Industrial Control System

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Industrial Control System

Industrial

Control System

Instrumentation of a technological process

dr inż. Anna Czemplik

(na prawach rękopisu)

1


Introduction

Industrial Control System (ICS) usually performs the following tasks:

1) an instrumentation of a technological process

2) a data acquisition and a process control

3) a data transfer

4) human-machine interface (HMI)

These tasks correspond to the following subsystems of ICS:

I. measuring devices and actuators

II. field devices

III. a communication infrastructure

IV. supervisory computers (a complex system)

2


Contents

The main topics:

I. Measurement devices

II. Actuating elements

III. Field devices - controllers

IV. Communication networks

V. SCADA & DCS

}

Instrumentation of a technological process

15 hours

}

15 hours

What is it?

How does it work?

Definition

Principle of operation

Main features

3


Topics

I. Measurement devices

Ia. Sensors

• Ia-1. Thermometers: a) Expansion thermometer; b) Pressure spring thermometer; c) Resistance thermometer; d) Thermoelectric thermometer;

e) Optical thermometer. Selection of thermometer and measuring circuit

• Ia-2. Displacement and force sensors: a) Resistance displacement sensor; b) Piezoelectric displacement sensor; c) Piezomagnetic

displacement sensor; d) Inductive displacement sensor; e) Capacitive displacement sensor; f) Hall effect displacement sensor

• Ia-3. Manometer: a) Hydrostatic manometer; b) Hydraulic manometer; c) Elastic manometer; d) Electronic manometer (strain gauge, inductive,

capacitive); e) Manometer with a force sensor. Selection of manometer

• Ia-4. Level indicator: a) Water-level indicator; b) Float level gauge: c) Hydrostatic level gauge; d) Displacer level gauge; e) Ultrasonic level

gauge; f) Radar level gauge; h) Another l.g. (capacitive, eletrical, thermometer). Selection of level gauge

• Ia-5 Flowmeter: a) Differential pressure flowmeter; b) Rotameter; c) Velocity-type flowmeter; d) Positive displacement flowmeter; e) Inductive

flowmeter; f) Ultrasonic flowmeter; g) Calorimetric flowmeter; h) Vortex flowmeter; i) Coriolis flowmeter; j) Open channel flowmeter. Selection of

flow meter

• Ia-6. Speed: a) Tachometer; b) Digital speed sensor

• Ia-7. Relays: a) Non electrical relays; b) Electrical relays and switches

• Ia-8. Physico-chemical properties: a) pH meter, ...

Ia. Sensor selection

Ib. Converters

• Ib-1. Measuring converters – types and selection

• Ib-3. Separating converters

• Ib-4. Analog-to-digital converters

• Ib-5. Digital-to-analog converters

Measuring devices; Instrumentation and control tag

II. Actuating elements

IIa. Final control element

• IIa-1. Valves

• IIa-2. Pumps

IIb. Actuators

• IIb-1. Pneumatic actuator

• IIb-3. Electric actuator - Electric motors: a) Brushed DC electric motor; b) Synchronous motor; c) Inductive motor

4

Electro-mechanical drive system


Fundamental definitions

Block diagram of a control system

A/D

converter

supervisor system

communication system

controller

control devices

D/A

converter

measurement

devices

measuring

converter

sensor

(sensing element)

actuator

(actuating driver)

final control

element

actuating

equipment

plant

5


Part I. Measurement devices

I. Measurement devices

a) Sensors

1 - temperature,

2 - displacement,

3 - pressure,

4 - level,

5 - flow,

6 - rotational speed,

7 - relays,

8 - electrochemical

b) Converters

1 - measuring,

2 - signaling,

3 - separating,

4 - A/D,

5 - D/A

c) Control engineering design

c

A/D

converter

measuring

converter

sensor

(sensing element)

supervisor system

communication system

controller

plant

D/A

converter

actuator

(actuating driver)

final control

element

I. Measuring devices/ 6


Ia. Sensors

Sensor (transducer, measuring converter) - a device that converts a physical (chemical,

biological, ...) quantity into another signal (usually an electric signal). There are many

types of sensors base on different principles.

Measurement of a basic physical quantity:

1) temperature

2) displacement and force (also stress, strain)

3) pressure (also pressure difference)

4) level (also volume)

5) flow

6) rotational speed

and another:

7) relays (two-stage transducer)

8) electrochemical transducer

} amount-related measurement

} force-related measurement

More: http://www.omega.com/literature/transactions/

http://www.sensorland.com/

Ia. Measuring devices/ 7


Ia-1a. Expansion thermometer

Principle: Thermal expansion of liquids or solid bodies

liquid-in-glass

e.g. mercury-in-gas thermometer

or alcohol thermometer

mechanical

• bi-metallic

• elongation-type

The difference in thermal expansion in the two metals

leads to a difference of lenght or to a twist of element

in proportion to the temperature

With increasing temperature,

the volume of liquid expands

and the meniscus moves up the capillary.

Ia. Measuring devices/ 8


Ia-1b. Pressure spring thermometer

(gas thermometer)

Principle: The relation between temperature and pressure in a constant volume

Types:

• liquid filled (mercury, ethyl alcohol, ...)

