guard indication

[richerdick]

I wish to improve the indication for which guards are open on a machine at work.
The current system includes several guards in series driving a dual loop safety relay, the safety relay then goes on to drive an ouput to a plc that indicates that there is a guard open within a specific zone of guards.
I want a more specific indication than this, so I would like to use a small stand alone plc to recieve inputs from the 24v of the existing guard loop after each switch, then go on to drive output leds or something to indicate where the loop is broken specifically. This is not something i have seen done before but is it bad practise or not advisable for any specific reason.?
Your thoughts please

 

[itsmoked]

It has some issues with reliability. What if the "small plc" goes psycho and lies about the guards being down? Just straight switches that must all be closed reduces the possibilities dramatically.

Any chance you can leave all that as is, and just add more switches? One for each guard of interest? You could then actually increase the safety level by adding one of the "small PLC's" dry contacts into the safety chain duplicating the per-gate checking.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[richerdick]

I was trying to keep cost down, I can get a small smart relay type plc for say £100, i would need to change the limit switches to add contacts and there are quite a few so the cost would be far greater. I am more woried about whether there are regulations that prevent me from interfacing a PLC with the main stop/guard circuit or whether this is acceptable. From what i can see an Input would actually have to souce 24v to make the circuit more dangerous which seems very unlikely?

 

[jraef]

You cannot run the safety circuits themselves through a non-safety rated PLC, and none of those cheap "smart relays" are rated as such.

One option is to use ASi Safe. You would use ASi inputs on your switches that go into an ASi Safety Relay, then there are ASi modules for the some of the programmable relays that can monitor the status of the safety inputs over the ASi network, without affecting the safetry operation of the Safety Relay. The Siemens LOGO! can do this.


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[whitehendrix]

i agree with jraef.

depending on the program capabilities, you can indicate an opened zone thru dedicated inputs PER zone (obviously)
or incorporate safety relays with multiple-input sensing.. the ones i worked with (can't recall the manufacture.. my apologies..) were like an AND gate, and needed to see the switch loops closed in order for its output to go high for the PLC. there was one per "zone" and 2 inputs.. a front and back gate.. this was brick making machinery, and was a mess of guards and gates, so knowing exactly what gate was opened was imperitive, as was the safety of the workers. i would'nt trust a "dedicated" (non-safety) PLC to do te job of a specific device.

good luck and have fun!

 

[waross]

Run a wire from each switch to an indicator lamp. In normal operation all lamps will be illuminated. When a switch opens, the associated lamp and all downstream lamps will extinguish. This will show the first guard that is open. In the event of more than one guard open, it will indicate (by the absence of illumination) the first open guard and when that guard is closed it will indicate the next open guard.
Not perfect but quick and cheap and safe.

Bill
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