Servo Actuator

The servo actuator is a standard part used in radio-controlled model cars, aircraft, etc.  We have modified it to make it compatible with the Lego parts used in your robot, as shown below.

The rotating output shaft has a gear wheel, which can mesh with other Lego gears.  Alternatively, you can fit an axle to the gear wheel and drive something with that.  The mounting pins on both sides of the servo body allow it to be installed with the output shaft vertical or horizontal, to suit different applications. 

With the Lego motors, you can control the speed and direction of rotation.  With the servo actuator, you can control the position, or angle, of the output shaft.  The shaft can only rotate through about 200°, but you can control its angle very precisely.

The servo moves relatively slowly - it takes about 1 second to turn through its full range of movement.  However, the torque available is relatively high, at about 0.3Nm.  This means, for example, that it could lift a mass of 300g at the end of a 10cm arm.  Using this much torque is probably not practical - it would require very strong arm construction and connection to the servo, and very strong mounting of the servo on the robot.

Connecting to the Handy-Board 

The Handy-Board expansion board (upper board) has connectors for 6 servo actuators, numbered 0 to 5, just below the left edge of the display.  You should use the servo 0 pins, as shown in the picture below.

Warning:  The servo cable must be connected correctly - black wire on the left, white wire on the right, as shown.  We have glued a red plate on one side of the connector to help you to get it right, but you can still destroy your servo actuator if you are not careful.  Always switch off the Handyboard before connecting the servo actuator, and double-check that it is connected correctly before switching on again.

Using the Servo

To control the position of a servo, simply assign a value to the global variable  servo0. 

Valid values are in the range from some hundreds to a few thousand -  the exact limits will depend on your particular servo actuator.  It is important not to assign a value which tries to push the servo actuator beyond its range of movement - it may damage itself trying to do the impossible!  Values in the range 800 to 4000 should be safe.  The sample program below will allow you to find the limits of movement of your servo.

To enable and disable servos, use the init_expbd_servos() function:
init_expbd_servos(1)  enables all servos
init_expbd_servos(0)  disables all servos

Always assign a safe value to  servo0  before you enable the servos.  Otherwise, you have no idea what position the servo will try to take on when it is enabled - it could destroy itself! 

Connect your servo to connector 0.  Pressing the STOP button will increase the value assigned to  servo0  in steps of 20.  Pressing the START button will reduce the value.  Note the values corresponding to the limits of movement of your servo.

void main()
  {
    servo0 = 1500;            //set servo 0 to a safe position
    init_expbd_servos(1);     //enable servos
    printf("Servo at %d\n", servo0);

    while(1)
      {
        if(start_button())
          {
            servo0 = servo0 - 20;  //decrease servo value
           }
        if(stop_button())
          {
             servo0 = servo0 + 20;  //increase servo value
           }
        printf("Servo at %d\n",servo0);
        sleep(0.1);
       } // end of while - repeat forever
    }