Mendel Mono Strange Motor Behavior, Won't Home

printer: Mendel Mono kit-build from RepRapPro.com
software used: Pronterface & Slic3r
description of problem:
I have successfully printed about a half dozen objects, mostly octopi, so far with satisfactory results, until now - when I send the HOME ALL command, I get some very strange behavior from the Mendel. Sometimes, it works ok, but for the most part, my steppers will stutter and whine - the z motors will do a turn, pause, turn, pause, almost bouncing all the way down to close to the endstop, if not all the way to the end stop. Sometimes the X-axis will head PAST its home and grind like crazy. I have run a M119 check and can confirm that all three mechanical end stop switches are functioning, I get an "H" when they are depressed and "L" when they are open. I have managed to run a small print successfully by simply setting the nozzle at the lower left corner of the printbed before initializing and connecting with pronterface - when I did that it seemed to think that it was homed and ready to go. (The print was small so it didn't have to be oriented perfectly to fit on the bed.)

For an example of the servo motor whining/"bouncing" issue, please check out this quick video: http://www.youtube.com/watch?v=COn9vs93c8g

The device just doesn't seem to know where it is - the GET POS button gives me all sorts of strange numbers that don't seem right to me, for instance, the x-axis will be posistioned over the edge of the plate and give me a value of 0 (offset to home is X-50), but then I can move the carriage with the directional tool in pronterface only 1 mm left and get an x-value of -70, which just doesn't seem right to me.

Also weird behavior - once or twice I have tried to send a command to increase the Z height and the machine has moved up and to the right (X positive) at the same time and I don't think I hit HOME ALL, though maybe I did.


I have tensioned the belts, made sure I was aligned and level, and just about everything else I could think of to get back to where I was but no luck. I looked into the post regarding the false triggering of an endstop, but I don't have a oscilloscope so I can't do all the troubleshooting they suggest. Has anyone there any experience with this type of error that might be able to help me out please? I would like to get back to octopus production as soon as possible.

Sounds like your motor current adjustment is too high. On my kit, the polu drivers all came with the adjustment pot at the half way point. My motors behaved like that the first time I tried to move them.

What I did, was rotate all of the drivers pots to minimum, all the way to the left. Then I rotated them up about 1/8th a turn as a starting point. Most of the motors moved well at that setting initially. over time, I had to tweak the Z and Y motors up a bit due to missed steps during prints. I tweaked them in very tiny amounts until I had smooth operation. It's a sensitive adjustment.

Unplugged everything and tried backing off on the trimpots, but that didn't seem to fix anything on restart. I have noticed a new strange behavior though, not sure if it was happening before or not. Sometimes in Pronterface when I send a command like X+10 the Y axis moves! That is not right! Then repeat the command and the X axis will move. I checked before and after POSITION numbers and even when the Y axis moves the machine only tracks a change in the X position.

Methinks my Melzi controller is shot. Anyone else concur? I mean, the Y axis should move when the Y axis is told to move, not when a command is sent to the X axis motor, that's just silliness. Update: Also, my extruder server is shuddering when she operates now, both fwd and reverse.

UPDATE: Other fixes I have tried:
I went through and checked all my measurements to ensure I was all lined up, square, level, etc. All OK within a nose hair +/- tolerance. I hit them with a little green threadlocker as I noticed some had loosened more than I liked.
I twisted the mechanical endstop wire pairs to maybe cut down on noise.
I checked all my motor lead connections to make sure they were snug biting the tinned wires nice-nice.
I disconnected my belts to make sure movement of the bearings on the x & y threaded rods was smooth, no problems.
I adjusted my belts, they are snug but not overtights, I've had previous success this tension.
Made sure my Z carriage was level and plum and my spinny rods were level and lubricated lightly with some actuator lube.

Same dumb issues, no attempts to print since the machine won't even home through Pronterface yet

Edited 2 time(s). Last edit at 09/01/2013 04:10PM by junkelsplitsy.

I watched your video. I don't think the driver electronics are fried on your controller, but there's probably something wrong with the micro controller. Either CPU, or the interface between the CPU and the analogue electronics. I'm not familiar with the board you mention, I run a RAMPS1.4 which has all of the analogue electronics on a separate board that plugs into the top of the arduino board.

At the least, you might try reloading firmware onto your board, maybe it became corrupted. If your board consists of two boards that are mated with a series of header pins, get a magnifying glass and inspect the solder joints for all of the pins. Make sure you don't have a bad ground connection somewhere causing floating ground issues.

But from the video, it looks like the drivers are driving the motors fine, they're just getting mixed up signaling from the CPU.

Ok, just went and looked at the Melzi board. Kinda neat that it's an all in one design like that. Assuming that it came with heatsinks for the stepper drivers.

Yeah, try reloading your firmware. Maybe the code on the micro-controller got corrupted.