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.