Re: Range-finding ultrasonic / infrared

From: Robert Monsen (rcsurname_at_comcast.net)
Date: 01/04/05 

Date: Tue, 04 Jan 2005 06:29:55 GMT

Danny T wrote:
> What's the best (cheapest, quickest, easiest) way to detect obstacles
> for a small robot? The book I have starts with infrared, and later
> replaces them with ultrasonic. Infrared is way cheaper, but I don't full
> understand how it works..
>
> I've got some phototransistors here, that seem to work exaclty like
> LDRs, they just change the resistance with the light - except they also
> work with infrared (as well as normal light). I've seen infrared LEDs,
> and infrared phototransistors - am I right in thinking these are simple
> filtered to only detect IR, instead of normal light?
>
> Would these be accurate enough to be able to get a range, or would it
> simple be a yes/no if something's there? Ideally, I want to be able to
> detect a) if there's anything within, say, 6 inches, and also if there's
> anything within, say, 3 inches. That way, as things get near, I'll start
> turning my robot, but if it doesn't turn quick enough, it'll increase
> the turn when it gets closer.
>
> Thanks,

You can buy units that are designed as IR distance sensors. They give
you either a voltage (which you can read with the comparator or a/d) or
a binary output. The Sharp GP2D120 comes to mind. Here is a datasheet:

http://www.technologicalarts.com/myfiles/data/gp2d120.pdf

You can get them at the various robotics webstores. However, they are
cheaper at Arrow electronics. Make sure you get some JST connectors,
because they are a bit difficult to run without them. Acroname sells
them with the connectors and wires for $12.50 plus shipping:

http://www.acroname.com/robotics/parts/R146-GP2D120.html

I've built robots with these before, and they work pretty well. One
issue they have is that they are *really* directional. This is an issue
when you are trying to detect obstacles, and mounting becomes critical.
That is one thing to say for ultrasonics.

You can also just use a bump sensor, like the roomba.

http://www.irobot.com/home.cfm 

-- 
Regards,
   Robert Monsen
"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

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