Line No. 7 starts with bar stock and performs several operations to make little widgets that are then shipped to a specific customer as components in one of their products. The widgets are counted by photoeye as they enter the final stage, which is a spray and dry prior to packing. They then hit a carousel from which they are dropped into a waiting box. When the weight of the box reaches the desired target, the carousel stops dispensing. The box is rotated out to head to the sealing and labeling stage for shipment, and another box is put in its place. This is all automated.
Daily, the total produced (and counted by the photoeye) is compared to the total shipped (as per weight). There is often a small discrepancy, and it’s attributed to widgets being slung off the line at some point between being counted and being weighed. But nobody can seem to find those lost widgets, and since these can’t be used or sold by employees, there’s no theft.
A recently hired production superintendant who now is in charge of this line also noticed that the discrepancy isn’t always less at the box. Sometimes, fewer are counted by the photoeye than are weighed in the box. She wants this resolved. What steps do you take?
Answer to Quiz. You could have a problem at the photoeye, the weigh station, or both. Let’s start with the photoeye. First, check that the lens is clean. Then check that the eye is aimed correctly. As a widget passes in front of the eye, where is the eye looking? Ideally, it will be at center mass in both the horizontal and vertical planes.
Take some time to watch the widgets move in front of the eye. An oblong object that spins could be counted twice (leading edge creates one count, then trailing edge the second). If this is happening, then change the vantage point of the photoeye so the spin doesn’t matter.
Excess vibration on the line could also cause double counting or skip counting. If the PLC has a slight delay added to prevent double counting, it might prevent a valid count sometimes. Rather than mitigate the results of the vibration problem in the PLC, reduce the vibration by adding bracing and vibration dampeners where needed. Take care with vibration dampening devices, as they can create a gap in the equipment grounding (bonding) path; jumper around them to preserve the electrical continuity, as needed.
At the weighing end, you need to ensure the box is sitting correctly on the scale, the tare weight is correctly entered, and the box is still sitting correctly on the scale after it’s been filled. If you have a variety of boxes being used instead of a standard carton size, you could have enough deviation in tare weight to throw off the count. Find out what’s being used throughout the day and from day to day.
Another possibility is that each widget does not weigh the same. For example, you could have a few from which the process material wasn’t completely removed. This not only will throw the count off, but it’s unlikely the customer appreciates seeing that.
If all of this checks out, what else might explain the deviations? Remember the equipment grounding (bonding) issue that may result from the use of vibration dampers unless you jumper around them? You may have this issue due to other causes, so perform a visual inspection to look for such problems, and then correct them with bonding jumpers or other means.
You may have current inducted into signal wiring due to its proximity to power wiring. Check to ensure that none of your signal wiring from the weigh station or the photoeye is run in the same raceway as any power wiring (especially the power wiring of motor). Regardless of any preventive measures taken during installation, the only realistic ways to protect against magnetic inductance are to add distance between the two sets of conductors and to reduce how parallel they are.
Is there still a discrepancy? Install a second photo eye to count widgets as they drop from the carousel into the box. If the counts between the two photoeyes are consistently the same, then the widgets and/or the boxes are varying in weight or the scale has poor repeatability. Test the scale by adding and removing the same weight twenty times to see if all twenty measurements agree; if not, replace the scale.
If the two photoeyes do not agree, you probably have a widget loss if the second photo eye produces a lower count. If it’s the other way around, you have some problem in how those photoeyes are set up. Getting this sorted will just take a little experimentation.
