[owhite]

Anyone with any experience with capacitive proximity sensors?

Are they accurate, precise, cheap, easy to work with and sensitive to different materials?

My application is that I'd like to detect if my cutting head is at least 1 mm away from the sheet metal it is cutting. Distances closer than 1mm would flip the e-switch.

My price range is under $10^2.



Owen

[Al_The_Man]

Owen, Do you include hall-effect inductive proximity sensors in the question?, you can get them for ferrous or non ferrous metal, the range usually depends on the diameter of the sensor and also wether it is shielded or non-shielded (non-shielded have a larger range), Shielded means the metal jacket runs right to the end of the head. They come in all voltage ranges and you can get two wire or three wire ( 3 wire usually are for higher current applications).
Ebay is usually a source for that price range.
Al

[owhite]

Al,

Great, I didnt know there were hall effect sensors that worked with metals, I thought you'd have to wave a magnet in front of them. My laser wont cut aluminum so I dont have to worry about non-ferrous metals.

Do you know if they're precise?

Owen

[Al_The_Man]

I have found the repeatability very good on the ones I have used.
Al

[NeoMoses]

I've dealt with some very accurate proximity sensors, but they were in the $10^3 price range, probably a bit pricey for what you're wanting.

What is this for? Perhaps we could give you some other ideas for sensor types.

[owhite]

Regarding the queston about the application: I want to detect if a laser cutting head comes in or near contact with the material that it is cutting.

a rough sketch of my system is here:

http://nilno.com/laser_dir/optics-Model.png

The issue is that the cutting head may come in contact with materials such as sheet metal on the cnc table. This is a concern because the metal may deform from the heat of the laser. Contact with the cutting head will threaten the optics that are really expensive. I have heard that if the everything is set up just right the metal doesnt get particularly hot, I just havent had the experience of everything being set up just right when I've done cnc before :-)

The laser head has to come in quite close to the part, so I'm looking for something that is sensitive and precise. I found this link:

http://www.qprox.com/products/qt310_320.php

which seems interesting because it has a calibration setting which I assume would mean I could bring my z-axis down to the point where I want to set as stop, hit calibrate, and then perform the run.

Its all based on emi which seems kind of worriesome, I'd hate to be in the middle of a long run and have it shut off because of sunspot activity.

I might go with the obvious alternative -- a continuity test. I wont always be cutting metal, but in those cases I may just have a probe on the head and an aligator clip on the sheet metal. Fortunately my laser will be mounted on a gantry projecting off the wall and there will not be continuity between the optical cutting head and the x-y table.

But I would prefer not using the continuity check in cases when I'm cutting wood and acrylic -- less subject to warpage though.

[owhite]

Hey, and then there's these units:

http://www.aculux.com./LaserRangerPricing.aspx

a precision optical range sensor. Check out the precision on that thing.

You could do range checking -and- laser guided digitizing. I wrote them for quote. Betcha its woo-woo chicka chicka boom mucho pricymo. But fun.

Owen

[Al_The_Man]

I gather what you are looking for is a automatic height sensing method, I am not sure how commercial lazer units do this, I know plasma cutters monitor the arc voltage and operate the Z axis servo accordingly. This may be worth looking in to.
Al