Arduino

Continuing with my diversion into the robotic implementation of my vision, I bought a cheap and popular robot controller to play with. Arduino is an open-source hardware and software project for prototyping electronics projects. I've seen it mentioned a few times, so I decided to get my hands on one.

There are number of versions of their microcontroller boards. Their common features are USB communication with a computer for power and for programming, a microcontroller chip to run the programming, and a lot of input and output pins to connect various sensors and actuators.

I opted for the Mega2560 version because it happens to offer a second serial interface (the first one being used for the USB connection). If you've read my previous entry, you'll see that Dynamixel uses a RS485 serial communications protocol, so if I ever wanted to use an Arduino board to talk to a Dynamixel, it would be needed. I'm not sure I'll ever want to do that, as it isn't really designed for it. But I wanted that flexibility.

My first impression is that this thing is extremely easy to use. The open-source software offers a programming environment that is very similar to C, with extensions that make sense for programming hardware. It also comes with a huge number of samples that take you from a first timer to some complicated projects.

I wanted something easy. And I wanted to use a servo. So I bought a cheap beginners kit of electronics accessories for Arduino that included a cheap servo. This is of the analog PWM variety, and it lists its key specs on the side: 21 oz-inches and 0.12 s/60deg. So that's comparable in speed to the Dynamixels I am considering, but much much weaker in power.

I first used a sample program that blinked an LED on the board. Simple. Then I jumped in and used the code from a servo example to make my own modification. I want to control the servo with a pushbutton. When I push the button, it rotates to 90 degrees as fast as it can. When I let go of the button, it rotates back to its starting position. Here is the Arduino code.

// control a servo with a pushbutton

#include <servo.h>
 
Servo myservo;  // create servo object to control a servo 
                // a maximum of eight servo objects can be created 

// constants won't change. They're used here to 
// set pin numbers:
const int buttonPin = 2;     // the number of the pushbutton pin
const int ledPin =  13;      // the number of the LED pin
const int servoPin = 9;

// variables will change:
int buttonState = 0;         // variable for reading the pushbutton status
int servoAngle = 0;

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);      
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);     
  myservo.attach(servoPin);
}

void loop(){
  // read the state of the pushbutton value:
  buttonState = digitalRead(buttonPin);

  // check if the pushbutton is pressed.
  // if it is, the buttonState is HIGH:
  if (buttonState == HIGH) {     
    // turn LED on:    
    digitalWrite(ledPin, HIGH);  
    servoAngle = 90;
  } 
  else {
    // turn LED off:
    digitalWrite(ledPin, LOW); 
    servoAngle = 0;
  }
  // write the servo location
  myservo.write(servoAngle);
  // i'm worried about making the servo freak out
  // so give it a delay here so it doesn't get too many commands
  delay(50);
}

That's it. And it worked, first try. That's why Arduino is so great: I got it to work on the first try. Here's a little video of the setup (a hybrid of a push-button example and a servo example) and me pushing the button.

Yeah, I know it's not that exciting. But at least I have now used a servo. Check that box.

You can see how fast 0.12 s/60deg is. It's medium-fast. Not blinding. Probably fast enough for most of my ping pong tasks, but faster might be better. The servo has a very short arm on it, which makes it feel quite strong (probably about 21 oz worth!).

Anyway, that's all I've done with the Arduino for now. I have a Dynamixel on order, and will play with that next. I'll be controlling that over USB using their USB2Dynamixel adapter, instead of using a micrcontroller like this. At least for now.