The next generation of automobiles is a major pillar of the future market. Consumers are not only interested in high-performance electric vehicles and hybrid vehicles, but also in the concept of smart mobility and the next generation of intelligent connected vehicles in recent years.
Intelligent connected vehicles refer to a type of car that uses advanced devices to collect various information about the vehicle's status and the road conditions, and integrates communication technologies to improve driving safety.
In other words, future cars will be able to see in all directions and hear from all sides.
The core technology driving intelligent connectivity and fully automated driving is simply referred to as V2X:
Vehicle-to-Vehicle (V2V) communication
Vehicles and infrastructure communicate with each other via networks (V2I: Vehicle to Infrastructure).
V2X technology builds an efficient transportation system that can quickly obtain real-time road conditions and effectively prevent traffic accidents, achieving the highest level of fully automated driving (ADAS).
Below, we will introduce you to the basics, latest advancements, and increasingly important applications of Murata ultrasonic sensors in ADAS.
1. What is an ultrasonic sensor?
Ultrasound generally refers to sound waves with frequencies exceeding the low-frequency audible range (0 Hz to 20 kHz) of the human auditory system.
v: Speed of sound 346 m/s (@25℃) f: Frequency λ: wavelength
The principle of ultrasonic wave generation is based on the characteristic of piezoelectric ceramics to expand and contract according to the direction of voltage. The electrical signal is converted into ceramic vibration, and the repeated expansion and contraction of the ceramic causes the air to vibrate, thus emitting ultrasonic waves.
In practical applications, ultrasonic sensors apply electrical signals to ultrasonic transducers (transmitters), which emit ultrasonic waves by expanding and contracting piezoelectric ceramics. The ultrasonic waves are then received by receivers and converted into electrical signals for analysis and various applications.
The speed of sound in air is affected by temperature. The speed of sound in air is C = 331.5 + 0.6T [m/s], where C is the speed of sound in air and T is the air temperature (°C).
Since acoustic impedance Zo = ρ･C (where ρ is the density of the medium and C is the speed of sound in the medium), reflection increases when the acoustic impedance difference between the media is large, and transmission occurs when the acoustic impedance difference is small.
Different objects, due to their different constituent materials, possess their own inherent and different acoustic impedances and speeds of sound. In other words, the speed of sound and acoustic impedance in a medium are numerical values that represent the ease with which sound propagates. Conversely, by transmitting and collecting these parameters, we can determine the spatial conditions of the "obstacles" or interfaces that sound waves encounter.
Ultrasonic sensors exhibit a conical directivity starting from the top surface, defined as the directionality from the frontal sound pressure level to -6 dB. Compared to optical sensors, the directivity is wider, but it becomes sharper at higher frequencies.
◆ Resolution: The higher the frequency, the higher the resolution.
◆ Attenuation: The higher the frequency, the greater the attenuation and the shorter the distance to reach the destination.
Ultrasonic waves have the characteristics that their propagation speed varies depending on the propagation medium, and their directionality and resolution vary depending on the frequency. These properties affect the measurement distance, accuracy, and detection range.
3. Applications of ultrasound
For example, distance can be calculated by measuring the time it takes for an ultrasonic wave to reflect: measure the time from the start of emission to the receipt of the reflected wave, then: time x speed of sound = distance to the object (round trip).
Schematic diagram of the principle of ultrasonic distance detection
Applying the same propagation principle, the presence of an object can be detected by identifying whether a reflected wave is present. Therefore, the presence of an object can be detected non-contactly, and the presence of an object can be detected over a wide range within a directional range.
Next, let's look at the application of ultrasonic sensors in automobiles.
4. Automotive PAS Applications
Ultrasonic sensors were among the earliest applications of driver assistance systems. Parking assist systems (PAS) in automobiles use ultrasonic sensors to measure the distance between the vehicle and obstacles (such as walls) and notify the driver of the approach level.
A pulse signal is emitted, and the ultrasonic wave is reflected from the obstacle. The distance is measured based on the time until the reflected wave is received.
When transmitting a pulse signal, a clock pulse will be sent, and the clock will be used to measure time.
Reflection time (s) x speed of sound (340m/s) = distance to obstacle (round trip)
The characteristics required for PAS applications are not only the ability to detect large-area obstacles such as rear walls in real time, but also the ability to determine the impact of "curbstones"; in addition, the vehicle-mounted ultrasonic sensor should cover the largest possible area with a single device to reduce the number of devices.
Finally, the PAS system also needs to be able to detect extremely close distances, which places higher demands on the directional performance of the ultrasonic sensors used, namely, narrower vertical direction, wider horizontal direction, and shorter reverberation time.
5. Automotive ADAS Applications
Level 3 autonomous driving is a key component of ADAS technology (Video 1). Starting with ADAS Level 3, the vehicle is truly "handed over" to software, algorithms, and sensors, making the performance, reliability, and accuracy of these products and technologies crucial. However, current industry technology capabilities can still introduce errors and inaccuracies, and cannot prevent accidents caused by unexpected road conditions.
When ADAS reaches Level 4 (advanced autonomous driving), the driver will rarely participate in the driving process, and the vehicle will enter a fully automatic cruise state. ADAS needs to monitor the vehicle's surrounding environment in real time, using various sensors such as radar, lidar, cameras, and ultrasonic sensors.
At this point, not only are there very high requirements for the performance and quality of individual sensors, but also for the combination of software/algorithms. By combining, optimizing, and comprehensively analyzing the data from various sensors, the detection range can be expanded, blind spots can be shortened, and the safety of autonomous driving can be improved.
Murata's sensing technology and piezoelectric ceramic processing technology have long been used in parking assistance systems (PAS). Murata's ultrasonic sensors, which can monitor vehicles within a few meters, have long been used in automatic parking systems, including low-speed autonomous driving.
In the era of advanced autonomous driving, the automotive industry has increasingly higher requirements for ultrasonic sensors in order to improve safety. Not only do they need to improve the reliability of the products themselves and expand the detection range (especially at close range around the vehicle), but they also need to use coding technology to enable multiple ultrasonic probes to detect the surrounding environment simultaneously in order to quickly detect the situation around the vehicle.
To meet market demands and achieve superior durability and stable close-range detection for ultrasonic sensors, Murata has been improving its products and manufacturing processes in recent years. The goal is to reduce the close-range detection distance of a single ultrasonic probe from the current 25cm to below 10cm.
