Advanced process control

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Thread Starter

Arvind Sharma

I have been trying to identify process improvement opportunities in our batch plant where advanced process control can make an impact. I am currently implementing a Controllogix solution to a Waste water treatment
project. The Analog to Digital ratio is high, and there are quite a few PID loops. Most of the PIDs are for controlling the pH with a given set
point. pH is non linear and the rate of flow of waste water varies (comes in batches). This would be a good opportunity to try out some advanced
control algorithms, and I have been trying to figure out an appropriate software that would interface with AB Controllogix. Our vendors suggested using RSTune or RSLoop Optimizer. RSLoop Optimizer seems to be a good fit. I was wondering if one of you Process control gurus could shed some light on appropriate software packages. Is RSLoop Optimization the best Advanced
Process Control software in the market for our situation? Should I start thinking along the lines of Adaptive controllers? What other packages
should I be aware of? This would help us out immensely.

Thanks,

Arvind
 
Ph system is nonlinear control problem, the best way is to use Neural Networks. I am sure.

If you need help about neural net Control, let me know.
 
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pH control is highly non-linear. The process gain can increase or decrease over three orders of magnitude. What RSLoop Optimizer (AB licensed version of ExperTune) will do for you is linearize the controller input so that the PID will see a linear character of the process and hopefully maintain desired performance throughout the full range of operation. This may work if your process is stationary.

A non-stationary process will change its character within the same operating regime due to chemical or mechanical changes or disturbances to the process. If your process is non-stationary, the model of your process that linearizes it will become inaccurate and the controller performance will degrade (become too aggressive or too sluggish). If that is the case, you will want to find an adaptive controller. One that either updates it's tunings based on real-time process information or feedback of it's own performance, or one that updates a model of the process that is either used to tune a control algorithm or is used in a Model Predictive Control scheme. The latter is where on-line learning neural networks are often used. There has been lots of academic research in that area (including my graduate research) but I have not been active in that area and am not familiar with current commercial products. I have heard of Gensym's G2 expert and neural systems, but I am not familiar with their capabilities (www.gensym.com).

Another company to look into is NeuralWare (www.neuralware.com). They had been bought by Aspen Tech., but I think they are their own company again. They have neural network modeling software that can produce the code that implements the model, but you then have to implement that code somehow in your control system. They may have better tools now. I used it about seven years ago.

Good Luck! I am interested in hearing what approach you select and implement. Please let me know.

Chip Hinde
[email protected]
 
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Eduardo Manuel C. Cipriano

I think it's better to use these products because it is 100% compatible with your Hardware which is ControLogix RStune and RSloop are developed by Rockwell Automation and I'm pretty sure you wont be having problems in compatibility issues rather than using other Vendors

[email protected]yokogawa.com.ph

Eduardo Manuel C. Cipriano
Sr. System Engr.
Yokogawa Phils. Inc.
 
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We are the creators of both RSTune and RS Loop Optimizer. RS Loop Optimizer is the only one that supports ProcessLogix.

RS Loop Optimizer includes a linearizer for pH Control. See:

http://www.expertune.com/advanced.html#pH
I suggest using this with Feedforward. For a good reference on pH, see Greg Shinskey's book "pH and pION Control in Process and Waste Streams", 1973, John Wiley & Sons.

For a complete comparison of RSTune and RS Loop Optimizer features see:

http://www.expertune.com/CompareStdAdv.html
John Gerry
ExperTune Inc.


 
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This may sound stupid but you will find that you can improve your plant efficiency 20% with very little change in process control. Turn off your
computer and go out into the factory floor. Talk with your technicians. Watch your operators. Find out why zones are in Manual instead of
Automatic. Just watch your operators. If they adjust the same setting several times in an hour then ask why. Have lunch with them and joke about
running the process. Stay late and note the difference between first shift and third shift. You will be amazed at how much you can improve your facility by just doing things the way they were meant to be done.

Step two is to look at your sensors. You can't control what you can't measure. Many process problems are associated with poor thermometry.
Simply moving a sensor can make an improvement on its own.

I worked in one plant that improved their efficiency from 60% to 90% simply by painting the machines. The employees worked harder because they felt better about the plant. I also worked with some PhD's who had grand ideas for improved algorithms but had never been on the plant floor. Basically they didn't have a clue about the machine.

