive fluids, fluids with abrasive particles, as it is likea noncontact sensor.


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Orifice Plate and Venturi Flow

[Ref.] Kilian


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Turbine and Magnetic

[Ref.] Kilian


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Ultrasonic Flow Sensors

Ultrasonic flow meters measure fluid velocity by passinghigh frequency sound waves through the fluid.

They operate by measuring the transmission time

difference of an ultrasonic beam passed through ahomogeneous fluid contained in a pipe at both an upstreamand downstream location.


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Flow Sensors for Solids


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Level Sensors

Level sensors are used to measure the fluid level intanks.

There are discrete and continuous level sensors


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Discrete Level Sensor

[Ref.] Kilian


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Continuous Level Sensor

[Ref.] Kilian


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Temperature Sensors

The most common temperature sensors are: Thermocouples Thermisters Resistance Temperature Detectors (RTD) Infrared types.

Thermocouples are the most versatile, inexpensive, andhave a wide range (up to 1200 C typical). A thermocouplesimply consists of two dissimilar metal wires joined atthe ends to create the sensing junction. When used in

conjunction with a reference junction, the temperaturedifference between the reference junction and the actualtemperature shows up as a voltage potential.


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Temperature Sensors

Thermistors are semiconductor devices whose resistancechanges as the temperature changes. They are good for veryhigh sensitivity measurements in a limited range of up to 100C. The relationship between the temperature and theresistance is nonlinear.

TheRTDs use the phenomenon that the resistance of a metalchanges with temperature. They are, however, linear over a

wide range and most stable

Infrared type sensors use the radiation heat to sense thetemperature from a distance. These noncontact sensors canalso be used to sense a field of vision to generate a thermalmap of a surface


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Temperature Sensors

Temperature measurement is based on one of thefollowing principles.

1. Material expansion based on change in length,volume, or pressure.

2. Based on the change in electrical resistance.

3. Based on contact voltage between two dissimilar

metals.

4. Based on changes in radiated energy.


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Thermocouple

[Ref.] Kilian


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Thermocouple Chart


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Standard Thermocouple Characteristics


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RTD

RTD is a length of wire whose resistance is a function oftemperature.

It consists of a wire that is wound in the shape of a coil toachieve small size and improve thermal conductivity.


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Thermistor

[Ref.] Kilian

Thermistor operation relies on the principle of change in semiconductorresistance with change in temperature


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LM35 Temperature Sensor


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LM35 Temperature Sensor


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Proximity Sensors

They are used to sense the proximity of an object relative to anotherobject. They usually provide ON/Off signal indicating the presenceor absence of an object.

Examples: Capacitance, inductance, photoelectric, and hall effect

Capacitancetypes are similar to inductance except the proximityof an object changes the gap and affects the capacitance.

Inductanceproximity sensors consist of a coil wound around asoft iron core. The inductance of the sensor changes when a ferrous

object is in its proximity. This change is converted to a voltage-triggered switch.


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Capacitive Proximity Sensor

[Ref.] Bartlet


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Inductive Proximity Sensor

[Ref.] Bartlet


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Proximity Sensors

Photoelectric sensors are normally aligned with aninfrared light source. The proximity of a movingobject interrupts the light beam causing the voltagelevel to change.

Hall effectvoltage is produced when a current-carrying conductor is exposed to a transverse

magnetic field. The voltage is proportional totransverse distance between the hall effect sensorand an object in its proximity


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Photo-detectors

[Ref.] Kilian


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Hall Effect

Hall effect transducers are used to measure position, displacement,level, and flow. They can be used as an analog motion sensing device as well as a digital device.

The Hall effect occurs when a strip of conducting material carriescurrent in the presence of a transverse magnetic field.

An electron of charge, e, traveling in a magnetic field, B, with avelocity v, experiences a Lorenz force F, and it is represented byF =e ( v x B)

An electric field, known as Halls field, counterbalances Lorenzs

force and is represented by an electric potential. The voltageproduced may be used to produce field strength or a current.


