Juggling percentages

[acollins 0]

Hello everyone, I'm new to this forum and Allen Bradley. I'm using RSLogix5000 (light version) to control a brewing process. The process requires a timed Boil-Off. This means that the customer eneters a time (90 minutes) and a percentage for the Boil-Off (7%), the means of control is a modulating steam valve that has to keep the kettle above boiling the entire time, but add BTUs to increase the Boil-Off if it gets behind. I have figured out how to get my percentages for time and volume. I can subtract the percentages from each other and try to maintain a 0 difference since I want them to track together so that at the end of the Boil-Off time (0%) the Volume left to Boil-Off should be 0%. If I can keep the difference between percentages at 0 I should be able to arrive where I want. My questions (after all of that) is can I use '0' as a setpoint for a PID? Is there a better way of doing this? Thanks, Art Collins

[TConnolly 13]

Greeting Art, and welcome to the forum. I don't know much about brewing processes but my first thought is to use a high-select PID setup. In a high-select, two PID loops run in parallel, one is controlling the boil temperature, the other the boil-off. Only one PID has control of the valve at any given time. In a high-select, that is the PID that produced the highest CV. The lower PID's CV is then updated to match. There is a technote in the AB knowledge base at http://www.rockwellautomation.com/knowledgebase/, search for technote ID 21693. that covers how to set up and use a low select PID. The high select will be basically the same. The Technote covers using it in a PLC/5 but the ladder PID in the CLX is not much different. Edited 23 Jul 2008 by Alaric

[acollins 0]

Thanks for the tip Alaric, I truly appreciate any and all help! I don't understand how doing the process this way (with 2 PIDs) will get the percentage of Boil-Off complete at the same time as the length of boil timer gets done. What would the competing PID setpoints be? Art

[BobLfoot 302]

Are you basically saying for the given 90 min / 7 pct example that 45 mintues into the boil-off you want 3.5% boiled off and 3.5% to go?

[acollins 0]

That's correct. The customer has the ability to set a given percentage of boil-off as well as a time period for the media to boil. Both need to meet at the end of the set time period.

[TConnolly 13]

As I said, I'm not that familiar with brewing processes, but I'll try and explain my thoughts. I understand that the brew must be kept at boiling. This will require a certain energy input to achieve. Once its boiling, incresing energy input doesn't raise the temperature, but it does change the boil off rate. The first PID loop is a temperature control loop. Its purpose is to keep the brew boiling. Now here is where I start making some assumptions (dangerous, I know). I assume that you use some kind of setpoint ramp function to set the amount of boil off, so over 90 minutes the boil off setpoint ramps from 0 to 7%. As time progresses, if the boil off falls behind the setpoint, the second PID loop will add energy. If it gets ahead of setpoint, then the second loop will decrease energy - however, it can't go below the energy to keep the brew boiling, the first loop will see to that. The program looks at which one of the two PID's generated the highest CV value and sends that value to the steam valve. Because its a boiling mixture you are actually going to have to use a modified high select - that is, as long as the mixture is still boiling you may actually need to switch to a low select on the boil off control so that the CV can actually be decreased if you are leading the boil off setpoint. I hope that made sense.

[BobLfoot 302]

Just one more stupid question I promise LOL. Does the 90 minutes include the ramp to boil temp time or does the 90 minutes begin once boiling is reached?

[Ken Moore 23]

I think this is a CIP (Clean in Place) task, so the time would start once the process reached the boil point. Just a guess, who knows?

[acollins 0]

The time starts at the boil, as does the boil-off. Can I use the difference in boil-off percentage and count-down percentage in one of the PIDs with 0 as the setpoint? The other PID as stated would keep the media at the boil. As long as the two percentage were in sinc (0 difference) it would be the lead. If the two percentage get out of sinc the second PID would kick in to maintain a 0 difference. Art

[BobLfoot 302]

Not as lot of PID experience but from what others have posted I would suggest you try something like below: PID One - Regulates Steam to maintain a minimum boil ie hold vat at boil temp. PID TWO - regulates to a setpoint of remaining volume. IF remaining volume is above desired the output from PID is additive to PID one to increase boil off. Yes PID two will have a changing setpoint from your boil-off ramp calculation. Hope this is clear.