Messages

441

General Mach Discussion / Re: Gcode Primer Manual

« on: December 14, 2011, 11:55:29 AM »

I feel obliged to point out that manual is copyrighted material....

442

General Mach Discussion / Re: G31 Probe problem

« on: December 13, 2011, 12:37:00 PM »

So that leaves us with the question of "When does IsMoving work as a sync function and when does it not?".
Alas, I don't have an answer for that question.

Unfortunately, I think Mach3 has a pretty high "entropy", and each version has its own unique quirks.  You may be able to state positively how a specific version behaves, but you can't necessarily assume ANY other version will behave the same way.  When it gets down to specific behaviors, the first question you have to ask is "What version of Mach3 are you using?", because behaviors change (at times, seemingly randomly) from version to version.  Sometimes the change is intentional (though even these are not necessarily documented), sometimes it's an unintended side-effect of some other change.  But I always dread changing versions, because I know there will ALWAYS be differences from whatever version I was running.

Regards,
Ray L.

443

General Mach Discussion / Re: wrong activelow

« on: December 09, 2011, 12:46:50 PM »

You are almost certainly not meeting the Step/Dir timing requirements of your motor drivers.  You need to understand what it requires, and make the Mach3 timing meet them  This may require finding an oscilloscope so you can see what the actual timing looks like.  I doubt you'll do any better with any other controller software.

As far as "active low",  most motor drivers actually step on the rising edge of the step pulse, so each pulse will consist of first a high-to-low transition, then a low-to-high transition.  Setting Mach3 to "active low" will do this.  Any transition on the Dir line must typically occur AHEAD of the low-to-high transition on Step, though not all drivers require this, and will allow both to transition at the same time.

You also need to make sure your pulse width is adequate.  Many break-out boards, particularly with optical isolators, will severely distort the Step pulses, or even filter them out entirely, at even fairly moderate step rates.

Whatever your problem is, I can pretty much guarantee is is NOT software - it is either hardware, or configuration.

Regards,
Ray L.

444

General Mach Discussion / Re: Run from here.......

« on: December 08, 2011, 11:04:44 PM »

SetNextLine does nothing to the feedrate - it will start running from the line you specify, using whatever feedrate is currently in effect.  I use it all the time, and have never once seen it do anything different.

Regards,
Ray L.

445

General Mach Discussion / Re: Brushless DC Servo Spindle Speed Control w/ Smooth Stepper and Leadshine

« on: December 07, 2011, 11:43:10 AM »

For a rotary axis, isn't acceleration in RPM/sec^2?  If so, 500 is not very high.

Revs per minute per sec per sec? That would be a 3rd order derivative, which would be jerk but with mixed time units - curious.

Hmmmm....  I wonder what the units are then?

Well whatever it is it has to be in terms of angular-displacement/sec^2 so revs/sec^2 or radians/sec^2 or degrees/sec^2 perhaps.

Ian

Yes, you're correct - Units would be revs/sec^2 or RPM/sec (which would be odd....).

Regards,
Ray L.

446

General Mach Discussion / Re: Steppers are too slow

« on: December 07, 2011, 11:30:29 AM »

That's not how choppers typically work.  They normally run at a constant switching rate, with the pulse width varying based on commanded output.  Current limiting will prematurely turn off the output current, ONLY if the limit is exceeded.  At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance .  The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.

Ray - I'm not sure where your reply contradicts anything I said. If you check out the attached pic I think I described it adequately for the purposes of the thread. However if we're being Mr. Picky... 

Current limiting will prematurely turn off the output current

Actually current limiting turns off the output VOLTAGE. current decays as a result.

because the current is not turned on long enough to saturate the coil, due to the coil inductance

As above, it's because the VOLTAGE is not turned on long enough to allow the current to rise to saturation because of the effects of inductance. We use a larger than rated VOLTAGE to "combat" inductance by reducing the rise time of the current and hence why we need current LIMITING in the first place.

But enough of this merry banter... now to the bit in your reply which is really pertinent to the point we were discussing... (please read post #33)

What I was asking was... please explain how the LOAD on the stepper motor affects the coil current? and in particular because you've said it here... please explain how a LOW load would result in a lower coil current. You may well be correct - as I said earlier in the thread I've had this stated to me before but no one has ever explained (or been able to explain) why. Personally I don't get it - and that's why I'd LOVE to have someone explain this.

