Got a bit further with the etch a sketch at the last WIP night at Foulab.  There are at least two components to the backlash problem. One - a curable one - was that the way I was using the AccelStepper and AFMotor libraries was not ideal.  I was releasing the motor immediately after finishing the stepping.  This caused the last step to be left incomplete, at least some of the time.  Introducing a 500ms delay before releasing the motor cleared this up.

This was detectable by writing a very simple program that took just a few steps (in this case 3) very slowly.  Then I could count the steps by eye, and see that some were missed.

That cleared up a lot of the irregular wobbling.  However, there is real backlash in the etch a sketch mechanism. When the direction of movement is changed, there are nearly 10 steps where there is basically no movement, and another 5 or so where the movement is proportionally reduced.   It would appear that this is due to stretch inside the etch a sketch itself, not in the gearing.  It persists even if the other axis is moved in between the forward and reverse movements.

You can see this in the picture - each step in the zigzag is ostensibly the same number of steps.  The squished ends are due to this backlash.

Just to be clear, i moved the cursor down and left to the bottom corner of the drawing shown, then shook the sketch.  Then the motors were moved in a zigzag to the right, then the reverse pattern.  The squishing at the ends is due to the backlash - note that the backlash in each direction is almost perfectly equal, meaning that the ends line up.  Gives hope for correcting it.

Backlash complications.

Did a little work on the etch a sketch device at Work in Progress night at Foulab yesterday.  Nothing amazing to report, but was able to observe more clearly the strange backlash problems that it has.   The following pic shows a square wave of 100 steps up, and 50 steps over, followed by one back moving -100 steps in y and -50 in x each time.  It should produce a nice level series of even rectangles.   Obviously it does not - theres a lot of drift.  At least some of the drift seems to be hysteresis in the movement, and it may also be skipping a step occasionally, especially the vertical axis.

Well, such is the nature of work in progress.

Bracket for Prusa

Nearly a year since the last post.  Oops.  It was a busy year.  Among other things, built a 3D printer, something that will probably feature in future posts on this blog.  Its a Prusa Mendel I3 - built it at
Voxel Factory / Foulab Prusa Mendel Build Party


Whats the first thing everyone prints on a 3D printer?  Parts for the 3D printer of course.  I am no exception. First non-calibration print - a bracket to hold the LCD screen on.

Files on thingiverse: http://www.thingiverse.com/thing:62091

Alright, its pretty minor, but I have to ease back into this blog thing.