This time, you will use the spectrum analyzer to automatically apply a range of sinusoidal inputs to the motor. The magnitude and phase will be computed automatically.
Turn off the power supply, oscilloscope, and fcn. gen., and turn on the spectrum analyzer. | |
Attach a "T" connector to the output of the spectrum analyzer. | |
Connect one side of the "T" connector to input 1 of the spectrum analyzer with a co-ax cable. This allows the spectrum analyzer to read it's own output. | |
Detach the co-ax/pigtail cable from the fcn. gen. and connect it to the free end of the "T" connector on the spectrum analyzer output. | |
Detach the co-ax cable from the input 2 of the oscilloscope and attach it to input 2 of the spectrum analyzer. |
Press Inst Mode button. | |
Select Swept Sine. | |
Press Freq button. | |
Set Start to 0.2Hz. | |
Set Stop to 100Hz. | |
Set Sweep to AUTO, LIN, and UP. | |
Select Res. Setup. | |
Set Auto Res. to ON. | |
Set Min Res to 101 pts/sweep. | |
Set Max % change to 2.5%. | |
Press Source button. | |
Set Level to 200mVpk | |
Press Scale button. | |
Set Autoscale to ON. | |
Turn on power supply. | |
Press Start button. Motor will begin to oscillate back and forth. The frequency will gradually increase (it spends more time at lower frequencies). The whole process takes several minutes. Wait until the analysis is complete. | |
Press Disp Format button. | |
Print this screen. | |
If you have time, repeat the analysis between frequencies of 1Hz and 1000Hz. |
The lab writeup is due on the Tuesday 10 February. Writeups can be turned in jointly or individually. Your lab writeup should include the following information. Be sure to justify all of your comments.
The final value of the output. | |
The time it takes to reach 95% of the final value. | |
The initial slope of the graph after the step. | |
The location where the initial slope line intersects the final value line. |
Using the manual data, plot Magnitude (in dB) vs. frequency, with frequency in rad/s on a log scale. | |
Also plot Phase Angle vs. frequency, with frequency in rad/s on a log scale. | |
Using the bode command in Matlab, plot these same plots using the transfer function obtained in the step response portion of the lab. | |
Compare the frequency plots from Matlab, from the manual experiment, and from the automated experiment both qualitatively and quantitatively. Remember that the automatically generated plots are plotted with frequency in Hz, not rad/s. How closely do they match? Are there any extra features in the experimental plots? If so, explain why there may be extra features. | |
How might these high-frequency artifacts effect a control system? | |
If you generated an automated plot over higher frequencies, describe what is happening at higher frequencies. |
The magnitude (in dB) at low frequencies is equal to 20*log(K*K_{a}). from this, calculate K. | |
Compare this to the step response result. |
Fit a -20dB/decade sloped line to the downward-sloping portion of the magnitude plot. Mark a horizontal line at the magnitude where the low frequency magnitude lies. The frequency where these two lines cross is called the break frequency. What is this frequency? | |
Compute the time constant as the reciprocal of the break frequency (in rad/s). | |
How does this compare to the step response result? |