Subject:
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Re: lasers and RCX
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 5 Jun 2002 06:23:44 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail.net%ihatespam%>
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Reply-To:
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sjbaker1@airmail[Spamless].net
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Viewed:
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703 times
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Dan Novy wrote:
> Years ago, there was a thread about hooking up lasers to the RCX and
> using then for signaling and proximity sensing. Aside from the dire
> warnings (Don't blind the cat!), does anybody remember this thread?
> I've recently come into ownership of a few hundred cheap 1.5 volt lasers
> and was wondering if anybody has any cool projects with the RCX and low
> power lasers. Links would be lovely.
We did talk about it for quite a while on this list - but I'm not sure
that anything much came of it.
I tried putting a laser and a light sensor right next to one-another
high up in one corner of the room on an azimuth/elevation mount controlled by
an RCX - the mobile robot (with a second RCX) was going to have some
kind of retro-reflector on top.
The stationary RCX would scan the floor of the room in a zig-zag pattern:
---->---->----
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----<----<----
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---->---->----
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----<----<----
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---->---->----
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----<----<----
...when it's light sensor sees the reflection of the laser in the robot's
retro-reflector, the stationary RCX sends the position that it knows the
laser is pointing at via an IR signal to the RCX in the robot. Hence, every
time the laser passes by, the robot knows *exactly* where it is.
Between scans and position updates, the robot can use dead-reckoning with
wheel sensors or whatever. If it doesn't get an update within a reasonable
amount of time (eg if it drives under a table) it can back-track until it
re-acquires the laser.
You could get fancy - so that once the laser has 'hit' the robot, it could
scan outwards in a tight spiral around it's last known position. That should
allow you to track the robot quite accurately - even with relatively slow
laser movement.
I think you could get the robot position accurate to a few inches - updated
every couple of seconds with a technique like this...standard dead reckoning
will keep you in the right place.
That's the theory.
In practice, the only retro-reflectors that looked remotely like they'd
work are:
* Three mirrors at right angles. It was too hard to mount the mirrors
at *exactly* right angles - so the reflected laser beam didn't bounce
back into the light sensor reliably enough.
* A bicycle safety reflector disk. This actually worked quite well - but
caused zillions of stray laser reflections at all sorts of angles - which
confuses the algorithm - and is incredibly dangerous for anyone standing
in the room.
Other objects in the room would occasionally generate false reflections - but
that's very rare.
Given the problem with retro-reflectors, I thought to put the light sensor
on the robot. When the laser hits the top of the robot, it would light
up the sensor. The RCX on the robot could then send an IR message to the
stationary RCX to say "OK - I've seen the light - where am I now?"...or
perhaps the stationary RCX could continually broadcast the location of
the laser by IR whilst scanning the floor. That would allow you to run
multiple robots - each one ignoring the IR messages until it sees the
laser light - at which point it knows exactly where it is.
This would be like having your own personal room-sized GPS system!
But Remember: LASER POINTERS CAN HARM YOUR EYESIGHT!!
WHEN YOUR EYESIGHT IS DAMAGED, YOU FEEL NO PAIN AND
THE EFFECT IS DELAYED - SO YOU DON'T EVEN KNOW IT
HAPPENED UNTIL IT'S TOO LATE.
With that in mind, I realised that even if I got this to work, I wouldn't
be able to actually *use* it for anything - so I abandoned this experiment.
Even a cheap laser pointer has a tight enough beam and enough brightness
to generate a 100% reading on the Lego light sensor even when it's shone
across a 15 foot room and back. Hence, we really need a far dimmer laser
than a laser pointer so it would be safe and yet still bright enough to be
detectable.
It's quite hard to dim down a laser without making the beam scatter outwards -
which pretty much destroys the accuracy of the tracking and brings back the
eyesight issues.
Someone with more patience might have better luck...but *please* be careful
with those laser pointers.
----------------------------- Steve Baker -------------------------------
Mail : <sjbaker1@airmail.net> WorkMail: <sjbaker@link.com>
URLs : http://www.sjbaker.org
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Message is in Reply To:
 | | lasers and RCX
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| Years ago, there was a thread about hooking up lasers to the RCX and using then for signaling and proximity sensing. Aside from the dire warnings (Don't blind the cat!), does anybody remember this thread? I've recently come into ownership of a few (...) (22 years ago, 5-Jun-02, to lugnet.robotics)
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