NC Controller difference

[curtisturner]

This might be a dumb question but here it goes.

What is the real difference between the following 2 (other than price)
PC and a couple of Gecko Drives some Servo's and Mach 3

And

A NC Controller like Fanuc or Allen Bradley

Thanks
Curtis

[Al_The_Man]

The main difference is the former is a PC based Software controller and the servo's are not closed back to the control.
Fanuc, Mitsubishi etc and PC based systems that use a controller card like Galil, the motion control is closed back to the CNC controller, in these PC based systems the PC really just acts as a HMI and the card takes care of closing the loop.
In an open loop system (to the controller) the CNC controller has no idea where the servo's are positioned.
Al.

[curtisturner]

I understand the Open / Closed loop

I was more intrested in
Interpolation (speed / Smoothness)
Look ahead (speed / ability)
Constant Velocity ??

my reason for asking is that my current setup is using mach 3 and i do not seem to get the above ablities the way i would think.

Example:
Interpolation.. My axis move at a higher speed than i would need, but add in intrpolation and speed goes south quick (slow )

Constant Velocity.. My axis seem to almost step through smaller movements with a split second pause between them (g-code lines)

Is this the downfall of using a Mach3 / Gecko style system

FYI i am running 1200oz/In Steppers and a 68V PS (10A)
And yes I have asked ArtofCNC no real fixes.

Thanks
Curtis

[NC Cams]

Mach is a step/direction system and it doesn't close the loop between what the steppers ultiametely do and what they're told to do.

Sort of like driving a car down the street via radio control - do you feel the acceleration in the seat of your pants if you pull the throttel trigger harde??? The radion (computer) tells the motor to speed up and slow down but can't never feel if it did it or not. That's the easiest way to look at it. Even if you use a servo card with Mach, you don't get closed loop feedback to the PC.

Yes the PC sent out a directive via the LPT port but until/unless the error gets too bad, it continues sending out pulses fat dumb and happy until the servo card sees a sufficient error. ONLY then, will the card notify the PC, again that will be recognzed when the next LPT port call occurs. Although the encoder feeds back to the servo card,the card doesn't feed an error back to the PC until/unless it gets too far behind.

In my servo system, the moment the drive faults (over current, no feedback signal, lag singal, whatever) IT STOPS. It doesn't do so many steps, gets so far behind then says, "we have problem, better stop because the problem finally got TOO bad".

Look ahead functions and interpolation (which any computer time so as to look ahead and calculations to happen for it to work), will bring thruput to a crawl. This is especially true depending on how your processor does background calculations and the priority it gives to them.

Instead of interpolation, try doing point to point milling in very, VERY small steps. WE do it on cam profiles and you can't tell that we've got teeny tiny flats on what should be a full radiused surface.If you make 2880 flat cuts to create a circle,you can't tell it after you buff it with 320 then 600 then scotchbrite.

At 360 cuts, the thing looks like a diamond disco ball with all the facets. Sounds like a machine gun when a roller runs along the surface.

We have the ability to do constant surface speed tool path cutting via S speed changes for every single cut point. The RAMDRIVE program in DOS does the look ahead, the controller sends out the code, the servo amp responds and if it does so properly, the cutter keeps mozying right allong. We run real slow (inches per minute and real high spindle speeds cause we're looking for mirror like finish).

Why an it move so slow and smooth at CSS?

Because a 12 bit ANALOG voltage is being sent to the servo amp (in increments of 0.004vdc, +/-10vdc full scale). Between the possibiltity of the stepper hittiing a resonance frequency and or the step function not being sufficient to provide adequate control at small incremental changes, it is possible for the system to hang up or default to a safe mode or the motor starts to "shiver" and that isn't what you want/need.

In your case, you're still dealing with a step funtion. You may have reached the limitation of your stepper motors and/or the limit for a particular step/direction system. Steppers do amazing things. But, they can't be turned into servos no matter how hard you may wish or try.

[fkaCarel]

For starters: it's not a dumb question, but you must have the opportunity to look on both sides of the fence. Fanuc etc. are dedicated and embedded controllers. This means that you cannot do much more with it than controlling coordinates on a machine. A PC is an office machine which gives you, in the end, on screen, the sum of 1 + 1.

