Refractive index is taken at 90 degrees and changes with change in angle of incidence.
In other words if you use a laser set at an angle of 45 degrees there will be reflection off the surface of most any material. It is just a matter of setting an optical sensor in the correct place.
n=c/v
Where,
n is the refractive index
c is the velocity of light in a vacuum ( 3 × 108 m/s)
v is the velocity of light in a substance
The cheap device mentioned in the link below works as an underwater distance sensor, 50mm to 2000mm, so meets your 6 cm criteria, barely.
It is intended to measure tank water level through the tank bottom wall, but @wvmarle tested it by holding it in water, pointing it down, and it worked. He even wrote a library for it.
For your application, it may not provide measurements quickly enough. The link has the sample rate.
Thanks but thats a bit too expensive for me. we have a budget of £150 for everything and using £40 on 2 sensors doesnt give us enough freedome for other components.
I cant tell if this is a joke lol. The largest things I've ever seen levitated with standing waves are polystyrene balls and the vehicle will be a touch larger than that. Is it possible with larger objects?
With large flat areas the separation would modulate the acoustic spectrum from ambient noise due to standing waves. In theory that just needs a hydrophone, amplifier, ADC and FFT, but I'm sure there are many complications.
Could you explain this a bit more? I had to look up what FFT meant and if it means fast fourier transform we havent covered anything to do with fourier transforms yet but I'd be up for learning more about it.
No need to measure.
Draw water in from the front, push some out the back for propulsion and the remainder out from jets in the sides. It'll behave like a hover craft whenever it gets close to the sides.
Thats a good idea that I didn't think of but part of the design brief is that it has to have a biomimetic propulsions system. We are using a very similar design to Festo's BionicFinWave so this isn't really possible I don't think.
What other essential information is missing from your stated project requirements?
Wikipedia will tell you that fish avoid bumping into things by sensing pressure along their lateral line.
Creating an artificial version on the cheap is going to be a challenge. Start with a literature search to see what sensitivity is required. If you cannot find any numbers then do a lab experiment using a sensor embedded in the channel wall.
You could still use sound ... well, not ultrasonics, but audible sound and a chirp Chirp - Wikipedia - and only one trunsductor/speaker but an array of receivers. Most likely you'd need something more beefy than an uno to do the DSP calculation. "Give me a ping Vasili, one ping only please."
Could you explain how this would work? Just had a look through the wikipedia article and i cant see anything about distance measurement but I might have missed it.