Very jerky stepper movement

Hi all,

My Printer:
'Official' Prusa i3
Mini-Rambo
E3D V6 hotend
Running Octoprint on a Pi

You've solved many of my problems without me having to start a topic but I need a little advice with my latest issue. I bought a Prusa i3 about 3 weeks ago and have upgraded the heated bed to aluminium. I've been getting better and better prints out of it and I've learnt loads with every issue I run into. But last night...

Halfway through printing, my Y axis went crazy; it had no idea where it was and was making a very loud vibrating noise (much, much louder than skipping teeth) and my aluminium bed was making a hell of noise; presumably vibrating against the Prusa Y carriage mount or the subsequent linear bearings. The noise persists when jogging around manually so I assumed I had a catastrophic mechanical issue. I unhooked the belt under the plate and pushed the carriage by hand: almost perfect, just a tiny left to right twisting motion across the X axis.

So then I assumed it was the stepper. I dismounted it ready to take it down for a like for like replacement. But when I turned it by hand it felt fine, like every other stepper I've had. No mechanical issues at least. So, just as a last gasp check, I swapped the steppers for the X and Y and lo and behold, the strange noise and awful jerky movement was now across the X axis.

So, I'm now convinced the following are fine:
* Y and X railings/mechanics
* Both stepper motors
* No belt teeth damage

I thought that since I'd been tinkering in the Marlin config, I'd reflash it with the default load-out supplied with my printer. I didn't expect this to work since I've never touched the stepper current digi-pots and sure enough, it changed nothing.I did up the current limits with M907 but it just whines more when holding.

So now I can add the Marlin firmware to the list of things I know not to be the issue.

That only leaves my Mini-Rambo, no? The X and Y ref test points seemed to be shared so can't be useful in debugging, what can I do since the Allegro chips are tiny surface mount jobs?

Has anyone seen something similar before? The driver obviously isn;t broken altogether as it does attempt the drive in the right directions, but something is clearly very wrong and it MUST be the chips unless I'm going mad. Which is possible...

I can make a video of it in 'action' but will have to wait until tomorrow for my neighbours' sake.
John

Probably the driver has failed. Or you have a bad connection at one end of the stepper motor cable.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks dc42, It must be the former as swapping the steppers transfers the issue. I feared it was the Rambo, has anyone had any success with replacing the driver chips? I'm confident of getting a new chip reflowed but haven't ever tried to get a knackered chip off the board. Any tips would be greatly appreciated, else I'll use the smallest tip I've got for my heat gun and see what happens. It's useless now, what damage can I do?

John

What about microstepping jumpers? Are there any on the MiniRumba?
Maybe they lost contact and now the stepper runs full-step mode?
-Olaf

Hi Olaf,

There is nothing to tweak on the (Mini)Rambo, no jumpers, nothing. See this photo. I doubt it's the micro-stepping but can't say for certain, I would expect much less noise and vibration though if the mode has somehow gone back to standard instead of 1/16th.

John

Before desoldering the driver, I would check the output pins for cold solder joints or broken traces.

With a hotair gun, you can easily blow away all the surrounding smd parts. Maybe you have better luck with heat from below?
-Olaf

Thanks for the tip. As I say, I haven't had much experience with desoldering SMD but my current plan is to use some gaffer tape to protect the surrounding parts. I am thinking about using a few layers of tape cut into about 20mmx20mm square with another square cutout (slightly larger than the chip). So the chip is the only thing exposed by the tape. My theory is that it should provide a basic heat shield and also serve to keep the surrounding components in position if the heat shield fails! I've been re-watching some Dave Jones videos on the EEVBlog about desoldering and I think my heat gun is too brutal for this job - I have heat control but no fan speed control.

I'll have a look tomorrow to see if I can get a well-priced heat station but since I've been doing electronics for 15 years and haven't needed one, it would have to be pretty cheap to outweigh just buying a RAMPS to replace the Rambo.

The 'heat from below' idea sounds like a good idea also. Thanks Olaf!

