Speed Sensor Anyone???

[ScaleRobotics]

Here's a little bit about the equations: http://www.engineeringtoolbox.com/pi...bes-d_612.html

You could use the same sensor you used for your depth sensor. And use your depth sensor as the 2nd differential sensor. Connect one to the pitot tube, subtract the readings from the depth sensor, and use the equation. You would need to have the sensors at the same depth, since you can measure a half inch difference. And your depth sensor should probably sense pressure at the side of the hull, I would think.

[Ramius]

Hi guys! Thank you! It all helps and it is wonderful to work with such great minds! Over the years there have been several "challenges". First was how to transmit video from underwater! Most people have use 2.4 Ghz which just happens to be the resonant frequency of water! So when their antenna becomes submerged all they end up doing is "heating" the water. Then there is focusing the transmit power up to the surface and not down to the fish. With a unique antenna design this problem became solved. One picture below shows the transmitter (gold looking box) and the chain is a chain drive to raise and lower the items to make things look real. The periscope goes up, down, and turns seperately. The camera used is called a "snake" camera, high resolution, color, and 1/2" square pointed up the periscope tube. There is drawing of the lenses in the periscope tube and the prism corrects the "mirror" image since on one mirror is being used. There is also a picture of the compass and you may notice the A/D is mounted directly below the compass sensor thus reducing any signal noise. The torpedo pictures give you an idea of their construction and size. The launch tube has an optical sensor so the launch air pressure is shut off once the torpedo leaves the tube. Another picture shows the launch method where the torpedo is pressurized and the top of the launch tube allows air pressure to release the torpedo. Just in case you are wondering, yes, you can have exploding war heads, or you can replace the nose with foam from a foam paint brush soaking in water color. So if you see ducks with small orange circles on their butts you will know it was me! Lol Best, Ed

PERISCOPE LENS.pdf

[boroko]

Hi All,

This may sound a bit ignorant, but it seems to me that the speed would be directly proportional to the shaft speed. There would definitely be some slip, but the "load" of the craft should be consistant and once you figured the correlation of revs to distance, you wouldn't need to directly measure speed.

Maybe I'm missing something, but it sure seems like the logic is there.

Good Luck
bo

[Ramius]

Hi Bo!
Thank you and there is no such thing as "ignorant" as we all see situations differently which makes for the stimulation of great ideas! Counting the revs is a great idea and this sub has two motors. I guess you could add the two motor RPM's together and then divide them by 2 for average RPM. In a turn you speed up one motor and slow down the other. While I do not have an answer as to why this is not used in aircraft (for example) it seems there must be a reason. Aircraft seem to use a pitot tube while boats seem to use either pressure or a "paddle wheel" system. Maybe it has more to do with "flow" such as a "head wind" in aircraft? Best, Ed

[ardhuru]

While I do not have an answer as to why this is not used in aircraft (for example) it seems there must be a reason.

Wouldnt you get a wrong reading after factoring in the tail/head wind? The RPM measuring technique would work only in the absence of the above. For the same RPM, your actual speed would be different depending on whether the wind is aiding or resisting you.

[SteveB]

Some clarifications regarding speed and airplanes (simplified). Aircraft measure speed in many different ways (mostly in knots, which is annotated by the K prefix).

KTAS = True Airspeed - This is the aircraft relative to the air mass.

KGS = Groundspeed - This is TAS corrected for the movement of the air mass (headwind/tailwind).

KIAS = Indicated Airspeed - Speed measured by the Pitot-Static System.

KCAS = Calibrated Airspeed - IAS corrected for installation/instrument/position errors.

If the air mass is stationary, GS = TAS = how fast you see the airplane fly by as you stand on the ground.
CAS ~ IAS. These numbers are normally pretty close (depending on aircraft), and often times used interchangeably in non-critical situations.
Typically, CAS, TAS, and GS need some form of 'advanced' instrumentation to read the cockpit, such as GPS for GS, or an Air Data Computer + GPS + Flight Management System for TAS and CAS.

CAS/IAS differ from TAS, often by significant amounts, due to variations in the air mass, such as decrease in air density with increase in altitude. However, it is because IAS is affected by density that it gives a better picture of the aerodynamics of the aircraft and speed/mass/volume of the air flowing over the wing. Whether the aircraft is at 1000' or 30,000' in altitude, at 175 KIAS, air flowing over the wing is aerodynamically equivalent, however the TAS at 30,000' will be much greater.

So, why not measure the rotation speed of the prop to get the speed of the aircraft? Because, the aerodynamic forces on the aircraft and propeller change with the speed, altitude, weight, etc. So a fixed pitch propeller spinning at 2000 rpm at sea level on a standard day, no wind, with the aircraft total gross weight (TGW) of 3000lbs and a new paint job may fly at 150 KIAS (and ~150 TAS = 150 GS). But, at 5000' on a hot summer day with TGW of 4000lbs and having not been washed in months may only fly at 130 KIAS. But, TAS would be more like 160 knots, and there is a 25 knot tailwind, so the GS would be 185 KTAS!


And that is the simple answer


I should add that with a ship/submarine, you will encounter the same issue with counting prop revs, although to a lesser degree due to smaller variations in density and speed, but weight would make a much bigger difference because of the much larger range in weight capacities of a ship.

[Ramius]

Thanks Steve!
I have been flying airplanes since 1973 and helicopters since 1994 (the real ones not the models) so I agree with you completely. Typically in most lakes or even a swimming pool the water is pretty much stationary. I will probably take an extra depth sensor and spin it in a plastic trash can filled with water at different speeds and see what "head-on" pressure readings I get. Hopefully there will be major changes in the readings? My fear is that the pressure at slow speeds will be to small to see. There are just so many details to work out on this sub project. For example, no one seems to know if First Person View goggles display video in underscan or overscan. This makes a different with trying to use an On-Screen Display character generator as with overscan as with a CRT TV you do not have the first column available and thus you are limited as to how many columns are useable! So many questions, so little time! Lol Best, Ed