Miniature ultrasonic sensors make object detection easier

Ultrasonic sensors are most often used for object detection and perception, distance calculation and measurement, dynamic object detection and perception, and other applications. Because the object will reflect when detected by an ultrasonic wave, this sensor can be used as an alarm device for the detection of intruders, sensor for automatic doors, and other objects. In this paper, a new kind of subminiature open-type ultrasonic sensor from Murata is introduced.

Miniature ultrasonic sensor improves design flexibility

First, some common terms are briefly explained. An ultrasonic transducer is a kind of transducer that can convert ultrasonic signals into electrical signals, or vice versa, and an ultrasonic transceiver is a kind of transducer that can both transmit and receive ultrasonic waves. Another related common term is “ultrasonic sensor”, which is essentially a transceiver that works like radar and sonar, and determines the distance of a target by measuring the time interval between transmission and reception.

The ultrasonic sensor is provided with a composite oscillating body that combines the oscillator piezoelectric ceramics with the resonator, which is attached to the base in an enclosed in the metal cap. The resonator to effectively radiate ultrasonic waves generated by the oscillating body into the air and gather ultrasonic waves from the air at the center of the oscillator.

"Sound pressure" is the most important parameter of the open-structure ultrasonic sensor. In distance measurement applications, the sensing area of the sensor elongates with the increase of the sound pressure of the ultrasonic sensor.

The SMD (surface mounted device) type ultrasonic sensor can measure the range of the intermediate level because it is capable of controlling the sound pressure loss resulting from miniaturization. The miniature SMD type ultrasonic sensor has minimized the installation space and improved the design flexibility.

The MA40H1S-R is an ultrasonic transducer launched by Murata, which can transmit ultrasonic waves into the air, receive ultrasonic waves through a transducer, and detect an object or distance to the target through the propagation time of the reflected sound. The MA40H1S-R is of compact construction, with a surface area of only 5.2mm x 5.2mm and a thickness of 1.15mm, enabling it to be installed in a confined space or to improve design through its functions. The open-type structure MA40H1S-R is suitable for consumer applications, not for outdoor applications or automotive applications.

The MA40H1S-R has a frequency of 40kHz, directivity of 80° (typical value), sensitivity greater than -65dB (0dB = 1V/Pa), minimum sound pressure level (S.P.L.) of 95dB (0dB = 20uPa), dimension of 5.2 x 5.2 x 1.15mm, operating temperature of -20~60, capacitance of 4500pF, capacitance tolerance of ±20%, and maximum input voltage of 7.2Vp-p (at 40kHz, square wave). The sensor is surface-mounted (solder-mount by reflow), although it is not waterproof, it can achieve waterproof effects through other housing components. In terms of development tools, the driver IC of the ultrasonic sensor provided by Texas Instruments (TI) can assist in the evaluation and circuit design of the ultrasonic sensor.

The MA40H1S-R can adjust the driving voltage and the quantity of driving waves, and its shortest detection distance is about 15cm, rendering it difficult to detect a distance less than 10cm and impossible to completely eliminate the influence of reverberation. The ultrasonic waves used in the MA40H1S-R will not interfere with remote communication in the 40kHz wave frequency, and has no effect on electromagnetic waves. However, when a 40kHz signal line operates near an ultrasonic sensor, the noise from the signal line may be superimposed on the sensor signal and reduce the signal-to-noise ratio (SNR). As the signal line of the receiving sensor has high impedance, it is recommended to connect and shield it with the shortest length possible to minimize the influences of crosstalk and external noise.

The MA40H1S-R can detect areas of the human body where sound waves are easily reflected, such as the face and hands. Clothes absorb sound, making it more difficult to detect the human body. Compared with infrared sensors, ultrasonic sensors can detect static objects and measure their distance. Ultrasound wave can also penetrate film, so if the film is thin enough for ultrasound wave to penetrate, the object can be detected after being placed at a short distance.

Foreign matter (such as snow or dust) attached to the sensor surface will reflect the sound waves, inevitably leading to false detection. Dirt and dust attached on the surface will reduce the sound pressure and change the sound distribution due to scattering, which will inevitably affect the detection area. As a result, the MA40H1S-R cannot be used in an environment where water droplets will become attached to the sensor.

 

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