EXPERIMENT 6 CLOSED-LOOP TEMPERATURE CONTROL OF AN ELECTRICAL HEATER

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1 YEDITEPE UNIVERSITY ENGINEERING & ARCHITECTURE FACULTY INDUSTRIAL ELECTRONICS LABORATORY EE 432 INDUSTRIAL ELECTRONICS EXPERIMENT 6 CLOSED-LOOP TEMPERATURE CONTROL OF AN ELECTRICAL HEATER Introduction: The main objective in this exeriment is to: Learn the basics of Oen-Loo and Closed-Loo temerature control. Observe the effects of each controller (P-Controller, I-Controller, D- Controller), on the dynamic behavior of the system, searately. Learn the basics of PID (Proortional, Integral, Derivative) controllers and a tuning method for finding the values of the controller constants. Equiments: Table 1. List of Equiments DC Power Suly Reference Variable Generator Temerature Controlled System Power Amlifier PID Controller Metra Hit 25S Multimeter (x 2) General Information: Oen and Closed Loo Control of Systems: Each system has at least one inut and one outut. The systems outut is called the rocess variable and this rocess variable (PV) can be changed to a desired value by setting the inut signal to a secific value. Figure 1 shows the system block diagram and the equation reresenting its inut to its outut is given below. Figure 1. System to be controlled Equation defining the system outut (Y(s): rocess variable) to the inut signal (X(s): inut signal) is called the transfer function. EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 1/9 Last udated March 29, :15 PM by D. Yildirim

2 Ys () Hs () (1) X() s If the system outut is controlled, by an oerator changing the inut signal manually, to have a desired value then; the system is said to be controlled in Oen-Loo. Figure 2. Closed-Loo controlled System Figure 2 shows a system controlled in Closed-Loo form. A sensor is used to samle the outut signal; this samle is subtracted from a reference value giving an error (E(s)). The reference value set to be roortional to desired outut value. The system is automatically controlled by a controller according to the error signal. Once the error is set zero the system outut (Y(s)) has reached its desired value (Y ref (s)). Proortional - Integral - Derivative (PID) Controller: In revious classes** you have designed a closed-loo system using roortional (P) controllers. You have exerimented and investigated that the P-controller can reduce the effects of disturbances and maintain an automatic control for a system. You have also seen that there was an unwanted nonzero steady-state error. You also figured out that trying to reduce the steady state error by increasing the gain of the P-controller would affect the settling time of the system in a bad manner. In this work you will see that these roblems (Steady-State Error and Settling Time) can solve by adding integral controller (I-controller) and a derivative controller (Dcontroller). The PID controller has a transfer equation as shown below: k 1 I C( s) k kd s k 1 TD s s TI s : k (2) This is main term defining the gain of the controller, also known as the gain of the P-controller. ki s 1 or : T s I EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 2/9 Last udated March 29, :15 PM by D. Yildirim

3 The second term in equation defines the I-controllers. This controller includes a term roortional to the integral of the error (E(s)) signal. By doing this it is ossible to eliminate steady-state error. k s D or T s : D The third term in equation defines the D-controller. Addition of this term in to the controller stage rovides an imrovement in the dynamic resonse of the outut. This controller can reduce the maximum overshoots and settling time of the outut. Sohisticated methods are available to develo a controller that will meet the steadystate and transients secifications for both tracking the inut references and rejecting disturbances. These methods require that the designer has either a dynamic model of the system or a detailed frequency resonse over a substantial range of frequencies. Both of these can be very difficult to obtain for some systems. This roblem engineers to develo some sohisticated methods in the identification of the system model. These methods lead to finding the necessary coefficients for a PID controller (k, T I, T D ). A general name for finding these arameters using some secial techniques is called Tuning. Ziegler-Nichols PID Tuning Method: J. G. Ziegler and N. B. Nicholas (1942, 1943) recognized that the ste resonse of a large number of systems exhibit an outut curve as shown in Figure 3. This S-shae curve is characteristic for many systems and can be generated by running the system in oen-loo configuration for a constant inut. X(t) Y(t) Y(t) Figure 3. S-Shae Curve at Outut for a Ste Inut This S-shae curve can be aroximated into 1 st order system with a time delay of L shown in Equation 3: sl Y() s K e Hs () (3) X ( s) T s 1 EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 3/9 Last udated March 29, :15 PM by D. Yildirim

