Resolvers to Encoders

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01 Nov 2011 23:14 #14498 by SRT
I need to change out my current Resolvers for encoders. My question is can I use absolute encoders and eliminate the homing process? Does anybody have any opinons on this? Sorry, I posted in the wrong section earlier.
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02 Nov 2011 00:05 #14501 by BigJohnT
You have several options.... resolver to encoder converters from iirc both Mesa and Pico. Absolute encoders I not sure that is a good option...

John
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02 Nov 2011 13:27 #14513 by andypugh
SRT wrote:

I need to change out my current Resolvers for encoders. My question is can I use absolute encoders and eliminate the homing process? Does anybody have any opinons on this? Sorry, I posted in the wrong section earlier.


Resolvers are actually very nice transducers, with a lot of positive features.
EMC2 interaction with absolute encoders isn't particularly good. You might be better with one of the resolver interfaces.

Pico do an interface board which makes a resolver look like an encoder:
www.pico-systems.com/resolver.html
$150 per channel.

Mesa (http://http://www.mesanet.com/ ) do a board that plugs into their FPGA interface cards and gives you 6 channels of resolver data for $184. Look for the 7i49 under "Anything IO daughter cards". It does only work with the Mesa FPGA cards, and the driver for it is only in the "master" (development) branch of EMC2, though this is available as pre-built package from the buildbot. This has a direct, absolute, digital interface into EMC2.

A much cheaper (and rather inferior) solution based on an Arduino is here: wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Res...oQuadratureConverter
3 channels for $20. Not to be confused with the professional solutions.
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02 Nov 2011 22:41 #14533 by SRT
I am really learning a lot. I can't wait to actually get started and make something happen. Thanks for the answers without the help on here I am sure that this would be out of the question to take this on.
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08 Nov 2011 19:55 #14695 by jmelson
SRT wrote:

I need to change out my current Resolvers for encoders. My question is can I use absolute encoders and eliminate the homing process? Does anybody have any opinons on this? Sorry, I posted in the
wrong section earlier.

Pico Systems resolver to quadrature converter produces 4096 counts/rev, and an index pulse.
How will you read absolute encoders into EMC? Generally, unless you write a new driver,
EMC only can read incremental encoders, which by definition are not absolute.

Note that some linear encoders that are CALLED absolute are really quadrature incremental
encoders that have a different spacing between each index pulse. Their own DRO boxes
know haw to handle that, but you still have to move the slide some distance to pick up
two index pulses to calibrate the pseudo-absolute logic.

What is the problem with homing? It takes about 30 seconds when I am starting up
EMC, and I can do othjer stuff while it is homing.

Jon
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08 Nov 2011 20:25 #14698 by PCW
The Mesa resolver converter/driver does provide a HAL absolute position pin, but note that in the normal situation with a resolver on the motor shaft the resolver output is only absolute over one turn so homing is still required.

As an aside, some Robots use dual resolvers (one on the joint for coarse absolute position and one on the (geared down) motor shaft for fine position) to avoid homing at startup.
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09 Nov 2011 13:35 #14714 by SRT
Homing is not that big of a deal. I did not know that these converters were avalivable and I thought that I woul have to change out the physical resolver. I have absolute encoders on a different machine that I had professionally retro-fit (Fagor) and it is kind of nice. I am still trying to learn everything I can before I get in over my head, for instance how would I know that EMC did not handle absolute encoders very well?

Thanks for all of the help!
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09 Nov 2011 21:16 #14728 by jmelson
SRT wrote:

Homing is not that big of a deal. I did not know that these converters were avalivable and I thought that I woul have to change out the physical resolver. I have absolute encoders on a different machine that I had professionally retro-fit (Fagor) and it is kind of nice. I am still trying to learn everything I can before I get in over my head, for instance how would I know that EMC did not handle absolute encoders very well?

Thanks for all of the help!

The problem with absolute encoders is there are no standards. Incremental encoders have been pretty standard
for years, quadrature digital signals, the only thing that can vary, usually, is the count per unit of movement.

Absolute encoders have a variety of systems. They can be serial (Fanuc, for example) or parallel, and report
at fixed intervals or upon request. They can be true absolute, where position is known without backup
power, or phony absolute, like Fanuc, where last position is saved by a backup battery when power
is off, but position is lost when the encoder is disconnected or if the machine is moved while
power is off. In any case, different resolution, different encoding schemes and transmission protocols
means that there is no standard input hardware for absolute encoders.


Jon
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