Generic Measurement Question

Hi guys,
I have a pretty generic question here. How do you (in your experience) go about validating your counting systems in production?

For example, lets say you have a manufacturing line that produces x widgets per hour. Your current process is somewhat naive, assuming you put a widget out every time your conveyor belt moves 2 inches and you measure how far your conveyor belt moves.

Now lets say you put a fancy ultrasonic sensor on your line and start measuring widgets as they cross the path of the sensor.

Then the business says "hey, how sure are you that the sensor is counting right? How do we know it's staying calibrated...", etc.

My question to you is whether you come across this situation very often. Sure, you could just count every widget off the line but what if the business wants to check the old method against the new continuously, on the fly?

Your thoughts are appreciated!

On some inspection machines I worked on in the past they had procedures to follow at the change of every shift. Generally this consisted of putting known good and bad parts at varying ends of the tolerance ranges through the machine and confirming that the good parts were let through and the bad parts were rejected.

I suppose counting is one of the most basic forms of inspection. I recon you could have a procedure that a person verifies the count every so often by a manual count of product over a range. It could help find mis-adjusted sensor or loose wire or bad input point on the controller.

Now, how you come up with the frequency of this check and how many widgets to count during the check I have no way to figure out.

Norm's last point should be your first question. What is the intent of the check and how accurate does it need to be?

If you want to confirm operation simply confirming that five widgets are correctly counted at the beginning of a shift would fit the bill. If you are looking for six sigma accuracy someone will be counting all month. There is significant ground in between.

One of our customers measuring lineal footage would check a 1000 foot run against a calibrated hand-held measurement device once every month and that met their requirements. Ultimately, you need to define what you will use as your basis since only you know the process.

Keith

REHJ said:

A good way to ensure you are counting parts correctly can be to use your control logic to count a cycle. Better than just a sensor counting parts but it can take into account all the moves it takes to make (1) part. For example it takes this arm forward one time then this arm forward one time and the conveyor to move forward one time and then the sensor making. This can be counted as (1) part. This can eliminate high false counts by someone flagging the sensor. To eliminate low false counts you can check this by redundant logic using different methods and comparing values constantly. Another method of measurement and checks can be to weight parts in and out of the line. The widget will weigh (x) pounds. Total weight / (x) pounds = number of widgets. Usually very accurate.

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For a manufacturing process for a car part, this makes sense, but for super-high volume parts or product that comes out in a continuous stream, its hard to find parts of the process that have a discrete state (in our case, there is no assembly, its just finished product moving along a belt).

However, I like the idea of finding some part of the process that has a discrete state for each part coming out and latching onto that as a 'sanity check' for my sensor.