Encoding position of item on conveyor with idler belt

[VELCROW]

I'm working on a system where we are laser-marking an item as it goes by on a conveyor. To better control the velocity, we are locally driving one side of the item with a urethane round belt. Normally I would use a skid plate backer, with a friction wheel and encoder on the other side to tell the laser when to fire, but the print length is longer than the area a friction wheel can hit. I started with another urethane belt with an encoder on the trailing pulley for the back side, but it was too stretchy, and took a set which provided uneven torque to move it, which meant some slipping. So I am using a timing belt for the back side, which has an encoder on the trailing pulley. There may be more than one item between the belts at one time.

Here are my questions:
-Has anyone done anything like this before (a friction belt for encoding), and is there a better way?
-if not, would you recommend lots of toothed idlers to support the timing belt with low tension, or just end pulleys and very high tension?

Thanks

 

[MikeHalloran]

Maybe a photo or drawing would help.


Mike Halloran
Pembroke Pines, FL, USA

 

[VELCROW]

Yeah, I should have put that in. Here's a rough sketch, with idlers and without.

 

[MikeHalloran]

I think you'll be happier with the idlers, or a polished stainless shoe, applying pressure to the toothed side of the timing belt to keep it in contact with the 'item'. The shoe can work at modest speeds because the belt teeth are usually faced with nylon. Not so much for molded urethane belts.

Super high belt tension won't bother the timing belt much, but it will bother the bearings on the encoder and idler pulleys, and make the passively driven belt susceptible to jamming from the odd piece of paper or other detritus falling on the belt teeth.

I think you can order timing belts with high friction backs, too.



Mike Halloran
Pembroke Pines, FL, USA