Omni robots use wheels with rollers positioned at 90 degrees to the main axis of rotation to allow them to slide forward, backward, left, and right. Given this ability, “lingib” realized that if the controls were setup correctly — and a actuated pen holder was added — one of these robots could make a sort of free-form plotting device with a theoretically unlimited drawing area.
What he came up with uses four 28BYJ-48 stepper motors along with ULN2003 drivers to move the robot about, along with an Arduino Uno and HC-06 Bluetooth module for control. To plot with the device, the user connects via a tablet or android smartphone, which sends it commands as described in this separate write-up. In order to save I/O, two sets of opposite steppers are wired together, enabling sliding motion in the X/Y plane, but no rotation, unnecessary in this application.
As seen in the video, the robot is able to plot straight and curved lines, the letters “ABC,” and presumably anything else you can think of. Accuracy is reportedly quite good provided the wheels don’t slip.
To combat potential misalignment or uneven ground — thus slippage — the robot is equipped with a central pivot point that lets one set of two wheels rotate with respect to the other. Having made an omni-bot myself, I know that this can be a real problem, This seemingly small detail could be the most important point to note out of the whole build if you want to create an omni-chassis yourself!