In the first
week, we got the Lissajous figure using Matlab as shown in the second picture
in the first blog. The area of the shape surrounded by the blue line represents
the work done by the DBD plasma reactor in one period. However, the shape is
not smooth enough. This is because the output signals were interfered by some interference
such as noise. To calculate the area accurately, we should smooth the curve first.
Thus, in the second week, we wrote a program to take the average of every 200 points as a sample
and use the new chosen samples to draw the Lissajous figure. As shown in the
follow picture, the curve became much smoother than the last one.
The next problem
needed to be figure out was how to obtain the frequency of the signal. The shape
of the output signal is similar to the sin wave. Firstly, our thought was to
set a fixed voltage value as reference. When this value appeared a third time,
we could take this time interval as the period. Then the frequency could be
achieved by the equation f=1/T. However, in a short time interval, the voltage
values were extremely close to each other. Therefore, it was difficult to
calculate the period accurately by this way, and we gave up this method. Secondly,
we wanted to get a fitted curve by the cftool in the Matlab. Afterwards, we did
not use this ready-made cftool, because we preferred to write the program to
achieve the frequency. Finally, we discussed this troublesome problem with our
supervisor Dr. Tu. And he told us we could use fast fourier transform (fft) to
cope with it. Then, we searched lots of information about fft from the internet
and tried to compile the program. Unfortunately, it did not work. It might be
something wrong with the program.
Thus, the
main task of the next week is to get the frequency by fft method.
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