How do diffuse, retroreflective, and through-beam ultrasonic sensors differ?

Ultrasonic frequencies are above the human audio range — higher than 20 kHz. Operating in this frequency range, ultrasonic sensors are commonly used for measuring proximity (distance), determining the level in storage tanks, detecting objects, and more. Ultrasonic sensors provide a non-contact sensing technique by measuring the time it takes for sound to travel to and reflect off the surface (time of flight).

Unlike optical sensors that use the reflection of light (traveling at 3*10⁸ m/s), ultrasonic sensors use sound waves that travel at about 343 m/s. However, ultrasonic sensors can provide stable detection of uneven surfaces, liquids, clear or colored objects, and measurements of objects in dirty, dusty, and even misty environments. Common types or modes of ultrasonic sensors include diffuse (reflection), retroreflective (reflex or diffuse reflection barrier), and through-beam or thru-beam designs. However, ultrasound (medical applications) sensors and sonar (underwater applications) sensors also operate at ultrasonic frequencies but will not be discussed in this blog.

Diffuse mode sensors

With a diffuse mode ultrasonic sensor, the transmitter, receiver, and typically the additional circuitry are all located in the same housing. When the transmitted audio signal reflects or echoes off a target and is detected by the receiver, the integrated circuitry provides a binary switch or an analog or digital signal, depending on the design and application requirements. The transmitted sound expands in a cone shape and can have a dead zone or minimum sensing range. Common applications for diffuse mode ultrasonic sensors include level measurements in a tank or silo since these measurements change relatively slowly.

Retroreflective sensors

Adding a permanently installed reflector or a reference reflector to constantly reflect the transmitted signal turns a diffuse mode sensor into a retroreflective or reflective barrier design. The permanent reflection can come from a variety of surfaces, including a plate made of different hard (sound-reflecting) materials or a background such as a wall or fixed part of the machinery.

With this mode, the sensor can reliably sense objects with surfaces that do not consistently reflect sound, such as inclined and sound-absorbing surfaces. This operating mode avoids the issue of a dead band. Since the sensor does not switch continuously between emitting and receiving modes, its response time and detection range are much improved.

Ultrasonic through-beam sensors

When a separate ultrasonic transmitter and receiver are mounted across from each other on a direct line of sight axis, they create a through-beam sensor. An object passing through the beam interrupts the sound path and causes the sensor to change state, providing a switching function. Through-beam ultrasonic sensors are commonly found in counting applications in factories, such as counting bottles on an assembly line.

Ultrasonic sensor disclaimers/caveats

While ultrasonic sensors have some very positive benefits, they also have a few negatives. They cannot operate in a vacuum since they need air for transmission. Without temperature compensation, a temperature change of 5 to 10 °C can degrade the sensor’s accuracy. Soft materials can also impact their accuracy. Acoustic noise near the ultrasonic sensor’s operating frequency can also cause interference. Finally, small objects and certain object shapes can be hard to detect.

References

Image source: Ultrasonic Sensor FAQ: Differences between Diffuse Mode Sensor, Retroreflective Sensor, and Thru-Beam Sensor
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