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Hello all, i am new to the Arduino world. I've been assigned to a project, that has to do with length measurement. What i really want is to design something to count the length of a rope that is on a plastic reel. is there a sensor that the rope can go through a custom wheel, and then base on the wheel perimeter, count the length of the rope?
Any guidance is much appreciated

Regards,
Nicos
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you could use a encoder.
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as i am new to this, could you elaborate more on this please?
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I don't think you connected the grounds, Dave.
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I remember a science programme on TV a while back, about an immediate-post-Soviet research station in Antarctica, taking ice core samples from great depth in the ice-cap.
The cable attached to sampler passed over a pulley, where a simple switch was pressed each revolution by a peg attached to the pulley.
The switch was wired across the '=' key of a cheap, off-the-shelf calculator!
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Yes. This is a very nice simple project for a new Arduino user. There are many ways to detect when the custom wheel has made a rotation. The simplest would be a momentary contact switch that is activated by something protruding from the wheel. The rest is simple math. Depending on the method you choose to detect a rotation, you may have to consider contact bounce. - Scotty
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where a simple switch was pressed each revolution by a peg attached to the pulley.
The switch was wired across the '=' key of a cheap, off-the-shelf calculator!

I thought about this, but i have a problem with this concept. Based on your example they could measure the depth. What if i want to know how much line is left until i get the ice samples when the ice sample is on its way to the surface
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I don't think you connected the grounds, Dave.
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Well, one direction ADDS to the count, the other SUBTRACTS.
Calculators with constants can do one or the other.

If you want to know direction and rate, you need a quadrature encoder.
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What if i want to know how much line is left until i get the ice samples when the ice sample is on its way to the surface
If you can count the outgoing rope you can count the incoming rope. At most you need a method of knowing which way the rope is going. Are you controlling the rope or just measuring?

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Rob
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the people at see use to use knots in the rope and they looked how many knots of line went in the see at a certain speed.

in the digital age you might be able to place markers on/in the rope
- either metal to be detected by a metal detector
- or barcodes
- or RFID

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Actually the application is a little bit different. We have several large reels with different diameters of rope. We have ro cut the rope as long as customer wants.
So what i want to do is to
1: count the rope on the reel by reeling out the line
2: re count the rope by reeling in (just to make sure reading 1 was correct)
3: measure 100 feet and cut.
4: measure 150 feet and cut
5: substract 250 feet from average of 1 and 2 so we know how much rope is left on the reel

Sounds easy but i am sure its not an easy task

Thanks again
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The only way I can see to do that is as AWOL said, run the rope over a pulley and count the revolutions. If there are several ropes and the reels are next to each other you could have several pulleys all fixed to an axle and an encoder (real or home-made with a switch) on the end of the axle. Then calibrate for each rope size.

The only problem I think is that with no load on the rope you could get slippage, maybe some sort of spring-loaded idler wheel to keep pressure on the rope and pulley.

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Rob
« Last Edit: May 01, 2012, 05:04:47 pm by Graynomad » Logged

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Rather than count revolutions, which give you low precision, put a rotary optical encoder on a shaft with a rubber cylinder that presses against the rope and another cylinder, not necessarily rubber.   The diameter of the rubber cylinder can be large, to handle all sizes of ropes.
For each revolution the encoder puts out multiple pulses, which your arduino counts.
Use an interrupt routine that triggers on any type of encoder pulse, determines the direction up or down, and modifies a variable called Length.
Your main loop can read a button and reset Length, display Length on an lcd panel, or even read a keypad preset and then set a light or beep when it goes down to zero.

These encoders are slightly expensive.  I would not use the $10 detent-type encoders popularly used for user knob input with Arduinos.
If this is a onesy project, there are some bargains on ebay.  (Like 280873016907 )
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