Interesting but some of us do this every day in their own field. What kind of systems do you think are notoriously difficult?
An applied controls book would be interesting. People are alway looking for application examples in their own field. It takes a lot of work to publish a book. Tim would be the expert on this. I know that even magazine articles take a lot of work.
A lot of books are NOT about the practicalities - Some people write books on control just to show their proficiency in Mathematics.
I can think of many BUT
Astrom and Hagglung have provided plently of material here - which I an in possession of
The idea I have is to target energy usage as the primary optimisation - using a variety of controllers to acchieve this - without introducing operator - setup difficulties.
My language thoughts on this are to transform the more complex controller types to an interface that emulates a PID controller for the operator - but the underlying controller will actually be more complex (not just for the sake of it)
That immediately introduces what many practitioners will perceive as a fairly artificial criterion. Fact is that for process plants, energy conservation tends to be a second-tier incentive for good control.
In fact, one often wants control goals that _maximize_ the energy consumption -- at least the energy consumed by the actuators. This is because you're often putting a controller in there to increase bandwidth, and that means pushing your actuator hard.
Where I've seen energy conservation used as a parameter it had more to do with fooling an otherwise robustness-agnostic algorithm into toning down it's control rule in hopes that the result wouldn't send a real system flying into wild oscillations.
The ones I find hardest are the ones that are designed by engineers who really aren't control engineers, then are presented to me with the comment "we just know this should work right, but we can't figure out why it doesn't" :-).
I have been thinking that the world needs "The Art of Control Systems Engineering" in a book, similar to "The Art of Electronics" by Horowitz and Hill.
Most of the control books out there don't talk about the down & dirty aspects of sticky or compliant actuators, sensors with dead spots or inconsistent quantization, mechanisms that change their behavior if the bearings aren't preloaded or the bolts aren't snugged tight, and all those other 'touchy-feely' aspects of control. Even mine only devotes a chapter or two to coping with these effects, and the effects themselves could have been given more print space if I had wanted a much longer book.
This is not something that I could write by myself, and I'm not sure that it could be done with less than four people (which would be a nightmare). Why? I know a fairly large corner of low-end aerospace and high-end embedded control, but I have no practical experience with industrial control such as Peter does. Folks like Peter have experience with industrial control, but I'll bet you that neither of us has much mileage with Matlab-generated pipe dreams, and how to make them really work in actual control systems. If there is anyone out there who could write the whole book from practical experience, they could charge $500 an hour for their time, and they'd never want to do the writing.
Perhaps when I'm old enough to retire I'll know enough to start...
Books certainly take me a lot of work. I find the actual writing part easy; my Master's thesis is long enough for a small book, and it was not a big deal, because I just took the fallout from designing a radio and I wrote it down in a (hopefully) clear and concise manner.
"Applied Control Theory for Embedded Systems", however, took significantly more work, but not in the writing. What had me tearing my hair out, off and on for years, was designing the flow of the book so that it would be a self-study guide, deciding what I could leave out and what I must put in, and designing all of the examples so they actually worked like they said they did.
I know of people who just write and write and write. I stand in awe of them. I simply don't know how they do it, at least if they are writing good books.
"Applied Control Theory for Embedded Systems" --
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"A DGPS/Radiobeacon Receiver for Minimum Shift Keying with Soft Decision Capabilities: A Thesis Submitted to the Faculty of the Worcester Polytechnic Institute" --
On Mon, 15 Jan 2007 08:29:05 -0800, Tim Wescott proclaimed to the world:
I will give an example that I think demonstrates what you are saying in mixed terms, at least for me. Tell me if this fits what you are asserting.
We were redesigning a control system for a pumping station. The station has VS pumps that take from a 30 ft wet well/storage tank. The old system used PID controls of the three pumps in cascade. When a single pump reached 80% capacity, then the controls would add another pump and bring them to 40% each. During peak flows, the main system would change the set point for tank level from the normal 20 ft mark, down to 10 ft gradually over a period of 8 hrs or so. This scheme was in use to spread peak loads out over the low times. The system also had a algorithm that lowered the power consumption during peak periods by overriding the set points if the total plant was coming close to the peak power value set by the utility. They were under contract which penalized you if you exceeded the peak.
Anyway, during the redesign, it was argued for doing away with PID at the pumps because they had problems with stability. It place of PID the other controls engineer on staff to the engineering firm insisted that straight proportional control was all they needed and trying to keep a set level was needless. I argued that even though the system would be more stable, you would give away tank storage capacity equal to the range of the proportional system. He argued this would be slight because the proportional would be set at three feet. The system was pushing it's capacity for storage anyway, but now it appeared to be alright to give away 10% of that because they had trouble getting PID stable.
