Lecture11_Resistive_Transducerx

Lecture 11 Resistive Resist ive Transd Transducer ucer

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Lecture11:ResistiveTransducer

Resistive transducer 

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The resistance of a transducer varies as the physical quantity varies (e.g. temperature or displacement) As values values of R varies, value value of V and i also varies varies Two basic basic devices devices for for measurement measurement of temperatures are RTD and thermistor


Resistivee Transduce Resistiv Transducerr a)Thermistor

• Semiconductor dev device • The The res resist istan ance ce value value of the the therm thermist istor or cha change ngess according to temperature • Incr Increa ease se in tem tempe pera ratur turee cau cause sess a decr decrea ease se in resistance

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• The The rel relat ation ion betw betwee een n the the temp temper erat atur uree and and the resistance RT = RT

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• • •

exp( β (

T

−

1 T 1

))

The resistance value at the temperature T T : The tem tem erature ure [K] th e reference R1: The resistance value at the temperature • T 1: The reference temperature [K] typically, 25°C is used • β : The coefficient of thermistor. 4

RT :

1


Thermistor resistance versus temperature is highly nonlinear and usually has a negativeslope. FIGURE 4.5

Curtis Johnson

Lecture11:ResistiveTransducer 5 Process Control Instrumentation Technology, 8e]

Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458

Thermistor Characteristics 

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Sensit Sensitivi ivity ty – change in in resista resistance nce 10% 10% per 0C, for nominal resistance of 10kΩ may may cchange hange 1 kΩ for 10C Constructi Construction on – semico semicondu nducto ctorr in various various forms forms discs, beads, rods Range - -200C to 1000C Response Response time – depends depends on on quality quality of material material Signal Signal condit condition ioning ing - bridge


Thermistor: Construction and symbols

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Advantages: • Low co cost, sm small si size • High output voltage • Fast response Disadvantages: • Highly no nonlinear • Rest Restri rict cted ed to re rela lati tive vely ly low low temp temper erat atur uree

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b) Resistive Resistiv e Temperature Temperature Detector (R (RTD)

• Electr Electrica icall resi resista stance nce is a function function of metal metal temper temperatu ature re • As temp temper erat atur uree incre increas ases es,, the res resist istan ance ce inc incre reas ases es • Resi Resist stan ance ce tem tempe pera ratu ture re rel relat atio ions nshi hip: p: • R = R0(1+ α ∆T ) with R = resistance of the conductor at temperature t emperature t0C R0 = ambience resistance (at reference point) α = temperature coefficients of resistance ∆T = difference between temperature at t and ambience 9


FIGURE 4.2

Metal resistance increases almost linearly with temperature.

Curtis Johnson

Process Control Instrumentation Technology, 8e]

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Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common Resistance Materials for RTDs:

• Plat Platin inum um (mos (mostt popu popula larr aand nd acc accur urat ate) e) • Nickel • Copper • Balco (rare) • Tungsten (r (rare)

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Sensitivity An estimate of RTD sensitivity is noted by value of α Pla Platinu tinum m – 0.00 0.004/ 4/0C Nick Nickel el – 0.00 0.005/ 5/0C For 100 100Ω platinum RTD, a change of 0.4Ω if temperature temperature is changed changed by 10C Range P atinum RT RTD –1 –100 to 65 650 C Nickel Nickel RTD RTD – 180 to 300 3000C Response time 0.5 to 5 s or more, slowness due to thermal conductivity   

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• Temper emperat atur uree ra range nge (f (fro rom m -200 -200 to 8500 C) • Advantages: • rela relati tivel vely y imm immune une to elec electr tric ical al noise noise and therefore well suited for temperature measurement in industrial environments , more linear, linear, more accurate • Disa Disadv dvan anttages ages:: Expe Expens nsiv ivee • Very small small fracti fractional onal changes changes of resist resistanc ancee with temperature, bridge circuit is needed 15


FIGURE 4.4:Note

the compensation lines in this typical RTD signalsignal conditioning circuit.

Curtis Johnson

Lecture11:ResistiveTransducer 16 Process Control Instrumentation Technology, 8e]

Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All ri hts reserve reserved. d.

Signal conditioning Bridge circuit Compensation line in R3 leg is required required ame res s ance c ange ue o eg cause no ne shift in the bridge null  

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Dissipation Constant RTD is a resistance, resistance, there is an I 2R power power dissipate dissipatedd by the device cause a slight heating effect, called selfheating Cause erroneous reading, therefore current of RTD must be kept low and constant to avoid self-heating

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D

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RTD It relates power required to raise RTD 100 C For P D = 25mW/0C: If I 2R power loses in RTD equal 25 mW, RTD will be heated 10C Lecture11:ResistiveTransducer

Dissipation Dissipati on constant (cont.) 

