A design profile of ultrasonic sensors

Article By : Jean-Jaques (JJ) DeLisle

Though ultrasonic sensing systems have been around for decades, more modern versions are emerging while using synchronized arrays of sensors or with configurable characteristics such as adjustable sound cones.

There is a lot of buzz about electromagnetic (EM) detection and ranging sensor technology, which encompass radar, lidar, infrared (IR), ultra-wide bandwidth (UWB), and many others. However, there are still many applications where ultrasonic transducers and sensors are viable. For many applications where there could be optically or EM-obscured targets, or when the targets may be transparent to certain EM wavelengths, ultrasonic sensors still provide precision detection, though often not with the comparable refresh rates as EM distance and ranging sensing technologies.

Use cases include material handling, industrial and agricultural machinery, and industrial and logistics processing and operations. Though ultrasonic sensing systems have been around for decades, there are more modern methods of employing ultrasonic sensors, specifically in groups as multiplexing, synchronized arrays of sensors or with configurable characteristics such as adjustable sound cones.

This blog aims to explain the basics of ultrasonic sensors and detail some modern ultrasonic sensing technologies and use cases.

Ultrasonic sensor basics

Most ultrasonic sensors used in industrial applications and robotics are based on a theory that is quite similar to the radar technology. In essence, a pulse is emitted (transmitted) from an ultrasonic transducer, generally between tens to hundreds of kilohertz, and the reflected pulsed signal is detected (received) by the same device.

The time difference between the transmitted and received pulse, as well as the conditions of the air channel—namely, speed of sound, temperature, and humidity—can be used to accurately calculate the distance between the ultrasonic sensor and the target. Though it’s possible with a simple ultrasonic transducer, additional electronics that calculate this distance automatically and output either an analog or digital signal are often integrated into an ultrasonic sensor module for ease of use and installation.

Figure 1 This is how an ultrasonic sensor, a system that can emit and receive ultrasonic waves, is used to sense the distance to and from an object. Source: OnScale

Just as with radars, clutter and other channel degrading factors can limit the accuracy and effectiveness of ultrasonic sensors. However, most modern ultrasonic sensors are relatively immune to contaminants like fog, humidity, dust, dirt, and industrial debris and particles that are in reasonable amounts. As sound doesn’t respond to color or feature sizes that aren’t incredibly minute or reflective, as are EM detecting and ranging sensors, ultrasonic sensors are generally insensitive to color, EM transparency or coatings if they are not acoustically absorbing. Moreover, ultrasonic sensors can be used to detect liquids, and virtually any material that isn’t acoustically absorbative substantially.

Ultrasonic sensors typically operate in a few specific modes. Distance-to-object (DtO) mode provides a signal that indicates a detection and a distance to the object as soon as the object is within range. This mode is also known as simple switching point, or sensor-on-object mode. Window mode (Wnd) is a mode that only triggers detection or distance readings when an object is within a set range, or window, in relation to the sensor.

Lastly, there is object-between-sensor-and-background (ObSB) mode, which requires a step to calibrate the sensor background reference. Whenever an object passes in the path of the sensor and background and changes the acoustic characteristics, the detection and ranging functions of the sensor are triggered. ObSB can be an extremely sensitive mode that can detect thin, flat, or acoustically absorptive objects as any difference from the background reference reading can indicate a detection.

The switching frequency, or switching speed, of an ultrasonic sensor dictates the range of the sensor, while a sensor’s operating frequency determines the sensitivity and accuracy of the sensor to some degree.

Ultrasonic sensor operation

As mentioned above, ultrasonic sensors can be configured in a variety of modes, and they can be paired with other sensors to achieve a variety of design goals. For instance, ultrasonic sensors are available with adjustable sound cones, and the readings can be filtered, multiplex/synchronized, and configured as arrays. Or they can be equipped with more advanced electronics that use sophisticated algorithms to perform advanced signature detection, as in radars.

Figure 2 Ultrasonic sensors can be configured in a variety of modes. Source: OnScale

In multiplexing or synchronization mode, a series of ultrasonic sensors are installed to enable detection in a larger coverage area. However, with multiple ultrasonic sensors in the same vicinity, there is the potential for crosstalk and a higher noise level from multiple sensor signals. Adjustable sound cone techniques can be used to mitigate this to some degree, where synchronization is less susceptible to crosstalk than multiplexing. Here, the sensor timing can be adjusted to simultaneously transmit and change receive timings to avoid crosstalk.

With ultrasonic sensor multiplexing, if the sensors are configured around a detection area that a target is passing through, the sensors could be used to effectively measure the dimensions of the object. As long as the multiplexed sensors are configured in such a way that their sound cones don’t interfere, or are staggered in timing adequately to prevent this, crosstalk generally isn’t an issue. This could be useful in a logistics fulfillment centers where shipping boxes could be accurately measured and correctly sorted, stacked, or handled by appropriate machinery.

Recent technology advances

Much like active electronically scanned arrays (AESA) or phased-array antenna technology, ultrasonic sensors can be put in arrays that allow far more complex detection techniques, including direction or even direction-of-travel for advanced tracking. This technology typically requires a linear array of ultrasonic sensors, though 2D arrays are also possible. Additional processing electronics are needed to handle the array processing needs, which typically aren’t extensive in terms of processing intensity given the relatively low switching frequencies of ultrasonic sensors.

With modulated transducer signals and advanced detection electronics, not only is detection and ranging possible, but there is also the potential to analyze the acoustic signature of a target and determine characteristics of the object. With a suitably-trained artificial intelligence (AI) and machine learning (ML) algorithms, it’s possible that the analysis, characterization, and identification can be done automatically without operator intervention.

 

Figure 3 The integration of ML/AI algorithms in ultrasonic sensor designs is bolstering their analysis, characterization, and identification capabilities. Source: OnScale

A significant drawback of ultrasonic sensor technology is the relative size of these sensors compared to EM detection and ranging sensors. However, recent innovations have led to the development of capacitive micro mechanical ultrasonic transducers (CMUTs), which are far more compact than traditional piezo-based ultrasonic sensors. CMUTs can also potentially operate at higher frequencies than piezo-ultrasonic transducers, which may enable greater accuracy and shorter-range operation.

To counteract the need for a voltage biasing during transmit and receive operation, a new technique using a two-level pulse scheme has been employed to eliminate the need for external bias components and allow for a deeper level of integration and a smaller footprint.

 

This article was originally published on Planet Analog.

Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.

 

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