On Jan 26, 2012, at 9:09 PM, Greg Bernard wrote: > Has anyone had experience with capacitive encoders? I found these while > browsing CNC4PC: > http://www.cnc4pc.com/Store/osc/product_info.php?cPath=64_78&products_id=363 > They seem to be very affordable.
Yes. They work just fine. We just completed a servo motor gantry machine that has these encoders on 3 of the 4 joints. I have also used them on stepper motors on a small mill for detecting faults (if I lose steps) and for it's DRO. During our tuning of the servo system we observed a small position error at beginning and end of acceleration while jogging (an error of about 0.003"). Jon Elson posted that he had observed this behavior with the CUI AMT-102 encoders and that it was due to their interpolation of the data not being as robust as other encoders. If you search the list archives for the "Subject: Encoders" and Jon Elson and CUI you will find it. However, firstly, if you run into this problem CUI claims you can improve the time constant by removing a jumper (at the risk of increased signal noise). See CUI link below. We removed this jumper and did see a small improvement in the error on initial acceleration and the blip mostly went away on deceleration. But we still had a very small position error (less than 0.002") so we decided to buy a US Digital Optical Encoder (see US Digital Link below) and try it on our X axis. It turns out that US Digital makes a version of this encoder that is pin compatible with the CUI encoder so it was a very easy thing to swap out. If the US Digital Encoder improved things at all it was barely perceptible (certainly less than 0.001). We still have a very tiny blip at the start of acceleration even with the US Digital device. The USD encoder was almost 3 times the price of the AMT encoder. While we kept the US Digital one on the X axis, if I had to make a choice I would go with the CUI encoders due to their price. There is one thing about them that leaves a bit to be desired. They attach to a motor shaft using a plastic collar. You get a bunch of different size and you pick the one for you motor shaft size. While these work ok, they aren't always the tightest fit. We had some tuning issues early on with the servo machine and Mariss at Gecko suggested that we put a drop of super glue on the collar (at the shaft) in the little channels there. I did that, and on one axis it made a large improvement. I think during quick acceleration it was slipping a bit. The collar was stuck well on there with the glue, but it didn't take a whole lot of prying to get it off when we wanted to try the US Digital encoder - the super glue works up to a point on smooth metal. BTW, Digikey also sells them as well, and for a bit less: http://search.digikey.com/us/en/products/AMT102-V%20KIT/102-1307-ND/827015 CUI Link: http://www.amtencoder.com/Resources/Frequently-Asked-Questions#3 US Digital: http://www.usdigital.com/products/encoders/incremental/rotary/kit/e5 Tom ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
