Unity gain F/B v. Compensator in F/B

I'm sure this is a naive question but I have been using software ( provided by a kind gentleman on this NG, was it Jerry? ) called Rootloc, which plots
out root locii and Bode plots etc, but only for unity gain feedback systems, i.e. the Open Loop forward function is G(s) so Rootloc calculates G(s)/[1+G(s)].
If I cascade a pole-zero compensator Gc(s) with the process function, then take unity gain feedback to the summing point this is fine. The closed loop function is G(s)Gc(s)/[1+G(s)Gc(s)]
Surely though this will not always be possible, for instance where the process function is mechanical, and the compensator is electronic, the only sensible place to put the compensator is in the feedback loop.
I then get a different closed loop response, G(s)/[1+G(s)Gc(s)].
This gives me the same denominator ( characteristic equation as before ) but a different numerator ( different zero ). Therefore the root locus plot is the same in both cases ( based on characteristic equation ), but the frequency response will be altered ( as the zero influences the magnitudes of the poles )
Thus a compensation network for a process will be different depending on whether it is in forward cascade or in the feedback path. Can anyone confirm my feedback 101 analysis, and suggest a source ( er, free would be nice of course ) for a program that can handle frequency response plots for CLOSED LOOP transfer function inputs, rather than one that accepts the open loop transfer function and assumes unity gain feedback?
I've had a look, but haven't found anything that actually works,
cheers,
Andy.
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Andy wrote:

Correct
A compensation network in the forward or feedback path will be the same if you locate the poles in the same place, and will generally have to be the same for the same gain & phase margins (your open-loop Bode plot remains the same, just as your root locus plot does). As you point out arranging some or all of your compensation in the feedback will change the system characteristics. Sometimes this will be for the better, sometimes for the worse.
I use SciLab (http://www.scilab.org ). It's a close cousin to Matlab, with a similar execution window/scripting environment. In my opinion it is better in many ways; it's primary drawback for me is that it's graphical simulator is far more difficult to edit than Simulink. It is a _very technical_ environment, but for control system stuff it can't be beat:
- you can enter a transfer function directly, e.g. H = %s / (%s + 10); - you can cascade two transfer functions, e.g. G = H * (1 / (%s + 20); - you can apply feedback, e.g. G_cl = G /. 1; - you can make Bode, Nyquist, Evans (root-locus) and Nichols plots with simple calls - It will do automatic exact conversions between s-domain to z domain (assuming a zero-order hold). - It supports state-space representations (good for maintaining accuracy with high order systems). - etc.
I like it a LOT; to my knowledge it has all the functionality of the Matlab with the Control Systems toolbox, it's free and the English-language newsgroup is quite responsive. I use it almost exclusively, with only occasional excursions to MathCad to solve symbolic problems.
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Tim, here's a *really* naive question: Can you give the curious an example of the kind of real-world project you might actually use this stuff on??
I'm a "Black Box with A/T" person myself and spend most of my time dreaming up better ways to connect one black box to another. I often see adverts for MathCad and MatLab and lug them straight in the bin - although now I come to think of it, maybe we did do some MatLab at Uni - but that was an awfully long time ago.. ;-)
Cameron:-)
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Cameron Dorrough wrote:

_If_ you're working with a system that is well behaved enough that you can make a decent model of it then SciLab (or MatLab) will help you build the model, analyze the model, simulate the model and then make pretty graphs to convince your boss that you know what you're doing. Excel will make pretty graphs, too, but they don't help much later in the project when the system doesn't work the way you thought. MathCad is _much_ better on the analysis side than MatLab -- it's basically a free-form spread sheet rather than a command line with a programming language, which is what MatLab or SciLab is.
I think you come from the process control industry, yes? Where the controllers are all accessible, tuning is a normal part of maintenance, and the equipment costs more than the engineering? If so then these tools would only be a real help when you're commissioning a plant or large part of one, particularly if you're not certain if it's actually going to work with the control strategy that you've cooked up.
Where I end up using these tools is for OEM systems where a number of them are going to be built and the customer needs to build controllers to one design and the tuning is fixed. In that case then I need to be able to predict the system performance over manufacturing variations and aging, because a system that isn't up to snuff at build time or after wear isn't "in need of tuning" -- it's "faulty", and if it's faulty because of the tuning then it's a design error on my part.
If you never find yourself staring at Excel thinking "God this is lame" then you don't need MathCad. If you never find yourself writing little programs in C or BASIC to do simulations or analyze a system then you don't need SciLab or MatLab.
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proclaimed to the world:

