Bono: a robot that draws on walls

Last week at Quark, on the library lawns, a whiteboard drew the Batman logo by itself while a crowd ate breakfast around it. The whiteboard had help: two stepper motors, two cords, a pen in a 3D printed carriage, and a laptop. The machine is called Bono, it is our take on the polargraph, a wall-drawing robot, and this post is how it works and what it taught us.

Two motors and gravity make three axes

A polargraph has no rails. Two motors sit at the top corners of the board, each winding a cord, and the pen carriage, the gondola, hangs where the two cords meet. That is the entire mechanism, and the geometry does the rest: the pair of cord lengths fixes the gondola’s position the way two compass arcs fix a point, so the machine’s native coordinates are cord lengths, and Cartesian coordinates exist only in software.

That makes the inverse kinematics the whole game. Each cord length is the hypotenuse of a triangle between its motor and the pen, so moving in a straight line requires both motors to follow continuously changing, nonlinear speed profiles. The Makelangelo firmware handles it the way all plotter firmware does, by chopping every line into short segments and re-solving the triangles at each one. A straight line on the board is a thousand small lies agreed upon by two motors.

Gravity is the third actuator and it works for free. It keeps both cords taut, because a motor can wind a cord but nothing can push one, and it pulls the gondola down into the V where the geometry wants it. It also swings the gondola like a pendulum the moment the motors accelerate harder than the pendulum’s natural frequency forgives, which is why the acceleration setting matters more than any other number in the firmware.

Bottles again

The cords carry counterweights, and ours are water bottles, which regular readers will recognize as this team’s answer to most of engineering. The bottles keep the slack side of each cord under tension so the printed ball-chain pulleys cannot skip, and the water level is a tuning knob: top up for coarse changes, sip for fine ones. Half full worked best, and deciding that took an afternoon we do not regret.

The workspace has a shape nobody advertises. Near the top corners one cord goes nearly horizontal, and a horizontal cord needs enormous tension to hold weight, so the gondola wobbles. Near the bottom the V gets wide and shallow and position resolution degrades. The honest drawing area is the middle band of the board, roughly the middle sixty percent, and every image we plotted got placed there.

The electronics grew up mid-project

The expense diary tells the story better than memory does. The first driver purchase is an L293D shield, the second, eleven days later, is a RAMPS 1.4 with proper stepper drivers, and the upgrade bought two things: current limiting that stopped the motors cooking at idle, and sixteenth-microstepping. Full steps put a visible scallop into every curve, the pen tracing the motor’s cogging one step at a time; microstepping dissolved it. The final stack is two NEMA 17 steppers, the RAMPS board on an Arduino Mega, an SMPS with cooling fans, and printed mounts for all of it, ₹20,372 all told, with the 3D printing again costing more than the electronics.

On the software side the Polargraph controller does what the Quark crowd saw: import a bitmap, click draw. Under the click, it maps pixel brightness to shading density, so dark regions get tight scribbles and light regions get sparse ones, and the servo on the gondola lifts the pen between strokes. A whiteboard marker draws it all, which was the pitch: notice boards and lecture diagrams drawn by a machine whose only consumable costs thirty rupees.

That is the gondola shading a dark patch, cords in a V above it, pen down. The sound in the room is two steppers singing at different pitches, which is the sound of the triangles being re-solved.

What the wall taught

1. Gravity is a free actuator with opinions. It tensions the cords and it swings the gondola, and the fix lives in the acceleration cap, never in the PID sense of control, because there is no feedback to close.
2. Scalloped curves are full-step cogging made visible in ink. Sixteenth-microstepping through real stepper drivers erased what no amount of pen pressure could hide.
3. The workspace is a band, and art placed in the top corners wobbles. Plan for the middle sixty percent of the board and the machine looks twice as precise.
4. A counterweight you can drink is a counterweight you can tune. Water bottles beat fixed weights because the mass is adjustable in ten-gram sips.
5. A robot drawing Batman holds a crowd that a robot drawing calibration grids never gathers. The demo is chosen by the audience, and the audience votes with its feet.
6. Buy your mistakes quickly. Two motor drivers eleven days apart looks like waste in the expense sheet and reads as tuition everywhere else; the ₹20,372 diary is the most honest project log we keep.

Bono shared its Quark table with the submarine and the scanner, and of the three machines, the one that just draws cartoons got the biggest crowd. There is a lesson in that too.