Sensors for Robotics

Week 3 • CMPSC 304 Robotic Agents

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The Robot Control Loop

PERCEPTION
→
DECISION
→
ACTUATION
  • Perception: Sense the environment (sensors) 👈 This week!
  • Decision: Process information and plan actions
  • Actuation: Execute actions (motors)

This cycle repeats continuously while the robot operates

What Are Sensors?

Sensor: A device that converts physical phenomena into measurable signals

Why robots need sensors:

  • Perception: Understand the environment
  • Navigation: Know where you are and where to go
  • Interaction: Respond to changes
  • Safety: Detect obstacles and hazards

Sensor Categories

Proprioceptive

Sense internal state

  • Wheel encoders
  • Motor current
  • Battery voltage
  • IMU (orientation)

Exteroceptive

Sense external environment

  • Distance sensors
  • Cameras
  • Temperature
  • Light sensors
Both types are essential for autonomous behavior!

Key Sensor Characteristics

Property Definition Example
Range Min/max measurable values 2-400 cm
Resolution Smallest detectable change 1 mm
Accuracy How close to true value ±3 mm
Precision Repeatability ±1 mm
Response Time How fast it updates 50 ms

Accuracy vs. Precision

Accurate

Close to true value

Measuring 10cm as 10.1cm

Precise

Measurements are consistent

Always getting 9.5cm (even if wrong)

Ideal: Both accurate AND precise!
But precision is easier to achieve than accuracy

Ultrasonic Distance Sensors

Principle: Measure time for sound waves to bounce back

HC-SR04 Specifications:

  • Range: 2 cm to 400 cm
  • Resolution: 0.3 cm
  • Angle: 15° cone
  • Frequency: 40 kHz (ultrasonic)
Formula: distance = (time × speed_of_sound) / 2
Speed of sound ≈ 343 m/s at 20°C

HC-SR04 Wiring

HC-SR04 Pin Arduino Pin Purpose
VCC 5V Power
TRIG Any digital pin (e.g., 9) Trigger pulse
ECHO Any digital pin (e.g., 10) Echo response
GND GND Ground
Note: Send 10µs HIGH pulse to TRIG, then read ECHO duration

Arduino Code: HC-SR04

#define TRIG_PIN 9
#define ECHO_PIN 10

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
}

void loop() {
  // Send trigger pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  
  // Measure echo duration
  long duration = pulseIn(ECHO_PIN, HIGH);
  
  // Calculate distance in cm
  float distance = duration * 0.034 / 2;
  
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  
  delay(100);
}

Ultrasonic Sensor Limitations

  • ✗ Soft/angled surfaces: Absorb or deflect sound
  • ✗ Small objects: May not reflect enough sound
  • ✗ Narrow beam: 15° cone misses objects outside
  • ✗ Speed: ~50ms per measurement
  • ✗ Temperature: Speed of sound varies
✓ Best for: Detecting walls, large obstacles, measuring room dimensions

Infrared (IR) Sensors

Principle: Emit IR light and measure reflection

Two main types:

Digital (Binary)

  • Obstacle: YES/NO
  • Adjustable threshold
  • Simple, fast

Analog (Distance)

  • Continuous distance
  • Voltage varies with distance
  • Typical range: 10-80 cm

IR Digital Sensor Wiring

IR Sensor Pin Arduino Pin
VCC 5V
OUT Any digital pin (e.g., 2)
GND GND 
void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT);
}

void loop() {
  int obstacle = digitalRead(2);
  if (obstacle == LOW) {
    Serial.println("Obstacle detected!");
  }
  delay(100);
}

IR vs. Ultrasonic Comparison

Feature Ultrasonic IR
Range 2-400 cm 10-80 cm
Speed ~50 ms ~20 ms
Cost $2-5 $1-3
Ambient Light ✓ No effect ✗ Can interfere
Best Use Walls, distance Obstacle detect

Environmental Sensors

Beyond distance sensing, robots need to sense their environment:

  • Temperature: TMP36 sensor
  • Light: Photoresistor modules
  • Moisture: Soil moisture sensors
Today: Quick overview, then Activity 3 hands-on

Temperature: TMP36

Principle: Voltage output proportional to temperature

Key Points:

  • Range: -40°C to +125°C
  • Output: 10 mV per °C, 500 mV offset at 0°C
  • Formula: temp_C = (voltage - 0.5) × 100
  • Wiring: 3 pins - VCC (red), Vout (yellow), GND (black)
Tip: Check for loose connections if readings jump wildly!

Light: Photoresistor Module

Principle: Resistance changes with light intensity

Key Points:

  • 4-pin module: VCC, GND, AO (analog), DO (digital)
  • Use AO pin for analog readings (0-1023)
  • Higher values = more light
  • You'll need to determine threshold ranges (dark/bright)

Moisture: Soil Sensors

Principle: Measure soil conductivity (wetness)

Key Points:

  • 3 pins: VCC, AOUT (analog), GND
  • Output: Analog voltage (0-1023)
  • Higher value = drier (less conductivity)
  • You'll need to calibrate ranges (wet/dry)
Calibration tip: Test in air (dry) vs water (wet)

Activity 3: Environmental Sensing

Due: Thursday, Feb 6 at 11:00 AM

Your Tasks:

  • Wire all 3 sensors (temperature, light, moisture)
  • Get Serial Monitor output working
  • Run 2+ experiments per sensor
  • Document findings in experiments.md
  • Push to GitHub + demonstrate to instructor
Note: Sensor issues? Document troubleshooting efforts!