# teach me PID

Transfer function: s^2+196/(s^2+100) Design a PID controller such that the first spike in the frequency response plot(bode diagram)is smoothed out.
The step response is oscillatory, but the Z-N tuning rules require it to be monotonic. The nyquist plot is singular due to poles on im axis, but Z-N rules require it to intersect the negative real axis. Can a PID controller be designed using Z-N rules?
Thanks
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Just went through older topics n found expert's comments on ZN tuning.....Does a better tuning method exist?
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Depends on whether you're talking about tuning on paper or tuning a real system.
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Scott
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You know darn well this is a student question. One can tell from the transfer function. So how do you calculate the gains on paper?
Peter Nachtwey
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This looks strange to me.

This sounds like home work to me.

Yep, one can see the damping factor is 0.

Yes, but I don't think they will work well for this.

Yes, this is obviously a homework problem. How do you get a transfer function where the order of the numerator is the same as then denominator?
I didn't have time to look at this in detail and take into account the zeros. I can place the three closed loop poles at -10 with the gains of Ki=5.012 Kp=1.02 and Kd=0.153. I used pole placement. If this is a student problem then the answer will not do you much good without knowing how to place the poles. One word of warning. I ignored the s^2 in the numerator. The best solution may require separate gains for the forward path so the result is a two degrees of freedom PID.
Look up pole placement or look for some of my threads where a mention Ackermanns method.
Peter Nachtwey
Peter Nachtwey
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What about dividing through by the denominator, and getting something along the lines of 1 + [As+B]/[s^2 + 100] instead of the transfer function given. It looks like it either turn into a mess, or turn into a nifty superposition problem.
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Scott
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On Wed, 09 Jan 2008 03:46:44 -0800, snovite wrote:

Would you like us to CC your professor with our responses?
Z-N rules are for tuning a system when you don't know, and never want to find out, the transfer function. If you have the transfer function already, then you can come up with a controller using pole placement.
Like Peter, I see some practical trouble involved with the improper fraction -- you're specifying a system that responds immediately, which simply doesn't happen in real life. Even if the above is an accurate model in that the real thing responded much faster than any controller you could devise, you'd probably be smart to intentionally add some lag to your controller, to prevent the controller's unmodeled dynamics from causing oscillation.
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Tim Wescott
Control systems and communications consulting
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Voltage divider??
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Scott
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On Thu, 10 Jan 2008 13:02:40 +0000, Scott Seidman wrote:

Divide it's length by the speed of light -- at best you'll see frequency dependent effects starting at around 1/10 the resulting frequency, depending on the physical design you may see them at much lower frequencies.
A better example for my purposes would be the op-amp. You can design circuits with it _assuming_ infinite gain, and do things like using resistive dividers for feedback. Many people do, in fact, and you can run into serious trouble with stability if you get too clever. This happens because while most op-amps these days have been carefully designed to be stable in a wide variety of circuits they cannot be stable in all; their propensity toward stability leads designers who aren't obsessive about it to fail to check, and they find those cases where the regular op-amp _isn't_ stable.
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Tim Wescott
Control systems and communications consulting
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Tim Wescott wrote:
...

meters -------------- is time. But what's a topsy-turvy among friends? meters/second
Jerry
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Engineering is the art of making what you want from things you can get.
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Pole placement doesn't work because the numerator is the same order as the denominator so ignore my previous answer. I used by last ditch tecnique which is to find the gains that minimize the sum of squarederror,SSE, between a desired closed loop transfer function's frequency response and the actual closed loop transfer function's frequency response.
The first .pdf shows how to smooth out the spike caused by the imaginary poles. It doesn't do anything about the imaginary zeros which cause the dip in the response.. The first .pdf calculates the gains and generates a Bode plot.
The next .pdf plots the poles and zeros. Notice the poles are in a safe location and clustered relatively close to the desired location ot -2PI. This removes the spike caused by the imaginary poles. ftp://ftp.deltacompsys.com/public/NG/Mathcad%20-%20T0C1-Min%20Bode%20snovite%20B.pdf
Notice that the proportional gain is negative. The proportional gain can be negative IF it is only used in the feed back path. This means the controller needs to be a I-PD controller.
This problem is not a good one for learning PID tuning, gain calculations or anything but frustration. It is a flawed problem so the answers aren't that good either.
Peter Nachtwey