It depends on what you mean in multiple head weighing, the only ones I have been involved in are where there is only one fill at a time but 2,3 or 4 intermediate pre-hoppers are used, the pre-hoppers are filled with a potion of the required weight & the program calculates which of these hoppers make up the closest required weight these are then dropped into a combining chute into the package. this sort of weighing requires high speed weighing cells & program.
I suppose you mean more than one linear feed weighers but the program selects the first one to reach the required drop weight. then this should be pretty easy just a matter of building a queue of the linear weighers, in other words a loop through the number of weighers, put them into a shift register however, the shift register should move down when the queue output is empty in other words rather than shift the queue only on first to reach weight but also shift down the queue if the next but one position is empty. this can be done quite simply assume the system has 4 weighers, then make the queue 5, when a feeder has the required weight move it into the queue, if the next position is empty, shift it down, until all but the first register is zero, when a fill is requested, the product is dropped into the pack then the value of that register is set to 0, thus allowing the positions to be shifted down the queue.
lets assume at start, the queue shift register is 0, & the linear feeders are filling with weight, when one gets to the required weight, move it into position 0 of the shift register, the program checks every scan if the value in shift position 1 is 0 & moves the value in position 0 into position 1, if the value in position 2 is 0 move from position 2 & so on until any positions in the shift register are filled with the weights, this requires a two dimentional shift register as you need to know the feeder number.
like this
Shift queue (2 dim shift register)
....Weight, Feeder No.
0 30kg, 2
1 0,0
2 0,0
3 0,0
4 0,0
5 0,0
It moves the value in register 0 down until it is in position 5 like so
0 0,0
1 0,0
2 0,0
3 0,0
4 0,0
5 30, 2 (this register is then used as the information for the fill
When another feeder reaches the required weight shift it down the queue.
so you could end up with the following:
1 0,0
2 30,4
3 30,3
4 30,1
5 30,2
This is just to show how if it could not fill the container so you end up with a full queue of all the feeders, then as the containers are filled the register location 5 is set to 0 & the others are shifted down.
As you probably only have a limited number of positions due to the number of feeders there is probably no need to use a shift register function you could just use simple logic perhaps in a loop i.e. if pos 5 is 0, shift position 4 to 5, position 3, 4 & so on
