Need encoder for linescan camera

[TommyBoy]

Hi all,

We have an application were we need to inspect 10m-long steel I-beams.

The I-beams are moving forward with an irregular speed under the camera.

We want to use a linescan camera, but the camera needs a triggering input

to be able to grab an image every 0.1 mm. There is nothing available now

to measure the position accuratly. So we need to install an encoder and

connect it to the linescan camera.

We can't find such an encoder, because it must be contactless.

Need something with the optical mouse principle.

Please advise!

Thx,

Thomas

Message Edited by TommyBoy on 11-21-2005 04:24 PM

[Rethunk]

I looked for just such a sensor about a year ago, and I also came up empty handed. Although for proprietary reasons you may not be able to reveal many details regarding the inspection requirements, knowing a little bit more about the physical properties of the I-beam, its motion, and the means of conveyance would be helpful.

• What is the maximum speed of the I-beam? the maximum rate of change of acceleration?
  
• How is the I-beam conveyed?
  
• Does the I-beam move, rock, or vibrate perpendicular to the direction of motion?
  
• What is the roughness and reflectivity of the I-beam (in qualitative terms)?
  
• How expensive could the triggering mechanism be? (For instance, would it be feasible to add a second linescan camera, lighting, lens, and processor to the project budget?)
  

As it happens, I saw your post on the machinevisiononline.org message board as well. Since you may visit this board more frequently, I decided to post a reply here as well.

[TommyBoy]

Hi!

thanks for your quick answer.

I can't give you very much numbers, but I can say that the application is a big hydrolic machine that

punches holes in the I beams. These move with a irregular speed on the machine. There a 2 motors to

move the beams: at the start the first motor is used to push the beam in the machine, and in the second half

of the application the 2nd motor starts pulling the beam. Max speed is about 5 cm/s. The machine is built to avoid

rocking of the beam. The beam is plain steel. If necessary a second camera shouldn't be a problem.

Thanks again!

Thomas

[AnalogKid2DigitalMan]

Laser doppler velocimetry may suit your needs, you may wan to Google it. Here is an example of such a unit with quadrature outputs:

http://www.betalasermike.com/Literature/USLiterature/ls4000_series_brochure.pdf

I presume mounting an encoder to the drive motor shafts is out of the question (i.e. slippage)?

[sch]

Given your description of the application - do you need to be so fussy.  A spring loaded rubber roller, friction driven by the beam and coupled to a standard encoder would give reasonable accuracy of feed rate. Enough to do any surface inspection with a Linescan camera - it all depends on the relative sizes of features you are trying to detect on the beam. I would go for the simple solution first - grab some images and then see if you can reliably work with them - but perhaps you have already done this?

Charles