Homebuilt Rov Forum

Does anyone know any measuring tools that would be effective to measure underwater between two points? I thought about regulating the voltage to motors and recording how long it took to go 1 foot, then seeing how long it took to get from point A to B and do the math, but there are many variables that could throw this off such as current, etc. Being as accurate as possible would be great.

Generally speaking, for longer distances Sonar and transponders are the most commonly used method. Not nearly as accurate, but another method might be to keep track of how much tether is paid out. If you know what the current's speed and direction is, you could also use a "knotmeter" and then get a rough calculation of what the vehicle speed is. Lots of ways to do it, just depends on how accurate you need it to be and how much you've got to spend.

ruler?

≤ 5 cm off true length would be great. Our team has a budget of about $300, but of course we would love cheaper solutions.
We know the object is between 2.25 and 3.75 m.

I'm assuming you're competing in the MATE competition? Is your team budget $300 for this one tool, or for your whole ROV?

Yes, we are competing in the MATE competition. Our budget is $300 for this one tool. We raised a lot of money in a fundraiser (selling candy). We also still have a lot of salvageable parts from last year's ROV like the Arduino, frame pieces, and bilge pumps (though i would like to upgrade to brushless motors).

How 'bout if you put a laser measuring tool (like this one by Bosch: http://www.amazon.com/Bosch-GLR225-Lase ... B003NSBG5Y ) into a watertight case? Since this is for MATE the water clarity should be pretty good. Not sure how it will effect the laser, but it probably will work.

If using a standard lasertool for measuring distances underwater you have to correct it, because the speed of light is not the same in the water as in air or vacuum.
The speed of light is a factor 1.333 slower through water @ 20 degree celcius then through a vacuum.

Actually I do not think you mean a vacuum. a vacuum is something where no matter exists. In common air, matter does exist and it has a factor of 1.0003 slower. Not much but different. You were right about the water though, being a factor of 1.33. I think if we order this Bosch laser measuring tool, we would have to collaborate it to see how much distance, d, 1 second, s, equals.

Lets say 1 second = 5 cm for this example.
d(cm) = s(sec)
5(cm) = 1(sec)
To correct this for use underwater use we would have to use the factor of 1.33 and multiply the seconds.
d(cm) = s((sec) * 1.33)
5(cm) = 1.33(sec)

Therefore, a measurement that took 1 seconds in air would take 1.33 seconds in water.
This is also providing that the tool calculates how long before a beam of laser light hits it back. If the tool uses other arithmetic, this formula won't work. Does anyone see any problem with this formula, and maybe how we could implement it?

I see no reason why it wouldn't work, with the assumption that the tool works as previously mentioned. Would you plan to manually perform the conversion, or have a computer do it for you? The only problem I could see is that you may have a bit of trouble hitting the piece of PVC with the laser.

Light can travel in a vacuum - it is electrical and magnetic energy; it does not have mass.

Good luck in the competition!