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Messages - joeaverage

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5371
General Mach Discussion / Re: Steppers have no torque.
« on: February 05, 2018, 09:48:58 PM »
Hi,
I have read that stepper magnets can be irrevocably demagnetised if the stepper is disassembled. It always makes me wonder how they make them I the  first place?
Do they assemble them real quick while the magnets are still hot and before all the goodness drains out of them?

Any permanent magnet can be demagnetized if a sufficient magnetizing force is applied contrary to its field, if memory serves the parameter which measures that threshold
is called coercivity. Ferrite magnets have vey useful coercivity, about 1/2-3/4 Tesla but rare earth magnets are better again 1-1.5 Tesla. You have to try that much
harder to demagnetize a rare earth magnet.

I think the risk comes when you disassemble a stepper the magnet is now dislocated from its soft iron magnetic circuit. It doesn't suddenly lose magnetism it just that
it doesn't have the easy magnetic conditions it is accustomed to. This is in effect a counter field equal to the remenance of its own field. In the case of ferrites
the difference between it remenance and its coercivity is fairly small, about 0.1 Tesla. Any additional demagnetizing flux due to a current in a winding could cause a
ferrite to demagnetize. Rare earth magnets the difference between remanance and coercivity is about 1/4-1/3 Tesla and so unlikely to encounter flux levels
sufficient to demagnetize certainly by accident.

My contention is that unless a ferrite is removed from its soft iron magnetic circuit or was deliberately spun in generator mode well in excess of it ratings that it
will not demagnetize and not therefore have a 'lifespan'. Whether my contention is correct is another matter!

Craig

5372
Mach4 General Discussion / Re: Goto work 0
« on: February 05, 2018, 09:22:30 PM »
Hi,
I looked (actually drooled) at DMM when I buying a servo for a spindle. In the end the 1.8kW Allen Bradley servo and drive turned up on Trade-me, a New Zealand
auction site and I bought pretty well, about $700USD including shipping. Shipping to New Zealand for something like a servo and drive can be over $100USD in
itself. So despite being second hand I'm very happy with it. It also explains why I had to buy the tuning software, had I bought new the software would be included.

Its the first 'modern' servo I've really had anything to do with and man they are great!. As you say all the flexibility of pulse/analogue modes, torque/velocity/position
modes, all the monitoring functions and the list goes on.
What really impresses me is that this thing can maintain angular position right through to full revs and rated and more torque with a following error of only
(20 / 8000)*360=0.9 degree and a zero position error of (4 / 8000)*360= 0.18 degrees. Way, way, WAY overkill for a spindle but really impressed by the technology.
This particular servo is ten years old and the design is older still. If its as capable as this then truly modern servos are surely better....

Craig

5373
Mach4 General Discussion / Re: Goto work 0
« on: February 05, 2018, 08:29:47 PM »
Hi,
kool, you have a solution, but do you have an answer?

May I suggest putting a LED or LEDs on screen and attach the input signals which are associated with your alarm outputs. It may give you a valuable insight
into the behavior of your machine but also the nature of the firmware fault.

If you are using wx4.set then the machine diagnostics page has six LEDs for inputs INPUT#0 to INPUT#5 so would be a fairly simple exercise.

What servos are you using? I have been spoiled by Allen Bradley, they have software, which I had to buy much to my disgust, that lives on my PC which I can
program all the parameters and features of the servo, simulate how it runs and when I'm happy download the whole shooting match to the EPROM memory
on the drive. I can unplug the drive from the PC and use it as a standalone drive. I thought it was a bit over the top when I first looked at it and trying to
justify spending a couple of hundred bucks on it but I'm glad I did.

Of course Allen Bradley are far from the only good servos out there but given I've got this software and how easy they are to use I'll be buying more of them,
they can be reasonbly priced second hand off EBay.

Amongst the things you can program is an analogue output that reflects the current following error, fascinating watching the error loop close when you offer up
a motion profile.

Craig

5374
General Mach Discussion / Re: Steppers have no torque.
« on: February 05, 2018, 07:31:04 PM »
Hi,
there are other ways to measure the motor current.

I have a high bandwidth current clamp from DC to over 1MHz, very useful for such tasks but not usually in the realm of hobbyists kit.
You could put a low value resistance, say 0.1Ohm, in the wire and measure the voltage drop with an oscilloscope, should avoid the disadvantage of driving
an analogue meter in reverse. Also with the bandwidth available with an oscilloscope you should be able to see the signature of PWM that generated the current.

Another possibility is to find the shunt resistor on the driver circuit board. All the current to the motor will pass through a low value resistor and the circuit will
measure the voltage drop to control its PWM loop. If you can identify the shunt you could measure the voltage drop with an oscilloscope with the same advatages
as above. Will require that you study the board and trace out the circuit to the extent that you can identify the shunt resistor. That requires some knowledge
and experience, its part of my training and very much a part of my job which is why I mention it, you may have similar skills/experience and could therefore
adopt this very simple test.

Craig

5375
General Mach Discussion / Re: Steppers have no torque.
« on: February 05, 2018, 06:39:05 PM »
Hi,
another fairly simple test is to install an ammeter in one wire of the stepper.

The stepper driver will be using a sort of pulse width modulation technique to regulate the current. If the full voltage of the DC supply were hooked to the stepper
the current would be so great that the motor would be destroyed in short order. For the purposes of this test a good-old rugged analogue meter is best.
Note also that the current in any given wire will reverse direction with each step, so your meter will try to read backwards, provided the current is not too
high relative to the rating of the meter it will probably withstand being driven backwards.

