Parking sensors use a type of sonar. The term sonar is an acronym for sound navigation and radar; it's used for calculating the distance and/or direction of an object from the time it takes for a sound wave to travel to the target and back. An ultrasonic sensor is a speaker or microphone that emits or receives ultrasound. There is also a type that can handle both emission and reception. Vehicle parking sensors are equipped with this type of sensor.
Ultrasound sensors initially found use in vehicles for detecting obstacles when parking (Figure 1) but it is now evolving into an automatic parking system.
This system controls steering, acceleration and braking automatically, based on the parking zone and location information gained from the ultrasonic sensor, to achieve parallel parking and garage parking.
In the case of the rear sonar, two to four ultrasonic sensors are mounted on the rear bumper to detect an obstacle up to 2 to 2.5m away. The distance is communicated to the driver in real time using varying buzzer sounds. Even a wire fence can be detected if it is close enough.
The main characteristics of ultrasonic sensors for rear sonar are directivity, ringing time, sensitivity and sound pressure.
Directivity of an ultrasonic sensor corresponds to the size and shape of the vibrating surface (that is emitting the ultrasound) and the frequency at which it vibrates. Figure 2 indicates directivity of a disc type transducer.