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Topic: Accurate rangefinding with two sensors? (Read 566 times) previous topic - next topic

normanc

Thanks to all your help, I've gotten my ultrasonic rangefinders working.

Now I'm trying to derive the math behind using two sensors in parallel to accurately map an object.  Using only one sensor would give me a single distance, with no idea of angle.  With two of them, at a set distance apart and fancing the same direction, I should be able to figure out the exact distance and angle relative to a point.

How do I go about doing this?  I tried using right triangles and pythagorean's theorem but it ended with a trivial solution.

When I try to google triangulation methods, most of the equations involve knowing the angles (relative to each sensor) beforehand.  How can I do this with just two distances?

Thanks for any help!

GoForSmoke

My ultrasonics have a 15 degree spread. There will be no detailed map.

What you might look up is 3D scanner. Makerbot has Open Source 3D scanner.


I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

normanc

I'm using large objects with SRF08 sensors... they have extremely high spread for large objects (I've tested this with chairs and people).

So I have two values and a known separation... I'm trying to figure out a way to do the math to get the angles.

I'm thinking I can use something like this: http://www.cs.rit.edu/~ark/543/module05/trilateration.pdf

See section 4.  Delta is known (separation between two sensors).  ar and br are known from the sensors.  It makes two circles, and you can solve for s and t.  With s, t, ar and br, you can build a triangle and solve for the angles, allowing for triangulation.

Am I wrong in thinking this?  However, it seems like this would only work IF the object was in-between both sensors.

GoForSmoke

The picture will be fuzzy at best. No points, just plate-size dots if you're lucky.

Maybe someone else wants to pour through the code but you're working with water-bombs not squirt guns so I fail to see how you expect to use linear equations to solve anything.



I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

normanc

I can't imagine that someone else has not figured out a semi-reliable way of direction with two sensor.  However, I'm having a hard time finding an example of how to do it.  I don't need pin-point precision, but I would like to be able to do something that say, the car bumpers can do.  At least on the car I've driven, with 4 sensors facing forward they can give you the general direction and proximity of an object.  I understand with two the resolution is much lower but I don't need pin-point precision, just accuracy.

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