This is my understanding of your requirement.
When a batch of cement goes away, you need to subtract it from the total. When the new amount of cement comes in, you need to add it to that total. The amount of the cement to be subtracted is stored in PLC's memory. So I assume that there is a logic value to indicate you that it's time to subtract. Right?
It's a little difficult to do so in Lookout, since lookout doesn't support X=X+n.
After doing some investigation, I think you can use Accumulator object. For example, pot1 is the amount to be subtracted from total. Pb1 is for pot1 to indicate you when to subtract. Pot2 is the manually input value. Pb2 is for pot2 to tell you to add the value.
You can create two Accumulator objects. One is for all subtraction. Aother is for all increase. So, for Accumulator1, Input=Pot1, Sample=Pb1. For Accumulator2, input=pot2, sample=pb2. You can create another pb for reset. Then the total=Accumulator2-Accumulator1.
If the number is too large, you can divide them by 1000 to do all those things.
You can also reset these two Accumulator to zero. But take down the current value first. After reset, input the current value to pot2.
If in your system, there is no pb1. If you just subtract the value at a certain rate, just modify Accumulator1 a little.
Let me know if this is what you want.
Ryan Shi
National Instruments
