Lecture 2 Daq

8/3/2019 Lecture 2 Daq

1/45

Mid Term 35%

Assignments 5%

Quizzes 10%

Final 50%

Assignment # 1 - P 1.14 till P 1.24 (Curtis)

Due Date: 19 July 2011


2/45

A typical process transducer.

A measurement transducer typically consists of a sensingelement combined with a driving element (transmitter).


3/45

Transducers for process measurements convert the

magnitude of a process variable (e.g., flow rate,

pressure, temperature, level, or concentration) into a

signal that can be sent directly to the controller. The sensing element is required to convert the

measured quantity, that is, the process variable, into

some quantity more appropriate for mechanical or

electrical processing within the transducer.


4/45

Mechanical transducers are those in whichthe output signals are mechanical by nature.For example, a bimetallic element reacts tochanges in temperature and its response is in

the form of mechanical displacement. Electrical transducers respond to

nonelectrical quantities but develop outputsignals, which are electrical by nature.

Example: In resistance thermometer element,a change in temperature is recognized as achange in the resistance of the element.


5/45

Passive electrical transducers are based on the

variation of resistance, inductance or capacitance due

to application of any external stimulus.

Active electrical transducers provide electrical signals

in the form of voltages or currents, dc or ac. They

function as sources of energy, though very small, and

do not need excitation from an external source of

electrical power. Examples are thermocouples andpiezoelectric crystals.


6/45

The degree of hotness of a body, substance, or medium;

a physical property related to the average kinetic

energy of the atoms or molecules of a substance.


7/45


8/45

Resistivity of metals is a function of temperature. Platinum often used since it can be used for a wide

temperature range and has excellent stability. Nickel ornickel alloys are used as well, but they arent as accurate.

In several common configurations, the platinum wire is

exposed directly to air (called a bird-cage element),wound around a bobbin and then sealed in molten glass,or threaded through a ceramic cylinder.

Metal film RTDs are new. To make these, a platinum ormetal-glass slurry film is deposited onto a ceramicsubstrate. The substrate is then etched with a laser.

These RTDs are very small but arent as stable (andhence accurate).

RTDs are more accurate but also larger and moreexpensive than thermocouples.


9/45

How it works: Utilizes the fact that

resistance of a metalchanges withtemperature.

Make up: Traditionally made up of

platinum, nickel, iron orcopper wound around aninsulator.

Temperature range: From about -196C to

482C.

Thin Film RTD


10/45


11/45

Wire wound and thin filmdevices.

Nearly linear over a wide

range of temperatures.

Can be made small enoughto have response times of a

fraction of a second.

Require an electrical current

to produce a voltage drop

across the sensor


12/45

Advantages:

Stable

Very accurate

Change in resistanceis linear

Disadvantages:

Expensive

Current source

required Small change in

resistance

Self heating

Less rugged than

thermocouples.


13/45

Air conditioning and

refrigeration servicing

Furnace servicing

Foodservice processingMedical research

Textile production


14/45

Several different bridge circuits are used to determine

the resistance. Bridge circuits help improve the

accuracy of the measurements significantly. Bridge

output voltage is a function of the RTD resistance.


15/45

Published equations relating bridge voltage totemperature can be used.


16/45

RTDs are more fragile than thermocouples. An external current must be supplied to the RTD. This current

can heat the RTD, altering the results. For situations with high

heat transfer coefficients, this error is small since the heat is

dissipated to air. For small diameter thermocouples and stillair this error is the largest. Use the largest RTD possible and

smallest external current possible to minimize this error.

Be careful about the way you set up your measurement

device. Attaching it can change the voltage.

When the platinum is connected to copper connectors, a

voltage difference will occur (as in thermocouples). This

voltage must be subtracted off.


17/45

How it works: Like the RTD a thermistor

uses the fact thatresistance of asemiconductor changes

with temperature.

Make up: Generally made up of

semiconductor materials

Temperature Range: About -45C - 150C

Thermistor


18/45

Thermistors measure the change in resistance withtemperature.

Thermistors are very sensitive (up to 100 times more than

RTDs and 1000 times more than thermocouples) and can

detect very small changes in temperature. They are also

very fast. Due to their speed, they are used for precision

temperature control and any time very small temperature

differences must be detected.

They are made of ceramic semiconductor material (metaloxides).

The change in thermistor resistance with temperature is

very non-linear.


19/45

A semiconductor used as a temperature sensor.

Mixture of metal oxides pressed into a bead, wafer or othershape.

Beads can be very small, less than 1 mm in some cases.

The resistance decreases as temperature increases, negativetemperature coefficient (NTC) thermistor.


20/45

Most are seen in medicalequipment markets.

Thermistors are also

used are for engine

coolant, oil, and air

temperature

measurement in the

transportation industry.


