Question on system design

clavius

Member
Join Date
Jul 2008
Location
Massachusetts
Posts
8
Hi All,
In the near future I will be retrofitting an quite old piece of equipment with a new control cabinet. This is a machine that was decommissioned and the controls cannibalized for other systems. The original system used a now obsolete PLC and so I'll be building an entire new cabinet.

This is a largish piece of gear used in a pretty harsh environment, think like a mine or a quarry.

I have the schematics for the original cabinet, and can largely duplicate what was there using a modern PLC, and can get through the programming.

I have a question on the design of the original system. This unit had the typical assortment of limit switch inputs, panel switches and buttons, and the like. An interesting thing about this was that every external PLC input is isolated with a relay.

For example, all of the external limit switch inputs are 120Vac powered, but rather than those inputs directly connecting to a PLC input (which are 120V I/O modules), they power a small relay coil, and the relay is what supplies the 120V input to the PLC.

I have not had a lot of exposure to systems built for these sort of environments. Is this typical practice? Is this just to protect the PLC from outside world?

I plan to be using 24V for all of the external switching, etc., but would it be smart to replicate this in the new system? Appreciate any thoughts.
 
I have a customer that insisted to have 24VDC outputs relay that controlled 24VDC relays that controlled 230VAC contactors to pilot motors.

He claimed that it reduced the size of the 24VDC power supply and limited the number of breakdowns. Personnally I was not convinced.
 
Thanks for the replies. In my case, pretty much all of the original cabinet guts are gone. So I am essentially starting form scratch, so there are no original relays to save. If I only had to replace/upgrade the PLC I would probably just leave them. I guess I am just trying to decide if it is worth the effort to duplicate that arrangement. It's more parts/work, but I have no issue with that if there is some compelling reason to set it up that way.

Iner, I think I agree. It seems a lot of extra stuff to limit the size of the 24V power supply.
 
In some designs power can come from multiple sources to a panel. Relays are used for potential free outputs. Not sure if it applies to your case though.
 
In some designs power can come from multiple sources to a panel. Relays are used for potential free outputs. Not sure if it applies to your case though.

Understood. That is the case here only on a few inputs, and they are pretty obvious in the schematic.

It just seemed slightly odd to me that there are many inputs here from things like limit and flow switches that energize a 120VAC relay coil, and the contacts of that relay will close and supply 120VAC to an associated PLC input. The 120V line is the same one in every instance.

This is a system that was built in the late 1980's and its (almost) twin is still running, so I can't fault the robustness of the thing. Just seems unusual to me and I can't quite follow the thinking behind it.

Thanks!
 
Pointless overkill. Maybe to justify cost.
Do everything 24DC, with buttons direct to PLC. Do most buttons on an HMI.
Maximize software, minimize hardware. That's what PLC's and HMI's are for. I would use pushbuttons for frequent operations like Start and Stop, to minimize wear on HMI membrane.
 
And I beg to differ....I try not to do anything on the hmi thats relatively frequent...start, stop, alarm reset. If your machine cycles every few minutes or so, guess where the hmi will wear out quickly? I have some equipment that cycles, then the operator opens a guard (creating an alarm that needs to be reset)...yes, some of you will quip that alarm messages should automatically reset....thats great until you have an intermittent fault that causes the machine to stop and goes away by itself....and no, not every hmi has an alarm history built in.

As for 24vdc outputs to relays to contactor coils, I do this also especially for larger contactors. The outputs are more susceptible to failure on handling the current surges. And more cumbersome to replace than a small relay. If your handling loads more than 0.5a, many transistor outputs won't even support them. If you're cycling the output many times during normal operation, you'd want to use a transistor output and probably trigger a relay. If the relay fails, its a $3 part and any maintenance person can swap.
bob
 
Personally, I always use PLC with 24VDC inputs, and transistor outputs (24VDC), always connected through relays with mechanical "flag" for forcing, and dedicated power supply only for PLC and its I/Os. Transistor outputs have much longer life, but they can't handle higher currents. And option to force input or output is very useful in real life situation.
 