John

Before you try to remove the chip, check the cable - or perhaps you swapped the X and Y motor connections at the electronics end already, so you know the cable is OK. Also check the board for bad joints on the motor connector and on the driver chip pins.

Getting an old SMD chip off the board is easier in my experience than fitting the new one, because it doesn't matter if you overheat the old chip. The main thing to watch is that you do not lift any traces when the chip comes off. Use a small hot air blower of an appropriate size, make sure the solder is fully molten all the way round, and lift it off square. If you have an old hard disk drive lying around, practice on that.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Good advice. I have checked the cables by swapping the steppers at the board end but haven't checked for dry joints yet. I will do so before I attempt anything drastic.

I've used Pololu A4988 boards in various projects over the years and I've always heatsinked the chip. The Rambo has no heatsinks and there is no real airflow over the board. Could this be related? Obviously too late now if so, but it might prevent it happening again. I only got about 3 weeks use out of this board and Ultimachine's reputation is much better than that. So I'm assuming its a dodgy one off or the 'Official' Prusa i3 has cooling/airflow issues - see link. Is that fair?

John

Edited 1 time(s). Last edit at 12/13/2015 06:22AM by Jelby-John.

If the board is only 3 weeks old then I suggest you ask for a replacement under warranty.

These driver chips are designed to be cooled through the PCB, so all-in-one boards usually cool them much better than stepsticks do, because a stepstick doesn't have enough PCB area. Stick on heatsinks are ineffective because the plastic top of the chip is a poor heat conductor. If the chips get hot then I suggest you either turn the current down, or use a fan to blow air from the edge of the PCB so that air flows over both sides. If the electronics is mounted horizontally in a confined space with little convection, such as under the bed of a delta, then it is wise to use a fan anyway

The driver chips on all-in-one boards are usually very reliable, much more reliable than stepsticks according to a company I know that sells both. But you need to guard against bad connections in the motor cables, because an intermittent connection can damage them.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks David (just found out who you are!),

Given what you said about bad mounting position, the board on the Prusa is fine (just mounted vertically on the frame behind the X stepper). Good points about the heat dissipation and I hadn't really thought about the ineffectiveness of stick-on heatsinks before; makes sense. Bad connection along the cable is very unlikely given that it affects a stepper which doesn't move at all (and has a short, straight cable run). They were cable tied but not even tightly let alone overly tight. The motors don't have connectors on the stepper end so I can't just change cables unfortunately.

Warranty! Why didn't I think of that? Voiding warranties has been a long standing hobby of mine but I surely can't have done anything to void this one! I'll drop an email to PrusaResearch first but I imagine I'll have to contact Ultimachine directly. Whether or not I can resist trying to fix it myself is another matter...

John

Edited 1 time(s). Last edit at 12/13/2015 06:52AM by Jelby-John.

Quote
Jelby-John
Bad connection along the cable is very unlikely given that it affects a stepper which doesn't move at all (and has a short, straight cable run).

It doesn't matter whether the motor moves frequently or not: breaking the connection while a motor is energised is probably the most common way of damaging a driver. It is easy to get a bad crimp connection where the connector is crimped to the end of the cable, especially if it wasn't made with exactly the right crimp tool.

Edited 1 time(s). Last edit at 12/13/2015 01:21PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

David,

Fair points but that leaves me in a bind inasmuch as if I try swapping the X & Y over (as I have done before), if there is a bad connection along the Y stepper cable, I risk damaging the X driver to prove the theory. It's a good point though and since it's a possibility, I'd like to somehow find out if it's so because if I buy a new board, and the connections are at fault, I could do the same damage to my new board.

I've had my meter reading ohms across each pole at the plug end (thin probes) and wriggled everything a lot. I do get small variations but no shorts or OLs. The cables do not feel high quality at all, almost waxy on the outer insulation but the plug itself seems sound. I can see the crimps holding the cable nicely from the top and it looks fine, insulation stripped at right point and crimps interfacing the conductors.

