PLC Logic

panthripu

Member
Join Date
Mar 2004
Posts
228
Hi
Your suggestions are required in following problem.
We have 5 machines in differnt locations. Due to technical reasons , i dont want to start any two machine at same time.
So what we did is, get feedback for ON from each machine. If any machine is switched ON by operator , we dont permit another one to start untill two minutes. The one who are already ON does not matter.
But this way , in some scenerio ,when lets say all machines are off and one operator starts the machines, the other four are not permitted logically to switch ON for two minutes. Now other four operators are ready to switch on and waiting for permission.As soon 2 minutes are complete, they try to switch ON , so who ever do is fast will start machine and other three will wait now for next 2 minutes.
The problem is , in this situation when they get permission, if two operators starts at the same time, because of scan time and feedback delay, both machine starts together. :mad:
Conditions: we dont want to give priority to any one of the machine. It is batch process , so who ever is ready should be able to start.
We are also planning to send permission on pulse basis which will keep on changing between the machines which are off and ready to switch ON , so that two operator does not get permission at same time.

I will appriciate any other idea here in this forum.
 
It appears that your issue is latency, which mixed with virtually instant local control is causing issues...

Consider doing things differently...
Build a central control system to manage all machine start requests. It sounds like you have most of this anyway.

Have individual start buttons request a start permission and only start the machine if permission is received whilst the button is still pressed and a watchdog timer is still running.

Now all machines are being started by one central system, within which the latency issue does not occur.
If multiple start requests are received simultaneously you can pick one, by whatever criteria you want, and ignore/suppress the rest.

If an operator were to wedge in the start button the machine wouldn't start because the watchdog timer would have expired.

It would be wise to add some sort of feedback, even if that is only a lamp indicating Clear To Start, on each machine.

Hope that helps,
Al
 

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