The caption for Fig. 4 on the following page:
is "Setup for Measuring Open-Loop Response". I thought I understood the
term "open-loop" until I saw this diagram. I'm having trouble thinking
of the pictured setup as measuring the open-loop response of the system,
since the loop is, well, /closed/.
Thanks in advance,
On Mon, 16 Nov 2009 23:54:45 -0800, Rick Armstrong wrote:
Looks like I should be reviewing that article for clarity. Goody, more
In an ideal world you wouldn't have that complicated a setup. In an
ideal world you'd just drive your desired signal into the controller, and
look at the output from the plant.
But this is the real world, where we have noise, plus offsets and other
worse nonlinearities (for a simple example try driving a double
integrator with a sine or cosine in the time domain -- you'll see what I
mean). So we need a way to get the system to settle to it's normal
operating conditions, while still allowing us to take sufficient
measurements to find the open-loop response.
What that setup does is allow you to operate the system in closed-loop
(which is often the only way you _can_ operate a system to get a decent
measurement), yet still collect enough data to determine the open-loop
gain and phase.
Essentially you're exciting it with a sine wave (the input to the
rightmost summation block), and you're taking the ratio of the two
outputs (u_b and y). The gain and phase shift from u_b to y is the ideal-
world open-loop response of the plant cascaded with the controller, even
if you could never get the system to operate in open-loop mode.
Ok, thanks. I had the right idea in mind, then. I was just hung up on
> Looks like I should be reviewing that article for clarity.
No, I think the chapter is plenty enlightening.
By the way, I've just about got my act together to the point where I can
start taking response measurements of my quad copter in the see-saw test
P.S. And thanks, Dave y for chiming in.
I didn't wade through the article above, but there's various
configurations that can be used to measure an open loop frequency
response. The old 'HP' has an application note on this. I don't have
the title or link right at the moment, but you can search it out if
you try hard enough.
On Tue, 17 Nov 2009 12:14:31 -0500, dave y. wrote:
I learned that method from the HP control systems analyzer that was
designed to use it (and reference the thing's manual, in fact) -- the
article would probably bear more than a passing resemblance to the
One thing I kinda remember is some measurement configurations are
easier to achieve than others, and it's temping to take the easy way
out, but the easy ones have a bias in the measurement. I think it
came down to you had to break the loop to inject a signal vs. just
measuring the system as is. Wish I could remember what that note was.
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