Lecture 9 Introduction to Measurements Process Control Prof. - - PowerPoint PPT Presentation

▶
lecture 9 introduction to measurements
SMART_READER_LITE
LIVE PREVIEW

Lecture 9 Introduction to Measurements Process Control Prof. - - PowerPoint PPT Presentation

Sep 08, 2023 120 likes •431 views

Lecture 9 Introduction to Measurements Process Control Prof. Kannan M. Moudgalya IIT Bombay Wednesday, 7 August 2013 1/26 Process Control Introduction to Measurements Outline 1. Thermal measurements 2. Other measurements 3. About

slide-1
SLIDE 1

Lecture 9 Introduction to Measurements

Process Control

  • Prof. Kannan M. Moudgalya

IIT Bombay Wednesday, 7 August 2013

1/26 Process Control Introduction to Measurements

slide-2
SLIDE 2

Outline

  • 1. Thermal measurements
  • 2. Other measurements
  • 3. About measurements

2/26 Process Control Introduction to Measurements

slide-3
SLIDE 3

Outline: Components of Process Control Instrumentation System

  • 1. Sensor/transducer
  • 2. A/D and D/A converter
  • 3. Transmitter
  • 4. Feedback controller
  • 5. End control element

3/26 Process Control Introduction to Measurements

slide-4
SLIDE 4
  • 1. Thermal Measurements

4/26 Process Control Introduction to Measurements

slide-5
SLIDE 5

A Sample Control System

Q(t) = x(t)h(t) h(t) Qi(t) FT FC Q(t) = x(t)h(t) h(t) Qi(t) FC LT

◮ Sensor: converts flowrate, height into voltage ◮ Transmitter ensures signal is conveyed without

loss of information

◮ Red dashed lines: transmission line ◮ Controller is indicated by FC ◮ End control element is the valve

5/26 Process Control Introduction to Measurements

slide-6
SLIDE 6

Distillation Column: A Control Strategy

? LT PT Feed Coolant Exit Reflux, R Bottoms: B, xB Distillate: D, xD Heat hD hB AT AT LT

6/26 Process Control Introduction to Measurements

slide-7
SLIDE 7

Computer Controlled System

Multiple measurements and multiple actuators

7/26 Process Control Introduction to Measurements

slide-8
SLIDE 8

Temperature Sensors

◮ Thermocouple ◮ Thermistor ◮ Resistance temperature detector (RTD)

◮ Also known as resistance thermometer

◮ Pyrometer

8/26 Process Control Introduction to Measurements

slide-9
SLIDE 9

Thermocouple

9/26 Process Control Introduction to Measurements

slide-10
SLIDE 10

Thermocouple

A thermocouple has the following useful property:

  • 1. Very accurate, almost a reference measurement
  • 2. Large output, voltage amplification is not

required

  • 3. Can measure large temperature ranges
  • 4. Highly linear output, external linearisation is

not required Answer: 3, large range

10/26 Process Control Introduction to Measurements

slide-11
SLIDE 11

Thermistor

11/26 Process Control Introduction to Measurements

slide-12
SLIDE 12

A typical bridge circuit

Supply 1.00 k ohm 0.29 k ohm 1.22 k ohm Thermistor Vout 2.56 V

12/26 Process Control Introduction to Measurements

slide-13
SLIDE 13

Thermistor

A thermistor has the following useful property:

  • 1. Highly sensitive in a small temperature range
  • 2. Can measure large temperature ranges
  • 3. Highly linear output, external linearisation is

not required

  • 4. Very accurate, almost a reference measurement

Answer: 1, sensitive in small ranges

13/26 Process Control Introduction to Measurements

slide-14
SLIDE 14

Resistance temperature detector (RTD)

14/26 Process Control Introduction to Measurements

slide-15
SLIDE 15

RTD

An RTD has the following useful property:

  • 1. Suitable for a small temperature range only
  • 2. Stable, accurate, repeatable
  • 3. Highly rugged
  • 4. Another name for this is Pt100

Answer: 2, accurate, repeatable, a standard

15/26 Process Control Introduction to Measurements

slide-16
SLIDE 16

A typical bridge circuit

E 10 ohm 10 ohm 10 ohm RTD 100 ohm

16/26 Process Control Introduction to Measurements

slide-17
SLIDE 17
  • 2. Other Measurements

17/26 Process Control Introduction to Measurements

slide-18
SLIDE 18

Differential pressure gauge

◮ Mount one or more strain gauges ◮ Pressure difference

◮ Deflection ⇒ Stress ⇒ Strain ◮ ⇒ Change of resistance ◮ ⇒ Wheatstone bridge, ∆V 18/26 Process Control Introduction to Measurements

slide-19
SLIDE 19

Flow Rate Measurement

◮ Orifice meter ◮ Venturi meter ◮ Turbine meter ◮ Thermal mass meter

◮ Heated filament in flow path ◮ Mounted on Wheatstone bridge

◮ Coriolis meter

19/26 Process Control Introduction to Measurements

slide-20
SLIDE 20

Liquid level measurement

◮ Through pressure measurement

◮ Deflection of diaphragm ◮ Strain gauge based ◮ Capacitance based

◮ Time of path based

◮ Radar, ultrasound 20/26 Process Control Introduction to Measurements

slide-21
SLIDE 21
slide-22
SLIDE 22
  • 3. General

22/26 Process Control Introduction to Measurements

slide-23
SLIDE 23

Precision and Accuracy in Measurements

◮ Accuracy in measurements:

How close a measurement is to the actual value

◮ Precision ≃ repeatability

Poor precision xx zz zz y y xx y Good precision Good accuracy Good precision Poor accuracy Good accuracy

23/26 Process Control Introduction to Measurements

slide-24
SLIDE 24

Accuracy and Error

◮ The word accuracy is misused

◮ ±5% accuracy actually means ±5% inaccuracy

◮ Error: difference between actual and measured

◮ Can be given as %FS - full scale ◮ or as %RTI - referred to input 24/26 Process Control Introduction to Measurements

slide-25
SLIDE 25

Items of Interest in a Sensor

  • 1. Measurement range (Span)
  • 2. Sensitivity
  • 3. Response time
  • 4. Accuracy, repeatability
  • 5. Reliability
  • 6. Materials of construction
  • 7. Electrical classification (e.g. explosive, etc.)
  • 8. Invasive/Non invasive
  • 9. Cost

25/26 Process Control Introduction to Measurements

slide-26
SLIDE 26

Standard instrumentation signal levels

  • 1. 3-15 psig (pneumatic)
  • 2. 1-5 mA
  • 3. 4-20 mA (most popular)
  • 4. 10-50 mA
  • 5. 0-5 Voltage, DC (VDC)
  • 6. ±10 V, DC

26/26 Process Control Introduction to Measurements

slide-27
SLIDE 27

Sensor/Transducer, Transmitter

◮ Sensor/Transducer

◮ To sense physical variables:

pressure, flow rate, temperature, etc.

◮ Convert into electrical quantity

◮ Transmitter

◮ Raises levels to suitable strength ◮ to prevent loss of information during transmission 27/26 Process Control Introduction to Measurements

slide-28
SLIDE 28

What we learnt today

◮ Measurements

28/26 Process Control Introduction to Measurements

slide-29
SLIDE 29

Thank you

29/26 Process Control Introduction to Measurements