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Transcript
```ENT 364/4 – Control Systems
Laboratory Module
EXPERIMENT
TEMPERATURE CONTROL
1.
OBJECTIVES
The students should be able to:

Explain the theory that governs the principles of operation of Thermocouple
(TC) and Resistance Temperature Detector (RTD).

Explain the process of instrument calibration.

Demonstrate the skills for the measurement of temperature using thermocouple
(TC) and Resistance Temperature Detector (RTD) and to calibrate a
Temperature Controller.

Study a typical oven heating process

Recognize the Ziegler Nichols Process Reaction Method to tune the controller
for optimal performance in the oven heating system

2.
Evaluating responses for different PID setting.
INTRODUCTION
From the industrial view point, temperature measurement is one of the important
measurements. The resistance temperature detector (RTD) and thermocouple (TC) are
the major devices of the temperature measuring instrument. Temperature measurement is
important for monitoring and control purposes.
2.1 Thermocouple Thermometers
A thermocouple thermometer has a thermocouple sensing element that produce an
electromotive force (emf) which is connected to a device that is capable of measuring the
emf and displaying the results in an equivalent temperature units.
If the 2 junctions of a closed circuit formed by joining the 2 dissimilar metals are to be
maintained at different temperatures, an emf which is proportional to the difference of
the Seebeck coefficients is produced . If the temperature of one junction is fixed at some
known value, then the temperature of the other junction can be determined by measuring
the emf generated.
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This formed the basic principle of thermoelectric thermometry. The junction with the
fixed temperature is known as the reference junction and is normally kept at 0 o C (ice
point). The other junction will be the measuring junction.
2.1.1
Thermocouple Types
The letter that designate the commonly used thermocouple types was introduced
by ISA and adopted in 1964 as ANS (American National Standard). Table 1 lists
some common types of thermocouple standards.
Designation by
+ Wire
- Wire
μV/oF
ISA Type
B
Pt70-Rh30
Pt94-Rh6
3.6
E
Chromel
Constantan 15.24
J
Iron
Constantan 14.35
K
Chromel
Alumel
9.24
R
Pt67-Rh13
Platinum
3.8
S
Pt90-Rh10
Platinum
3.7
T
Copper
Constantan 8.35
Y
Iron
Constantan 22.33
Table 1: Commonly used Thermocouple types
2.1.2
When the measuring instrument is directly connected between the hot and cold
junctions of the thermocouple, it is not always a practical as the measuring
junction may be a distance away from the measuring unit and thus connection
wires is required. The same type of material as in the thermocouple should be
used for the connecting wires so that an extra emfs generated due to dissimilarity
of metals are minimized. Compensation leads have a thermoelectric behavior
similar to that of the thermocouple leads over a narrow temperature range
usually it is less than 50 o C are commonly available.
2.1.3
Reference (Cold) Junction
For the purpose of calibration of thermocouples, the reference junction is kept in
an ice bath. However, most thermocouple nowadays has the cold junction
compensation incorporated. The cold junction is made to remain at a constant
temperature by attaching to it a thick copper block called isothermal block and its
temperature is measured with an electric temperature sensor. A silicon sensor is
used. A correction to the emf generated between the measuring and cold
junctions is calculated using the measured temperature of the cold junction and
standard temperature-millovolts tables ( equivalent regression equations)..and is
carry out by a microprocessor incorporated inside the instr4ument.
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2.1.4
Laboratory Module
Law of Intermediate Metals
If AB thermocouple and another with AB junctions with another metal C
incorporated somewhere in between, then the net emf will be the same in both
the cases.
If EAB is the emf generated by the AB thermocouple and EBC is that by the BC
thermocouple, then the emf generated by the AC thermocouple will be E AB+EBC.
2.1.5
Measurement of thermocouple output
The thermocouple output is in the order of millivolt range, high input impedance
measurement device is required. In calibration, the output emf is measured using
a digital voltmeter or potentiometer. In most industrial cases, the output is either
send into a digital temperature display or a temperature transmitter. A basic
configuration layout is shown in fig 1.
