PLC Redundancy

First set of questions. Does the switch from the primary PLC to the secondary PLC have to be "Bumpless"? How critical is the need for bumpless control transfer? Is there any life or enviromental concerns with your proccess?

Do you already have any PLC's or compenents or is this from the ground up design?

Hi There. Thanx for the reply.

The system is going to control a common HVAC system.

However, this system is going to be installed in an oil & gas facility. The specs specifically require hot PLC redundancy. I would suspect there is no critical need for bumpless control transfer.

Thanx for your time.

Nicolas.

Given bumpless transfer is not required just about any PLC would work , but it will require some skill on your part. Automation Direct PLC's should have a model that would sufice your needs you will have to determine which model is required by the size of I/O demands and funtions required. If you are not experienced in PLC progamming and control technology you should get some professional assistance as several issues could exist in your application , such as type of switch over required and evirnment -- explosive gasses in some or all areas? Jon Equipment Technology , Spokane

I am currently tendering the project.

Initial design was introducing Siemens S7-2xx ( CPU 226 ).

the client however requires hot redundancy. I have to provide a solution introducing something with the same cost if possible, but being redundant. i am not skilled at all, and i was forced into this situation.

I am looking all over the net trying to find a REDUNDANT device with the Siemens S7-2xx characteristics.

Is the Rockwell controllogix 1756 a feasible solution?

thanx for your time,

Nicolas.

Control Logix is a feasible solution. Since your application is not critical and you are looking for a lo cost solution why can you workout of RA SLC Platfom with flex I/Os

Regards,

A K Saseendran

ControlLogix does allow you to have multiple processors in the same rack but is not going to be anywhere near the price point of a S7-2XX. I believe AB does have a way to set up MicroLogix 1500 in a redundant fashion which might get you a bit closer to your price constraint. Good luck!

Shawn

"I believe AB does have a way to set up..."

AB stands for what?

allen bradley?

Sorry, yes AB is Allen-Bradley, the hardware side of the "logix" family of PLC's.

Shawn

Also too is Allen-Bradley Control Logix 5000 PLC is also "hot swappalbe", meaning its design to swap out I/O modules without power down. I don't know if thats what you are looking for.

MidniteFighter

First let me say...... This is not the recommended method and it assumes that the HVAC system is small and you have spare I/O, plus you have someone that can do basic programming for PLC's and possibly SCADA's.

Install two identical PLC's, Daisy chain the the I/O between the two PLC's. Decide which one is to be primary and which one is secondary. Write your logic for the primary so that it outputs a "Hi" on a spare digital output point when in a healthy running state and send that to the secondary unit. Have the logic written in the secondary so that when it see's the heathly "Hi" from the primary it will disable it's outputs but if for any reason the primary stops sending the "Hi" it will take over.

Those that write PLC logic know that there are a few other things to be mindfull of to keep the PLC's at least partly sync'd but this can work and if your using a SCADA you can use it keep the setpoints updated on both PLC's.

Like I said at the start this is not a recommended method but it will get you by on the cheap.

In the world of PLC's when using the term redundant and adding the term reliable to it will rarely equal cheap. Your best bet in my opinion is to re-bid or do a change order and purchase equipment better suited to a redundant configuration.

I am trying to implement redundancy by having two identical Omron PLCs connected to remote I/O.

You mentioned write your logic for the primary so that it outputs a "Hi" on a spare digital output point when in a healthy running state and send that to the secondary unit. Have the logic written in the secondary so that when it see's the heathly "Hi" from the primary it will disable it's outputs but if for any reason the primary stops sending the "Hi" it will take over.

Question:

1. What do you mean by outputs a "Hi" on a spare digital output point? D you means turn "on" the spare digital output point?

2. What do you mean by secondary disable its outputs? Do you mean secondonary do not communicate with the I/O modules?

Do you have any sample PLC ladder program for this?

You should look at the Honeywell HC900 with C70 redundant controllers.

"I have to provide a solution introducing something with the same cost if possible, but being redundant. i am not skilled at all, and i was forced into this situation."

Ahhhh, the old Engineer's Lament!

"I need you do do twice as much with the same budget or the same amount with 1/2 the budget."

The reality is, redundant backup is going to cost you more. The S7-200 is the least expensive of all of the Siemens PLCs, the entry level product if you will. Redundant backup with bumpless transfer is never an "entry level" function, it requires very specialized capabilities to avoid bogging down processors while they look at 2 sets of I/O, etc. etc. The problem is, you don't necessarily need it to be bumpless, but that is the exception to the rule, so most PLCs that have redundancy are also going to be capable of bumpless transfer and hence more expensive. In fact, the S7-300 uses the same basic internal hardware as the 400, but even it is not capable of bumpless transfer because it involves so much processing power.

I like the idea expressed above to just put in two S7-200s and be done with it. You will waste a lot more time trying to find something cheaper and then re-writing all of your code to make it work.

Either that, or get a better definition from your customer as to what they mean by "redundant". It could just mean they want a spare CPU with all the programming loaded in it sitting in a box at the bottom of the cabinet. That's what several of my clients meant by that (I found out the hard way).

