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## gocupi: Go + Raspberry Pi polargraph/drawbot #piday #raspberrypi @Raspberry_Pi

The gocupi is an awesome polargraph made using a Raspberry Pi. Check out the full tutorial on the gocupi site.

Basic polargraph description

Two stepper motors move a pen hanging from threads to draw stuff out on a whiteboard. A program written in Go runs on the Pi, it sends movement commands over serial to an arduino, which then pulses the step pin on the stepper drivers to make the stepper motors move.

This project is different from most other Polargraphs in that there is no step generation code on the arduino, everything is calculated in Go and then the arduino just receives a stream of step deltas that it stores in a memory buffer and then executes. Since all logic is written in Go running on the Pi it allows using more advanced interpolation models for smooth drawing, not needing to use fixed point or single precision floats for calculations, not needing to reflash the arduino often when making code changes, etc.

Design description

In the Go program, there are several channels that form a pipeline where separate functions execute the different pipeline stages. All of the stages are run in different goroutines so that they execute concurrently.

• The first stage in the pipeline is generating X,Y coordinates, it can either read those points from an svg file, gcode file, mouse data, or generate them according to an algorithm(such as hilbert space filling curve, spiral, circle, parabolic graph, etc).
• The second stage takes an X,Y coordinate and interpolates the movement from the previous X,Y position to the new position by evaluating the pen position every 2 milliseconds. It takes into account acceleration, entry speed, and exit speed so that it can slow down the pen smoothly before the end of the current line segment if needed. It calculates how much the stepper motors need to turn to move the pen to the interpolated X,Y location over those 2 milliseconds.
• The final stage takes the step commands and writes them over serial to the arduino. The arduino first sends a byte requesting a certain amount of data when the buffer has enough room, then the raspberry pi sends that much data to the arduino.
• The arduino has a 1KB buffer of step commands and uses simple linear interpolation to see if it should generate a pulse at the current time to move the stepper motor one step in a particular direction.

In order to generate single line art drawings(like the raspberry pi logo shown below) I followed the makerbot and eggbot tutorials which show how to convert a grayscale image to a stippled image to a path to an svg file.

Stepper motor – NEMA-17 size – 200 steps/rev, 12V 350mA: A stepper motor to satisfy all your robotics needs! This 4-wire bipolar stepper has 1.8° per step for smooth motion and a nice holding torque. The motor was specified to have a max current of 350mA so that it could be driven easily with an Adafruit motor shield for Arduino (or other motor driver) and a wall adapter or lead-acid battery. Read more.

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