So, I'm going to keep my fingers crossed that the stepper is not dodgy. I can't know for sure but unless you know of any others basic tests, I'll have to assume. The only issue that leaves me with is not knowing what caused the Y driver to fail but it could just be a dodgy chip I guess. I think I'll get myself a RAMPS whilst the warranty process goes on (going to build a dual extruder machine next year so the new board won;t go to waste).

Thanks for your input David, much appreciated. BTW your IR probe looks intriguing, I've recently fitted an inductive sensor to my i3 but I really like the look of what you've done there. I reckon that'll go nicely on my build next year.

John

John, it sounds to me that you have done the right things to eliminate the possibility of a bad cable or motor. So I guess it was either infant mortality of the chip, or the chips are running too hot and you need to reduce the current or cool the board. What do you have the current set to?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

David,

The current on the Mini-Rambo was just set to the default in the advanced config, so about 0.75A. I upped it to about 1A via M907 (value = 185) but it made no difference to the movement, just whined slightly more when holding position.

From the config:

// Motor Current setting (Only functional when motor driver current ref pins are connected to a digital trimpot on supported boards)
#define DIGIPOT_MOTOR_CURRENT {135,135,135,135,135} // Values 0-255 (RAMBO 135 = ~0.75A, 185 = ~1A)

I've been out this morning to see the friendly fellas at BcnDynamics and nearly found enough Spanish to get myself a RAMPS 1.4 with a Mega and 5 Pololu boards - "He roto mi rambo. El eje 'Y' es....(blows a raspberry)". So after 5 minutes, some pointing, english, accidental french and bad spanish, I got what I wanted. So this afternoon, I'm going to have fun configging that (my PRUSA was all pre-configured so I've not yet delved into Marlin as much as I'd like to).

I've just noticed some melting on the heated bed connector when I was disconnecting my Mini-Rambo. If I need help on that, I'll start a new topic as it's certainly not Stepper related! I don't believe in coincidences like these so this for me is evidence towards over-heating in general and linked to my Y axis driver failure. I'm going to print a little cabinet for my ramps and build in a fan and some decent airflow (maybe cable trunking surrounding the board to make the wiring nice and neat).

Many thanks for your input David (and Olaf).

John

Edited 1 time(s). Last edit at 12/14/2015 08:16AM by Jelby-John.

OK, so now you have the fun of working around the design flaws in RAMPS. I've posted about those elsewhere so I won't repeat them here.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Haha, I'll find them as I need them I'm sure. I've already drawn up a 6 MOSFET board to do the fans properly as that seems under-done on the RAMPS. Will build for 2 extruder fans, 2 cooling fans, 1 electronics fan and my LED strip lighting. For now though, I'm sticking with the single extruder so will have the extruder fan always on and use D9 for the cooling fan. I imagine I'll run into other issues but I'm about to start hooking it all up so I'm trying to think positively.

Thanks!

If you look at the driver datasheet, they are supposed to be cooled via thermal pad on the bottom. It is advised against to cool them with heatsink on top of the package, as it provides a lot of stress on the chip package and cause failed drivers.
RAMBo mini has thermal vias and 4 layer pcb to cool it down the proper way with the whole PCB.

The current for rambo mini is set in pins.h, it is not set with digipot but thru PWM.

If the connector for your heatbed starts to melt, it is very likely caused by making the terminal on the connector not tight enough or not pluging it completely in. Once it starts it is best to solder it on the back of the board as the connector oxidises inside and tightening won't help anymore.

To your original problem, the driver is 99% fine, it is the motor connector or cable. Sometimes it gets broken during printer assembly when one of the four leads gets broken. Did you try switching the motors to see if the issue transfers?
We ship out 300+ printers a month and I haven't seen a blown driver on RAMBo. It is sad for me to see it replaced with fake RAMPS board :-(

I can't find any posts from you on official forum or even email requesting help. We sell replacements and even hold 2 year warranty so don't hesitate to contact us thru our official channels, we even have 24/7 live chat on our websites. It was a coincidence I found this topic ;-)

Jo Prusa

Wow Mr Prusa, I'm sorry I made you sad. Maybe I can make you feel better by telling you that your printer is great!

