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RenaissanceTinker

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Reply with quote  #31 
RE: DRV8825 stepper drivers
I found this and wondered if it may be an issue for milling as speeds would/ should be drastically reduced.   
http://cabristor.blogspot.com/2015/02/drv8825-missing-steps.html

There should probably be a software fix for this "dead zone" too.  I don't really grasp current control from outside the driver.  I thought the coils were just switched on and off.  So I am lost as to where the sine waves are captured.  Does the controller do this to "smooth" out between steps?  
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dewhisna

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Reply with quote  #32 
Hmmm...  Interesting article.  Though he doesn't say what his microstep size is.  I'm assuming he's trying to do 1/32nd stepping.  The X/Y on the BoXZY runs at 1/4th steps, the Z at 1/8th steps, and the extruder at 1/16th, so it's probably immune from most of the dead-zone issue he's describing.  But might be an issue if we ever experiment with 1/32nd stepping.

My understanding of how these modern stepper drivers work is that they modulate the outputs in a sine-wave in order to do the microstepping by causing it to move by the phase angle difference between the waveform in the two windings.

Back in the "old days" of stepper control, it was just on/off pulses.  But that only yields four combinations of on/off patterns or eight if you have center-tapped windings and feed the center-tap with the positive voltage (usually through a "brake resistor" for setting the holding current limit) and switch the other leads with low-side drivers.

By using full-bridges instead and shifting the phase angle difference in the two windings, it can artificially create additional step points and do it with non-center-tapped windings.  And since it's actively controlling the current and has the full output bridge, it eliminates the need for the "brake resistors" and things that used to have to be used for holding current control.

Right or wrong, that's my understanding of how it works.  Though I will say that the BoXZY runs much better with the DRV8825 drivers...  though I've also swapped out my power supply with the same 200W HP supply that Gary mentioned in one of his posts on this forum, which was another important modification as it cleans up the supply voltage quite a bit (and noticeably so).

This past weekend I've printed probably 30 objects, including a huge 160mm x 160mm part that took the entire printable bed area and that previously failed with the A4988 drivers from missed steps.  Absolutely no problems with the DRV8255...  It's running so much more smoothly (and quietly) with the DRV8825 drivers I no longer believe it's critical to change out my linear bearings.  I still have what appears to be some slight linear bearing problems, but it's no longer causing print issues, so I think I can put off the bearings for a bit longer.

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lakewaybob

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Reply with quote  #33 
I started having this problem yesterday. All of the sudden, the printer just "shifted" in the x direction, but only after printing several layers. Interestingly it was consistent which seems strange. I replaced the x-driver and the problem went away. Seems that the A4988 boards are marginal at best. I've been through about 4 of them so far. In one case the A4988 would only work in one direction and in another case, the part worked for a while then suddenly emitted smoke and died! I've ordered some DRV8825 drivers and hope they are more robust.
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dewhisna

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Reply with quote  #34 
While you are waiting for your DRV8255 chips to arrive, you may want to try bumping your X and Y acceleration from 100 mm/s^2 down to 80, if you have something you need to get printed right away.  It actually doesn't slow your overall print time down very much.  That initially let me get several prints to run with the A4988's that absolutely wouldn't print otherwise.  Doing infills on tight radius curves and/or circles where it's constantly changing direction seems to be the worst.

In fact, I initially thought changing the acceleration totally fixed mine -- until it popped up again a number of weeks later.  I've now done over 250 hours of printing since switching to the DRV8825's and so far (knock on wood), no issues.

The acceleration values are in eeprom and settable via a couple of gcode commands that you can send via the 'manual control' tab.  If you scroll up a page or so on this thread, you'll see directions for how to do that...

Welcome to the skipping-steps club!  There seems to be only two classes of users -- ones that have experienced the problem and ones that are going to experience it.

