Understanding Bode Plots, Part 4: Complex Systems
So far, we have looked at the asymptotic behavior of first order constructs, like pure integrators or single poles and zeros. Once you start working with typical dynamic systems, it is very likely that you will have to deal with higher order polynomial expressions.
The trick to dealing with those is remembering that any polynomial, no matter the order, can always be factored into a bunch of first order constructs which will correspond to the real roots, and a number of second order constructs which will correspond to complex conjugate pairs of roots. Typical examples of second order systems are mass spring dampers and RLC circuits.
Both of this, depending on the ratio of either the damping to the mass or the resistance to the inductance, will have a pair of complex conjugate roots. Anyways, if we calculate the roots of that quadratic polynomial using the standard formula-- minus b plus minus square root of whatever-- we find that the complex conjugate pair of roots will have this form.
Note that these roots will only be a complex conjugate pair as long as the damping ratio zeta is less than 1. Anything greater than 1, and both roots will become real numbers, which means that the system will behave as the product of two first order poles.
As we did before to calculate the frequency response, we replace s by jw in the transfer function. So both the magnitude and the phase will be approximately 0. Finally, when the frequency w is much larger than the natural frequency the quadratic term will dominate. When taking the log, the square will come out and multiply the 20, so the magnitude will asymptotically approach a straight line with a slope of dBs per decade.
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The phase will go to degrees because G will now fall on the negative real axis. This adjusted value is what is called a damped natural frequency. And note that, in bode plot single pole case, the magnitude of the resonant peak will go towards infinity. A small value of the damping ratio means a higher and sharper resonant peak as well as a sharper shift in the phase.
We just saw how a function like Bode in MATLAB can quickly and easily create a frequency response plot directly from the dynamic equations of, or the input, output transfer functions of our system.
You can see that as we increase zeta, the magnitude of the resonant peak comes down and the phase transition becomes smoother. Here I want to highlight the damping ratio of 0.
This damping makes the magnitude -3 dBs at the natural frequency. At this point let me go back to our interactive design tool because there are a couple of additional things I want to highlight.
First, let me bring in a pair of complex conjugate poles, and I am going to place them close to 10 radians per second. Let me just make sure that I set the natural frequency to exactly I notice that, because I am starting with a damping ratio of 1, my polynomial is the product of two real roots.
As soon as I change the damping to any value lower than 1, let's say 0. If I choose a smaller damping ratio, what you're going to see is a sharper and higher magnitude peak.
In this particular case, I picked 0. Log of 10 is 1, times 20 is 20 dBs.
If I change the natural frequency, all I am doing is shifting the location of that peak. Now, if I want to increase or decrease the gain because of the properties of logarithms, we're just superimposing the effects of these two constructs on one graph.
In this case, low of a gain of 10 is 1, times 20 is 20 dBs. So all that has happened is the entire magnitude trace has been shifted up by Notice that the phase does not get affected at all. Let me add a zero now, at around 10 radians per second.
Understanding Bode Plots, Part 3: Simple Systems
Let me freiburg frauen kennenlernen make sure that my 0 is exactly at 10 radians. And the phase that was climbs up to Similarly, if I add auf der suche nach einem mann berlin pole, let's say at radians per second-- again, let me make sure it is actually at -- now the dBs per decade became at that 0, and then goes back to again after the new pole.
So, using this concept of superposition I can easily construct any transfer function that I am interested in studying. All I need to do is break down or factor the transfer function into smaller constructs, and then graphically add all of those traces together.
Understanding this simple concept can bode plot single pole quite powerful because it allows us to get a good idea of the primary dynamics of our system just by observing the magnitude and phase traces in a Bode diagram. Related Products.