I have a customer that insisted to have 24VDC outputs relay that controlled 24VDC relays that controlled 230VAC contactors to pilot motors.

He claimed that it reduced the size of the 24VDC power supply and limited the number of breakdowns. Personnally I was not convinced.

Thats a new one on me from the cr@p customers come up with.

Why would you stress about the size of a 24vdc supply and be fine with having a cabinet full of relays like its from the 1950's!
 
Thats a new one on me from the cr@p customers come up with.

Why would you stress about the size of a 24vdc supply and be fine with having a cabinet full of relays like its from the 1950's!

This customer is led by a old guard style man, nearing retirement. He also tried to fight off the idea of Ethernet/IP drives.

With this relay/contactors requirement and others stuffs, we ended with a full 4 double doors main cabinet.

In the same time, we had a same budget, more complicated 15 axis machine that ended with a main cabinet of only one door due to the use of distributed I/O and Profinet.... But some people still like big cabinets apparently...
 
Well I agree with rswolff001, and others.
Dont stress your HMI screen
Use buffer relays on everything except local volt free inputs.
(SSR' if you have to do PWM)
Employ surge protection on any long, or exposed, external circuits.
If you are buffering inputs then you only need to protect 24DC out and ground for all your inputs because the relay isolates individual inputs - much cheaper than protecting every input.

HOWEVER

The way things used to be done is almost certainly not how you would need to do it now, especially in a mine or any other dusty industrial environment...
You need to be making decisions about electrical design, including the enclosure specs, based on safety requirements and probably SIL assesment.

Here there be demons.....

I am not saying don't do it just make sure you do it right with appropriate kit.
There are things you simply cant d now that would have been common place not too many years ago.

It inst something to be frightened of but it most certainly needs to be understood and respected.

Hope this helps,
Al
 
And I beg to differ....I try not to do anything on the hmi thats relatively frequent...start, stop, alarm reset. If your machine cycles every few minutes or so, guess where the hmi will wear out quickly? I have some equipment that cycles, then the operator opens a guard (creating an alarm that needs to be reset)...yes, some of you will quip that alarm messages should automatically reset....thats great until you have an intermittent fault that causes the machine to stop and goes away by itself....and no, not every hmi has an alarm history built in.

As for 24vdc outputs to relays to contactor coils, I do this also especially for larger contactors. The outputs are more susceptible to failure on handling the current surges. And more cumbersome to replace than a small relay. If your handling loads more than 0.5a, many transistor outputs won't even support them. If you're cycling the output many times during normal operation, you'd want to use a transistor output and probably trigger a relay. If the relay fails, its a $3 part and any maintenance person can swap.
bob
We don't differ. I'm saying you should use mechanical buttons, not the HMI, for frequent operations.
I took apart a Siemens S7-1200 CPU. I think the outputs were rated about 1 Amp or so. They used 12 Amp MOSFETs. 3-4x is a good safety margin for Industrial devices, so this PLC would reliably handle loads up to it's rating and beyond. A relay will fail much sooner.
Other PLC's don't have this much overkill, but I'm confident in using pure solid state, and relays only when I must. You can put diodes, caps, etc. across relay contacts, and they will still have a short life with inductive loads.
The P2000 from Automation Direct has hot-swap capability. Just as fast or faster compared to a socketed relay. We don't care much about a $3.00 relay vs a $100.00 module. Both are considered low cost.
 
The very first thing you have to do is get the specifications of the environment and your customer specifications!
it doesn't matter what the equipment is, you need the environment specifications. Things has changed from 40 years ago.

if this is a mine, you have to design for hazardous environment.
a quarry might be different.

if its a dusty environment, you need purged / pressurized enclosures.
In dusty environment, HMI terminals will accumulate dust and will create problems.

Too many unanswered Questions.

James
 

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