Parking sensors are proximity sensors for road vehicles which can alert the driver to unseen obstacles during parking manoeuvres.
Parking sensors generally fall into two categories.
1. Electromagnetic parking sensors
These rely on the vehicle moving slowly and smoothly towards the object to be avoided. Once detected the obstacle, if the vehicle momentarily stops on its approach, the sensor continues to give signal of presence of the obstacle. If the vehicle then resumes its manoeuvre the alarm signal becomes more and more impressive as the obstacle approaches. Electromagnetic parking sensors are often sold as not requiring any holes to be drilled offering a unique design that discretely mounts on the inner side of the bumper preserving the 'new factory look' of your vehicle
2. Ultrasonic parking sensors
The rest of this posting refers to ultrasonic parking sensors only.
The ultrasonic sensors are currently available in several brands of cars, with a variety of brand names such as Parktronic and Parking Aid. Some systems are also available as additional upgrade kits for later installation.
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Parking sensor systems use ultrasonic proximity detectors embedded in the front and/or rear bumpers, to measure the distances to nearby objects at low level. The sensors measure the time taken for each sound pulse to be reflected back to the receiver.[1][2]
Depending on the speed of the vehicle and the distance to the obstacle, the system will warn the driver by visual and/or audible means about the risk of collision. The feedback to the driver will generally indicate the direction and proximity of the obstacle.
Sensors are usually fitted to the rear of a vehicle but may also may be fitted to the front.
Rear sensors are activated when reverse gear is selected and then deactivated as soon as any other gear or neutral or park is selected.
Front sensors are generally activated by pressing a button and then automatically deactivated when the vehicle exceeds a certain speed, this is to avoid nuisance warnings in slow moving traffic.
The most common form of feedback to the driver in a car with parking sensors is audible "beeps" and/or tones. Generally, the frequency of the beep indicates distance from an obstruction, with the beeps becoming faster the closer the vehicle moves to an object. A continuous tone may be heard when the vehicle is extremely close, often warning a driver to stop immediately to avoid collision.
Some systems use visual aids as well as or instead of audible tones, such as LED or LCD readouts to indicate distance from an object. The direction and distance to the obstacle is indicated by the location and strength of the warning symbols.
Since the system relies on the reflection of sound waves, it may not detect some items that are not flat or large enough to reflect sound, for example a narrow pole or a longitudinal object pointed directly at the vehicle or near an object.
Some objects such as skips (UK) or dumpsters (USA) may have flat surfaces that are angled from the vertical. These objects can behave in "stealth mode" by deflecting the return sound waves away from the sensors causing them not to be detected.
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