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. 1 ENT 364/4 – Control Systems Laboratory Module 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 Extension and Compensating Leads 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. 2 ENT 364/4 – Control Systems 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 3 ENT 364/4 – Control Systems Laboratory Module 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 4 ENT 364/4 – Control Systems Laboratory Module 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 not all instruments are adjustable. Instruments that are non-adjustable, corrections are 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 5 ENT 364/4 – Control Systems Laboratory Module 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 nc (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,, 6 ENT 364/4 – Control Systems Laboratory Module 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. 7 ENT 364/4 – Control Systems 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. 8 ENT 364/4 – Control Systems 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. 9 ENT 364/4 – Control Systems 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 10 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 T link. 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 : 42C 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 calculated in your report) 4). 11 ENT 364/4 – Control Systems 5. Laboratory Module RESULT Result for Experiment 4.1: Table 2: Hysterisis Data for UT321 controller Upscale Reading Calibrator Source Temperature (mA) Indicator(ºC) 4.00 8.00 12.00 16.00 20.00 Down Scale Reading Calibrator Source Temperature (mA) Indicator (ºC) 20.00 16.00 12.00 8.00 4.00 Table 3: Hysterisis Data for UR1000 Recorder Upscale Reading Calibrator Source Temperature (mA) Indicator (ºC) 5.6 6.88 8.16 9.44 10.72 12.00 Down Scale Reading Calibrator Source Temperature (mA) Indicator (ºC) 12.0 10.72 8.16 9.44 6.88 5.60 12 ENT 364/4 – Control Systems Laboratory Module Fig 6: Hysterisis Graph for UT321 Controller Fig 7: Hysterisis Graph for UR1000 Recorder 13 ENT 364/4 – Control Systems Laboratory Module Result for Experiment 4.2: Fig 8: Process Reaction Curve with Thermocouple input/RTD Table 6: Process Reaction Curve Optimization with TC input/RTD Dead Time = 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 14 ENT 364/4 – Control Systems Laboratory Module P Control using Thermocouple input Type K/RTD PI Control using Thermocouple input Type K/RTD 15 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 . 16 ENT 364/4 – Control Systems 7. 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