Please apply your idea to this system. I assume you would say in my system that I wanted to maximize energy usage of the pump system because it responded faster and to a greater degree.
Here I am assuming you are arguing with me in my case, but you can see that energy conservation needs to be better defined.
I find it hard to believe that a tank level control can't be made stable.
Again, this happens when there isn't a good understanding of what the system is doing.
I don't think anyone is arguing the point that one can use the tank as an accumulator so that peak demands are reduced. This is done all the time in hydraulic control systems. It makes good sense. Why size the motors or pumps for peak loads when they can be run at an average load? This just takes a little more design work up front to optimize performance.
I agree with Tim about the bandwidth. I have only been asked once about LQR type of control in 25 years and I don't think that guy really know what he was asking for. Most of the time I get asked about how fast can I move from here to there and there are no concerns about the power required. A lot of my customers are happy to run the system in saturation until the last millisecond before ramping down to a nice stable stop with no overshoot.
It's still a second-tier incentive for good control. If energy saving was a plant's primary incentive, the solution is easy - turn off the power and go home. You can't get much better than that...
The amount of inflow is uncontrolled, but the maximum rate of level change is known: the interceptor is only so big, and flows by gravity. The rate at which pumped flow can be allowed to change is very limited. Do not underestimate the water-hammer effect in a few miles of 1-meter pipe.
I think that energy spent in the actuators is a small part of the energy of the plant. Moving water is much more costly than moving valves.
Not sure where your comin from Cameron - Sure - Energy saving is a secondary consideration - BUT - tell your top management you can save them x amount of dinari per blip and see what happens -
How can you save energy on a motion application? The amount of work required to move an object from point a to point b will not change no matter what control algorithm you use.
Energy will not be saved by the control algorithm. Motion control requires that energy added to the system is equal to the work required on a millisecond to millisecond basis. I don't care what control algorithm you use that balance must exist if one is going to follow a motion profile.
P=F*v P=M*a*v
Since the acceleration and velocity are carefully controlled at each point along the path the power required at each point will be the same and the energy required is just the sum of the power required at all the points.
The power or energy optimization is done at design time by reducing mass or friction.
On 16 Jan 2007 02:40:05 -0800, "Peter Nachtwey" proclaimed to the world:
And it can, but the problems came from this being a 1970 built system with three pumps that cut in and out. You need a control that takes this into account. You need a system that takes kilotons of inertial energy into account. You just can't try to stop 6 miles of water in a three foot pipe to come to a stop very quickly. Early on, they cracked a check valve trying. I have been in the pump pit when a pump is shut down and water hammer slams the valve shut. It is scary. The building shakes to the point where dust rains down from the roof.
This is not what I was asking or saying. Of course accumulators work. That is what the tanks were there for. I was saying that control redesign would make 10% of the tank capacity unusable for accumulation. They used tank storage to keep from going over a peak, set by the power company. Going over the peak during certain hours of the day caused them to get fined by the contract. They used storage tanks hold wastewater during the day and pump it at night when energy costs were lower too.
But I was talking about an industry where energy conservation has a lot to do with the process. Bandwidth has little to do with anything. The OP brought up different concerns his design software looked at. I don't know what kind of controls it was intended to be used with.
Ok, so you don't want the motion control guys to win. That doesn't sound fair. ;) Fine. So what do you have in mind?
On a motion control application, like a flying shear, one can save energy by minimizing the length of travel required to synchronize and make the cut and get back. Minimizing the required distance minimizes the required energy. The question is how much does each part of the motion controller contribute to the minimizing the required motion. Is the the PID? Feedback resolution? Or is the motion profile itself. Maybe it is all of the above and more.
Perhaps someone knows of an application where the control algorithm is the significant contributor to efficiency.
Off the top of my head there is generalised minimum variance control where a cost function is used to minimise the effort and the errort simultaneously.....this is a fairly complex affair but the idea is nevertheless important....it has many successful applications that apply to energy saving and improved accuracy (crushing of limestone, manufacture of paper, focusing of telescopes....)
No doubt someone has written a neuro-fuzzy-generalised minimum variance-self-tuning -PID controller....:-)
Not everyone agrees with such uneccessary complexity....i guess it is application specific...but ....my original questions were really regarding saving energy whilst NOT introducing complexity for the operator.....
The PID is the workhorse......and is understood universally...this is true ...but is is it the most cost efficient....consider a 1% increase in efficiency for allthe PID controllers in a large chemical processing plant...over the lifetime....
I am suggesting that it would be worthwhile to, at least, consider..the implications of introducing a more complex controller (for a set typeof processing models...benchmarks) and write the software in such a way that the tuning be indistinguisable from a normal PID.....a tall order perhaps...
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