Over the last few weeks I’ve been messing around quite a bit with the Processing programming language. I was drawn to it initially because it is a close cousin to the Arduino language and programming environment (which is based on Processing), so naturally I wanted to find a way to combine the two in a project.

Background

What I had in mind, of course, was an outlandishly-complicated, full-screen Processing user interface, with dials and button and knobs and sliders and big maps and graphs of data readouts and a little videoport showing the view from the Kinect-based eyes of some hardy Arduino-powered robot as it trundled bravely into the caldera of a smoking volcano.

But alas, I thought it would be better to start small — with one little graphical slider control, one Arduino, and one LED. The slider would command a brightness level for the LED, serial communication would do the talking, and pulse-width modulation (PWM) would do the legwork. In my quest for the ultimate ready-made user interface, however, I did stumble across the awesome-looking and fully-featured controlP5 GUI library for Processing.

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Although the Arduino platform is ideal for standalone applications, it really comes to life when interfaced with a PC. Connect Arduino to a personal computer and you instantly add a ton of versatility and processing power to your project.

This tutorial will describe how to use Arduino to control a bank of four independent RC servos with your PC (or Mac, or *nix Box), using a USB cable and a modular Arduino-Python software stack.

The following discussion builds upon concepts presented in two previous articles, “Arduino Serial Servo Control” and “Joystick Control of a Servo.” As always, comments, critiques, or suggestions for improving or adapting this code are welcome and appreciated.

Project Outline

The primary goal for this project was to create a software stack that allows simple and flexible control of multiple servos from any type of Python script.

The solution has two basic components: (1) an Arduino sketch that waits for serial input from a connected PC, then moves each servo to its commanded position, and; (2) a Python module on the PC that opens the serial connection and formats the data packets expected by the Arduino.

Any other Python program written to sit on top of these two layers need not worry about the messy details of serial communication, but rather can just say something like, “Move servo #2 to 90 degrees.” Or, more precisely:

servo.move(2,90)

Easy, right? Let’s get started.

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One of the cool features of the Arduino platform is its ability to talk to other electronic devices using standard protocols. The big draw of physical computing, in my opinion, is the power it gives you to affect a limitless range of real-world objects with your PC, rather than just boring old monitors and printers.

This short tutorial will demonstrate one way to use Arduino to control a servo motor with a PC, using a USB cable and the Arduino’s serial library. It will in no way attempt to be an introduction to asynchronous serial communication, since such topics are better addressed elsewhere.

RC servos are comprised of a DC motor mechanically linked to a potentiometer. Pulse-width modulation (PWM) signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the DC motor is powered until the potentiometer reaches the value corresponding to the commanded position.

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