help with shift register

Hi All

I have set up a simple bit shift register use fc92.

Basically I have a three pick and place units.

The first picks up a bottom case using a vacuum valve.

The second picks up a pcb

The third picks up the top.

I have used the shift register so that if the vacuum swith is not activated in the first PP then the pcb or the top are not activated.

This all works fine. The problem I have is that the pcb board falls off the vacuum head sometimes and the top is still put on because the bit has already been shifted.

If I change when I look at the vacuum switch output the shift register seems to stop working. Is it something to do with the scan time?

bump

Are they controlled by the same PLC?

Is this correct, the first part is put on and a bit is set at the start of the shift register. At the end of each cycle the bits shift, if a '1' appears at the point of other two stations they perform their function.

If the suction is not good enough for the middle station, then either increase it, if not possible look ahead, if the bit ls '0' in the position upstream of its station, don't pick one up.

Why is the last station feeding, the shift and checking of the register should be done on a one shot basis.

Sorry I didn't explain it very well. The first part is put on and the bit is set from a vacuum switch on the first part. This enables the second part to be put on. When the second part is put on a bit is set in another shift register, this then enables the third part to be picked up when it needs to. The vacuum switches are working correctly. What happens is we have some of the second parts stick in the nest that holds them. The vacuum switch is being made but the part is falling off when it is picked out of the nest.By this time the bit has already been set to '1'. Due to the design of the second part we can't get around the fact that sometimes the part is not pick. However because the bit is being set it then puts the third part on. What I want to try to do is look at the vacuum switch at a specific time in the cycle to determine whether to set the bit or not. I have tried something but the shift register doesn't work at all.

Yes this is all run from one PLC

saleshark said:

Due to the design of the second part we can't get around the fact that sometimes the part is not pick.

Click to expand...

So we agree, You are wanting to solve a bad design with a programming change. Not always possible.

saleshark said:

The problem I have is that the pcb board falls off the vacuum head sometimes and the top is still put on because the bit has already been shifted.

Click to expand...

saleshark said:

If I change when I look at the vacuum switch output the shift register seems to stop working. Is it something to do with the scan time?

Click to expand...

I believe it has to do with the timing of the shift register clock pulse.

saleshark said:

Sorry I didn't explain it very well. The first part is put on and the bit is set from a vacuum switch on the first part. This enables the second part to be put on. When the second part is put on a bit is set in another shift register, this then enables the third part to be picked up when it needs to. The vacuum switches are working correctly. What happens is we have some of the second parts stick in the nest that holds them. The vacuum switch is being made but the part is falling off when it is picked out of the nest.By this time the bit has already been set to '1'.

Click to expand...

This sounds like you need to reset the bit after it has already been shifted.