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Author Topic: Opinion and advice - explain relationship of encoder to kernel speed  (Read 17012 times)

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Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #40 on: April 16, 2009, 11:07:18 AM »
I see your point, but I think I need to clarify.

As far as I can tell (and I may be wrong on this) ANY servo controller/driver that is connected to MACH does what you describe, however the point I am making is that MACH, as the 'intelligence' does not have access to the 'knowledge' that the axis is falling behind, and can take no action until it gets a fault, at which point it is too late and everything stops.

If the computer software (MACH in our example) was DIRECTLY watching the encoders, it would know how closely the axis was keeping pace and could make and automatic or programmed pre-programmed decision as to what action to take. Bringing everything to a screeching halt via a fault could certainly be one of those actions.

I can speculate that abandoning the current part and continuing with the next part could be one action. Changing to a new sharp tool could be one. Lowering the feed rate could be one. Warning the operator that 'X" error is present at 'X" part of the code.

In other words, MACH is isolated from the 'action' by a 'translator' that accepts step and direction commands to control a servo motor. It is a way og getting things done, but it does prevent MAC from doing more sophisticated functions because it simply does not have the available data to monitor what's goint on . .  even on a servo driven machine tool.

Maybe someone with more detailed experience migt chime in here and flesh this out more, but really I have what I need for now.

My decision is this:

I have just purchased a NEMA 23 servo with matching 300 line encoder and a Gecko 320 all from Keling. If I can get that all working with my 4th axis as I think I can, then I will consider gettng a larger motor and building a proper 4th axis head with tapered roller bearings, etc. What I will have is a working prototype to play with and work out the programming details. 

I'm listing the current 600 oz-in NEMA 34 stepper and the Gecko 203v in the for sale section in the forum. If nobody here want them, I'll throw them on Ebay later.

Time for a new thread on this project! Thanks very much to everyone for the help, knowledge and advice!!

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #41 on: April 16, 2009, 05:53:36 PM »
I see your point, but I think I need to clarify.

As far as I can tell (and I may be wrong on this) ANY servo controller/driver that is connected to MACH does what you describe, however the point I am making is that MACH, as the 'intelligence' does not have access to the 'knowledge' that the axis is falling behind, and can take no action until it gets a fault, at which point it is too late and everything stops.


 I think we may have to just agree to disagree but will try one last time as I am a stuborn bas%43d :)
With my servo drive (others may not be the same) Mach does not need to know what is happening for my machine to be closed loop. My drive knows where the axis is and it knows where Mach told it to go to and the timescale it had to get there. The drive will compensate as best it can to do that and if properly tuned that is exactly what it will do unless there is some abnormal condition such as a crash.
 If it was Mach rather than the drive doing the compensating then exactly the same would happen, no difference at all except that you could use "dumb" amplifiers.
  If Mach was doing the watching and altering then it would still require the tuning to be correct and motors correctly sized or it would still fault and nothing you could do to stop it.
 I am not sure what the Geckos following error is like, if I recall it may be fixed so that could be one downfall. On my drives I can set the following error to whatever I wish, mine are set to 20 counts and I have 8000 per rev and one rev is 5mm, so if my drives dont keep my axis within 0.0125mm then my drive will fault, this will disable the other drives and inform Mach that they have faulted and Mach will halt the code running and go into reset. I have never had a following error fault yet other than when I crashed during beta testing of some hardware.
Hood
« Last Edit: April 16, 2009, 07:25:01 PM by Hood »

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #42 on: April 17, 2009, 03:53:30 AM »
Hood,

We don't disagree on how the systems operate. I completely understand how your system (and actually any servo system using Mach3) functions and as I said, I consider it acceptable as a practical matter for my purposes.

What makes an interesting debate is your comment: "Mach does not need to know what is happening for my machine to be closed loop."

What we may agree to disagree on is simply a question of symantics in what constitutes 'Closed loop'. One way to think about it is that in the case of Mach3 and servo motors, the loop is only 'closed' back to the servo drives (amplifiers?), and not all the way back to Mach.  That arrangement precludes certain functions to take place. Whether those functions are usefull or neccessary is another legitimate debate, but the fact that the 'loop' does not reach all the way back to Mach is not debatable, unless one was to argue that a simple fault constitutes a 'control loop'.

I will agree with Webster's definition of 'closed loop'
Main Entry: closed loop
Function: noun
Date: 1951
: an automatic control system in which an operation, process, or mechanism is regulated by feedback



I made no implication that proper sizing or tuning of the motors had anything to do with defining what constitutes feedback.

