Sense, think, act (the control loop)

Learn

What you’ll learn

• The input → process → output model
• Why a robot repeats this loop over and over
• How to describe any robot behaviour as a loop

Input, process, output

Computer scientists call sense–think–act by another name: input → process → output (IPO).

   ┌──────────┐     ┌───────────┐     ┌──────────┐
   │  INPUT   │ ──▶ │  PROCESS  │ ──▶ │  OUTPUT  │
   │ (sense)  │     │  (think)  │     │  (act)   │
   └──────────┘     └───────────┘     └──────────┘
        ▲                                   │
        └────────── the world changes ◀─────┘

The arrow coming back is the important part. When the robot acts, the world changes (it moves closer to the wall, for example). Next time it senses, it sees a new situation and may decide something different. This repeating cycle is called a control loop.

Open loop vs closed loop

• Open loop: the robot acts without checking the result. “Drive forward for 3 seconds.” If a wheel slips, it never knows.
• Closed loop: the robot checks a sensor and corrects itself. “Drive forward until the distance sensor reads 10 cm.”

Closed-loop control is what makes robots reliable. Most of this course builds towards it.

Note

A washing-machine timer is open loop. A thermostat is closed loop: it measures the temperature, then switches the heating on or off.

Examples

Example: a thermostat

Step	What happens
Input	Temperature sensor reads 18 °C
Process	Rule: “if colder than 20 °C, heat on; otherwise heat off”
Output	Heater switches on
Loop	A few minutes later it reads 20.5 °C, so the heater switches off

Example: a line-following robot

Step	What happens
Input	Left line sensor sees black, right sees white
Process	Rule: “line is to the left, so turn left”
Output	Left motor slows down, right motor keeps going
Loop	Next reading: both sensors see white, so drive straight

Example: the same loop in code

function loop() {            // runs again and again
  const d = distance();      // INPUT: read the sensor
  if (d > 20) {              // PROCESS: decide
    motors(50, 50);          // OUTPUT: drive
  } else {
    motors(0, 0);            // OUTPUT: stop
  }
}

A worked example: robot vacuum

Step	What happens
Input	Bump sensor is pressed
Process	Rule: “if bumped, back up and turn”
Output	Wheels reverse for 0.5 s, then spin to turn
Loop	Sense again: bump sensor now not pressed → drive forward

A real robot runs this loop hundreds of times per second. That’s why it looks like it reacts instantly.

Practice

Try it

The virtual robot below starts with open-loop code: it drives forward for 5 seconds without looking.

1. Press ▶ Run. Does it stop before the box, or crash into it?

2. Now make it closed loop: delete the code and type this instead, then Run again.

   function loop() {
     if (distance() > 20) {   // SENSE + THINK
       motors(50, 50);        // ACT: drive
     } else {
       motors(0, 0);          // ACT: stop
     }
   }

3. Watch the distance reading under the arena while it drives. Which part of the code is the input, the process and the output?

You’ll write much more of this in the Programming and Control sections. You can always come back to the full Robot simulator.

Open loop vs closed loop

▶ Run↺ Reset

Your code (JavaScript)// OPEN LOOP: drive forward for 5 seconds, without checking any sensor. function loop() { if (millis() < 5000) { motors(60, 60); } else { motors(0, 0); } }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Stop further awayEasy

Change the closed-loop code so the robot stops 40 cm from the box instead of 20 cm.

Show an answer

Change the number in the condition: if (distance() > 40).

Slow down before stoppingMedium

Make the robot drive fast when the box is far away and slowly when it’s closer than 50 cm, then stop at 20 cm. Use if, else if and else with two different distances.

Show an answer

function loop() {
  if (distance() > 50) motors(70, 70);
  else if (distance() > 20) motors(25, 25);
  else motors(0, 0);
}

Never get stuckHard

Make the robot explore the whole room without ever stopping: when something is close, turn away and carry on.

Show an answer

function loop() {
  if (distance() < 25) motors(-40, 40); // spin left until the way is clear
  else motors(60, 60);
}

Each time round the loop the robot re-checks, so it only turns for as long as the obstacle is in front.

Quick check

1. In input → process → output, which part is the robot’s "thinking"?

InputProcessOutputPower

2. "Drive forward for exactly 3 seconds" is an example of…

Closed-loop controlOpen-loop controlA sensorAn actuator

3. Why does a robot repeat the loop many times per second?

To use up the batteryBecause the world changes after each action, so it must keep re-checkingBecause computers can only do one thingIt doesn’t, it runs the loop once

Check my answers