Work summary of week 4

In the last week, we obtained the frequency f of the signal by fast fourier transform (fft) method. However, the resolution ratio of the frequency is 10kHz. This is because resolution ratio equals to sampling frequency (2*10^9Hz) divided by sampling points(2*10^5). Thus, we regarded f as 40kHz. In week 2, the smooth Q-U Lissajous was achieved. That graph contains about three and a half periods of data, so there are three curves mixed with each other. What we want is the Q-U Lissajous figure with just one period, so we modified the program. If we use the former 57000 data points to plot the graph, it seems like a closed curve for one period as shown below.

 

 

Next, the total work of one period, i.e. the area surrounded by the curve, could be calculated by the following equation,

According to the above equation, we wrote a program to compute the work and the result is 0.0016J. Then, the discharge power of the DBD plasma reactor could be gained according to

So far, the most important part of the project was completed. To make it more impeccable, some characteristics of the input signals should be showed. In week 1, the waveform of the applied voltage and the capacitor voltage had been displayed. Thus, we wrote a program to gain the maximum value, minimum value, mean value, rms value and peak-to-peak value of the input signals. Matlab has the ready-made functions of these characteristics, so it was easy to compile the program.  

 

Finally, to display the information intuitively, it was essential to create an interface to integrate all the results and figures together. This could be done by GUI of Matlab, but we had never used it before. Hence, further work should be done to learn how to use GUI to make the interface. The main task of the 5^th week is to show all the information on one interface.  

Work summary of week 3

As stated in the previous blogs, the main problem we are facing is how to calculate the frequency of the signal. This is very important because to calculate the discharge power, the equation used includes the value of frequency and the sum of the products of the instantaneous voltage across the reactor and the change of the accumulated charge on the capacitor.

It used to be a problem as how to calculate the frequency using fast fourier transform before because each time we tried to generate the plot of the signal in the frequency domain using matlab, it turns out to be something like what the following figure shows.

In the figure there are two apparent frequency peaks, one is at around 0Hz and the other is about 2*10^9Hz. It is normal that there are two frequency peaks with the same amplitude in the frequency domain for one signal because it is such a characteristic for the fourier transform. Usually we would take the first half of the plot as our results. However, this time the frequency of the first peak is so close to 0Hz and it influences the results seriously. Thinking about the problem for a long time, I find there is a possible reason that the frequency of the first peak is so close to 0Hz might be that the range of the x-axis is too large. Therefore to solve the problem, I bound the border of the x-axis to a rather smaller range and suddenly the plot turns out to be successful and it can be seen in the following figure.

With this step successful, we are able to march on to the next step in the next week which is about to find the x-axis value of the first peak in the frequency domain which should be the frequency we are searching for and also the frequency of the target signal.

Work summary of week 2

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.

Work summary of week 1 and before

Getting the topic of this experiment, I, Liang Tang and my teammate Chunyang Song started to think about the topic together with searching for the related information about the topic. The topic of our year 2 project is "Online measurement of the discharge power in a DBD plasma reactor".

Before meeting our project tutor, we had already gained much knowledge about the topic by searching on the internet. The information we got related to the topic can be found via the following links:
http://en.wikipedia.org/wiki/Dielectric_barrier_discharge
http://hal.archives-ouvertes.fr/docs/00/25/55/61/PDF/ajp-jp4199707C405.pdf

Then on the 11 of December we first met our project tutor, Dr. Tu, and learned that our topic is about using the software Matlab to calculate the discharge power in a typical DBD(dielectric-barrier discharge) plasma reactor. Moreover, we got a few files from Dr. Tu of the background knowledge related to the DBD plasma reactor and some conventional rules of calculating the discharge power.

Before the experiment period in week 1, we have been quite familiar with the DBD plasma reactor and the method of calculating the discharge power. Then on last Friday, which is the experiment day, we met our project tutor again and got the input data of the project.

According to the information, to calculate the discharge power of a DBD plasma reactor, the circuit needs to be connected as follows:

Then with the voltage across the plasma reactor and the instantaneous charge on the capacitor, the discharge power of the plasma reactor can be calculated. Luckily, the applied voltage, the capacitor valtage and the capacitance are known value to this experiment, and the problem that the experimenters need to face is how to handle the data so as to get the value of the discharge power.

Firstly, we drawed the waveform of the appiled voltage and capacitor voltage shown as below.

  
With the input data given, we eventually got the Lissajous figure using Matlab, as shown in the following figure.

This is ought to be the first step of calculating the discharge power of a DBD plasma reactor according to the information we gathered.

However, as there were too many groups of data(approximately 200000 groups), we did not know how to calculate the frequency of the given signal, which stopped us from going further.

After the experiment time, we decided to review the information we had and try to find out the method of calculating the frequency of the voltage signal using Matlab.