PLC vs relay interlocks

g.mccormick

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Jul 2012
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A little background. I work in the engine testing testcell world. I am currently in the process of designing the testcell controls for a new cell of ours. This cell will be originally slated for doing burner testing on aftertreatment systems (customer is supplying a 3rd party hot gas generating burner we are providing the facitlities and cell).

We have a software product that does the full data acquisition and control of hte test cell, normally we use products from Beckhoff, Gantner, Opto22 for IO. The "IO" is usually just used as IO with no control aspects, all control from our PC. With that, it has been normal convention to use a lot of hardwired relay interlocks for differing levels of permissive. This is done usually since our system is flexible enough that someone can change spec files to alter sequencing, etc. The hardwired also incorporates a small PLC that's sole purpose is to receive pulses and do an orderly shutdown of the cell if the pulses stop (Watchdog).

On this cell, I am using Bechoff as the IO, but I am also going to utilize the PLC to handle all of the permissive interlocking that would normally be done in relay hardwire.

The estops, gas safety, and flame detection is handled through safety relays.



Question: Does anyone really rely on hardwired relay interlocks when using a PLC? Its hard to break old habits, and I keep wanting to put in relays where PLC logic will go. The only thing that I can see as a potential benefit of the relays is that it is easier to see what is stoping the process (open the panel, follow the schemits, see whta relay is off).

The fix for this, I believe will be better status between the PLC and my system. Allowing for more statuses (DI's from my system point of view, but not mapped to physical DI's on the Bechoff, just internal logic).

So, long boring story short, other than Estop related, does anyone really use physical relay interlocking?
 
Yes, motor starters, Large valves, and other devices can require holding currents that exceed the plc output, not to mention that the surge current is even higher that the holding current.

I also use interface relays for arc flash safety.

I take that into account for all projects.

regards,
james
 
I don't mean useing interface relays to protect outputs/ increase current handling. I mean more of the interlocking of logic.
 
All interlocking should be done in the PLC. Unless its related to safety such as emergency stops which should have a dedicated safety relay. It's much easier to diagnose issues in the PLC instead of sticking your head inside a control cabinet. Also there is less hardware to malfunction. And less equipment to wire and take up cabinet space.
 
mccormick: I can understand how you may be irritated by the added trouble of trying to find out what is not working by sifting through the code.

What we do is create a separate routine in the program called STATUS
create NOP rungs and put all the bits you want to monitor on those rungs as open or closed contacts.


I set up my status rungs so that the contacts are "lit up" (Highlighted) whenever there is a fault or a problem.

A quick glance at the STATUS Routine, and I immediately find the problem by the one bit/contact that is lit up in the entire routine.


If you don't want to use a laptop then use a HMI/Panel with a status screen. Easy and very handy/fast.
 
I plan to bring the PLC logical status bits into my system as digital inputs. They won't be physical DI, but will appear as such.
 
beckhoff has safety i/o so you are sure the relay is working proper.
so yes i would go for plc interlock.
use ladder software and copy the ladder to a display so you are sure not missing anything.
(and yes i also use some lights to get a sequence to start main engines for russian engineers.
 

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