.
Isothermal
Block
Chromel
Copper
Display
Or
Indicator
Alumel
Compensation
Circuit
p-n junction
temperature sensor
Fig 1: Thermocouple in Industrial Instrumentation
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2.2 Resistance Temperature Thermometer
The resistance of a conductor varies when its temperature changes. The
temperature coefficient of resistance define the magnitude of electrical resistance
that change with respect to a 1 degree change in the temperature of that
conducting material. For most material the temperature coefficient is constant
over some range of temperatures and is positive. The most commonly use
resistance material is platinum which exhibits a large temperature range
The change in resistance is a function of temperature coefficient of resistance
designated as α which represent the slope of the expressed by the general
equation;
Rt  R0 (1  T )
(1)
dR0
 R0
dT
where
Rt - resistance at temperature T
Ro - resistance at reference temperature of 0 oC
 - temperature coefficient of resistance
(2)
The equation shows a linear function , it provides a fairly accurate calculation of
the resistance at its operating temperature. However the empirical equation that
governs it is;
Rt  R0 (1  T  T 2  T 3  ...............)
(3)
β is obtainable from the manufacturer and γ is the coefficient for higher of
temperature where extreme accuracy.
2.2.1
Types of RTD Temperature Measurement System
One of the major causes of error in resistance thermometers is the change in
ambient temperature which affect the resistance of the connection wires between
the resistance thermometer and the Wheatstone Bridge which is used for
measuring the change in resistance. Using the three wires or four wire
connections can minimize this error as shown in Fig 2. In the three wire
connections any change in the resistance of lead 2 is added to the thermometer
resistance. However, this is balanced by the equal change in the resistance of
lead 1 which is added to the reference resistor. In the four wire connection
change in resistance of lead 1 and lead 3 are compensated by that of lead 2 and
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4, since the former adds top the thermometer resistance while the later adds to
the reference resistor.
Fig 2: Types of RTD Measurement System
2.3 Instrument Calibration
Calibration involved the adjustment of the measuring instrument or the device. However
determined at time when they are calibrated. Therefore when measurements are made
using a non-adjustable instrument, the procedure is to use the corrections given in the
calibration certificate. The corrections are determined by the difference in readings
between the Unit Under Test (UUT) and the Master Standard Unit (MSU).
The Test Uncertainty Ratio (TUR) signify the accuracy of the UUT to the accuracy of the
MSU given by;
TUR= accuracy of the MSU
accuracy of the UUT
A TUR of one order of the magnitude of 10:1 is ideal. However, this is difficult to
achieve since the UUT has high accuracy nowadays due to technology development as
such a TUR of 4:1 is recommended
2.3.1
Measurement Uncertainty
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Mean value

x
1 i 
  xi 
n 1 
(1)
Variance
 2
1  i 

 
xi  x  


n  1  1 
 
2
(2)
Variance of the mean x

 2 ( x)
 2 ( x) 
n
(3)
Standard Uncertainty

 ( x)
u A ( x)   ( x) 
n
2.3.2
(4)
Degree of freedom
The degree of freedom in a measurement is associated with the variability of the
distribution. The smaller the sample size, the lower is the degree of freedom and higher is
the variability.
v  nc
(5)
where n is the number of observation and c is the number of constraints.
To obtain a normal distribution the sample size should approach infinity.
For finite sample sizes the degree of freedom is n-1, where n is the sample size.
The final measurement result could be expressed then as:


x  x   ( x)
(6)
Measurement uncertainty by combining the individual uncertainty contributions:
The total system uncertainty is made up of three components namely,,
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The uncertainty due to spread of the measurement data
Uncertainty contribution due to the resolution of the UUT
Uncertainty contribution due to the uncertainty in the MSU.
