# Length measurement

#### Abhishek Bansal

Oct 29, 2015
3
Can someone tell me how to measure length of PVC insulated copper wire. Since copper gauge(diameter) is not fixed electronic methods generally do not give highly accurate results.

Say wire of length 500 m +- 40 cm needs to be measured.
I have a cable manufacturing unit so even small amount of inaccurate result, on large scale creates a problem.

#### shrtrnd

Jan 15, 2010
3,826
I use a TDR (Time Domain Reflectometer), but I've had it for over 20 years and it was pretty expensive.
I just did a quick google search, and there are a lot of hand-held wire-length measuring instruments on the market now for from $200 (US) and up. Google what you want. #### hevans1944 ##### Hop - AC8NS Jun 21, 2012 4,739 If you are manufacturing the cables, perhaps you can measure the length with a pair of spring-loaded rollers with a revolution counter attached to one roller. Just place the wire between the rollers and pull the wire, winding it up on a spool perhaps. Such a device is available commercially, but they appear to be simple enough to make your own. #### Abhishek Bansal Oct 29, 2015 3 I use a TDR (Time Domain Reflectometer), but I've had it for over 20 years and it was pretty expensive. I just did a quick google search, and there are a lot of hand-held wire-length measuring instruments on the market now for from$200 (US) and up.

The problem with these handheld device is their accuracy. Like we pack wires in 200-500 m length. Even 1 m extra on large scale cost us a lot. We rather go with layman's approach of measuring it physically (making marking on wall and then measuring)

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)
Just for egs wire of 2.2 mm Diameter needs to be cut at 497 for 500 m. But with these techniques we are not quite sure about the length , that's why we wanted a method/instrument to cross check our length

#### Abhishek Bansal

Oct 29, 2015
3
If you are manufacturing the cables, perhaps you can measure the length with a pair of spring-loaded rollers with a revolution counter attached to one roller. Just place the wire between the rollers and pull the wire, winding it up on a spool perhaps. Such a device is available commercially, but they appear to be simple enough to make your own.

hevans1944 : First of all thank you so much for your interest. Can you please check my reply above to shrtrnd . I guess this problem would come with this instrument as well. We really need accurate results

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,739
hevans1944 : First of all thank you so much for your interest. Can you please check my reply above to shrtrnd . I guess this problem would come with this instrument as well. We really need accurate results
I can think of two other approaches that could work. The first (and easiest) is to make the wheel over which you pull the cable, and whose revolution you count, really large. Ten meters diameter maybe. Then small variations in cable diameter will have less effect on the measurement.

The second approach is to build a pinch-and-pull mechanism that operates on a linear slide. It would mechanically grip the cable at one end of the slide and then traverse a precisely known distance, for example, one meter. The gripper would then be released from the cable and returned to its starting position, whereupon it would grip the cable again. Multiple the number of back and forth cycles by the distance through which the cable is moved to determine the length of cable measured.

A variation on the second approach would mount two grippers on a continuous chain, like a bicycle chain, between two sprockets a known distance apart. When the cable has been pulled through the distance between the sprockets, a solenoid releases the first gripper and another solenoid engages the second gripper. The timing of the solenoids would be controlled by optical interrupters attached to the gripper mechanism. The two grippers continuously circulate with the chain around the two sprockets, alternately gripping and releasing the cable. You might also want to incorporate a braking mechanism for the chain to stop each gripper at precisely known positions while one gripper releases and the other gripper engages.

This could be an interesting mechanical design problem. It should be possible to eliminate the solenoids and use cams to release and engage spring-loaded grippers on the cable. Might still want a mechanism to halt the chain as one gripper releases and the other grabs the cable, some sort of clutch mechanism perhaps.

#### bigone5500

Apr 9, 2014
712
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 $.02. #### hevans1944 ##### Hop - AC8NS Jun 21, 2012 4,739 A precision rotary encoder of sorts with a precision machined roller mounted could possibly be a solution. An encoder with 100 pulses/revolution or more would be accurate enough I would think. You can do the math to find out what wheel diameter would be most appropriate. The wheel in which the encoder is mounted could have a light knurling to eliminate slippage. Fast google search got this: http://www.gurley.com/Encoders/rotinc.htm This post cost a total of$.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.

#### bigone5500

Apr 9, 2014
712
There is also the use of lasers to measure the length. Two of them could be used in conjunction to both measure the linear movement and the diameter. An algorithm of sorts could be written to offset the varying diameter as opposed to the amount of wire travelling through the beam. I'm sure this is probably the most accurate method available.

As @hevans1944 said about the commercial solution, there will most definitely be an accuracy tolerance with this type of setup. ±0.05% is very accurate so you may not be gaining much by going with a laser measuring setup.

This is all my brain has to offer so good luck to you.

#### shumifan50

Jan 16, 2014
579
It would seem to me that an additional 1m of cable supplied on an order of 500m would equate to good customer service ensuring you do not sell short. That is equivalent to a 0.2% discount or add 0.2% to the price.

Replies
11
Views
1K
Replies
12
Views
1K
Replies
7
Views
1K
Replies
7
Views
729
Replies
2
Views
716