On Thursday 18 October 2007, Kirk Wallace wrote: >On Thu, 2007-10-18 at 00:17 -0400, Jim Coleman wrote: >> On 10/18/07, Jon Elson <[EMAIL PROTECTED]> wrote: >> >> Kirk Wallace wrote: >> > Thanks again Jon and Chris. What should have been obvious >> >> turns > >... snip > >> interference sources like motor-VFD cables and any other that >> might have 120 or 240 VAC on them. >> >> Jon >> >> >> in my hitachi seiki manual, it says that twisted pair wire should be >> used for the encoders. not sure if this would help eliminate noise in >> your situation or not. > >I used the existing cable which has four shielded twisted pairs. I have >one pair for each A, B, I and power.
How about the ends of the shielding wrap at the machine? They should be well grounded at the computer interface end and get as close as possible to the encoder, but should probably be isolated from the machine frame to avoid picking up noise or enabling a ground loop, which with variable phase angle thyristor speed controls on the spindle motor, can contain a lot of hf noises. And if the encoder has a ground, can it be isolated from the machine and tied to one of the shields? I'm thinking of the usual fiber or possibly nylon washers used for transistor isolation as a method of isolating the encoder pcb mounting from the machine frame. The mica sheets for such aren't all that thick if the washer thickness might be a problem in maintaining the spacings. A possible test to see if its VFD generated noise would be to see if it largely goes away when the motor is at full speed, and gets progressively worse as the speed is lowered. It may get better when the motor is at creep speeds too depending on how much losses there are mechanically, and the VFD itself. Some are thyristor based which would act like that, and some are basicly switching regulators driving dc motors, with the switching being done at several kilohertz. Those types could get noisier as the speed goes up, or if a hand is laid on the chuck to load the motor and make it work harder. Basically, look for a correlation between spindle speed and the extra noise pulses. An oscilloscope with at least a 20 megahertz bandwidth can be handier than bottled beer for this sort of troubleshooting. >> also what about an inductor or capacitor at the encoder in its power >> line to help smooth any noise introduced there? > >I have a short run between my encoder and a differential driver. I am >wondering about whether I should have some sort of filters on the high >impedance inputs of the driver. The thing is that with 50k pulses at >3,000 I'm not sure how much of a filter I could use. This might be a good place to use a piece of that shielded cable too. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If God had not given us sticky tape, it would have been necessary to invent it. ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
