To measure circumference

Refer the attached image,this is i want to do,measuring circumference of large shell.
For calculating this, im going to fix a sensor stationary and rotating shell, in this the sensor points from position say P0,now the shell rotates P0,P1,P2,sensor keeps tracking…After attaining the initial P0,the sensor stops,length is measured is that the circumference.
is this concept possible to implement?
i am new to arduino…kindly help.
is there any other method to calculate circumference of a big size shell?

Circumference-01.jpg

Unless you know that the tube, of whatever size it is, has completed a full rotation, and only one rotation, the time between the measurements will be useless information.

If the tube has a known diameter, the circumference is known. If the tube does not have a known diameter, but you know the circumference, based on knowing how fast the tube was rotating and how long it took to make one revolution, you can calculate the diameter.

It is not clear exactly what you are measuring and what you are trying to calculate.

Use a tape measure?

i want a system which calculates circumference of large size circular/elliptical object.

How it can be automized?

Do you plan to turn objects around that are too big to measure with a 10 to 30 meter tape measure?

Start looking at how 3D scanners work.

neepu:
is there any other method to calculate circumference of a big size shell?

If building size then aerial photo might do.

Put a wheel on a stick, roll it around the girth to be measured and count revolutions.
The same is used to measure roads and property lines. You can buy them already made.

How big is you object? a foot across? a mile?

regards

Allan

i decide to go this encoders,as study says distance calculation of mouse varied with speed.
There are several encoders availabe in market.
should encoder suit my application for length measurement ie circumference.
Output of encoder can me converted to distance?
you have any idea?

What encoder?
How big the objects to measure?
My mind-reading cap is still in the shop being fixed by magical elves so I have to ask.

neepu:
Refer the attached image,this is i want to do,measuring circumference of large shell.
For calculating this, im going to fix a sensor stationary and rotating shell, in this the sensor points from position say P0,now the shell rotates P0,P1,P2,sensor keeps tracking...After attaining the initial P0,the sensor stops,length is measured is that the circumference.
is this concept possible to implement?
i am new to arduino..kindly help.
is there any other method to calculate circumference of a big size shell?

Use one of these, no calculation needed.

It is of 600m diameter..A very big tube,,, what is the cap you have mentioned

neepu:
what is the cap you have mentioned

The cap is one that allows me to read minds.
It is entirely fictional, part of a deep sarcasm, the words "magical elves" should be a big clue.

You need to think along the lines of a CMM with at least two components.

A wheel for measuring around the shape which must be rotated and a quill type sensor tp measure the high point of the shape.

The shape should rotate until the high point is found and at that point it should zero the wheel
It should rotate from there until the high point is registered again and at that point the calculation should be done for the actual circumference.

There may need to be an offset in place for the wheel Dia etc.
The shape may even have to over run in rotation to ensure its fully measured but readings taken should be able to match the high point with the wheel position and logged to make the calculation.

for accuracy a high count "ROTARY" encoder required for the wheel and a high count "LINEAR" encoder (or resistive) encoder for the quill.

A bit rudimentary I know but what do you want for free and a few seconds in MS paint.

The other method is to use a non contact system where the part if measured by knowing the rotation speeds and using some form of laser or other distance measuring device to again pick up the high point.
problem with that is it might bump and move during rotation giving a few errors.

Something 200 meters across, you don't turn. You move around it, on a rail or track if necessary and measure in to the object directly to center. If it reflects, an ultrasonic sensor may be enough.

3D scanners use lasers and can be scaled up. Take scans from 4 sides and integrate on a PC.
You can model real objects so why not a big shell? Scan at night when no one is around.

Hi GFS

I thought it was 600 mm...wow then I go with your idea of a scanner in the middle.

I would prob just call for a surveyor and total station...quick cheap effective.

There are others I just call that as an example

OP stated 600m and has not confirmed error. First description used the word shell.

I think that there was a siege mortar that fired 60cm shells but 600m dia TUBE? Industrial scale storage?

If it is 600M diameter this is the sort of thing to use.

http://www.ebay.com/itm/322313439614?euid=e4f714dcf2c94724a6ccc89fa3bbb34c&bu=43106377121

If 600 mm same but with a smaller wheel i would suggest.

You will however need to know when a complete revolution has been completed.

For an elliptical shape the mechanics will be more complex.

Either way the mechanical arrangement is unlikely to be off the shelf.

An optical mouse in close contact may work , it may also be possible to draw a line so the mouse detects when a complete revolution has occurred.

EDIT
wrong link
http://www.screwfix.com/p/forge-steel-measuring-wheel/6998C?kpid=6998C&cm_mmc=Google--Product%20Listing%20Ads--Sales%20Tracking-_-sales%20tracking%20url&gclid=CMKFoa2pjdACFQ06GwodDfEP5g

Apologies

GoForSmoke:
I think that there was a siege mortar that fired 60cm shells but 600m dia TUBE? Industrial scale storage?

Even storage tends to limit tank size to 100M dia for practical reasons.

EDIT

I am wondering if the purpose of the project is to measure the diameter of a tree trunk but OP seems to be somewhat unforthcoming.