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Subsea laser options?


#1

I’m looking for a pair of lasers depth rated at >100m. Any ideas? The only ones I’ve managed to find are well over $1000 each.

In the meantime, I’ve drawn up a design but I don’t want to blow a load of money on CNC and sapphire glass until I’ve exhausted other options.


(Tim Pierce) #2

I’m planning on putting two of them in a 2" blue robotics tube mounted sideways under the main pressure hull. Power out from there to the main hull via a penetrator, etc. At this point I think i’ll have them just come on with the lights since I am running low on channels to use. I sourced the lasers from the OpenROV kit, and I plan on using their LED board in that tube as well just for additional lighting. Total cost should be around $100 or so, and it should work fine to at least 100m or so. The smaller diameter hull should actually be able to take more pressure than the main ROV hull anyways. you could even put more than 2 lasers, maybe make a 4 point grid, etc.


(Roy Petter Dyrdahl Torgersen) #3

Hey, we’ve had some success using a regular red dot laser pointer diodes (like the ones used on the OpenROVs) inside a Blue Robotics 6mm penetrator. The laser diode normally costs a couple of bucks for a bag of 10, and the BR penetrators, well, you know where to find them.
If you have access to a laser cutter you can cut a transparent acrylic disc to cover the laser diode. You will also have to esnure all cavities are filled with epoxy. We use Devcon 2t epoxy, but the stuff sold by BR is also good.

A bit finnicky work, specially getting the laser aligned, but material budget is probably less then 10 USD


(undersearobotics.com) #4

@roy - Any photos of your lasers you can share?


(Roy Petter Dyrdahl Torgersen) #5

Sure, I’ll take a couple of pics when I get into the office tomorrow.


#6

This is my design, it incorporates 4 o’ring seals, sapphire glass window and will accept a standard BR penetrator at the rear. We estimate that it will achieve a depth rating of about 500m using marine grade aluminium, considerably more if we use SS. The overall dimensions are 107 x 25mm but, having just got the laser modules, we can probably shorten the main barrel by about 20mm. The lasers are 5mW 535nm (red) from Roithner and run on 3.3V, they also have columnating lenses so you can get the beam pin sharp.

We will be running two tubes paralleled 150mm apart so that we can get accurate measurements of corrosion, concretion, etc. This explains why the design may seem to be a slight overkill.


(Roy Petter Dyrdahl Torgersen) #7

@paul-unterweiser

@Sarawak_ROV that looks super-pro. I’d love to see it finished.


#8

@Roy, yeah I’d love to see them finished too but we’re pretty tight on the budget right now!

All the same, I like your concept because you have the lasers on the camera tilt. We won’t get that because both our camera options will be behind the dome. Our v3 has the camera and lasers on the gimbal but we’re a long way away from that at the moment.


(undersearobotics.com) #9

@roy - Thanks! What holds the clear acrylic to the penetrator, epoxy?


(Roy Petter Dyrdahl Torgersen) #10

We used Devcon 2t epoxy, maybe also a couple of drops of superglue


(Kaos) #11

Apinex.Com have 100m rated green lasers. Green is better at depth than red. Battery powered and long lasting. There is a recommendation from a rov firm using them. I intend to buy 2 of them in the next few months and build the mounts.
Hope this helps


(Pierson) #12

Hard to see what exactly is going on with that CAD Sarawak, but that lense sealing mechanism looks hard to fab, and harder to keep sealed. Mind detailing it a little further? We build larger assemblies like that with mcmasters “clear gauge glass” for our cameras. Found they tend to prefer fixed sealing, like potting the lense, and face seals with it tend to leak. and We don’t go anywhere near the depths you guys do.


#13

@Kaos, these must be a fairly new product because I looked at this site last year. But many thanks all the same. It’s interesting what you said about green/red at depth, I had reasoned that red would be better because of the absence of ambient red light at depth. I discussed it with a work dive supervisor and he said his divers use both but red stood out better against biomass concretions.

The only downside with the Apinex units is that they would need to be on all the time which wouldn’t be acceptable for diver overwatch operations, neither do the give any indication of the approximate battery life. Nonetheless, these would definitely suit our purposes in the short term.

@Pierson, the laser will sit in the main barrel and the end cap will screw to the front, clamping the sapphire between two O rings (see below). The design is loosely based on some commercial units ($1000+) and we spent quite a lot of time working on the O ring specifications, these call for the CNC tolerances to be +/- 0.1mm (which is why the machining could be expensive).

