Sense, Think, Act: What Makes a Robot
A drill is a machine; a thermostat is almost a robot. The difference is a loop that reads the world before pushing on it — and how often that loop runs decides everything.
One loop, three verbs
Strip any robot to its skeleton — a vacuum, a rover, a factory arm — and the same three-step loop is running underneath:
- Sense — read something about the world: a distance, an angle, a brightness.
- Think — compare the reading to what you want and decide what to do about the difference.
- Act — push on the world with a motor, then go straight back to step one.
That last clause is the whole trick. A machine acts; a robot acts, checks what happened, and corrects. The loop is called feedback, and Unit 3 is devoted to doing it well — but every lesson between here and there is really about feeding this loop better senses and stronger muscles.
The loop has a heartbeat
How often the loop runs is its loop rate, and it is a spec, not a detail. A thermostat deciding every few seconds is fine — rooms drift slowly. A two-wheeled robot balancing on the spot must sense and correct hundreds of times per second, because it starts falling between the corrections:
This is why robots keep a small, single-minded computer close to the motors. It does nothing clever — it just never, ever misses a beat.
Where the loop lives
In a real robot the loop is split across parts you will meet all course: sensors feed a microcontroller (the metronome that never misses a millisecond), which drives motors through power electronics; bigger thoughts — maps, plans, vision — live upstairs in a larger computer that talks to the metronome. The split matters: thinking may hesitate, but the loop may not.
It senses (water level), decides (program step), acts (spin). Most engineers still say no — its loop never looks at the result of its actions and corrects. It follows a recipe. The line is fuzzy, and the useful question is never “is it a robot?” but “where is its feedback loop, and how fast?”
⚗️ Lab — The Bump Bot
A one-sensor robot in a corridor: it drives until its rangefinder says stop. You set the speed and the stop distance — and how often the loop runs.
- With a fast loop, it parks neatly at any speed. Now slow the loop down and watch the same settings smack the wall.
- Find the highest speed each loop rate can survive. That number is why balance robots run at hundreds of hertz.
- Set the stop distance to zero: sense–think–act becomes think-too-late.