# Distance measurement with laser

1. Feb 6, 2004

### Cliff_J

How would go about measuring distance with a laser?

I'd like to construct a zero-contact method of measuring distance much like the commercial units designed to allow digitizing of a model as a retrofit on a 3-axis CNC. My understanding is that they position the X-Y and then determine the Z for each point. I figure with an inexpensive consumer laser and something like a PIC microprocessor I might construct a low-budget form of this.

I remember a physics lab where we measured the velocity of a moving sled using a laser, IIRC we just measured the doppler so no help in this application. So a pulse of the laser and determining the time to return much like SONAR seems plausible.

Am I in way over my head?

Cliff

2. Feb 6, 2004

### LURCH

There is some usefull info at this site ; enough to verify that you seem to be on the right track. Commercially available laser range finders appear to use a highspeed clock to measure the return time of the signal. Unfortunately, it also states that this amazing technology will give you readings accurate to within "plus or minus two yards/meters", is that accurate enough for your needs?

If not, you may want to look at some form of triangulation or even interferometry to make finer measurements.

3. Feb 6, 2004

### Integral

Staff Emeritus
The laser distance sensors I have dealt with use trig, there is a slight angle on the transmitted beam, the sensor is able to register the location of the reflected beam, this information is sufficient to compute the distance quite accurately.

I do not think timing pulses would work for the short distances involved in a CNC mill.

Check this site for more information. These sensors will generally have a long range low accuracy mode or a short range high accuracy mode. Of course longer range higher accuracy means more .

Given the right sensors it would indeed be possible to build your own, not sure if you would be able to match the quality and features of the commercial models. They have a significant amount of electronics on board.

4. Feb 6, 2004

### Cliff_J

Ok, thanks for the responses.

I looked up interferometry and sure enough I'd done that in a physics lab as well, kinda need a mirror on a stable plane to get that to work, a bit tough for reverse engineering. But it would definitely be precise...

That Keyence site has an excellent technical document in PDF that explains their use of inductive eddy currents sensors, laser triangulation with a CCD, and what they call 'confocal' with a half mirror and a focusing lens to sense the reflected focused spot. I like the last one, requires the least amount of circuitry and maybe I can find some old surplus lenses I can acutate with a stepper motor and make work. Hmmmm

Great ideas and links guys. Nice to find this forum.

Oh, and other ideas welcome as well!!
Cliff