Designing a Redundant PLC System - AB option

A client of ours recently requested us to look into an old compressor control panel using an AB SLC500/02 with the view to building in redundancy. The AB PLC sits in an old panel beside a Mitsubishi FX2-128MR PLC.

The Client also requires that the same compressor sends back status information to the control room which is basically a Honeywell DCS system. My first thought is - why not extend the honeywell system to the compressor unit and viola - redundancy finished. Potential difficulty for the client is the cost from Honeywell - they charge an arm and a leg to walk on site.

What I have been looking at in order to build a budget price is to hook up to the honeywell system via modbus. In order to do so I was considering the Contrologix redundancy in the control room with a Flexlogix option in the compressor panel. The option to elimanate the Mitsubishi PLC and integrate it to the new design would also help O&M. The Client has indicated 2 other compressors in the same area that they will want to integrate later and my idea was to extend the controllogix bus out to both of these panels on this new design.

Has anybody any recommendations on my considered design. AB is not a requirement but I just considerded it as the client is currently using a product line. As the SLC500 doesnt offer redundancy (correct me if Im wrong), it effectively is to be replaced.

Happy new year to all,

AC

Actually, reading into the Mitsubishi PLC details (FX2-128MR) it appears to be a motor controller which would not be wise to try and replace. It is worth noting - there are no diagrams, no PLC programe details - nada - only the physical instalation. We basically have to backward engineer the panel and the download the software and determine the process in discussion with the operators.....

Oh boy! Redundacy again.

How redundant does the client require the system to be?

There are a multitude of semi solutions out there and some semi solutions you can basically build up yourself.

Personally, I find redundant systems are not as redundant as people think. For example, you can use a PLC with redundant power supplies and processors and 1 rack system. The rack system is not redundant.

You can use 2 separate PLCs with their own I/O systems with one locking the other out until the first one fails. How do you feed the I/O? Use relays? A whole lot of single points of failure are introduced to make the PLCs redundant.

You can use a true redundant PLC where there are 2 power supplies and 2 processors with a hot changeover unit and each CPU has it's own rack system. Very expensive and you still require 2 separate sets of signals for the I/O. You either finish up with a pile of relays again - lots more single points of failure - or a lot more sensors on the machine etc. If one sensor fails the 2 PLCs then receive conficting information and how do you program that out?

I personally feel the best solution is plenty of spares, a spare processor with the program already loaded and spare power supplies and I/O cards. Someone also needs to know how to use the spares.

By the way, I have not seen a PLC failure with my favourite brand for many years. I use PNP (positive switching) inputs and NPN (negative switching) transistor outputs and all relays/indicating lights off the output cards are fully diode protected. Have not even seen an I/O card fail for years. Have seen plenty of sensors fail in the field though.
The biggest cause of failure I have seen of PLC inputs is someone connecting 240/110 VAC to 24 VDC inputs. The biggest cause of failure I have seen of output cards is internal relays failing when they wear out or a short causing the relay to blow up. That I why I use transistor outputs with external relays hanging off them to drive the load. Lot easier to replace an external relay than one on a PCB. The indicating lights I use are the IDEC (Izumi) ones as they have full diode protection. The other advantage is that the LEDs are BA9S types and can be replaced from the front of the panel. Those stupid European ones from Schneider and Sprechur & Schuh (Allan Bradley) have to be replaced from inside the switchboard and working in live boards is becoming a real problem. I have also had problems with 240 VAC lights from both those manufacturers blowing up. Nothing like getting a call, driving 4-5 hours and finding one of these things has blown up taking out sensing fuses and starting a generator which will then never turn off. Drive back, buy a new LED unit, drive down again and fix the problem. 2 days lost due to a blown full voltage LED. I have NEVER had one of the IDEC ones blow up yet, or ever had to replace one.

Unless a client can give me a very good reason, and after a lot of discussion as to how redundancy is to be achieved with respect to the I/O system (lots of relays for the inputs are not the answer), I always suggest they just carry a heap of spares. However, some people cannot see the wood for the trees and still insist on heaps of relays. Cannot see the point quite frankly. They waste a lot of money trying to achieve redundancy and then screw you over your price to design and implement the system. I have NEVER seen a system where someone has been able to design out all single points of failure. One can only minimise them by good design.

BobB,

I can see your point of view. For the SLC500 family, I was wondering what is the MTBF number. This is a fair number to base the need for redundancy in a system. From reading the boards here (I trawled back thru the history looking for a similar redundancy question), it appears that SLC500 has been around a long time and should at this stage be extremely reliable. I worked quite a bit with Siemens stuff and to tell you the truth it just hums along once commissioned.

I have felt like ringing the Client and telling him we are in a futile exercise of providing him very little extra for what could be a lot of money.....I know he wont understand and wont listen. He wants redundancy!

Cheers in anycase for your tips.

AC

Can only suggest you print off my notes to see if he can see sense from that.

Good luck!