Resistive Transducer Lecture 11 Resistive Transducer Lecture11:ResistiveTransducer 1
Oct 03, 2014
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
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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
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resistance
• The relation between the temperature and the resistance
• RT: The resistance value at the temperature T• T: The temperature [K]
))11
(exp(1
1 TTRR TT −= β
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• 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]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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
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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
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Advantages:
• Low cost, small size
• High output voltage
• Fast response
Disadvantages:
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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 )
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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.
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Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
Common Resistance Materials for RTDs:• Platinum (most popular and accurate)
• Nickel
• Copper
• Balco (rare)
• Tungsten (rare)
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• 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
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Platinum RTD –100 to 6500C Nickel RTD – 180 to 3000CResponse time 0.5 to 5 s or more, slowness due to thermal conductivity
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• 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
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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]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Signal conditioning
Bridge circuit
Compensation line in R3 leg is required
Same resistance change due to RTD leg cause no net
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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
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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
DP
PT =∆
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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
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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
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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(
10 =+
=
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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
030.0
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c)Potentiometer • Displacement sensor – converts linear or
angular motion into a changing in resistor
• Simple potentiometric displacement sensor
• Voltage divider:
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VRk
RV
TH
THD 10
)5.3( +Ω=
FIGURE 5.1 Potentiometric displacement sensor.
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Curtis JohnsonProcess Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458All rights reserved.
Voltage E is applied to resistor with length L
Measure displacement, generate output e (Ohm’s Law)
x
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L
xEe =
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
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Advantages:
Cheap, easy to use, adjustable
Problem:
Mechanical wear, friction in wiper, high electronic noise
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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
L
xEe =
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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
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at 20°C. Evaluate the effects of self-heating
More on potentiometer
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Potentiometer
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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.
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Rotary Potentiometer
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100 K Potentiometer
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Potentiometer Foot Paddle
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The slide potentiometer changes its resistance linearly with position. The slide potentiometer has about 60 mm (2.3 inches) of travel, and a nominal resistance of 10k ohms ± 20%.
System Components:
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PIC Microcontroller:
Potentiometer: the potentiometer will control the rpm of the stepper motor. This setting will be read by the A-to-D on the PIC.Stepper Motor:
Stepper Motor Controller:
Motor Potentiometer AssembliesMotor Potentiometer Assemblies have become extremely popular
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Motor Potentiometer Assemblies have become extremely popular with system designers. Today, Betatronix can supply the complete motor-pot assembly or mount the potentiometer to the motor at our facility.
End of Lecture 11
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