Sensors for Mapping

http://www.robot-electronics.co.uk/htm/reducing_sidelobes_of_srf10.htm

--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear

     Sure.  Using Moravec's certainty grids as you move around will produce
an image with more resolution than the beam width.  It's neat to watch
those images appear.  I did that back in the 1980s.

     It's also possible to go to much higher frequencies, like 1MHz,
and get a 2 degree sonar beam width.  Cybermotion used to have a device
like that.  The problem is that with a narrow beam width, you can't
collect data very rapidly; you only get one point per round trip time,
so scanning takes many seconds.  So that's a dead end.

     Hobby robotics sonars are still as dumb as they were twenty years
ago.  One would expect better systems, with multiple sensors and a DSP,
but it's not happening.

     Vision-based systems are starting to work; look up "Simultaneous
Localization and Mapping" (SLAM).   Structured vision, like
triangulation-based rangefinders, is useful at short range.

     True small-scale time of flight systems exist; see
"http://www.swissranger.ch" .  There's no reason that thing
needs to be expensive, although it currently is.

                    John Nagle

I am curious to know if there are any low-cost [I.e. under $500]
phased-array sonar systems that are designed with a lot of on-board
processing (via a DSP).  I mean something very roughly comparable to
the CMU cam, where a lot of functionality is built in.  For sonar, I
can see the possibility of a system with 3 microphones and one emitter
that will send the user a simple 2D grid showing the intensity of the
reflected signal. Such a system may also provide the ability to do very
fast scans of an area.

Even if such a device is not available as a kit, perhaps a university
has documented such a project enough that an average robotics hobbiest
could put it together without know a lot of about digital signal
processors.

Joe Dunfee

There is a company here in Massachusetts called Massa Corp, they build
phased array sonar imaging systems for the US Navy. Sonar is their
specialty. I am pretty sure they have a web site. You may want to
google for them and see if they also sell to the commercial market. I
am sure they would, just as a matter of doing business.

I know you can use an array (matrix, say 8X8 transducers or so) and get
an image. I don't know the exact details how. I figure, if I want an
image, I will just use a photo, or video cam (on land that is, I don't
know what your application is).

hth,
Joe

They didn't show anything about phased arrays on their web page.  They
didn't have a web store, and obviously deal one-on-one with
manufacturers.


I am not sure we are talking about the same thing... the "image" I am
talking about is a 3-d map of obsticals. While this is sometimes done
with stereo video images, by triangulating common points, it is
definitely a challenge to do.

I am working on modifying a Roomba robotic floor vacuum so it can map
out its surroundings.

Joe Dunfee

wrote:

Hello Vaibhav ,

I am working on a sonar project for my small outdoor bot. I am using
40Khz piezo type transducers available from Jameco. I have not done any
measurements on cone angle, at least not quantitative. My system is
still in breadboard stage, but when I was testing it a few days ago, I
found that at a range of 10 meters point to point (ie, I was aiming the
transmitter at the receiver), I could turn the transmitter an estimated
30 degrees or so and the receiver would still respond (I have a beeper
on it to alert me).

Now, there's a few ideas I have that I am going to try. First, I am
going to use 2 transmitters, instead of one, placed on either side of
the receiver but inside a 'waveguide' . This is for rangefinding. I
read one of the articles posted earlier and someone talked of putting
the transmitter in a tube lined with foil. OK, as sort of a waveguide
(or in RF it would be an antenna).

The wavelength of 40Khz ultrasound traveling at, say 340 meters/sec is
about 8.5 millimeters. That is the best length of your waveguide, at
least according to physics (ie, 1 wavelength) . So that was something
else I was going to try. Now, I don't know if it is going to help
things any, but just thought I would mention it, since this thread
caught my attention.

A waveguide is similar to an antenna but is used on the longer
wavelengths (like microwave), but sound is not an electromagnetic wave,
it is a pressure wave. It needs a medium to travel in. I will also be
experimenting with how far the transmitters need to be from the
receiver, and at what angle they should be at (I don't thing pointing
straight ahead is the best angle to use, necessarily, especially for
'stereo' sonar) in order to maximize my rangefinding ability.

Have fun with your project,

Best Regards,
Joe