Re: Nyquist by hand

pnachtwey wrote:
On Jun 15, 11:56 am, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
pnachtwey wrote:
On Jun 15, 7:41 am, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
pnachtwey wrote:
On Jun 14, 6:01 pm, Jerry Avins <j...@xxxxxxxx> wrote:
dave y. wrote:
...
I would recommend Scilab, because it is very powerful and entirely
free alternative to expensive Matlab.
Best regards,
Asim Vodencarevic
A comment--I would think everyone here is taking | has taken
one or more control system classes. Therefore I would assume
everyone has at least a student version of Matlab available.
Isn't this correct?
I graduated in 1962. There was no Matlab then.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
I graduated in 1975. There was no Matlab then either that I know of.
Go ahead and post Matlab script. Scilab can translate most Matlab to
Scilab.
Does anybody really use Nyquist or Nichols plots? I know poor
students are forced to waste their time with them. I have never seen
the need.
Peter Nachtwey
Even in 1988, when I got my degree, one was expected to construct
Nyquist plots with a calculator and graph paper.
I use Nyquist plots all the time. If you're going to do
frequency-domain tuning, then you have to use Nyquist and Bode plots
extensively.
I can see using Bode plots. It would be much easier to see the
inflection points on a Bode plot.
You can glance at a Nyquist plot and see the maximum sensitivity -- just
look at how close the trace gets to the -1 point. Bode is better for
seeing the phase and gain crossings, for predicting how fast your system
will operate, and for getting an idea of how you next need to adjust the
controller (or that you're done tuning that particular controller topology).

I usually plot both when I'm tuning a system.
So what do you do with it?

I use the Nyquist plot to see how close to the 3dB or 6dB sensitivity circle I am, and I use the Bode plot to guide me in tuning my gains.

How do you calculate the gains then?

I use a heuristically guided stochastic conjecture refinement process (I make a wild-ass guess, then I use my experience to guess again until I get it right).

You
have never said. I still prefer my pole and zero placement.

Which requires that you make an educated guess at the plant model, as well as placing (guessed-at) constraints on your target pole locations. At that point, if someone say "how much can things vary before this system won't perform to spec" I can't see how you can do anything but shrug.

I prefer to put my guesses where they don't have as much effect on the stability analysis.

But to each their own -- if you believe it works best, it probably does for you.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

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