Write 2 pages thesis on the topic will be provide. Procedures and Results The flow response and the levels of the loops were examined by making changes to the set point at each loop during the familiarization process. While the flow control set point was set at 2.5%, the level was kept at 60%. Following system stabilization another flow controller set point was introduced at 3% and the flow valve was found to be closed from 30.1 to 47.5%. Additionally the level valve had also closed from 47.7% to 25.6%. A procedure similar to the flow control response was used for the level loop response test. The set point in the level was decreased from 3 to 3% and following system stabilization the flow set point percentage was found to remain constant but the position of the flow valve had closed slightly from 30.1 to 47.5%. Additionally, the level valve had closed from 32.7 to 25.6%. The results were recorded.
The level of the tank and the capacity section was ascertained with the help of a meter stick attached to the tank and the reading was compared to that of a controller. The compared levels ((%) vs. level) was plotted in meters and from the figure the line of best fit as determined by the transmitter gain was found to be1.206 %/cm.
The next part of the experiment involved comparison of readings taken from four different flow measuring devices to a standard change in volume over time. While the flow rates of the rotameter and transmitter were read in USGPM, those of the controller and console were taken as percent rates. With gradual increase in the height over time the changes noted in the height was used for volume calculation. Appendix 2 includes the flow rates and after conversion of the actual flow rates to USGPM, a comparison between all the actual, rotameter and transmitter flow rates indicated a mutual agreement in the rates. The flow rates of the console and controller were closer as they used the same controller.
The characteristics of the valve were determined for the flow and level value by increasing the flow percentage and taking the valve position values from the display controller. A plot of the Fractional Flow (Flow %/Max Flow %) vs. fractional valve opening which is indicative of the flow and level valve can be used to determine valve characteristics such as to quick-opening, linear, or equal-percentage relationships. In this case, a plot of both valves showed an almost linear valve character except for a slight distortion in the quick opening which can be attributed to drop in pressure across the value. While the valves had identical and linear characters originally, following installation and use a slight distortion in their characteristics would have ensued. The results are also indicative of the gains for the various valve positions. There is a higher capacity for flow rate in the level valve as compared to the flow valve and this explains the reason why it is not completely opened all along as it allows the same flow at lower valve opening.
The PI controller was further examined in order to determine that it functioned as per the equation 1 in the theory. In this effect a source of error and output trajectory was generated for the PI controller by making changes in the set point and analyzing the controller output values in accordance to the time. Thus in this section the main purpose was determining the existence of an offset for the proportional controller. The tank can be prevented from emptying by turning off the integral mode and introduction of a 60% to 40% flow into the system. The system gain was increased at every run and the corresponding level offset values were recorded both prior and after the run. Additionally the level controller valve position was noted both prior and after the change of flow. In addition, the valve position for the level controller was recorded before and after the flow change. While the theoretical values of the offsets can be calculated from equation, the practical results provide evidence for the decrease in offset values with increasing gains.
This section explains the fine tuning of the PID controller which is necessary to achieve a quicker and reliable response. The experiment is begun by turning off all the integral and derivative modes and then slowly introducing a low gain to the controller. Following introduction of a set point change the response of the system was observed. In case the system remained stable, the gain doubled and a further change in set point was introduced into the system. The gain was reduced to the original level and kept constant when the system became unstable. Further integral actions were introduced and the same procedures were followed. The integral values were reduced for each run and system stabilization was observed. Derivative actions were introduced in the end which reduced the stability of the system. From the results it was observed that upon increasing the gain in the absence of integral and derivative control the system became unstable. However, when the gain was maintained at a constant level and when maintaining the integral value at half the system was found to be more stable. The presence of any derivative control caused the system to become unstable immediately.
This section examines the effective behavior of the level loop following changes in the controller gain. In order to eliminate any offset the integral was suitably fixed at a value and a 10% change in the set point was introduced into the system and the subsequent values in the level output and time were recorded. Upon plotting of the level output and time, the original set point was at 40% and 30%. This is an indication of system impair that nullifies the system and this trend continues. It can be further observed that an increase in controller gain is indicative of a higher damping ratio which causes lesser overshoot and a quicker response when nearing the set point. The percent overshoot was found to be 63% as indicated in Figure 1.14 in the lab manual which has resulted in a providing a damping ratio of 0.12. A similar procedure was conducted and the resulting natural frequency and damping ratio were determined both theoretically and practically and the results recorded.
Both the electronic and flow level trainer is intended for students and has been designed for their demonstration as it allows them to have an hands on experience in opening and closing the loop level control and thus in manipulating the measurements. Such manipulations can be done using a controller that is run with a microprocessor. A delta V Emerson system which is popular in the industry is used and a digital instrumentation is used in the system. The electronic system has been part of the technology since its inception with the MS windows platform has been used to control the input and output devices which are actually placed on the flow level trainer. The control calculations are performed in this area instead of the computer which only delivers current to the control region. The input modules present in the control rack convert the current signal to the digital format. The measurements are then processed based on the programmed algorithm and the data is sent to the output modules. The output module later translates the digital data into current which is then sent to the actuators and valves. As the system is protected by a password it has to be booted by the instructor. Once the tank is checked for the presence of required quantity of water the open directory icon is selected followed by the flow level trainer. Few other options can be chosen such as the controller face plate manual which can be selected if the control output is set manually and the option Auto is selected following which the controller resets itself to the set point. Other options include detail, primary control, trend, control studio and alarm knowledge icons with each having their own uses. The majority of the work carried out in this lab involves the controller face-plate, the detail and tuning plot windows. The flow was controlled using an electric flow trainer. In the 1-2 experiments the volume is proportional to the change height as the fluid occupies more space with the container having the same cross-sectional area. The slope obtained in the graph for experiment 2 was used in calculations for the 6th experiment. With increase in the console flow there is a subsequent increase in the flow valve and level valve and the speed of response of the flow loop must be faster compared to that of the level loop. The output value was calculated in experiment 3 and was not required to be calculated. When the system shows zero error the effective controller output is found to be equal to m. From experiment 4 it is learnt that setting the gain at a high level result in control loop instability and with an increase in the level gain there is a decrease in the offset. The results from experiment 5 were checked visually and at the ultimate gain value with 0 derivative the integral the system is under-damped. Doubling of the value of I, which caused the system to become under-damped, it results in over-damping of the system. Experiment 5 includes all the other visual observations and in the last experiment the natural frequency was found to be ideal, but the damping ratio was found to be lesser than the ideal value. The natural frequency was found to increase in proportion to the level gain in order to stabilize the system.
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