A precision rotary encoder of sorts with a precision machined roller mounted could possibly be a solution. An encoder with 100 pulses/revolution or more would be accurate enough I would think. You can do the math to find out what wheel diameter would be most appropriate. The wheel in which the encoder is mounted could have a light knurling to eliminate slippage.
Fast google search got this:
http://www.gurley.com/Encoders/rotinc.htm
This post cost a total of $.02.
Nice try,
@bigone5500. Your suggestion has caused me to rethink this problem. The cable diameter varies as
@Abhishek Bansal explained in post #4 above, and according to him that creates a problem:
We have counters and pulleys with fixed circumference but the main problem with them is their count keeps on varying as thickness of wire changes(since circumference of pulley changes as wire rolls over it)
He doesn't explain why the wire diameter is varying... perhaps it has something to do with how the individual strands are laid and the outer jacket is applied... nor does he explain why a variation in cable diameter should cause a problem. The diameter of the pulley
does not change as a wire tangent to the circumference is pulled across it.
I don't understand why a variation in cable diameter would cause a problem. The cable should always be tangent to the circumference of the "pulley" so the cable diameter has no effect on how far the "pulley" rotates as the cable is pulled across it. I have seen wire length measuring devices used with all manner of wire and cable, from single strands to cables as thick as my thumb and larger. The diameter of the wire or cable has absolutely
NOTHING to do with the length measurement.
But perhaps physics and geometry are different in India than here in the States. That is why I suggested increasing the diameter of the "pulley" whose revolutions the counter is recording. If the wire diameter was having an effect, then increasing the diameter of the pulley would reduce such an effect. You could also add a rotary encoder as
@bigone5500 suggested to increase resolution, although a gear-train, driven by the pulley axle, and incrementing an "odometer" type counter would accomplish the same thing.
I now suspect that whatever "counters and pulleys with fixed circumference" are being used, the mechanical arrangement is faulty. Perhaps the wire is being pulled across a pulley with a "V" groove. This is not a correct arrangement. The "pulley" must have a constant diameter. A knurled surface on the "pulley" would help prevent slippage between the wire and the "pulley" surface the wire is pulled across. A spring-loaded "idler" pulley must be used to press the wire or cable against the measuring pulley to ensure there is no slippage between the measuring pulley surface and the wire or cable surface.
As I mentioned in post #3,
there is a commercial solution that measures with ±0.05% accuracy. For 500 meters of cable, this is an error of ±0.25 meters or ±25 centimeters. This is not rocket science. Any competent mechanical engineer should be able to custom-design a cable length measuring system based on pulling a wire or cable, without slippage, tangent to a freely rotating cylinder. Such a system is NOT sensitive to wire or cable diameter.