Ping sensor at higher frequency ?

Dries

Hi all,

The Ping ultrasonic sensor is measuring at 40kHz. To make an echo sounder based on the basic stamp which can measure the depth in water, like lakes and rivers, I'm looking for a solution where the sensor is IP68 watertight at the front side, and the preferred measuring frequency is around 650 kHz. Is there somebody with an idea or suggestion how to realize this ? Has also to work with applications where the bottomlayer is a little bit "fluffy" caused by floating sludge.

Thanks.

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Dries Nelemans

Chris Savage

Hello,
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·· You’re going to need to use the type of transducer used in a ‘fish-finder’ and probably create a custom controller based on an SX chip to send and receive pulses at that frequency.

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Chris Savage
Parallax Tech Support

Tracy Allen

Hi Dries,

Me again. Most of the ultrasonic depth sensors I know of bounce the ultrasound off the top surface of the water, from ultrasound passing through an air column above the water. Chris Nafis posted a nice project using the Ping for that purpose, and also for snow, which can have a "fluffy" surface. www.howmuchsnow.com/ping/. You can also find info on that project with a search of the forum here.

Why 650khz? The attenuation per unit distance is quite high at that frequency. The wavelength at 40khz is laround 0.7cm, and that sort of defines the limit of resolution. Although a phase sensitive system could do better, given a non-fluffy signal.

Are you thinking of having a sensor immersed in water (Your comment about IP68) at the bottom, with reflections back from the top surface? . There might be something you could salvage from an ultrasonic cleaner tank or an ultrasonic vaporizer. Those are impedance matched to transmit in water.

Unidata makes a "starflow" system that sits on the bottom of a water channel and uses ultrasonic doppler to measure the flow rate. It also measures depth, but I think that is hydrostatic.

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Tracy Allen
www.emesystems.com

Dries

The sensor has to be immersed around 5 cm at the water surface, facing down and measure the distance to the bottom or sludge layer. Two reasons for the high frequency. (1) The higher the frequency, the lower the dead band of the sensor and (2) with high frequency it's easier to detect the sludge layer on the bottom, low frequencies (with higher power) blow through the layer. Like bass tones from your neighbour goes through the wall, higher tones are more reflected (or absorbed).

/Dries

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Dries Nelemans

Tracy Allen

So is the ultrasound is going to measure the thickness of the sludge layer then? A nasty problem! I know the higher frequencies are used for medical imaging sensors for the reason of short wavelength and diffraction in tissues. There is an Australian company, "Hexamite" that makes ultrasonic locating devices, and I recall they do have some underwater transducers, but maybe not that high in frequency.

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Tracy Allen
www.emesystems.com

Dries

I remember the datasheets of hexamite and will check the possibilities. However this transducers cannot be connected directly with the basic stamp like the Ping and I don't know how to make the electronics to drive this equipment. (Frequency signal, burst voltage etc ?) Finaly I want to measure the sludgelevel in clarifiers and thickeners. Normaly we do this with portable turbidity meters, well you know about it...

/Dries

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Dries Nelemans

Tracy Allen

Well, looking at the Hexamite web page, they now have a 200khz underwater sensor www.hexamite.com/hx1200tr.htm, IP68 of course. They also list a 495khz underwater transducer (HX1495), but I couldn't find much promotion that. There is a short application note on transmission properties in sea water. For Stamp interface, they also have interfaces (IP68) that produce an RS232 ranging output, but I don't know if that would present the information you need. Their stuff is not cheap, but it is fascinating what they claim to do.

One alternative to echo depth sounding might be to monitor echo phase instead of echo time. If you have a pair of transducers, excite one with a sine wave, and hook the receiver to an oscilloscope, synced to the transmitter. The phase seen on screen will be related to the depth to the bottom of the channel (a constant?). That would be modulated by the depth and density of the sludge. The circuitry required would be a synchronous detector feeding an analog-to-digital converter. Each zero of phase represents 1/2 wavelength of the operating frequency. Is is not an absolute meaurement like echo, but more like an incremental encoder wheel.

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Tracy Allen
www.emesystems.com

stamptrol

Dries,
Depending on how much time and money you want to put into the sensor design/construction/testing, you might consider a ready-to-go marine depth sounder with dual-frequency transducer. They are particularly set up to see both the 'fluffy' layer and the 'solid' layer underneath.
Many of the Lowrance brand (and others) have serial data output which the Stamp's handle very easily.

I've used this setup to measure the sludge layer in municipal sewage lagoons.

Cheers

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Tom Sisk

http://www.siskconsult.com
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Dries

Hi Tracy,
Didn't found time to check the hexamite information but I will do soon. I prefer a "complete system" that if possible directly can be connected to the basic stamp. Maybe this is an utopia....Measuring phase shift instead of time, maybe a good idea when echo fails but I'm afraid that developing this will take a lot of time
/Dries

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Dries Nelemans, Holland

Dries

Hi Tom,
Can you tell me more about the municipal sewage lagoon application ?
Have you measured the sludgelayer in water, with the sensor immersed in the "clear water" zone.
BR, Dries

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Dries Nelemans, Holland