[Gdeaver]

One problem with the ultra sonic sensor is the wide cone of detection. For example, say there is a 3" post in front of the robot 1 meter out and then there is a wall 3 meters out. You'll get a ping from the post and a ping from the wall. You'll know very precisely how far out the post is and the wall, but you'll know nothing about the width of the post. You might also want to get an IR proximity sensor like Sharp GP2D12. It has a more focused cone of detection but is not as accurate in range. If it was mounted on a servo, it could scan and define the width of the post. You would have more info to avoid the post.