Proximity sensors??

[Arbutus]

The application is a precision core drill for boring glass blocks.

The machine must detect the glass surface within -0.000/+0.001 (or better) repeatability. This positions the tool correctly. It is important that there is no contact with the glass surface.

I have never used these and have some questions before finalising the design.

How 'precise' are these sensors?
Are they adjustable for sensitivity or trip point in software?
Does particulate debris such as sub-micron glass dust affect the sensor?

Any and all advice appreciated!

Don

 

[DavidR8]

I use inductive proximity switches as limit switches on my CNC router. The switches are from Automation Direct and they were not cheap. One thing I noticed is that they work fantastic on steel but lousy on aluminum.
You will definitely need capacitive switches to sense glass.

 

[Janger]

Don - just curious why do you need such precision? What is the purpose of the glass blocks if you can say? Why can’t the surface be touched? A renishaw probe would give you the accuracy but they touch of course. Interesting problem.

 

[Arbutus]

Don - just curious why do you need such precision? What is the purpose of the glass blocks if you can say? Why can’t the surface be touched? A renishaw probe would give you the accuracy but they touch of course. Interesting problem.

I cant say much here - the application is for the fiber optic manufacturing industry. One glass block is 1000x300x300mm annealed extremely pure glass. One slug costs about $20,000 before it is bored. End faces are prepped and cannot be contaminated by debris or scratching. Everything else is under NDA!

Don

 

[Arbutus]

I use inductive proximity switches as limit switches on my CNC router. The switches are from Automation Direct and they were not cheap. One thing I noticed is that they work fantastic on steel but lousy on aluminum.
You will definitely need capacitive switches to sense glass.

I'm looking at some capacitive sensors now. They do have adjustment for the trip point, and there are all kinds of connection and hookup options. I'll have to buy one to test it properly since the specs are a bit vague regarding repeatability.

Not cheap though!

 

[RobinHood]

Would there be any optical sensors that could do the positioning?

It would meet the criteria of ”no touch”.

 

[Arbutus]

I was using the Sharp reflective modules which are quite precise but the concern was the clarity of the glass which doesn't trigger the sensor unless the glass is dirty enough to reflect the IR. There are some other options for optical detection which I'll explore as well. So far the capacitive proximity sensor seems appropriate and reliable.

 

[Canadium]

Could you use a temporary coating on the glass to make it detectable and remove the coating when finished?

 

[van123d]

In my career I have a similar but different issue. I need to measure lens actuator movements at a micron level accuracy but I cannot contact the lens as it would interfere with the measurement and the glass surface is transparent. I use an opaque PVD coating on the front lens surface and a Keyence laser displacement sensor.

In my case the coating is permanent but I believe you can find a coating that could be removed with something like acetone or plasma cleaning. PVD coating is likely overkill, if you can find something in an aerosol I doubt you would build up enough coating to exceed your tolerance.

 

[Susquatch]

If capacitance doesn't provide enough resolution, consider testing the glass for bandwidth specific reflectivity? Most glass will reflect certain frequencies in the IR band.

Also incident angle for laser light. This approach could also improve resolution.

Or perhaps a diffraction method that looks at the gap to determine width. This would also have high resolution.

Oh yes! And how about air gap? Basically you put a regulated air flow into a nozzle to sense back pressure. This kind of system has been used in bores to detect sub micron distances for years.

Just a few thought starters.

 

[Janger]

And again I am awestruck at the depth of knowledge and experience by forum members. Hobby machinists my ass.