Ultrasonic Sensors Explained: Four Types, Four Applications, One Core Technology

1.Introduction
2.Why go for ultrasonic sensor over optical sensor?
3.Ultrasonic Distance Sensor
4.Ultrasonic Level Sensor
5.Ultrasonic Flow Sensors
6.Ultrasonic Proximity Sensors
7.Future Trends
8.Conclusion

Introduction
Ultrasonic sensing is a very simple concept. You send out a high frequency pulse of sound. That pulse goes out there through the air and it hits the ground and it bounces back. You measure how long it takes to do the round trip. Starting from that time, you calculate distance, level, flow or just the existence of an object.
The physics of industrial sensors are simple, but the engineering of enabling accurate, reliable ultrasonic sensing in the real world isn't.

Why go for ultrasonic sensor over optical sensor?
Optical sensors have a tough time in many real industrial environments. Low lighting, heavy dust, steam, smoke and transparent or highly reflective targets are all problematic for infrared or laser based systems. Ultrasonic sensors can manage all of these situations well.
Transparent targets-Glass bottles, clear film and plastic containers resemble nothing to most optical sensors. These can be detected reliably by ultrasonic sensors.
Dark surfaces-Black rubber matte coatings and dark fabrics absorb infrared light. Sound waves don't care what color they are.
Harsh environments-Dust, smoke, and steam affects the light beams. They have very little impact on the ultrasonic sensor functionality.
Non-contact, no wear-No physical contact with the target, no mechanical wear and no risk of contamination.
Bestar has experience in the design of acoustic components for decades. Its expertise core covers piezoelectric ceramic transducers and high sensitivity acoustic modules-the hardware at the core of every ultrasonic sensor. That foundation is the basis for the specific suggestions about practical recommendations throughout this blog.

Ultrasonic Distance Sensor
1.How it works
Ultrasonic distance sensors use time-of-flight (ToF) method. The transducer sends out a brief pulse followed by listening for the echo. The time elapsed is multiplied by the speed of sound and divided by two by the sensor's processor. The resultant is the distance to the nearest reflecting surface.
Speed of sound varies with temperature, so production-grade sensors are provided with a temperature compensation circuit. Without it, a 68°F(20°C) change in ambient temperature would allow for about 3.5% error in measurement - enough to cause positioning failures in tight tolerance automation equipment.
2.Key applications
Robot Obstacle Avoidance
AGVs and collaborative robots with ultrasonic distance sensor for real-time proximity sensors. The sensors feed the motion controller with constant distance information so the robot can slow, stop or divert before hitting something. Ultrasonic sensor has an advantage over infrared sensor in the warehouse setting where floors are dusty and fork tines and pallets and racking are all different colors and materials.
Automotive Parking Assist
Reversing radar and automatic parking systems use ultrasonic sensors fitted in the bumpers at the front and rear. They detect obstacles at short-range, usually 7.87in(20cm) to 98.43in(2.5m) and feed the data to the ADAS controller. The technology is well-established, low cost and operates well in rain, mud and low-light conditions where camera-based systems break down.

Ultrasonic Level Sensor
1.How it works
Ultrasonic Level sensors are mounted at the top of a tank, silo or open channel. They point downward. The sensor sends out a pulse, the pulse is sent down to the liquid or material surface, and the echo travels back up. The sensor measures that travel time and calculates the distance from sensor to surface. Subtract that from the known height of the tank and you've got the fill level.
The method is fully non-contact. The sensor never comes into contact with the liquid. There are no floats, no pressure ports and no moving parts to corrode, clog or wear out.
Foam, turbulence, and high walls of the tank can scatter the echo and compromise accuracy. Sensor placement angle, beam width and signal processing algorithms are all important in these conditions. Bestar's engineering team will offer professional advice for echo processing of non-standard vessel geometries.
2.Key applications
Industrial Tank Monitoring
Chemical plants, water treatment facilities and food and beverage producers use ultrasonic level sensors to continuously monitor inventory. Because there is no contact with the process fluid, the same sensor hardware can be used for tanks with corrosive acids, clean water or cooking oil, with only a housing material change.
Smart City Infrastructure
Ultrasonic sensors in stormwater drains, sewers and retention ponds to monitor water levels 24 hours a day. When levels are close to flooding, the system sends this out as automated alerts to operators. The sensors operate on low power wireless protocols, are battery powered for years, with no maintenance access, important for installations in confined or hard to access locations.

