How to convert Twist msg to PWM signals?

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This will convert the Twist to Ackermann messages: http://wiki.ros.org/teb_local_planner...

def convert_trans_rot_vel_to_steering_angle(v, omega, wheelbase):
  if omega == 0 or v == 0:
    return 0

  radius = v / omega
  return math.atan(wheelbase / radius)

v = data.linear.x
steering_angle = convert_trans_rot_vel_to_steering_angle(v, data.angular.z, wheelbase)

This will convert angular velocity to steering angle.
Once you have the steering angle you can send the servo PWM value that corresponds to that angle. Or put a control loop around your steering angle sensor.