Download Lecture11 Resistive Transducer

Lecture 11
Resistive Transducer
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Lecture11:ResistiveTransducer
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
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Lecture11:ResistiveTransducer
Resistive Transducer
a)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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Lecture11:ResistiveTransducer
• The relation between the temperature and the
resistance
1 1
RT  RT 1 exp(  (  ))
T T1
• RT: The resistance value at the temperature T
• T: The temperature [K]
• R1: The resistance value at the reference
temperature
• T1: The reference temperature [K] typically, 25°C
is used
• : The coefficient of thermistor.
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Lecture11:ResistiveTransducer
FIGURE 4.5
Thermistor resistance versus temperature is highly
nonlinear and usually has a negativeslope.
Curtis Johnson
Lecture11:ResistiveTransducer
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Process
Control Instrumentation
Technology, 8e]
Copyright ©2006 by Pearson
Education, Inc.
Upper Saddle River, New
Jersey 07458
Thermistor Characteristics
 Sensitivity – change in resistance 10% per 0C, for
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nominal resistance of 10k may change 1 k for 10C
Construction – semiconductor in various forms discs,
beads, rods
Range - -200C to 1000C
Response time – depends on quality of material
Signal conditioning - bridge
Lecture11:ResistiveTransducer
Thermistor: Construction and symbols
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Lecture11:ResistiveTransducer
Advantages:
• Low cost, small size
• High output voltage
• Fast response
Disadvantages:
• Highly nonlinear
• Restricted to relatively low temperature
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Lecture11:ResistiveTransducer
b) Resistive Temperature Detector (RTD)
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Electrical resistance is a function of metal temperature
As temperature increases, the resistance increases
Resistance temperature relationship:
R = R0(1+  T )
with R = resistance of the conductor at temperature t0C
R0 = ambience resistance (at reference point)
 = temperature coefficients of resistance
T = difference between temperature at t and
ambience
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Lecture11:ResistiveTransducer
FIGURE 4.2
Metal resistance increases almost linearly with temperature.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
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Lecture11:ResistiveTransducer
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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Lecture11:ResistiveTransducer
Sensitivity
 An estimate of RTD sensitivity is noted by value of 
 Platinum – 0.004/0C Nickel – 0.005/0C
 For 100 platinum RTD, a change of 0.4 if
temperature is changed by 10C
Range
 Platinum RTD –100 to 6500C
 Nickel RTD – 180 to 3000C
Response time
 0.5 to 5 s or more, slowness due to thermal conductivity
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Lecture11:ResistiveTransducer
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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
more linear, more accurate
• Disadvantages: Expensive
• Very small fractional changes of resistance
with temperature, bridge circuit is needed
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Lecture11:ResistiveTransducer
FIGURE 4.4:Note the compensation lines in this typical RTD signalconditioning circuit.
Curtis Johnson
Lecture11:ResistiveTransducer
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Process Control Instrumentation
Technology, 8e]
Copyright ©2006 by Pearson
Education, Inc.
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 shift in the
bridge null
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Lecture11:ResistiveTransducer
Dissipation Constant
 RTD is a resistance, there is an I2R power dissipated by the
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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 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
Lecture11:ResistiveTransducer
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:
P
T 
PD
 T = temp rise of self-heating
 P = power dissipated by RTD from circuit in W
 PD = dissipation constant of RTD in W/0C
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Example 4.7
 An RTD has 0=0.005/0C, R = 500 , and a dissipation
constant of PD = 30mW/0C at 200C. The RTD is used in a
bridge circuit such as that in previous figure, 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
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Lecture11:ResistiveTransducer
Solution
 Find RTD resistance at 00C without dissipation effect
R = R0(1+  T ) =500(1+ 0.005(0-20))
RRTD = 450 
 Without considering self heating, for the bridge to null
R3 = 450  (from R1R4 = R2R3)
 Self-heating effects??
P = I2R
 Calculate the current I from bridge
 Power dissipated from RTD
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Lecture11:ResistiveTransducer
Voltage supply V = 10V, R1
= R2 = 500  and R3 = a
variable resistor to null the
bridge
Current I is calculated:
I
I
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Lecture11:ResistiveTransducer
10
 0.011A
(500  450)
Solution (cont.)
 Therefore power dissipated in RTD:
P = (0.01)2(450) = 0.054 W
 Find the temperature rise
P = TPD
 Temperature rise:
0.054
T 
 1.80 C
0.030
 Thus, RTD is not actually at bath temperature of 00C but at 1.80C
 Resistance of RTD
R = R0(1+  T ) =500(1+ 0.005(1.8-20))
RRTD = 454.5 
 Therefore, bridge will null with R3 = 454.5 
Lecture11:ResistiveTransducer
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c)Potentiometer
• Displacement sensor – converts linear or
angular motion into a changing in resistor
• Simple potentiometric displacement sensor
• Voltage divider:
RTH
VD 
10V
(3.5k  RTH )
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Lecture11:ResistiveTransducer
FIGURE 5.1
Potentiometric displacement sensor.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
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Lecture11:ResistiveTransducer
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
 Voltage E is applied to resistor with length L
 Measure displacement, generate output e (Ohm’s Law)
x
eE
L
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Lecture11:ResistiveTransducer
Resistive Sensors - Potentiometers
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/
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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Lecture11:ResistiveTransducer
Example:
The value of R is 100k 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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Lecture11:ResistiveTransducer
Solution
x
eE
L
 The output voltage e = 9V(1.5/2) = 6.75 V
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Lecture11:ResistiveTransducer
Example 4.8:
A thermistor is to monitor room temperature. It has a resistance
of 3.5k 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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Lecture11:ResistiveTransducer
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.
Lecture11:ResistiveTransducer
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%.
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Lecture11:ResistiveTransducer
System Components:
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:
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Lecture11:ResistiveTransducer
Motor Potentiometer Assemblies
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.
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End of Lecture 11
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