Lecture11_Resistive_Transducerx

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Resistive Transducer

Lecture 11

Resistive Transducer

Lecture11:ResistiveTransducer1

Resistive transducer

The resistance of a transducer varies as the physical quantity varies (e.g. temperature or displacement)

As values of R varies, value of V and i also varies

Two basic devices for measurement of temperatures are RTD and thermistor

Resistive Transducera)Thermistor

• Semiconductor device

• The resistance value of the thermistor changes according to temperature

• Increase in temperature causes a decrease in resistance

resistance

• The relation between the temperature and the resistance

• RT: The resistance value at the temperature T• T: The temperature [K]

(exp(1

1 TTRR TT −= β

• T: The temperature [K]• R1: The resistance value at the reference

temperature • T1: The reference temperature [K] typically, 25°C

is used• β: The coefficient of thermistor.

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

Lecture11:ResistiveTransducer5Curtis JohnsonProcess Control Instrumentation Technology, 8e]

Upper Saddle River, New Jersey 07458

Thermistor Characteristics

Sensitivity – change in resistance 10% per 0C, for nominal resistance of 10kΩ may change 1 kΩ for 10C

Construction – semiconductor in various forms

Construction – semiconductor in various forms discs, beads, rods

Range - -200C to 1000C

Response time – depends on quality of material

Signal conditioning - bridge

Thermistor: Construction and symbols

Advantages:

• Low cost, small size

• High output voltage

• Fast response

Disadvantages:

• Highly nonlinear

• Restricted to relatively low temperature

b) Resistive Temperature Detector (RTD)

• Electrical resistance is a function of metal temperature• As temperature increases, the resistance increases• Resistance temperature relationship:• R = R0(1+ α ∆T )

with R = resistance of the conductor at temperature t0CR0 = ambience resistance (at reference point)α = temperature coefficients of resistance∆T = difference between temperature at t and

ambience

FIGURE 4.2FIGURE 4.2 Metal resistance increases almost linearly with temperature. Metal resistance increases almost linearly with temperature.

Curtis JohnsonProcess Control Instrumentation Technology, 8e]

Common Resistance Materials for RTDs:• Platinum (most popular and accurate)

• Nickel

• Copper

• Balco (rare)

• Tungsten (rare)

Sensitivity An estimate of RTD sensitivity is noted by value of α Platinum – 0.004/0C Nickel – 0.005/0C For 100Ω platinum RTD, a change of 0.4Ω if temperature is changed by 10C

Range Platinum RTD –100 to 6500C

Platinum RTD –100 to 6500C Nickel RTD – 180 to 3000CResponse time 0.5 to 5 s or more, slowness due to thermal conductivity

• Temperature range (from -200 to 8500 C)

• Advantages:

• relatively immune to electrical noise and therefore well suited for temperature measurement in industrial environments More stable, have an output response that is

More stable, have an output response that is more linear, more accurate

• Disadvantages: Expensive

• Very small fractional changes of resistance with temperature, bridge circuit is needed

FIGURE 4.4:FIGURE 4.4:Note the compensation lines in this typical RTD signalNote the compensation lines in this typical RTD signal--conditioning circuit. conditioning circuit.

Lecture11:ResistiveTransducer16Curtis JohnsonProcess Control Instrumentation Technology, 8e]

Upper Saddle River, New Jersey 07458

Signal conditioning

Bridge circuit

Compensation line in R3 leg is required

Same resistance change due to RTD leg cause no net

Same resistance change due to RTD leg cause no net shift in the bridge null

Dissipation Constant

RTD is a resistance, there is an I2R power dissipated by the device cause a slight heating effect, called self-heating

Cause erroneous reading, therefore current of RTD must be kept low and constant to avoid self-heating

Dissipation constant or P is usually in the specs of

Dissipation constant or PD is usually in the specs of RTD

It relates power required to raise RTD 100 C For PD = 25mW/

0C: If I2R power loses in RTD equal 25 mW, RTD will be heated 10C

Dissipation constant (cont.)

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

Difference in capacity of medium to carry heat away from device

The self-heating temperature rise can be found: The self-heating temperature rise can be found:

∆T = temp rise of self-heating P = power dissipated by RTD from circuit in W

PD = dissipation constant of RTD in W/0C

PT =∆

Lecture11:ResistiveTransducer 19

Example 4.7

An RTD has α0=0.005/0C, R = 500 Ω, and a

dissipation constant of PD = 30mW/0C at 200C.

The RTD is used in a bridge circuit such as that in previous Figure 4.4, with R1 = R2 = 500 Ω and

previous Figure 4.4, with R1 = R2 = 500 and R3 a variable resister to null the bridge. 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 effectR = R0(1+ α ∆T ) =500(1+ 0.005(0-20)) RRTD = 450 Ω

Without considering self heating, for the bridge to null

Without considering self heating, for the bridge to null R3 = 450 Ω (from R1R4 = R2R3)

Self-heating effects?? Power dissipated from RTD P = I2R Calculate the current I from bridge

Voltage supply V = 10V, R1= R2 = 500 Ω and R3 = avariable resistor to null thebridge

Current I is calculated:

IAI 011.0

)450500(

Solution (cont.) Therefore power dissipated in RTD:

P = (0.01)2(450) = 0.054 W Find the temperature rise P = ∆TPD Temperature rise:

Thus, RTD is not actually at bath temperature of 00C but

CT 08.1030.0

054.0 ==∆ Thus, RTD is not actually at bath temperature of 00C but 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 23

c)Potentiometer • Displacement sensor – converts linear or

angular motion into a changing in resistor

• Simple potentiometric displacement sensor

• Voltage divider:

THD 10

)5.3( +Ω=

FIGURE 5.1 Potentiometric displacement sensor.

Voltage E is applied to resistor with length L

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

Resistive Sensors - Potentiometers

Translational and Rotational Potentiometers

Translational or angular displacement is proportional to resistance.

Lecture11:ResistiveTransducer27 Taken from www.fyslab.hut.fi/kurssit/Tfy-3.441/ luennot/Luento3.pdf

Advantages:

Cheap, easy to use, adjustable

Problem:

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

Solution

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

Example 4.8:

A thermistor is to monitor room temperature. It has a resistance of 3.5kΩ at 20°C with a slope of -10%/°C. It is proposed to use the thermistor in the divider of Figure below to provide a voltage of 5.0 V at 20°C. Evaluate the effects of self-heating

at 20°C. Evaluate the effects of self-heating

Lecture11_Resistive_Transducerx

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