Hi guys

I have a machine here that i am upgrading it has 22 guard door safety sensors on it (its a very long machine with many doors) i am thinking f putting a PLC onto this machine as a lot of the hardware is very old and some of it is not even made any more! Anyways my question is do i need to give each of these safety sensors there own input on the PLC or can i group them together into some sort of safety relay that will use up less inputs on the PLC but still stop the machine if any of the doors is open? I have attached an image of the current sensors that are being used.

 

You could use the N.C. contact on them and series all of them up to one input.
The one problem is that it is then difficult to identify one that is stuck open etc.
You could use a safety relay but not really necessary the safety is in the switch which usually requires a special striker that operates the switch sequentially to prevent manual tampering/overide etc.
Telemecanique sell or (used to) sell safety door switches of a similar kind XCSA series.
Max.

 

Thanks Max

One of the problems is that the doors are wired in series already and the exact problem happens that you described on a regular basis hence why i want to give each door its own output lamp too with a number to tell which one it is exactly that is open. Is there any other piece of kit that you have experienced that could do this over a PLC?

 

For monitoring each door SW and providing a indicator for each then you are pretty much committed to some kind of multi I/O controller, such as the PLC you have mentioned already, I know of no single piece of equipment designed just specifically to do this.
Max.

 

How many contacts are there in each sensor?
If you have a spare contact you can run this to switch power to an LED indicator.

 

With lots of safety inputs, you might consider a dedicated PLC just for safety.

 

yeah i was thinking that too i'm looking at the Mitsubishi ALPHA 2 logic controller for this its nice and compact and comes in a good starter kit with SW and cable included will definitely help with my budget too but the only thing i have just noticed is that the input expansion modules are digital will this matter for what i am doing?

 

I use Mitsubishi and really like them, the I/O is normally all digital (1 or 0) unless you get a 'special' option module.
BTW, the likes of Maple Systems make a digital display for most PLC's you could program a message indicating door number if you wished.
Max.

 

The simplest way to reduce the input requirements is to divide them into zones then give each door of each zone a specific resistance value when open that's part of a voltage divider creating a specific voltage 'door ID tag' level the PLC looks at.

It's not perfect but with the right logic it can be effective at determining is any single door in a zone is open and possibly two if you set your specific door resistance values up so that no matter what combination of doors is open no two combinations give the exact same reference voltage.

 

@Jonplynch the machine operated by a single operator or one at each station or fully automatic?
IOW, if auto or single operator, the local digital display method may work rather than checking the light on each station.
This will decrease the amount of outputs drastically.
Max.

 

Yes it is operated by single operator fully automatic

 

Yes it is operated by single operator fully automatic

Then a local indication at the operator station rather than an output/light at each door may be a the preferred way to go?
Max.

 

Hello,

As @MaxHeadRoom said, the operators panel could have an image of the machine and indicators wich door is not closed/safe.
Here is an example of a welding facility:
http://weldingproductivity.com/article/staying-safe/

Bertus

 

Yes it is operated by single operator fully automatic

So it sounds from the OP that the system is controlled now by hard wired logic etc?
If so It sounds a good candidate for a PLC retro-fit/upgrade.
Max.

 

Yeah it does one thing I forgot was that all the sensors on the doors are 230vac where as I want a Plc with 24vdc supply should I change the sensors to 24vdc?

 

Probably because 230vac is used through the switches, all you need to do is re-feed them with 24vdc.
230vac is the maximum rating of the switches.
If needing to detect each sw then you will need a input for each.
Max.

 

The way I am used to seeing it done, one set of NO (closed when door shut) contacts are wired in series to a single input of a safety relay or safety PLC. Then the another set of NO (closed when door shut) contacts [or NC (open when door shut)] from each switch are ran to the PLC to indicate which door is open. This way the safety circuit and the indicator circuit are separated. Your 21 series switches have 2 sets of NC and one set of NO contacts, so this should be totally possible.

 

So i can series wire the 22 switches to a single safety relay with one set of the contacts on each sensor and then use the other set of contacts on each sensor to its own input of the PLC which will allow each door its own output alarm or lamp?

 

So i can series wire the 22 switches to a single safety relay with one set of the contacts on each sensor and then use the other set of contacts on each sensor to its own input of the PLC which will allow each door its own output alarm or lamp?

yes

 

So i can series wire the 22 switches to a single safety relay with one set of the contacts on each sensor and then use the other set of contacts on each sensor to its own input of the PLC which will allow each door its own output alarm or lamp?

I think I stated this back at post #5?