Firstly the melted heated bed connector. I don't feel entitled to come back to you directly because I have changed the bed to an aluminium one (and have had issues getting it up to ABS temperatures - can only get it to about 102 deg so far). I also don't consider my setup regular because the room temperature varies wildly and the air is foten dusty. I had been meaning to sort out a fan for the vertically-mounted electronics and a new enclosure to keep the dust out but I got distracted printing pretty vases. :| As for the aluminium bed, I am thinking about getting an SSR and a 24v supply but it seems like overkill. Maybe making an enclosure? I'm in no rush to decide.

You won't find any posts on your forum from me, I haven't yet decided on what to do about the Mini-Rambo. Let me get this straight - I am not disappointed or complaining. Money aside, I love issues like this because I learn a lot. I was going to buy a RAMPS anyway because no matter how nice the RAMBO is, I'm a long-time Arduino fan and the RAMPS feels more maker-y, less consumer. And I intend to build a machine next year. Ideally, I'd have liked a proper RAMPS board but I wanted it today. I like official stuff where possible, hence getting your printer.

However, I have to disagree with your assertion that the stepper, cable or connector is at fault. I don't see how swapping the X & Y connectors on the Rambo would transfer the fault to the other axis. i.e. both X & Y steppers acted in the same way when driven by the Y axis A4988. I'd love to think it was fine but I just can't see how it could be. Either way, I have had 3 weeks great printing out of it so it was not a shipping fault and it arrived here in working order. I'm not looking to blame anyone but if I had to point fingers, I guess it would be at the A4988 manufacturer or the PCB assemblers.

I just want to get printing again as soon as possible. I'm about to delve into the steps per mm config for the RAMPS setup and intend to use your calculator (I haven't had a reason to yet). Sorry to make you sad Jo, please cheer up! For the record, the RAMPS is made by 'big-tree-tech.com'. I know nothing about these guys but I just hope the board works OK. I'll find out in the next few hours!

Thanks to all (and a pleasure to get such high profile assistance, I'm loving this community and that's rare for me).

John

P.S. I apologise if my posts questioned your printer's cooling. As my earlier post states, the vertical mounting combined with dc42's info about the thermal vias and dissipation put my mind at rest on that front. Another reason why you haven't heard from me on your site, I don't consider it your fault.

I'm having a similar issue with my Mini-Rambo board as well. Mine is a homebuilt Lulzbot Mini.

Anyway, the extruder was working fine, except today when i had a failed print.but not as expected. It seems the extruder gear was not moving well and had a kind of jerky motion to it so it was not extruding correctly. I figured it was probably the little gear that was loose and i think it was a little loose, but that did not fix the problem.

After i shutdown my machine and repowered it to test again the motor now does not have the jerky motion, but it makes a strange higher pitch noise that i have never heard before (almost sounds like the motor is broken), but the gear moves, although i suspect it's not moving as far as it should. Like perhaps not enough current is getting to the motor.

I tried switching the cable to the x-axis driver and "moving my x-axis" to test the extruder and the extruder works fine and actually sounds different. Considering the motor works fine on a different driver and there is an audible difference i am assuming my extruder driver on my Mini Rambo board is in the process of going bad and probably at this point unusable. My Mini Rambo does not have heat sinks on the stepper drivers, but i was seriously considering buying some the other day. Did it just get too hot and fry itself?

Thoughts? Is it time for a new Mini Rambo board? Is there anyone who does Mini Rambo repair work that would be cheaper than buying a whole new board? I don't have the tools or the expertise to replace a surface mount driver chip, but that's probably all that this board really needs to be functional again. I hate to just throw an 80% working board in the junk pile.

I may attempt to reflow and replace the driver if i can reasonably think i can pull it off, but i probably would be willing to ship the board off to someone else if they were offering such a repair service.