Experiment 1 PID regulating of water level of a single water tank system

Experiment 1 PID regulating of water level of a single tank systemObjective1.This experiment helps students learn the structure of a single loop feedback system and itsworking principle.2.It also helps students study the step response of the system with P, PI or PID regulation.3.It helps students study the anti disturbance performance of the P, PI or PID regulated system.4.Students do qualitative analysis of influence of parameter variation on the system’sperformance if P, PI and PID regulators are used respectively.Equipment1.The process control equipment THKGK‐1 consisting of GK‐02, GK‐03, GK‐04, and GK‐07 .2.MultimeterputerTheoryIntroduction of the single tank water level control systemThe single tank water level control system is shown in Fig. 7‐1 and Fig. 7‐2. This is a single loop feedback control system whose objective is to keep the water level equaling to the required value while minimize the error caused by inner or exterior disturbance. Single loop control system is widely applied because of its simple structure, low cost, convenient operation and also because it satisfies many general processes.Figure 7‐1 The block diagram of the single tank water level control systemOnce the hardware of a single loop system is set up, the controlling quality depends substantially on the selection of the regulators’ parameters. Appropriate parameters could resultin satisfying controlling effect. Otherwise, if improper controlling parameters are employed, the regulating effect becomes bad or the system may even break down. Therefore tuning the regulator parameters is very important work after the system is set up.When the system is switched from manual operation to automatical operation it must not induce turbulence, which requires that the regulator output can follow the manual control andensure that no difference between measurement value and required value exits at the switching instant.Generally the system with a P regulator has static error. The value of proportional band affects the quantity of static error and also relates closely to the dynamic performance of the system. Because of the function of the integral, a PI regulator contributes zero static error to the system. Moreover, appropriate value of parameters and can also provide good dynamic performance. A PID regulator can not only eliminate the static error but also improve the dynamic performance. With the unit step input, the response of P , PI and PID regulating system are shown respectively as the three curves in Fig. 7‐3.Figure 7‐2 Sketch map of the water tank systemFigure 7‐3 Step response under P ,PI and PID regulating are shown as curve ①,②, and ③Experiment Content and Procedure1. Proportional (P) regulating① Connect the modules to form a single loop feedback system, referring to Fig. 7‐1. Theprocess is the top one of the three water tanks, and the controlled variable is the water level .② Start the devices and instruments, and tune the zero point and the gain of the sensor’soutput.③ Guided by the teacher, power on the control screen of the microprocessor, and start thecomputer supervising system. Prepare for the recording of transient curve.④ With the loop opened, tune the controlled variable manually to the desired value by theT Tank 2TransducerMotorPumpPID regulatorReference inputTank 1ValveDisturbance“manual operation” switch of the regulator. Normally the water level is controlled to be at 50% of the tank height.⑤Observe the curve being shown on the computer screen, and wait for the curve to reachthe desired value, then switch the regulator to proportional regulating situation—the integral time constant is set to the maximum value and the integral and derivative switches rest at “off” position. After that, set the proportional band a middle value, push the “direct‐inverse” switch to “inverse” position, push the “manual” switch of the regulator to “automatical” position. Such that the system works in a closed loop condition.⑥Wait for a while for the water level to reach a stable state, then add a disturbance to theprocess ( usually a change of the reference input by positive or negative 10% magnitude is taken as a disturbance). The water level varies because of the disturbance. Observe the fluctuation of the water level caused by the disturbance till it become stable again.⑦Reduce the proportional band , repeat step 6, observe the transient curve on thescreen, and record the curve.⑧Increase the proportional band , repeat step 6, observe the transient curve on thescreen, and record the curve.⑨Select an appropriate value of so that a satisfying process curve could be obtained.Record this value of .⑩Pay attention that students must wait for the system to achieve a stable state every time they want to execute a new operation.2.Proportional and integral (PI) regulating①Based on the P regulating, an integral function is added in the regulator (the “integral”knob is tuned to a middle position and the “integral” switch is set at “on”). Observe whether or not the controlled variable (the water level) can reach the reference input, so that to verify that the PI regulating could remove the static error.②Set a middle value to the proportional band and keep it unchanged. Set integraltime constant T a large value, and give a step disturbance to the process and observe the curve of the output signal (water level). Set integral time constant a small value, and givea step disturbance to the process and observe the curve of the output signal (water level).Record the overshoots in both cases in table 1.Table 1 The overshoot referring to different values of and a constant value of Integral time constant Ti Large value Small valueOvershoot Mp③Set the integral time constant a middle value and keep it unchanged. Set a largevalue, and give a step disturbance to the process and observe the curve of the output signal (water level). Set a small value, and give a step disturbance to the process and observe the curve of the output signal (water level). Record the overshoots in both cases in table2.Table 2 The overshoot referring to different values of and a constant value of Proportional band Large value Small valueOvershoot Mp④Select appropriate value for and so that the system has a satisfying transientoutput signal’s curve when it is exposed to a step disturbance. This curve could be obtained by changing the reference input from 50% to 60% of full range.3.Proportional, integral and derivative (PID) regulating① Based on the preceding PI regulating, a derivative function is added in the regulator,that means the “D” switch is turned on. A disturbance with the same magnitude of that in preceding PI regulating is given to the process. Record the dynamic response curve of the controlled variable. Compare the curve with that one obtained in preceding PI regulating, and get a conclusion of influence of derivative control on system performance.②Select appropriate value of , and so that the step response of the system is asatisfying transient curve ( The step input can be implemented by changing the reference input from 50% to 60% of the full range).③Record all the transient curves by the software in the computer and analyze theperformance of different regulators.Attention①When the circuit is connected, it must be checked and approved by the teacher beforeit is powered on.②It must be supervised by the teacher when students start the computer and themicroprocessor screen.③If the parameters are not appropriate, the system may work improperly.Requirement for the report①Plot the block diagram of the single tank water level control system.②Practice switching from manual control to automatical control and describe the way tomake sure that the output variable does not change sharply at the moment of switching.③Record or draw the step response curves referring to different values at P regulatingmode.④Record or draw the step response curves referring to different values and constantat P regulating mode. Also record or draw the step response curves referring to different values and constant .⑤Draw the step response curves of PID regulating, and analyze the function of Dregulating.⑥Compare the static errors and dynamic performance of the system obtained at P, PI andPID regulating mode.Questions①How to reduce or eliminate static error? Is it possible for the proportional regulating toeliminate the static error?②Do qualitative analysis of the influence of parameters variation of , and onthe system performance.。