Ultrasonic Sensors Knowledge (Part 5): Simple Installation and Assembly

The objects being detected can enter the sound beam from any side. The anticipated detection points can be accurately approximated by using the ranges and response curves specified in the ultrasonic sensor data sheets.

Ultrasonic sensors can detect solids, liquids, and powders. The surface properties of the object are critical for the sensor’s echo evaluation. Level, smooth surfaces at right angles to the beam produce ideal reflections. The angular deviation of the measurement plate must not exceed 3° to ensure reliable detection.

Material properties such as transparency, color, or surface treatment (polished or matte) have no effect on detection reliability. Rough surfaces reflect the acoustic energy in multiple directions. This decreases the overall detection range. Conversely, rough surfaces allow greater angular deviation due to the predominantly diffuse reflection of the ultrasound signal.

This characteristic can be used to detect filling levels or piles of coarse-grained materials with an angular deviation of up to 45° (at a reduced range).

The following objects can be detected particularly well:

• All smooth and solid objects aligned perpendicular to the beam angle
• All solid objects with a surface roughness that cause diffuse reflections and that are randomly aligned
• Surfaces of liquids at an angle <3° from the beam axis

The following materials are poorly detected:

• Materials that absorb ultrasonic signals, such as felt, cotton, rough textiles, or foam
• Materials with temperatures of more than 100° C

In these cases, a thru-beam sensor may be required for such materials.       

The response curve of ultrasonic sensors is referred to as a sound beam. Objects are detected within the sound beam if they reflect sufficient sound back to the ultrasonic sensor. The response curve depends on the reflective properties of the object. Therefore, sound beam diagrams are provided for various standard objects in the data sheets. The sound beam has no precisely defined limits and can change due to environmental influences such as temperature or humidity.

If unwanted acoustically reflective objects are present in an application, there must be clearance around the sound beam. This is the only way to avoid incorrect switching caused by unintended reflections.

Response curve 2 (25 mm round bar) can be used for orientation in the case of small, round, or poorly reflective objects. It can also be used for smooth surfaces that are mounted parallel to the ultrasonic sensor’s beam direction (container inner wall, pipes). Clearance corresponding to at least response curve 1 (flat panel 100 mm x 100 mm) must be provided for larger objects with good reflection properties (interfering edges).

If clearances cannot be maintained, many Pepperl+Fuchs ultrasonic sensors provide the option of modifying the sound beam. This procedure can be performed by using Teach-in buttons or with a programming interface and software. The software can be used to selectively suppress many interfering objects within the detection range (fixed-target suppression).

In order to prevent crosstalk of ultrasonic sensors that work at the same detection range and are mounted in parallel, the minimum distances between sensors must be observed, as specified in the following illustrations. The specified values in the displayed table are to be taken as guidelines. They apply if the beam angles are aligned parallel to one another and the surfaces of the objects are at right angles to the beam angle axes. The actual required spacing (X) depends on the alignment, type, and the surface of the target located within the sound beam.

The intervals indicated in the table below must be observed for opposite mounting. If interference occurs, it may be necessary to increase the spacing (X) or, where appropriate, activate the synchronizing or multiplexing feature. Synchronized and unsynchronized ultrasonic sensors must not be mounted opposite each other.