The combined uncertainty Uc in the measurement could be obtained.
uc  (u12  u22  u32  u42 )
(7)
The effective degree of freedom
uc4
neff 
c1u14 c2u24 c3u34 c4u44
(



)
n1
n2
n3
n4
(8)
where c1, c2, c3 and c4 are the sensitivity coefficients. If the neff is as greater than 2100, it
could be considered as infinity in the t-distribution chart to determine the value of
coverage constant (k).
The overall measurement uncertainty will be:
u  uc .k
(9)
The final measurement result could be expressed as:

x  x u
(10)
The procedure for calibration can be easily shown through the flow chart in fig 3. Firstly,
you must determine the appropriate Master standard. This can be easily evaluated using
the TUR formula. Measurement of the UUT is taken several times to determine the
spread of measurement repeatability . This is also known as draft calibration. Then a 5
point check is done throughout the full scale range to determine the linearity and
hysterisis of the instrument. The total measurement is then calculated and reported as a
means of measurement confidence.
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Laboratory Module
Start
Select MSU
Determine
TUR
Draft UUT
Calibration
5 Points
Calibration
Method
Uncertainty
Measurement
Calculation
End
Fig. 3: Calibration Procedures
2.4 Process Reaction Method –Ziegler Nichols
Tuning is the skill of selecting values for the tuning parameters P, T I, and T D so that the
controller could eliminate the error quickly to reach stability.
The open-loop method is based on the results of a step test for which the controller is
manually forced to increase its output rapidly. A response chart of the process variable is
known as the Process Reaction Curve.
A sloped line drawn tangent to the reaction curve at its steepest point or point of
inflexion shows how fast the process reacted to the step change in the controller's output.
The inverse of this line's slope is the process time constant T which measures the severity
of the lag.
The reaction curve also shows how long it took for the process to demonstrate its initial
reaction to the step (the equivalent dead time L) and how much the process variable
increased relative to the size of the step (the process gain Kp). Ziegler and Nichols
determined that the best settings for the tuning parameters P, T I, and T D could be
computed from T, L, and Kp as shown in table 4:
Refer to Fig 4 for the type of response plot.
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Laboratory Module
Fig 4: Response Plot of Ziegler Nichols Method 2,Open Loop
Controller Mode
Proportional
Integral Time
Derivative
Band PB (%)
TI (sec)
Time
TD (sec)
P
100* Kp*L/T
off
off
PI
110*Kp*L/T)
3.3*L
off
PID
83*Kp*L/T
2.0*L
0.5L
Table 4: Ziegler Nichols Method 2
Once these parameter settings have been set into the PID formula and the controller
returned to automatic mode, the controller should be able to eliminate future errors and
achieve stability.
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Laboratory Module
3.
COMPONENT AND EQUIPMENT
1.
2.
Temperature Control System Chamber - YTCS-02
Handy Calibration – CA71
4.
PROCEDURE
4.1 Checking of Temperature Sensing Device
1. Use the source output of the CA71 Calibrator. Select the Current output range.
Disconnect the Jumper link and connect the CA71 to the right side of the jumper
link as shown in Fig 5.
2. Set the Temperature Controller to Manual mode by pressing the A/M key selector
switch .The Manual Indicator light will be lighted. Set the output of the heater
control to O=0 to avoid unnecessary heating of the oven chamber.
3. Conduct a 5 point check (refer result table) on the controller and recorder.
Tabulate the results in Table 2 and 3 and plot the hysterisis graph of the
instruments in Fig 6 and 7.
A calibrator CA71 is used to source current into the recorder and controller. The mA
current is used as the output from the type T Thermocouple and PT100 RTD transducers.
The range for both transducers are 0-100ºC is 4-20mA
The following set-up as shown in Fig 5.
Fig 5: Instrument calibration
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ENT 364/4 – Control Systems
Laboratory Module
4.2 Heating System with Thermocouple
1).
2).
3).
