I made a sand drawing robot with 50 miniature servos and a two half tanks, I use an Arduino Mega 2560 to read from an sd card a text file with the desired image and send the servo position data to another two Arduino Mega that then send the adequate signals to the servos. 5 DC to DC converters connected to a 6S LiPo battery provide enough power to support the stalling current of all the 50 servos at once. Two half tanks provide enough grip and torque to drag all the servos through the sand. The entire thing weighs around 14 Kg in working order (with the batteries on) so I had to build a tailor made trailer to cary the thing as there's quite a walking distance to the test field.
I made another version a couple of years ago that can be seen here:
youtube.com/watch?v=7T1esQgRwrM
This new version draws way faster than the previous one but the older one had way more contrast, it is time to think on an MkIII