Ultrasonic Sensors

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 Sensors work?

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:

 Axial and Cross Sectional beam profiles

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.

Problems & Concerns

However, Ultrasonic sensors too aren’t free of all the problems. The speed of sound in a medium increases as the temperature of the medium increases. Thus even when the target has remained in the same place, it may now seem that it has shifted to a place closer to the sensor. Air currents due to varied reasons may disturb the path of the wave which could lead to ‘Missed Detection’ or a wrong measurement.

Acoustic noise like high pitched sounds created due to whistling or hissing of valves and pneumatic devices at the frequency close to the operating frequency may interfere with the output of the sensor. Electrical noise also affects the performance of the sensor. These may generate artifacts which are not a true representation of the imaged object. Just like the vision starts to blur when the distance of the object from the eye gets too small for the eyes to see it, ultrasonic devices also have a ‘dead zone’ where the sensor cannot reliably make measurements. This happens due to a phenomenon called ringing which is the continuous vibration of the transducer after emitting the pulse. Thus when the distance is too small, the transducer has not yet come to rest to be able to differentiate between the vibration due to the incident radiation or the oscillation from the electrical excitation. The dangers of Ultrasonic waves are also well founded. If the intensity is too high, it can cause human tissues to heat and may cause ruptures in people exposed to it. Ethical issues like fetus identification and resulting abortions in medical field are also a widespread concern.

Applications

The applications of ultrasonic sensors can be classified on the basis of the property that they exploit. These can be summarized as:

Domain	Parameter	Applications
Time	Tile-of-Flight, Velocity	Density, Thickness, Flaw Detection, Anisotropy, Robotics, Remote Sensing etc.
Attenuation	Fluctuations in reflected and Transmitted Signals	Defect characterization, microstructures, interface analysis
Frequency	Ultrasonic Spectroscopy	Microstructure, grain size, porosity, phase analysis.
Image	Time-of-Flight, velocity, attenuation mapping in Raster C-Scan or SARs	Surface and internal Defect imaging, density, velocity, 2D and 3D imaging.

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.

                                               Flaw Detection Using Ultrasonic Sensors

Future

Non destructive testing and flaw detection uses ultrasonic waves in various modes like the Longitudinal (L-wave) mode and the Shear (S-wave) mode to detect flaws in materials. With the advances in Science, new materials offering increased performance at lower voltages like the capacitive micromachined ultrasonic transducers (CMUTs) are being developed which are expected to have higher bandwidth and greater potential for integration with electronic circuits.

These devices provide non-invasive measures for the detection of problems in all kinds of materials, be it a living tissue or non-living manufactured goods. With a healthy history of being able to detect many problems which otherwise left the doctors dazed and the problem untreated, ultrasonic sensors do offer a lot of promise even in the coming times. The environmental and psychological effects of exposure to EM-radiation being rigorously being put under the scanner, ultrasonic applications are expected to thrive and offer a substantial alternative to the contemporary technologies.