Strange Z axis behavior
November 06, 2011 02:00AM

Im getting weird stuff going on with my Z axis which might be two (unrelated?) problems.

When I use pronterface to go up or down by increments of 10 it will sometimes go down by a lot, other times it goes down by only one or two millimetres. Any idea what may be causing something like this?

Secondly, when I go and click print, the extruder head raises up by some strange amount and starts printing into thin air - to amusing and frustrating results. How is home determined in pronterface? Why is the first thing it does raise the head?

Thanks in advance!
Re: Strange Z axis behavior
November 06, 2011 12:55PM
It sounds as if your z-axis is missing steps. Can you give it a bit more current by turning the pololu/stepstick potentiomenter 90 degres clockwise? Is that helping?

Regarding your second question: Are pronterface home buttons working as expected on each axis?

Re: Strange Z axis behavior
November 06, 2011 11:14PM
Hello everyone I am new to this forum. Please advise me.



Edited 1 time(s). Last edit at 11/11/2011 10:38AM by xsnoopx.
Re: Strange Z axis behavior
November 07, 2011 01:35AM

So, I solved the second half. RAMPs default home is whatever it boots up as. So I needed to set the extruder to the correct home then reboot the RAMPs.

I'm having major problems with my motors though. My motor specs are below. If I turn the pololu too far clockwise, it starts jumping and ticking out loud. If its too far counterclockwise, it just doesnt move. Either way, it seems to move in random amounts, not the amount specified. Ive worked out that its a problem in all axis. I have also tried it without the heated bed on, so should be plenty of current... Getting really nervous/desperate now, I hope after all this effort I can get it working!

Motor Specifications:
Nema size 17 (42x42x34)
5mm shaft
15mm flat section

Step angle 1.8deg (200 steps per revolution)
Step angle accuracy +-5%
Temperature rise 80 deg C
Ambient temperature range -10 to +50 deg C
Insulation resistance 100M ohms min, 500 VDC
Dielectric strength 500VAC for 1 min
Radial play 0.02mm max (450g load)
End play 0.08mm max (450g load)

Rated voltage 5.0V
Rated current/phase 0.9A
Winding resistance/phase 6.3 ohms
Holding torque (estimated) 1500 g-cm (0.147 Nm)
Inductance 10.0 mH
Rotor inertia 40 g-cm^3
Detent torque 100 g-cm
Weight 220g
Re: Strange Z axis behavior
November 07, 2011 04:38AM
When you have the current too high for the Pololu it will overheat and shut down, then it cools and restarts. That is the jumping and ticking. You need heatsinks and / or a fan to keep the Pololu's cool if you want to run that much current, but your motors are only rated for 0.9A and you should be able to do that without heatsinks. Set the Vref (measured at the wiper of the pot) to 0.36V to get 0.9A.

Your motors are only about 1/3 of the torque we normally use though, so they may get too hot when run at their maximum current and you may struggle for torque on the extruder.

Re: Strange Z axis behavior
November 07, 2011 07:16AM
Are you sure your steppers are connected right? If so, are you sure the Pololu driving it is not faulty? (You can exchange it for the one driving other axis that are known to work ok).

I have had one defective Stepstick that exhibit an erratic behaviour. It just didn't work ok, but it made the motor kind of rattle and move but not smoothly.

Re: Strange Z axis behavior
November 07, 2011 03:00PM
I'm a little unsure about how to set the Vref to 0.36V. Is this simply adjusting the trimpot?

It sounds like the Pololu is doing the shutdown/restart thing, athough I felt the driver chip and it did not seem hot. Is it possible it is cutting out for some reason other than heat?

The X axis motor certainly gets too hot to touch during operation. Does this mean that these motors will be unsuitable? I dont think its related to the problem though, as the Y axis is relatively cool.

I am using a wades extruder with a motor with more torque,so hopefully that one will be ok.
Re: Strange Z axis behavior
November 07, 2011 04:25PM
You set vref by measuring the voltage on the wiper of the pot. I.e. put one multimeter probe on the wiper and the other on ground. There is also a test point nearby on genuine Pololus.

Note stepsicks have a different Vref values than genuine Pololus.

I can't think of why it would shut down and restart other than when too hot, assuming all the connections are good. If it steps occasionally when it should be stationary it could be because of noise due to a bad ground, but that should not be the case with a RAMPs shield.

Motors current ratings are for a temperature rise of 80C with both coils on 100%. When you set vref to .36v that is actually the one coil on peak current so the temperature rise should be half that so they should run at 60C. OK for ABS brackets, but not PLA.

I don't know if you will have enough torque. I have only ever used 0.42Nm rated for 1.65A but under run with 1A peak. They have plenty of torque for the axes and run cool.

Re: Strange Z axis behavior
November 08, 2011 07:38AM
I remember that when my motors were connected backwards that they would act much the same as you describe. Can you try flipping the motor connections over to see what happens?

Re: Strange Z axis behavior
November 08, 2011 02:39PM
I'm not sure what you mean by "backwards". They are connected such that they move in the correct direction. As far as I'm aware, there is no polarity or difference in plugging in coils in either order - in fact the Mendel instructions say to swap the wires around if it moves in the wrong way. Could someone with motor skills confirm this?
Re: Strange Z axis behavior
November 08, 2011 03:08PM
I think this is the case with RAMPS. You flip the connection and you invert the motor direction. Maybe other electronics have a different pinout with a different result.

Re: Strange Z axis behavior
November 18, 2011 06:46PM
So, I have just brought 4 new motors and 1 new Pololu from lulzbot and... the exact same problem. Clicking in Pronterface "X -100" will sometimes move it 1mm sometimes the full amount. With a new motor and new Pololu. So logically, it must either be a faulty ramps or a faulty version of Sprinter firmware? I wish there was an easier way to debug this problem - its getting impractical to just throw away $130 on a hunch. Please god there must be someone who knows what this problem is?!?
Re: Strange Z axis behavior
November 19, 2011 04:50AM
Have you tried the test code in the Pre-flight section of the wiki? it moves each motor backwards and forward.

Re: Strange Z axis behavior
November 19, 2011 02:04PM
Solved my problem after talking to the awesome dudes on irc. It was crosstalk causing false triggering on endstops. My suggestion - update the buiild instructions so it doesnt have all the wires being grouped together with spiral wrap.
Re: Strange Z axis behavior
November 24, 2011 02:24AM
Hi Blob,

I am having some irradiate axis issues too. Are you saying that the end stop wires were too close (an EMI/noise issues)? Are you using RAMPS and if so what version? Also what were the critical points they needed to be separated at since they come together (on RAMPS) at the board.

Re: Strange Z axis behavior
November 24, 2011 01:33PM
I use RAMPS 1.4. Basically the end stop wires cannot be tied (using spiral wrap or whatever) to the motor cables. I found once I separated them out so they just dangled loosely about 2 - 3 inches apart from the motor cables then all of my (endstop related) problems just went away.

Of course, that just left the *other* problems. The RepRap is a project consisting of nuts, bolts, and problems.
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