Conveyor Belt Tracking using Servo Motor

[idir.mahrouche@gmail]

Hello,

I have a servo motor running a conveyor belt system. I do not have the exact circumference of the head pully and therefore I get some shift whenever I run the conveyor for one cycle.

I do have a sensor seeing the 47 (full cycle) mounted containers on the belt.

What would be the best method to track the position of each container at any moment while conveyor is running.

Regards,

 

[willxfmr]

Knowing exactly how far the belt moves per revolution of the motor, and tracking exactly where on the belt a box enters is probably your best bet. You could also add a zero flag to the belt to eliminate any compounding error caused by belt or drive system slippage.

 

[parky]

A simplest form of an encoder would be best, for example a toothed wheel with proximity on it, depending on the number of teeth & speed you can get quite good resolution without the need for an encoder however, to many pulses per second may require you to use high speed counters/inputs, have done this many times, a system I did some years ago used this method to track boxes on a very long conveyor with 17 takeoff points, a bar code reader read the barcodes at the start of the conveyor, the data was put into a shift register & shifted along by the conveyor pulses from the proximity, it turned out to be about 5 pulses per box so effectively the conveyor was split into x number of registers, when the register came into the window for a diverter, it was compared with the required barcode for that divert & would divert it, it can be done with a bit shift if you do not need any data. because the pulses from the toothed whell correspond to the conveyor position even if it speeds up or down or falters the box position is still correct.
With a bit shift every time you get a pulse & a box is present at the input a "1" is put in a shift register, if no box then a 0, if this is done on a oneshot off a sensor then you get the position of the begining of the box.
So in simple form, let's assume you have a conveyor that is 5 metres long, the number of pulses is (to make it simple) is 500 per full rotation of the conveyor that equates to 100 pulses per metre, or 10mm so that would be your resolution. pretty good for tracking without the need of an encoder

 

[joseph_e2]

If there's significant slippage between the motor and belt, you could also do something I've done to track metal strip through a rolling mill. Get a resolver or encoder with a wheel of known diameter that rides on the conveyor belt itself. You're then looking at the conveyor belt, not any of the drive mechanism. You can then do your tracking directly.
If there isn't slip between the servo motor and belt (I think there really shouldn't be), the best option is to find the actual circumference of the head pulley since the servo motor should already have feedback built into it.

 

[arpus4KM]

A simplest form of an encoder would be best, for example a toothed wheel with proximity on it, depending on the number of teeth & speed you can get quite good resolution without the need for an encoder however, to many pulses per second may require you to use high speed counters/inputs, have done this many times, a system I did some years ago used this method to track boxes on a very long conveyor with 17 takeoff points, a bar code reader read the barcodes at the start of the conveyor, the data was put into a shift register & shifted along by the conveyor pulses from the proximity, it turned out to be about 5 pulses per box so effectively the conveyor was split into x number of registers, when the register came into the window for a diverter, it was compared with the required barcode for that divert & would divert it, it can be done with a bit shift if you do not need any data. because the pulses from the toothed whell correspond to the conveyor position even if it speeds up or down or falters the box position is still correct.
With a bit shift every time you get a pulse & a box is present at the input a "1" is put in a shift register, if no box then a 0, if this is done on a oneshot off a sensor then you get the position of the begining of the box.
So in simple form, let's assume you have a conveyor that is 5 metres long, the number of pulses is (to make it simple) is 500 per full rotation of the conveyor that equates to 100 pulses per metre, or 10mm so that would be your resolution. pretty good for tracking without the need of an encoder

This is exactly the way a few machines I help customers with currently were done by the OEM.

6 different push points. Barcode gets scanned, and then they don't bother with a BSL. but instead take that barcode and place it into 10 points of an array thats 400+ long.

the "encoder" is a prox triggered by a star wheel that has about 1" gap between each prox trigger. so they get a 1" resolution.

When the box enters the zone it triggers the entry prox, which MOVs the barcode into the Array. Every time the Prox triggers, it MOVs the array into a temporary Array, and then immediately below (scan top to bottom) this MOV is another MOV that sends it back to the original +1 location. essentially just shifting the barcodes through. Then each lane diverter has that barcode assigned to it, so there's no need to have multiple BSL for each code, it literally just BSL's the barcode through an Array.

 

[parky]

Yes that exactly, The point was that if the boxes on the OP's conveyor are not coded or need to be tracked using codes but purely position i.e. actual position & it is there not empty then bit shift is all that is needed.