PLC & Warning Lights

[dale1627]

I know that you should not use a non-safety rated PLC for safety applications, but I was wondering if anyone saw anything wrong with the application that I am looking at. I have machine which has four different sections that can be started independently of each other. I need to install flicker lights and warning buzzers to indicate when a section is about to start. Would it be acceptable to use a small PLC to do this versus using real world timers and relays or a safety rated PLC. The PLC will do nothing more than create a three second delay while the start PB is pressed before the section will start. It will also sound a warning buzzer and flash a warning light during this three seconds. In no way will it be tied to the e-stop circuit.

Dale

 

[brucechase]

What currently is controlling your machine now? Can you just use that control to sound your alarm? If there is already a plc in that machine, you can use that. Also, I've chnaged some of my thinking on startup warnings. On some of my large systems, I used to push the start button, flash the lights, and sound the horn. After 5 seconds, everything would automatically start.


I have since changed this to require a second push of the start button to start the system. The reasoning is that if during this 5 seconds you hear someone start hollering "Wait!" the system won't still start. I understand that most people would understand to hit the stop or e-stop button (and also if there is someone in danger that they should be locked out), but what about a situation in which a life isn't in danger, but the machime might not be ready. That way, a simple lack of the second push will prevent the machine from starting.

I am sure there will be plenty of members here that don't agree with this. In either case, my answer to your question is that it is acceptable to use a small plc or timer to do what you want. There is no reason for a safety plc. I would use a timer if all it is going to do is delay a signal for 3 seconds, I would not spend the money on a plc for that.

 

[bledet]

I agree with brucechase. If the plc is going to be used only to create the delay and give warning, there are better (less expensive) ways to accomplish your goals. Sveral manufaturors make 'smart' relays these days which hold several lines of ladder logic. You could even design a redundant system with them, just in case.

brucechase, I like your thinking on machine startup. I would, however, make a small change to your philosophy. I would require the button to be held for the entire 5 seconds. This would eliminate issues of the timing of the second button press.

 

[dale1627]

Bruce: there is a plc already but I do not have the software for it. I have a paper copy of the program and never needed to make a program change to warant purchasing the software. The reason to use a plc versus real timers is because I need to do a seperate buzzer and lights for four sections. An AD 05 for $99 is almost cheaper and easier to use. Plus I can use the plc for a PM time notification.
I plan to set it up for a one push start. Operator presses and hold the PB for the three second time before the machine starts. Let go of the PB during the buzzer/light time and they'll have to start over. Thanks for the thoughts.

Dale