21/45


22/45

Advantages:

Very sensitive (has

the largest output

change from inputtemperature)

Quick response

More accurate than

RTD and

Thermocouples

Disadvantages:

Output is a non-linear

function

Limited temperaturerange.

Require a current

source

Self heating

Fragile


23/45

How it works: Made up of two different

metals joined at one endto produce a smallvoltage at a given

temperature. Make up:

Made of up two differentmetals. Ex: A type J ismade up of Iron and

Constantan. Temperature Range

Type J: - 190C to 750C

A few Thermocouples


24/45

Thermocouples

Two dissimilar metals are joined together

Change in temperature at junction generatesvoltage

Smaller size compared to thermistors Fast response time (time constant as small as 1 ms)

Ease of fabrication, long term stability

Low sensitivity, small output voltage

Need reference temperature


25/45

Seebeck EffectAn electromotive force (EMF) exists in a loop

made oftwo dissimilar metals when the two

junctions are different temperatures.

T1 T2

A

B


26/45

T1 T2

A

B

+

_+

_

pAB pBA

Peltier EMF Voltage at the junction of two dissimilar

metals

Denoted aspAB, pBA


27/45

Seebeck effect

If two wires of dissimilar metals are joined at both ends

and one end is heated, current will flow.

If the circuit is broken, there will be an open circuit

voltage across the wires. Voltage is a function of temperature and metal types.

For small Ts, the relationship with temperature is

linear

For larger Ts, non-linearities may occur.

2 1whereT T T T


28/45


29/45


30/45

Type B very poor below 50C; reference junction temperature notimportant since voltage output is about the same from 0 to 42 C

Type E good for low temperatures since dV/dT (a) is high for low

temperatures

Type J cheap because one wire is iron; high sensitivity but also

high uncertainty (iron impurities cause inaccuracy)

Type T good accuracy but low max temperature (400 C); one

lead is copper, making connections easier; watch for heat being

conducted along the copper wire, changing your surface temp

Type K popular type since it has decent accuracy and a wide

temperature range; some instability (drift) over time

Type N most stable over time when exposed to elevated

temperatures for long periods


31/45

Appendix 3 of book gives the output voltage ofseveral thermocouples (TC) types over a rangeof temperature in 5C increments. In each case,the TC reference temperature is 0C. The

temperature is in C and the output is in mV.Each column is in 5C increments from thetemperature of that row.


32/45


33/45


34/45


35/45


36/45

Plastic injection moldingmachinery

Food processing

equipment

Deicing

Semiconductor processing

Heat treating

Medical equipment

Industrial heat treating

Packaging equipment


37/45

Simple, Rugged High temperature operation

Low cost

No resistance lead wire

problems

Point temperature sensing

Fastest response to

temperature changes

Least stable, leastrepeatable

Low sensitivity to small

temperature changes

Extension wire must be ofthe same thermocouple

type

Wire may pick up radiated

electrical noise if notshielded

Lowest accuracy

Advantages Disadvantages


38/45


39/45

Cost thermocouples are cheapest by far, followed by RTDs. Accuracy RTDs or thermistors.

Sensitivity thermistors.

Speed thermistors.

Stability at high temperatures not thermistors.

Size thermocouples and thermistors can be made quite

small.

Temperature range thermocouples have the highest range,

followed by RTDs.

Ruggedness thermocouples are best if your system will be

taking a lot of abuse.


40/45

Bimetallic Strip

Application

Thermostat (makes or

breaks electrical

connection withdeflection)

Metal A

Metal B

)]T-(T1[ 00 LL


41/45

Minco Pyrotek

Omega

Watlow Texas Instrument

National Semiconductor

Maxim


42/45

I2C Interface -55 to 125C range

1 accuracy (3 max)

0.0625C resolution

2.7 to 5.5 operating voltage

45 to 75 A operating current,0.1 to 1A shutdown current

40ms/320ms conversionrate(9/12 bit)

25/3 conversions per second(9/12 bit)

Online sample request 6 pin SO23 package

Needs 400kHz clock for I2CInterface


43/45

I2C Interface

-55 to 125C range

1 accuracy (2 max)

5/0.0625C resolution(625/626)

3.0 to 5.5 operating voltage 250A to 1mA operating current,

1A shutdown current

133ms conversion rate

Online sample request

6 pin SO23 package


44/45

I2C Interface

-55 to 125C range

2/ 1 accuracy

9 bits/ 12 bits or 0.0625Cresolution

3/3.3 to 5.5 operating voltage 0.25 to 0.5 A operating current,

4/5A shutdown current

100ms/400ms conversionrate(9/12 bit)

Online sample request

8 pin SOP package

Needs 400kHz clock for I2CInterface


45/45

Texas Instrument TMP100/101 Small package

Can operate on lower voltage

Low power consumption Accurate

Lecture 2 Daq

Documents

Transcript of Lecture 2 Daq