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DPRG: Sonar users: share your ideas!

Subject: DPRG: Sonar users: share your ideas!
From: David Philip Anderson dpa at io.isem.smu.edu
Date: Mon Mar 8 18:33:12 CST 1999

Howdy DPRG.

>So, here's the deal: Have any of you sonar users noticed apparent loss of
>sensitivity or accuracy in scanning hard surfaces like kitchen cabinets as
>opposed to softer surfaces like our styrofoam walls?

Looks like Ed, Dan, Daniel, and Kip have pretty much covered what I might
have suggested in the way of possible sonar signal ideas.  My general
experience has been that hard surfaces present a _bigger_ challenge than
soft because of the increased number of specular reflections that can produce
multi-path errors, like trying to take IR readings in a room full of

I tend to agree with Dan that irregular kitchen cabinets are more likely
to return good signals directly to the sonar, while the smooth DPRG
contest walls may give back stronger multi-path signals than direct signals.
So the sonar bounces off the wall at a steep angle, then off the far end of
the course, and finally back to the sonar, making it look like the wall
is a long way off.

With SR04, this gets worse as the robot gets closer to the wall, and the
angle of return is more and more oblique.   For this reason I've found
that navigating on sonar alone is pretty tough.  With sonar and IR
running, the IR "subsumes" the sonar as the robot approaches the wall and
pushes it away, effectively (hopefully) canceling out the sonar errors.

The arrangement currently running in SR04 orders the priority like this:

	1. Bumper:	collision recovery	(closest)
	2. IR:		collision avoidance	(near)
	3. Sonar:	Path finding		(far)

Sonar gets to choose the path of the robot only if the IR sees that
the near-path is clear.  Objects that the IR misses (toes) must be dealt 
with by the bumper.  Because IR can see a wide field of view all at once,
it seems more robust at manuvering the robot in close quarters.  If it
still runs the robot into something, then the Sensor-of-Last-Resort (bumper)
subsumes them both.  

- --------------------------------------------------------------------------

SR04 Sonar and IR coverage:

20 feet  ------------------------------------- limit of sonar

    \                   |                   /
     \                  |                  /
      \                 |                 /
       \    Left Sonar  |  Right Sonar   /
        \   ~15 degrees |  ~15 degrees  /
         \              |              /
30 inches-\-------------|-------------/------- limit of IR
           \        . . | . .        /
            \   . "     |     " .   /
             \.         |         ./
   Left IR  . \         |         / .   Right IR
   ~90 deg .   \        |        /   .  ~90 deg
          .     \       |       /     .
          .      \      |      /      .
           .      \     |     /      .
              .    \    |    /    .
                 .  \   |   /  .
                 ====\==|==/====    <--Front Bumper
                 ||           ||
                 ||   Robot   ||
                 ||           ||
                  \           /

		  (not to scale)

- --------------------------------------------------------------------------



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