• gas filled (nitrogen, argon, helium)

• vapor pressure (volatile liquid)

capillary

manometer

(spring-type pressure gauge)

thermometric

bulb

The bulb is immersed in a heated substance.

The liquid (gas) expands causing the pressure spring to unwind.

Ia. Measuring devices/ 9


Ia-1c. Resistance thermometer

Principle: Temperature dependence of resistance

metallic

Pt 100, Ni 100 (i.e. 0ºC = 100Ω )

bifilar winding of metallic t.

• platinum

• nickel

• copper

Metal resistance increases under the influence of temperature

thermistor (semiconductor)

NTC, PTC, CTC

R

platinum

PTC

NTC

ceramic

PTC

Temperature dependence of resistance

T

Ia. Measuring devices/ 10


Ia-1d. Thermoelectric thermometer

metal A

T 1

T 2

metal B

(thermocouple, thermoelement)

Principle: Thermoelectric effect - if junctions of two different metal have a different

temperatures than a voltage is generated.

E

[mV]

1

1) Fe-konstantan

2) NiCr – Ni

3) PtRh - Pt

E=f(T 1 -T 2 )

2

3

measuring junction

(hot junction)

connecting

head

500 1000 1500

°C

The construction of joint and shield Ia. Measuring devices/ 11


Ia-1e. Optical thermometer

(pyrometer)

Principle: Measurement of thermal radiation emitted by any matter with a temperature

greater than 0ºK

global radiation

monochromatic

two-colour

Measuring area

Non-contacting measurement

based on an optical system and a detector

Ia. Measuring devices/ 12


1) Temperature range:

Ia-1. Selection of thermometer

0 ºC

expansion -200 500

pressure spring -50 700

resistance -270 900

thermoelectric -100 1600

optical 400

(The main selection criteria)

2700

2) Contact or non-contact

Ia. Measuring devices/ 13


Ia-1. Measuring circuit

for the resistance thermometer

R1 R2

2-wire circuit

U z R 0

R w R t

R3

connecting wiring

The connecting wiring

are added to

the measured resistance

R1 R2

3-wire circuit

U z R 0

R w R t

R3

connecting wiring

The connecting wiring

occur in

two legs of the bridge

Ia. Measuring devices/ 14


Ia-1. Measuring circuit

for the thermoelectric t.

Simple circuit

hot

junction

e

cold junction

t 0

R1

R2

mV

Rt

R3

U z

Circuit with the cold junction compensation

hot

junction

e 3

A

e 1

B

e 2

A

e 4

t 0

thermostat

cold junctions

C

C

e=e1-e2 (e2=const), e3 = - e4

mV

Ia. Measuring devices/ 15


Ia-2. Displacement and force sensors

Measurements of displacement (position) and force are similar on account of that the

displacement is a result of some force.

The force-related measurements contain a wide gamut of sensors for measuring:

• stress (calculated by dividing the force applied by the unit area)

• strain (defined as the deformation per unit length)

• weight (the force on the object due to gravity)

• acceleration (accompanied by a force)

• torque (moment of force)

• pressure (by definition the force per unit area)

Ia. Measuring devices/ 16


Ia-2a. Resistance displacement sensor

Principle: The resistance depends on the geometry of the resistor (and the resistivity of

the material)

potentiometer

• linear

• rotary

l

ϕ

The linear or angular motion of a wiper

is converted into a changing resistance of potentiometer

strain gauge

R=rl

l

R=rϕ

The gauge is attached to the object by a suitable

adhesive. As the object is deformed, the foil is deformed,

causing its electrical resistance to change.

Ia. Measuring devices/ 17


Ia-2b. Piezoelectric displacement sensor

Principle: Piezoelectric effect - some materials (e.g. quartz) generate a voltage under

influence of a mechanical stress.

In most cases, the same element can be used as:

• piezo sensor that converts mechanical energy into electrical energy (it is referred to as "generators“)

• piezo actuators that converts electrical energy to mechanical energy (it is referred to as "motors“)

polarization

disk compressed:

generated voltage

has the same polarity

as poling voltage

disk stretched:

generated voltage

has polarity opposite

that of poling voltage

applied voltage

has the same polarity

as poling voltage:

disk lengthens

applied voltage

has polarity opposite

that of poling voltage:

disk shortens

[http://www.americanpiezo.com/knowledge-center/piezo-theory/piezoelectricity.html]

Ia. Measuring devices/ 18


Ia-2c. Piezomagnetic displacement sensor

Principle: Mechanical strain has an influence on a magnetization of ferromagnetic

materials

coil sensor

F

Measurement of inductance

L

transformer-type sensor

F

mA


Measurement of current in the secondary circuit

consisting of two push-pull winding

(a differential measurement)

Ia. Measuring devices/ 19


Ia-2d. Inductive displacement sensor

Principle: Displacement of a part of core involves changes in inductance

coil sensor


L≈sµ 0 W 2 /δ

s

δ

differential coil sensor




s

δ

1) Measurement of inductance

2) Measurement of eddy current

(eddy current linear encoder)

transformer-type sensor

1 ∼ x

δ

2 ∼

differential transformer-type sensor

2

U 2


3

x

The target is part of magnetic circuit

The primary winding are energised with a

constant amplitude A.C. supply. This

produces an alternating magnetic field in the

core and induces a signal into the secondary

winding (2,3) depending on the position of

the core.

