Long Range Narrow Beam Ultrasonic

Back in 2010 in this thread http://arduino.cc/forum/index.php/topic,19878.0.html there was some discussion of Long Range Narrow Beam ultrasonic measurement. I have an interest in this with respect to caving and was wondering if any of the original posters got anywhere with it.

Also in reply #11 in that thread Crimony floats the idea of an array of transducers to narrow the beam. I assume that a line of sensors would provide a beam narrowed in one dimension and that it would take a 2D array to get narrowing in two dimentions. Do the same equations apply for an array? Does the array need to be filled in or would a cross pattern of sensors do it?

assume that a line of sensors would provide a beam narrowed in one dimension and that it would take a 2D array to get narrowing in two dimentions

No it will work in two dimensions with a one dimensional array, in the same sort of way as a yagi antenna.

I wish I could talk you out of caving. Rather not say why, just that cool can turn to horror beyond telling and a lifetime of regret. Everybody's safe till they're not.

Grumpy_Mike - Thanks for the reply but I would need some convincing. As I see it the thing is that a radio wave is polarised and so a Yagi only needs to work in the direction of the polarisation but the ultrasonic is not polarised and so needs to be 2 dimentional.

GoForSomke - not really the place to discuss the merits of caving vs other activities but that stats show that caving (at least the sort of UK caving I do) is pretty safe. Driving to the caves is much more dangerous.

sirch:
As I see it the thing is that a radio wave is polarised and so a Yagi only needs to work in the direction of the polarisation but the ultrasonic is not polarised and so needs to be 2 dimentional.

I've no practical experience, but the way you describe it seems correct to me. I assume you would orient the emitter array towards the target so you're just using the phased array for beam forming and not for steering. In that case the emitter would need to output a contracting circular pattern on the output array. If you can't afford to populate the whole area I suspect it would be more effective to populate several concentric circles rather than radial lines, if you see what I mean. This would also be simpler on the control side, since each concentric circle would conceptually correspond to a single logical output from the timing system.

The important thing is that it is a wave, what the wave is composed of does not affect the maths of the Interferance and reinforcement that goes on to make a beam.
However you have not said what you want to do with it. Is it a cave survey sort of thing?

Where in the UK do you cave. I did most of mine in the Youkshire Dales but I have been to the Mendips and to Southern Ireland and the charastics of the caves are very different in all three places.

Just toying with some cave measuring ideas, I mainly cave in the Dales.

Anyway, I had a conversation with a guy who used to design phased arrays to go on satellites and he said to think of it as an aperture. So a linear array can be thought of as a slot where the propagated beam pattern is roughly a cylinder with its axis along the slot. A cross would be the sum of an X-axis cylinder and a Y-axis cylinder and so would have a strong central lobe. When I get my act together I will probably do some experiments.

I think you would be better off using lasers rather than sound. A frend of mine was once hit in a cave by a bat. Apparently they turn off their sonar off in a cave as things never change and they can remember where the walls are.

Grumpy_Mike:
I think you would be better off using lasers rather than sound. A frend of mine was once hit in a cave by a bat. Apparently they turn off their sonar off in a cave as things never change and they can remember where the walls are.

And the bleeding echoes get too confusing. That must have been a real bad day for the poor bat.

Did you know that there are blind people who navigate by clicking their tongue and listening to the echoes? Really, good enough to ride off-road bicycles down hill trails.

Grumpy_Mike:
I think you would be better off using lasers rather than sound. A frend of mine was once hit in a cave by a bat. Apparently they turn off their sonar off in a cave as things never change and they can remember where the walls are.

Yes, I have developed a laser based system (see the CREG Journal a couple of issues ago). The thing with laser systems is they are generally beyond me to integrate into a larger system (can't build a modulated or time-of-flight system and hacking something on to a ready build system is very hard, I've tried).

What I am looking at making at the moment is a profiler - i.e. something that can scan the passage cross section. I am pretty much there with the first part or it which is an ultrasonic/radio system where there is a base station that sends out an ultrasonic ping and the "profiler" that receives it. The idea being that the profiler sends a radio signal to the base station and starts timing. The base station receives the RF signal and sends an ultrasonic ping. The time between the profiler sending the RF and receiving the ping is proportional to distance. This means that the base station can be set up at a known survey point and the profiler moved along the passage and the distance that the profiler is from the survey point can be found.

The second part of the system is to actually scan the profile. My parameters for this are for a range around 0.6m to 5m and initially I was going to use a laser/camera triangulation approach. This has the added advantage that you also get images of the profile for drawing up maps and verification. The drawback is that the accuracy/resolution drops off with distance and with small distances between the laser and camera lens.

I too had ruled out Ultrasonic due to spurious echos etc. but then came across this array technique for narrowing the beam so it seemed worth prototyping it just to see how it went.

There are very few bats in the caves I visit (I've never see any) and certainly not in the scrotty little digs that I want to survey.

The bats I have seen are in the caves arround the Ribbleshead viaduct.

I can't see you being able to measure time of flight for RF.

Any way good luck with it.

sirch:
Grumpy_Mike - Thanks for the reply but I would need some convincing. As I see it the thing is that a radio wave is polarised and so a Yagi only needs to work in the direction of the polarisation but the ultrasonic is not polarised and so needs to be 2 dimentional.

Sirch, I agree with you. The array you are talking about is more like a diffraction grating or a synthetic aperture radar than a yagi array.

PS - there are a links to a couple of papers at Synthetic aperture sonar - Wikipedia.

PPS - a phased array Beamforming - Wikipedia is closer to what you are talking about.

dc42 - thanks for that, those links are really helpful.

Just thought I'd report back.

I've made a small hexagonal array of 7 transducers spaced 2Lambda apart (1 in the middle and 6 around) and it narrows the beam width from 70 degrees to about 30 degrees. Curiously there is a dip on the bore-sight.

Not that a small dip on the bore-sight really matters for my application but my thinking at the moment is that there is some acoustic coupling going on through the board that is causing interference and hence the dip.

My cheap unit has the transducers in open cylinders and has a 15 degree spread.

sirch:
I've made a small hexagonal array of 7 transducers spaced 2Lambda apart (1 in the middle and 6 around) and it narrows the beam width from 70 degrees to about 30 degrees. Curiously there is a dip on the bore-sight.

Did you make sure all the transducers are connected in the same phase?

Are the fronts of all the transducers flush with each other, in a plane perpendicular to the beam?

How far away from the transducers did you measure the beam width? If you measured it close to the transducers, you may need to set the center transducer back slightly, to equalize the distances between the transducers and the center of the beam at the measurement point, in order to avoid a dip in the centre.

It's many years since I studied physics, so I'm not sure that there isn't some mathematical reason why you should expect a dip in the center; however it doesn't sound right to me.

The transducers are soldered flush on to a board and wired in parallel so they are reasonably in a plane and in phase as far as I can tell. When I get chance I'm going to try driving them at a slightly different frequency to reduce the chance of a standing wave since they are about 2 lambda apart.

I calculated that the far-field is about 2 feet away, I did the measurements about a foot away so I might try measuring further away.

OK tried doing the measurement in the far-field and the dip turns out to be just a near field effect. Also the -6dB beam width is 11degrees in the far field which is good.