4 In this method, controller first ut in manual mode where the outut is set to 20%. The controller is allowed to oerate at this ercentage of outut until the rocess reached a steady-state temerature allowing to temerature to line u. If the variation is lotted with resect to time, this means that the grah will show a constant line. This can be seen in Figure 4 as a straight line before the 10% variation is introduced. The next ste in the rocess is to increase the outut by 10% and record the variation with resect to time at the temerature outut using an A/D converter and a comuter where tyical variation is illustrated in Figure 4. Figure 4. S-Shae Curve at Outut for a Ste Inut Next ste would be the determination of rocess gain K which is the amount of change in the rocess variable ( PV) divided by the amount of ercentage change to the outut signal from 0-100% ( Outut). K PV Outut (4) Using Table 2, it is ossible to tune the PID controller just by looking at the curve in Figure 4. Please note that these values are first estimates for the controller. You will then lay with these arameters to obtain the desired characteristics. The effect of changing PID arameters searately on the resonse of system is summarized in Table 3. Table 2. Ziegler-Nicholas PID Tuning Controller Tye P-controller I-controller D-Controller P TC K DT - - TC PI 0.9 K DT TC PID 1.2 K DT 3 DT - 2 DT 0.5 DT EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 4/9 Last udated March 29, :15 PM by D. Yildirim

5 Table 3. The effect of changing PID arameters on the system resonse. Procedure of Exeriment: 1. Oen-loo oeration: Determination of heater characteristics Circuit Set-u: Assemble the circuit shown in Figure 5. Make sure that the PID controller is OFF, i.e., V ref is directly connected to ower amlifier. Set V ref to zero volts and aly ower to your circuit. You should see zero volts across heater (lam). No light will be emitted from lam. Write down the voltage at the outut of the sensor (voltmeter V tem ) and room temerature (T amb ) at this condition. Fill in values in Table 4. Set the fan adjustment knob to minimum value, i.e., equals to 1 and make sure that mechanical louver is closed, i.e., in osition 0. You should also connect reference voltage V ref and sensor outut voltage V tem to the Channel A and B analog inuts of the Cassy A/D converter, resectively. Turn-on the comuter and start Cassy rogram. Click on the left to circle in box named LD of Figure 5a to activate that channel and set its arameters according to Figure 5b. Reeat rocedure for other channel. Click on the Dislay Measuring Parameters button. Set samling interval to 0.5 sec and total number of samles to maximum value. Make sure that the switch on reference voltage regulator is in osition to zero volts. Adjust V ref to 10V this will aly 20V across the lam. Start the samling rocess by ressing F9 key. You should see on comuter screen 0V reference voltage and about 2.1V for the sensor outut voltage. 0.5 minutes after you start samling throw the switch on reference voltage generator to 10V. On the comuter screen you will see a ste change in reference voltage. Lam will be emitting high intensity light at this moment. Test time for this art should be long enough to obtain a nearly constant heater temerature (no large changes in temerature). When temerature levels out to a steady state value (near straight line) you can sto samling by ressing F9 key again. Write down in Table 4, time and reference voltage at the end of this art. Plot the variations of temerature in Celsius (T tem ) with time in your reort using the sensor conversion ratio. EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 5/9 Last udated March 29, :15 PM by D. Yildirim

