Exercise 6-3 Level Process Control EXERCISE OBJECTIVE In this exercise, you will perform PID control of level process. You will use the open-loop step response method to tune the controller. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: The open-loop Ziegler-Nichols method DISCUSSION This exercise presents one more method of tuning controller. This method is bsed on the chrcteristics of the process rection tht re redily obtined from the response to step chnge. The exercise then pplies the open-loop Ziegler-Nichols method to level process. The open-loop Ziegler-Nichols method This method of controller tuning ws developed in 1942 by John G. Ziegler nd Nthniel B. Nichols. It enbles the opertor to clculte the P, I, nd D tuning constnts required for P, PI, or PID control of process bsed on the open-loop response of the process to step chnge in the set point. The open-loop step response method is performed ccording to the following procedure: 1. With the controller in open-loop mode, crete step chnge in controller output. The resulting chnge in controlled vrible should be typicl of the expected use of the system. You cn use clibrtor insted of the controller to crete step chnge. 2. Bsed on the response curve of the controlled vrible, determine the process gin, the ded time, nd the time constnt of the process. Clculte the vlue of the prmeter 3. Using the process chrcteristics found in step 2, clculte the tuning constnts of the controller s shown in the following tble. Festo Didctic 87996-00 299
Ex. 6-3 Level Process Control Procedure Outline Tble 6-3. Control prmeters for the open-loop Ziegler-Nichols tuning method (nonintercting idel controller). Mode Proportionl Gin Integrl Time Derivtive Time P - - PI - PID Once the tuning constnts of the controller re djusted to the clculted vlues nd the controller is returned to the closed-loop mode, typicl chnge in the set point should produce the desired qurter-mplitude decy response. The controller should lso be ble to correct for lod chnges rpidly, without excessive overshooting or oscilltion of the controlled vrible. Note, however, tht smll redjustments of the P, I, nd D tuning constnts my be required to obtin the optimum controller setting. It is importnt to note tht the formuls given bove pply only to non-intercting, idel controllers. Other formuls must be used for series or non-intercting prllel controllers. An dvntge of the open-loop step response method is tht the process needs to be disturbed only once to obtin the required process chrcteristics. On the other hnd, the determintion of precise process prmeters requires few clcultions nd, often, some djustments. PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up nd connections Purging ir from the pressure line Trnsmitter clibrtion Level process chrcteriztion Clcultion of PI nd PID prmeters PID control of the level process End of the exercise PROCEDURE Set up nd connections In the following exercise, you will chrcterize level process nd use the openloop Ziegler-Nichols method to set the PID controller. A solenoid-operted vlve, FCV1, will be used to crete sudden chnge in process lod. 1. Set up the level process shown in Figure 6-31. Mount the DP trnsmitter nd the column on the expnding work surfce so tht the pressure ports of the DP trnsmitter re pproximtely one row of perfortions bove the bottom of the column. Connect the rotmeter outlet to the column port with pipe tht extends down into the column. 300 Festo Didctic 87996-00
Ex. 6-3 Level Process Control Procedure Block the unused hose port of the column using provided plug. Firmly tighten the top cp. Connect the terminls of the solenoid-operted vlve (FCV1) to the 24 V dc power supply, which sets the vlve in the fully open position. The column will first be operted in the pressurized mode in order to purge ir from the hoses between the bottom of the column nd the pumping unit. Tip : use two short hoses with joint here SP Figure 6-31. PID control of level process. 2. Power up the DP trnsmitter. 3. Mke sure the reservoir of the pumping unit is filled with bout 12 liters (3.2 gllons) of wter. Mke sure the bffle plte is properly instlled t the bottom of the reservoir. 4. On the pumping unit, djust vlves HV1 to HV3 s follows: Open HV1 completely. Close HV2 completely. Set HV3 for directing the full reservoir flow to the pump inlet. 5. Turn on the pumping unit. Festo Didctic 87996-00 301
Ex. 6-3 Level Process Control Procedure Purging ir from the pressure line 6. With the controller in the mnul (open-loop) mode, set the controller output to 100%. This cuses the wter level to rise in the column. 7. Bleed ir from the high-pressure line of the DP trnsmitter. Using plstic tube with mle quick-connect fitting t one end, perform the following steps: While directing the bre end of the tube into the reservoir of the pumping unit, connect the tube mle fitting into the bleeding port below the highpressure port. This cuses wter from the line to flow towrds the reservoir. When constnt strem of wter is flowing out of the tube nd no ir bubbles re present, disconnect the tube from the bleeding port nd return it to the storge loction. 8. Close vlve HV1 completely, then disconnect the solenoid-operted vlve from the power supply. This cuses the wter level to rise further in the column. 9. Set the controller output to 0% to stop the pump. 10. Remove the plug t the top of the column nd connect this port to the remining uxiliry return port on the pumping unit, using n extr-long hose. 11. Set the controller output to 50% to mke the wter level rise in the column, then open vlve HV1 completely. From now on, it is importnt tht the wter level in the column not be llowed to fll below 4 cm (1.5 in), s this cuses ir to enter the line between the bottom of the column nd the pumping unit, which in turn prevents the wter in the column from dropping below certin level when the pump speed is reduced or the pump is stopped. Should this sitution occur, purge ir from this line by plcing the column in the pressurized mode gin. Trnsmitter clibrtion In steps 12 through 18, you will be djusting the ZERO nd SPAN knobs of the DP trnsmitter so tht its output current vries between 4 ma nd 20 ma when the level of the wter in the column is vried between 5 cm nd 55 cm (2 in nd 22 in). 12. Connect multimeter to the 4-20 ma output of the DP trnsmitter. 302 Festo Didctic 87996-00
