AB PLC Control redundancy solution

[mikey431 3]

I want to know if there is any "cheap and dirty" solution for A-B PLC controller redundancy system ? I wanted to use controllogix L55 but it going to cost a lot of money to use ControlLogix redundancy system.As described in the A-B manual, i have to have to use ControlNet network with 2 same controller racks consisting of 2 controllers ,2 CNB modules, 2 SRM modules...Anybody has a better idea...

[Contr_Conn 4]

FlexLogix has hot standby support with I/O on Devicenet. Not exactly redundancy, but it may work for you.

[gravitar 3]

In the cheap-n-dirty category, I recall several of the stamping presses at a shop I once worked for were wired with a pair of redundant fixed SLCs. I can't recall the exact configuration but I believe the inputs were wired in parallel, except for a pair of inputs that were wired to a pair of outputs on the other SLC (and vice-versa) for a "heartbeat". Can't remember exactly what they were doing with the outputs.. I think they were wired in series so both had to close before you'd get an energized circuit

[Contr_Conn 4]

For FlexLogix Easy Backup see tn 24180 or DeviceNet Bible

[BobLfoot 301]

What are you trying to achieve with redundancy? I/O points and cards fail far more often than CPUs.

[mikey431 3]

unfortunately, it is customer spec...

[mikey431 3]

Besides being phasing out, I was told it is not really reliable ..I am not sure how much of truth regards this matter

[Contr_Conn 4]

Flexlogix still active product, V16 will be last version that it is supported. As for stablity, I am not aware of any issues. Dr. Devicenet Bob Law is behind it. But you are getting what you are paying for. You asked for a cheap solution.

[BobLfoot 301]

If the customer has specified it then they must be prepared to pay for it. Word of advice from a programmer who has written several redundant system programs for CLGX. Get and follow the Redundant programming guide from rockwell. You'll be surprised how a "simple edit" can blow your scan time out of the water. Also avoid the SSV/GSV instructions unless absolutely necessary and then trigger them only as needed. Rockwell CLGX Redundant processors are a great thing when they work, but a nightmare when you try to make them work and cut corners at the same time.

[gravitar 3]

What about the red safety PLCs? (Can't remember the name, I think it is Guard Logix?) Don't those have built-in redundancy? I'm sure they don't fit the "cheap-n-dirty" category though :) I would suspect that any reliable level of redundancy would tend not to be cheap..

[Contr_Conn 4]

Guardlogix is not cheap and it's not redundand - it uses 2 processors, they must agree, if they don't then it shuts down. This is a nature of safety: any fault must shut system down. With redundancy if one PLC failed, another takes over.

[gravitar 3]

So what he's looking for is "hot backup" (like the PLC-3 could do)? Wasn't there an option for the PLC-5 to do that also? I guess if he needs it to be "cheap" he could see if this backup communication system is on ebay. I know you can get some good deals on PLC 5 processors there..

[rpraveenkum 5]

what about Slc 5xx with BSN?

[Mark 0]

I just went through redundancy questions with a customer. The specification: "Two fully redundant "hot standby" PLC's in a common control panel with Panel View Plus 1000 screen." Not specified, but I later found out comms were to be Ethernet\IP. Prime solution from A-B was Control Logix. Alternative is SLC500, but what throws it for a loop is the Ethernet\IP and the expertise required to get it going. Eventually the customer settled on meeting the specification by going with Control Logix and absorbing the cost.

[Contr_Conn 4]

ControlLogix redundancy does I/O only via ControlNet With redundancy Ethernet/IP can be used with HMI/Scada only

[BobLfoot 301]

You may want to cover the following with your customer as well. He wants redundant CLGX PLCs in the same cabinet. 1. Does he have redundant power sources back to the local utility or UPS support for the PLCs. 2. With redundant system you'll have 2 CNET Coax cables to each rack. Are you running the coaxes in the same or seperate conduits and on the same or individual routes. On steroids the CLGX redundancy system would look like follows: PLC A 1. Utility Substation A feeds power thru plant Substation A to MCC A to UPS A to PLC A. 2. Utility Substation B feeds power thru plant Substation B to MCC B to UPS A to PLC A. 3. PLC A resides in Cabinet A 4. PLC A Rack has CPU, ENET, CNB & SRM Modules PLC B 1. Utility Substation A feeds power thru plant Substation A to MCC A to UPS B to PLC B. 2. Utility Substation B feeds power thru plant Substation B to MCC B to UPS B to PLC B. 3. PLC B resides in Cabinet B 4. PLC B Rack has CPU, ENET, CNB & SRM Modules CNET IO Portion of System 1. Cnet Coax A runs in its own conduit to each and every IO Rack. 2. Cnet Coax B runs in its own conduit to each and every IO Rack. Also redundant HMIs with automatic switch over would be needed.

[paulengr 44]

We just went through the redundancy arguments recently for our plant. This is how we came down on it: 1. Redundant power supplies: I can only recall one bad PLC power supply in several years. It still works, but the power switch went bad on it. We've had numerous power quality issues, partly because of a P.F. around 0.68 (this is not a mis-statement), partly from an overloaded transformer, and partly from utility interruptions. Putting UPS's on the PLC's with network monitoring cards (so that the UPS can e-mail somebody when there is a problem such as a bad battery, and so that the PLC can monitor what's going on) has effectively eliminated the power problem. 2. Redundant PLC's: Having a PLC go bad is even more rare. I really can't think of a time where this has happened that wasn't a result of some electrical problem which usually took out more than the PLC. Using distributed IO is a great way of helping avoid this problem. 3. Redundant I/O cabling: OK, now here we have had lots of issues. It's a foundry. So anything not designed to withstand 3000 F tends to get scorched at one point or another. An neandrathals with jack hammers just don't watch where they are aiming them. In this respect, there are two routes. We can install controlnet or we can use Ethernet with redundant rings. For the money, it seems that Ethernet seems to be the way to go. It's faster and not proprietary, and the wiring is less expensive. Sure we don't have redundancy to the device level, but either way, the devices are in cabinets and a 12" pigtail from a tiny Ethernet switch isn't any worse than Controlnet wiring in terms of vulnerability. Plus, I have half the wiring. Plus some devices (PC's, Control-Logix with multiple Ethernet cards installed) can still have redundant cabling at the device level. 4. Redundant devices: Nice when you can make it work. We use it in a few places. There are two forms. You can have a safety-type situation (A and B must agree), or a fail-over situation (A or B must be true). This is a programming/process design problem for the most part. In the end, redundant CPU's seems to make sense only if you've already spent gobs of money on redundancy against every other conceivable problem, or where the expense of replacing components (very remote sites) makes having the redundancy worthwhile. In the case of where I work, redundant network cabling and UPS's (no redundancy, just a UPS so that I have clean power with battery backup) takes care of the majority of the problems that can be solved with redundancy. As much as I like the idea, I just haven't been able to come up with a scenario where redundant PLC's makes financial or engineering sense.

[mikey431 3]

Thank for all your opinions.We would probably stick with ControlLogix redundancy and have customer pay for some of the cost...

[BobLfoot 301]

Glad to help. Mikey. Please keep us posted how it goes. Just for a target the last CLGX redundant system I worked on was V15 and had 14 CNET racks and a scan time of 28ms.