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Stick for referential measurement?

Hello all and thank you for reading my post.

We are using the BlueROV2 (the octarotor one) to make underwater inspections of submerged infrastructure and we want to measure through the video diameters or lengths. I know some people here that has added laser pointers set up in parallel so they can use the beams as a measurement, but we are not there yet, so we thought isntalling a removable rod with perpendicular rulers at the point. Something like this:

Knowing the rod length and the scaled rulers we can easily get referential measurements that can be judged in the post analysis.

Did anyone try something similar? is it a god/bad idea? and where in the ROUV can we install something like this?

Thank you for your time

Hi @Javi,

The main issues I can think of with something like this are:

  • the rod rotating or getting bent (attachment and rod material and form are important for these)
  • getting the rod/rulers caught on the objects being measured
  • getting the tether caught on the rod/rulers
  • fitting the ROV in tight spaces (e.g. if trying to inspect close together dock pilings)

Assuming you can work around the issues, it’s likely ok (it at least can work in theory)

There’s not really a great place to install a tube. You’ll have to make some kind of mount for it to attach to the bottom or top panel, or attach it with multiple bolts/screws through the tube if your tubes can handle having holes and a reduced cross-section (noting the added stress concentration and likelihood of bends/breakage at these points).

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Thank you for your response @EliotBR , we will take another round of thinking. I still want to add DIY lasers tho, but it seems like an arduous task.

Measure sticks are often used in oil and gas industry on workclass ROVs. Made of plastic or metal. Often made onboard by the crew. So we make it out of what we find. And normally hold it in the manipulators.
Of course the get bent and so on but like all equipment you put on the vehicle it will be up to the pilot to do a good job with it.
Always a challenge :stuck_out_tongue_winking_eye:

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The approach you take depends on your budget (time and money), what it is you’re measuring, and the required precision.

A few possible approaches:

  • Fixed lasers and a camera feed can be effective for measuring planar (flat) surfaces, but the calibration complexity is tied significantly to how well you can mechanically align the laser beams and camera, and non-planar surfaces gets complicated really quickly. Wider lasers gives more pixels in between, so higher measurement resolution, but then can’t measure smaller objects and can be more difficult to get suitable alignment between the lasers (harder to make a stiff single-body mount)
  • There’s also the time of flight approach, using an imaging sonar or lidar, but that’s then quite a bit harder/more complicated to align correctly with the video feed if you’re trying to measure visible cracks or discolouration.
  • If you’re measuring things that are less wide than the ROV and want to trade visual/software complexity for mechanical/electronic complexity you could set up a waterproof box with two parallel lasers mounted on a rail and actuated to move symmetrically. That allows you to use some form of physical distance sensor/estimation (magnetic/resistive linear, or just using a stepper motor for control and counting the steps) for the distance between the lasers, and then just use the camera to get the distance equal to what you’re measuring. Note that having moving lasers means it’s extra hard to keep them aligned, but it could be sufficient for the precision you need at the measuring distances you’re expecting to use.
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