Industrial Automation 2

8/2/2019 Industrial Automation 2

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1/52 2.1 InstrumentationIndustrial Automation

2.1.3.3 Hydraulic measurements

Flow, Mass Flow, Level, Pressure, Conductivity, pH-Sensor, Viscosity, Humidity,

special requirements: intrinsic safety = explosive environment, sea floor = high pressure


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Level measurement

pulsed laser

load cell

pulsed microwave

nuclear

ultrasonic (40-60 kHz)

low power ultrasonic

detectorrow

see Control Engineering, Aug 2003

F = mg


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Flow measurement

Distinguish:volumetric flow ( m 3 /s)

mass flow: (kg / s)identical when the density of the liquid is constant

main methods:-floater-turbine

-pressure difference-vortex-temperature gradient-ultrasonic-electrodynamics


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Flow velocity measurement: differential pressure

occultation(Verengung)

membrane

the flow velocity is proportional to the square root of the pressure difference

piezo-electricsensor

p2 - p 1 = r v21

2 (Bernoulli effect)

p2 p1

v

fluid ofviscosity r

occultation(Blende)


5/405/52 2.1 InstrumentationIndustrial Automation

Flow measurement

Other means:

Magnetic-dynamicCoriolisUltra-sound



Flow measurement in a plant


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Pressure Detectors



Absolute pressure - psia

Gage pressure referencedto atmospheric (approx

14.7 psia) - psig

Differential pressure

difference between twopressures - psid

Types of Pressure Measurements



Capacitance Pressure Transducer

Piezoelectric Pressure Transducer

Strain gage pressure transducer

Types of Pressure Sensing Elements



Bellow-Type Detectors

System pressure is applied tothe internal volume of abellows and mechanicallinkage assembly.

As pressure changes, thebellows and linkage assemblymove to cause an electricalsignal to be produced or tocause a gauge pointer to move.



Bourdon Tube-Type Detectors

System pressure is applied tothe inside of a slightly flattened

arcshaped tube. As pressureincreases, the tube tends torestore to its original roundcross-section. This change incross-section causes the tubeto straighten.

Since the tube is permanentlyfastened at one end, the tip ofthe tube traces a curve that isthe result of the change inangular position with respect tothe center. The tip movementcan then be used to position apointer or to develop anelectrical signal.


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Pressure Detector Functional Uses Pressure detectors perform the following basic functions:

- Indication

- Alarm

- Control

If a pressure detector becomes inoperative:

- A spare detector element may be used (if installed).

- A local mechanical pressure gauge can be used (if available).

- A precision pressure gauge may be installed in the system.

Environmental concerns:

- Atmospheric pressure

- Ambient temperature

- Humidity


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PRESSURE DETECTION CIRCUITRY

Resistance-Type Transducers

A strain gauge measures the pressureapplied to a fine wire. The fine wire isusually arranged in the form of a grid. Thepressure change causes a resistancechange due to the distortion of the wire.

This change in resistance is used as thevariable resistance in a bridge circuit thatprovides an electrical signal for indicationof pressure.


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Re: Strain gauge &Wheatstone Bridge Definition

Definition of Strain


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Definition of Strain

Strain is a dimension-less ratio= delta length / lengthStrain is described with units of strain

Usually we measure microstrains a microstrain is where the ratio ofchange in length to length is .000001

In working with microstrains, amplification of strain gage values is very

important

Because hi gain amplification is being used, low pass filtering is importantto remove noise


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Bonded Metal Strain Gage

As the material to which the gage is bonded increases inlength (tension), the cross sectional area of the wire in thestrain gage decreases. As area decreases, the resistanceincreases because resistance is inversely proportional to wirecross sectional area


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Wheatstone Bridge


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Quarter Bridge 1 active arm


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Half Bridge 2 active arms


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Full Bridge 4 active arms

i l l i


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Typical Implementation

L d C ll Li F


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Load Cells Linear Force

Full bridge strain gage to measure strain in materialdue to applied force

M i Li P i i


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Measuring Linear Position Direct change to an electrical element

Moving a wiper/contact to change the resistance in a variable resistor(potentiometer)

Moving a core within an electromagnet (Linear Variable Differential Transformer(LVDT))

Sending a signal and measuring time of flight or time to receive reflection Synchronized reception of time of flight Global Positioning System Reflected radio waves RADAR (radio detection and ranging) Reflected sound waves SONAR Reflected LASER light LIDAR and reflectometery

Shadowing Physical object blocks transmission of a localized sensor

Infrared

Visible Light (Linear Position Encoder)