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Hall Effect Sensors

[Ref.] Kilian

h


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Light Sensors

Light intensity and full field vision are two importantmeasurements used in many control applications.

Phototransistors, photoresistors, andphotodiodes aresome of the more common type of light intensity sensors.

When the photoresistor is exposed to light, its resistancedrops in proportion to the intensity of light. Wheninterfaced with a circuit and balanced, the change in lightintensity will show up as change in voltage.

These sensors are simple, reliable, and cheap, usedwidely for measuring light intensity.

i h


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Light Sensors

O i l l d l


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Optical Slotted Coupler

R S Ul i


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Range Sensors: Ultrasonic

R S L


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Range Sensors: Laser

S M i l S


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Smart Material Sensors

New smart materials are being used as sensors.

Examples are: Optic fibers, piezoelectric, andmagnetostrictive materials

Optic fibers Can be used to sense strain, liquid level, force, and temperature with

very high resolution. They have found numerous applications in smart structure applications

such as damage sensors, vibration sensors, and cure-monitoring sensors.

The basic principle of operation of an embedded optic fiberused to sense displacement, force, or temperature. Therelative change in the transmitted intensity or spectrum isproportional to the change in the sensed parameter.

O ti l Fib S i


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Optical Fiber Sensing

Pi l t i T d


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Piezoelectric Transducer

Piezoelectric materials, when subjected to mechanicalforce or stress along specific planes, generate electriccharge.

The best-known natural material is quartz crystal (SiO2).

Rochelle salt is also considered a natural piezoelectricmaterial.

Mi d N S


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Micro and NanoSensors

Microsensors (MEMS) are the miniaturized version of theconventional macrosensors with improved performance andreduced cost.

Silicon micromachining technology has helped the development ofmany microsensors and continues to be one of the most activeresearch and development topics in this area.

Vision microsensors have found applications in medical technology. Afiberscope of approximately 0.2 mm in diameter has been developed to inspect

flaws inside tubes. A microtactile sensor, which uses laser light to detect the contact between a

catheter and the inner wall of blood vessels during insertion that has sensitivityin the range of 1 mN.

Similarly, the progress made in the area of nanotechnology hasfuelled the development of nanosensors. These are relatively newsensors that take one step further in the direction of miniaturizationand are expected to open new avenues for sensing applications.

Si l C diti i


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Signal Conditioning

Output from a sensors require signal processingbefore sent to the controller. Such as Amplification

Filtering

Demodulation Linearization

Some sensors are available with integrated signal

conditioners, such as the microsensors. All theelectronics are integrated into one microcircuit andcan be directly interfaced with the controllers

C lib ti


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Calibration

The sensor manufacturer usually provides thecalibration curves. If the sensors are stable with nodrift, there is no need to recalibrate. However, oftenthe sensor may have to be recalibrated after

integrating it with a signal conditioning system.

This essentially requires that a known input signal isprovided to the sensor and its output recorded to

establish a correct output scale. This process provesthe ability to measure reliably and enhances theconfidence.

C lib ti


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Calibration

If the sensor is used to measure a time-varying input,dynamic calibration becomes necessary.

Use of sinusoidal inputs is the most simple andreliable way of dynamic calibration.

Another test is looking at the transient behavior of

step response.

S


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Summary

Sensors and transducers are essential elements inmechatronic systems because they are used to measurethe controlled variable and transmit to the controller asfeedback

Sensors can be divided according to the variable that theymeasure: Position/Velocity Acceleration Force/Torque/Pressure

Flow Temperature Proximity/Range

References


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References

Mechatronics System Design 2ndedition by Shettyand Kolk. Cenage Learning 2011

Modern Control Technology: Components andSystems 2ndedition by Kilian. Delmer Publication

Documents

4. Sensors and Transducers Overview