Inexpensive Depth Sensing?

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sircastor
 
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Inexpensive Depth Sensing?

Post by sircastor »

Does anyone know a good method or an inexpensive component for measuring depth in water? My mom does a robotics competition every year with her students - It's a submersible tethered robot. It has to do a number of things, but one thing that has held them back is depth sensing. They need the robot to semi-accurately report how deep it is. This is in a swimming pool, I think the lowest it goes is probably 10-15 feet.
Any ideas or suggestions would be welcome.

123Splat
 
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Re: Inexpensive Depth Sensing?

Post by 123Splat »

Think 'Fish finder", as in Magellan, or Lowrance.

Captain, SONAR, we have a contact, bering ....

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adafruit_support_bill
 
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Re: Inexpensive Depth Sensing?

Post by adafruit_support_bill »

You will want a pressure sensor capable of at least 20 psi. In small quantities you should be able to find one for about $15.

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zener
 
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Re: Inexpensive Depth Sensing?

Post by zener »

Shouldn't 10 PSI be enough? 10 PSI would give around 23ft range I think.

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adafruit_support_bill
 
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Re: Inexpensive Depth Sensing?

Post by adafruit_support_bill »

10 PSI would give around 23ft range I think.
That would be relative to normal atmospheric pressure at the surface. (~14.7). The absolute pressure would be the sum of the two.

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zener
 
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Re: Inexpensive Depth Sensing?

Post by zener »

So I think you are saying:

1) Use an absolute sensor

2) An absolute sensor rated for 20 PSI can only take an additional 5.3 PSI of pressure

I was thinking use a guage type and just plug one side. Either way you will always have some cal issue due to barometric pressure changes. I believe 1 inch (Hg) of barometer change equals about 14" of level change. So you could be off by a couple of feet max which might not be an issue. Only solution to that is either have another sensor measuring atmospheric pressure and use that as a reference, or use a differential type sensor that has a breather tube which is kept above water. That is common in industry but probably not practical in this application. I suppose you could also just have a setting for whatever the barometer is that day. And there is altitude of course...

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lyndon
 
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Re: Inexpensive Depth Sensing?

Post by lyndon »

Why don't you just zero the reading whenever the robot is out of the water or on the surface?

spacecowboy
 
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Re: Inexpensive Depth Sensing?

Post by spacecowboy »

Try the Freescale MPX Series of differential pressure sensors. I've played around with them before and have seen them used in student-built ROVs. DiYDrones uses them for Airspeed sensors and has a handy breakout board for some of the differential versions.

Have the reference side of the sensor referenced to a sealed container that's pressurized at 1 atm or the surface pressure. You can use this calculator here to play around with the depths that your sensor would need to function at. Then you can calibrate ADC values to a look up table--or maybe just use Excel and some readings to come up with an equation for depth.

The datasheets give a sensitivity of 0.2 mV/kPA, so you will want around 14-16 bits. There are a lot of nice ADCs that come with an appropriate sampling rate.
If your current ADC doesn't have enough bits (MPX Series sensors tend to have sensitivities around 0.2 mV/kPA) then you could try oversampling to boost the effective number of bits. However you need 2^(2*N) samples to get N additional bits of effective resolution so I'm not sure how cost effective oversampling a faster ADC would be vs just buying a 16 bit ADC.

It's probably going to be a good idea to just build an analog front end, add some offset and amplification, just so the output of the sensor in the range you are interested in matches the range of your ADC

123Splat
 
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Re: Inexpensive Depth Sensing?

Post by 123Splat »

Since nobody seems interrested in digital echo location/sensing, how about a digital or analog media isolated preasure sensor ( http://www.meas-spec.com/product/t_product.aspx?id=2880 )? About $10 USD. 0.5 to 4.5 Vout

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zener
 
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Re: Inexpensive Depth Sensing?

Post by zener »

That looks really great! But where can I get it for 10.00?

spacecowboy
 
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Re: Inexpensive Depth Sensing?

Post by spacecowboy »

The Freescale MPX2200 DP will give you the pressure span you need to go to about 30 ft--as long as you reference it to Atmospheric Pressure. If you want absolute pressure, Freescale has the 41-MPX2202ASX or similar. All the pressure sensors in that range are going to be about $10-11 USD apiece although you might get Free Swag if you ask Freescale nicely. I do believe Mouser also carriers a board for mounting the surface mount versions of the sensors, also at about 10 dollars.

Mouser Part #: 841-MPX2200DP
Manufacturer Part #:
MPX2200DP

I also recall that the MPX sensors isolate the pressure from the gauge side, so you're probably not going to have flooding issues with one. Just run a tube to the inside of your ROV and a tube to the outside if you're using a differential type. With a screw mount type sensor like the MPX2202ASX you could simply drill a small hole through your hull for the pressure port, and then use the screw-mounts & appropriate nuts/bolts/lock washers to sandwich a watertight gasket between the sensor and your hull like this.

Image

Only downside of this scheme is you need a 10V supply and you still only get a 40 mV full-scale span (meaning the voltage will only swing 40 mV as the pressure increases from 0 to 200 kPA

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riaz
 
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Re: Inexpensive Depth Sensing?

Post by riaz »

i am trying to build a less expensive pressure sensor for fluid flow in pipes. Where can i find a pcb or arduino board for MPX2200DP?

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adafruit_support_bill
 
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Re: Inexpensive Depth Sensing?

Post by adafruit_support_bill »

I don't believe there are any off-the -shelf boards for that. But the MPX series have analog outputs are are fairly simple to interface. You can either wire it directly or solder it to a proto-shield.
https://www.adafruit.com/products/2077

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riaz
 
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Re: Inexpensive Depth Sensing?

Post by riaz »

so you mean follow this: http://cache.freescale.com/files/sensor ... AN1324.pdf to design a board using the proto shield?

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adafruit_support_bill
 
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Re: Inexpensive Depth Sensing?

Post by adafruit_support_bill »

They have sensors with 0-5v outputs also. You can connect them directly to an analog pin with no additional circuitry.
http://cache.freescale.com/files/sensor ... PX5100.pdf

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