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« 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.