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Author Topic: Digitizing  (Read 79299 times)

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Re: Digitizing
« Reply #80 on: August 23, 2008, 06:25:38 PM »
Very nice, Bertho! I like the simplicity. Your questions are ones I have been asking myself for some time. The contact resistance varies dramatically with different materials and the amount of time in use (number of actuations) according to my tests. I used a circuit board in my original design and soldered the balls to the board, using small drilled holes to align the balls. The biggest problem I had with ball and rod contacts was that after about 100,000 samples (that's 100,000 switch operations) the contact resistance would increase so much that the op amp circuit would start to fail to switch its output state. At first, when I started testing my probes they were great, but after hundreds of thousands of samples they started to fail to close enough to work reliably. I couldn't accept those statistics, otherwise I would be selling them for cheap! Logic tells me that the problem is that the contacts don't wipe (self-clean) and a buildup of carbon occurs. This even happens at the extremely low current I was using (less than 10 microamps). Cleaning the contacts with some fine steel wool fixes the problem for up to another 100,000 point samples, but I still find this unacceptable.

The spring pressure is also something to be reckoned with. If it is too weak, as you know, the resistance increases. However, making it too strong means you are applying more force to the probe tip and the object being scanned. Objects made of softer material may not stand up well to these higher forces. Also, for really fine scanning with a needle-sharp tip, you will definitely leave a mark on all but the hardest of metals each time a sample is taken. Of course hardly anyone will be using such fine point probes for everyday use. I use them when I am testing my probes in order to get the finest resolution I can in my test results. If you want to scan an object that is a model and made from wood or clay, then the spring force may become a factor due to deformation of the object.

I am currently working on new designs. One uses a piezo-electric sensor and the other uses miniature off-the-shelf normally closed switches. The piezo element tends to deform over time and I have scrapped that idea for now, although it did perform pretty well for a while. The switch system is working even better than I suspected it would. Instead of using three rods I have gone to five. It still uses the rods to re-center the probe after each point sample. I use five switches that are operated by a ring that lays on top of the five radial rods. My thinking in using off-the-shelf switches is that, being made with excellent switch contact materials, they will survive longer before their resistance goes too high. I would like to get 3 million operations between servicing the probe. For my off-the-shelf switch model, servicing would mean replacing the switches or perhaps the whole circuit board with the switches pre-mounted just to make it easier for the user. Or perhaps just call the probes disposable and toss them after 3 million point samples and buy a new one.

My goal is to build and sell them for under $100, with accuracy and repeatability of under 0.0002" and have them last for 3 million operations.
-Mark
Re: Digitizing
« Reply #81 on: August 23, 2008, 07:32:41 PM »
Thanks Mark,
What are the commercial probes doing that is different?  "Normal" usage might not include vertical surface probing to generate a cloud.  If they are only used for measuring features or references the number of actuation will be very low.
Bertho
Re: Digitizing
« Reply #82 on: August 23, 2008, 09:14:40 PM »
Yes (regarding measuring and referencing), that's a good point! I don't really know how most people would use them, but as a hobbyist I think I would want to do 3D point clouds to copy shapes. Also, my tests have huge numbers of point samples and maybe the the ordinary, everyday scan that one might usually do would be a much larger grid with much fewer sample points. Maybe I've been making good probes all along and didn't know it!

From the information I've seen on commercial probes of the type that use balls and rods I don't think they are doing it any different basically. Maybe they are using some special materials? I just don't know.

Here is a link to an interesting PowerPoint presentation from Renishaw, a maker of probes. It does have a fair amount of information and its worth a look (if you haven't found it already):
http://www.renishaw.com/media/ppt/en/744f3d51d1b944caa6c8bba90515b9a8.ppt

« Last Edit: August 23, 2008, 09:16:59 PM by TetraLite »
-Mark
Re: Digitizing
« Reply #83 on: August 23, 2008, 10:55:34 PM »
I remember seeing the Renishaw presentation  a year ago or so and I had forgotten about it.  It is very well made!
Three items caught my attention:
They monitor the increase of resistance and at a defined point they turn off the current before the contacts actually open.
They are talking about 10g force.  Very low!
They guarantee 10 million life cycles.

I think the normal industrial use for the probes is to verify that machined parts meet the specification.  In other words, selectively measure a hole size and its location in reference to other holes and features.  No scanning at all.
Bertho
Re: Digitizing
« Reply #84 on: August 23, 2008, 11:29:00 PM »
You mentioned: "They monitor the increase of resistance and at a defined point they turn off the current before the contacts actually open."

AHA!!! That could be really easily done with a very small microcontroller chip with an A to D input (I have some and I have the capability of programming them). By stopping the current before the contacts fully open it would stop them from arcing, which causes carbon to build up. And indeed, the amount of force needed to move the contacts only enough to detect increased resistance would be very small, but of course, the machine does not stop instantly, so more force would continue to be applied until the Z axis drive stopped moving. But cutting off the current before the contacts fully open is very interesting. This is a very neat idea and I just might pursue it. The PIC12F615 or PIC12F675 microcontrollers are 8-pin DIP packages with a 10-bit A/D converter and could be built right into the probe. This would require three wires to hook it up: one for +5 volts, one for ground and one for the output signal which would not need amplification and could drive a parallel port directly. A tiny potentiometer could be added for fine tuning the trigger level but may not be needed. I might give it a try soon.
-Mark

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Re: Digitizing
« Reply #85 on: August 24, 2008, 07:27:40 AM »
Hey Mark, was hopeing you would see that reply from Bertho. Hopefully this will be what you have been looking for. I have my fingers crossed for ya.  :)

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

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Re: Digitizing
« Reply #86 on: August 24, 2008, 07:32:19 AM »
Bertho, Nice looking probe you have there too.

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Digitizing
« Reply #87 on: August 24, 2008, 01:01:35 PM »
Now that I have point clouds from scanning I need to first make them easy visible to see any obvious defects and then be able to import them into a CAD program to scale, tweak and adjust the scanned data, and finally machine the updated data.  With Mach-3, LazyCAM, MeshCAM, and Leading Edge I think I am OK on the output side but what suggestions do you have for the cloud conversion?   I have seen:

Global Mapper:  http://www.globalmapper.com/index_data.html  which looks very good for the visualization and it is also tempting since I have been doing a lot of design work with GPS and:

VRMesh:  http://www.vrmesh.com/default.asp which does the 3-D import and repair.
Both look like excellent programs but expensive to just play with.  Any suggested low-cost alternatives?
TIA
Bertho
Re: Digitizing
« Reply #88 on: August 27, 2008, 07:05:15 PM »
Here is an update on my scanning adventure:
After hunting high and low for software to check out my probe with, I finally found several free and extremely low cost solutions and I have updated the info with samples on the website. See:
http://www.vinland.com/Touch-Probe.html
Bertho

Re: Digitizing
« Reply #89 on: June 30, 2009, 12:50:22 AM »
Hi Guys:

  I promised soem photos of the digitized prop..

 65000 points, 6 hours. .5mm grid..

Art


I know this is an old thread, can you tell me how you avioded the dreaded two point per probe bug?  my points clouds are useless, my probe hence is not usable, need to probe some parts badely.