Lecture - 3 Measurement Systems Characteristics

Instuctional Objectives

 After learning the lesson students should be able to:

• Define and explain major static characteristics parameters for sensors and instruments

• Understand the process of calibration and subsequent characterisation of errors

• Describe the response of first and second order instrument to step and sinusoidal inputs

• Interpret typical industrial sensor specifications

 Table of contents

1.General structure of measurement system

2.Weight measurement system

3.Importance of Sensing in Automation

• Product Quality Control

• Manufacturing Process Controll

• Process Monitoring and Supervision

4.Instrument Characteristics

• Static characteristics of an instrument are concerned only with the staedy state reading

• Static characteristics are improtant for indicating instruments or for instruments where the dynamics is very fast

• Static and Dynamic characteristics are important for sensors which are used in control loops

5.Calibration

• Procedure that involves a comparison of the particular instrument with either (1)a primary standard (2) a secondary standard with a higher accurancy than the instrument is to be calibrated

6.Calibration of an instrument

7.Span

8.Accuracy

9.Linearity

• The calibration curve of a real instrument is typically , not a exactly straight line

• For ease of use it is instrument is considered lineearly related to the quantity being measured

• The linearity spec. indicates the deviation of the calibration curve from a good fit straight line of it

10.Definition of non-linearity

11. Sensitivity

• The slope of a static calibration curve evaluated at an input value is the static sensitivity

12.Repeatability of Precision

13.Resolution

14.Dead Zone

15.Hysteresis -Chart

16.Fig 1.4-Chart

17.Bias /Offset  Sensitivity/Gain error

18.Bias and Gain Error

19.Drift

• The calibration of an instrument is usually performed under controlledd conditions

• As variations occur in these conditions and alco with passage of time , the instrument characteristics change

• Typical factors for which drift is characterized are temperature and time

20.Dynamic Characteristics

21.Zero Order Instrument

27.First Order Instrument

28.Step Input

29.Response of a first-order element to a unit step

30.Sinusoidal Input.Frequency response of an element with linear dynamics

31.Fig.Frequency response characteristics of first-order element

32.Second Order Instrument/Sensor

• A second order sensor is modeled by a second differential equation

• Accelerometers,diapharm pressure transducer, mercury in glass manometers are few examples of second order system

33.Step Input

34.Dynamic Characteristics of Measurement Sysytems

35.Fig.Frequency response characteristics of second-order element- Chart

36.Example Specificaton of Industrial Sensor

37.Industrial Flow Meter sensor

38.Lesson Reviev

• Significance and Structure

• Static Characteristics

• Dynamic characteristics of zeroth, first and second order instruments

39.Poinst to Ponder

• What is the difference between accuracy and linearity?

• What is the difference between dead zone and resolution?