garageFab.cc

To investigate the plotter stalling problems that were encountered during recent tests, I had ordered Keling stepper motors - one of the official choices of the Reprap project. They are rated for 166 oz*in of holding torque (vs 60 oz*in in Alltronics). They also cost about as much more. When the Keling steppers arrived, I assembled the plotter again. 

For some reason, the maximum feedrate that Keling steppers gave without stalling was 300 IPM, and that was on single-axis movements (like "G1 X10" or "G1 Y6"). Alltronics steppers could do about 600 IPM on single-axis movements in previous tests. I thought this was strange and decided to install Alltronics back to compare.

The shaft on Alltronics is 1/2" longer than the shaft on Keling stepper. I did not want to move the rods, so I just put the motor on ~1/2" standoffs (which consisted of three 3/16" nylon spacers, see pics). 

Except for the motors and the way they were mounted, everything else remained the same. I was in for a surprise when the test pattern was executed cleanly at 400 IPM. For some reason, Alltronics steppers gave better performance, at least by this metric. I ran another, longer test pattern, and it was also performed fine. See the video below:

I did not see stalling on either Keling or Alltronics steppers (except on feedrates over 300/400 IPM respectively); nor did I see the pauses - which were seen in the test a couple of weeks back. The differences between the tests are as follows:

2 weeks ago

1. Teflon sliding elements on Y axis
2. Steel screws on shaft mounts
3. Alltronics steppers mounted directly to frame
4. SMD v1.2 

these tests

1. UHMWPE sliding elements on Y axis
2. Nylon screws on shaft mounts
3. Keling steppers mounted directly to frame / Alltronics steppers mounted on standoffs
4. SMD v1.1 

I don't think #1 and #2 have anything to do with it. #3 is a possibility - I think plastic standoffs dampen the vibrations somewhat. I had not actually used SMD v1.2 before, so it may be a culprit. The schematic for v1.2 slightly differs from v1.1 - the master/slave circuit in v1.2 is hardwired, and R4/R5 from current sensing circuit are not needed according to v1.2 board build instructions.

Later, I soldered couple more SMDv1.2 boards, and tested them with the stepper exerciser routine. While one board worked OK, the other one displayed pausing/stalling behavior very similar to what I saw during plotter tests 2 weeks ago. So at this point I'm pretty sure it's the board. I'm going to see if master/slave jumper on SMDv1.1 or soldering R4,5 on SMDv1.2 will change anything, so more on this later..