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William K.
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Platinum
Re: New servos verus no servos
William K.   9/17/2013 11:18:31 PM
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Rob,

I have been wondering about that as well, since an "instant" stop would require a huge comversion of rotational inertia into some other form of energy. But I have seen some quite fast clutches that can decouple a big flywheel from a load, and then re-connect it, very rapidly. That is a lot more believe-able. Instant decelleration is just two amazing to accept.

Rob Spiegel
User Rank
Blogger
Re: Nice round-up
Rob Spiegel   9/12/2013 11:42:10 AM
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Thanks for the link, Elizabeth. The advances in plant robotics has been significant. This particular article was looking at motion control through the eyes of servo technology. Robotics is definitely a worthy subject for a coming article.

Rob Spiegel
User Rank
Blogger
Re: New servos verus no servos
Rob Spiegel   9/12/2013 11:27:58 AM
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Good point about the drunks, William K. However, some of the safety advances actually improve the safety of the equipment. One example is that fly wheels now come to an instant stop.

William K.
User Rank
Platinum
Re: New servos verus no servos
William K.   9/10/2013 9:06:11 AM
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78RPM, one of the failure modes that happens is for the controller to "wander off someplace" and ignore interrupts. Another failure is for it to simply stop executing the program, and, in the case of network connected systems, for something of greater importance to come along and take over completely, leaving the control program suspended. Of course, that last failure mode is primarily related to controllers running microsoft windows operating systems.

My point is that in order to be adequately reliable the emergency stop system must be able to do more than just request a stop, it must be able to force that stop without the assistance or consent of the software. As ajn example, consider those cars with the "unintended accelleration" situation, where pressing the big red button had no effect. A hardware shutdown would have switched off the engine no matter what, but instead the software decided to kep the engine running because "clearly the driver made an error in requesting an engine shutdown." In many plants the failure of the E-Stop button to halt machine operation would be sufficient cause to reject the machine until such a flaw was corrected. The emergency stop function is different from the "stop" request in that it need not be orderly. It is an emergency function. 

So the question is about how removing that emergency function could improve a machine's efficiency.

Elizabeth M
User Rank
Blogger
Re: New servos verus no servos
Elizabeth M   9/10/2013 5:29:12 AM
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Hey, 78RPM, thanks for that link. I actually wrote that article (which I'm sure you can see by the byline :)) and yes it's definitely apropos to the discussion here.

Elizabeth M
User Rank
Blogger
Re: Nice round-up
Elizabeth M   9/10/2013 4:42:50 AM
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Sure thing, Rob. You still covered a lot of ground here. I think a story on robotics could be totally separate. In fact, I just did one a little bit similar...here's another link for you :): http://www.designnews.com/author.asp?section_id=1386&doc_id=267058

78RPM
User Rank
Gold
Re: New servos verus no servos
78RPM   9/9/2013 11:04:17 PM
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One more thing: I noticed this article/ad in the margin of this discussion
Motion Controller Simplifies Design of Automation Systems

http://www.designnews.com/document.asp?doc_id=267511& and it offers E-STOP input. It requires not programming. I didn't believe it until I watched the company's video.

Pretty cool.

78RPM
User Rank
Gold
Re: New servos verus no servos
78RPM   9/9/2013 11:01:09 PM
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William K, I'm not sure I understand your concern. Modern MCUs have multiple layers of interrupts. Couldn't the STOP button have a high priority Interrupt? Use of State Machines and Real Time Kernels could resolve the safety issue while promoting energy efficiency -- and I'm sure the plant owner would welcome the increased electric efficiency.  And I don't think we have to be worried about "rules" to protect safety.  Most employers welcome increased safety as a way of lowering their insurance costs, not to mention their genuine compassion for their employees.

William K.
User Rank
Platinum
Re: New servos verus no servos
William K.   9/9/2013 7:47:51 PM
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Rob, safety systems are another area altogather, and they have made some veru fundamemtal changes in the way things work. With the single-driver-mechanically -linked systems, hitting the E-STOP button halted the prime mover and things came to a fairly quick stop all in synchronization. But now with the splitting up into many diffeent segments, each driven by it's own servomotor, an emergency stop must be choreographed in order to avoid breaking the machine. So it may not provide the anticipated advantage as far as safety operation goes. Of ourse, there is none of the inertia from the linking mechanismas, which is a secondary benefit of using individual servos.

Unfortunately, from many aspects, modern machine safety requirements seem to be aimed at protecting a bunch of stupid drunks bent on self destruction. At least that is what quite a few of the rules seem to be aimed at.

Rob Spiegel
User Rank
Blogger
Re: New servos verus no servos
Rob Spiegel   9/9/2013 11:11:37 AM
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Very good point, William K. Safety systems are also becoming very sophisticated and are often programmed so that machine crashes (with other machines, with themselves, or with people) are less likely.  But safety systems can't anticipate all the changes that can come through programming.

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