Hi, to create a Nyquist plot from the measured voltage and current data obtained in the time domain, you need to perform a Fourier Transform to convert the data into the frequency domain. However, simply taking the FFT of the voltage and current signals might not provide accurate impedance values for a fuel cell.
You may consider the following approaches to calculate the Nyquist plot from your measured data:
- Preprocess the data:
- Ensure that the voltage and current signals have the same length and are properly aligned.
- Apply any necessary filtering or preprocessing techniques to remove noise or artifacts from the signals.
2. Perform a Fourier Transform:
- Use a suitable Fourier Transform method, such as the Fast Fourier Transform (FFT), to convert the voltage and current signals from the time domain to the frequency domain.
- Apply a windowing function if needed to reduce spectral leakage.
3. Calculate impedance:
- Divide the voltage spectrum by the current spectrum to obtain the complex impedance at each frequency bin.
- Note that impedance can be calculated as impedance = voltage_fft ./ current_fft, assuming the signals are properly scaled and aligned.
4. Plot the Nyquist plot:
- Create a scatter plot using the real and imaginary parts of the impedance values.
- Use the plot function to plot real(impedance) on the x-axis and imag(impedance) on the y-axis.
Following is an example code snippet illustrating the steps mentioned above:
voltage_fft = fft(voltage);
current_fft = fft(current);
impedance = voltage_fft ./ current_fft;
plot(real(impedance), imag(impedance), 'o');
ylabel('Imaginary Part');
