Sense, think, act

A robot vacuum bumps gently along a wall, a warehouse cart follows a painted line, a rover crawls across Mars. They look very different, but under the covers they all run the same loop. Learn this loop once and every lesson in this lab will fit into it.

A robot is a machine that senses its surroundings, runs a program to choose what to do, and moves something to act on that choice, over and over. Three parts make that possible:

• Sensors measure the world and the robot itself: distance to a wall, wheel rotation, tilt, light.
• A controller is the software (running on a small computer) that turns sensor numbers into commands. It is a set of rules and calculations written by people.
• Actuators are the parts that create motion, most often electric motors that spin wheels.

The loop. The three parts are joined in a cycle that repeats many times per second:

1. Sense: read the sensors.
2. Think: the program compares what it measured with what it wants and works out a command.
3. Act: send the command to the motors.

Then the robot's motion changes what the sensors see, and the cycle starts again. Because the result of the action is measured and fed back into the next decision, this is called a feedback loop. Feedback is the single most important idea in robotics. A robot with no feedback is just a toy that replays a recording, as you will see in the last lesson of this world.

Autonomous or remote controlled? In a remote-controlled robot a person closes the loop: you watch, you decide, you press the stick. In an autonomous robot the program closes the loop by itself. Most real robots are in between. A robot vacuum is autonomous for cleaning but you start it. A Mars rover is given a plan by people each day and then carries it out on its own (World 7 explains why).

Mobile robots versus arms. The Robot Arm lab studies an arm bolted to a table: its base never moves, so the big question is where the hand is, given the joint angles. A mobile robot moves through the world. That adds new questions that this lab is about: How do wheels make it steer? Where is it now? What does it see? Where is everything else? How does it get from here to there?