Anybody come across an ultrasonic sensor that has a very narrow beam width? I've tried a few different ones that haven't been thin enough for my application. I'm trying to measure the distance from a large object through a tunnel that is 30 feet long and 10 feet in diameter. All the other sensors worked great if the distance was less than 10 feet, but after that they seemed to record misreads off the side of the tunnel rather than the object that was at a farther distance. I've looked into Laser and IR but can't find sensors that are under $100 that have a distance of around 30 feet when used in the real world (not using bright white reflective targets).
You are fighting physics. Any wave launched will disperse, even if you can focus it. Ultrasonic 'lenses' do exist but it is not something you can just knock up.
I understand that the beam pattern is going to grow as it is something that is impossible to stop. I just didn't know if anyone found a relatively cheap sensor that had a very narrow beam width and a long range. Thanks!
You might be able to use the ultrasonic part of the below gizmo.
http://www.engineersedge.com/catalog/product_info.php/cPath/34/products_id/82
Yeah I would recommend trying to find a commercial device which has a serial connection or similar which you can take data from.
Probably not a lot in it price wise and it works right out the box.
Mowcius
yes I saw that but then thought is :-
distance from a large object through a tunnel that is 30 feet long and 10 feet in diameter.
considered indoors or outdoors.
It was the same when I used to go caving, I never did decide if it was an indoor or outdoor sport?
It was the same when I used to go caving, I never did decide if it was an indoor or outdoor sport?
Does your cave have a door?
No? ;D
Guess it is ok to use it in an indoor shower. 
Guess it is ok to use it in an indoor shower.
No doubt it has specs on max humidity though...
Does your cave have a door?
Actually yes it had one. Valley entrance in the Kingsdale Valley in the Yorkshire dales. You walk through a field and there is an oil drum on a slight mound. You take the lid off and crawl in.
Also there is this one:-
Hi eSietsema,
A while back I posted a DIY Ultrasonic Ranger project you might want to experiment with. Right now I'm getting 15 feet of range without any type of transmit pulse amplification.
For about $20 in parts and an Arduino you can build an ultrasonic ranger that you can experiment with. Try adding a transmit amlifier or step-up transformer to extend your range.
Quick Start: http://majolsurf.net/wordpress/?page_id=829
How It Works PDF: http://majolsurf.net/projects/documents/100830%20Build%20Your%20Own%20Ultrasonic...
Parts:
(1) Maxbotix-UT Ultrasonic Transducer (sparkfun)
(2) OpAmp Modules BOB-09816 (sparkfun)
(1) 64 Logic Cell CPLD (or Amani/Kemani Dev Kit)
(2) 0.1uF Ceramic Capacitors
(1) 4.7k Resistor
(1) Breadboard
You might consider an array of transducers, if you drive them all in phase you will end up with a narrow beam (effectively a phased-array sonar). The wavelength of a 40kHz sound wave is 8.6mm, so placing those MaxSonar transducers as close together as you can (about 16.4mm) gives an array spacing of approximately 2*lambda.
The first zero in the transducer array pattern is at (sin theta = lambda / ((2N +1) * d))) where d is the distance between the array elements and N is the number of elements (linearly). So for d = 2lambda (ie. the closest you can space these transducers):
For N=2, theta = 5.75 degrees, or beamwidth ~11 degrees.
for N=4, theta = 3.2 degrees, beamwidth = ~6 degrees.
for N=8, theta = 1.7, beamwidth = ~3 degrees.
Note that these are array factor improvements, the MaxSonar transducer looks like it has a beamwidth of around 30 degrees, this value is modulated onto the above improvement factor. The take-home result is that doubling the number of array elements results in an approximate halving of the effective beamwidth.
If you were trying to steer the beam electronically, you would get severe grating lobes due to the wide array spacing. If you're only going to drive the whole thing in phase (ie beam on the boresight, you don't need to worry.
Actually yes it had one. Valley entrance in the Kingsdale Valley in the Yorkshire dales. You walk through a field and there is an oil drum on a slight mound. You take the lid off and crawl in.
Also there is this one:-
Yeah I have to admit I have seen a number like that.
Mowcius
You might consider an array of transducers,
Just like to add that this is a very good idea.