Ia. Measuring devices/ 20


Ia-2e. Capacitive displacement sensor

Principle: Displacement of a capacitor plate involves changes in capacity

parallel-plate c.

rotary c.

cylindrical c.

x

differential parallel-plate c.

ϕ

x

x

Sensor

x

Target

The target is one plate of the capacitor

Ia. Measuring devices/ 21


Ia-2f. Hall effect displacement sensor

Principle: Hall effect – an electric current in the conductor placed in a magnetic field

causes a voltage difference (the Hall voltage)

N

-

U H =kIB

I+

S

x

U H

The Hall voltage is developed between the two

edges of a current-carrying conductor whose faces

are perpendicular to an applied current flow.

Ia. Measuring devices/ 22


Ia-3a. Hydrostatic manometer

(Hydrostatic pressure gauge)

Principle: Hydrostatic equilibrium between the pressure and the hydrostatic force per

unit area at the base of a column of fluid

U-tube

(U-pipe)

p 1 p 2

h

p 1 -p 2 =gh(ρ 1 -ρ 2 )

ρ 2 ρ 1

The pressure is indicated by the difference in levels in

the two arms of the tube

float sensor

p

The pressure causes a change of liquid

level and the float transfers it to an

indicator

Pipes and tubes are not the same

Pipe: The purpose with a pipe is the transport of a fluid like water, oil or similar, and the most import property is the capacity or the inside

diameter.

Tube: The nominal dimensions of tubes are based on the outside diameter. The inside diameter of a tube will depend on the thickness of the

tube. The thickness is often specified as a gauge.

Ia. Measuring devices/ 23


Ia-3b. Hydraulic manometer

(Hydraulic pressure gauge)

Principle: Hydraulic equilibrium of the pressure and another force

piston pressure gauge

y

bell pressure gauge

(bell-type manometer)

p

ϕ

The balance the

force of pressure

and bouyant force

p

p

cy+mg=pA

The force of pressure

is in balance with the spring

p 1 p 2

ring differential manometer

(ring balance)

p

p 1 p 1

2

p 2

p 2

p 1 -p 2 =ghρ

Ia. Measuring devices/ 24


Ia-3c. Elastic manometer

(Spring-type pressure gauge)

Principle: Equilibrium of the pressure and a spring force

Bourdon tube

(tube pressure gauge,

spiral pressure gauge)

The curved tube is open to external

pressure input on one end and is

coupled mechanically to an indicating

needle on the other end. The external

pressure is guided into the tube and

causes it to flex.

p

diaphragm pressure gauge

p

Sensor uses the deflection

of a flexible membrane

that separates regions of

different pressure

bellows pressure gauge

p

The bellows is stretched on

pressure influence

Ia. Measuring devices/ 25


Ia-3d. Electronic manometer

Principle: Conversion the pressure to a displacement or a mechanical stress and next an

electric measurement of this displacement.

strain gauge

(measurement of resistance)

Displacement sensors and manometers

differ in a process connection

inductive

capacitive

(measurement of inductance)

(measurement of capacity)

differential capacitive

p

p 2

p 1

Differential manometer with

an elastic membrane is a

double capacitor

Ia. Measuring devices/ 26


Ia-3e. Manometer with a force sensor

Principle: Conversion the pressure to a force or a strain and next an electric

measurement this parameter.

with piezoresistive

strain gauge

with piezoelectric sensor

with strain gauge

with strain gauge

Ia. Measuring devices/ 27


Ia-3. Selection of manometer

1) Type of measured pressure

• absolute pressure is zero-referenced against a perfect vacuum, so it is equal to

gauge pressure plus atmospheric pressure.

• gauge pressure is zero-referenced against ambient air pressure, so it is equal to

absolute pressure minus atmospheric pressure.

• differential pressure is the difference in pressure between two points.

2) Compromise between an accuracy and susceptibility to overload

3) Inertia – measurement of slow/fast pressure changes

(The main selection criteria)

4) Process connection

A 1 2

A 1 2

h 2 -h 1

h h 2

2

h 1

h1

h 2 -h 1

p 1 =p 2 p 1 >p 2

h 2 -h 1 – the pressure arisen from a liquid flow

Assumption: no pipe resistance

Ia. Measuring devices/ 28


Ia-4a. Water-level indicator

Principle: Communicating vessels

glass level gauge

(tubular level gauge)

magnetic level indicator

It is perfect for high temperature and pressure

applications in case sight glasses and indicating

glass parts cannot be used for safety reasons

Ia. Measuring devices/ 29


Ia-4b. Float level gauge

Principle: Liquid level float is buoyant in liquid and indicates the level

inductive

Float causes a displacement

of a coil core

resistance

Float moves

on a linear resistor

Ia. Measuring devices/ 30


Ia-4c. Hydrostatic level gauge

(Manometric level gauge)

Principle: The static pressure in the bottom is proportional to the liquid column in the tank

∆P

p atm

h 1

∆P=p 1 -p atm

∆P=gρh

The pressure at a given depth in a static liquid

depends upon the density of the liquid and the

distance below the surface of the liquid

plus any pressure acting on the surface of the

liquid

bell-type

capacitive

∆P

Ia. Measuring devices/ 31


Ia-4d. Displacer level gauge

(Buoyancy transmitter)

Principle: Archimedes' Principle - a body which is completely or partially submerged in

a fluid experiences an upward force (the buoyant force)

F

Weighing of

the displacer element

Buoyant force moves a coil core

and changes its inductance

Intelligent level transmitters based on

Archimedes buoyancy principle are

designed to measure liquid level,

interface and density.

