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Centering object on spinning platform

Centering object on spinning platform

Centering object on spinning platform

(OP)
I've been trying to come up with a good solution to a unique problem.  I have a spinning shaft with a plate attached, and i need to place an object on that plate and have it be centered.

The shaft spin is controlled via a laptop and is used for photography - the problem we haven't been able to find a solution for is trying to perfectly center that product on the plate.  Right now when the shaft spins, if the product isn't centered up exactly it is very obvious in the photos.  

Objects placed on the plate very greatly, sometimes they are cylindrical and sometimes they aren't even a standard geometrical shape.  I'm looking for any suggestions on how to solve this problem - we've tried several different methods and haven't came up with anything good.

RE: Centering object on spinning platform

What is you definition of "centered?"  

There could be a number of different meanings, center of gravity, optical centroid, etc.

TTFN



RE: Centering object on spinning platform

Put a sticker on the object placed where you want the 'centre' to be. Make sure that that sticker is on the axis of rotation of the platform. Remove sticker. Take film.

I suspect, as IRstuff hints, that in practice what you mean by 'centre' is not a definable thing.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Centering object on spinning platform

Do you really need the whatever "centered", or do you need its surface to be a constant distance from the focal plain of the camera?

RE: Centering object on spinning platform

(OP)
I would defined 'centered' as the geometric center of the object being placed on the center of the spinning plate.

Since the objects we're taking photos of aren't always square or cylindrical, we can't really keep the lense a constant distance from the object.  Picture something like a sculpture of a horse with a base and trying to take photos from every angle.  

Bottom line is we just need these series of photo's to look fluid and pleasing for people. Whatever achieves that result is what we're looking for.  Thanks for all your suggestions thus far.

RE: Centering object on spinning platform

Can the plate be modified so that it tapers down to the center? Picture a very shallow cone turned upside down. Any object placed on it will gravitate towards the center as the plate spins.

RE: Centering object on spinning platform

(OP)
Scastillo - I don't think that would work since the center of gravity on the object isn't always the geometric center.  Interesting idea though - thanks for the suggestion.  

RE: Centering object on spinning platform

How about using a curved "scooter" or shepherds' crook to pull the object toward the center little by little as it turns - while you watch through the viewfinder and decide when it looks "centered enough."

RE: Centering object on spinning platform

(OP)
ivymike - That's something similar to what we've tried - it's a very time consuming process attempting to adjust an item as it spins and make it look right.  1mm makes a big difference in this project and it's hard to accurately make such minute adjustments as the product spins.  Thanks for your suggestion

RE: Centering object on spinning platform

You can buy software that finds the geometric center of an object.  Programs like Matlab can do that with some programming.  All that's required is an image of the object in a clear, or bland, background.  Segment, or threshold, the object from its background and run a geometric centroid of the object.

TTFN



RE: Centering object on spinning platform

I still the the desired centre is not necessarily definable.

eg take the letter C. You want the centre of the circle, not its mass centroid, or its geometrical centre.

I'd think about a remote controlled x y table if this is a big deal.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Centering object on spinning platform

What is your definition of geometric center?

I can't find a decent one anywhere.  However, I seems that not all objects have one.

Greg's suggestion of an X-Y table is where I was headed when I asked about constant distance.

RE: Centering object on spinning platform

A geometrical center should be the center of mass of the 2-D projection of the object onto a plane.

TTFN



RE: Centering object on spinning platform

Ok, how about a teacup?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Centering object on spinning platform

Russian, English, or Japanese?

RE: Centering object on spinning platform

Wouldn't matter.  The point is that the teacup would be photographed, rendering it into a 2-D projection onto the camera's focal plane.  Each unique orientation of the teacup's image would have a different geometrical center.

TTFN



RE: Centering object on spinning platform

And yet I'm sure the OP would want the rim of the cup to be concentric as it rotates. So no mathematical procedure will find this desired centre, in general.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Centering object on spinning platform

Desires are, in general, not mathematical.

