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Author Topic: Opinion and advice - explain relationship of encoder to kernel speed  (Read 17031 times)

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Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #30 on: April 15, 2009, 12:51:25 PM »
You mentioned you were in a hurry.

In a hurry + waiting on software guys = how old are you now . .  you may not live long enough . . . ;)

If you did rig a DPDT switch, you could prove the theory, get your immediate need satisfied, and it would not interfere with any miracles that come down the road from the programers.

Just a thought . . .

Offline Dan13

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #31 on: April 15, 2009, 02:26:37 PM »
You are right, but arranging a hardware solution would also take time...

As I said, I think it should be an easy fix for Greg to enable setting same pins for two axes. I will see first how it goes with this software fix...

Daniel

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #32 on: April 15, 2009, 03:45:39 PM »
I'm a new kid on the block, so I'm afraid I don't know all the players around here, but I think I recall the name Greg being associated with the smooth stepper.

Just so I have this straight in my mind (I'm easily confused  :'()  . . .

The hold up is with implimenting something in smooth stepper, not in MACH, and the operation at issue here is hard tapping on a lathe. Is that correct?

If so , could someone elaborate on this a little for those of us who are newbies?

Thanks.

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #33 on: April 15, 2009, 04:16:12 PM »
Yes Greg is Mr SmoothStepper :)

SwapAxis is not at this time supported with the SmoothStepper (possibly will soon but no promises, depends on complexity)

The SmoothStepper protects you from assigning the same pins for different axis but that is what Daniel is wanting as he can do a workaround to use the spindle as an A axis by doing that.

Yes rigid tapping or even indexing of the spindle is the goal.

Hood

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #34 on: April 15, 2009, 04:47:02 PM »
Thanks, Hood, that's a bit clearer.

I've done a fair amount of programming in my younger days (back when you HAD to because there weren't numerous apps for every possible need as there is today), and just this morning, I got a chance to watch some of the MACH tutorial vids and got a peek at the scripting.

My goal is to add a fully capable 'A' axis spindle to my little mill. Since machining centers have long had a similar arrangement, it stands to reason that I won't exactly have to re-invent the wheel to get that working.  I just need to do some more homework on the subject and build something to test.

Based on what I have learned so far, I'm inclined to go forward with my intention to abandon the current single pulse indexed variable speed spindle scheme altogether and just move forward with a full servo driven A axis and just work out the programming. When I have more time to research MACH's scripting and Brain capabilities, I'll have a better idea of how automated that process can be.

I may be mistaken on this as I only had a moment to peruse the videos, but it appears you can replace at least some of the MACH G-code interpreter with scripts. Certainly you can make up totally new commands via the scripting facility.

All that is missing is a command for continuous spinning of the 'A' axis for general turning . . and  . . . (famous last words) . . that looks doable to me at the moment.

Bottom line is that all of the pieces already exist to build a functional soultion for my purposes. It is only a question of how automated the G-code 'conversion' can be made.





Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #35 on: April 15, 2009, 05:35:20 PM »
Are you using the SmoothStepper?
Hood

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #36 on: April 16, 2009, 05:31:43 AM »
Not using smooth stepper at this point. I'm not seeing an advantage to it in my specific situation. Note that I am not experienced in CNC so that should not be interpreted as a negative comment.  

I looked more into the (original topic of this thread) issue of speed and find that even my old  850mghz dual P3 server gets an 'excellent' rating at 65k. My newer computers handle 100k no sweat.

Also turns out the Gecko driver I was planing to use (320) has an optional multiplier (340) that mutiplies the pulse rate and is specifically designed for high count encoders to be used with slow kernel speeds.  

Something else that has come to light from my research into this is that Mach does not read the encoder. I'm not sure yet how much of a limitation this is, but at a minimum, it prevents the software from doing anything about cutting problems in real time.

More homework is needed on that and I don't have a lot of time to spend on this, but if that limitation turns out to be a critcal in terms on servo function, that may be reason to investigate alternatives to MACH for control software.

I've been reading quit a lot on servo vs stepper and I'm pretty much sold on servo advantages at this point. I am noticing that the negatives for servos come from users and/or manuf who have already chosen steppers, long ago in most cases. I'm old and have been around forums since before the web so I am very ware of the 'emporer's new clothes' syndrome and also that if advice gets repeated enough times, it morphs into 'common knowledge' and continues to get repeated long after it is no longer valid, if indeed it ever was.

IN my reading so far, the major argument agains servos has been cost, and I have to assume that this was true at some point, but as a newbee, I have no predudices or pre-conceived notions, so I look at the current available choices. What I observe relative to appropriate alternatives in my case is this:

Nema 23 stepper $50 + Gecko 203v $150 = $200 per axis.

Nema 23 servo $70 + encoder $30 + Gecko 320 $115 = 215 per axis


That difference is insignificant, so I consider that cost argument to be outdated and invalid.

