Pepperl+Fuchs Blog

4 Key Attributes for Selecting Ultrasonic Sensors

Posted by Austin Phillips on Fri, Mar 03, 2017

Every sensing technology has its own set of unique capabilities that allow it to perform reliably in certain areas. In dirty environments, a photoelectric sensor’s light beam may fail to return to the receiver due to a covered lens or reflector. However, ultrasonic sensors thrive in dirty and dusty environments, since sound waves are unaffected by dust particles floating in the air or adhering to the sensing face. Industries that commonly use ultrasonic sensors include agriculture, food and beverage, and packaging. The following key attributes will help you select the right ultrasonic sensor for your application.

4 Key Attributes for Selecting Ultrasonic Sensors1. Mode

There are three ultrasonic sensor detection modes: diffuse, thru-beam, and retroreflective. Selecting the proper one is critical and will help you narrow your search.

Diffuse is most commonly used. In this mode, the transducer transmits a series of pulses and uses the echo to determine object presence and distance. An example of an ultrasonic diffuse application is the detection of boats on a log flume ride to avoid collisions. Inductive sensors cannot be used here, because the boats are floating in water with gaps on either side and are too far away from the track. Photoelectric sensors would also not fare well due to exposure to weather and the varying colors and shapes of the boats.

Thru-beam ultrasonic sensors have two components: the emitter and the receiver. When the sound beam between the two is interrupted, the output is triggered. Common applications for thru-beam ultrasonic sensors are double sheet and splice detection in the printing industry. The emitter and receiver constantly monitor the entire length of a material passing between them. A double sheet is detected if the echo received by the receiver from the emitter experiences a decrease.  A splice is detected when material runs out, which would stop the process so that the old sheet and the new sheet can be combined.

In retroreflective mode, sound waves are bounced off a permanent reflector, like a floor or a wall. This mode works well for objects that are hard to detect in diffuse mode because if the echo is lost due to being deflected, the output is triggered and an object is confirmed present.

An application for a retroreflective ultrasonic sensor is the detection of a car on an assembly line. The floor below the car is established, and as the car passes underneath the sensor, the echo returns sooner than it would from the floor. The quicker echo signals that a car has been detected. The benefit of using retroreflective mode is that each car is only detected once. A retroreflective ultrasonic may detect three separate objects for each car as there are three sections of metal and two sections of smooth, slanted, glass on every car that may deflect the echo away from the receiver.

Vehicle detection with ultrasonic sensors

2. Housing

Housings come in many different shapes, sizes, and materials. Selecting the correct one depends on the application and environment. The main housing types are cylindrical (12 mm to 30 mm), flat pack, and cube style. Most cylindrical housings are threaded for easy mounting—which is as simple as drilling out the proper size hole and fastening a nut on each side of the sensor. Sensing faces fixed at a right angle are available for applications that are tight on space and do not have room for a horizontally mounted sensor. The most common housing materials are plastic and metal. If your application requires a durable sensor that can withstand high torque, choose a metal housing. Otherwise, choose a plastic housing to reduce costs.

Cylindrical housing ultrasonic sensors

Some of our ultrasonic sensors come with additional features such as a PTFE-coated transducer, or our 100% stainless steel hygienic design for tough conditions. The PTFE-coated transducer provides chemical resistance and corrosion protection. The hygienic design model is commonly used in the food industry. Rather than a cylindrical sensor with a threaded housing, a hygienic design features a food-grade smooth housing. The smooth housing is suitable for high pressure washdowns and does not allow bacteria to grow as it could in a threaded housing’s grooves.

stainless steel hygienic design ultrasonic sensors3. Output Type

The output types available for ultrasonic sensors are analog and switch point. An analog output has a current or voltage signal that changes proportional to the distance being measured. High and low analog endpoints are set within the sensor’s operation range. An object detected at a distance in between the two endpoints will correspond to a current signal (4 mA … 20 mA) or a voltage signal (0 V … 10 V), which corresponds to a distance measurement. Switch point is also known by its types: NPN, PNP, or relay contact. Switch point sensors work by turning the output on or off at a set distance or window. The distance is set by pushbuttons, potentiometers, or software. For example, the turning on and off of a switch output can indicate when a train has left a station.

4. Measurement Range

Our ultrasonic sensor operating distances range from 10 to 10,000 mm (1/2 inch to 33 feet). Similar to the above attributes, the right measurement range for you depends on your application. The smaller the transducer, the smaller the operating range. Also, keep in mind that all ultrasonic sensors have a dead band in front of the sensing face. Dead bands occur because the transducer has to dampen before the echoes return or else the transducer’s ringing will interfere with them. The dampening process still takes a measurement of time to take place; for that reason, objects that are too close to the transducer may return the echo before the transducer is ready to receive it.

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Topics: Ultrasonic Sensors

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