Determine how round a circle is

[garethhunt]

Hi,

I just purchased a NI 1742 smart camera. I want to detect flashes, shorts, and out of round, from injection molded parts. We mold plastics caps and jars. I am looking at a 53mm plastic cap, firtsly I want to detect if the cap is warped. If the part is warped, it will jam a machine. I can find the circular edge with Visioion builder, but I am having a hard time finding information on the other tools. Does anyone have any examples about determining circle quality? Is there a way to measure the radius of the part in a 1 degree increment? I must detect the warped cap on the fly, the cap travels through it's own distance in about 20mS. My ejection air valve has a 6mS response, I want to make sure that the inspection does not take much oonger than a mS or 2.

Is there a way to determine how circular a circle is in a % form? If not, what are my options for measuring this?

Thanks

Gareth

[TiTou]

You can use the Find circular edge, it has "roudness" output, the context help doesn't give details about how it is calculated... I assume you could find that in the Vision Concept document though.

And I'm petty sure there are examples in LabVIEW to show how to use it.

edit : I knew : LabVIEW folder\examples\Vision\3. Applications\Battery Clamp Example.llb\Battery Clamp Inspection Example.vi

Hope this help

[garethhunt]

I am using Vision builder, not LabView. I can set a pass fail for Maximum Deviation (Pixels). Is there any way to convert this into a % round? I need to be able to understand the limits that I set. See the attachement. I have 2 find circular edge functions. The first one just finds the circular edge. The second one has the max deviation set to 1.00 pixels. Sure I can make the system fail if I present the camera such that the cap appears as an elipse.

[TiTou]

Oh... ok, what version of Vision Builder do you have? I thought in the most recent version there was a roundness output you could use... I've just checked in 2011 SP1 and there is not.

The context help (ctrl+H) explains what the "deviation" is but it's not roundness. I'm afraid the only way you have to get the roundness is to develop you own custom step... do you have the SDK?

[garethhunt]

What's SDK? I just have the software which came with the NI 1742 camera.

[TiTou]

oh, ok... the sdk is a LabView extension that lets you build your own custom step for Vision Builder, I think I can find a circular edge step with circularity for Vision Builder 3.6, becuse i'm pretty sure one of my coleagues had to do it for a project, that's why I asked which version you have.

[garethhunt]

The version is 2011 SP1 - Build 20120119174623.

I'm new to Vision Builder and vision systems. I'm sure that this system can do what I want. I just need pointing in the right direction.

Thanks!

[nil27]

Hi,

I had done a similar project and I had used to formula 2(a-b)/(a+b), where a and b are major and minor axis.From this we can find out of roundness. So if you get 0, it means that its perfectly round or a perfect circle. 

To get the two axis you can use fix ellipse from IMAQ to get major and minor axis. 

Hope this will be helpful.