Ultrasonic sensors have been widely used in various distance measurement and object detection applications, and now can also be used in automatic doors, anti-theft systems, and other products that need to sense the distance to the object and detect the existence of a human body. This article will show you the open-structure ultrasonic sensor launched by Murata to understand its product features and applications.
Sound pressure levels affect the detection distance
An ultrasonic sensor with an open structure can be used in the application of indoor distance measurement. This kind of ultrasonic sensor is formed by bonding a metal plate and piezoelectric ceramics, in which the composite vibrating body combining a vibrator with a resonator is elastically fixed on the base and can be incorporated into the housing.
The resonator can efficiently radiate ultrasonic waves generated by vibration into the air. In addition, in order to concentrate the ultrasonic waves in the air to the center of the oscillator, the oscillator is designed as funnel-shaped to facilitate the concentration of sound waves.
Sound pressure is the most important specification for open-structure ultrasonic sensors. For example, for distance measurement, the measurable distance is extended as the sound pressure increases. The ultrasonic sensor of a surface mount device (SMD) is also an option, with a miniaturized design that controls sound pressure loss and provides a measurement distance in the intermediate range. The miniaturized SMD ultrasonic sensors can reduce the installation space and improve the design flexibility.
Improve sensor performance by reducing resonance time
The transmitting-receiving ultrasonic sensors can transmit and receive acoustic signals, so that even if it stops receive signals, the mechanism that emits sound waves in the sensor will continue to vibrate for a short period of time due to inertia and stop completely. At this time, if the detection distance is too close, the emitted sound wave will quickly travel back to the sensor from the object, and the mechanism that emits the sound wave in the sensor has not stopped vibrating, and the received sound wave cannot be obtained before the vibration stops. The vibration time here is called after-vibration time. If the after-vibration time is too long, close-range detection cannot be performed. To decrease the impact of resonance on the sensor, the sensor should be fixed in such a way that it does not interfere with its own vibration. When the sensor is embedded into the housing, elastic materials such as rubber or sponge that do not affect the sensor vibration can be covered around the sensor. Materials with less temperature-based variation in rigidity should be selected.
For reverberation suppression, it is necessary to ensure that the sensor vibration will not be transmitted to the board or housing. Otherwise, the housing will vibrate even if the reverberation of the sensor converges, resulting in a longer reverberation time.
In addition, the upper part of the bevel (top surface) and the directional range of the sensor must be built into an open space in order not to interfere with ultrasonic emission. Since the sound waves emitted by ultrasonic sensors travel through the air, the housing (if any) at the top or in the directional range will reflect the sound waves, making it difficult to properly measure the distance to the object. Thus, the directivity can be controlled by attaching a horn to the sensor. The directionality decreases with the widening of the horn opening and the sound pressure on the front increases with the lengthening of the horn.
Ultrasonic sensors in support of consumer applications
With many years of experience in sensor development, Murata has also launched a number of ultrasonic sensors, including the MA40S4S/MA40S4R, an open type ultrasonic sensor that can support general electronic equipment as well as distance measuring, object detection, and human body sensing. The sensor is compact/light-weight, highly sensitive, and can emit ultrasonic waves in the air and then detect the reflected waves.
The MA40S4S/MA40S4R is an ultrasonic transducer suitable for a variety of detections, where the MA40S4S is used to transmit ultrasonic waves and MA40S4R is used to receive ultrasonic waves. MA40S4S/MA40S4R consists of piezoelectric ceramics, metal plates, resonators, and a resin housing. The resonator is funnel-shaped, allowing for effective emission of the ultrasonic waves generated by the vibrations of the resonator to the air (or by concentrating the ultrasonic waves in the air to the center of the resonance). Sound pressure level (S.P.L.) is the most important characteristic of the ultrasonic transducer. For example, in distance measurement applications, transducers with a higher sound pressure level can detect longer distances. However, the MA40S4S/MA40S4R with an open structure is not available for outdoor and automotive electronics applications and only supports consumer applications.
The MA40S4S/MA40S4R has a center frequency of 40kHz and a directivity of 80 degrees (typical). The typical sensitivity of an MA40S4R is -63dB (0dB=10V/Pa), the typical sound pressure level of an MA40S4S is 120dB (0dB=20μPa), and the maximum input voltage is 20Vp-p at a 40kHz square continuous wave, with a size of Φ9.9x7.1 mm and operating temperature of -40℃ to +85℃.
SMD ultrasonic sensors for use in narrow spaces
In addition, Murata has introduced the MA40H1S-R, an SMD-type ultrasonic sensor with an open type that allows for installation in narrow spaces or improvement in design through its characteristics, measuring just 5.2mm x 5.2mm in size and 1.15mm thick. Although the sensor is not waterproof, it can meet waterproof requirements through the housing assembly.
The MA40H1S-R has a center frequency of 40kHz and a directivity of 80 degrees (typical), with a typical sensitivity of -65dB (0dB=1V/Pa), minimum sound pressure level of 95dB (0dB=20uPa), operating temperature of -20℃ to 60℃, rated voltage of 6.6Vp-p at a 40kHz square wave, and maximum rated voltage of 7.2Vp-p. The sensor is also provided with an open type structure not available for outdoor and automotive electronics applications and supports only consumer applications.
Conclusion
A wide variety of ultrasonic sensors enable customers to choose from different specifications according to the differences in application demand. The open-structure ultrasonic sensor from Murata introduced in this article, with excellent product performance, is suitable for indoor distance measuring, object detection, or human body sensing, and is an ideal choice for the development of related products and applications.