Ultrasonic Flow Sensors
1.How it works
Ultrasonic flow sensors are based on the transit time method. Two transducers are located on the pipe, one upstream and one downstream. A series of transducers take turns sending and receiving. A pulse sent downstream (in the direction of flow) can travel faster than a pulse sent upstream (against the flow) The time difference is measured by the sensor. A little time difference indicates the small velocity of flow. High flow velocity corresponds to a large time difference. The processor translates that velocity to a flow rate.
Clamp-on variations are attached to the outside of the pipe. They require no pipe cutters, no process shutdowns and no wetted parts. This makes them particularly suitable for retrofitting existing systems or even applications which involve aggressive, hygienic or high purity fluids.
2.Key applications
Smart Water and Gas Meters
Residential and commercial meters for water and gas are making the transition from mechanical turbine and diaphragm meters to ultrasonic. There are no rotating parts to wear out, there are no moving seals to degrade and there is no potential for measurement drift due to mechanical wear over time. Ultrasonic meters are also more accurate for low-flow condition measurements than their mechanical counterparts, relevant for leak detection applications.
Medical Monitors for Infusions
Non-invasive ultrasonic flow sensor can be used to monitor infusion rates of drugs through an IV tube in real-time. The clamp-on transducer pair attaches to the outside of the tubing. There is no contact with the fluid and no chance of contamination. The sensors can accurately detect extremely low flow rates and detect blockages, air bubbles or flow interruptions quickly enough to sound an alarm before patient safety is affected.
Need a pair of transducers for your flow meter design? Bestar's engineering team can help recommend the appropriate frequency, size and beam geometry for your pipe diameter and fluid type.

Ultrasonic Proximity Sensors
1.How it works
Ultrasonic proximity sensors need not know the distance to a target, they just need to get close enough. They just have to know whether or not an object is there in a specific defined zone. The sensor keeps an eye on the echo signal at all times. When the time for the return of the echo is in the detection window, the sensor gives alarm. When this is not the case, the output remains off.
Detection range and trigger level are adjustable. This adjustability makes proximity sensors practical for a very broad range of object sizes and surface types as well as detection distances on the same platform.
2.Key applications
Automated Production Lines
Transparent packaging, clear film, colorless labels and black rubber components are always problematic for optical proximity sensors. Ultrasonic proximity sensors provide sensing of all of these without adjustment. On a food packaging line run which clear blister packs through, there is one ultrasonic sensor instead of different optical sensors with various wavelengths and reflector configurations.
Stack Height Control
Ultrasonic proximity sensors are used to check the height of the stacks of material on conveyor belt, pallets and storage racking. When a pile of corks reaches a predetermined height, the sensor activates the stop signal or the divert sign. The sensors process cardboard, fabric, shrink-wrapped bundles and other non-reflective materials without calibration dependent on surface as in the case of optical systems.

Future Trends
Miniaturized and MEMS ultrasonic sensors
Piezoelectric MEMS ultrasonic transducers (PMUT) are creating the acoustic element on a silicon chip by semiconductor processes. The result is a smaller and less expensive by volume and easier to integrate with signal processing electronics than a traditional ceramic disc. PMUT-based sensors are already appearing in consumer devices and are making their way gradually into industrial and auto-industries where size and price are of critical importance.
Low-power designs for IoT
Remote monitoring applications,  such as water level sensors in rural infrastructure, silo monitors in agricultural facilities, condition monitoring sensors in difficult-to-access locations, all require sensors to run for years on a single battery. This requirement is pushing ultrasonic sensor design toward burst-mode, aggressive sleep cycling and stripped-down signal chains that have only the power required to perform each measurement event.

Conclusion
The four ultrasonic sensor types of this blog are not universal. Each one is optimized for a specific measurement task and there's still one to fit each and every industrial requirements.
Ultrasonic distance sensors take care of positioning and obstacle detection as well as measurement of proximity for robotics and automotive systems.
Ultrasonic level sensors oversee fill levels uninterruptedly in tanks, silos and open channels without having to come in contact with the process medium.
Ultrasonic flow sensors determine the fluid velocity in pipes without error, have no moving parts and suffer no pressure drop where they are read.
Reliable ultrasonic proximity sensors detect the presence of objects on materials and geometries that defeat the use of optical methods.
Together these four types satisfy most of the sensing requirements in modern industrial automation, smart infrastructure and connected devices. They are complementary technologies and in the quality of the acoustic transducer at their core, they are the same.

Bestar designs and makes the piezoelectric transducers and acoustic modules that are the basis for that. With deep expertise in frequency tuning, beam geometry, environmental sealing and module integration, Bestar supports OEM customer support as well as system integrators with the building of sensing solutions that are reliable across the range of conditions with industrial applications.
Whether you are specifying a single transducer for an existing design or developing a whole new sensing module from the ground up, Bestar's engineering team is available to help match to the right acoustic component to your application requirements.