An ultrasonic proximity sensor uses a piezoelectric transducer to send and detect sound waves. Transducer generate high frequency sound waves and evaluate the echo by the detector which is received back after reflecting off the target. Sensors calculate the time interval between sending the signal and receiving the echo to determine the distance to the target. When the target enters the operating range the output switches. The ultrasonic proximity switches are equipped with temperature sensors and a compensation circuit, in order to be able to compensate for changes in operating distance caused by temperature fluctuations. The ultrasonic sensor can work in diffuse, reflex or thru-beam mode.
Thru-Beam: In this case the emitter and detector are 2 separate units. The emitter emits the light which is detected by the detector. A target is detected when it passes in-between the emitter and detector.
Diffuse Reflective: In this case the emitter and detector are put in the single package in such a way that their field of view cross. Here the emitter continuously emits the light. When the target comes within the operating range of the sensor the light from the emitter is reflected off the target and detected by the detector. 
Retro-Reflective: the main components of this sensor are the emitter, detector and the Retro-reflector. The emitter and the detector are in the same package. The Retro-reflector is placed little far from the sensor. The light from the emitter is reflected off the Retro-reflector and detected by the detector. When the target passes between the sensor and the Retro-reflector the beam is not reflected back to the detector. Here the problem can be that the beam could reflect from the target itself. For this the polarising filter is used in the sensor. Hence only the light reflected by the retro-reflector is detected by detector
The advantages of an Ultrasonic proximity sensor are
Below are suggested links for more information
Thru-Beam: In this case the emitter and detector are 2 separate units. The emitter emits the light which is detected by the detector. A target is detected when it passes in-between the emitter and detector.
Diffuse Reflective: In this case the emitter and detector are put in the single package in such a way that their field of view cross. Here the emitter continuously emits the light. When the target comes within the operating range of the sensor the light from the emitter is reflected off the target and detected by the detector. Retro-Reflective: the main components of this sensor are the emitter, detector and the Retro-reflector. The emitter and the detector are in the same package. The Retro-reflector is placed little far from the sensor. The light from the emitter is reflected off the Retro-reflector and detected by the detector. When the target passes between the sensor and the Retro-reflector the beam is not reflected back to the detector. Here the problem can be that the beam could reflect from the target itself. For this the polarising filter is used in the sensor. Hence only the light reflected by the retro-reflector is detected by detectorThe advantages of an Ultrasonic proximity sensor are- No physical contact with the object to be detected, therefore, no friction and wear
- Unlimited operating cycles since there is no mechanical contact with the target
- Ultrasonic proximity sensors are not affected by target colour or atmospheric dust, snow, rain..etc
- Can work in adverse conditions
- Sensing distance is more compared to inductive or capacitive proximity sensors
- The targets to be detected can be in the solid, liquid, granular or powder state.
- The sensor has a blind zone of several millimetres in front of it
- Proximity detection
Below are suggested links for more information
