Re: [CAD_CAM_EDM_DRO] true closed loop vs. hobby closed loop

This is my perspective on things; but following error is a fact of life
due to the way controls operate. There are algorithms to reduce the
following error but it will always exist. The issue is to avoid
excessive following error, which involves mechanical and electrical
tuning of your machine.

There really is nothing that can be called "true" closed loop. There
are many ways to close the control loop, but once closed it is in fact a
"closed loop". Some servo systems allow an additional parameter that
works in conjunction with the PID terms to remove some of the following
error, these are often called feed forward systems. Even here if you
try to operate outside the systems capabilities you will have following
error. This is one of the reasons for maximum acceleration and
velocity values that a controller maintains.

Thanks
Dave


snagglexr650 wrote:

>Hello,
> I work in a machine shop and we have several retrofitted
>lathes/mills using OpenCNC and various motors/drives including
>Yaskawa servo motors/drives, we also have a Haas VF4 VMC. These are
>all high dollar, industrial level cnc controls, with "true" closed
>loop feedback from the encoders.
>
> It seems to be widely considered that true closed loop actually
>allows the computer control to self-adjust the motors if error in
>positional accuracy has occured. However, all the machines we have,
>including the Haas (not a retrofit) have an allowable following error
>(FE) on each axis. If that FE is exceeded, the drive faults out and
>cuts power to the servo motor. If the positional error falls within
>that FE range, then inaccuracy does occur, it's happened to me.
>Furthermore, this FE is used in axis crash situations. Say you crash
>Z into your vice/part/table, as the axis stops moving due to hitting
>a solid object, FE is exceeded and the drive faults. Attempted
>positional compensation via the control would simply drive the axis
>into the part harder and cause more damage to the machine.
>
> Now, with that in mind, compare the +/- 128 counts 'FE' on the
>Gecko drives. If positional error exceeds 128 counts, then the drive
>faults out and servos stop.
>
> In either scenario, it appears that the same thing happens, and
>that even in the case of 'true' closed loop controls, there is no
>attempt to correct the positional accuracy, just a fault.
>
> My question is where is the difference? I know software is the
>key to closing the loop. Is it simply that with Geckos, your
>step/direction software will happily continue to run the program
>without knowledge of the drive faults, and that on the Haas
>control/OpenCNC software control the software is aware of the fault
>and stops running the program and displays an 'excessive following
>error exceeded' message?
>
> Is this true? If not, then please explain to me the difference
>between hobby closed loop via Gecko Rutex etc... and
>industrial 'true' closed loop via our Haas VMC and OpenCNC retrofit
>control.
>
>
>
>
>
>
>
>
>
>Addresses:
>FAQ: http://www.ktmarketing.com/faq.html
>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
>Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
>Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
>List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
>Moderator: pentam@... indigo_red@... davemucha@... [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
>
>
>
>
>
>
>
>
>