Hey guys can anyone help me out with this project, i am trying to make a optical range finder that can detect an object about 50cm away. im unsure as to what method would be best for this. any help is much appreciated. Thanks

 

Well, your qestion is not very specific.. let me put it this way:

There are two types of IR sensor:
- sensor to measure the distance in a specific range
- Proximity sensor to detect the presense of body in a specific range without any detail about the exact distance within that range

Which one are you trying to build?

Note that the reflectivity of object varies depending on their color and surface finishing, so measuring the intensity of reflected ligh is not always a good idea...

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Ibrahim Kamal
check my electronics and robotics page: http://www.ikalogic.com/

 

thanks for the reply, im trying to actually measure the distance so it is basically a range finder. i know using time of flight is not an option because of the short distances i want to measure, btw ignore 50cm it may be a few metres but not far away enough to use time of flight. i was just wondering if there is any other methods or property of light that i can use to obtain the distance. also i dont want to use triangulation.

 

In the topic at
Measure angles and distances
j.p.bill wrote
The optical rangefinders used in WWI & II used a fixed and moving mirror to get range. The angular motion of the moving mirror gave the tangent function, where the near side is known (the baseline of the rangefinder, or distance between mirrors. Good out to 25,000 yards or so. Needs a really good geartrain, though.

Shine a laser at the object, then scan the field with a phototransistor in a tube until it is detected. Use that angle to determine distance?

I just noticed that you DON'T want to use triangulation.

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Take a look at the Sharp GP2xxx family of sensors. For example: http://info.hobbyengineering.com/specs/gp2d120.pdf

You can get preset distance sensors or ones that give an analog level that is proportional to the distance.

 

Does anyone know anything about using phase as a method of finding the range . I have found only three different ways of measuring distance: time of flight, triangulation and phase/beam modulation.

 

I believe phase is possible but at the ranges you are talking it is dicey. Vary the period between pulses and look for a point where the received pulse is coincident with the outgoing pulse. If you start with a long period and shorten it until you see the match, the pulse period is 2X the time of flight.

However, light travels around 30 CM per nanosecond. to measure 50 CM (100 CM round trip, 3.333 nS), you would need to be able to generate pulses with a period of better than 3.3nS to some accuracy. Not impossible. You would have to have a pretty sensitive detector to pick up the really short pulses (~1 nS, I'd guess). Typical phototransistors have rise times in the uS range so they are out. Avalanche photo diodes are pretty fast but quite pricey - this one http://www.pacific-sensor.com/pdf/AD-100-8-TO52-S1.pdf is $80 from Mouser. I'm sure there are shielding/interference issues.

I'd suggest using ultrasonics if it's a cost sensitive application. Those can meet your needs cheaply.

 

Thanks to everyone who posted, i have found a circuit which uses phase to measure distances from 1 m to 15m with infrared light. For those who are interested check the book called "Optoelectronics theory and practice" by Allan Chappell

 

could you post that circuit.. or the URL.. ?


thx

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Ibrahim Kamal
check my electronics and robotics page: http://www.ikalogic.com/

 

I have not been able to find anything similar on the web and the circuit is fairly large. I will try to post some sort of block diagram when i get the chance