Programming Question: Part Removal without Sensor

[Jieve]

Hello Guys,

I’m curious how the pros on this board would program in the following situation.

Take the case where it is required that the machine be stopped and a part be removed by hand at step 4 of a 10 step process, because of say, a broken part, but there is no sensor detecting that the part was actually removed. There is only a message on the HMI telling the operator or maintenance personnel to remove the part. If the part is not removed, the operator simply resets the cycle and the process is restarted (again, there’s no way to tell it was removed), it will cause the machine to crash (in other words, parts to break and the line to back up).

How would you program the equipment in this circumstance? Would you require the fault to only be acknowledged by someone with higher authorization? Of course I’m sure this is customer dependent and maybe a stupid question, but is it acceptable as a programmer to allow that to happen if there is no means of detecting the removal of the part?

Thanks.

 

[James Mcquade]

is there no way to put in a sensor - photo, prox, limit switch?
what about a camera?
is there no way to allow the machine to skip processing the part and remove it at the end?

if it is critical, install a musical tone or other alarm.
then use 2 key switches.

the operator removes the key from the main operator control (turns the machine from run to stop) and removes the key.
he then takes the key to the removal station and turns the key to allow the safety door to be opened. remove the part and place it in a fixture with sensors to identify the part and press a pb.

extreme, but its not full proof.

regards,
james

 

[JerryH]

We have several places here where the operator must manually remove parts from the line. At each location the procedure is the same.

1) When a part removal is necessary the machine is cycle stopped and a fault code is displayed prompting the operator to go to the machine.
2) The PLC program must see the door open (Master is OFF of course)
3) The part is removed from the pallet.
4) While the door is open, the part must be dropped down a chute (Usually made from drain pipe) with a sensor that detects a part is passing by.
5) The door can be closed and the machine restarted.

The Japanese have a name for this, they call it Pokyoke. A system to verify NG parts are removed from the process.

Of course this can be cheated but an operator would lose their job for not doing their job here.

 

[nwboson]

I worked on a (wood grading) machine that had this challenge. It was recycled wood so it was virtually impossible to prevent some wood from jamming the machine occasionally - and then completely screwing up the tracking of the boards.

Since the operators would always need to inform me of the problem somehow, I would just show all 10 (in your example) of the parts on the HMI. There would be 10 green dots shown on the conveyor on the HMI, for example. When the operator paused the machine to remove the offending part, they were allowed to touch any of the 10 parts to remove the bad part (from the software - there really was no tracking in the middle of all the conveyors). It was a little tricky because, in my case, there could be anywhere from 1 to 10 parts on the conveyor line at once so the HMI would have to dynamically show how many there were on the conveyors - and the operator would have to count the parts on the line, then remove the correct one from the HMI.

Once the operators were trained, this manual HMI removal method worked great. The operators learned how to do this in a hurry: if they forgot, the wood got sorted wrong further down the line and the managers would notice fairly quickly that the sorting was messed up. And then the operator would have to manually sort the stacked wood, trashing their productivity and being no darn fun - a real incentive to get it right.

 

[Jieve]

Guys,

Thanks for the responses and insight!

 

[iant]

depending on your process ( I think JerryH nailed it )
you could use a shift register
If the part is a fail - allow it to pass through the system
without further operations on it then reject it at the output