What kind of sensor can used for measuring tape

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
562
I have application, where need to know how much elastic thread has been taken out from the device. Something like this:

What kind of sensor is used here, to exactly measure how much thread is taken out in millimeter?
 

MrSoftware

Joined Oct 29, 2013
1,935
At first glance it looks optical to me. Follow the yellow string, see where it wraps around the pulley with all of the spikes on it, for lack of a better word. Notice where the spikes go between the black piece of plastic on the PCB. That is most likely an IR sensor, with an IR LED on one side and an IR receiver on the other. The little spikes on the wheel break the IR beam as they go past. They are just counting how many spikes have gone by. Knowing the diameter of the wheel where the string wraps around it and how far apart the spikes are, just do the math to calculate how much string has gone out.
 

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
562
Yes its not elastic thread.(wrong word), its string, same as used in video in post #1.
I need to measure how much string moves out and in for some measurement.
Since string will be used for measuring circumference so cannot use those metal type measuring inch,


Is there any such readymade sensor aseembly availble which directly gives some digital/analog signal on how much thread moves in/out.
 

Reloadron

Joined Jan 15, 2015
5,667
Is there any such readymade sensor aseembly availble which directly gives some digital/analog signal on how much thread moves in/out.
Yes, several. You may want to Google "String Pot". I have used them accurately out to 10 feet (3.048 meters). You may also want to look at how the wire feed systems work on tig welding where how much wire is fed and at what speed. They count up and down just like a string pot.

The video looks to use maybe an optical encoder on a small pulley of known diameter, they get N number of pulses per revolution which increment or decrement a counter.

Ron
 

MrSoftware

Joined Oct 29, 2013
1,935
The least expensive method I can think of is the way it was done in your video, with a notched wheel and an optical sensor. Or the wheel attached to an encoder. Functionally the string pot looks like it will do what you want technically, but a quick google shows that they can be pricy.
 

Tonyr1084

Joined Sep 24, 2015
5,196
When measuring circumferences I measure the diameter then multiply that by Pi. (3.14158)

If you don't want to or can't measure diameter then you can use a flat tape. Those regular tape measures are really difficult to measure circ's. Small diameters I use my wife's sewing tape. It's cloth and can wrap around something.

Or finally, take ANY piece of string and wrap it around what you're measuring. Take a marker and put a mark on the two strings as they overlap. Then straighten them out and measure that distance.

You CAN do it electronically if you wish, but the simplest methods almost always seem to work the best.

What are the typical and max circumferences you want / need to measure?
 

MisterBill2

Joined Jan 23, 2018
7,234
The type of device referenced as a "string pot" can certainly be used to measure diameters. There is a similar device that uses a rotary incremental encoder that can be had with one pulse per millimeter output. This can be fed to a suitable up/down counter giving a convenient digital display . They are not cheap but very reliable and quite accurate
 

djsfantasi

Joined Apr 11, 2010
6,947
Go to any store similar to a CVS, Walmart or Dollar Store. Buy an emergency sewing repair kit which includes a cloth tape measure.

Use that.
 

MrChips

Joined Oct 2, 2009
22,067
Take a piece of string and wrap it around the cylinder. Mark off one circumference.
Measure the length of your string with your metal tape measure.
 

MisterBill2

Joined Jan 23, 2018
7,234
In post #1 there was a requirement of accuracy within one millimeter, about 0.024 inches. A string will usually stretch more than that, and a cloth measuring tape can not be read that closely. THAT is why I suggested the stringpot with the encoder option.
 

Reloadron

Joined Jan 15, 2015
5,667
In post #1 there was a requirement of accuracy within one millimeter, about 0.024 inches. A string will usually stretch more than that, and a cloth measuring tape can not be read that closely. THAT is why I suggested the stringpot with the encoder option.
Well then with that uncertainty the cost just went up. String pots I used had a stainless steel wire. Still have an old 8' one. Unimeasure makes great stuff which includes a matching price tag. I agree, you can have accuracy and resolution but it doesn't come cheap (inexpensive). :)

Ron
 

Bernard

Joined Aug 7, 2008
5,680
There are flat steel & plastic retractable tapes, 1/4" wide; I have 5, 39", 6' & 10'. I also made 2, one for measuring cap wire( dynamite ) 0 to 300 ft. mechanical read out & 1/4 in cable 0 to 2500 ft. coupled to chart drive.
 

scorbin1

Joined Dec 24, 2019
44
If you want accurate measurement and it absolutely needs to be a home brew electronic device, I would go for a quadrature encoder. Would likely need a uController to go with it, but TI sells their MSP430 line of controllers very cheap(used to be around $5-10 for a dev board) and they are very capable. That would reduce error incurred by potentially missing pulses if you pull it out quickly.
 
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