Subject:
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Re: optical mouse chips as vision sensors?
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 9 Jul 2002 12:18:34 GMT
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Original-From:
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Steve Baker <sjbaker1@!AvoidSpam!airmail.net>
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Reply-To:
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SJBAKER1@saynotospamAIRMAIL.NET
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Viewed:
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618 times
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Mike Payson wrote:
> > I think you could also do this correctly with a single sensor (for a
> > typical two-wheel-plus-caster/skid robot) if you mounted it far from the
> > center of rotation of the robot. Then, the lateral motion of the mouse
> > chip would give you a pretty accurate idea of the rotational speed of your
> > robot.
>
> Hmmm... I don't see how that would work. How would a single sensor be able to
> differentiate between rotational movement and straight forward & back
> movement?
The mouse's camera chip tells you how fast the beast is moving forwards/backwards
and how fast left/right - as two separate sets of numbers.
If you mounted it on a boom (say) six inches ahead of the wheels of a typical
two-wheel-plus-caster robot then when the robot moves forwards/backwards,
the mouse chip will report forward or backward motion and no sideways
motion at all. As you turn, the mouse chip will be moving sideways (and
possible forwards or backwards too) at a rate of six inches times the
sine of the angle turned through in each sampling period...
[ It'll also be rotated through that same angle - but it will be unable
to report that. Hopefully its image processor will tolerate a little
rotation in the image it sees - when I move a mouse, it does rotate a
little on the mousepad - so it has to be somewhat tolerant. ]
Since the mouse chp produces data at least 40 times a second, the robot
will likely have only rotated a few degrees in each sample period
and we can use the mathematical approximation that sin(theta)==theta
(in radians) for small angles. Hence we get the rotational speed
by multiplying the sideways motion reported by the mouse by a
simple constant that depends only on the distance from the center
of rotation of the robot to the mouse chip.
This should allow you to detect both rotation and forward/reverse
motion with a single mouse chip.
You'd need to mount the chip as far from the 'center of rotation'
of your robot as possible in order to make best use of it's maximum
range of output for very slow rotational speeds of the robot. I would
expect that just sticking the chip a few inches out on the front of the
robot would work just fine in most cases.
> I suppose you would be able to guess since you would know what
> state the robot was in at any time (turning or straight), but it seems that
> this would introduce a rather large margin of error. Or am I missing
> something?
Yeah - you're missing something! :-)
This scheme should allow you to steer and move at the same time with a
single mouse chip PROVIDING that we can assume that the robot never
moves sideways without steering.
----------------------------- Steve Baker -------------------------------
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Message is in Reply To:
 | | Re: optical mouse chips as vision sensors?
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| (...) Hmmm... I don't see how that would work. How would a single sensor be able to differentiate between rotational movement and straight forward & back movement? I suppose you would be able to guess since you would know what state the robot was in (...) (22 years ago, 9-Jul-02, to lugnet.robotics)
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