I have been hearing a lot about "dedicated safety PLCs". I am curious as to why you would need a dedicated PLC to perform safety tasks, and if you happen to be using regular safety switches and contacts, what would be the proper way to interface them into a PLC? This issue gets kind of gray and confusing for me.
As safety regulations change and more safety devices need to be installed, the task of hard-wiring a safety circuit with the proper relays and dedicated controllers becomes more complicated. For example, light curtains have different requirements than safety mats, Emergency Stops, or Two-hand controls.
Let's say you have an application where you have a light curtain, an E-Stop, and, say a safety mat. Now, let's say the light curtain needs to be muted because product needs to pass through it without shutting off the machine, but it can only be muted WHILE the product is passing through it AND the E-Stop and Safety Mat still must be able to shut the machine down at all times. You need three safety relays (one for each device) and a lot of hard-wiring to make this happen. You could be over $1,000 in costs very easily between parts and labor.
So, instead, you buy a safety controller, like a
Guardmaster 440C. All of your devices, resets, lights, etc. wired into the Safety relay, and then you PROGRAM the safety circuit. Now, you are using specialized software that only allows you to program in certain ways and will not let you compile and download if you break the "rules." This of course isn't foolproof. You can program an unsafe safety circuit and you can do it improperly...but you can also designed an improper hard-wired circuit as well. The big thing here is that the program can't easily by changed, and you have to have the proper elements for it to work. The program is also "locked down" when it's downloaded so that changes can be tracked. The one I used for example gave me a unique code every time I downloaded, so that if there was a claim that the machine did something unsafe, I can pull up the program, look at the code, and verify that the program is exactly the same as when I last downloaded it. If not, then I know it has been tampered with.
By using a programmable safety controller in this application, you cut down on costs: design time, wiring time, and components. You cut down on the complexity of the wiring which reduces troubleshooting time. Those are just a few advantages.