Ultrasonic sensor:
Bats are wonderful creatures. Blind from the eyes and yet a vision so precise that could distinguish between a moth and a broken leaf even when flying at full speed. No doubt the vision is sharper than ours and is much beyond human capabilities of seeing, but is certainly not beyond our understanding. Ultrasonic ranging is the technique used by bats and many other creatures of the animal kingdom for navigational purposes. In a bid to imitate the ways of nature to obtain an edge over everything, we humans have not only understood it but have successfully imitated some of these manifestations and harnessed their potential to the greatest extent
History:
The history dates back to 1790, when Lazzaro Spallanzani first discovered that bats maneuvered in flight using their hearing rather than sight. Jean-Daniel Colladon in 1826 discovered sonography using an underwater bell, successfully and accurately determining the speed of sound in water. Thereafter, the study and research work in this field went on slowly until 1881 when Pierre Curie’s discovery set the stage for modern ultrasound transducers. He found out the relationship between electrical voltage and pressure on crystalline material. The unfortunate Titanic accident spurred rigorous interest into this field as a result of which Paul Langevin invented the hydrophone to detect icebergs. It was the first ultrasonic transducer. The hydrophone could send and receive low frequency sound waves and was later used in the detection of submarines in the World War 1.
On a note parallel to the SONAR, medical research also started taking interest in ultrasonics. In late 1930’s Dr. Karl Dussik used a technique called hyperphonography which recorded echoes of ultrasonic waves on a sensitive paper. This technique was used to produce ultrasound pictures of the brain to help detect tumors and marked the birth of ultrasound imaging. After that, many scientists like Ian Donald, Douglas Howry, Joseph Holmes, John Wild and John Reid improved upon the various aspects of ultrasonic sensors in the medical field which enabled diagnosis of stomach cancers, ovarian cysts, detection of twin pregnancies, tumors etc. Industry too did not waste time in jumping on to the bandwagon and soon developed techniques like ultrasonic welding and non destructive testing at the outset of the 1960s.
How ultrasonic sensor works:
Ultrasonic sensors are devices that use electrical–mechanical energy transformation, the mechanical energy being in the form of ultrasonic waves, to measure distance from the sensor to the target object. Ultrasonic waves are longitudinal mechanical waves which travel as a succession of compressions and rarefactions along the direction of wave propagation through the medium. Any sound wave above the human auditory range of 20,000 Hz is called ultrasound. Depending on the type of application, the range of frequencies has been broadly categorized as shown in the figure below
When ultrasonic waves are incident on an object, diffused reflection of the energy takes place over a wide solid angle which might be as high as 180 degrees. Thus some fraction of the incident energy is reflected back to the transducer in the form of echoes and is detected. The distance to the object (L) can then be calculated through the speed of ultrasonic waves (v) in the medium by the relation
Where‘t’ is the time taken by the wave to reach back to the sensor and ‘’ is the angle between the horizontal and the path taken as shown in the figure. If the object is in motion, instruments based on Doppler shift are used.
Generating ultrasonic waves:
For the generation of such mechanical waves, movement of some surface like a diaphragm is required which can then induce the motion to the medium in front of it in the form of compression and rarefaction. Piezoelectric materials operating in the motor mode and magnetostrictive materials have been widely employed in the generation of ultrasonic waves at frequency ranges of 1-20 MHz and 20-40 kHz respectively. The sensors employ piezoelectric ceramic transducers which flex when an electric signal is applied to them. These are connected to an electronic oscillator whose output generates the oscillating voltages at the required frequency. Materials like Lead Zirconate Titanate are popular piezoelectric materials used in medical ultrasound imaging. For best results, the frequency of the applied oscillations must be equal to the natural frequency of the ceramic, which produces oscillations readily through resonance. It offers maximum sensitivity and efficiency when operated at resonance.
Piezoelectricity being a reversible phenomenon produces electrical voltages when ultrasonic waves reflect back from the target and impinge upon the ceramic structure. In this way, a transducer may work both as a transmitter and a receiver in pulsed mode. When continuous measurement of distances is required, separate transducers may be used for transmission and reception. The sensors when used in industry are generally employed in arrays which may be mechanical arrays consisting of oscillating or rotating sensors, or electronic arrays which may be linear, curved or phased. To visualize the output of an ultrasonic sensor, displays of different kind are used whose shape depends on the type of transducer array used and the function.
A sectored Field of View is produced by mechanical arrays and curved and phased electronic arrays, while a linear field is generated by linear arrays. The display modes may be linear graphical plotting with amplitude on y-axis and time on x-axis called Amplitude mode or A-mode, or intensity modulated B-scans where the brightness of a spot indicates the amplitude of reflected waves. Other modes include M-mode, Doppler (D) Mode etc.
