Start & Stop machines from HMI

rich_p

Member
Join Date
May 2009
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in a house
Posts
5
Hi Everyone!

In the past I've always used hard wired push buttons into PLC inputs to start and stop machines. The start button a N/O contact and the stop button N/C. I've always assumed the stop button is a n/c contact so that it fails safe.

Anyhow now I've been asked to do a way with the hard wired push buttons and put some buttons on a HMI to start/stop some conveyors. Is this safe? Just wondering how and if others do this? Note: EStop is still hardwired ;o)!
Do you use a seperate address for the start and stop pb's or do you just set/reset a bit? What happens if the HMI fails? Should you always use an 'HMI alive' flag that if fails switches off plant?

Cheers
 
I don't see a safety issue as long as you keep the hardwired E-Stop. I prefer separate momentary Start and Stop buttons instead of the set reset bits - if you loose comms with the HMI these can get out of synch with the PLC.
 
Hi Everyone!

In the past I've always used hard wired push buttons into PLC inputs to start and stop machines. The start button a N/O contact and the stop button N/C. I've always assumed the stop button is a n/c contact so that it fails safe.

Anyhow now I've been asked to do a way with the hard wired push buttons and put some buttons on a HMI to start/stop some conveyors. Is this safe? Just wondering how and if others do this? Note: EStop is still hardwired ;o)!
Do you use a seperate address for the start and stop pb's or do you just set/reset a bit? What happens if the HMI fails? Should you always use an 'HMI alive' flag that if fails switches off plant?

Cheers

It shouldn't be a problem, as long as the E-stop is hardwired, and safely stops the machine, without using the PLC. Which should be the case anyway. An output can weld closed, and the E-stop, should still stop the machine. Usually my rule, is that if a button is used a lot, then I use a physical switch. If it's only used rarely, then it goes on the HMI. It mostly depends on the convenience for the operator (is it more convenient to hit the button without looking... do they need tactile feedback, that the button was pressed) I recently added a physical button to a machine, because the operator was complaining about having to hold the button for a few seconds at times. They just weren't hitting the screen right.

-MUR
 
What happens if the HMI fails?
You are stuck. You can still stop your machine by means of E-stop (not the most benevolent way of stopping but better than nothing) and you have no way to restart. Until you find a replacement HMI, install it, wire it and download the program. Which, even if you have a spare in stock, still may hold production for a couple of hours.

That is the reason I don't like start and stop on HMI only; pushbuttons are cheap and allow to keep running while a failed HMI is being replaced. True, you cannot do any changeovers or manual operations during that time, but you still can run. Making profit for the company, that is.
 

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