Messages

1251

General Mach Discussion / Re: 3.042.026 Fixtures broken

« on: April 18, 2009, 05:01:00 AM »

Thanks, seems to be working now in my office.

I'll give it a go on the actual machine today and post back if the problem persists

1252

General Mach Discussion / Re: Opinion and advice - explain relationship of encoder to kernel speed

« 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  .

This has been a very interesting thread. Thanks again!

1253

General Mach Discussion / Re: Opinion and advice - explain relationship of encoder to kernel speed

« 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 

1254

General Mach Discussion / Re: Opinion and advice - explain relationship of encoder to kernel speed

« 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!!

1255

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 16, 2009, 10:44:26 AM »

Thanks to all for the info and advice!

Here is what I have decided to do:

For now I am going to buy a 36v NEMA 23 servo motor, matching 300 line encoder and a Gecko 320

I'll use the power supply that I have which one of you graciously calculated for me that I still have some 'room' on.
 
Hopefully I will be able to get everthing proved out and working good and then I can think about getting a bigger motor and probably a separate power supply for it.

1256

General Mach Discussion / Re: 3.042.026 Fixtures broken

« on: April 16, 2009, 10:39:32 AM »

I did not specifically select any profile. I just installed it on top of an existing .025, so it would be whatever the default it, I suppose.

I did not add any new screens . . . I don't even know how to do that yet . . 

1257

General Mach Discussion / Re: Opinion and advice - explain relationship of encoder to kernel speed

« 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!!" 

1258

General Mach Discussion / Re: Opinion and advice - explain relationship of encoder to kernel speed

« 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.

1259

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 16, 2009, 05:01:43 AM »

MeanWell is based in Taiwan (China).

My Keling PS does not say MeanWell anywhere on the ourside, but the Keling par numbers exactly shdow those of MenWell. THe physical appearance as well as every spec also matches exactly. All good evidence that Meanwell may manuf the PS for Keling.

The Keling mdel 320-36 that I have is working fine so far. I like the automatic thermostatically controlled fan.

Anybody have good or bad things to say about Kelings's servo motors?

1260

General Mach Discussion / Re: Opinion and advice - explain relationship of encoder to kernel speed

« 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.