Laser

Magnetic

Note: with a gear and a gear track, you can convert a linear motion into rotary motion

P iti T d P i P f C


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Position Transducer Price vs Performance Curve

String Potentiometer


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String Potentiometer

Fromwww.spaceagecontrol.com

bl ff l


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Linear Variable DifferentialTransformer (LVDT)

LVDT core centered no signal

Typically core is attached by a shaft to theobject whose position is being measured

Core left magnitude is a function ofposition in same phase as Ein

Core right magnitude is afunction of position in oppositephase as Ein

Time of Flight One Way


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Time of Flight One Way

A signal (or a string of signals) is sent at synchronized time. By determiningtime of flight and dividing by the speed of transmission (speed of light forradio, speed of sound for audio), you can determine the range

Classic example is determining the distance in miles to lightning bymeasuring the difference between the arrival of the light and the sound anddividing by 5

In GPS, we receive a signal along with timing and he location information forthe satellites so we can determine location by time of flight

By receiving signals from 3 satellites we can determine location on surface ofthe earth. By receiving signals from 4 satellites we can determine position andaltitude

Sat 1

RangeSat 2Range

Sat 3Range

GPS Location is wherethree range circlesintersect

Time of Flight - Two way


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g y

Two way position determination involves generating a signaland transmitting it

Signal is either reflected back off the target (passive) or

received and transmitted back by a transponder on the target(interrogated-active)

When the signal is received back at the original transmittingstation, the time is divided by two to generate a time of flightmeasurement

A person shouting down a well and hearing an echo back is aprimitive two way time of flight ranging system

RADAR is Radio Detection and Ranging using radio frequencywaves as the signal

SONAR is Sound Navigation and Ranging active sonargenerates a ping which is reflected back by objects and canbe used to determine range

LIDAR is Light Detection and Ranging and uses a laser beam A home laser rangefinder is a primitive LIDAR

Graphic from howstuffworks.com

Using Blockage - Linear Position Encoder


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Using Blockage Linear Position Encoder

Light sensor(s) that detect the pattern

The pattern is typically a Gray code (only one bit difference between adjacentmeasurements)

Gray code decoded into a distance

Drawing from Alexander Khazan, Transducers and their elements courtesyof www.ni.com Sensors Fundamentals

Angular Position Synchro-Resolver
http://www.ni.com/http://www.ni.com/


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Angular Position Synchro-Resolver WW2 era technology still used in applications today where

ac power is readily available

From www.controlsciences.com

Rotor of resolver follows rotor of synchro

Optical Position Encoder Angular


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Optical Position Encoder AngularPosition


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Incremental Position Encoder from Industrial Electronics by Thomas Kissell located at www.ni.com

Zero location


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Quadrature Encoder using two tracks, 90 degrees out of phase we can determine direction of motion by the order inwhich the tracks sequence


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Early optical encoders when thesewere started up, they had to be rotatedto find the zero location these arecalled incremental encoders

disk

Index (or zero) mark

Pulse Train Output

Graphics from Industrial Electronics by Thomas Kissell located at www.ni.com www.ni.com


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More modern encoder always output currentposition based on encoded bit pattern in thewheel these are called absolute positionencoders position is always determined atpower up

Hall effect switches are often used to resolvelocation

from Industrial Electronics by Thomas Kissell located at www.ni.com

Measurement of Linear Velocity


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Measurement of Linear Velocity

Direct using Doppler

By converting to rotary motion and determiningangular rate tachometer either by countingposition pulses in a fixed time or by using agenerator to generate an analog voltageproportional to rotation

By differentiating a high accuracy estimate ofposition enhances noise

Doppler Determination of Linear Velocity


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pp y

By using a counter to determine frequency, we can compute rangerate the rate by which something is moving towards us or away fromus by

Speed of transmission is speed of light for rf of laser, speed of soundfor sound or pressure waves

By using three observing stations we can determine the velocity in atwo dimensional orthogonal system (x,y).

Doppler effect fromhowstuffworks.com

dt

dS freq freq sourcercvr

*iontransmissof speed

11(

Angular Rate determination


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g Of a shaft

Count pulses to determine rate from a position encoder Generate a voltage by using a dc motor as a generator to generate

a voltage proportional to speed or rotation

Of a body determine rotation rate use a gyro Mechanical gyroscope Ring laser gyroscope Fiber optic gyroscope Solid state gyroscopes (Micro Electro Mechanical Systems MEMS)

Servotek DC Motor Generator Tachometer

Magnetic Rotary Encoder


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Magnetic Rotary Encoder Angular Rate

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Industrial Automation 2