To drive the main motors of the robot form the Arduino, I'm using the Ardumoto board from SparkFun.
The Ardumoto comes assembled (it uses surface mount components), making it very easy to use. All that's necessary to get it going is to solder on some appropriate connectors. I've used a separate 6v battery pack to power the motors connected to the Vin socket on the Ardumoto, while isolating the Vin pin from the Arduino by bending it outwards (as the Arduino is currently running from USB there is no power on the Vin pin):
A simple test program is listed below to drive the motors using PWM speed control by sending simple serial commands to the Arduino.
Update: I've now added servo control to the Ardumoto here.
// Test program for SparkFun Ardumoto board // Copyright (c) 2009 mechomaniac.com // To use, connect the Arduino to a computer and send commands using a serial terminal. // eg AR40# motor A forwards with a speed of 40 #define PwmPinMotorA 10 #define PwmPinMotorB 11 #define DirectionPinMotorA 12 #define DirectionPinMotorB 13 #define SerialSpeed 9600 #define BufferLength 16 #define LineEnd '#' char inputBuffer[BufferLength]; void setup() { // motor pins must be outputs pinMode(PwmPinMotorA, OUTPUT); pinMode(PwmPinMotorB, OUTPUT); pinMode(DirectionPinMotorA, OUTPUT); pinMode(DirectionPinMotorB, OUTPUT); Serial.begin(SerialSpeed); } // process a command string void HandleCommand(char* input, int length) { Serial.println(input); if (length < 2) { // not a valid command return; } int value = 0; // calculate number following command if (length > 2) { value = atoi(&input[2]); } int* command = (int*)input; // check commands // note that the two bytes are swapped, ie 'RA' means command AR switch(*command) { case 'FA': // motor A forwards analogWrite(PwmPinMotorA, value); digitalWrite(DirectionPinMotorA, HIGH); break; case 'RA': // motor A reverse analogWrite(PwmPinMotorA, value); digitalWrite(DirectionPinMotorA, LOW); break; case 'FB': // motor B forwards analogWrite(PwmPinMotorB, value); digitalWrite(DirectionPinMotorB, LOW); break; case 'RB': // motor B reverse analogWrite(PwmPinMotorB, value); digitalWrite(DirectionPinMotorB, HIGH); break; default: break; } } void loop() { // get a command string form the serial port int inputLength = 0; do { while (!Serial.available()); // wait for input inputBuffer[inputLength] = Serial.read(); // read it in } while (inputBuffer[inputLength] != LineEnd && ++inputLength < BufferLength); inputBuffer[inputLength] = 0; // add null terminator HandleCommand(inputBuffer, inputLength); }