Ideal microstep setting
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@bot:
Peridot has some great points, as usual. I've heard, and confirmed with some experimentation, that 4x microstepping is about the limit of "gained resolution," as in, beyond that, don't expect the additional microsteps to register consistently. As peridot says, the benefit is smoothness of motion, lowered resonance, etc.
….Worth pointing out I think is that on a delta, microstepping beyond 4x is important. This is because the Z resolution and hence layer height is much coarser than in a Cartesian or CoreXY printer with a leadscrew or threaded rod driving the Z axis. For example, my delta had a resolution of 80 1/16 microsteps/mm when I was using 1.8deg motors. That's 0.2mm per full step, or 0.05mm per quarter step. Changes in Z height of 0.05 mm are going to have a big effect on layer height and hence under/over extrusion. I now use 0.9deg/step motors, and with the 1/16 microstepping I was using when the printer was controlled by a Duet 0.8.5, the improvement in print quality could be seen if I looked closely enough.
I totally agree with you there. I'm not sure if you remember a series a posts I made on google groups about the "lava lamp" of resolution on delta printers, but my observation was likely caused by the varying resolution, and the variable ability for the motors to maintain torque causing inconsistent z height. I want to experiment with increasing the physical resolution and/or improving the power curve on deltas, but right now I'm going cartesian.
On my delta I have 80 micron full steps, and therefore 20 micron quarter steps. I do not have reliable or consistent z movement below 20 micron increments. Do you think going from 12v to 24v may improve the microstepping torque, and therefore attainable microstepping resolution?
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How come you have 80 micron full steps - are you using leadscrews instead of belts?
I can jog the effector on my delta in steps of 6.25um, in other words it response to each 1/16 microstep. I haven't tried to measure how even the microsteps are.
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I have 16 tooth pulleys with 2mm pitch belt, 0.9 degree motors.
I can also sometimes jog my effector in .01mm increments, but then in other areas of the bed, those increments do nothing until a big jump up. I probe my bed manually at about 28 points, and I jog up and down and measure with feeler gauges or with a dial indicator in place of the hot end. The 20 micron increments are almost always reliable (though not completely accurate), while 10 micron increments work about 50% of the time, more or less.
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Vertical resolution is the most regular of the delta steps - a vertical move is always simply the same number of steps on all three motors. (This is why it is safe to home by moving all three motors straight up as far as they'll go - no matter where the delta is this will home it without horizontal excursions.) Of course, depending on your horizontal position your three motors will be at different places within a step, so microstep inaccuracies may result in some horizontal motion as well as vertical inaccuracy.
That said, I too have noticed some weirdnesses with small vertical steps appearing to do nothing, both with my previous RAMPS hardware and my current Duet WiFi. Of course my printer does not have terribly good mechanical resolution, and instead of feeler gauges I'm using paper. But it's interesting to hear your reports of peculiarities. Steppers are supposed to be so simple!
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@bot:
There is a line of servo motors that accept step/dir signals from any typical stepper driver, called clearpath motors. They are only available as small as NEMA 23, and are quite pricey, but for a large delta or something, I would love to try them some day.
Not to take us too far afield, but I quite like the Mechaduino project, which is apparently now shipping Kickstarter orders. Relatively affordable servos with ~0.1 degree resolution and full torque/velocity/position control. They do take step/direction input, but I suspect that this interface might not improve on a microstepped stepper; for really good performance you'd want to have the motion planner send target velocity/acceleration information to the motors as well. And anyway, it's not clear to me whether these motors will lead to any real improvement in printer reliability. Fortunately some of the smart tinkerers on the deltabots Google Group ordered sets and are planning to experiment.
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Although I like the idea of using position feedback, I would want better than 0.1deg resolution for a delta printer. My 0.9deg steppers move the effector on individual 1/16 microsteps, which equates to 0.5625deg resolution. Print quality was quite good when I was using 1.8deg motors but the moire on the sides of flat surfaces is noticeably less with 0.9deg motors.
Sending target velocity and acceleration info to the drivers precludes the use of segmentation-free delta movement as used by RepRapFirmware, unless the servo controller can compute delta motion itself. What processor does the Mechaduino use?
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Although I like the idea of using position feedback, I would want better than 0.1deg resolution for a delta printer. My 0.9deg steppers move the effector on individual 1/16 microsteps, which equates to 0.5625deg resolution. Print quality was quite good when I was using 1.8deg motors but the moire on the sides of flat surfaces is noticeably less with 0.9deg motors.
I think you dropped a zero, but yes, the accuracy they claim is several microsteps of your current stepper. Though that's accuracy - the "steps" of position sensing are actually 0.02 degrees, there are just residual nonlinearities so that those steps are not all that evenly spaced. Which is true of microsteps too.
That said, the moire patterns are the result of taking discrete steps, as I understand it, and position/velocity/acceleration control would (could) result in genuine straight lines, not digital approximations. Which is why I suspect that a step/direction interface is not going to be satisfactory, or at least, not that much of an improvement over what we have now.
Sending target velocity and acceleration info to the drivers precludes the use of segmentation-free delta movement as used by RepRapFirmware, unless the servo controller can compute delta motion itself. What processor does the Mechaduino use?
It's an ARM M0+ SAM D21 (Arduino Zero compatible). So it can probably be flashed to handle the conversions itself. Failing that, it might be adequate to use segments that were cubic splines (constant-acceleration) - I believe it may natively support "follow cubic spline" commands?
Clearly getting the most out of Mechaduino for a 3D printer is going to take a lot of work from people who understand motor control and motion planning. If the only change is smoother, faster prints, I'm not sure whether it's worth it. The real appeal of servos is the hope that they can improve print reliability, and for that I think just hooking them up with a step-direction interface will be an adequate test.
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It might be of interest - I've used these in my Mill (with Mach3). They accept Step/Dir inputs and once the PID's are tuned, give impressive results.
http://www.cncdrive.com/DG4S_08020.html
Compared to most other Servo controllers, they are very good value!I converted a 4 Axis Bridgeport Interact 1 Mk2 - and can achieve 100mm/s. It's not bad considering the table has to weigh quarter of a ton!
Maybe a bit OTT for a printer - but if you want high speed, high precision and near silent operation, maybe not!
I've tried 3D Printing on it, attaching a hot end to the spindle and printing on to a (cold) sheet of glass, covered in tape on the bed - and the results were pretty good. I only ran 50mm/s but the prints were largely free from motion artifacts except where it needed high acceleration. Unfortunately, I need the mill as a mill - otherwise I'd be tempted to build a more solid implementation (heated bed etc). It did show that a rigid structure with zero backlash and servos could pay dividends in quality.
Si
Si
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Which servos do you use with those boards? I've never used a servo before, so I wouldn't know where to start looking. Your mill sounds pretty cool. I want to make something like that some day.
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Clearpath, not for your typical printer though. They are pricey but the SDSK known as step direction stepper killers are drop in replacements for steppers. The caveat is your build needs to be large to make them worthwhile as they are Nema 23… I have a build I am planning around them which will use several independent XYZ carriages that move over a large 3x 5 foot fixed build plate using 3to1 gear reduction on linear profile rails and ball screws
