Introduction
Smart parking systems need to detect whether a parking space is occupied. While inductive loop detectors and ultrasonic sensors are common, PIR sensors offer a low-cost alternative for detecting vehicles.
How PIR Detects Vehicles
Vehicles, especially after being driven, are warm. The engine, exhaust, and even tires radiate heat. A PIR sensor above or beside a parking space can detect the presence of a warm vehicle.
Limitation: A cold vehicle (parked for hours) may not be detectable. This limits PIR to detecting recent arrival, not long-term occupancy.
Applications
Entry/Exit Counting
PIR sensors at garage entrances and exits can count vehicles entering and leaving, providing real-time occupancy counts.
Space Occupancy (Short-term)
In short-term parking (e.g., supermarket), PIR can detect if a space is occupied by a recently arrived vehicle.
Security
Detect unauthorized movement in parking areas after hours.
Sensor Placement
- Above each space: Ceiling-mounted sensor looking down at parking space.
- At entrance/exit: Side-mounted curtain sensor to detect passing vehicles.
- Pole-mounted: In outdoor lots, sensors on poles angled toward parking spaces.
Challenges
Cold Vehicles
A vehicle that has been parked for hours may be at ambient temperature and invisible to PIR. Solution: Use in conjunction with other sensors (ultrasonic, magnetic) for reliable occupancy detection.
Environmental Factors
- Sunlight: Heating of ground or vehicles can cause false triggers.
- Rain: Water on sensor lens blocks IR.
- Temperature changes: Rapid ambient changes can mimic vehicle arrival.
Differentiation
PIR cannot distinguish between a vehicle and a person standing in the space. May cause false occupancy readings.
Dual-Technology Approach
A robust parking sensor often combines:
- PIR: Detects warm vehicle arrival.
- Ultrasonic: Measures distance to confirm object presence.
- Magnetometer: Detects large metal object (vehicle).
Case Study: Shopping Mall Parking
A shopping mall installed PIR sensors above each space in the short-term parking area. The system displayed available spaces on digital signs. Accuracy was 85% for recently parked vehicles but dropped to 60% for vehicles parked >2 hours. They added ultrasonic sensors to improve accuracy to 95%.
Conclusion
PIR sensors can be used in parking systems, but they work best for detecting recent arrivals and should be combined with other technologies for reliable long-term occupancy detection.