Dissipation constant is specified under 2 conditions: free air and well-stirred oil bath

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Difference in capacity of o f medium to carry carr y heat awa awayy from device

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The The sel selff-he heaatin tin tem tem era eratur ture rise rise can can be be fou found nd:: ∆T =

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P P D

∆T =

temp rise of self-heating power er dissipated dissipated by RTD RTD from circuit circuit in W P = pow P D = dissipation constant of RTD in W/0C Lecture11:ResistiveTransducer

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Example 4.7 

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An RTD has α 0=0.005/0C, R = 500 Ω, and a dissipation constant of P D = 30mW/0C at 200C. The RTD RTD is used in a bridge circuit such suc h as that in previous Figure 4.4, with R1 = R2 = 500 Ω and R3 a variable resister to null the bridge. br idge. If the supply is 10 V and RTD is placed in a bath at 00C, find the value of R3 to null the bridge


Solution 

Find RTD resistance at 00C without dissipation effect R = R0(1+ α ∆T ) =500(1+ 0.005(0-20)) RRTD = 450 Ω

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Without consideri rinn self heatin for the bri ridd e to null R3 = 450 Ω (from R1R4 = R2R3)

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Self-heating effects?? Power dissipated from RTD P = I 2R Calculate the current I from bridge

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Voltage supply V = 10V, R 1 = R 2 = 500 Ω and R 3 = a variable resistor to null the bridge

I is calculated: Current I is

I

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I =

10 (500 + 450)

=

0.011 A

Solution (cont.) 

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Therefore power dissipated in RTD: P = (0.01)2(450) = 0.054 W Find the temperature rise P = ∆TP D Temperature rise: ∆T =

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0.054 0.030

0

= 1.8 C

us, s not actua y at at temperature o at 1.80C Resistance of RTD R = R0(1+ α ∆T ) =500(1+ 0.005(1.8-20)) RRTD = 454.5 Ω Therefore, bridge will null with R3 = 454.5 Ω Lecture11:ResistiveTransducer

ut

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c)Potentiometer

• Disp Displa lace ceme ment nt sens sensor or – conv conver erts ts line linear ar or angular motion into a changing in resistor • Simpl Simplee pote potent ntio iome metr tric ic displ displac aceme ement nt sens sensor or • Volt oltage di divide viderr: V D =

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RTH

(3.5k Ω + RTH )


10V

FIGURE 5.1

Potentiometric displacement sensor sensor..

Curtis Johnson

Process Control Instrumentation Technology, 8e]

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Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

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Voltage E is applied to resistor with length L

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Measure displacement, generate output e (Ohm’s Law)

e = E

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x L

Resistive Sensors - Potentiomet Potentiometers ers Translational and Rotational

Potentiometers Translational or angular displacement is proportional to resistance.

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Lecture11:ResistiveTransducer Taken from www.fyslab.hut.fi/kurssit/Tfy-3.441/ www.fyslab.hut.fi/kurssit/Tfy-3.441/ luennot/Luento3.pdf

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Advantages: 

Cheap, easy to use, use, adjustable

Problem: 

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Mechanical wear, friction in wiper, high electronic noise


Example: The value of R is 100kΩ and the maximum displacement is 2.0cm. If E = 9V and x is 1.5 cm, determine the value of output voltage e

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Solution e = E

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x L

The output voltage e = 9V(1.5/2) = 6.75 V


Example 4.8: A thermistor is to monitor monitor room room temperat temperature ure.. It has a resistance of 3.5kΩ at 20°C with a slope of 10%/°C. It is propose proposedd to use the thermistor thermistor in the divider of Figure below to provide a voltage of 5.0 V ° .

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More on potentiometer

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Potentiometer 35


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The potentiometer on the MCBXC866 board connects to port 2, pin 6 (P2.6) for generating analog voltage to the on-chip ADC. The analog input is AIN6 and the voltage range is 0-5.0 VDC. Lecture11:ResistiveTransducer

Rotary Potentiometer 37


100 K Potentiometer 38


Potentiometer Foot Paddle 39


The slide potentiometer changes its resistance linearly with position. The slide potentiometer has about 60 6 0 mm (2.3 inches) of travel, and a nominal resistance of 10k ohms hms ± 20% 20%. 40


System Components: PIC Microcontroller: Microcontroller: Potentiometer: the potentiometer will control the rpm of the stepper motor. motor. This setting set ting will be read by the t he A-to-D on the PIC. Stepper Motor: Stepper Motor Controller: 41


Motor Potentiometer Pot entiometer Assemblies

with system designers. Today, Betatronix can supply the complete motor-pot motor-pot assembly or mount the potentiometer to the motor at our facility.

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End of Lecture 11

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Lecture11_Resistive_Transducerx

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