Hypotrochoid-drawing robot


Agnewgraph I is a device that traces Spirograph curves (technically, hypotrochoids). Unlike other systems that compute and plot hypotrochoids mathematically, Agnewgraph I operates a physical Spirograph much as a human being does. In order to do so, Agnewgraph I contains two electromechanical subsystems–a force-sensing pen and a pen transport mechanism–plus a Parallax Propeller chip as the brains.

The force-sensing pen consists of a pen mounted to an upside-down analog joystick. The joystick has springs to return it to its neutral position. A force exerted at the pen tip will displace the joystick from neutral. Measuring the resistances of the joystick’s potentiometers yields a fair approximation of the force vector at the pen tip. (As it happens, the joystick’s springs are not stiff enough for Spirographic purposes so I augmented them with some rubber bands.)

The pen transport mechanism consists of two improvised linear actuators, each consisting of a stepper motor, some aluminum tubing, and some Kevlar thread wound around the motor shaft. The actuators pivot around two fixed points and they meet at the pen.


My interests include writing, electronics, RPGs, scifi, hackers & hackerspaces, 3D printing, building sets & toys. @johnbaichtal nerdage.net

View more articles by John Baichtal