Either that, or get a better definition from your customer as to what they mean by "redundant". It could just mean they want a spare CPU with all the programming loaded in it sitting in a box at the bottom of the cabinet. That's what several of my clients meant by that (I found out the hard way).

Good point. I work as an on-site contractor at a semi-conductor manufacturing site were redundacy is critical so I to often forget that some customers have different definitions.

If they are requiring a "HOT-BACKUP" then they are implying a bumpless transfer. Modicon has one of the best (in my opinion) but they are also very expensive so that bit of information will not help you at all.

I strongly advise against the suggestion to connect the IO in parallel. In my opinion it offers a real possibility of tearing apart the equipment being controlled or otherwise causing the system to become FUBAR. (FUBAR = [messed] Up Beyond All Recognition)

Is Seimens the required manufacturer? If not then call up the Allen Bradley rep and give them the requirements and go from there. I would do the same with Modicon.

I strongly advise against the suggestion to connect the IO in parallel. In my opinion it offers a real possibility of tearing apart the equipment being controlled or otherwise causing the system to become FUBAR. (FUBAR = [messed] Up Beyond All Recognition)

As I said in my post it's not recommended and without having an PI&D in hand at a minimum I can not make any promises to reliability. With that said... On a simple system with basic on/off control it is fairly simple to write the logic to protect the equipment. If "Bumpless" transfer of control is not a requirement then it's a simple matter of writing the logic with a couple of timers and condition rungs that set the system to a safe state before running in Auto. Would I do this on a chemical delivery system,,,,,NO, what about where 30h.p. motors are running pump....NO. But on a small cheap system that has a fairly benign/harmless "Modes of Failure", then yes it is a viable method even if it not recommended. The key is properly setting up the safety interlocks whether through hardwiring and/or programming.

To the original poster..... It is your responsibility to create a safe system. My suggestion is based on the assumption that you have someone that understands PLC's and has a clear understanding of safety. I am also assuming that your HVAC system is a small unit (servicing a 5000 sqft area or smaller) with basic air handling controls. If your system is larger or is of substatial value spend the money on more capable proccessors.

I didn't mean to offend you with my post and I did realize that you qualified the solution with "not recommended". Maybe if I had written, "I agree with Richard L. that the dual PLC/shared I/O solution is not recommended and strongly advise against it." that would have conveyed my intent better. I agree with you that without more information, like a P&ID, we can't help too much. That being said, the assumptions being made don't seem to be based upon the limited information provided.

1) We don't know how many I/O are in the HVAC system so we can't begin to evaluate the potential complexity. Therefore we don't know if it is a small cheap system, a medium sized system or a large expensive one.

2) We do know it is in an oil and gas facility but we don't know whether that means an equipment area (storage/transmission/loading/pumping) or an office area or both.

3) The requirement as stated is for "hot redundancy". Without any further information that means $$ and the customer wanting to insure the system does not go down. I do know that, depending on the system size and design, suddenly shutting blowers/fans down or closing airways can cause duct work to collapse or come apart at the seams. That, of course, is what you refer to with the statement regarding hardwiring and safety interlocks. We don't know if the system heating and cooling is being provided by refrigerant or by steam and chilled water. All together that means we don't know what the potential "Modes of Failure" are.

4) The original post along with some of the further clarifications gives me the impression that the job was "sold" at a low price (without redundancy), either stand alone or as part of a much larger project and now the poster has inherited the HVAC portion and has to make it happen.

5) It would seem that there is no PLC/controls knowledgable person available to him otherwise the post would not have been made.

Again, I don't mean to be talking (writing) down to or otherwise offending you, or anyone else. I am only offering my opinion based on the information provided and the input from others like yourself. The major drawback to these blogs is that no one can hear the voice tone or see the body language to help with evaluating the mindset of the person writing.

οκ.

guys, you have already helped me a lot. the thing is that I am new in this position ( 2 weeks ).

many thanks, i am looking into the info u provided. if there is something more, i shall come back.

again, many thanks,

N.

No offense taken. My writing style is very short and in your face so it comes across harsh sometimes, it is I who should apologize. My intent was to present another way to evaluate the system. I take safety very serious but I also try to apply real world what ifs and work my solutions from there.

Again no offense taken.

Rich

In all cases I am aware of there is always some loss of complete redundency un less ALL I/O is duplicated down to to the physical sensory perception of the process so it can reach the sublime and even the rediculas , I dont beleive that any HVAC system would require complete redundency. and never is going to be seen in any level close to the S7-2XX. The sugestion of remote I/O w/ 2 cpus comes the closest to true redundency without going in high end equipment , again this requires expertise and time to impliment and a high degree of "what if". Jon

Just so you know, you can do what is called a "warm backup" with the S7-300 PLCs from Siemens, they are about 1/2 the price of the 400s. It just requires 2 CPUs and a software package. Each CPU is not scanning the I/O continuously as it would in a hot backup, but both contain the same program and the secondary takes over from the primary in a fraction of a second. That would undoubtedly be fast enough for an HVAC system.

Here is a link to the manual for this from Siemens;

http://support.automation.siemens.com/WW/view/en/1137637

How embarrassing... I looked right past that a dozen time.