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Triangulation in car ultrasonic park-assist system

Posted: 05 Jun 2013     Print Version  Bookmark and Share

Keywords:time-of-flight  ultrasonic park assist  ECU  transducers  triangulation algorithm 

In automotive ultrasonic distance-ranging applications, ultrasonic sound wave time-of-flight (TOF) is used to calculate the distance to objects from the passenger vehicle. This distance is used to assist the driver in parking the vehicle, identifying parking spots, or detecting objects in the driver's blind spots.

A key metric used to characterise the performance of automotive distance-measuring systems is accuracy of the measured distance.

A number of factors influence the accuracy of the distance measured. These factors include the object itself, the distance between the vehicle and the object, the transducer characteristics and the electronics used to drive the transducer and signal chain used to process the echo signals.

In this article, we investigate the use of triangulation. Specifically, we look at whether triangulation is effective in improving the accuracy of the distance measured.

Ultrasonic park assist systems
An ultrasonic park assist (UPA) system consists of an UPA ECU (electronic control unit), and multiple smart ultrasonic transducers. Ultrasonic transducers typically are installed in front and rear bumpers, and wing mirrors of an automobile. Up to four transducers can be installed in each bumper.

Measuring the distance
Figure 1 shows a passenger car with two ultrasonic transducers in the car's rear bumper. Further, this picture also shows an object (such as a parking pole) behind the car.

Figure 1: Ultrasonic park assist system.

In an ultrasonic park assist system with multiple smart ultrasonic transducers, the UPA ECU coordinates the measurement of distance. In a typical scenario, the UPA ECU first commands "ultrasonic transducer 1" to measure the distance. It then commands "ultrasonic transducer 2" to measure the distance. This sequence is cycled through as long as the park assist system is activated. An example scenario in which the transducers in the rear bumper are activated is when the driver selects reverse gear to back the vehicle into a parking spot.

Inherently inaccurate
The time-cycled distance algorithm described in the previous section is inherently inaccurate. This is because the correct distance between the vehicle and the object is not the distance measured by the transducers from themselves to the object. Rather, the correct distance between the vehicle and the object is the closest distance from the bumper to the object, which is along the perpendicular line from the bumper to the object.

Figure 2 geometrically illustrates this inherent inaccuracy. In this figure, T1 and T2 at a distance x from each other, are the two ultrasonic transducers in the car's rear bumper.

The ultrasonic transducer T1 measures distance l1 while the transducer T2 measures distance l2, while the correct distance between the vehicle and object is d.

Figure 2: Geometry of the ultrasonic transducers and the object.

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