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I am looking for a reference design for a laser range estimator. I actually don't want to measure the absolute distance but the relative position of the object that is interest. i.e. if it is getting closer or not.

The range of the object is between 50 to 20 cm and object has a movement of a few millimeters and i like to measure this movement using laser. Ultrasound is not an option due to the speed of the object.

Any ideas or pointers?

Thx, Frank.

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It can definitely be a triangulation with linear sensor. Can be a lot of aspects involved, like safety, noises, mechanical, optical, thermal drifts, calibration, speed, statistics etc. Watch out for patents, if it is a real product. –  user924 Jul 8 '11 at 15:27
    
What is the speed range (and frequency, if applicable) of the movement? Will a "draw wire encoder" work for your application? –  rwong Aug 7 '11 at 0:25

4 Answers 4

Your task screams "Please, use interferometer" here :-) Basically, you measure phase difference between laser itself & reflected beam through interference.

This way you may get precision down to 0.1 micron, but you should be ready to count all these 10'000 phase changes per second (I.e. if you skip one - you've got 0.7 micron error).

Speed of tracked object is virtually unlimited.

But this is not going to be weekend project :-)

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1  
I don't know. Yes, laser interferometer is the answer, but I have the impression OP knew that already. He did say "laser", didn't he? Maybe he forgot the right term. If these presumptions of mine are right it may be a little thin to be a good answer. Info on how to detect phase changes would be welcome. –  Federico Russo Jul 5 '11 at 8:26
    
Well, usual laser range finders work on time delay to get to the target and back - this way it's hard to measure small distances. –  BarsMonster Jul 5 '11 at 8:29
    
@BarsMonseter, someone asks for a reference design, you suggest a different technology, but still no reference design! This assumes the OP does not know what the best option is, which is normally a fine assumption to make, but it does not answer anything. "Question updated: Where are reference designs for interferometer." –  Kortuk Jul 5 '11 at 9:18
    
interferometer is the answer I know.. I have looked at a few designs where they use caps/switches and flip flops to measure discharge etc to deduce distance. I need something that is proven to work so that I can start from there and improve. The critical point also, I don't really care about the actual distance, I need the delta distance over time. I hope this requirement would simplify the design. Open to ideas and solutions. –  Frank Jul 5 '11 at 11:48
    
The output from an interferometer is a sinusoid where the period is proportional to distance. As the target moves, the intensity at the detector varies from peak (constructive interference) to trough (destructive interference). You have to detect a) which way the target is moving and b) how many interference cycles it's moved. The difficulty of the electronics side of this is very closely related to how fast you expect the target to move. –  Optimal Cynic Jul 5 '11 at 18:50

Check out this blog which catalogues the beta-testing of a laser-range-finder module with some extensive design notes here.

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Great resource thanks.. –  Frank Jul 7 '11 at 4:14
    
@Frank, no problem! –  vicatcu Jul 8 '11 at 4:54

At that distance, laser triangulation might be a viable alternative.

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An answer this brief looks more like a comment. Can you elaborate to make it more acceptable as an answer? –  Federico Russo Jul 6 '11 at 8:16
    
I looked at triangulation.. If I can find a way to do triangulation with Photo Detectors as opposed to CMOS sensor sure, I will go for that. So far all examples are using cmos sensors.. Not suitable for me.. –  Frank Jul 7 '11 at 4:14

If you have some flexibility in your solution, then I'd recommend using one of Sharp's infrared rangefinder units. They are robust and fairly cheap. They use infrared diodes, not a laser, but if you just need optical sensing, that should work.

Sharp infrared sensor

You can buy units with varying distance ranges (5 cm out to 100 cm or so), and with a digital or analog output. Sparkfun and Trossen Robotics (among many) sells them. Note that these sensor "wrap around" when you measure a distance less than the specified distance. For most applications, you can just set the sensor back, such that no object can come closer than the minimum distance, but it's something to consider.

You also will need to make sure they have the bandwidth you need. They aren't the speediest of sensors, so if your object is moving very fast, they might not be a viable option.

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If ultrasound won't work due to the speed of the object, and a movement of a few millimeters needs to be detected, these won't work at all. These are a classic hobbyist/hacker optical sensing solution for basic tasks, but they're not for everything. –  Kevin Vermeer Jan 11 '12 at 19:02
    
Regarding speed: This could affect the ultrasonic via doppler shift. He doesn't necessarily mean it needs to sample quickly. Regarding precision: that might be an issue, but it's not clear from the OP. I don't see the need for a downvote :( –  gallamine Jan 11 '12 at 22:53

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