Sensor

in a side-and-bottom chamber

Sensor

without

chamber

Ia. Measuring devices/ 32


Ia-4e. Ultrasonic level gauge

Principle: Reflection of high frequency acoustic waves

The sensors emit waves (20 kHz to 200 kHz) that are reflected

back to and detected by the emitting transducer. The elapsed time

period between transmission and reception of the signal - at the

speed of sound - is measured and calculated as a distance and

computed into level or volume.

In order to improve the accuracy of

measurement it is important to take into

account a moisture, temperature, and

pressure changing speed of sound.

Example applications

Ia. Measuring devices/ 33


Ia-4f. Radar level gauge

Principle: Reflection of microwaves

(Microwave sensor)

The sensors emit waves (1 GHz to 30 GHz ) and measure the time

period between transmission and reception of the signal.

Speed of microwave is independent of moist,

vaporous, dusty, and temperature environments

Ia. Measuring devices/ 34


Ia-4h. Another level gauge

Principle: Property of sounder depends on its draught in liquid

Capacitive l.g.

Sounder is a long capacitor and his capacity depends on draught

Electrical variable-resistance l.g.

Sounder is a superconductor and his resistance

depends on draught in a low temperature liquid

Thermometer level gauge

Draught cools the measuring element, e.g. the resistance thermometer

Ia. Measuring devices/ 35


Ia-4. Selection of level gauge

(the main selection criteria)

1) Type of medium (phase): liquid, solid or slurry

2) Properties of medium, e.g. dielectric constant, density

3) Conditions of measurement, e.g. temperature, pressure (or vacuum)

Ia. Measuring devices/ 36


Ia-5a. Differential pressure flowmeter

(Orifice flowmeter)

Principle: Bernoulli’s principle – an obstruction inserted in the flow causes a pressure

drop proportional to the square flow speed.

orifice plate

flow nozzle

∆p

Pressure sensor measures the differential pressure before and within the constriction

Venturi tube

(Venturi meter)

Nozzle and tube offer advantages over orifice plates in that they require less

upstream piping and incur lower permanent pressure loss.

Ia. Measuring devices/ 37


Ia-5b. Rotameter

Principle: Balance between the flowing force and the weight of the float

The rotameter consists of a vertically oriented glass (or plastic) tube with a larger

end at the top.

The substance flows through the meter vertically from bottom to top and lifts the

float proportionally to the flow quantity.

plastic

metal

glass

Ia. Measuring devices/ 38


lade f.

Ia-5c. Velocity-type flowmeter

counter

(Rotating meter)

Principle: The fluid flow actuates the movement of blades, screw or turbine-type impeller

proportionally to flow rate.

The flow is calculated by

measuring and integrating

the flow speed over the

flow area

screw f.

1

2 impeller

impeller

counter

turbine f.

Ia. Measuring devices/ 39


Ia-5d. Positive displacement flowmeter

Principle: Counting repeatedly the filling and discharging of known fixed volumes

A typical positive displacement flowmeter comprises a chamber that obstructs the

flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to

create fixed-volume discrete parcels from the passing fluid.

piston

four-way

valve

Piston is operated to fill a cylinder with the fluid

and then discharge the fluid. Each stroke

represents a finite measurement of the fluid

See also: http://www.efunda.com/designstandards/sensors/flowmeters/flowmeter_pd.cfm

Ia. Measuring devices/ 40


Ia-5e. Inductive flowmeter

(electromagnetive)

Principle: Faraday's law of electromagnetic induction - when a conductor moves through

a magnetic field then a voltage will be induced

electromagnet

electrode

The liquid serves as the conductor and the

magnetic field is created by energized coils

outside the flow tube. The inducted voltage is

detected with the aid of an electrode.

It can only be used for electrical conductive fluids as water.

Ia. Measuring devices/ 41


Ia-5f. Ultrasonic flowmeter

(Ultrasonic Doppler flowmeter)

Principle: Doppler effect - The frequency of the reflected signal is modified by the

velocity and direction of the fluid flow

scheme

Z V

By measuring the frequency shift between the

ultrasonic frequency source, the receiver, and the

fluid carrier, the relative motion are measured.

Doppler meters may be used where other meters don't work.

It can be installed outside the pipes (do not obstruct the flow )

It is sensitive to changes in density and temperature the fluid.

Ia. Measuring devices/ 42


Ia-5g. Calorimetric flowmeter

Principle: Intensity of cooling depend on the flow rate of the fluid

Two temperature sensors are in close contact with the fluid but thermal insulated from each

other. The flowing fluid cools both sensors but one of the two sensors is constantly heated.

The temperature difference between the two sensors is proportional to the flow rate.