Cheers,
Mitch
 
If you are going for APC, sit and talk with your people about all the things that they want, that are likely to improve performance. Make a list of items that are required. Check for availability of readymade software's.

But the best strategy is to make the APC software inhouse due to the complexities involved.

Some notes.

Advanced Process Control is a very complex subject. Here you are talking about utilizing logic and software and probably also adding some field hardware to improve economics and/or controllability of process.

Unless there is a company which has made a software for your process, and for your size, the chances of an off the shelf product working well is very limited. In fact almost impossible.

A lot of customization needs to be done to any standard software. People have to be trained to operate the new system properly, else you will find that the new system is generally offline and slowly termed a failure. One very popular APC installation in one of m Ex-companies is lying unused. One of the VP's that I met some time back was lamenting on such a collosal waste of money. And it was very difficult to convince him that a system that works for a few days, should work in the long run, perhaps with a bit of troubleshooting.

Assess what other changes are likely in your plant (capacity changes and other modifications) that may adversely affect the system that you chose and so on.

You can also look at business.vsnl.com/deneb_solutions and find APC text for Profile based controls.

Anand

 
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Bruce Durdle

And another point about advanced process control schemes - they have to be operated by the usual guys on the control panel, long after the boffins who dreamt them up have written their PhD papers and moved on to greater things. They should not need continual high-tech input to get them on line, maintain them, or make minor changes.

Bruce.
 
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Jeffrey D. Brandt

Mitch:
Its called the Hawthorne Effect, named after the area the original experiment was performed. IE's and good managers are well aware of what it is and how it works. Probably, painting the machines
pink worked as well as moving the pay phones, or re-orienting the parking lot. The problem is that it never works for LONG.

I totally agree with your other points. Simple works, but, the problem with simple is that its not exciting to report, even when it DOES work.
I've found that its easier to request $2,000,000 for a new controller, than it is to ask for $2,000 to move a control pendant.

>I worked in one plant that improved their efficiency from 60% to 90% simply
>by painting the machines. The employees worked harder because they felt
>better about the plant. I also worked with some PhD's who had grand ideas
>for improved algorithms but had never been on the plant floor. Basically
>they didn't have a clue about the machine.

 
Bruce makes an interesting assertion with his statement; that the process to which the advanced controls are being applied remains constant. While this may be true in some facilities, in many more it is not. If the process is from a petroleum refining application (where I have most of my experience) the feedstock varies in quality from shipment to shipment, heat exchangers become fouled, or control valves are not as precise as they should be.

Three observations from my experience:
1) The Advanced Process Control MUST have a champion, someone who understands what it is doing, believes that it will work and knows how
to make it work when it isn't.
2) The Advanced Process control is usually inferrential in nature, and MUST have an experienced person (most likely at the engineer level) who can monitor and tweak the controls to match the actual conditions in the process.
3) This is usually not the same person.

I have seen the gamut of APC work it a small refinery. In one case, the operators loved it because it reduced their workload immensely and
improved quality of product. During implementation of this system, when the engineer reported the status of the project, the unit manager indicated that he could tell how far along the project was by looking at the laboratory results. On the second, the operators fought the controls, and now less than 20% of the advanced controls are on line.
NOTE: The first had a strong champion, the second, due to personnel relocation, did not. Draw your own conclusions.

John Beck
Control System Engineer, Retired
 
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Bruce Durdle

That's the "bike shed effect". One of my colleagues had to make a presentation to the board of a "(very) large English company, requesting approval to spend several million pounds. The item on the agenda previous to his was a case to build new bike sheds as more employees were afluent enough to afford bikes instead of having to walk. Total cost of this -
around 2,000 pounds.

The board spent about an hour discussing the bike shed issue before agreeing to pay for a much watered down version. My colleague thought he
would never get HIS case even listened to. He didn't - they just said "Mr X must have put a great deal of thought into it, and we are sure it will be a resounding success" or words to that effect. No further discussion - approved unanimously.

Moral - people who make decisions know about things like bike sheds and can make meaningful comments and be seen to be saving the company money. People who make decisions know bugger all about complex issues like Advanced Process Control, and don't want to display their ignorance - anyway, if it's a mess, they know who to sack.

Bruce.
 
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