TTFN



RE: Centering object on spinning platform

here's a procedure that might speed things up:

* Use a small aperture so that depth of field is large
* put a large thin "x" on the table
* put vertical marks every 90deg on the edges of a "coaster"
* put the coaster on the x so that the marks align
* put your object on the coaster
* turn the table and take one photo each time the x has an arm pointing at the camera (4 photos)
* Superimpose pairs of photos from 180deg orientations so that they appear to show rotation about the "correct" axis
* measure the distance between the marks on the coaster
* move the coaster 1/2 that distance relative to the marks on the table
* re-check

RE: Centering object on spinning platform

..how to know which way to move the coaster?

If you did your vertical marks in different colors for 180deg orientations (ie red opposite green), then you should be able to figure out the right way to move the object based on whether the red vertical mark was on the left or the right of the green vertical mark when you'd aligned the photos properly.

RE: Centering object on spinning platform

segment3,
Instead of rotating the subject, revolve the camera around the subject and use an auto focus feature to manage the depth issue.

Griffy

RE: Centering object on spinning platform

I thought about that one too, but you have the same fundamental problem (selecting an appropriate axis of rotation).

RE: Centering object on spinning platform

Mount camera on x-y table.  Make a plate that mounts onto the existing plate with legs so that the camera is between the two plates.  Mark o center point and graduation lines on the second plate.  Take a picture and take a look at it and adjust the x-y table based on how far off the mark is in the photo.

RE: Centering object on spinning platform

I think you'd still end up with an object that turns about the wrong axis... unless I'm missing something here.

RE: Centering object on spinning platform

You would need to put the object on the x-y table, which is put on the turntable.

TTFN



RE: Centering object on spinning platform

Second plate is rigidly mounted (but removable) above the rotary table surface.  Its center mark is aligned with the axis of rotation of the rotary table.  The camera is mounted in-between on an xy table which is mounted to the rotary table.

RE: Centering object on spinning platform

If the camera is ultimately mounted on the rotary table, then there is no relative rotation between the camera and the object.

TTFN



RE: Centering object on spinning platform

Camera is mounted on xy table. Second plate is not.

RE: Centering object on spinning platform

Interesting problem.  I think your solution lies in a plane that is perpendicular to your photographic plane.  Basically, what I'd suggest is to find the smallest cylinder that can circumscribe the object; the center of that cylinder should be your center of rotation of the subject piece.  You then align the center of the cylinder with the center of the rotary table.  For situations where you want the rotational center to be the dominant feature, such as the rim of the teacup, then you'd use the body of the cup (excluding the handle) as the object to be circumscribed.

Try this; Mount a camera vertically above the table, centered on the rotational axis of the table.  From a scientific supply company, you can get assorted filters that provide a geometric pattern such as a series of concentric rings, for mounting on the camera.  Adjust the X&Y location of the workpiece until it is entirely within the same / smallest ring on the grid.
If you are in a high-volume situation, I'd suggest using a vision-inspection system for the overhead camera; the software can generate the smallest circumscribed cylinder and give X/Y coordinates of the cylinder center relative to the center of the rotary table.  You should even be able to tie in an encoded bed to automatically center it based on the feedback from the vision system.

It would be appreciated if you let the group know how you make out on this.  Tks.  Jim

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

RE: Centering object on spinning platform

Are you photographing so many differently shaped components that it's worth the hassle and expense of coming up with some automated solution rather than the photographer simply adjusting the position of the component until it looks good?

For an automated solution, an idea I had was instead of moving the component around on the table, can you put the camera on a slide and move the camera closer or further from the component using some kind of range finder?  As the component rotates, the range finder tells the camera to move closer to or further from the piece to maintain a set distance.  Or have an 'electric eye' that looks across the table perpendicular to the camera.  If the object blocks the beam, the camera and eye move out together until the beam is no long blocked by the object.  The camera could take a picture and it would always be the same distance from the side of the object nearest the camera.

Or what about using image editing software to crop the photos to make it look like the photo was taken from the same distance from center?  I assume some kind of 'auto-crop' feature could be used to make the object fill the same amount of space in all photos.

Bob


Bob

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