Ia. Measuring devices/ 43


Ia-5h. Vortex flowmeter

Principle: Karman effect - an obstruction in a fluid flow creates vortices in a downstream

flow

Vortices cause a local disturbance of pressure

detected by the sensor. Frequency of vortices is

proportional to the flow rate

Karman vortex street

Animation http://en.wikipedia.org/wiki/File:Vortex-street-animation.gif

Ia. Measuring devices/ 44


Ia-5i. Coriolis flowmeter

Principle: Coriolis effect -

It is a direct measurement mass (not sensitive to changes in pressure, temperature, viscosity and density )

The fluid runs through a U-shaped

tube that is caused to vibrate in an

angular harmonic oscillation. Due

to the Coriolis forces, an

additional vibration arise that

deform the tube

Animation: http://www.emersonprocess.com/micromotion/tutor/42_densityoperatingprincipal.htm

Ia. Measuring devices/ 45


Ia-5j. Open channel flowmeter

Principle: An obstruction inserted in the flow causes a backwater

A common method of measuring flow through an open channel is to

measure the height of the liquid as it passes over an obstruction as a

weir or flume in the channel.

Common used obstruction types:

• the sharp-crested weir,

• the V-notch weir,

• the Cipolletti weir,

• the rectangular-notch weir,

• the Parshall flume

• Venturi flume.

sharp-crested weir

c

V-notch weir

Venturi flume

Ia. Measuring devices/ 46


Ia-5. Selection of flow meter

1) If the flowrate information should be continuous or totalized?

2) Type of medium: steam, gas, liquid

3) Properties of medium: viscosity (Reynolds number), density

4) Conditions of measurement, e.g. pressure, temperature

(The main selection criteria)

5) Unit

• m 3 /s (volumetric flow rate, volume flow rate, rate of fluid flow, volume velocity)

• kg/s (mass flow rate)

Ia. Measuring devices/ 47


Ia-6a. Tachometer

(Rate generator)

Principle: A small ac/dc generator that develops an output voltage proportional to its rpm

The rotor of the tachometer is mechanically connected, directly or indirectly, to the load .

dc generator

ac generator

U 1

U zasil

e g

Φ 1

e g

The dc rate generator often has

permanent magnetic field excitation.

The ac rate generator field is excited

by a constant ac supply

The phase or polarity of output voltage (e g ) depends on the rotor's direction of rotation

Ia. Measuring devices/ 48


Ia-6b. Digital speed sensor

Principle: A pulse generator plus a pulse counter

photoelectric

transformer-type

f n

ω

ω

relay-type

(reed swich)

N S

f z U z

.

magnetoresitive

(rotational speed

sensor)

S N

f n

Ia. Measuring devices/ 49


Ia-7a. Non electrical relays

(Two-stage transducers)

Principle: Overflow of a definite input value causes an abrupt change of output value

(usually closure of contacts used e.g. to direct control)

Measurement sensors used to a detection of only two-stage.

liquid level switch, e.g.

- vibrating fork liquid level switch (submergence, filling)

- float switch (exceeding the level)

pressure rise relay, e.g.

U-tube (mercury join contacts after the overflow of pressure)

temperature rise relay, e.g.

- bimetallic switch (overflow of temperature)

acoustic

- microphone (detection of sound intensity, frequency)

light-, ...

- photodiode, photoresistor (proximity detector )

Ia. Measuring devices/ 50


Ia-7b. Electrical relays and switches

Principle: Relay - overflow of a definite input value causes a closure of contacts

Switch is operated by the motion of a machine part or presence of an object

electrical activated device

reed switch

N S

close relation

N S

turn

N S

tern

tern

limit switch

Ia. Measuring devices/ 51


Ia-8. Physico-chemical properties

The main type of measurement:

a) pH → pH meter

b) redox potential (oxidation/reduction potential, ORP) → ORP meter

c) humidity → hygrometer

d) oxygen (proportion of O 2

in the gas or liquid ) → oxygen meter, lambda sensor

e) conductivity → conductometer

f) suspension → densitometer, suspension turbidity meter

g) water hardness

h) concentration → refractometer

....

Ia. Measuring devices/ 52


Ia-8a. pH meter

Principle: The measurement bases on an electrode made of a doped glass membrane

that is sensitive to a specific ion

- concentration of hydrogen ions (H + )

- in practice from 10mol/l to 10-15 mol/l

10 -14

10 -7 mol/l 10 0

14 7 0

basic solution

H 2 O

pH

acidic solution

A typical pH probe consists of a

combination electrode, which combines

both the glass and reference electrodes

into one body. The probe produces a

small voltage (about 0.06 volt per pH

unit) that is measured and displayed as

pH units by the meter

The ph meter requires a cleaning and a frequent calibration

because the glass electrode does not give

a reproducible e.m.f. over longer periods of time

Ia. Measuring devices/ 53


Ia. Sensor selection

The main directions

Procedure of the sensor selection:

1) statement of the main requirements for sensor

2) review of available sensors from the point of view of fulfilment of requirements

Requirements:

measurement value (temperature, pressure, ..)

range

accuracy class

mounting of the instrument (process connection, location)

frequency response

environmental condition (e.g. Ex, dustiness, moisture)

operational reliability (e.g. periodical calibration)

dimensions, weight

complexity of additional equipment

qualification of service staff (method of calibration or programming)

price of sensor and an additional apparatus

resolution of the measured signal

Ia. Measuring devices/ 54


Ib. Converters

Definition and classification

Classification of converters according to function:

1) measuring

• conversion of a sensor signal into a standard signal

• typical electric standard signals: 0-5mA, 0-10mA, 0-20mA, 4-20mA, 0-10V

2) signaling

• matching circut - exchange of one standard to another

• current-current, voltage-voltage, current-voltage, voltage-current, currentpressure

(intersystem converter);

3) separating

• assurance of the galvanic isolation between functional sections of system

• the same standard of input and output and the gain equal 1

4) analog-to-digital converter (ADC, A/D, A to D)

• conversion a continuous quantity to a discrete time representation in digital form

• typically the digital output is a two’s complement binary number

5) digital-to-analog converter (DAC, D/A, D to A)

• conversion of a digital (usually binary) code to an analog signal (current, voltage

or electric charge)

Ib. Measuring devices/ 55


Ib-1. Measuring converters

a sensor signal to the standard signal

According to input value:

• converter of force, voltage, resistance, pressure

According to principle of operation:

• parametric, generating

According to construction (electric circuit)

• open circuit (without a feedback)

• close circuit (with a feedback)

According to modulation of output signal:

a) modulation of direct current level

b) frequency modulation

c) discrete output with modulation pulse-width (PWM)

Ib. Measuring devices/ 56


Ib-1a. Measuring converters

with modulation of direct current level

Examples:

measuring converter of a resistance

measuring converter of a small voltage

R r

U s

R b

U s

R f

R r

I out

U s

R s

R b

R f

R s – resistance of sensor; U s – voltage of sensor

R b – balancing resistance of the circuit; R f – feedback resistance; R r – receiver

Ib. Measuring devices/ 57


Ib-1b. Measuring converters

with frequency modulation

Types:

generating

• tachometer generator

modulating

• digital speed measurement

} position-type

oscillatory with a forced vibration

oscillatory with a free vibration

in MC OS C out

out

f

=

1

2

F

ml

AC

A

l

F

MC – matching, AC – activation

OS - oscillatory system

C out – output converter

string-type converter

(the force F into the frequency f)

Ib. Measuring devices/ 58


Ib-1c. Measuring converters

with pulse-width modulation

Example:

U out

U A

T

LG

C

U

U X

U LG

U A

U LG

U X

U C

LG – linear generator (g. of linear signal)

U A – activation; C - comparator

U x – input voltage

U out – output voltage

U out

t i

Ib. Measuring devices/ 59


Ib-1. Measuring converter selection

Requirements:

a suitable static characteristic (linear or non-linear)

stability of characteristic

a small conversion error (e.g.


Disturbing signals:

serial voltages

Ib-3. Separating converters

• result of a inductive coupling between two wires

• primarily frequency of 50Hz and 100Hz

Suppression with the help of low-pass filter

passing the measured signal (frequency


Ib-3. Separating converters

Application of galvanic separation

UP U I

I I

sys 1

I 2

UP U I

I I

I 1

galvanic separation

200

100

200

2

6

3

200

200

4

5

transformer-type

Realization of galvanic separation

GZ

optoelectronic

I in

x in

= ∼

M

TO


D

=

x out

I out

Ib. Measuring devices/ 62


Ib-4. Analog-to-digital converters

A/D

Conversion process

u(t)

Analog signal

(continues in both

time and amplitude)

Sampling

u[k]

Sampled-date signal

(discrete in time and

continues in amplitude)

Quantization

Discrete time

discrete amplitude

signal

Encoding

U x

Parameters of A/D converter:

• range of input signal

• resolution (bits) - quantization error (%)

10 bits = 210 = 1024 qantums= 0,1%

12 bits = 212 = 4096 qantums = 0,025%

• sampling rate (sampling frequency)

• conversion time (for one sample)

If a single converter services n inputs, than sampling rate = 1 / (n*conversion time)

Digital signal

t

Minimum of sampling rate (Shannon-Kotielnikov sampling theorem*)

- theoretically: f s >= 2f w

f s – sampling rate,

- practically: f s >= 2f b , (f b =10f w )

f x – the highest frequency of the original signal

f b – used pass band (gain>=0.7)

*Shannon-Kotelnikov, Whittaker–Nyquist–Kotelnikov–Shannon

Ib. Measuring devices/ 63


Ib-4a. Analog-to-digital converters

Integrating ADC (dual slope ADC)

U x

U w U 1

US

Types

U 1

K U 2 U 3

L

T 1 T 2

t

GW

U 2

U

N x = N x

t

max

U w

U 3

N max N x t

U x Uw

T 1 T 2

Ib. Measuring devices/ 64


Ib-4b. Analog-to-digital converters

ramp-compare ADC

U

U X

frequency-type ADC

c fx

Forming Gate Counter

frequency

input

Controler

Cancel

Types

N x

U out

t i

t

Pulse generator

counting f x

for a determinate

time period

a direct-conversion ADC (flash ADC)

a successive-approximation ADC

a delta-encoded ADC or counter-ramp

a pipeline ADC (a subranging quantizer)

a sigma-delta ADC (a delta-sigma ADC)

a time-interleaved ADC

an ADC with intermediate FM stage

...

