By |September 17, 2018|Categories: Industrial Sensors, Knowledge Base|

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

Different operating modes can be achieved with ultrasonic sensors with switching output by appropriate construction and configuration. Learn more in this blog article about how diffuse mode sensors, retroreflective sensors, and ultrasonic thru-beam sensors work.

1. What Are the Differences Between Ultrasonic Diffuse Mode Sensor, Retroreflective Sensor, and Thru-beam Sensor?

In the diffuse mode sensor, the ultrasonic transducer is used as an emitter and receiver and is usually contained in a housing shared with the electronics for level measurement. Diffuse mode sensors require emitted sound waves to be reflected (echoed) from an object back to the ultrasonic sensor. As soon as an object is in the switching range of the sensor, the distance to the object is determined or a switching operation is triggered.

In retroreflective sensors , the ultrasonic transducer is used as an emitter and receiver. However, unlike diffuse mode sensors, the ultrasonic signal is constantly reflected by a permanently installed reflector—the reference reflector. A metal or plastic plate or a background (e.g., wall, conveyor belt, or floor) can be used as a reflector. When operating as a retroreflective sensor, objects between the ultrasonic sensor and the reflector are detected in any orientation. This includes angled objects that deflect the sound.

The ultrasonic thru-beam sensor features two ultrasonic transducers, one of which acts as an emitter and the other as a receiver. The electronics for level measurement are located in the receiver, along with the switching outputs. For optimal use, the ultrasonic sensors are mounted on an axis exactly opposite each other. As soon as an object interrupts the sound path, the switching output of the ultrasonic sensor changes its status. The sensitivity of the receiver can generally be adjusted via a teach-in function or a potentiometer for different intervals between the emitter and receiver, and/or for different object sizes.


Level measurement in a tank using an ultrasonic sensor as a reflection sensor.

Which Application Areas Are Best Suited to Using Diffuse Mode Sensors, Retroreflective Sensors, and Thru-beam Sensors?

Ultrasonic diffuse mode sensors are especially suitable for level measurement tasks since fill levels in a tank or silo usually change relatively slowly. The switching frequency or the required response time is therefore significantly lower than for object detection in the thru-beam sensor operating mode.

Ultrasonic retroreflective sensors are ideal for objects that do not reliably offer a surface that reflects sound, such as inclined and sound-absorbing surfaces. This operating mode bypasses the issue of a dead band.

It doubles the detection range and enables objects to be reliably detected at significantly greater sensor distances. Since the sensors do not need to switch continuously between emitting and receiving mode, their response behavior is significantly quicker. This results in a much higher switching frequency. The process of counting passing bottles, for example, requires a very high switching frequency. Ultrasonic sensors that are operated as thru-beam sensors are a suitable solution for this.


Reliable detection of an oblique object on a conveyor belt using the ultrasonic sensor operated as a retroreflective sensor.

Can Objects Enter the Radius of the Sound Cone From Any Direction?

The objects being detected can enter the sound cone from any side. The expected switch points can be determined using the detection ranges and response curves specified in the datasheets. The response curve shown in the datasheet is based on room temperature and the diameter of the curve changes as the respective temperature changes. The resulting deviations in the responsiveness of the sensor may need to be considered if the object enters the sound cone radially.

Take a closer look at how ultrasonic sensors work in our animation:


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Technology Guide Double Material Detection



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