Our Drive Function

Many teams have a drive function like this:

// tank_drive.c

void tank_drive() {
  motor[DriveL] = joystick.joy1y1;
  motor[DriveR] = joystick.joy1y2;

And that drive function is good because it only takes up two lines of code, but it’s bad because it’s difficult to use.  It assigns the y-value of the left joystick to the left drive motor and the right drive motor to the y-value of the right joystick.  This makes turning very confusing.

The first priority in writing a drive function should always be make it convenient for the driver.  This is a drive function that we recommend:

// arcade_drive.c

void arcade_drive() {
  motor[DriveR] = joystick.joy1y1 - joystick.joy1x1;
  motor[DriveL] = joystick.joy1y1 + joystick.joy1x1;

This is an arcade drive function where the joystick makes the robot behave like a joystick in a video game.  If you push forward on the joystick, the robot goes forward; push the joystick down and to the left, the robot rotates to the left while moving backwards.

This method works very well for us, but even it can be improved. The Logitec controllers employed by FTC do not always center at (0, 0). Sometimes they’ll give some minute value like (2, -1). Although setting your drive motors to that much is unlikely to move your robot, it will try and move you motors to no avail. That’s bad because it puts stress on you motors, which can lead to smoking. Here’s our solution:

// arcade_drive.c

int J1Y1() { return joystick.joy1_y1; }
int J1X1() { return joystick.joy1_x1; }

void abs_tank_drive() {
  int y_pow = J1Y1();
  int x_pow = J1X1();

  if (abs(J1Y1()) < 10) {
    y_pow = 0;

  if (abs(J1X1()) < 10) {
    x_pow = 0;

  motor[DriveR] = y_pow - x_pow;
  motor[DriveL] = y_pow + x_pow;

This will be easiest for you drivers and easiest on your motors.

Here’s a video showing the function at work (ignore the trumpeting in the background):

3 thoughts on “Our Drive Function

  1. Correct me if i’m wrong, but using this code, if you push the joystick to the right it returns a value of 127.
    Using the formula, the left motor=-100 and the right motor=100. Would this not make the robot spin left?
    I think this might be the way to go…


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