Ib. Measuring devices/ 65


Ib-5. Digital-to-analog converters

In: number X= X 0

2 0 + X 1

2 1 + ... + X n

2 n

weigh-resistive

U

2 1 R 2 2 R 2 i R

1 1 1

0 0 0

X 0 X 1 X i

U out

if X i

=1 then switch=1

U

2 1 R

2 2 R

2 i R

DAC

Out: signal U out

U out

voltage ladder

2R

2R 2R 2R

U

2R

2R

U

1 1 1

0 X 0 0

0 X 1 X i

U out

2R

2R

U out

...

Ib. Measuring devices/ 66


Ic. Measuring devices

classical measuring transducer (converter)

= sensor [+ measuring converter]

A

D

controller

sensor.

measuring c. separating c. A/D

intelligent measuring transducer (converter)

= sensor + separator + ADC + uP = communication port

A

D

µP

controller

sensor communication port

Ic. Measuring devices/ 67


Ic. Instrumentation and control tag

Graphical symbols and identifying letters in control engineering design

TRCA

154

H

process parameter

function: R - recording

I - indication

C - control

A - alarm

alarm

specification

TRCA

154

programmable

device

D – density

F – flow rate

G – distance, length, position

L – level

P – pressure

Q – material properties

T – temperature

W – velocity, mass

TRCA

154

configurable

device

For further details, see DIN 19227

More: http://www.samson.de→Services→Technical Information

Ic. Measuring devices/ 68


Ic. Instrumentation and control tag

US Standards:

ANSI Y32.2.3 Graphical Symbols for Pipe Fittings, Valves and Piping

ANSI Y32.2.11 Graphical Symbols for Process Flow Diagrams

ISA 5.5 Graphical Symbols for Process Displays

Standards

British Standards:

BS: 1646 1-4 Symbolic Representation for Process Measurement, Control

Function and Instrumentation

German Standards:

DIN 19227 P1-P3 Graphical Symbols and Identifying Letters for Process

Measurement and Control Functions

Polish Standards:

PN-M-42007 (archive)

More: http://enormy.pl

http://www.samson.de→Services→Technical Information

Ic. Measuring devices/ 69


Part II. Actuating equipment

II. Actuating equipment

a) Final control element

1 – valve

2 - pump

b) Actuator

1 - electrical motors

2 - pneumatic actuator

3 - hydraulic actuator

c

A/D

converter

measuring

converter

sensor

(sensing element)

supervisor system

communication system

controller

D/A

converter

actuator

(actuating driver)

final control

element

controling

system

plant

controlled system

Examples:

Actuating equipment (final control equipment): Control valve:

- final control element - valve (closure element, body of valve)

- actuator - actuating driver

- positioner (measuring element)

Body of valve manipulates the mass and energy flow.

The opening or closing of control valve is usually done by electrical, hydraulic or pneumatic actuator.

Positioner is used to control the opening or closing of the actuator based on electric, or pneumatic signals.

IIa. Actuating equipment/ 70


IIa. Final control element

Final control element – a part of the controlled system that manipulates the mass and

energy flow.

Basic type of final control element:

1) valves 2) pumps

Classification valves according to function:

• control valve

• throttling (choke) valve

• gate (sluice) valve

• safety-valve

• reflux valve

Classification pumps according to principle of operation:

• positive displacement pump

• impulse pump

• velocity pump

• gravity pump

Classification valves according to construction:

• ball (globe)

• rotary (butterfly)

• knife

• neadle

• flap

IIa. Actuating equipment/ 71


IIa-1. Valves

Valve – a device that manipulates the mass flow on basis of a throttling.

ball valve

(globe valve)

Inside a spherical disc

Features: simplicity, sealing

V-port ball valve

(V-notch valve; a segmented ball valve )

Inside a spherical disc with a notch

Features: simplicity, sealing, precise control

butterfly valve

(quarter-turn valve)

Inside a metal disc mounted on a rod

and positioned in the center of the pipe

Features: low cost, light

knife gate valve

More: http://www.valtorc.com→Valves

IIa. Actuating equipment/ 72


IIa-2. Pumps

Pomp – a device used to move fluids (liquids, gases, slurries) by mechanical actions

(often a reciprocating or rotary mechanism).

positive displacement pump

velocity pump

The pump moves a fluid by trapping a fixed

amount of it and then forcing (displacing) that

trapped volume into the discharge pipe

(rotodynamic pump, dynamic pump)

The pump increases the flow velocity thereby

kinetic energy and this energy is converted to

pressure

screw

centrifugal

lobe

Operation under closed valve conditions

The positive displacement pump physically displaces the fluid

resulting in a continual build up in pressure and finally

mechanical failure of either pipeline or pump

The velocity pump can be safely operated

under closed valve conditions

impulse pump

The pump use pressure created by gas (usually air) and pushing part of the liquid upwards

IIa. Actuating equipment/ 73


IIb. Actuators

(effectors, servomotor)

Actuator - a type of motor for moving or controlling a mechanism (final control element).

It is operated by a source of energy (an electric current, hydraulic fluid pressure,

pneumatic pressure) and converts this energy into some kind of motion.