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gregket1

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Reply with quote  #35 
So where did you get the heat sinks? Pololu doesn't offer them and won't recommend a source.
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dewhisna

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Reply with quote  #36 
Actually, I got the cheap Chinese knockoff DRV8825 boards on Amazon...  They seem to average between $10 and $20 for a pack of 5 (whereas authentic Pololu boards are about that much per board):

Just go to Amazon and search DRV8825.  Here's a few entries from my search:

https://www.amazon.com/StepStick-DRV8825-Stepper-Driver-Printer/dp/B00S3Q9YZA/
https://www.amazon.com/3D-CAM-StepStick-Electronics-Robotics/dp/B010MZ8T5K/
https://www.amazon.com/Hobbypower-StepStick-4-layer-DRV8825-Stepper/dp/B00NCSK6T2/

I think mine were the "Hobbypower" ones, but I'm not 100% sure.  They came with the heatsinks.

Now one caveat was that the heatsinks that came with my DRV8825 boards didn't have the peel-and-stick thermal tape.  I had to use some thermal glue and glue them on.  Now Gary, another Forum member, ordered some and he said his came with the peel-and-stick back.

I first tried using a 2-part thermal epoxy and for whatever reason, it failed to set up properly and never cured.  I had some issues with their syringes and made a bit of a mess when dispensing it and may not have gotten the correct 4-to-1 ratio mix of the two parts.

I instead ended up using "Thermal Plaster" like this:
https://www.amazon.com/Thermal-Conductive-Heatsink-Adhesive-Compound/dp/B00KAOP4SY/

Though shop around there for it because that link is one tube for $5.  When I got mine, I found a seller that had 5 for about $8 and got it since it was the better deal.  I don't see it listed any more, but I do see one that's 10 tubes for $14.  I don't know what its shelf-life is though.  And do keep in mind that a very little bit goes a long, long way.  If the shelf-life is good, I may never make it off the first tube.

The thermal plaster worked well, but before it came, I actually stole some heatsinks out of several A4988 boards I had on-hand (also Chinese knock-offs).  They had the peel-and-stick back.  I'm working on several different projects that need stepper drivers so I have like 20 of the A4988 boards here and about 15 of the DRV8825 boards...

If you already have the driver boards and no heatsinks, search for "GPU RAM heatsink" or "video RAM heatsink" on Amazon and you'll easily find a pack of 10-20 heatsinks that are about the right size.  You can open the electronics bay cover on your BoXZY and see the heatsink on the original A4988 boards if you want to measure and get dimensions.

Most of the cooling actually comes from the pads themselves, so be sure to look for driver boards with 4-layer 2oz or better copper with a nice heatsink cooling pad area for the driver chip.  Mounting the heatsink to the top of the chip is better than nothing, but the ideal place would be the underneath part on the copper pad that's on the board itself.  In fact, Pololu even makes a version of the board that's inverted so that the parts are on the bottom and the heatsink pad is on the top so that it can be mounted there.

Unfortunately, the capacitors and other components are right in the way on the Ultimaker board for either the inverted board or putting a heatsink on the bottom to work (plus the limited airflow around them in the BoXZY if they were to be on the bottom).  So putting it on top of the chip is about as good as it gets.  Plus the chip claims to be able to go to like 2amps without having a heatsink if the PCB pad area is sufficient.  So it shouldn't be too critical.

The most important things to remember is 1) never install or remove the driver boards with the power supply on (or even with the USB cable connected to the computer), and 2) make certain you don't install the driver boards in backwards, as doing so will put the +19v motor voltage on the wrong pin and will fry the board and maybe even damage the Ultimaker board too...  Don't go by pictures or part location.  The pins on the driver board are labeled on the silkscreen (usually on the bottom side) and the "en" pin is labeled on the Ultimaker silkscreen.  Make sure they are lined up.  And the extruder driver side is turned different from the X/Y/Z axis driver side.


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dewhisna

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Reply with quote  #37 
Well, the new DRV8825's are a definite improvement.  They are more quiet and run cooler.  And they let your BoXZY power right through many "linear bearings are failing" scenarios.  But most likely, those of you experiencing this step skipping issue are actually having linear bearing issues.

