The stepper motors in the sand table were always operating right at the limits of their performance and I had to run many different patterns and tweak the acceleration and jerk settings to ensure reliable operation, and even with all that testing, every once in a while I found a pattern that just pushed the motors over the edge and stalled them out making horrible noise and ruining the pattern drawn in the sand.  Also, the 3D printer pulleys with tiny bearings were squeaking a lot.

I decided to try using servomotors in place of the steppers.  Servomotors have what seems to be low torque specs, but the torque is constant up to a few thousand rpm, which sounds perfect for a high speed sand table.  The servomotors also turn much smoother than steppers so there is liable to be less motor noise and vibration. 

I ordered an expansion board for the Duet controller ($30) and a pair of 75W, iHSV42-40-07-24 NEMA-17 servomotors ($180) from China. and went to work on the table.  I redesigned the motor mounts, corner pulley blocks, and Y axis bearing blocks replacing the tiny-bearing 3D printer pulleys with stacked F625 bearings.  After a few tests I ended up using two 24V power supplies in the system- one rated for 150W connected to one motor and the other rated for 200W powering the other motor, the controller board, and the LED lighting.

The motors I used are spec'd for constant 0.185 Nm torque up to 3000 rpm. I used 16 tooth drive pulleys so achieving the 500 mm/sec I used to use with the steppers requires spinning the servomotors at 957 rpm- well below their 3000 rpm spec.  

I did some speed and acceleration tests using the bare mechanism without the sandbox and was impressed with the results.  

The Spice Must Flow gets servomotors! from Mark Rehorst on Vimeo.

Next , I added a Blue LED in place of the magnet and tried some light painting.

After playing with that for a while I decided it was time to draw some patterns in sand.  I put the sandbox back on the table and started searching for the speed and acceleration limits.  I haven't found them yet!  The video below is not sped up- it is real-time performance of the machine.

The fastest sand table in the known universe! from Mark Rehorst on Vimeo.

So far, the table can draw sand patterns at acceleration of 20,000 mm/sec^2 and 1500 mm/sec.  At more reasonable speeds and accelerations there are no longer any worries about the motors stalling out.  Also at the same speed and acceleration I used with the steppers, the servomotors are quieter.  At super high speeds and accelerations (and jerk speed of 200 mm/sec) the patterns lose some detail because the ball throws the sand around, but it's nice to know there's plenty of margin in the design.

I'll be making a blog post soon with more details, more photos, and more video.

I'm planning a total rebuild in a slightly smaller size to make transporting it easier, using a glass top, putting the top higher above the sand so that thrown sand is less likely to end up stuck to the glass, and hopefully, a more attractive appearance so it can be used like furniture.

In the future I may try mounting an airbrush on the magnet carriage and use it to paint patterns on paper, walls, or floors.