Speaking of tuning, I do not own a scope nor do I know anyone who owns a scope, but Gecko has a method of tuning that uses a volt meter. NOt as good as a scops, they say, but adequate. They ship my new drive and motor today so it won't be long till I can resume frying electronic components again . .  ::)

Incidentally, my understanding is that the Geckodrive is fixed at 128 steps befor faulting. Your setup is obviously quite a bit tighter . .   and tight is good  ;D







« Last Edit: April 17, 2009, 04:00:28 AM by simpson36 »

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #43 on: April 17, 2009, 04:21:20 AM »
 From what I hear most people have tuned the Geckos without the use of a scope, for my drives the config software has a built in scope. On the axis motors I just tuned them by ear and have since looked at the software scope and they are good. The spindle motor was a different story, it was extremely hard to tune and I needed to use the scope in the software to get it right, the reason was probably due to the size of the motor combined with the backlash in the gearing in the headstock and 30Kg of chuck being spun but now its tuned it works well :)

RE tuning and sizing, afraid if you dont have these two done properly then you will never get it to work no matter if it is the control or the drive that the loop is closed to. Unless of course you have the CNC Brain which supposedly can do this although I have not seen anyone reporting one working for any more than basic open loop movements yet let alone the advanced compensation and double closed loop  it is meant to be capable of.

128 counts seems quite a large amount, especially with a 300 line encoder (1200 count), on my setup that would be 0.5333333......mm, not good :(  I am thinking with motors tuned/sized (yes these chicken words again ;D )  properly then it is likely you will never see that large an error. Not sure if you can in any way monitor the following error with the Geckos, with mine the drive config software can monitor it and also gives the max and min errors
Hood

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #44 on: April 17, 2009, 06:33:48 AM »
"RE tuning and sizing, afraid if you dont have these two done properly then you will never get it to work no matter if it is the control or the drive that the loop is closed to."

Can't argue that. I'm looking forward to playing around with this.

What is a CNC Brain?

"128 counts seems quite a large amount, especially with a 300 line encoder (1200 count), on my setup that would be 0.5333333......mm, not good"

128 seems like a lot to me also, and as far as I know it is not adjustable. Fortunately in this application, it will be driving a lathe head/indexer and I'll have 12 steps per degree . . thats enough for my purpose, and it will allow my computer to get the motor going fast enough without resorting to Gecko's multipier tricks or being forced into a smooth stepper type of device.  For my purpose, accuracy is only important for indexing and threading. For indexing, there would theoretically be no error because the spindle would be stopped. For threading or hard tapping, even the max error of 10 degrees along the thread *probably* won't bother my work because I'm not doing precision threads. If that need ever did arise, I could simply change the disk on the encoder to a higher count (US Digital sells them separately) or 'back gear' the head.

Also the reality check is that I'm using a pretty crappy little Seig Mini which is not at all accurate to start with. Sort of like trying to slice off a nice piece of .040" mahogany veneer using a chain saw  :D.

This has been a very interesting thread. Thanks again!

« Last Edit: April 17, 2009, 06:37:56 AM by simpson36 »

Offline Dan13

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #45 on: April 17, 2009, 07:18:25 AM »
The CNC Brain is a pretty expensive motion controller, that can be interfaced with Mach3 (like the SmoothStepper), and has yet to prove itself being worth the money. It is supposed to provide double closed loop control. I think, if it proves itself, it's going to be the next thing for many CNC builders and OEMs that use Mach3.

You can check it here http://www.safeguardrobotics.com/default.aspx?tab=cncbrain

Daniel

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #46 on: April 17, 2009, 07:20:32 AM »
CNC Brain is a hardware device (and GUI) that was supposed to be capable of turning a slack piece of junk into a high precision super machine, do a search on CNC Zone and you will find lots of info.
Hood

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #47 on: April 17, 2009, 07:24:39 AM »
that can be interfaced with Mach3 (like the SmoothStepper),
Daniel

Afraid thats not the case, the CNC Brain needs to use its own GUI, any plans that the guy had to have a plugin for Mach have been dropped.

Hood

Offline Dan13

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #48 on: April 17, 2009, 07:40:22 AM »
Hood,

Thanks for the insight... and I was wondering why no one is using it with Mach3... :D

Daniel

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #49 on: April 17, 2009, 07:53:08 AM »
Hood,

Thanks for the insight... and I was wondering why no one is using it with Mach3... :D

Daniel

Even if there was such a plugin I dont think you would see anyone using it with Mach as there doesnt even seem to be anyone using it even with the guys own GUI, well a few have basic open loop movement but thats about as much as I have seen.

Hood