Last night, I was attempting to print the mounts for the laser head that JohnB posted (https://boxzyunofficial.forumchatter.com/post/alternate-boxzy-laser-power-system-8178082) and with less than 10 minutes left of printing, the Y-axis linear bearing completely seized.  After a loud "bang" when it finally released, it continued printing but was shifted about the most I've ever seen and on the Y-axis (most of my previous shifts have been on the X-axis).

So while I still stand behind and highly suggest the DRV8825 mod, I will warn you that there's a good chance you are having bearing or other drive system issues going on too...  My Y-axis linear bearing has even worn a flat spot on the front guide rod (on the underside near the middle).  I've got some replacement bearings, but need to order some rods so I can swap them out too -- no need putting good bearings on bad rods.

Here's a few pics of last night's skip...  And SO frustrating because the part was nearly done printing!!  Grrrr....


2016-07-17 02.19.23.jpg 

2016-07-17 02.20.04.jpg

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dewhisna

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Reply with quote  #38 
I've started a new topic, but it's related to this problem.  I think our problems might potentially be power supply related:

https://boxzyunofficial.forumchatter.com/post/pushing-power-supply-limits-overloading-supply-8187295

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gregket1

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Reply with quote  #39 
Ok, I'm confused. From reading the threads it seems like the different drivers should be set for different step sizes but when I took mine apart to put in the drv8825 boards all four of mine were originally set for 1/16-step (aka all traces intact and all jumpers on). What are they supposed to be set to? [image] 

Attached Images
jpeg image.jpeg (863.87 KB, 197 views)

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dewhisna

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Reply with quote  #40 
Unless you've modified your Ultimaker Board, the jumpers on the Ultimaker Board are just for looks.  This particular version of the Ultimaker Board has actual traces on the bottom of the board where each jumper is at, essentially keeping them "on" even if you remove them.  This is why the original A4988 boards have some of the pins cut on them (look carefully at the bottom of your A4988 boards and the pins coming out of them).

You have two choice -- you can either cut the pins on the DRV8825 boards or modify your Ultimaker board, by removing unnecessary traces between the jumpers to make the jumpers functional, and set the jumpers correctly.

For the DRV8825 boards, either cut the following pins on your DRV8825 boards, or if you've cut the traces on the bottom of the Ultimaker Board, remove the following jumpers:

X and Y axis boards : Cut MS1 and MS3 to select 1/4th steps
Z axis board : Cut MS3 to select 1/8th steps
Extruder board: Cut MS1 and MS2 to select 1/16th steps

These are all the same as the A4988 boards except for the Extruder driver.  The A4988 boards use all jumpers on (no pins cut) to select 1/16th steps.  The reason is that the A4988 doesn't support 1/32nd steps and the DRV8255 does, so the 1/16th selection is different between the two boards.

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gregket1

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Reply with quote  #41 
Thank you, Thank you, Thank you, Donna!!

For those making this modification in the future, be careful cutting the traces on the back because on the Y-axis there is a 4th trace right next to the traces you need to cut.  Mine now has a patch wire.

But, after your rear-traces are cut so your jumpers take precedence, here is what your board should look like:

IMG_1786.jpg 

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dewhisna

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Reply with quote  #42 
Well, I now think I know why the DRV8825 boards improved things for me, regarding the missing steps issue, but didn't completely solve it...  I'm posting this followup here to complete this thread to benefit readers that may not be following the other threads, but I don't want to cross-post the same thing on multiple threads.  So for the details, please read my posting regarding my stepper motor research and how it relates to the BoXZY's power supply here:

https://boxzyunofficial.forumchatter.com/post/pushing-power-supply-limits-overloading-supply-8187295?trail=15#15

and the BoXZY's Stepper Motor Specs here:
https://boxzyunofficial.forumchatter.com/post/boxzy-stepper-motor-specs-8253042?trail=15#1

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