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chee291_labview_project - Introduction In this project a...

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Introduction In this project, a hypothetical system simulated in National Instruments™ LabVIEW™ software [2] had to be modified to achieve particular results. The system consisted of a water tank in which two inlet valves supplied cold and hot water at constantly fluctuating temperatures [3] . The main purpose of the project was to design a software controller which would keep the tank height at 12 m and the tank temperature at 50˚C by automatically adjusting the flow rates of the two valves; this goal was accomplished by integrating a feedback system based on proportional control principles into the original simulation environment. Theoretical Background In process dynamics, there exist three basic modes of feedback controllers [1] : proportional, integral and derivative. Proportional control used here is based on reducing the error signal, e(t) , to zero [1] : e(t) = R(t) – B(t) , where R(t) – set point, B(t) – variable measurement. The controller’s output value is proportional to the error signal [1] : p(t) = p + K c e(t) , where p – bias value (current variable) and K c – controller gain constant. In integral controllers , the output is proportional to the error and its duration. Integrating the error over time gives the accumulation of uncorrected offset, which is then multiplied by the integral gain constant and added to the controller output [1] . In derivative controllers , the rate of change of the process error is determined from the slope of the error over time; this value is multiplied by the derivative gain to give the output component [1] . The controller gain directly adjusts its sensitivity to offset. Increasing the gains generally results in a less sluggish response, but for too large values of K c , the response may have an unwanted degree of oscillation or become unstable [1] . A midway K c value thus gives best results. Results Figure 2. Front panel of the program. In the front panel depicted above, the leftmost section is the process control section, while the rightmost one with the four plots is the graphing section. Both are described below.
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P a g e | 2 In the left bottom corner of the control section, user can adjust the temperature and height gain constants of the controllers. Although both the cold and hot water valves influence height and temperature values, the temperature gain was linked to the hot water valve, which was observed to affect the temperature property the most, while the height gain was linked to the cold water valve (see Discussion). The valve flow indicators are found above on the left; they represent the current values of flow in the valves. On the right, two graphical representations of the tank height and the thermometer
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chee291_labview_project - Introduction In this project a...

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