The advantage of a PC, fast execution of a part of a program or a whole program, disappears if it has to execute the result of a small program. This means: sending pulses to the parallel port in timeslices is crippling. The design is made to send all the data in a stream to the port. A printed page!

A real time processor derives the time from a timer. When the timer is zero or overflows, depending on the type of processor, a new task can be started. For example the generation of one step. When the task is done, the program waits for the next timer interrupt.

I cannot look in the Mach3 software, but it looks like it's pulse delays are generated by a loop. Load a number, decrement by one and exit if the number is zero.

Where the embedded controller can do initialising of new instructions, calculate interpolation and maintain constant velocity in its free time, for the PC everything is overhead. The to decrement number should be, but cannot be adjusted for the specific software action of the moment.

So the easy solution is: buy the fastest PC you can get. The number in the delay loop grows, the overhead is executed faster. This is noise reduction by numbers.

The costly solution is: buy a Fanuc and have a specialised controller.

The crazy solution is: do it yourself and invent new G-codes and new approaches.

[ger21]

Originally Posted by curtisturner Example: Interpolation.. My axis move at a higher speed than i would need, but add in intrpolation and speed goes south quick (slow )

What is the current set on the Geckos? Perhaps your power supply is the culprit when multiple motors are moving together.

Originally Posted by curtisturner Constant Velocity.. My axis seem to almost step through smaller movements with a split second pause between them (g-code lines) Is this the downfall of using a Mach3 / Gecko style system

Are you sure your in CV mode? Is there a G64 at the top of your screen? Or, are the segments very very small? There are literaly thousands of Mach users, and this is not a common problem that I've heard of. ( I read every post on the Mach Yahoo support group, about 100 a day)

Also, how fast is the PC your using?

Here's a link to a Mill controlled by mach3, but using a Gecko G100 to send the step and direction. Very smooth and quick.
http://machsupport.com/forum/index.php?topic=502.0

[NC Cams]

fkaCarel = I now know why it is so hard to do the simple task of retrofitting a PC that does CNC controls in place of a Fanuc controller.

I'm going to add that to my cheat sheet. Good info that comes up over and over.

[curtisturner]

As far as the power supply i tried 2 seperate PS's and is still a problem
I am using Gecko 201's
I also have 2 no name brand drivers that I had originally had the problem with and bought the Gecko's thinking it was the problem.

Yes i have G64 up there and in my G-code (just in case)
-- first thing i checked, Art worked on it with me going down the check list of the easy things and still nothing

I have a 1.6 Ghz with 768 Ram Computer

I also did not think this was normal, but cant get a straight answer.
Did not want to go the route of a Fanuc unless absolutely required.

Thanks
Curtis

[NC Cams]

curtisturner: you just may have to - go Fanuc

See post #5 above.

If you do, the Fanuc's will have servos instead of steppers and true closed loop feedback...

[curtisturner]

What is a decent price for a fanuc system 10 or 11
everything including the servo's APROX ?

CT-

[Al_The_Man]

Before you consider Fanuc, there are are many equal to the task. (Fanuc 10&11 are way outdated, I don't think you can buy them new).
I have found Fanuc not very receptive to the one-off Retro-fitter etc, they are interested in the OEM's, they are Expensive and support is lousy unless you are one of the above mentioned OEM's.
Anyone looking at an equivalent, I would suggest Fagor or Mitsubishi.
If you are a one off user, the things to consider is whether the company offers free instruction and/or development software, Fanuc charge for everything.
With these kind of systems, you have to be able to write the machine logic, i.e. the M,S,T codes, as these are machine unique.
This is usually written in ladder logic or some kind of boolean arithmetic (not really that hard).
Even if you buy a used system, the ladder etc would have to be modified to suit your machine.
Personally, I have found Mitsubishi support outstanding.(they are used by Mazak BTW).
You are looking at a minimum of $10~$14k.
Al.

[balsaman]

I would try a different PC, even if you gotta grab it outa your office temporarily. Don't change the control yet. The combo you are using is known to work and work well.

Eric