Lecture 2. In The Name of Allah. Instrumentation. Dr. Ali Karimpour Associate Professor Ferdowsi University of Mashhad

Transcription

1 In The Name of Allah Instrumentation Dr. Ali Karimpour Associate Professor Ferdowsi University of Mashhad

2 Transducer Specification Topics to be covered include: Introduction Transducers Transducer Specification Transducer Static Specification Calibration Accuracy Resolution Repeatability Hysteresis Linearity & sensitivity Transducer Dynamic Specification Time constant Dead time Settling time 2

3 Introduction (Transducer) Sensor: Sensors are often transducers in that they are devices that convert input energy of one form into output energy of another form. 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). 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 3

4 Introduction We need to control level We need to control flow We need to control temperature We need to control Composition We need to It is too complicated. 4

5 Introduction Control with digital supervisory control 5

6 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. 6

7 Some transducer or sensor Displacement sensors: Variable Dielectric Displacement Sensors Active or passive? 7

8 Some transducer or sensor Displacement sensors: Potentiometer (variable resistance) sensors Active or pasive? 8

9 Some transducer or sensor Displacement sensors: LVDT: Linear Variable Differential Transformer Active or passive sensor? 9

10 Some transducer or sensor Displacement sensors: Rotary contactless position sensor Active or passive sensor? 10

11 Some transducer or sensor Temperature Sensors: RTD: resistive temperature detectors PT100 Active or passive sensor? 11

12 Some transducer or sensor Temperature Sensors: Thermistor Active or passive sensor? 12

13 Some transducer or sensor Temperature Sensors: Thermocouple Active or passive sensor? 13

14 Some transducer or sensor Temperature Sensors: Bimetal Active or passive sensor? 14

15 Some transducer or sensor Force sensors (Stress, Mass, Weight,.) Motion sensors (Position, Speed, Acceleration, ) Fluid sensors (Pressure, Flow, Level, ).. 15

16 Transducer Specification Topics to be covered include: Introduction Transducers Transducer Specification Transducer Static Specification Calibration Accuracy Resolution Repeatability Hysteresis Linearity & sensitivity Transducer Dynamic Specification Time constant Dead time Settling time 16

17 Transducer Specification Transducer performance is described by the manufacturer in two sets of specification. 1- Static specification speak about steady state performance such as: Calibration Repeatability 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 17

18 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? 18

19 Transducer Static Specification Accuracy, Precision Accuracy is specified in terms of the percent error. Percent of full-scale output or as percent of the reading. More accurate صحت More precise دقت Precision is the spread of value obtained. Repeatability, Reproducibility Short term Long term 19

20 Transducer Static Specification 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.. 20

21 Transducer Static Specification 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. 21

22 Transducer Static Specification 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 pulses per revolution. 360/((0.02/1.25)*(180/π)))= pulses per revolution 22

23 Transducer Static Specification 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. 23

24 Transducer Static Specification 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 24

25 Transducer Static Specification Repeatability Some measure for repeatability. 25

26 Transducer Static Specification 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. 26

27 Transducer Static Specification 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. 27

28 Transducer Static Specification Linearity There are three ways to derive a linear formula for a sensor. End point linearity. Independent straight line linearity. Least square linearity. 28

29 Transducer Static Specification Linearity End point linearity. 29

30 Transducer Static Specification Linearity Independent straight line linearity. 30

31 31 Transducer Static Specification Linearity (least square method) b au y i i T i i i b a u y 1 N t T i i N N y N Z V ), ( ), ( 0 N V N Z d d N i T i y i N 1 2 N i i i N i T i i N y ˆ b a u i i, 1

32 Transducer Static Specification 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 32

33 Transducer Specification Topics to be covered include: Introduction Transducers Transducer Specification Transducer Static Specification Calibration Accuracy Resolution Repeatability Hysteresis Linearity & sensitivity Transducer Dynamic Specification Time constant Dead time Settling time 33

34 Transducer Specification Transducer performance is described by the manufacturer in two sets of specification. 1- Static specification speak about steady state performance such as: Calibration Repeatability 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.. 34

35 Transducer Dynamic Specification Rise time 35

36 Transducer Dynamic Specification Time constant (For first order transducers) 36

37 Transducer Dynamic Specification Dead time 37

38 Transducer Dynamic Specification Settling time (For second order transducers) 38

39 Transducer Dynamic Specification Cut off frequency 39

40 References Industrial Control Electronics J.M. Jacob, Prentice-Hall, 1989 مبانی اندازه گیری در سیستمهای ابزار دقیق حمید رضا تقی راد و سید علی سالمتی انتشارات دانشگاه صنعتی خواجه نصیرالدین طوسی 1392 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

41 Some Useful websites for the course