Since finding this group, one of the things I found interesting is the differences in outlook and attitude according to the particular industry and in general the shortness of temper. Perhaps this is our nature. I'm learning a lot and not just about controls.
Be well,
HoP
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HoPpeR trading at 1492 wrote:

You know, even on a second reading I can't see where I sound irritated -- or was the "temper" comment just general? USENET tends to foster that attitude -- the social punishment for blowing one's top is much less severe on a newsgroup, probably because much as we may want to neither of us will ever successfully poke the other in the nose to culminate an argument.
Yes, I have learned that "control", while usually applying the same principals, varies quite a bit in the details. I do control rules for itty bitty clean things that get made in quantity; you and (I believe) Cameron do control rules for great big dirty things that may be similar to other great big dirty things but really are one-offs. This leads to some pretty significant differences in how we conduct our day to day work, yet we are constrained by the same rules of physics.
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proclaimed to the world:

The comment was just general. A lot is perceived. Some is as you say, you can get away with it. A little is arrogance. I am and I try not to be. It's difficult being smart and not getting arrogant and we are all smart. I've had some very humbling things go on in my life and it has taught me some things. I don't mean my comments as criticism to anyone. Just an observation.

Industries also spawn their own nomenclature, different term for the same beast. Sometimes I find that two different industries think of the same thing completely different. It's interesting to see how things evolve. Frustrating too. The coal mining industry is a good example. You can step into a new industry and look the fool because of this.
Be well,
HoP
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HoPpeR trading at 1492 wrote:
...

Shortness of temper? Can you cite examples?
Sein Sie gesund,
Jerry
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Engineering is the art of making what you want from things you can get.

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proclaimed to the world:

I don't think this would be productive. As I said this may be perceived. I am accused of this often myself when actually it is my passion for the field so if I am making this mistake about others, it would be ironic.
If we were all in a room I would be able to read people meanings. Here I cannot. My own experience on the groups has taught me to always make an extra effort to make clear my meaning. I'm so focused on a subject that I do not take the time to see how something sounds to others and egos can be fragile. I've had this problem with customers where I said something that was taken differently than I mean. I try hard not to do this.
Be well,
HoP
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Yes. The equipment costs *way* more than the engineering - so much so that clients expect the controllers to run their plant straight out of the box. Well, hey, that's what the Sales Rep said, so it must be true! ;-)

I will remember that. Thanks!.
Cameron:-)
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On Thu, 10 Nov 2005 18:04:22 +1100, "Cameron Dorrough"

And they think that they guy that ends up making it all work does not deserve to be paid. Sound familiar?
Sorry, I've been sitting here doing all the maintenance on my families laptops for the past couple of nights and am a little tired. Another pet peeve.
Be well,
HoP
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Tim, Thanks for that, I downloaded the Scilab stuff. It's about as user-friendly as a chain-saw isn't it?! I'm hoping that it might make a bit more sense if I look at it again, but at the moment it looks as if I need to learn a whole programming language just to plot out a Bode plot. Ho hum,
Andy.
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Andy wrote:

I disagree. Chain saws usually come with instruction manuals.
H = syslin('c', (%s + 10) / (%s * (%s + 100)))
bode(H)
clf; // clears the figure
nyquist(H)
clf; evans(H)
You can zoom in on figures, or use the isoview command.
When I said highly technical that's what I was talking about -- there's a pretty high sill on the door getting in. Once you figure it out you can do some extremely heavy duty analysis, however.
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