Ex. 6-3 Level Process Control Procedure 13. Mke the following settings on the DP trnsmitter: ZERO djustment knob: MAX. SPAN djustment knob: MAX. LOW PASS FILTER switch: I (ON) 14. Adjust the controller output nd HV1 opening until the wter level is stble t 5 cm (2 in) in the column. This is the reference level. From now on, it is importnt tht the wter level in the column not be llowed to fll below 4 cm (1.5 in). 15. While observing the multimeter reding, turn the ZERO djustment knob of the DP trnsmitter counterclockwise to decrese the current nd stop turning it s soon s the multimeter reds 4.00 ma. 16. Redjust the controller output nd HV1 opening to rise nd stbilize the wter level to 55 cm (22 in) in the column. 17. Adjust the SPAN knob of the DP trnsmitter until the multimeter reds 20.0 ma. 18. Due to interction between the ZERO nd SPAN djustments, repet steps 14 through 17 until the DP trnsmitter output ctully vries between 4.00 ma nd 20.0 ma when the level of the wter is vried between 5 cm nd 55 cm (2 in nd 22 in). Level process chrcteriztion 19. Hve the following signls plotted on trend recorder: Controlled vrible, the DP trnsmitter output Mnipulted vrible, the controller output 20. Adjust the controller output until the wter level is stble t 5 cm (2 in) in the column. The controlled vrible should be t 0%. 21. Suddenly increse the controller output by 10% of spn. For exmple, if the controller output is t 39%, set it to 49%. 22. Once the controlled vrible hs stbilized, stop (puse) the trend recorder. Festo Didctic 87996-00 303
Ex. 6-3 Level Process Control Procedure 23. Determine the process gin, the time constnt nd the ded time by nlyzing the recorded trend. Record these vlues in Tble 6-4. Tble 6-4. Chrcteristics of the pressure process. Chrcteristic Vlue Process gin, Time constnt, Ded time, Clcultion of PI nd PID prmeters 24. Clculte the vlue of the prmeter. 25. Using the open-loop tuning formuls pproprite for the configurtion of the controller you re using, clculte the prmeters for PI control of the level process. 26. Using the open-loop tuning formuls pproprite for the configurtion of the controller you re using, clculte the prmeters for PID control of the level process. PID control of the level process 27. Hve the following signls plotted on the trend recorder of the controller: LVProSim Set point, () Controlled vrible,, the DP trnsmitter output From the Settings menu, chnge the smpling intervl to 200 ms. Refer to Appendix B for detils on how to use LVProSim. 304 Festo Didctic 87996-00
Ex. 6-3 Level Process Control Procedure PI control 28. Adjust the controller tuning constnts for PI control of the level process, ccording to the vlues clculted in step 25. 29. Adjust the set point to 30%, plce the controller in the utomtic (closedloop) mode, nd let the controlled vrible stbilize. 30. Crete sudden chnge in set point from 30% to 60% nd observe the response. Does the controlled vrible hve qurter-mplitude decy? Explin. 31. If necessry, try fine-tuning the P nd I constnts of the controller to reduce the stbiliztion time nd overshooting of the controlled vrible following 30-60% step chnge in set point. Record your observtions nd results. PID control 32. Adjust the controller tuning constnts for PID control of the level process, ccording to the vlues clculted in step 26. 33. Crete 30-60% step chnge in set point nd observe the controlled vrible response. Record your observtions. 34. Try fine-tuning the P, I, nd D constnts in order to reduce the stbiliztion time nd overshooting of the controlled vrible following 30-60% step chnge in set point. Record your observtions nd your results. Festo Didctic 87996-00 305
Ex. 6-3 Level Process Control Conclusion 35. Try different set point vlues by incresing or decresing the set point through 20% step chnges. Does the controller tuning remin cceptble over brod rnge of set points? Fine-tune, if necessry. 36. When the system is well tuned, djust the controller set point to 50%. The presence of residul wter in the overflow hose might cuse the process to be unstble even t low controller gin. 37. Power (open) the solenoid-operted vlve FCV1. Is the controller ble to rpidly correct for the lod chnge without oscilltion of the mnipulted vrible? Explin. End of the exercise 38. Stop the pump nd turn off the pumping unit. 39. Disconnect the circuit. Return the components nd hoses to their storge loction. 40. Wipe off ny wter from the floor nd the trining system. CONCLUSION In this exercise, you performed PID control of level process. You used the open-loop step response method s strting point for controller tuning, then you fine-tuned the controller to reduce the stbiliztion time nd overshooting. You observed tht the level process demonstrted conditionl stbility becuse it ws stble over some intermedite rnge of controller gin; but it ws unstble t both lower nd higher controller gin. You lso observed tht derivtive ction ws unnecessry in this process, since it tended to mplify the smll fluctutions in the controller output nd since it did not decrese the overshooting or the stbiliztion time of the controlled vrible. REVIEW QUESTIONS 1. Who developed the open-loop step response method? 306 Festo Didctic 87996-00
Ex. 6-3 Level Process Control Review Questions 2. Briefly describe the procedure to follow in pplying the open-loop step response method. 3. Which process chrcteristic determines both the integrl time nd derivtive time when using the open-loop step response method? 4. Cite n dvntge nd disdvntge of the tuning method presented in this exercise. 5. With the P, I, nd D controller settings used in the exercise, wht is the effect on the process response of using column of lrger dimeter? Festo Didctic 87996-00 307