Another couple of points should be made so that you can be sure that your measurement is accurate.

The first is that with microstepping, and just about all drives for CNC use it, the current builds up to its peak through each intervening microstep to its maximum
that occurs on the motors natural fullstep. I would recommend therefore to switch microstepping off for this measurement, that will means that any current
you measure will be the maximum the drive is capable of or configured for.

The second issue that it is common for stepper drives to reduce the current delivered to the stepper if the drive is sitting at idle for any length of time. So if
the stepper drives to a point and then sits there, still with its maximum current its going to get warm so if its stationary for more than a second or so the drive will
reduce its current by half to save the stepper from getting to hot. For this measurement you want to disable that current reduction feature so that you will
at all times get maximum current and therefore a realistic appreciation of the motor current.

Now you have to decide does the measured motor current correspond to the manufacturers rated current?

Craig

5376
Mach4 General Discussion / Re: How can I edit lua script?
« on: February 05, 2018, 03:48:59 PM »
Hi,
Daz has got you pointed in the right direction.

I see the code that you want to delete to make your machine do what you want.

Sometimes I done a similar thing only to find it had some consequence I didn't think about and I wanted the code back. I find it best now instead of deleting
the code 'comment it out'. That means putting "--" at the start of each line you want to 'delete' and Mach will ignore that lie as a comment. Should at a later date
you want to get the code back or see how NFS did it, its still there for you to see.

Craig

5377
Mach4 General Discussion / Re: Goto work 0
« on: February 05, 2018, 03:41:57 PM »
Hi Mark,
that is a very interesting problem...I don't know whats going on but a thought occurred to me that might describe the symptoms and a possible test.

You noted that with a late build of Mach4 that your servos would not move if you attempted to jog with an increment greater than 0.01.
Now you have found that when homing is active that the servos will move in discrete bursts yet in both circumstances the LEDs of the controller
the DROs and the oiler relay would all indicate that Mach had called for motion and passed that to the motion controller but for whatever reason
either the motion controller or servos failed to make the move.

I'm guessing that the motion controller is dumping a whole bunch of pulses to the servo drives and the drives are exceeding the 'following error' window
and stopping. I would expect Mach/ESS to issue pulses at a rate within the acceleration limits of the servo/drive but maybe is not under certain circumstances?

There are a few questions which could resolve whether this is in fact whats happening or not. I don't know what servos you have  or how sophisticated they're
wired. It is common for servos to have an alarm output which could trigger under a number of circumstances including following error. It is also common
that such an alarm would trigger an Estop. Is your machine wired this way? I have a servo as a spindle and haven't been bothered to wire its alarm output
back to my controller, if you have done the same then you may not realize that the servos are faulting 'following error'.

If they could be faulting without you being alerted to the fact then I would suggest either monitoring the alarm output, it might be as simple as an LED and a
resistor. Another way is to widen the following error window, if that is possible with your servos. My Allen Bradley servo allows me to program the following
error window, its default value is 20 encoder counts. That is to say that if the servo lags its input (from the controller) by more than 20 counts (8000 count/rev)
then it will fault. I could reduce that following error but then could expect it to fault more regularly, alternately I could program it to 4000 counts say and it would
not fault until it lagged 180 degrees from where it should be!

What I propose for the purposes of testing that you increase the following error window to some wide value to see whether the fault you've described changes.
What to do about it if it proves to be the case is a different matter.

Craig

5378
Mach4 General Discussion / Re: macro for Reference All Axes
« on: February 05, 2018, 05:32:03 AM »
Hi,
Quote
I think that there is some issue about putting a homing command in a macro in mach4.  I'm still trying to figure out what that's all about.
The problem with putting a homing command in a macro is that a macro is run by the interpreter whereas the homing command is initiated by the GUI.

There are two Lua chunks that make up Mach. One chunk is contains the Gcode interpreter. If you run a Gcode job it is this chunk of Lua code that does the business
which includes running macros. The common macros in Gcode files are M01, M02,M03,M06 etc. If you write your own macro it might be M120 say, and the Gcode interpreter
would open and run that macro.

The GUI is the other chunk and it is responsible for the screen and all its functions like Homing buttons. Homing functions are initiated from the GUI and passed to the motion
controller. The controller reports back to the calling function in the GUI.

These two chunks of Lua code cannot run at the same time.

When Mach is running control passes back and forth between these two chunks. The mechanisms for doing it are vital to Mach and determine a lot of its 'character'.
A good deal of Mach programming involves having both Lua chunks running their own bits of code and they have to combine to do the job that you want and constitutes
perhaps the biggest challenge in successfully programming complex behavior in Mach.

The simple expedient is 'Have the GUI do GUI things and have the Gcode Interpreter do Gcode Interpreter things', trying to combine them or worse have then swap
roles is likely to be very difficult if not impossible.

Craig




5379
Mach4 General Discussion / Re: Jogging and DRO errors
« on: February 05, 2018, 12:35:49 AM »
Hi,
kool your making progress. It would appear that you jog increments are such that they don't land on whole numbers and therefore subject to rounding errors.

Most importantly Mach will execute Gcode and be able, at least according to the DRO, to drive to the right location. The challenge now is to see to it that the
motion controller and axis drives follow suit.

Craig

5380
General Mach Discussion / Re: Mach 3 turn
« on: February 04, 2018, 10:19:20 PM »
Hi,
aren't the axes on a lathe X and Z?

Craig

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