Cheers

Ian

Your original post said:  "ONLY when the current through the motor reaches the set point will the chopper start to chop."  This is not necessarily correct, depending on exactly how the PWM is implemented.  The waveform you provided is correct for some PWM power supplies, but NOT necessarily for a PWM stepper motor drive running at a low duty cycle.  As I said, the H-bridge will be switched on and off at a fixed rate, based on the commanded duty cycle that will vary from near zero, to near 100% (again, sinusoidally, if micro-stepping is used).  Each pulse can be shortened if, and ONLY if, the corresponding coil reaches the limit current.  This will likely NOT happen at low duty cycles, as the pulse is not long enough for the coil to saturate.  At some greater duty cycle, limiting will start of occur, but that may not be until the duty cycle approaches 100%, heavily dependant upon the motor inductance, and voltage used.  The whole point of the chopper, as I said, is to modulate the RMS voltage across the coils, which implies a corresponding modulation of the RMS current.  The actual current is not directly controlled (beyond simple peak limiting).  If it operated as you suggest, the ripple in the coil current would be running essentially full-scale all the time, which would do some really serious heating in the motor.  The PWM is there to allow the current to be modulated, in part to reduce ripple, by lowering the peak currents.

For a stepper, I don't believe motor load does affect current, unless there is some small effect related to back-EMF/slip angle, but I'm not sure about that.  It's been a loooooong time since I studied the details of the operation of motors...

Regards,
Ray L.

447

General Mach Discussion / Re: Brushless DC Servo Spindle Speed Control w/ Smooth Stepper and Leadshine

« on: December 07, 2011, 10:16:32 AM »

Hood,

Hmmmm....  I wonder what the units are then?

In any case, if that were the problem here, the motor shouldn't be spinning at all, and the servo should simply be faulting instantly, no?  I don't see how it could just spin slowly, as reported.

Regards,
Ray L.

448

General Mach Discussion / Re: Brushless DC Servo Spindle Speed Control w/ Smooth Stepper and Leadshine

« on: December 07, 2011, 09:31:07 AM »

One thing is to check what you have in the SS config for the spindle frequency, for your 4000 steps per rev and 3000 rpm it will require 200KHz so use the 256KHz option.
Next thing to say is an acceleration of 500 sounds very high to me for a spindle, try more like 20 and see how you go then increase a bit if it works.

Hood

Hood,

For a rotary axis, isn't acceleration in RPM/sec^2?  If so, 500 is not very high.

Regards,
Ray L.

449

General Mach Discussion / Re: G31 Probe problem

« on: December 07, 2011, 02:19:05 AM »

Hi Ray -
Oh NO you don't - I won't let you blame me for that one! 
(well I guess you could, but I was not the instigator and am not responsible).

Dave,

Now that you've re-told the sordid tale, some of it does sound familiar.  Seems so long ago....  But I'll still blame you - It's just easier!  :-)

We'll have to get together for breakfast again sometime.  Maybe January?

Regards,
Ray L.

450

General Mach Discussion / Re: G31 Probe problem

« on: December 07, 2011, 02:14:47 AM »

March 17/2009 Release 3.042.023 -- Threading bug found and fixed in Driver... -- Probe error know kills script and file execution

Is it possible that this was not fixed or has crept back int later versions?

I'm pretty sure I was the one that isolated that problem with Brian and Greg.  I was having LOTs of probing problems back then - it would sometimes probe in the wrong direction, or on the wrong axis, or even on multiple axes at the same time!  And when a G31 failed, the script did NOT get terminated, so if you had a multi-move probing script, and ti failed, and you then re-ran it, the second instantiation would complete, THEN it would finish the first one!  However, since about that release, probing has actually been pretty solid for me.  Back then, I was breaking a probe about every week or two, but I haven't had that happen in almost two years now.

Sadly, Mach3 has a LONG way to go before it can be considered really robust, and hardly a day goes by that I don't get bit by one bug or another.  One of the reasons I'm now bringing up a KFlop on my machine (though it has a different set of issues....).

Regards,
Ray L.