how to calculate the length of cloth in a big roll using an encoder?? because in every rotation the radius of roll decreases and the length of cloth come out in every rotation is different...max at starting and min at end...

A pinch roller on the cloth itself rather than on the roll spindle.

The basis for some formulas used in web handling are that the side area of the web is equal to the side area of a roll.

Length = ( Pi / 4* Caliper )( D^2id - D^2od)

So you would have to have a count for each rotation and a variable length added to each count.
There is a lot of good information in Converting Magazine. And David R. Roisum used to post articles about things like this. He also has a set of books about web handling you may be interested in.

Thank you for the link.

As another option similar to the first, can you measure rotational speed of a shaft or gear feeding the cloth?

Check out www.sensorso.com. We have 1000s of speed sensors to choose from and are available from 8:00am-5:00pm mountain time to discuss the right sensor solution for any application.

Dear Vijayant, Instead of placing encoder on ROLL, you need to place it on NIP or Puller and you will get accurate length.

Better to ask an Unwinder/Rewinder Application developer....

While there are several good suggestions here, trying to measure the outfeed length of fabric based on the number of rotations of an uncoiler mandrel presents a host of problems. The first and most obvious is the non-linear equation that describes the relationship between feed length and the roll circumference as the roll diameter decreases. Howard has pointed you in the right direction but this is, at best, difficult to program in a PLC. The next concern is the potential for the fabric to stretch and the related tendency of the back tension to result in the tightening of the roll such that actual outfeed is greater than would be apparent from the mandrel rotation. The third concern is the slip on any drive roll that pulls the fabric off the uncoiler.

Depending on the nature of the fabric and the nature of the process I would suggest that any measurement of length be taken between driven rolls where the fabric can be maintained under a constant and relatively low tension. I would suggest a Doppler LASER or similar non-contact sensing technology in order to minimize the influence of the measuring device on the measurement.

There is a great deal more to your question than meets the eye.

Some non-contact laser systems such as the µSpeed from Scantron Ltd will give you super accuracy and a length output directly. However, en encoder is much cheaper indeed. Do not place it on the cloth roll, place it on a web-driven roll with a good wrap angle. Then you simply count the number of pulses, knowing the roll's actual circumference and the number of pulses/rev... Use a counter/timer for the count. The only problem is the rollover once the register is full. Simply reset it yourself well before the rollover and add the new pulse count.