To be honest, I’m not that struck on O rings either due to the maintenance issues, however all the pressure vessels I’ve ever worked with have used O rings so it was an immediate choice - but I’m always open to other ideas.


(Pierson) #14

This is purely constructive feedback / design discussion, on the sealing of viewports, as I don’t have any knowledge how much design experience you have with this and wouldn’t want to be presumptive please take this as no insult. I have no doubts that this design could work, and is at least theoretically sound. There are some aspects that concern me, from my experience.
You start by mentioning dimensional tolerance, while the dimensional tolerance in seals is important, of equal importance is the surface finish quality. Luckily this is a circular face seal, which with the proper trepanning bit finish quality would normally not be a major concern, however with this part as the first groove is at the bottom of that deep circular boar that may cause some issues, chatter from a long tool, and perhaps clearance. Overcomable issues most likely, but does not look fun to try, and hard to evaluate for yourself I imagine.
The free floating lens sandwiched between two face seals also gives me pause for concern. I guess with this size the likelihood of a leak caused by torsional displacement would be very low, we have typically 2-3" diameter viewports where this can be an issue. However face seals like a lot, a lot, of pressure. Seen face seal covers need torqued down so hard they bend the pressure rated metal covers. Maintaining/creating enough pressure to get consistent seal pressing between two o’rings seems like it would be a concern. I guess physics suggests that this should not be the case via newton’s 3rd, so you could make a case that it is a valid sealing configuration. Just never thought about it/seen it before and it gives me some concerns, might have to put that on the list to test. (Having gotten to this point I considered removing this concern but felt that it was worthy of discussion, even if it makes me look like a dolt.)

This may not be exactly applicable to this design, but we seal our view ports like this currently. We have been experimenting with different sealing compounds, but we find this works well.

So this does mean that the lense is permanently sealed into place, but it provides some advantages to the traditional face seal. One the seal is not reliant on face seals which require pristine conditions on both housing and lens. Let’s say one of the times you are opening the enclosure, or servicing the o-rings, the lens gets dropped and scratched, wouldn’t want that to ruin electronics on the next dive. Next is clamping force, face seals require immense pressure to remain sealed, this pressure can put strain on the lens. This method uses the oring as a supplementary seal and largely to keep the potting compound in place during assembly; the potting compound is doing the sealing. Wherever possible we prefer to use permanent sealing like this, it is much more stable especially with glass/acrylic/polycarb, (in order of increasing need).

Again a lot of this comes from being used to a slightly different scale, but the viewport problem has been kicking in my head for some time and I am taking advantage of this opportunity to get my point of view/solution out and hear/see others.


#15

@Pierson, don’t ever be worried about sharing your knowledge and I’m guessing a lot of people here could benefit from your experience. You have raised a number of issues that are not directly related to my original post but they are no less important to our collective ambitions.

It’s 02:00 here in Malaysia so please let me chew it over, in the morning I can give you a more detailed response.


#16

@Pierson When I was drawing our laser tube design, I used the Trelleborg calculator to specify the rebate dimensions and geometry, these are calculated in relation to the specific O-ring. Obviously every dimension associated with the O-ring rebate is critical and and it goes without saying that the surfaces should be machined to tolerance. The Trelleborg calculator is frustrating but once all the boxes are checked, you can be fairly sure that you have complied with (in our case ISO) specifications. I have shown the results for the radial seals in the image below, however, do note that the O-ring material and hardness parameters have been omitted.

Having satisfied all of the design rules, the dimensions seemed way too big so I had a very informative chat with a former colleague (who designs subsea controls enclosures) and he confirmed that the specification was right on the money. In light of this, I am satisfied that the design is fit for purpose.

The BR penetrator we included at the rear of our tube is not depth rated (it relies on the user’s ability to pot them) and it’s unreasonable to expect that they have been specified for anything greater than 15 Bar provided that the user has followed the instructions and used an appropriate potting compound. Therefore I designed our seals to out-perform that specification and we can expect a burst pressure way and above the capabilities of the other components of the ROV.

Nonetheless, we will not deploy the design until it has been pressure tested, there is a benchmark facility nearby which can provide us with the certificates we will require to operate the tubes in our target application, that will be a very interesting day!