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #37 on: April 16, 2009, 06:52:14 AM »
Ok so if you are not using the SS then the easiest option for you I would thinkĀ  is to use Swap Axis.

Mach can read encoders, just it can not be used to correct in real time, they are basically for monitoring purposes. Plugin or macro could be used to read the endcoders and fault if they differ by a set amount but by that time it may be too late.

If you use servos then Mach does not need to read the encoders as that is what the servo drives do, it is them that close the loop. If a drive faults for any reason then you can have Mach halt motion by sending a signal from the drive to Mach but that should only be in extreme cases as your drives should get a motion signal from Mach and should get the motor to that position by monitoring it and making sure they get there at the correct time, if they cant then they will fault. If they fault except from in abnormal conditions then your tuning or sizing will most likely be the cause of the problem.The industrial controls that keep track of position use "dumb" amplifiers to move the motor with an analogue signal so because of that the control needs to close the loop.
 

I am not a fan of electronic gearing as it can produce cogging but that will depend on how much you gear and the setup you have. Previous to the SS I was forced to use 1:2 electronic gearing in my drives to get even a semi decent rapid (3400mm/min) but I needed twice that to get to the machines original rapids and on very slow moves I would notice cogging when watching a clock (dti)
 Having said that it will all depend on your aplication and encoder count, some people swear by it, just I am not one.

Servo/stepper may be similar now I am not sure as the servos I use are certainly not that kind of price (new anyway ;) ) One thing to make sure is that you are looking at similar kw/torque motors when comparing prices.

Hood
« Last Edit: April 16, 2009, 06:54:52 AM by Hood »

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #38 on: April 16, 2009, 08:34:50 AM »
Hood, thanks for the replies. You always have good insight that gets me thinking.

My sense is that you have big machines, outside the capabilities of steppers. Beyond a certain point, steppers fall off the possibilities list and servos get stupidly expensive. I'm aware of this, but my focus is on my little baby mill, and at that level, (which seems to represents a very large percentage of MACH users), the pricing is very comperable. That's not my opinion, that's a contemprary fact.

I don't want to get into a servo/stepper debate , but I will say that in my opinion, it is not really reasonable to compare steppers and servos on a power basis as their characteristics are so vastly different. They really each have their own place. I think a lot of the criticism of steppers is the result of people not acknowledging (or not knowing) their limitations and using them in situations that really call for servos.

While I am a newbee to home grown CNC, I am quite familiar with industrial closed loop control schemes and one of your comments  has me a little confused.

I am aware of how the Gecko servo drivers take step and direction pulses from MACH and keep track internally of the desired position and fault if 'x' deviation occurs.

My understanding at the moment is that MACH only sees the fault condition and has no awareness of the real time performance of the axis. i.e. the axis may be on the ragged edge for a long time and not fault, but MACH would be as unaware of this as it would be of missed steps in a stepper.

The differnece being the servo would catch up and not require rehoming.

The similarity being an out-of-tollerance ruined part.

Commercial/Industrial systems, from what I can gather so far, first off are all servo driven, and the system computers read the encoders directly and control the motors, so there is no 'translator' like the Geckjo 320/340 or eqiv.  preventing the control system from knowing exactly what strain is on the system, if the axis are keeping pace, and if there needs to be some corrective aciton taken, even if that action is simply an operator warning.

While the significance of the effect is certainly debatable, the limitation in MACH either exists or it does not.

If I have it wrong, please explain where I've gone off the mark.

If I have it right, then it doesn't mean MACH is no good, it simply means that the system designer (including a rank ameteur like myself) should consider if this limitation would compromise the intended function of the machine. In my case, from what I have learned thus far, I would not consider it critcal and I would find the MACH/GECKO/SERO scheme adequate for my puposes. But that would be an informed decision.


Southern California machining center operator warning:

"Dude, we're bogus on Zed by 6, time to cruise the program, man. . . . hold it, chill . . .  awesome, we're righteous again .. . . party on, Dude!!" 

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #39 on: April 16, 2009, 09:22:00 AM »
I couldnt agree more with you regards Servos and Steppers, I put steppers on the coil winder I made even though the guy I made if for was willing to pay for industrial type AC servo system. I managed to convince him that it was just a total waste of time/money as a properly sized stepper would be , more than adequate for that application.


Now the servo and closed loop thing.

I do not have a great deal of knowledge on the industrial controls but from what I understand if the control  sees that the motor (from encoder or resolver/tach feedback) )is lagging it will send an analogue signal to the amplifier to boost the motor, if its going too fast it will reduce. If the Control cant get the motor to where it wants then it will fault with a following error or the drive may fault with an overcurrent situation.

 I dont know anything about the Geckos etc but I do know my "intelligent" servo drives do the same thing as the control does, ie if it gets a command from Mach to move to a position in a certain time it will move it there, reading the encoder for position and speed and vary the current and voltage accordingly to get the motor there. Again as with above if it physically cant do it it will fault with a following error or overcurrent error.




Hood