Lecture 2 In The Name of Allah - UM

Lecture 2

Dr. Ali Karimpour Feb 2015

Instrumentation

In The Name of Allah

Dr. Ali Karimpour

Associate Professor

Ferdowsi University of Mashhad

Lecture 2


2

Transducer Specification

Topics to be covered include:

Introduction

Transducers


Transducer Static Specification• Calibration

• Accuracy

• Resolution

• Repeatability

• Hysteresis

• Linearity & sensitivity

Transducer Dynamic Specification• Time constant

• Dead time

• Settling time


Introduction

Transducers

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Introduction (Transducer)

Transducer: Transducer or converter is device that receives information

as one form of an instrument signal and transmits an output signal

as another form(Often electrical).

Sensor: Sensors are often transducers in that they are devices that

convert input energy of one form into output energy of another form.

Transmitter: Transmitters (4 to 20 mA) convert physical measurements

in hazardous areas, such as pressure and flow, into electric signals

which can be transmitted to a controller in a safe area.

Type of Sensor:

Passive

Active

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Introduction

We need to control flow

We need to control

temperature

We need to control level

We need to control

Composition

We need to ……

It is too complicated.

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Introduction

Control with digital

supervisory control

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Introduction

Process with distributed

computer control

Fieldbus is a family of IPC

network protocols used for

real-time distributed control.

PROFIBUS (Process Field

Bus) is a standard for fieldbus

communication in automation

technology.

HMI is human machine

Interface.

HART (Highway Addressable

Remote Transducer Protocol).

Its most notable advantage is

that it can communicate over legacy 4-20 mA.

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Some transducer or sensor

Variable Dielectric Displacement Sensors

Displacement sensors:

Active or passive?

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Potentiometer (variable resistance) sensors


Active or pasive?

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LVDT: Linear Variable

Differential Transformer


Active or passive sensor?

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Rotary contactless position

sensor



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RTD: resistive temperature

detectors

Temperature Sensors:

PT100


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Thermistor



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Thermocouple



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Bimetal



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Force sensors (Stress, Mass, Weight,….)

Motion sensors (Position, Speed, Acceleration, …)

Fluid sensors (Pressure, Flow, Level,…)

………………..

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Introduction

Transducers



• Accuracy

• Resolution

• Repeatability

• Hysteresis



• Dead time

• Settling time

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Calibration

Repeatability

Transducer performance is described by the manufacturer

in two sets of specification.

1- Static specification speak about steady state performance such as:

Accuracy

Hysteresis

Resolution

Linearity & Sensitivity

2- Dynamic specification speak about how quickly the output changes:

Rise time

Time constant

Dead time

Damping

P.O.

Frequency response

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Transducer Static Specification

Calibration

A figure or table in which a known value of measurand are

applied and output reading are recorded.

Is it static or dynamic?

Is it preformed for some or?

Who do it?

Who check it?

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Accuracy, Precision

Accuracy is specified in terms of the percent error.

Percent of full-scale output or as percent of the reading.

More accurateصحت

More preciseدقت

Precision is the spread of value obtained.

Repeatability, Reproducibility

Short term Long term

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Accuracy

Accuracy is specified in terms of the percent error.

Percent of full-scale output or as percent of the reading.

Example 2-1: It is necessary to measure the torque applied

by a hydraulic motor to a shaft. An accuracy of 0.1 ft-lb is

desired. The maximum torque is 20 ft-lb.

A transducer with full-scale rating of 100 ft-lb and 0.2%

accuracy as percent of full-scale available. Is it ok?

Solution: No, since …..

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Resolution

Resolution is the smallest change in the input of the

transducer that leads to a change in the output.

An encoder with

90○ resolution.

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Resolution

Example 2-2: A 2.5 m long vane is rotated slowly in a

circle. The motor and gears are directly connected to a

rotary encoder. It is necessary to know the position of the

vane within 2 cm. What must be the resolution of the

optical encoder?

Answer: Choose an encoder with at least 392.6 pulses per

revolution.

360/((0.02/1.25)*(180/π)))= 392.6 pulses per revolution

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Resolution

Example 2-3: A temperature transducer output 10 mV/ ○C is

used to measure the temperature in a chamber that goes

from 0 to 100 ○C. Can an 8-bit A/D converter with a 5-V

full-scale input be used to produce a 1 ○C resolution?

Answer: In the first glance it appears that the it has not

enough resolution.

But with suitable design ( amplifying by 5) we find 0.39○ C

resolution.

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Repeatability, Reproducibility

It is a measure of how well the output returns to a given

value when the same precise input is applied several times.

Example 2-4: Three load cells are tested

for repeatability. The same50-kg weight

is placed on each load cell 10 times.

The resulting data are given in Table.

Discuss repeatability and accuracy of

each transducer.(Load sell is 0.2 mv/kg)

A: Accurate in some sense but not

repeatable.

B: Repeatable but not accurate.

C: Repeatable and accurate. Z&S

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Repeatability

Some measure for repeatability.

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Hysteresis

Careful observation of the

output/input relationship of a block

will sometimes reveal different

results as the signals vary in

direction of the movement.

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Linearity and Sensitivity

Often calibration curve is nonlinear. So a linear approximation is used.

Sensitivity is the slope of the

calibration graph.

What is output?

Is it voltage or current or … ?

Bias?

Signal conditioning circuits.

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Linearity

There are three ways to derive a linear formula for a sensor.

End point linearity.

Independent straight line linearity.

Least square linearity.

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Linearity

End point linearity.

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Linearity

Independent straight

line linearity.

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Linearity (least square method)

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Linearity

Example 2-5: A calibration record of a load cell is given in the table.

a) Draw calibration curve.

b) Find the end point linearity parameters and +

%FSO and –%FSO error.

c) Find the independent straight line linearity

parameters and + %FSO and –%FSO error.

d) Find the least square linearity parameters

and + %FSO and –%FSO error.

e) Compare b, c and d.

FSO: Full scale output

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Introduction

Transducers



• Accuracy

• Resolution

• Repeatability

• Hysteresis



• Dead time

• Settling time

Lecture 2


34


Calibration

Repeatability

Transducer performance is described by the manufacturer

in two sets of specification.

1- Static specification speak about steady state performance such as:

Accuracy

Hysteresis

Resolution

Linearity & Sensitivity

2- Dynamic specification speak about how quickly the output changes:

Rise time

Time constant

Dead time

Damping

P.O.

Frequency response

The purpose of the transducer is to sense changes in its input so …..

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Transducer Dynamic Specification

Rise time

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Time constant (For first order transducers)

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Dead time

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Settling time (For second order transducers)

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Cut off frequency

Lecture 2


References

Industrial Control Electronics J.M. Jacob, Prentice-Hall,

1989

مبانی اندازه گیری در سیستمهای ابزار دقیق، حمید رضا تقی راد و سید

1392علی سلامتی، انتشارات دانشگاه صنعتی خواجه نصیرالدین طوسی،

Principles and Practice of Automatic Process Control,

Third edition, C. A. Smith, A. Corripio, Wiley 2006

Instrument Control Engineers' Handbook Fourth

Edition, Béla G. Lipták, editor-in-chief. CRC Press 2003

40

Lecture 2


Some Useful websites for the course

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http://saba.kntu.ac.ir/eecd/ecourses/instrumentation.htm

http://profsite.um.ac.ir/~shoraka/Instrumentation.htm

http://karimpor.profcms.um.ac.ir/index.php/courses/10328

Documents

Lecture 2 In The Name of Allah - UM