Ultrasonic Sensors in Self-Driving Cars
What is an ultrasonic sensor?
Sonar stands for Sound Navigation And Ranging. It has been used in vehicles to support drivers with their driving tasks like parking and nearby obstacle detection.
Physics of an ultrasonic sensor:
The ultrasonic sensors send out short ultrasonic impulses which are reflected by obstacles. The echo signals are then received and processed. Within the plastic case of an ultrasonic sensor is themain components, the ultrasonic transducer. It consists of an aluminum pot with diaphragm containing a piezoceramic element). The sensor receives a digital transmit signal from the ECU. This causes the aluminum diaphragm to oscillate with square waves for about 300µsec with resonant frequency of about 48 kHz which results in emission of ultrasonic pulses. The diaphragm then relaxes for about 900µsec (during which no reception is possible) receives the reflected sound from an obstacle and vibrates. These vibrations are outputted by the piezoceramic element as analog signals, it’s then amplified and converted to a digital signal.
Types of ultrasonic sensor:
The ultrasonic sensor is available in three mechanically compatible sensor variants:
1. Detection range: max. 2.5 m, min. 15 cm; object presence 6 cm
2. Detection range: max. 4.5 m, min. 15 cm; object presence 3 cm
3. Detection range: max. 5.5 m, min. 15 cm; object presence 3 cmThere are different types of radars based on the range and beam angle. Ultrasonic sensors in self-driving cars usually have a wide horizontal sensing range (min. 15cm to max. 2–6m depending on sensor type) and a narrow vertical sensing range to avoid the earth reflections/
What are their strengths?
- They’re able to see through objects unlike LiDAR.
- They work normally in bad weather (rain, snow, dust) unlike LiDAR.
- They’re relatively cheap and inexpensive.
- They work well in fog and low light night time situations unlike cameras.
- Some newer versions have resoultions and object recognition capabilities comparable to LiDAR.
What are their weaknesses?
- They don’t have the resolution to detect small objects or multiple objects moving at fast speeds.
- Shorter field of view and accuracy compared to LiDAR.
- They can’t see color.
