A curiosity I'm having problems with

sparkie · Dec 19, 2014

So at work, on our main chain, we have an encoder disk with a set number of holes around the edge and a photoeye that sits there and is made and broken as holes and solid portions of the disk are passed between the transmitter and receiver. This is used to track trolleys as they travel down the chain. What I am curious about is how do you do the math to relate the number of holes and size of the disk to the pulses and speed of the chain, it is seeming to elude me. I would like to learn how to do the math for something such as this.

bernie_carlton · Dec 19, 2014

You will have to work with the mechanism which drives the encoder. How much movement rotates the encoder once? If you need help with that post back with details.

Then you divide that distance by the number of holes to get how much distance one cycle of PE On/Off represents.

danatomega · Dec 19, 2014

The size of the encoder disk is irrelevant.

Take the pitch diameter of the chain drive sprocket and multiply by Pi. That will give you the circumference or the distance that one revolution of the sprocket make the chain travel.

Divide that distance by the number of holes in the encoder disk to obtain the distance that a single encoder cycle (one on/off transition) is equal to.
edit: that wording didn't come out right ... from rising edge to rising edge might be more correct.

bernie_carlton · Dec 19, 2014

Take the pitch diameter of the chain drive sprocket and multiply by Pi. That will give you the circumference or the distance that one revolution of the sprocket make the chain travel.

I've found this to give inaccurate results. Multiply the number of teeth by the length of one link.

In the meantime I await the detailed description of the encoder drive mechanism, if the OP feels it is necessary.

danatomega · Dec 19, 2014

Bernie, you are correct that your method will provide accurate results, plus pitch diameter can be sometimes hard to determine...

bernie_carlton · Dec 19, 2014

In general, there are exception unfortunately, the 'Pitch' is the number divided by 80 to yield 'inches'. E.g. 40 'pitch' equals 40/80 inches or .5 inches.

Plastic · Dec 19, 2014

Don't caculate, Just Do It

Stop the conveyor.
Put a temporary counter in the pic to monitor the sensor.
Reset the counter accumulated to zero.
Put a registration mark (white mark) on the conveyor chain.
Run the conveyor for a known distance, use the maximum amount of linear distance possible. The stop the conveyor.
Check your counter accumulated value.
Use the acquired data to know your (counts per foot) or (counts per meter)

sparkie · Dec 19, 2014

Thanks for the info guys, I appreciate it. I was just wanting a better idea of the math behind this sort of thing before I build it to learn how to do it.

Seipy · Dec 26, 2014

To me it appears your question is rather simple. First you need to find the relationship of your "encoder wheel" to the drive mechanism on the chain. Each pulse on the encoder wheel = X amount of travel on the chain. you can have a counter that counts the number of holes each minute for your speed and then calculate. Or set up a timer to grab the time between pulses on your encoder wheel. I had a similar application and I had done both. Time between pulses. total pulses in 10 seconds, and total pulses in 1 minute. I added all the results together and divided by 3 to give me a good solid average.

sparkie · Dec 26, 2014

Thanks a ton. Speaking of that encoder disk, I ended up having to replace it! The chain broke and fell down, I had to replace the disk and the photoelectric sensor. I've actually fallen behind on my PLC studies, so I'm kind of sad

. They have been keeping me pretty busy.

A curiosity I'm having problems with

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