The parameterization of these sensors is generally done by monitoring the reflected and transmitted signals from the lateral an axial motion of transducer while keeping the target fixed in a specific medium (water in general). The sound beam diverges rapidly, hence care is taken that the transducer produces the smallest possible beams. The narrower the beam pattern, the more sensitive the sensor is. However, the angle possible between the transducer and the surface increases with the beam width. The beam patterns of the kind shown below are observed
The parameters on which the performance of an ultrasonic sensor is measured include bandwidth, attenuation, dynamic range and resolution like grayscale, axial and lateral resolution. Other parameters are Nominal Frequency, Peak Frequency, Bandwidth center Frequency, Pulse Width, sensitivity and Signal to Noise Ratio (SNR).
Importance of ultrasonic sensors:
There are a variety of sensors based on other physical transduction principles like the optical range finding sensors and the microwave based devices too. Then why should one use ultrasonic transducers in the first place, given that the speed of sound is very slow than the speed of electromagnetic waves? The answer lies in the question itself. Because the EM waves based devices are too fast. Being slower that the EM waves, the time taken by ultrasonic waves is much longer than that taken by the latter and hence its measurement can be done more easily and less expensively. Because these are based on sound waves rather than EM waves, these would work in places where the latter would not. For example, in the case of clear object detection and measurement of liquid levels or high glare environments, light based sensors would suffer greatly because of the transmittance of the target or the translucence of the propagating media. Ultrasonic devices being based upon sound propagation would remain practically unaffected. These also function well in wet environments where optical beams may suffer from refraction from the water droplets in the environment. On account of range and accuracy, the ultrasonic sensors may lie in between two EM wave based sensors, the Infrared rangefinders on the lower end and the LIDARs on the upper end. Not as accurate or long distance as the LIDARs, the Ultrasonic rangefinders fare better than the IR rangefinders which are highly susceptible to ambient conditions and require recalibration when environment changes. Further these devices offer advantage in medical imaging as compared to MRI or X-Ray scans due to inexpensiveness and portability. No harmful effects of ultrasonic waves at the intensity levels used have been detected in contrast to X-rays or radioactivity based methods and is particularly suited for imaging soft tissues.
Applications of ultrasonic sensor:
The applications of ultrasonic sensors can be classified on the basis of the property that they exploit. These can be summarized as
Research has been going on to overcome the problems of ultrasonic sensors, particularly in medical imaging where it is known as ultrasound. The artifacts of ultrasonic sensors like Acoustic shadowing and Acoustic Enhancement are being exploited to characterize tissues which allow the differentiation between solid and cystic tissues. The industry too has reaped the benefits from ultrasonic sensors in applications like plastic welding, jewelry cleaning, remote sensing and telemetry, assisted parking systems etc. Robotics has been known to use ultrasonic rangefinders as a favorite tool for distance ranging and mapping. Even the fashion industry is using ultrasonic sensors in hair styling like hair extension implants.
Working of ultrasonic sensors:
Ultrasonic sensors are devices that use electrical–mechanical energy transformation to measure distance from the sensor to the target object. Ultrasonic waves are longitudinal mechanical waves which travel as a sequence of compressions and rarefactions along the direction of wave propagation through the medium. Apart from distance measurement, they are also used in ultrasonic material testing (to detect cracks, air bubbles, and other flaws in the products), Object detection, position detection, ultrasonic mouse, etc.
These sensors are categorized in two types according to their working phenomenon – piezoelectric sensors and electrostatic sensors. Here we are discussing the ultrasonic sensor using the piezoelectric principle. Piezoelectric ultrasonic sensors use a piezoelectric material to generate the ultrasonic waves.
An ultrasonic sensor consists of a transmitter and receiver which are available as separate units or embedded together as single unit.
Figure 48 transmitter and receiver
The ultrasonic distance sensor provides precise, non‐contact distance
measurements from about 0.8 to 120 inches.
The ultrasonic sensor works by emitting a short ultrasonic burst of sound
(well above human hearing range) and then “listening” for the echo.
The ultrasonic sensor emits short bursts of sound and listens for this sound to echo off of nearby objects. The frequency of the sound is too high for humans to hear (it is ultrasonic). The ultrasonic sensor measures the time of flight of the sound burst. A user then computes the distance to an object using this time of flight and the speed of sound (1,126 ft/s).
This sensor uses ultrasonic sound to measure distance just like bats and dolphins do. Ultrasonic sound has such a high pitch that humans cannot hear it. This particular sensor sends out an ultrasonic sound that has a frequency of about 40kHz.
The sensor has two main parts:
A transducer that creates an ultrasonic sound and another listens to it's echo.
To use this sensor to measure distance, the robot's brain must measure the amount of time it takes for the ultrasonic sound to travel.
Sound travels at approximately 340 meters per second. This corresponds to about 29.412us (microseconds) per centimeter.
To measure the distance the sound has travelled we use the formula:
Distance = (Time x Speed of Sound) / 2.
The "2" is in the formula because the sound has to travel back and fourth. First the sound travels away from the sensor, then it bounces off of a surface and returns back.
The easy way to read the distance as centimeters is use the formula:
Centimeters = ((Microseconds / 2) / 29).
For example, if it takes 100us (microseconds) for the ultrasonic sound to bounce back, then the distance is ((100 / 2) / 29) centimeters or about 1.7 centimeters.
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