A whiteboard plotter is a popular project that has been done may times and in many different ways. Basically, two motors are used to move a marker across the surface of a whiteboard. I plan to start with some simple goals and incrementally add features as I gain a better understanding of the existing mechanism.
The idea came to me on 9 February 2017. I had recently acquired a 36″ × 24″ whiteboard to use as a visual aid for a class I am teaching. I thought a “simple” motion control project like this one would be a nice illustration of several design techniques that I wanted to show to the students. I spent about a week kind of day-dreaming about it and rounding up some various bits to get started. I started to document the project on Tuesday 14 February 2017, also celebrated here in the USA as Valentine’s Day.
The first pass will use the 36″ × 24″ whiteboard I am using for my class. The goal is to not make any sort of changes or modifications to the board and tripod stand themselves, so that they can go on being useful in their original state, even once my interest in this project has waned, as it inevitably will.
I obtained two NEMA 17 sized, bipolar stepper motors from BG Micro in Garland TX (part number MOT1100). These motors are probably overkill for this project, but they have a nice, square output shaft that I suspect will be simple to interface to a 3D-printed take-up spool. Also, it’s easier to make adapters and mounting flanges for standardized components, such as the NEMA 17 motor specification.
To drive the motors, I will first use a pair of L298-based motor control boards that I bought from China at some point in the past. The ones I have bear a strong but not identical resemblance to the one explained in the very-well-written Instructable by tronixlabs. Eventually I will replace these drivers with “proper” stepper motor controllers such as the RepRap A4988 used on the Arduino CNC Shield. I have ordered a couple of these shields (because yes, I plan on blowing up at least one of them at some point) from China but I’m not going to hold my breath waiting for them to arrive. Like the motors themselves, these driver boards are a bit of overkill for this project, but here they are. I also like that they will most likely drop the 12V supply to the nominal 10.4V or below needed for the motor coils, due to their source and sink saturation voltages. The current-carrying capacity is about 10× what is needed, which is in accordance with my preferred safety margins.
At the outset, it doesn’t look like there will be a need for a large number of I/O lines to control this plotter. I can see four output lines per stepper motor, with possibly another PWM line to control power output, for a total of 10 output lines. There will at some point be limit switches but I haven’t completely puzzled that. A standard Arduino Uno should do the trick. An Arduino Mega seems a bit too much, but I can keep that as a fall-back position in case I dream up some more interesting features that require more I/O lines.
Before I can wire up both motors, I need to wire up one motor and get the hang of spinning it around and around. I can use my bench power supply initially, then migrate to a 12V regulated power brick. I have lots of jumper wires here in the lab for just such lash-ups. This will be Experiment Number One.
Experiment Number One – Arduino Spins Stepper Motor
To be continued…