Instrumentation - Introduction

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Instrumentation - Introduction




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Slide1

Instrumentation - Introduction

10/9/2013

Ohio University - Dr. Cyders

1

Engineering measurements are usually taken by some form of transducer. A transducer transforms one form of energy into another – an example would be a thermocouple, which converts a temperature differential to an analog voltage.Analog – continuously variable, as opposed to discrete, which is quantified in finite units of data – examples would be a dimmer vs. a switch, or the audio signal from a vinyl record vs. that from a compact disc

Analog

1:19

Discrete

Slide2

Instrumentation - Introduction

10/9/2013

Ohio University - Dr. Cyders

2

Usually, engineering transducers are set up to either be read visually or to produce an electrical signal that can then be conditioned and converted to a discrete digital signal by a data acquisition system with an analog-to-digital converter (ADC). Digital signals can then be manipulated for analysis on a computer.

Analog

1:19

Discrete

Signal

Conditioning – altering a signal

using tools such as

filtering

and

amplification

to prepare it for further processing

Data Acquisition System (DAQ) – system used to collect and record analog signals, usually in digital (discrete) form

Analog-to-Digital Converter (ADC) – device that converts analog signals to digital (discrete) data

Slide3

Instrumentation – Analog Device

10/9/2013

Ohio University - Dr. Cyders

3

Picture: http://www.instrumart.com/assets/1008A1_200.jpg

Ashcroft 1008S Analog Pressure Gauge

Bourdon tube designFull Scale Range - 100 psiResolution - smallest measurement change in input variable that can be detected and indicated – 2 psiAccuracy – deviation of the reading from the real value - ±1.6% Full Scale (1.6 psi)Random or Human ErrorGraduations - ±1 psiParallax

Slide4

Instrumentation – Analog Device

10/9/2013

Ohio University - Dr. Cyders

4

SSI Technologies P51-100 Pressure TransducerStrain Gauge Diaphragm designFull Scale Range - 100 psigResolution – dependent on DAQAccuracy – ±0.5% Full Scale (0.5 psi)Random ErrorNoiseInput Voltage – voltage supplied to power transducer – 8 to 30 VDCOutput Voltage – 1 to 5VDC

Slide5

Instrumentation – A Brief Diversion - Number Systems

10/9/2013

Ohio University - Dr. Cyders

5

Think of the decimal number system (what we commonly use) as a series of switches. Each switch has ten settings, zero through 9. The first switch denotes the multiplier of 100, the second, of 101, and so on. The numbers are then added to calculate the total value represented. Example: 7*(102) + 4*(101) + 2*(100) = 742

102101100999888777666555444333222111000

Slide6

Instrumentation – A Brief Diversion - Number Systems

10/9/2013

Ohio University - Dr. Cyders

6

The binary system works the same way, but with switches having only two settings, 0 (off) and 1 (on). Because the switches have only two values, they are multiplied by successive powers of two.

24232221201111100000

Example: 1*(24) + 0*(23) + 1*(22) + 0*(21) + 1*(20) = 21

Slide7

Instrumentation – Digital Acquisition System

10/9/2013

Ohio University - Dr. Cyders

7

Analog-to-digital conversion works essentially the same way whether you’re reading a gauge visually or using an analog to digital converter. Since analog devices usually have theoretically continuous behavior, resolution is usually limited by the bit depth of the ADC.Bits are the most basic unit of memory, representing a binary number (0 or 1, transistor switch)The number of divisions over the full scale reading is equal to 2n, where n is the bit depth of the ADC.

Time [sec.]

Pressure [psig]

0.0049.550.1047.960.2049.160.3049.270.4048.12

5 bits

2

4

232221201111100000

Example: 5 bits gives 32 divisions (00000 through 11111)

Slide8

Instrumentation – Resolution

10/9/2013

Ohio University - Dr. Cyders

8

100 psi

0 psi

Analog Signal

0 V

5 V

111

000

3-bit Digital Signal

001

010

011

100

101

110

 

37.8 psi

42.8 psi

56.6 psi

2.25 V

2.88 V

2.83 V

1.89 V

2.14V

57.5 psi

45.0 psi

0 V

5 V

 

n

: Bit-depth of ADC (3 in this case)

V

D

: Full-scale input voltage on ADC (5 Volts in this case)

V

S

: Full-scale

output

voltage on

analog device

(5 Volts in this case

)

B

:

Range type (0 for unipolar, 1 for bipolar; 0 in this case)

R

FS

: Full-scale range of input value (100 psi in this case)

 

Slide9

Instrumentation – Resolution

10/9/2013

Ohio University - Dr. Cyders

9

100 psi

0 psi

Analog Signal

5 V

 

n

: Bit-depth of ADC (3 in this case)

V

D

: Full-scale input voltage on ADC (5 Volts in this case)VS : Full-scale output voltage on analog device (4 Volts in this case)B : Range type (0 for unipolar, 1 for bipolar; 0 in this case)RFS : Full-scale range of input value (100 psi in this case)

1 V

 

0 V

111

000

3-bit Digital Signal

001

010

011

100

101

110

5 V

 

Amp

100 psi

0 psi

Amplified Analog Signal

5 V

0 V

Slide10

Instrumentation – Digital Acquisition System

10/9/2013

Ohio University - Dr. Cyders

10

Example: An SSI P51-100 pressure transducer (0-100 psi, 1-5 V) is connected to a 10-bit ADC that can read 0-5V signals. The resolution on the ADC output for this range would be:This is known as zero-order uncertainty in the instrument. We generally combine error sources using a root-sum-square. We know of at least two major error sources: transducer accuracy and ADC resolution.

 

Transducer accuracy: ±0.5 psi (stated by manufacturer), error due to resolution: ±0.061 psi (half the resolution). Total uncertainty due to these two sources: Actual pressure: 46.90 psiMeasured pressure: 46.8 psi ±0.5 psi

 

Time [sec.]

Pressure [psig]

0.0049.550.1047.960.2049.160.3049.270.4048.12

Slide11

For more information, go to

http://www.library.ohiou.edu

and search the ALICE catalog for

The Uncertainty in Physical

Measurements

by

Fornasini

– it is an electronic resource you can access anywhere with your student login

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