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Messages - Chaoticone

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1601
Thanks, can you do the same program only the first move be in the same direction as it is going now but only by the amount you have BLC set up for each axis then feed to the original position in your test.  I want it to take 2 moves to get to the position.  First position will be equal to BLC, second will be remainder of the distance needed.

Brett

1602
General Mach Discussion / Re: Homing Sequence
« on: February 23, 2013, 08:32:39 PM »
Also, what version of Mach are you running?

Brett

1603
General Mach Discussion / Re: Homing Sequence
« on: February 23, 2013, 08:26:03 PM »
What hardware are you running?  Smooth Stepper? Do have any slaved axis?

Brett

1604
How many lines of look ahead do you have entered in general config.?  Can you try it with this set to 1?

Brett

1605
Quote
The y axis then lurches forward at a speed far greater than either the backlash compensation speed or the acceleration rate are set for.  The x axis is already backlash compensated, so there should be no 'rush' to get anything anywhere. It just lurches forward in the already compensated direction.


Where is the y going when it lurches forward? Y0, Y.5, Y-.5 ?

Brett

1606
MORE DATA:

sorry for spamming my own thread haha ;D

SO, that little jerking motion I mentioned that you can see when you slow EVERYTHING down to a crawl? thats NOT backlash compensation. It only shows up when backlash comp is turned on, but it is NOT backlash compensation.

Why do I say that?  I just paid closer attention, and the y axis DRO is incrementing during that brief jump. Then it paused and started to do its rapid. 

So mach 3 is throwing in an actual axis move right before it does its legitimate rapid, immediately after a backlash comp move.  No idea why. If there is a setting that affects this I would love to know about it, because it seems directly related to my problem. My motors are being commanded a very rapid series of moves at full speed ignoring the motor and blc settings.

That's interesting.

Quote
Do you know if mach 3 always does the axis backlash compensations sequentially rather than simultaneously

I do not.

Brett

 

1607
OK, Your right about the .3*90=27.  But, the next position it has to be at, at a certain time. However long it takes, taking into account your accel.,  it has to make the time difference up.  If that point is a long way off, it has more time to make up the lost time.  If it is a short move it needs to be there right now (because the other axis is) pushing your accel exponentially higher in order to achieve this, hence the lost steps.  

Brett  

1608
General Mach Discussion / Re: Homing Sequence
« on: February 23, 2013, 05:27:05 PM »
Yes, look in your ref all vb button.  Click on the user tab then edit button scpipt.  All vb buttons on the screen will begin to flash, raf all is one of them.  I think it will be 24 for z, 23 for y, 22 for z.  Just change the 23 and 22 around.  Watch it close and make sure it does what you want after you change and save your changes.

Brett

1609
The backlash comp speed is the speed at which the comp move will be made.  The speed is a percentage of rapid, not your accel.  even at 100% it will still take time to make that adjustment.  The only way to make that time up is to push the accel higher, just briefly.  If the next move is as small as your amount of backlash, where will it get the extra time?  

The settings in BLC do effect the speed but to my knowledge it only effects travel speed, not accel.  I think the accel is overridden briefly so that you end at the right place at the right time.  This is why you won't see it when just working one axis, it has all the time it wants to take to end at a certain position.  If it is has to meet another axis at another point at the same time it really has to push to make up for the lost time and motion the BLC move took.

Brett

1610
OK, you have done some really good test.  Do you see why I said it is your accel. now?  Bottom line is this, backlash is not cumulative, you only see backlash when you change directions or have an intermitten tool load (think of a 2 flute cutter at a very low rpm, I mean low, 30 RPM low) imagine how that may push the gantry and then it would spring back between tool contact time.  These are the only times you will see backlash.  If you can hold .004 with your machine your positional accuracy will be .004 if you drill 1 hole or 1000 if this inaccuracy is due to backlash alone. However, inaccuracy due to missed steps can add up, they can be cumulative but most often are random.  You have done the test now to know how severely under powered your machine is to be able to hold such a tight tolerance with speed.  Now don't take this wrong, I'm not criticizing your machine or your ability, I'm pretty impressed with both.  But, the simple fact is, we haven't figured out a way to beat the laws of physics.  You see, the amount of backlash your machine has is irrelevant to distance you ask it to move.  Backlash is just that, it is lost movement.  So if your running a square for instance at 30 IPM.  What ever accel you have tuned your motors to is what time will be allowed to slow the one axis to a stop and allow the other to get to speed (in exact stop, CV blends this transition).  When you enable backlash, your requesting that motor to move your machine the distance planned by the tool path + the distance in backlash comp. in the same amount of time.  Not only that, but the amount in backlash needs to be executed almost instantly else your tool path will suffer.  Imagine you had a bus and a corvette that had to run the same road course in the same amount of time.  That would be a big task in its self.  How much harder would it be to accomplish if every corner was so tight the bus had to stop and back up to stay on the road?  You see that lowering your accel to 1 your machine almost had the guts to cut the path you plotted for it.  You also see that changing BLC speed had little effect.  You could probably run the comp speed to 2 or 3 hundred % (I don't think mach respects anything over 100)  because usually in BLC moves, you aren't actually moving very much mass, your just taking up space.  But, just the inertia of a motor rotating and instantly having to reverse can be hard to hold.  Couple that with the fact that when both axis get to the other corner, they need to be in the right place you can see why high production, high precision machines cost 100s of thousands of $ and weigh 10s of tons with no axis having over 15 inches of travel and motors big enough to match the price tag.

Brett

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