R Rogers wrote: >While running the mill today and thinking about the runaway servo issue should >an encoder fail. I wondered if an electronic device could be designed that >mounts between the servo leads and the A/B signal from the encoder. It would >monitor the pulse train to the servo and the signals returning from the >encoder. Then be tied into the E-stop circuit of the control. If a pulse >stream was present and a A/B signal stream was absent it would break a >connection and make the E-stop circuit drop out. Having hard limits at the >extremeties of the envelope seem to be the only protection available and could >result in alot of damage before it ever got there. That would be great if >something would monitor and trip in a very short distance in the event of a >lost encoder. Or would a dithering servo situation make this impossible? If >someone designed one that worked, I'd be the first customer. Someone mentioned >that Geckos fault after a 128 step/count discrepancy. I've heard and seen >Marriss m ention > the runaway servo issue and dont recall him saying it will stop in a short > distance. He stresses hard limits at the ends of the travels. > > What Allen-Bradley did on the 7320 CNC control was VERY sophisticated. But, it was also necessary in the days when encoders had light BULBS in them, and they burned out every three months or so. First, they used differential encoders, and compared the A to A/, B to B/ and Z to Z/ with exclusive or gates. If any of these signals was the same polarity as its complement for more than a microsecond, that caused an E-stop. Also, it generated a velocity signal from the encoder, and compared that to the magnitude of the tachometer. If the tachometer signal exceeded the encoder velocity by 10%, it caused an E-stop. This was a pretty exhaustive safety system.
Your description above may not be workable. In certain cases, there can be dither or VERY small moves from the CNC that don't require a physical move (because they are within the deadband). Using the encoder signals to detect encoder failure is pretty hopeless. You need some other means to measure position or velocity as a check on the encoder. (Possibly armature voltage could be a good enough velocity reference, but you'd have to bring this differential signal with huge voltage pulses from the drive down to a level that could be used to compare to the encoder or step pulses.) Geckos will not fault in most cases of a failed encoder. The drive will see no movement, and go up to the current limit. It will ram the end of travel, and if nothing shuts it off, it will just sit there, cooking itself and the motor. In some cases, the abrupt stop at the end of travel will "surprise" the Gecko current limit with a sudden increase in motor current, and the drive will trip into fault status. But, that is WAY to late, of course. The analog servos on my Bridgeport are set in a wierd way so they WILL fault. I have the current limit set close enough to the overcurrent trip threshold so that if the amps ever go to full current limit, they will fault. In normal servo operation, the amps should NEVER reach current limit, as that introduces a hard non-linearity in the transfer function (input to output relationship) that can destabilize the servo loop. The current limit is really just to protect the motors, amps and other drive components. So, if the amp ever gets out of the linear part of the transfer function, it is already out of control in some manner, and a trip is probably a good thing to have happen. I can short the tachometers with a screwdriver, and the amps go "click". Or, I can unplug an encoder while the machine is supposed to be holding position, and it moves about 1/8" and the amps go click. I occasionally develop a bad contact somewhere, and the same thing happens immediately when I try to start up the machine. I have NEVER had a runaway, and things would likely get broken if I did. Jon Addresses: FAQ: http://www.ktmarketing.com/faq.html FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/ Post Messages: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] List owner: [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED] Moderator: [EMAIL PROTECTED] [EMAIL PROTECTED] [EMAIL PROTECTED] [Moderators] URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO OFF Topic POSTS: General Machining If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble. http://www.metalworking.com/news_servers.html http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list. NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........ bill List Mom List Owner Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