6 Calculate the temerature san (T san ) of this rocess by subtracting the end temerature from the value at the beginning of test. Write value in Table 4. At this time cool the heater, by turning fan knob to 10 and mechanical louver in osition 4. After heater is cooled down, turn them back into original values. Determine the reference voltage value that will oerate the heater at 20% of temerature san (T san ) and reeat the above stes for this oerating oint. Start the samling and look at on the oscilloscoe until you see a straight line at this reference voltage level. At this steady temerature oerating with 20% value, introduce additional 10% change into the reference voltage and record the temerature outut of heater. You should obtain a curve similar to the one given in Figure 4. Write down the values for your system in Table 5 with reference to Figure 4. Figure 4. Temerature control of electrical heater, oen-loo. (a) Figure 5. Cassy rogram channel setu. (b) 2. Closed-loo oeration of electrical heater Circuit Set-u: Assemble the circuit shown in Figure 6. Using the temerature-time curve determine PID coefficients emloying Ziegler- Nichols method. Write values in Table 6. Adjust reference voltage such that heater temerature is 60 o C. Connect reference voltage V ref and sensor outut voltage V tem to the Channel A and B analog inuts of the Cassy A/D converter, resectively. EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 6/9 Last udated March 29, :15 PM by D. Yildirim

7 Monitor and record the ste resonses on the Cassy rogram for the closed-loo system using P, PI and PID controller, searately. Include resonses of individual controllers in your reort. If necessary, fine tune these coefficients to obtain best erformance and write these new values in Table 7. Introduce some disturbances to system that oerates with PID control at a steady-state oint and see the resonse of system to these disturbances. One examle of such a disturbance would be to increase the seed of cooling fan form 1 to 3. Include the resonse curve in your reort. Conclusion: Figure 6. Temerature control of electrical heater, closed-loo. The transfer function of the system you will use in the lab is as follows: sl K e Hs () T s 1 (5) where, K: is the gain of the system L: delay time T: rise time Figure 7 shows the MatLab/Simulink model of this system due to the transfer function given in Equation 5. The time delay (T D ) oerator in the transfer function was modeled with a transort delay element with a value of 7.3 seconds, the rest was modeled with a transfer function block with K s of 5.8 and T r of 60 seconds. Since the temerature starts rising from the room temerature and it must be added to the outut. The summing network outut has two gain functions as you can see the uer one has a gain constant 1 where you can measure the real temerature, and the lower one has a gain of 0.1 modeling the sensor so that you can use in your feed-back circuit. EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 7/9 Last udated March 29, :15 PM by D. Yildirim

8 Figure 7. Model of temerature controlled system Design and tune a PID controller using the Ziegler-Nicholas PID tuning method that has been shown to you in your Industrial Electronic course. You must design the closed loo system with PID controller using Matlab/Simulink and verify that the controller works. From results make a fine tuning which gives the minimum overshoot and minimum settling time with minimum error. References: [1] T. E. Kissell, Industrial Electronics: Alications for Programmable Controllers, Instrumentation and Process Control and Electrical Machines and Motor Controls, Prentice Hall, 3 rd edition, [2] T. J. Maloney, Modern Industrial Electronics, 5th Ed., Prentice Hall, EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 8/9 Last udated March 29, :15 PM by D. Yildirim

9 E X P E R I M E N T R E S U L T S H E E T This form must be filled in using a PEN. Use of PENCIL IS NOT ALLOWED EXPERIMENT 5: CLOSED-LOOP TEMPERATURE CONTROL OF AN ELECTRICAL HEATER STUDENT NO STUDENT NAME SIGNATURE DATE 1 2 INSTRUCTOR APPROVAL 3 4 Start End Table 4: Time (min) T amb ( o C) V tem (V) T san ( o C) Table 5: K DT TC Table 6: Electrical heater controller arameters, initial values Proortional (P) Proortional-Integral (PI) Proortional-Integral-Derivative (PID) P I P I D Table 7: Electrical heater controller arameters, values after fine tuning Proortional (P) Proortional-Integral (PI) Proortional-Integral-Derivative (PID) P I P I D EE432 Industrial Electronics, Sring 2012 Exeriment 6, age 9/9 Last udated March 29, :15 PM by D. Yildirim