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Sensor Detection

The HC-SR04 ultrasonic sensor is the core component for obstacle detection in the Object Detection Car, enabling real-time distance measurement to ensure collision-free navigation. This section details the sensor’s operation, as described in the project documentation.

Sensor Overview

  • Type: Ultrasonic distance sensor.
  • Pins:
    • Trigger: Pin 5 (output, sends ultrasonic pulse).
    • Echo: Pin 18 (input, receives reflected pulse).
  • Range: 2 cm to 400 cm (effective up to 60 cm for this project).
  • Accuracy: ±3 mm.
  • Operation: Emits 40 kHz ultrasonic waves and measures the time for the echo to return.

Detection Process

  1. Pulse Transmission:

    • The ESP32 sends a 10 µs HIGH pulse to the Trigger pin.
    • This triggers the HC-SR04 to emit eight 40 kHz ultrasonic waves.

  2. Echo Reception:

    • The sensor waits for the waves to reflect off an obstacle and return.
    • The Echo pin goes HIGH for the duration of the round-trip.

  3. Distance Calculation:

    • The ESP32 measures the Echo pulse duration using pulseIn(ECHO_PIN, HIGH).
    • Distance is calculated as: 
      distance = (duration * 0.0343) / 2
      • 0.0343 cm/µs: Speed of sound (343 m/s) converted to cm/µs.
      • / 2: Accounts for round-trip travel.
    • Result is in centimeters.

  4. Obstacle Avoidance:

    • Measurements are taken every second.
    • If distance < 60 cm and > 0 cm:
      • The car moves backward for 500 ms.
      • The car stops.
      • Manual commands from the Android app are ignored.
    • A distance of 0 cm indicates an invalid reading (e.g., no echo).

Code Implementation

The following function in CarControl.ino handles sensor detection:

float getDistance() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  long duration = pulseIn(ECHO_PIN, HIGH);
  return (duration * 0.0343) / 2;
}
  • Logic: Ensures accurate pulse timing and converts duration to distance.
  • Usage: Called in the main loop() to check for obstacles.

Calibration

  • Environmental Factors: Temperature and humidity affect sound speed. The constant 0.0343 assumes 20°C; adjustments may be needed for extreme conditions.
  • Noise Reduction: Short delays (2 µs LOW, 10 µs HIGH) minimize interference.
  • False Positives: Invalid readings (e.g., 0 cm) are filtered to prevent erroneous avoidance actions.

Challenges

  • Reflective Surfaces: Glass or mirrors may cause weak echoes.
  • Soft Materials: Fabrics absorb waves, reducing detection reliability.
  • Narrow Objects: Thin obstacles may be missed due to the sensor’s beam angle.
  • Mitigation: Calibration and positioning optimize performance (see Solutions).

Integration

  • Hardware: Connected to ESP32 pins 5 (Trigger) and 18 (Echo).
  • Software: Integrated with motor control to trigger avoidance (see Movement Controls).
  • Feedback: Distance readings are printed to Serial Monitor for debugging.

For complete code, see CarControl.ino.