Actuator processes and amplifies the output signal of controller

Basic type of actuator:

1) pneumatic actuator

2) hydraulic actuator

3) motor-driven actuator (electrical servomotors)

Actuators are also known as:

• effectors (in robotics)

• servomotor – linear actuator, rotary actuator

IIb. Actuating equipment/ 74


IIb-1. Pneumatic actuator

Principle: Pneumatic actuator converts energy of compressed air into a mechanical motion

diaphragm actuator

bellow actuator

Actuator that has a chamber divided in half by a

diaphragm that separates areas with different

pressure levels.

pneumatic cylinder

Self-acting thermostatic actuator

(e.g. used for temperature control)

More: http://heating.danfoss.com

IIb. Actuating equipment/ 75


IIb-3. Electric actuator

Definitions:

• actuator - a device converting a low-power signal into a large-force displacement

(linear or rotary )

• motor – a device converting a heat, electrical energy, mechanical energy into energy

to drive machines (usually electrical energy into energy of rotational motion)

Actuator

• a large force

• a small velocity

Motor

• a small torque

• a high velocity

Electric actuator (servomotor):

a) an electric motor + a gear

b) an electric motor giving a suitable displacement (e.g. stepper motor)

IIb. Actuating equipment/ 76


IIb-3. Electric motors

General principle: Lorentz force - any current-carrying conductor placed within an

external magnetic field experiences a torque or force

Classification:

rotor

stator

[http://www.allaboutcircuits.com/vol_2/chpt_13/1.html]

IIb. Actuating equipment/ 77


IIb-3a. Brushed DC electric motor

Principle: Stator with a stationary magnets and the rotor powered from a DC power by

brushes and commutator.

Magnetic fields of the stator and the rotor interact and a generated torque causes a turn of the rotor.

The commutator consisted of a split ring reverses the current each half turn of the rotor.

permanent magnets

+ -

electromagnets

+ - +

+ -

brushes

M

N S

M

M

M

series m.

-

shunt m. separately excited m.

The type of connection determines the characteristics of the motor

Control:

- the sense of rotation depends on the polarity of the excitation winding – control by change of the polarity

- the rotational speed is proportional to the EMF in its coil - control by variable supply voltage, resistors or

electronic controls (e.g. PWM)

- the torque is proportional to the current

Advantages: low initial cost, high reliability, simple control of motor speed

Disadvantanges: sparking and wear of the electric contact commutator-brushes

IIb. Actuating equipment/ 78


IIb-3b. Synchronous motor

Principle: Rotor with a stationary magnets and the stator powered from the AC power

and generating a rotating magnetic field.

Electromagnets on the stator create the magnetic field which rotates in time with the oscillations of the

line current and the rotor turns in step with this field, at the same rate (the motor speed is synchronized

with the frequency the AC supply current )

three-phase s.m.

single phase s.m. stepper m.

M

M

N

Control:

- sense of rotation depends on the direction of rotating magnetic field – control by change of the phase order

- motor speed is synchronized with the supply frequency – control by a variable-frequency driver

Advantages: speed independent of the load,

accurate control in speed and position for stepper motor

Disadvantanges: above a certain size, synchronous motors are not self-starting motors

S

IIb. Actuating equipment/ 79


IIb-3c. Inductive motor

Principle: Rotor contained no powered circuit and the stator powered from the AC power

and generating a rotating magnetic field.

Electromagnets on the stator create the rotating magnetic field which induces an electric current in the

windings of rotor. Interaction between magnetic fields of stator and rotor produces a torque. The rotor

rotates at a slower speed than the stator field

squirrel-cage rotor induction motor

Windings of rotor in the form of cage (poured

or welded)

wound-rotor induction motor

Control:

- sense of rotation depends on the direction of rotating magnetic field – control by change of the phase order

- motor speed is proportional to supply frequency – control by a variable-frequency driver.

Advantages: ruggedness, simplicity, 90% of industrial motors are induction m. (mainly the squirrel-cage rotor)

Disadvantanges: hard starting (it is accompanied by inrush currents up to 7 times higher than running current)

V

Starting with:

• star-delta switch

• motor soft starter

U

(asynchronous motor)

M

V

Windings of rotor brought out via slip rings and

brushes which allows to connect a resistance during

start-up and to short-circuit windings during work.

W

U

M

W

IIb. Actuating equipment/ 80


IIb-3. Electro-mechanical drive system

A variable-frequency drive (VFD)

(adjustable-frequency drive, variable-speed drive, AC drive, micro drive, inverter drive)

- a type of adjustable-speed driver used to control AC motor speed and torque by

varying motor input frequency and voltage

electronic - frequency converter (frequency changer)

electromechanical = motor + generator

A motor soft starter

- a device used with AC electric motors to temporarily reduce the load and torque in the

powertrain of the motor during startup. This reduces the mechanical stress on the motor

and shaft, as well as the electrodynamic stresses on the attached power cables and

electrical distribution network, extending the lifespan of the system

IIb. Actuating equipment/ 81


Contents

The main topics:

I. Measurement devices

}

Instrumentation

II. Actuating elements

------------------------------------------

I. Field devices - controllers

II. Communication networks

III. SCADA & DCS

}

15

of a technological process

15 hours

hours

82

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