Anyone who works on in motion checkweigher and digital weighing equipment (such as a bench scale, laboratory balance, etc.) knows the relatively simple task of weight testing the scale with known test weights.
It is simple right? You turn off automatic zero tracking (AZT), verify that the scale is at center of zero, then apply known test weights and compare the displayed value against the actual weight applied. You check it at several different weight values and provided you have a good static scale, bingo you are done! Simple right?
Now, put the test weight in-motion across the scale and suddenly things get really confusing. You run the weight once and you are done right? WRONG!!!! This is where statistics rears it’s ugly head. Often when you run the weight across the in-motion checkweigher, it weighs different! What should you do!?
Steps To Success
- Same as the static scale, check zero both static and in-motion.
- Get a product (one single example) that represents the product that you intend to check weigh in-motion or dynamically. Then verify the exact weight of that product on a very accurate static scale. You now have what is known in the industry as a “test puck.”
- Turn the in-motion checkweigher on (run the belt) at the speed that you intend to use the in-motion checkweigher in production.
- Apply your test puck to the infeed belt well ahead of the weighing section of the in-motion checkweigher to allow relative motion of the test puck-to-belt motion to stabilize before the test puck arrives on the in-motion checkweigher’s live section. Then collect the test puck after the in-motion checkweigher’s live section once weighing is complete.
- Record the in-motion checkweighers in-motion value.
- Repeat steps 4 and 5 at least 32-100 times. (no I am not kidding!)
- Verify that zero did not change or you will need to start over.
- Verify that the test puck static weight is still the weight value when you started or you will need to start over.
- Analyze the results by determining the highest and lowest values, the difference is the spread, then average all the in-motion values and compare against the actual static weight of the test puck, oh yes don’t forget standard deviation calculations!
- Ask yourself, ARE YOU SATISFIED? If so, congratulations!! You are done (for now) as soon as you file the results to compare against future testing results.
Following the above steps, as tedious as they seem, will give you a clear picture of the true accuracy and repeatability of your in-motion checkweigher. Future testing with similar results will prove the performance is staying constant while variations with trends can indicate problems with the checkweigher.
Obviously, if any kind of change is made, it is time to test again. Be very careful to never change more than one thing at a time, as it becomes impossible to tell performance changes based on the “thing.”