Select the thermocouple type K sensing input and short the Thermocouple type
Use Group 1 (PID parameters) in the controller for this purpose. Set the OT
parameter in the I/O page to OT =0. The heater relay will be connected to the
time proportional relay output of the controller.
The operating condition of the oven heating system need to be selected. The
operating condition such as the oven operating temperature range and
disturbance.
Blower Speed Control :
40%
Damper
:
20%
Set point
:
42C
Output
:
0%
Set the controller to Manual mode ‘M’ . A green light will appear indicating
manual mode. Record the stable temperature reading(ie room temperature)
5). In MANUAL operation mode set MV Heater output to O=50. Use the up/down
arrow key. Maintain a steady state plot and at a graticule crossing then
introduce a step change of heater output to O=50 in the controller output MV.
This will increase the output to heater rod by 50%.
6). The process reaction curve will be obtained as shown in Figure 8. Considering
a system with a pure time delay plus a first-order lag. Using the point of
inflexion approximating the maximum process reaction rate with the following
steps.
7). Draw a tangent to the process reaction curve at its point of maximum rate ( i.e.
at the point of inflexion.)
8). Calculate the equivalent time delay or dead time L (the time in seconds between
the step change and the point where this tangent crosses the initial value of the
controlled variable), the equivalent time constant T, and the process gain Kp.
Refer to table 4 for the formula .Measure the x an y axis with a ruler. The
process gain Kp is determined as follows:
Kp = change in the final and initial steady state/ change in
manual output
Kp = ΔPc / ΔPm
9). Enter the new values in Table 6.
10). Use result in the table to observe the different between P, PI and PID. Paste all
the sample on your report.(Calculation of Percentage of Overshoot, Settling
time, time constant, Max Peak, Steady state error and Error of the system must
4).
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5.
Laboratory Module
RESULT
Result for Experiment 4.1:
Table 2: Hysterisis Data for UT321 controller
Calibrator Source
Temperature
(mA)
Indicator(ºC)
4.00
8.00
12.00
16.00
20.00
Calibrator Source
Temperature
(mA)
Indicator (ºC)
20.00
16.00
12.00
8.00
4.00
Table 3: Hysterisis Data for UR1000 Recorder
Calibrator Source
Temperature
(mA)
Indicator (ºC)
5.6
6.88
8.16
9.44
10.72
12.00
Calibrator Source
Temperature
(mA)
Indicator (ºC)
12.0
10.72
8.16
9.44
6.88
5.60
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Laboratory Module
Fig 6: Hysterisis Graph for UT321 Controller
Fig 7: Hysterisis Graph for UR1000 Recorder
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Result for Experiment 4.2:
Fig 8: Process Reaction Curve with Thermocouple input/RTD
Table 6: Process Reaction Curve Optimization with TC input/RTD
mm= 3600s x _____/100 =
Time Constant =
mm = 3600s x_____/100 =
Gain Kp =
Controller Mode
Proportional
Band PB (%)
P
PI
Integral Time
TI (sec)
off
s
s
Derivative Time
TD ( sec)
off
off
PID
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ENT 364/4 – Control Systems
Laboratory Module
P Control using Thermocouple input Type K/RTD
PI Control using Thermocouple input Type K/RTD
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ENT 364/4 – Control Systems
Laboratory Module
PID Control using Thermocouple input Type K/ RTD
Fig 10: Output Response for different control modes
Result for Experiment 4.3:
6.
DISCUSSION AND EVALUATION/EXERCISE
Temperature Measurement
1) Explain the principle of thermocouples. Give examples of a few types of
Thermocouples
2) Describe the cold junction compensation in instruments
3) Explain the principle of Resistance Temperature Detector (RTD). Give
examples of a few types of RTD
4) Compare the performance of RTD and Thermocouple from the process
reaction curve . Describe the differences
.
Method of Tuning.
5) Describe the influence of Proportional (P), Integral (I) and Deferential (D)
6) Describe P control, PI Control and PID Control. Which control method is
best suited for Temperature Control .
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7.
Laboratory Module
CONCLUSION
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